Dissertations / Theses on the topic 'Resistance to drought'

Drought stress was studied in rice (Oryza sativa) and maize (Zea mays) to identify drought-responsive genes and associated biological processes. One experiment with rice examined drought responses in vegetative and reproductive tissues and identified drought-responsive genes in each tissue type. The results showed that brief periods of acute drought stress at or near anthesis reduced photosynthetic efficiency and ultimately lowered grain yield. Yield was reduced as a result both of fewer spikelets developed and of lower spikelet fertility. Affymetrix arrays were used to analyze global gene expression in the transcriptomes of rice vegetative and reproductive tissue. Comparative analysis of the expressed genes indicated that the vegetative and reproductive tissues responded differently to drought stress. An experiment was conducted with maize, using GS-FLX pyrosequencing to identify differentially expressed genes in vegetative and reproductive tissues; and these results were compared with those from the just-described rice transcriptome. Some of the drought-responsive genes in the maize reproductive tissue were validated by quantitative real time polymerase chain reaction (qRT-PCR). The differentially expressed genes common to both maize and rice were further analyzed by gene ontology analysis to reveal core biological processes involved in drought responses. In both species, drought caused a transition from protein synthesis to degradation, and photosynthesis was one of the most severely affected metabolic pathways. In a validating experiment, a drought-responsive transcription factor found in rice and dubbed HIGHER YIELD RICE (HYR) was constitutively expressed in rice, and the transgenic HYR plants were studied. Under well-watered conditions, the HYR plants developed higher rates of photosynthesis, greater levels of soluble sugars (glucose, fructose, and sucrose), more biomass, and higher yield. They also exhibited a drought-resistant phenotype, with higher water use efficiency, photosynthesis, and relative leaf water content under drought stress. Taken together, these studies demonstrate the potential value of newer technologies for identifying genes that might impart drought resistance and for using such genes to make crops more productive either in the presence or in the absence of drought stress
Ph. D.

This thesis reports on an investigation of drought resistance of willow SRC genotypes. Experiments were conducted at Silsoe, Bedfordshire, in pots and field trials in 2002 and in lysimeters in 2003 to evaluate the range of water use efficiency (WUE) of 50 willows varieties (Salix sp.) and isolate morpho-physiological traits related to WUE and drought resistance. Within the genotype pool tested there was a wide range of responses. The results depict the morpho-physiology of an ideal candidate that plant breeding could produce for drier area of UK, which are summarised below. Its cuttings do not develop calluses when stored in darkness at +4°C. After planting, the candidate does not grow rapidly but has an early exponential phase of stem elongation, after a year of growth it has few stems per stool (< 5). Its long, narrow (Rl/w > 8) hairless leaves are characterised by small adaxial epidermal cells (AECS < 330µm2). The ideal candidate prioritises less biomass to its root system (root/shoot < 0.8) mainly in the top 0.2 m. When grown under optimum condition, the large leaf area has high stomatal conductance and leaf temperature. As water stress progresses, the leaf area decreases leaving little time for leaves to senesce and few yellow leaves remain on the stems. The stomatal conductance decreases slowly and the leaf temperature is almost unaffected. If water stress occurs before August the candidate is able to recover faster the initial physiological state and grow new leaves when re-watered. The results indicate that the best parents to produce such candidate are S. viminalis and S. schwerinii or their related hybrids. Water use (WU) of high yielding willow short rotation coppice hybrids is similar which indicates that the opportunity to reduce WU is limited and that productivity can be only improved by increasing WUE to produce above ground biomass and drought resistance. The current willow breeding programme has great chance to produce hybrids with high WUE however the production of a progeny population from high yielding hybrids that contrast widely in resistance to water stress is recommended. In theory, from such a population, valuable data on morpho-physiological traits related to drought resistance and high WUE can be collected and help genomics to develop quantitative trait loci to the condition that reference hybrids are grown along to quantify the level of water stress experienced by the planting.

In the first year, two upland rice varieties (Azucena and Bala), were screened for root response to drought at the West Africa Rice Development Association (WARDA), Cote d’Ivoire, in two fields of slightly different soil penetration resistance (PR) characteristics.  Changes in soil PR and soil water content were monitored during the drought period.  Root density and depth were significantly greater for Azucena than Bala, and on the irrigated plots compared to the droughted plots, although no consistent site differences in root density were observed.  At each site, on the droughted subplot, soil PR quickly increased near the surface (0-30 cm) in response to reduction in soil water content and soil matrix potential caused by root water extraction.  It is likely that this increase in PR would have either prevented or reduced the rate of downward growth of new roots entering or growing within this layer.  Under these conditions, varietal differences in root response to impedance would be important for drought avoidance. In the second year, also at WARDA, a mapping population based on a cross between Azucena and Bala were tested in two fields of contrasting soil physical properties and QTL for root density at 35 cm were identified.  There was no agreement between sites.  Site characterisation prior to field screening revealed the two sites to be very different in terms of soil texture and water relations.  These site differences would have restricted root growth in different ways and are likely to be reasons behind the lack of agreement in root density QTL between sites. In the third year, near-isogenic lines (NILs) differing only in single or multiple root growth QTL were screened in fields at the International Rice Research Institute (IRRI), Philippines, in soils of lower mechanical impedance than sites used at WARDA.  No major differences were observed for root density indicating the importance of interaction between root traits and the environment when considering contribution to drought resistance.

Desert willow and yellow bells had a higher capacity to avoid drought than fruitless mulberry under moist conditions, but the reverse was true under dry conditions. Desert willow and yellow bells had a higher tolerance to dehydration than fruitless mulberry, resulting in a higher relative resistance to drought under either moist or dry conditions. Detached leaves from desert willow and yellow bells plants grown under stress restricted water loss better than those grown under no stress. Fruitless mulberry leaves lost water at essentially the same rate regardless of irrigation.

This study aims: 1) To quantify the factors affecting the changes of site water status of the high sand tailings, 2) To investigate the growth and physiological responses of Acacia mangium and A. auriculiformis to the fluctuations of site water status on high sand dunes, and 3) To identify characteristics which may allow the trees to adapt to and survive on high sand tailings. In both shade and open plots on sand tailings, diurnal air water deficits would be anticipated particularly in sunny days and late afternoon in cloudy days. The results from dipwell showed that the main source of soil water supply to the sand tailings is solely dependent on the rainfall and not from surrounding mining pools. Furthermore, at 6 d after rainfall <37 mm, sand tailings at 0-15 cm depth had low sand suction showing soil water deficit to plant uptake. Hence, the rainfall distribution would determine the growth of acacias in sand tailings. Dry periods significantly reduced (P<0.05) the shoot length and stem diameter growth rates and leaf area of both acacias. Dry periods also significantly increased leaf and branch number shedding rates of both acacias. Generally, shading did not improve the growth rates of both acacias in dry periods but significantly delayed leaf shedding of the two acacias in dry periods. A. auriculiformis had significantly higher shoot length and stem diameter growth rates as the result of higher leaf and shoot numbers. The superiority of A. auriculiformis compared to A. mangium in adapting to high sand tailings was explained by its growth patterns that had characteristics of drought avoidance, and/or tolerance. Greater vegetative growth was observed in A. auriculiformis than A. mangium in wet periods. The effects of atmospheric and soil water fluctuations on some physiological responses of the two species were also examined.

Drought is an important abiotic stress that has limited rice yield in rainfed drought-prone ecosystems.  Its complexity has slowed progress in cultivar improvement.  Recent advances in the identification of quantitative trait loci (QTLs) governing the complex mechanisms of drought resistance have been facilitated by the development of molecular markers in rice. The present study focused on elucidating the causative physiology and genetics of drought avoidance-related QTL on chromosome 7 and others, which have been previously revealed in the Bala x Azucena mapping population under field drought environments.  Here the approaches conducted are (a) QTL mapping for morpho-physiological traits in controlled glasshouse conditions, (b) the development and testing of recombinant inbred near isogenic lines (RINILs) for the drought-avoidance QTL on chromosome 7 and (c) testing a positional candidate gene for that QTL using bioinformatics, physiological measurements and gene expression analysis. From the QTL data and comparative analysis of the results, regions on chromosomes 1, 3, 5 and 7 appear noteworthy in drought avoidance and can be considered candidates for marker-assisted selection.  RINILs on chromosome 7 confirmed that the QTL improves plant water status (relative water content) under drought stress.  Testing RINILs for a root growth QTL on chromosome 9 also confirmed original results of QTL mapping.  Differences between parental varieties Bala and Azucena in root hydraulic flow and the expression of OsPIP2a, an aquaporin known to improve hydraulic conductivity under stress, together with evidence of non-synonymous sequence differences in the gene indicate that OsPP2a is a promising candidate gene for the chromosome 7 QTL.  This can be tested in the future using RINILs developed here.

Drought stress is paramount in limiting worldwide maize (Zea mays L.) production and productivity. In addition, drought has limited expansion of maize production to marginal conditions in areas including western North Dakota (ND) and eastern Montana (MT) where short-season drought tolerance is essential. The development of new cultivars in these dryer areas has been slow in part due to unavailability of reliable field phenotyping methods for breeding purposes and existing insufficient information on the genetic control of agronomic and grain quality attributes under drought stress. Our objectives were (1) to investigate root and stalk quantitative traits as novel field phenotyping methods for drought resistance, (2) to examine the variability and genetic control of agronomic and grain compositional attributes across soil moisture regimes, and (3) to discuss implications of findings for developing new maize cultivars under drought stress environments. Ninety-four partial diallel crosses including 47 diverse maize inbred parents and checks were tested in 12 water stress (WS), well-watered (WW) and random drought (RDT) environments in 2013 and 2014. A new high throughput non-destructive maize phenotyping method for drought tolerance is proposed, including the quantitative traits visible brace root number count and its spread width. Additive gene action was predominant in most of the agronomic and grain quality attributes, while non-additive gene action was important for grain yield, root lodging, stalk diameter, and grain oil content. Root and stalk lodging were negatively associated (P < 0.01) with brace root count and spread width. The estimates of narrow sense heritability were higher for brace root spread width (0.23) and number of brace root count (0.24) as compared to grain yield (0.06) and root lodging (0.1) in WS environments. High relative narrow sense heritability (> 0.40) was estimated for grain compositional attributes except for grain protein and amino acids. Stress environments were largely associated for grain oil and total and extractable starch contents (> 0.60). A reciprocal recurrent selection program can be vital to develop cultivars with high grain yield and quality in drought stress.
North Dakota Corn Growers Association Minnesota Corn Growers Association

Doctor of Philosophy
Department of Horticulture, Forestry, and Recreation Resources
Dale Bremer
Greg Davis
In 2005, turfgrass was estimated to cover approximately 20 million ha of urbanized land. That area is increasing with rapid urbanization, stressing the importance of water conservation in the lawn and landscape industry. Turfgrasses have been identified for replacement by presumably more water-efficient ornamental plant species to conserve water. However, research comparing drought resistance and evapotranspiration (ET) of turfgrasses with ornamental landscape plants is limited. Two studies were conducted to evaluate water use and performance under drought stress of several ornamental and turfgrass species. An online course was developed to educate students about critical water issues related to irrigation in urbanizing watersheds. In a field study, ET was measured using lysimeters and plant water status was evaluated under deficit irrigation (100%, 60%, and 20% ET) in Festuca arundinacea Schreb., Buchloe dactyloides (Nutt.) Engelm. ‘Sharps Improved’, and Ajuga reptans L. ‘Bronze Beauty’. Evapotranspiration was similar between A. reptans and F. arundinacea, and was 32 and 35% greater than ET of B. dactyloides. In a greenhouse study, the performance of one turfgrass (Poa pratensis L. ‘Apollo’) and eight landscape species (Achillea millifolium L., Ajuga reptans L. ‘Bronze Beauty’, Liriope muscari Decne., Pachysandra terminalis Siebold and Zucc., Sedum album L., Thymus serpyllum L., Vinca major L., and Vinca minor L.) was evaluated during a severe dry down and subsequent recovery. S. album, L. muscari, and P. terminalis performed the best, requiring 86 to 254 d to decline to a quality rating of one (1-9 scale: 1=dead/dormant, 9=best quality). The remaining species required 52 to 63 d. The only species to recover were P. pratensis [46% pot cover (PC) after 60 days], S. album (38% PC), and V. major (35% PC). A survey was developed to measure student learning as it relates to the level of sense and meaning present in the content of a new online course entitled “Water Issues in the Lawn and Landscape.” Survey results were compared with student learning as measured through a post-test. Post-test scores declined as the difference between sense and meaning increased (r =-0.82; P=0.03), indicating student learning is higher when both sense and meaning are present.

The relevance of drought and salinity stress under Mediterranean conditions is reviewed and prospects for crop improvement against these constraints are discussed. Field trials under severe drought in Syria highlighted the importance of earliness to ensure satisfactory yields. Peduncle length and frost tolerance were also important attributes. Under more favourable conditions in Sicily, the yield components per se (number of spikes, number of kernels and kernel weight) had greater influence on genotype performance. At both locations of evaluation high yields were attained through different architectures of traits. Durum wheat genetic resources proved very variable. Genotypes were identified which could be used as donors of adaptive characters in breeding programmes. The CERES-Wheat growth model was used for the two locations, using historical weather data and two genotypes of known adaptation to the region. Early heading was a positive attribute, particularly in Syria. At both sites, lengthening of the grain filling period resulted in higher yields. Three sowing dates were simulated. "Early" sowing (1 November) had the highest simulated yield in both environments, suggesting a possible agronomic means to improve yields under stress. Simulated yields were in most cases within 15% of measured values when a comparison was possible. The ability to adjust osmotically was sought in seedlings artificially exposed to drought stress during early development. One entry appeared to possess this feature. However, another genotype, of known tolerance under real conditions, did not show this ability. Therefore, osmotic adjustment during early stages of ontogeny does not seem unequivocally able to identify the best genotypes under drought. Salt tolerance of durum wheat genetic resources was assessed measuring early growth under controlled environment. The data indicated that the results may be somewhat experiment-specific when using different growing techniques such as hydroponics and sand-culture. Finding tolerant tetraploid entries in terms of plant survival and ion uptake seemed difficult. However, variability existed and some entries, less susceptible than others, were noted. They could be used for breeding. For instance, they could be valuable recipient for the introgression of identified resistance mechanisms from other taxa.

Urban soils are typically degraded due to land disturbance. The poor quality physical and chemical properties of the soil can benefit from application of organic amendments. Local sources of such amendments are biosolids, which are treated domestic wastewater sludges. The objective of this experiment was to compare effects of various high quality biosolids-based soil amendments with synthetic fertilizer on the growth and quality of tall fescue (Schedonorus arundinaceus) under two different soil moisture regimes. The research site was a disturbed soil at the Virginia Tech Turfgrass Research Center in Blacksburg, Virginia. The experimental design was a split plot with irrigation regime as the main factor and soil amendments as the split factor. All treatments were arranged in four randomized complete blocks. The study was established in late summer 2013. Soil amendment treatments, applied prior to seeding in September 2013, were: 1) inorganic N, P, K applied according to soil test laboratory recommendations; 2) anaerobically digested, dewatered biosolids to supply agronomic N rate; 3) anaerobically digested, dewatered biosolids blended with sand and sawdust to supply agronomic N rate; 4) anaerobically digested, dewatered biosolids blended with sand and sawdust to supply agronomic P rate; and 5) composted biosolids to supply agronomic N rate. The agronomic N rate for the turfgrass was 224 kg of estimated plant available nitrogen (PAN) ha-1. Inorganic fertilizer was applied to supply annual P and K requirements prior to seeding in late summer, and the N was split into three application timings (September 2013, April 2014, and June 2014). Supplemental fertilizer N to achieve full agronomic N rate was applied to the treatment plots that received the agronomic P rate of blended biosolids-sand-sawdust. The area was seeded on September 13, 2013 with a tall fescue blend at a rate of 488 kg ha-1. Following full tall fescue establishment, in June 2014, two irrigation regimes, consisting of 0% and 80% evapotranspiration replacement every three days, were initiated. The study had three phases denoted as the pre-drought, drought, and recovery phases which started in April and concluded in August of 2014. Turfgrass color and quality, volumetric soil moisture percentage to a 5 cm depth, normalized difference vegetative index (NDVI), clipping yield, and turfgrass N uptake were measured bi-weekly throughout the growing season. During the first May through July 2014 irrigation season, results were that the fertilizer control consistently provided improved responses relative to the biosolids amended treatments. Clipping yield, quality, and NDVI were all significantly greater in the inorganic fertilizer treatment, but volumetric soil moisture percentages were slightly greater in the biosolids treatments. Turfgrass responses appeared to have been associated with plant available nitrogen, which was lower in the biosolids treatments than in the fertilizer treatment. Calculated PAN for the biosolids products was too low to achieve ideal turfgrass growth and quality. Improving the estimated PAN and/or splitting the organic amendment application times should improve the growth and quality of the turfgrass.
Master of Science

Includes bibliographical references (leaves 145-171).
Drought stress is often the most limiting factor to maize and sorghum production in the semi-arid areas. This study evaluates the physiological (water relations, gas exchange characteristics, membrane leakage), biochemical (antioxidant protection mechanisms and photosynthetic pigment compositions) and seed viability and quality response of maize (cv Melkassa-2) and sorghum cv Macia) after exposure to and recovery from pre and post-flowering dehydration in plants grown in a controlled environment growth chamber under constant environmental conditions (12/12h day/night, 28-32/17 °c day/night temperature, 60-80% RH and PPFD of 1200-1400 umol m-2 S-1), at the Department of Botany, University of Cape Town.

Sorghum is the fifth most important cereal in the world and a staple food for humans. It is also a source of food and fodder for animals. In addition to the abiotic stresses such as drought, parasitic weeds of the genus Striga cause losses in sorghum production in sub Saharan Africa. Striga asiatica is a parasitic weed that attacks cereals in low input agricultural systems and is distributed throughout semi- arid regions of Africa. Most sorghum producing farmers rely on tolerance for their harvests in Striga infested fields yet the stability of tolerance in the face of a changing climate (recurrent droughts), new farming systems (mulch based agriculture) and existence of various Striga strains needs further investigation. Reduced strigolactones production was also studied as a resistance mechanism. The first study was focused on the determination of tolerance of Striga asiatica infested sorghum under drought in a pot study. Five sorghum lines were subjected to infestation with Striga and some were not infested while watering was done at 50 percent field capacity (FC) and 100 percent FC. The results showed that the five sorghum lines differed significantly in chlorophyll content and Normalised Differential Vegetation Index (NDVI). Infection did not lower chlorophyll content when it co-occurred with drought across all sorghum lines. Drought and infestation had mutually exclusive effects on chlorophyll content and NDVI. Under infestation, internode length was similar both at 100 percent FC and at 50 percent FC while under uninfested conditions, 100 percent FC gave the longer internode compared to 50 percent FC. Both infestation and irrigation regime reduced the sorghum head weight, illustrating that the two effects have synergistic effects on sorghum head weight. The second study sought to determine the effects of mulching and infestation on sorghum spp tolerance to Striga asiatica. The experiments were carried out in the seasons 2013/14 and 2014/15 summer seasons. The results indicated that mulching increased chlorophyll content in the 2014/15 season which was a drier season compared to 2013/14. In the 2014/15 season, mulching increased chlorophyll content in all varieties except Ruzangwaya, Mukadziusaende and SC Sila. When the same varieties were infested under mulch and infested without mulch, the results showed that mulching overcomes the effects of infestation in some varieties. Mulch also negates the effect of Striga parasitism and results in yield maintenance in sorghum varieties. The third study sought to determine the stability of sorghum lines when exposed to two Striga asiatica lines sourced from two places which are 500 km apart in Zimbabwe. The two strains were termed the Chiundura and Rushinga strains, based on where they were sourced. The experiments were conducted at Henderson Research Station (HRS) at Mazoe and at Bindura University of Science Education (BUSE). The results showed differential virulence for some traits while the two strains were equally virulent for some traits. The two strains were equally virulent on all sorghum lines with respect to chlorophyll content. The different sorghum lines responded differently to the effects of the two strains. The effects of the two strains were generally similar for head index, root index, and leaf index at all sites. Generally the Chiundura strain was more virulent to sorghum lines compared to Rushinga strain, confirming the existence of physiological strains of Striga in Zimbabwe. Therefore physiological speciation of Striga asiatica exists and this adds a further dimension to the complexity of Striga management in the smallholder sector. Quantification of strigolactone production by different sorghum genotypes was conducted in the laboratory using the agar gel assay. The genotype Mukadziusaende produced significantly the least (P<0.01) quantities of strigolactones, as inferred from the maximum germination distance (MGD) from the sorghum root. The MGD was negatively correlated to tiller numbers illustrating that the more the strigolactones the less the tillering capacity. Tiller numbers and MGD can therefore be used to select for reduced strigolactones production.

To decrease dependency of fossil fuels and avoid direct competition with food crops, massive research efforts are investigating next-generation cellulose biofuel crops such as switchgrass (Panicum virgatum). A low-input, sustainable switchgrass production could be achieved by reducing traditional management practices though applying plant growth promoting rhizobacteria (PGPR), of which our understanding is still rather limited. To elucidate physiological mechanisms behind PGPR's beneficial effects, we inoculated switchgrass seedlings with Burkholderia phytofirmans strain PsJN. Two experiments were conducted to determine the initial and long-term responses of switchgrass to PsJN inoculation by tracking growth and leaf physiology. In a third experiments, we tested the effects of PsJN on growth and leaf-level physiology of switchgrass under a moderate pre-drought conditioning and a successive severe drought stress. PsJN inoculation increased biomass and promoted elongation of shoots within 17 days following inoculation. The enhanced root growth in PsJN inoculated plants lagged behind the shoot response, resulting in greater allocation to aboveground growth (p=0.0041). Lower specific root length (p=0.0158) and higher specific leaf weight (p=0.0029) were also observed in PsJN inoculated seedlings, indicating advanced development. Photosynthetic rates (Ps) were higher in PsJN inoculated seedlings after 17 days (54%, p=0.0016), which were related to higher stomatal conductance, greater water use efficiency, and lower non-stomatal limitation of Ps. These rapid changes in leaf physiology are at least partially responsible for switchgrass growth enhancement from PsJN treatment. The early growth enhancement in PsJN inoculated switchgrass linearly decreased with plant age. PsJN inoculation increased Ps of upper canopy leaves by 13.6% but reduced Ps of lower canopy leaves by 8.2%. Accelerated leaf senescence and early flowering were observed in PsJN-inoculated switchgrass, which might contribute to slightly lower aboveground biomass at final harvesting. Drought preconditioning increased Ps of PsJN-inoculated switchgrass during a later severe drought; whereas, control switchgrass only benefited from drought preconditioning when leaf water potential dropped below -1 MPa. This study verified early growth enhancement and accelerated development of switchgrass due to PsJN inoculation. Rapid improvement in leaf physiology is related to enhanced productivity. PsJN inoculation also improve drought tolerance of switchgrass.
Ph. D.

Bibliography: leaves 134-145. The improvement of drought tolerance of commercial soybean varieties via indirect selection for transpiration efficiency (TE) in breeding programs was investigated. The extent and nature of variation for TE among soybean genotypes were established through glasshouse experiments under well watered conditions, and confirmed in the field under contrasting water stress conditions. The results suggest that increasing TE will be a beneficial strategy to improve soybean grain yield at the crop level, and a protocol developed suited to indirect selection for high TE soybean genotypes under a range of environments. This will have immediate application in the development of soybean varieties specifically adapted to the dryland production areas of the Australian sub-tropics.

This dissertation is comprised of three essays that generate methods to measure the ex-ante economic impacts of agriculture technology innovations. The first essay entitled â Valuing Intellectual Property Rights in an Imperfectly Competitive Market: A Biopharming Applicationâ presents a method for valuing the intellectual property rights (IPRs) for an innovation that lowers product production costs below those associated with the patented process of a monopolist. The application to Glucocerebrosidase enzyme from transgenic tobacco suggests an intellectual property rights (IPRs) value of about $1.75 billion. Despite the innovatorâ s market power, significant surplus gains also accrue to consumers. Further, U.S. antitrust laws that prohibit IPRs acquisition by the current monopolist increase consumer welfare by almost 50 percent. The second essay entitled â Ex-Ante Analysis of the Benefits of Transgenic Drought Tolerance Research on Cereal Crops in Low-Income Countriesâ develops a framework to examine the ex-ante benefits of transgenic research on drought in eight low-income countries, including the benefits to producers and consumers from farm income stabilization and the potential magnitude of private sector profits from IPRs. The framework employs country-specific agroecological-drought risk zones and considers both yield increases and yield variance reductions when estimating producer and consumer benefits from research. Benefits from yield variance reductions are shown to be an important component of aggregate drought research benefits, representing 40 percent of total benefits across the eight countries. Further, estimated annual private sector benefits of $US 178 million suggest that significant incentives exist for private sector participation in transgenic drought tolerance research. The third essay entitled â Ex-Ante Evaluation of Alternative Strategies to Increase the Stability of Cropping Systems in Eastern and Central Africaâ examines the ex-ante economic impact of transgenic drought resistance maize breeding and of conventional maize, millet and sorghum drought resistance breeding in Kenya, Uganda, and the Amhara region in Ethiopia. An expected utility framework is combined with a partial equilibrium model and a spatial drought risk zonation scheme to estimate benefits from mean yield increases and yield variance reductions at the market level as well as at the household level for maize, millet and sorghum producers in the administrative regions of each country. Results suggest that annual ex-ante benefits of $87 million, $6.8 million and $4.8 million can be generated from public sector conventional breeding research on maize, sorghum and millet, respectively. Private sector transgenic drought tolerance research may also generate substantial benefits of $97 million for maize producers and consumers, particularly through the reduction of yield variance arising from drought, and an additional $21 million as profits from intellectual property rights protection.
Ph. D.

Introducció
L'aigua és un recurs i un factor limitant per la vida de les plantes. El transport d'aigua des de les arrels fins a les fulles té lloc fonamentalment a través del xilema. La capacitat d'aquest teixit per transportar aigua (i.e., la seva conductivitat hidràulica) no és constant sinó que depèn de les condicions en què es produeix el transport. En particular, quan els potencials hídrics es fan molt negatius (sequera) o les temperatures molt baixes (glaçada) augmenta el risc que la columna d'aigua a l'interior del xilema es trenqui, amb la consegüent pèrdua de conductivitat. La vulnerabilitat a aquesta disfunció del transport (embolisme) depèn de l'espècie i del teixit considerat, i ha estat reconeguda com un dels factors clau en la resposta de les plantes a diversos estrès ambientals. En aquesta tesi s'han estudiat les implicacions de l'embolisme al xilema per la resistència de diverses plantes llenyoses a l'eixut i a les glaçades, amb els següents objectius: (1) estudiar el mecanisme de formació de les embòlies a causa de les glaçades en Larrea tridentata, (2) comparar l'estructura i el funcionament del xilema d'arrels i branques, (3) testar l'existència d'un compromís entre eficiència i seguretat al sistema conductor, (4) estudiar els marges de seguretat als què operen les espècies de diferents comunitats, (5) caracteritzar les estratègies que les espècies estudiades fan servir per fer front a l'eixut, i (6) predir els efectes que el canvi climàtic pot tenir sobre les espècies estudiades.
Vulnerabilitat a l'embolisme induït per les glaçades en Larrea tridentata al desert de Chihuahua
En aquest treball s'estudia la vulnerabilitat a l'embolisme induït per glaçada en una població de Larrea tridentata al desert de Chihuahua (Nou Mèxic, USA). Al camp vam mesurar les temperatures de la fusta, l'aire i el sòl, els potencials hídrics al sòl i a les fulles, i l'embolisme nadiu present a rels i branques. Aquestes mesures les vam completar amb estudis de l'anatomia del xilema i experiments al laboratori en els quals una cambra aïllada connectada a un bany tèrmic s'utilitzava per congelar mostres d'arrels i branques en condicions controlades. Els nostres resultats els vam comparar amb els obtinguts per Pockman & Sperry (1997) en una població de L. tridentata al desert de Sonora. Encara que la població del desert de Chihuahua va resultar una mica més resistent a l'embolisme induït per congelació, l'impacte de les glaçades hi va ser també major perquè estava sotmesa a temperatures molt més baixes. Aquest resultat concorda amb l'observació d'una elevada mortalitat de branques a la població estudiada (ca. 50%), i suggereix que les baixes temperatures contribueixen a limitar la distribució actual de L. tridentata a la zona central de Nou Mèxic. Dins de plantes, les branques estaven més limitades per les glaçades que les arrels, al contrari del que passa amb l'embolisme induït per sequera. Els experiments al laboratori van corroborar les troballes de Pockman & Sperry (1997) quant a l'existència d'una relació entre la temperatura mínima de congelació i el % d'embolisme. La correlació que vam observar entre l'àrea de l'exoterma de baixes temperatures produïda durant els tractaments i el % d'embolisme resultant suggereix que la congelació de l'aigua a l'interior de les cèl.lules del parènquima estava associada amb l'ocurrència de l'embolisme. Aquesta troballa concorda amb evidències recents que apunten cap a un paper actiu de les cèl·lules vives en el transport d'aigua a llargues distàncies dins la planta.
Propietats hidràuliques d'arrels i branques de nou espècies llenyoses en un alzinar del nord-est d'Espanya
En aquest treball estudiem l'arquitectura hidràulica d'arrels i branques de nou espècies que conviuen a un alzinar del nord-est d'Espanya. Les espècies estudiades difereixen en les seves relacions filogenètiques, el seu origen biogeogràfic, el tipus de fusta i l'hàbit foliar. Per a cada espècie es va mesurar l'anatomia del xilema, la vulnerabilitat a l'embolisme induït per sequera, i la dinàmica estacional de les relacions hídriques. Les diferències en les propietats hidràuliques que vam trobar entre teixits són consistents amb els resultats obtinguts en estudis previs. El diàmetre dels conductes del xilema, la conductivitat hidràulica i la vulnerabilitat a l'embolisme van resultar majors a les arrels que a les branques. De resultes d'això, les arrels superficials operaven més a prop del seu límit hidràulic que les branques de diàmetre similar. Els nostres resultats també van mostrar grans diferències en les propietats hidràuliques entre espècies. La pressió a la qual es produïa un 50% d'embolisme va variar entre 1.3 i >8 MPa a les arrels i entre 3.1 i >8 MPa a les branques. De més a menys vulnerable a l'embolisme, les espècies estudiades eren: Quercus ilex = Acer monspessulanum = Arbutus unedo = Sorbus torminalis = Cistus laurifolius > Cistus albidus = Ilex aquifolium > Phillyrea latifolia > Juniperus oxycedrus. Totes les espècies van mostrar grans reduccions de la conductivitat estomàtica associades amb la progressió de l'eixut. Els marges de seguretat (definits com la diferència entre les pressions a les quals es produeix un 95% d'embolisme i els potencials hídrics mínims mesurats al camp) van resultar també molt diferents segons les espècies. Algunes d'elles (J. oxycedrus, I. aquifolium, P. latifolia) tenien un xilema desproporcionadament resistent a la cavitació. Vam trobar una relació no lineal negativa entre la conductivitat hidràulica i la resistència a la cavitació. La forma d'aquesta relació, una funció potencial amb exponent ca. -2, l'expliquem amb un model simple que assumeix una relació lineal entre el diàmetre dels vasos i la mida del seu porus més gran. Finalment, discutim les implicacions per la tolerància a la sequera del compromís que implica la relació anterior.
Arquitectura hidràulica de Pinus halepensis, P. pinea i Tetraclinis articulata en un ecosistema dunar de l'est d'Espanya
En aquest treball s'estudia l'arquitectura hidràulica de Pinus halepensis, P. pinea i Tetraclinis articulata en una zona de dunes de l'est d'Espanya. Les variables mesurades inclouen la vulnerabilitat a l'embolisme, la conductivitat hidràulica i la discriminació isotòpica foliar. Els potencials hídrics de les fulles es van avaluar també durant un període d'eixut extrem. Els resultats van mostrar que el diàmetre de les traqueides i la conductivitat hidràulica era sempre major a les arrels que a les branques. Les arrels eren també més vulnerables a l'embolisme i operaven més a prop del seu límit hidràulic (i.e., amb marges de seguretat més estrets). Encara que no la vam quantificar, vam observar una mortalitat extensiva d'arrels en els dos pins. T. articulata va resultar molt més resistent a l'embolisme que P. pinea i P. halepensis. Al mateix temps, T. articulata presentava potencials hídrics més baixos. De resultes d'això, els marges de seguretat eren raonablement similars per les tres espècies. L'estudi posa de manifest grans diferències en l'estratègia d'us de l'aigua entre T. articulata i els pins. Mentre l'estratègia de T. articulata era marcadament conservativa, basada en la poca vulnerabilitat del seu sistema conductor, els dos pins presentaven mecanismes reguladors a diferents nivells (i.e., embolisme, demografia de rels) que limitaven l'absorció d'aigua quan aquesta esdevenia escassa.
Mortalitat induïda per l'eixut i arquitectura hidràulica en poblacions de pins del nord-est de la Península Ibèrica
Els estius de 1994 i, en menor grau, 1998 van ser particularment secs a l'est d'Espanya. Aquest eixut va provocar danys importants en algunes espècies. En aquest treball s'estima la mortalitat associada a l'eixut en diverses poblacions de tres espècies de pins que coexisteixen a la zona d'estudi (Pinus nigra, P. pinaster i P. sylvestris). Per cada població, vam mesurar també la conductivitat hidràulica, la vulnerabilitat a l'embolisme i el gruix dels anells de creixement. Els resultats van mostrar que la mortalitat només va afectar P. sylvestris, i que l'afectació va ser diferent en dues poblacions d'aquesta espècie. Encara que les conductivitats hidràuliques màximes i les vulnerabilitats a l'embolisme van resultar molt semblants entre espècies i poblacions, aquestes diferien en altres aspectes de la seva arquitectura hidràulica. En particular, (1) la conductivitat hidràulica per unitat d'àrea foliar era menor en la població de P. sylvestris que va resultar més afectada per la sequera. Conductivitats específiques foliars baixes provoquen majors gradients de potencial hídric i, per tant, majors nivells d'embolisme (si les vulnerabilitats són iguals). Suggerim que aquesta diferència va ser la principal causant del patró de mortalitats que vam observar entre poblacions. (2) P. pinaster mostrava una major eficiència en l'ús de l'aigua (inferida a partir de la discriminació isotòpica del carboni a les fulles) que les altres dues espècies. Respecte a la resposta a la sequera a nivell de població, la població més afectada de P. sylvestris va augmentar lleugerament el creixement després de la sequera de 1994. Aquest resultat el relacionem amb una possible relaxació de la competència entre els individus supervivents. L'elevada taxa de mortalitat observada en aquest estudi suggereix que climes més secs, com els previstos per la majoria de models climàtics, poden posar en perill la supervivència de diverses poblacions de P. sylvestris a la conca mediterrània.
Flux de saba en tres espècies mediterrànies que coexisteixen, i la seva variació en funció de les condicions ambientals
En aquest treball s'estudien els patrons estacionals d'ús de l'aigua en tres espècies que conviuen en un alzinar del nord-est d'Espanya. Les tres espècies estudiades (Quercus ilex L., Phillyrea latifolia L. i Arbutus unedo L.) constitueixen *99% de l'àrea basal de l'alzinar. El període d'estudi va incloure les estacions seques dels anys 1999 i 2000. L'ús de l'aigua es va estimar mitjançant sensors de flux de saba de tipus Granier. També es van mesurar les variables meteorològiques estàndard, el contingut d'aigua del sòl, i els potencials hídrics a les fulles. Els fluxos de saba diaris (JS) van estar molt influïts per les variables ambientals. En general, JS es correlacionava sobretot amb el dèficit de pressió de vapor de l'atmosfera durant la primavera, mentre que durant l'estiu el dèficit hídric del sòl explicava la major part de la variància de JS. La reducció mitjana de JS entre el començament de l'estiu i el pic de l'eixut fou del 60% per A. unedo, del 48% per P. latifolia i del 80% per Q. ilex. Aquest estudi va formar part també d'un experiment de simulació de sequera. A. unedo va respondre a la reducció experimental de la disponibilitat d'aigua reduint el flux de saba durant l'estiu. Aquesta espècie també va modificar l'ús de l'aigua entre anys segons els diferents patrons de precipitació que hi van tenir lloc. La transpiració estimada a nivell de parcel.la durant l'any 1999, que fou relativament humit, va ser el 98% de la precipitació neta, corroborant que la comunitat estudiada estava limitada per la disponibilitat hídrica. Els nostres resultats, combinats amb investigacions prèvies realitzades a la mateixa zona, perfilen estratègies molt diferents per fer front a l'eixut en cadascuna de les tres espècies estudiades. A. unedo i Q. ilex van requerir un control estomàtic molt estricte per evitar potencials hídrics excessivament baixos que podien causar nivells perillosos d'embolisme. D'altra banda, la seva poca vulnerabilitat a l'embolisme va permetre a P. latifolia tenir un control estomàtic més lax i mantenir la transpiració a potencials hídrics més baixos que les altres dues espècies. Q. ilex va patir una reducció del flux de saba a llarg terme que relacionem amb els elevats nivells d'embolisme predits per aquesta espècie. En conjunt, els nostres resultats suggereixen que Q. ilex i, en menor grau, A. unedo, podrien estar al seu límit per fer front a l'eixut en diverses zones de la Mediterrània, en concordança amb els greus efectes que la sequera de 1994 va tenir en moltes poblacions de Q. ilex del nord-est d'Espanya. Aquests resultats són discutits en relació amb els possibles impactes que el canvi climàtic tindrà en els boscos actualment dominats per Q. ilex.
Un model hidràulic per predir la mortalitat induïda per l'eixut en plantes llenyoses: una aplicació a l'estudi dels efectes del canvi climàtic a la Mediterrània
Hi ha un creixent interès en l'estudi dels efectes del canvi climàtic sobre la vegetació. A la regió mediterrània, l'efecte dominant del canvi climàtic serà probablement a través de la modificació del balanç hídric. En el present treball presentem un model orientat a predir la mortalitat induïda per sequera en plantes llenyoses sotmeses a diferents règims climàtics. El model té una base fisiològica i simula el transport d'aigua a través de plantes individuals, les quals poden estar isolades o competir per un recurs hídric comú. Suposem que la mortalitat que es pot produir durant un episodi de sequera aguda es deu a la incapacitat de les plantes de transportar aigua fins les fulles. En el cas particular que estudiem aquí, es comparen dues espècies perennifòlies, Quercus ilex i Phillyrea latifolia, que van resultar diferentment afectades per l'eixut extrem que va tenir lloc a l'est d'Espanya l'estiu de 1994. Mentre en algunes poblacions de Q. ilex es van arribar a assecar completament el 80% dels individus, P. latifolia no va mostrar danys aparents. Aquestes dues espècies conviuen en un alzinar que va ser estudiat durant els anys 1999 i 2000. Les mesures de flux de saba obtingudes en aquest alzinar es van utilitzar per calibrar el model mitjançant una aproximació de tipus GLUE. L'única diferència entre espècies que es va introduir a priori va ser la major vulnerabilitat de Q. ilex a l'embolisme al xilema, basada en mesures realitzades a la mateixa zona d'estudi. Durant la calibració, la informació proporcionada per les mesures de flux de saba es va utilitzar per retenir els conjunts de paràmetres més versemblants per cada espècie, els quals van ser utilitzats en les simulacions subsegüents. El model va ser capaç de simular amb una exactitud raonable tant la transpiració com la mortalitat mesurada per ambdues espècies a l'àrea d'estudi. En les simulacions de canvi climàtic es van explorar dos factors: l'augment en la temperatura mitjana (+1.5, +3, +4.5ºC) a través del seu efecte en ET, i la durada de l'eixut estival. Sota qualsevol dels escenaris les mortalitats van ser majors per Q. ilex: mentre que els resultats van indicar que aquesta espècie era capaç de resistir amb mortalitats de menys del 5% sequeres de fins a 84-94 dies, la mortalitat de P. latifolia assolia el 5% entre els dies 133 i 150. Per eixuts de més de tres mesos, durada aproximada dels estius secs que es produeixen actualment a la zona d'estudi, la mortalitat de Q. ilex augmentava ràpidament. Aquests resultats són discutits en relació als possibles impactes a llarg termini que el canvi climàtic pot tenir sobre els alzinars.
Introduction
Water is a resource and a limiting factor for plants. Water transport from roots to leaves occurs mainly through the xylem. The capacity of this tissue to transport water (i.e., its hydraulic conductivity) it is not constant but varies as a function of the conditions at which the transport takes place. In particular, the probability that the water column inside the plant breaks increases when water potentials become very negative (drought) or when temperatures are very low (frost). This dysfunction, called xylem embolism, causes a decrease in the hydraulic conductivity of the tissue. The vulnerability to xylem embolism depends on the species and tissue considered, and has been recognized as one of the key factors in the response of plants to several environmental stresses. In this thesis we have studied the implications of xylem embolism for drought and frost resistance in several woody plants, with the following objectives: (1) to study the mechanism of freezing-induced xylem embolism in Larrea tridentata, (2) to compare xylem structure and function between roots and stems, specially with regard to their possible role in limiting water transport, (3) to test the existence of a trade-off between conducting efficiency and security in the xylem in species from a given community, and explore its possible causes and consequences, (4) to study the safety margins at which the different species of a given community operate, (5) to characterize the strategies to cope with drought stress of several Mediterranean species from different communities, (6) to predict the likely effects of climate change on the species studied.
The vulnerability to freezing-induced xylem cavitation of Larrea tridentata in the Chihuahuan desert
The vulnerability of Larrea tridentata to freezing-induced xylem embolism was studied in a Chihuahuan desert population (central New Mexico, USA). In the field we measured wood, air and soil temperatures, soil and leaf water potentials, and native embolism in roots and branches. These measurements were combined with anatomical studies and laboratory experiments in which an insulated chamber attached to a temperature bath was used to freeze stem and root samples under controlled conditions. We compared our results with the measurements by Pockman & Sperry (1997) in a Sonoran desert population of L. tridentata. The comparison showed that although the Chihuahuan population was slightly more resistant to freezing-induced xylem embolism, the impact of freezing was also greater in this population because of the much lower temperatures. This result agrees with the observation of ca. 50% stem mortality in the population studied, and suggests that low temperatures contribute to limit the present distribution of L. tridentata in central New Mexico. Within plants, stems were more limited by freezing than roots, as opposed to what happens with drought-induced embolism. Our laboratory experiments corroborated the findings by Pockman & Sperry (1997) regarding the existence of a relationship between minimum freezing temperature and % embolism. The correlation observed between the area of the low temperature exotherms produced during the freezing treatments and the resulting % embolism strongly suggested that the freezing of water inside parenchyma cells was associated with the occurrence of xylem embolism. This finding adds to the existing evidence supporting an active role of living cells in long-distance water transport.
Hydraulic properties of roots and stems of nine woody species from a Holm oak forest in NE Spain
The hydraulic architecture of roots and stems of nine woody species was studied in a Holm oak forest in NE Spain. The species studied differed in phylogenetic relationships, biogeographical origin, wood type and leaf-habit. Xylem anatomy, hydraulic conductivity, vulnerability to drought-induced xylem embolism, and seasonal water relations during a period of acute drought were measured. Between-tissues differences in hydraulic properties were consistent with previous findings reported in the literature. The diameter of xylem conduits, hydraulic conductivity and vulnerability to xylem embolism were always higher in roots than in stems. Surface roots were shown to live closer to their hydraulic limit than stems of similar diameter. Our results also showed large differences in hydraulic properties among species. The pressure at which a 50% of conductivity was lost due to embolism ranged between 1.3 and >8 MPa for roots and between 3.1 and >8 MPa for stems. Vulnerability to xylem embolism followed a pattern of Quercus ilex = Acer monspessulanum = Arbutus unedo = Sorbus torminalis = Cistus laurifolius > Cistus albidus = Ilex aquifolium > Phillyrea latifolia > Juniperus oxycedrus. All species showed large reductions in stomatal conductance associated with the progression of drought. Safety margins (defined as the difference between the pressure causing 95% embolism and minimum leaf water potentials under field conditions) differed markedly among species, with some of them (J. oxycedrus, I. aquifolium, P. latifolia) showing a xylem overly resistant to cavitation. A non-linear, negative relationship between hydraulic conductivity and resistance to cavitation was found when species and tissues were combined. The shape of this relationship, a power function with exponent ca. -2, is explained using a simple model that assumes a linear relationship between vessel diameter and the size of its larger pore. The implications of the trade-off implied by the above relationship are discussed in relation to drought tolerance of the species studied.
Hydraulic architecture of Pinus halepensis, P. pinea and Tetraclinis articulata in a dune ecosystem of Eastern Spain
The hydraulic architecture of Pinus pinea, P. halepensis and Tetraclinis articulata was studied in a coastal dune area from Eastern Spain. The measured variables include vulnerability to xylem embolism, hydraulic conductivity and carbon isotopic discrimination in leaves. Leaf water potentials were also monitored in the three studied populations during an extremely dry period. Our results showed that roots had always wider tracheids and higher hydraulic conductivity than branches. Roots were also more vulnerable to xylem embolism and operated closer to their hydraulic limit (i.e., with narrower safety margins). Although it was not quantified, extensive root mortality was observed in the two pines. T. articulata was much more resistant to embolism than P. pinea and P. halepensis. At the same time, T. articulata experienced lower water potentials. As a result, the safety margins were reasonably similar for the three species. The study outlined very different water-use strategies for T. articulata and the pines. Whereas T. articulata had a conservative strategy that relied on the low vulnerability of its conducting system, the two pines showed regulatory mechanisms at different levels (i.e., embolism, root demography) that constrained the absorption of water when it became scarce.
Drought-induced mortality and hydraulic architecture in pine populations of the NE Iberian Peninsula
The summers of 1994 and, to a lesser extent, 1998 were particularly dry in eastern Spain. As a result, several plant species were severely affected. We estimated drought-induced mortality in populations of three pine species that co-exist in the study area (Pinus nigra, P. pinaster and P. sylvestris). Hydraulic conductivity, vulnerability to xylem embolism, and tree-ring width were also measured for each population. Results showed that mortality only affected P. sylvestris, and that there were significant differences between two populations of this species. Although maximum hydraulic conductivity and vulnerability to embolism were almost identical among species and populations, they differed in other aspects of their hydraulic architecture. In particular: (1) hydraulic conductivity per unit of leaf area was lower in the most acutely affected P. sylvestris population. Lower leaf-specific conductivity causes higher water potential gradients and, hence, higher levels of embolism (if vulnerabilities are alike). We suggest that this difference was the main cause of the observed mortality pattern. (2) P. pinaster showed higher water-use efficiency (inferred from _13C data) than the other two species. Regarding the response to drought at the population level, the most affected P. sylvestris population slightly increased growth after the 1994 drought, which we relate to a relaxation of competition among surviving individuals. The important drought-induced mortality observed in the study area suggests that a drier climate (as predicted by climate change simulations) may endanger several P. sylvestris populations in the Mediterranean basin.
Sap flow of three co-occurring Mediterranean species under varying atmospheric and soil water conditions
We studied the seasonal patterns of water use of three woody species co-occurring in a Holm oak forest in NE Spain. The three species studied (Quercus ilex L., Phillyrea latifolia L. and Arbutus unedo L.) constitute *99% of the total basal area of the forest. The study period included the dry seasons of years 1999 and 2000. Water use was estimated by means of Granier-type sap flux sensors. Standard meteorological variables, soil water content, and leaf water potentials were also monitored. Daily sap flux (JS) was strongly influenced by environmental variables. In general, JS was mainly correlated with atmospheric water pressure deficit during the spring (wet period), while during the summer (dry period) soil moisture deficit explained most of the variance in JS. The average reduction in JS between early summer and the peak of the drought was 60% for A. unedo, 48% for P. latifolia, and 80% for Q. ilex. This study was also part of a drought simulation experiment. A. unedo responded to the experimental decrease in water availability reducing sap flux during the summer. This species also modified its water use between years according to their different seasonal patterns of precipitation. Estimated stand transpiration during the "relatively wet" 1999 year was 98% of net rainfall, further supporting that the community was limited by water availability. Our results, combined with previous studies in the same area, outlined very different strategies to cope with water shortage for the three species. A. unedo and Q. ilex needed a strong stomatal control over water loss to avoid low water potentials that could cause dangerous levels of xylem embolism. On the other hand, low vulnerability to xylem embolism allowed P. latifolia to have a less strict stomatal regulation and maintain transpiration at lower water potentials than the other two species. Q. ilex showed a long-term reduction of sap flux which we relate to the high levels of embolism predicted for this species. Our results suggest that Q. ilex and, to a lesser extent, A. unedo, may be at their limit to cope with water stress in many Mediterranean areas, in agreement with the acute impact that the 1994 drought had on Q. ilex populations in NE Spain. These results are discussed in relation to the possible impacts that climate change will have on Q. ilex-dominated forests.
A hydraulic model to predict drought-induced mortality in woody plants: an application to climate change in the Mediterranean
The potential effects of climate change on vegetation are of increasing concern. In the Mediterranean region, the dominant effect of climate change is expected to be the modification of water balance. In this paper we present a model developed to predict drought-induced mortality of woody plants under different climatic scenarios. The model is physiologically-based and simulates water transport within individual woody plants, which can be isolated or competing for a common water pool. The key assumption of the model is that, within a drought event, plant mortality is caused by the inability of plants to transport water to the leaves. In the particular application that we report in this study, we compare two evergreen species, Quercus ilex and Phillyrea latifolia, which were very differently affected by the acute drought that occurred in E Spain in summer 1994. While in some Q. ilex populations the amount of individuals that dried completely was up to 80%, P. latifolia showed no apparent damage. This two species grow together in a Holm-oak forest in NE Spain that was monitored during the years 1999 and 2000. A Generalised Likelihood Uncertainty Estimation (GLUE) approach was used to calibrate the model against sap-flow measurements. The only difference between species that was introduced a priori was that Q. ilex was more vulnerable to xylem embolism than P. latifolia (based on our own measurements in the study area). During the calibration the information provided by the measured sap flows was used to retain the more likely parameter sets for each species. These parameter sets were used in all the following simulations. The model was able to accurately simulate both transpiration dynamics and measured mortalities for the two species in the study area. In the simulations under climate change two factors were explored: the increase in mean temperature (+1.5, +3 and +4.5ºC) through its effect on ET, and the duration of summer drought. Under any of the scenarios, mortalities were much higher for Q. ilex: while this species was predicted to survive with less than 5% mortality droughts of up to 84-94 days, the mortality of P. latifolia reached 5% between the day 133 and the 150. For droughts longer than three months, which is approximately the current drought duration in the study area for dry years, the mortality of Q. ilex increased sharply. These results are discussed in relation to the possible long-term impacts of climate change on Q. ilex-dominated forests.

A set of 135 rice landraces from different geo-climatic regions of Sri Lanka were tested for their rooting characteristics with high throughput physiological screening methods. In the preliminary screening, nearly half of the variation in toxicity symptoms observed in all 135 landraces due to herbicide layer (TRIK, 100 mg / plant) buried at 30cm depth was explained by landrace. Deep rooted and shallow rooted landraces were distinguished by herbicide score. Selected 36 landraces with diverse herbicide scores were tested in hydroponics for 5 weeks for root traits. Root length and root thickness had significant genotypic differences explaining 60% and 30% of the variation. From 36, 12 landraces were then grown in rhizotrons for 6 weeks and their root characteristics were confirmed. Genotypic differences accounted for 62, 74 and 54% of variation in root length, root angle and root thickness. Root screening concluded that Podi wee-1, Sinnavellai, Niyan Wee-1 and 2 are the most deep rooting, drought resistant landraces among the set of 135 Sri Lankan landraces. These landraces showed better relative growth than shallow rooted landraces under drought environment. Cluster analysis of SNP data from 378 markers indicated that there are 6 different subpopulations among landraces. The most deep rooted, upland landrace Podi Wee-1was identified as a tropical japonica variety .Other deep rooted landraces, Sinnavellai, Niyan Wee-1 and Niyan Wee-2, were grouped as indica 2, a significantly different group from all other indicas due to their high mean herbicide score and high drought resistant properties. None of the OryzaSNP indica landraces grouped with the indica 2, suggesting that the indica 2 group identified here represents a local subpopulation of indica adapted to drought prone environments. It was proven that differences in rooting depth were associated with genetic variation both at the whole genome level.

The work described in this thesis aimed to identify new structural and regulatory genes involved in trichome development in tomato (Solanum lycopersicum). Trichomes are hairs that cover the aerial epidermis of most terrestrial plants, and they confer resistance to abiotic and biotic stresses. In species with glandular trichomes, such as tomato, trichomes can produce a vast array of specialised metabolites with diverse biological activities. I explored the genetic variation present in the wild tomato species Solanum pennellii, which has high trichome density and is tolerant to biotic and abiotic stresses, to find genes involved in trichome development. I determined the trichome phenotype of the S. pennellii x S. lycopersicum cv. M82 introgression lines over two generations and identified genomic regions potentially containing genes involved in trichome development. I tested candidate gene by transient virus-induced gene silencing (VIGS) in tomato and could identify three genes with a role in trichome formation: SlMIXTA-like, SlMX2 and SlCycB2. I generated transgenic lines overexpressing SlMIXTA-like as well as SlMIXTA-like knock-out mutants using the CRISPR/Cas9 system to characterise this gene functionally. These results, combined with promoter analysis and comparisons between the MIXTA-like structure and function in S. pennellii and S. lycopersicum, showed that SlMIXTA-like is a negative regulator of trichome initiation and a key factor in epidermal patterning in tomato. I used the CRISPR/Cas9 system to generate knock-out mutants of known regulators of trichome development in tomato (SlMX1, Woolly, Hairless, CD2 and DWARF). Analyses of these knock-out mutants contributed to clarifying their precise function in trichome development and led to the establishment of a better model for trichome initiation and morphogenesis in tomato. My research also showed a clear relationship between trichome density and tolerance to drought stress and whitefly herbivory, indicating that trichome-related traits can be targeted for agricultural improvement of tomato cultivars.

A four-year drought, increasing population and shifting climate has spurred water conservation practices within Virginia. Creeping bentgrass (Agrostis palustris â L93â ), Kentucky bluegrass (Poa pratensis â Midnightâ ), and tall fescue (Festuca arundinacea) Dominion blend were evaluated under deficit irrigation and upon exogenous application of plant growth stimulants (PGS), seaweed extract (SWE) + humic acid (HA), glycinebetaine (GB) and a commercial SWE product (PP). The objectives were to determine crop coefficients (K_c) for creeping bentgrass fairways and tall fescue home lawns, to determine if PGS application allowed for more water conservation, and to determine if they impacted physiological function and/or root morphology. A preliminary greenhouse experiment was conducted with creeping bentgrass and Kentucky bluegrass irrigated with 100%, 85% and 70% of evapotranspiration (ET). The study determined that an additional deficit irrigation level should be included for the field study and that GB application and 100% and 85% ET irrigation level produced the greatest creeping bentgrass root mass. The two â year field study evaluated creeping bentgrass and tall fescue. Tall fescue home lawns could be irrigated every five days with a K_c of 0.55 or once a week with a K_c of 0.70. Creeping bentgrass fairways could be irrigated every four days with a K_c of 0.85. Glycinebetaine application increased bentgrass rooting after planting and showed osmoprotectant properties. Another greenhouse study evaluated five GB rates on bentgrass and tall fescue. No differences were found between the five rates and concluded that the rate utilized in the field study may be appropriate for turfgrass application.
Master of Science

Doctor of Philosophy
Department of Biology
David C. Hartnett
Grazing and drought are instrumental in the development and maintenance of perennial grasslands. In this research I tested the belowground bud bank contribution to tallgrass prairie resistance and resilience when perturbed by grazing and drought. First, I tested the bud bank role in vegetation response to and recovery from severe drought (Chapter 2). I compared above- and belowground responses of experimentally droughted plots to ambient controls and irrigated plots during two years of severe drought and two years of recovery. I found that although aboveground net primary productivity declined 30-60% during drought, bud bank density and demography were insensitive to drought. These results suggest that grassland resistance and resilience when perturbed by drought may be mediated by stability of belowground bud banks. Second, I investigated vegetation and soil nutrient legacies following release from long-term grazing (Chapter 3). I documented a relatively rapid shift in aboveground vegetation within four years of grazer exclusion, with productivity, stem density, and diversity becoming relatively more similar to ungrazed than grazed prairie. The density and composition of the belowground bud bank and soil seed bank shifted more slowly, remaining more similar to grazed than ungrazed prairie. Responses of soil nutrients to removal of grazers varied, and in some cases was affected by recent fire history. These results demonstrate the contribution of belowground propagules to the maintenance of a diverse plant community both during grazing and after grazers are removed. Finally, I examined short-term vegetation responses to both drought and grazing (Chapter 4). Despite extreme drought and simulated grazing that reduced productivity and increased mortality of individual stems, the dominant C4 grasses maintained a stable bud bank. Aboveground net primary productivity and bud bank density of sedges and forbs, however, were reduced by both drought and grazing. This differential response of species to extreme drought and grazing led to shifts in community composition and species diversity over one growing season. Across drought and grazing treatments, live rhizome biomass was highly correlated with bud bank density and may be a useful, more easily measured index of bud bank density.

A transgenic Arabidopsis line containing a chimeric PR-1::luciferase (LUC) reporter gene was subjected to mutagenesis with activation tags. Screening of lines via high throughput LUC imaging identified a number of dominant Arabidopsis mutants that exhibited enhanced PR-1 gene expression. Here we report the characterisation of one of these mutants, designated activated disease resistance (adr) 1. This line showed constitutive expression of a number of key defence marker genes and accumulated salicylic acid, but not ethylene or jasmonic acid. Furthermore, adr1 plants exhibited resistance against the biotrophic pathogen Peronospora parasitica and Erysiphe cichoracearum but not the necrotrophic fungus Botrytis cineria. Analysis of a series of adr1 double mutants suggested that ADR1-mediated resistance against P. parasitica was SA-dependent, while resistance against E. cichoracearum was both SA-dependent and partially NPR1-dependent. The controlled, transient expression of ADR1 conveyed striking disease resistance in the absence of yield penalty, highlighting the potential utility of this gene in crop protection. Homologous genes were uncovered in many phylogenetically distant and agronomically important plant species; their sequence analysis revealed a number of consensus motifs unique and distinctive for the ADR1 family. Furthermore, we show that either constitutive or conditional enhanced expression of ADR1 conferred significant drought tolerance. Cross-tolerance was not a characteristic of adr1 plants; rather they showed increased sensitivity to thermal and salinity stress.  Hence, ADR1 activated signalling may antagonise some stress responses. Additionally, a large activation tagged population was screened in order to isolate novel mutants altered in disease susceptibility.  Here we report the characterisation of one mutant, designated activated disease susceptibility (ads) 1. This line was confirmed to concurrently exhibit increased susceptibility to hemi-biotrophic, necrotrophic and non-host pathogens.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
Among the principal native fruit trees in Northeastern Brazil, especially those found in the semi-arid areas, the umbu tree (Spondias tuberosa Arruda) represents itself as an important alternative as it well accepted by consumers and is a good produce in dry environments.Thus, the fruit trade fair or through cooperatives provides a source of supplementary income for small farmers. However, this income can be compromised by harvesting and excessive deforestation, which has intensified each year. Concern with population reduction of this species and by anthropic, Brazilian Institute for the Semi-Arid Tropic has developed studies on seedlings production, cultivation, and genetic inheritance preservation recovering genotypes with distinct morphological characteristics and deployment of a germplasm active bank provide the most promising for small producers, in addition to contributing to the reforestation of the Caatinga with a native species. Of the climatic factors limiting fruit species production in the semi-arid northeast, drought is the main factor, also allied to the growing problem of soil salinization, which has worsened each year. The mechanisms used by umbu tree to tolerate drought is not well elucidated and the physiological response before soil salinity is not yet known. Thus, the present work aimed to evaluate the physiological responses of umbu tree to drought and salt stresses. To evaluate drought responses, a project was developed in green house conditions using four grafted genotypes classified as giantumbu (BGU 44, BGU 48, BGU 50 and BGU 68) in order to evaluate the alterations on stomatal behavior, anatomical parameters, water relations and some biochemical aspects induced by intermittent drought and the possible genotypical variations. Transpiration (E) and diffusive resistance (rs) were measured daily after the beginning of the stress treatments by withholding water. When plants presented stomatal closure, the vases were re-watered and the water withhold again. This cycle was repeated for a 31 period days. The leaf water potential (Yw) was measured in four-hour intervals during a 24-hour period at the moment of the first stomatal closure and at the end of the experimental period. Total soluble carbohydrates (CHS), free amino acids (AA), protein (PROT) and proline (PRO) in leaves and roots were also measured. Certain regularity in the stomatal closure was observed among the watering period, showing differences between the species. The correlation with environmental factors suggest that, besides the water, stomatal behavior of BGU 44 and BGU 68 were influenced by Tar, RH and VPD, while the access BGU 50 were influenced by PAR and BGU 48 had no correlation with these environmental factors, suggesting that the water exerted the major influence in this genotype. Anatomical alterations in response to drought on stomatal density (DE) and reductions on stomatal index (IE) and stomatal aperture size (AO) were observed.The access BGU 48 maintained its anatomical features unaltered. There was an inversion in tissue proportion in BGU 44 under stress conditions, reducing the thickness of the spongy parenchyma and increasing palisade parenchyma. The inverse occurred with BGU 68 and theremaining genotypes continued unchanged. The lower Yw time of most of the genotypes was between 8h and 12h. The Yw of the stressed plants of BGU 44 and BGU 50 reduced significantly at 8h. The highest Yw was observed to BGU 68. The stress prolongation induced reductions in CHS content in the leaves of all genotypes. There were increases in the leaves to AA in BGUs 44 and 48, while BGUs 50 and 68 were reduced by about 40% and 43% respectively. BGU 44 and BGU 50 kept this behavior at the end of the experimental period.Significant differences in PROT content were not observed, but there were increases of 50% in PRO, except to BGU 50. Alterations on CHS, AA and PRO contents in the roots were verified and varied among the different genotypes. BGU 68 and BGU 50 were the most contrasting genotypes. In order to evaluate the salt stress responses in umbu plants a project was developed using seedlings propagated by seeds. Plants were grown in washed sand with Hoagland & Arnon nutrient solution without salt and with 25, 50, 75 and 100mM NaCl. Growth, Yw, E and rs were then evaluated. Na+, K+, Cl-, soluble carbohydrates and free amino acid contents were measured in several plant organs. Most variables were affected with salinity above 50 mM NaCl showing decreases in: number of leaves, plant height, stems diameter and dry masses and increases in root to shoot ratio. Reductions in pre-dawn leafwater potential (Ypd) were observed in plants submitted to 75 and 100 mM NaCl. Salt levels applied increased Na+ and Cl- contents in leaves. However, K+ content was not affected. A saturation to retain Na+ and Cl- in stems and roots was verified in treatments above 50 mM NaCl. These results allow us to say that there are physiological and anatomical differences among umbu tree genotypes; genotypes respond differently to intermittent drought; the turgor maintenance in umbu tree is relative to water storage in the xylopodium associated with the efficient stomatal closure mechanism and not by osmotic active solutes accumulation in either drought or salt stress conditions; due to the great variation found, the organic solutes accumulations did not demonstrate to be a good physiological trait as indicator to droughtand salt-tolerance in umbu plants. This specie tolerates salt levels until 50 mM NaCl withoutshowing significant physio-morphological alterations.
Dentre as principais fruteiras nativas do Nordeste, especialmente aquelas encontradas no semi-árido, o umbuzeiro (Spondias tuberosa Arruda) desponta com uma alternativa importante, por ser uma fruta bem aceita pelo consumidor e por ter uma boa produção em ambientes secos. Dessa forma, o comércio dos frutos em feiras livres ou através de cooperativas proporciona uma fonte de renda complementar para os pequenos agricultores. No entanto, essa renda pode ser comprometida pelo extrativismo e o desmatamento excessivos, que tem se intensificado a cada ano. Preocupada com a redução populacional desta espécie pela ação antrópica, a Embrapa Semi-Árido vem desenvolvendo estudos sobre produção de mudas, cultivo e preservação da herança genética, através da recuperação de acessos com características morfológicas distintas e a implantação de um banco ativo de germoplasma, para disponibilizar os mais promissores para os pequenos agricultores, além de contribuir com o reflorestamento da Caatinga com uma espécie nativa. Dos fatores climáticoslimitantes na produção de espécies frutíferas no semi-árido nordestino, a seca é o principal fator, aliado também ao problema crescente de salinização dos solos, que tem se agravado a cada ano. Os mecanismos utilizados pelo umbuzeiro paratolerar a seca ainda não estão completamente esclarecidos e não se conhece ainda as respostas fisiológicas frente a salinidade do solo. Desta forma, o presente trabalho objetivou avaliar as respostas fisiológicasdo umbuzeiro quando submetido às condições de seca e salinidade. Para avaliar as respostas à seca, desenvolveu-se um experimento em casa de vegetação utilizando mudas enxertadas de quatro acessos de umbuzeiro (acessos BGU 44, BGU 48, BGU 50 e BGU 68) classificados como umbu-gigante, com o objetivo de avaliar as alterações no comportamento estomático, parâmetros anatômicos, relações hídricas e alguns parâmetros bioquímicos induzidos pela seca intermitente, além das possíveis variações genotípicas. Foram efetuadas mensurações da transpiração (E) e da resistência difusiva (rs) diariamente após a suspensão da rega até ocorrer o fechamento estomático, momento em que as plantas foram re-irrigadas. A rega foi suspensa novamente até ocorrer novo fechamento estomático e este ciclo foi repetido por um período de 31 dias. O potencial hídrico foliar (Yw) foi determinado em dois cursos nictimerais (no momento do primeiro fechamento estomático e ao final do período experimental). Também foram avaliados os teores de carboidratos solúveis totais (CHS), aminoácidos livres (AA), proteína (PROT) e prolina (PRO) nas folhas e nas raízes, assim como alterações anatômicas. Os acessos apresentaram regularidade no período de fechamento estomático entre as regas, demonstrando diferenças intra-específicas. Houve correlação com as variáveis ambientais sugerindo que, além da água, o comportamento estomático dos acessos BGU 44 e BGU 68sofreram influência da Tar, UR e DPV, enquanto que o acesso BGU 50 sofreu influência do PAR e o BGU 48 não se correlacionou com os outros fatores, indicando que a água foi o fator que exerceu maior influência neste acesso. Alterações anatômicas em resposta à seca foramobservadas na densidade de estômatos (DE), reduções no índice estomático (IE) e na abertura do ostíolo (AO). O acesso BGU 48 manteve as características anatômicas inalteradas. Houve uma inversão na proporção dos tecidos do acesso BGU 44 quando sob estresse, diminuindo a espessura do parênquima lacunoso e aumentando o parênquima paliçádico. O inverso ocorreu com o BGU 68 e os demais acessos permaneceram inalterados. O horário de menor Yw para a maioria dos acessos foi entre 8h e 12h. O Yw das plantas estressadas do BGU 44 e BGU 50 foi reduzido significativamente às 8h. O BGU 68 apresentou os valores mais elevados de Yw. O prolongamento do estresse provocou reduções nos teores de CHS nas folhas de todos os acessos. Houve aumento no teor de AA nas folhas dos BGU’s 44 e 48, enquanto que os BGU’s 50 e 68 reduziram 40% e 43%, respectivamente. Ao final do período experimental esse comportamento se manteve para o BGU 44 e o BGU 50. Não houve diferença significativa para os teores de PROT nas folhas, mas houve aumento de 50% nos teores de PRO, exceto para o BGU 50. Foram verificadas alterações na concentração de CHS, AA e PRO nas raízes, com diferença entre os acessos. Os acessos BGU 68 e BGU 50 foram os mais contrastantes em condições de seca. Para avaliar as respostas do umbuzeiro ao estresse salino, foi desenvolvido um experimento utilizando-se plantas propagadas por sementes. As plantas foram cultivadas em areia lavada, regadas com solução nutritiva de Hoagland & Arnon, sem ecom adição de NaCl (25, 50, 75 e 100 mM). Avaliou-se o crescimento, o Yw, E e rs. O teor de Na+, K+, Cl-, carboidratos solúveis e aminoácidos livres foram dosados nos diversos órgãos da planta. A maioria das variáveis estudadas foi afetada em níveis de NaCl de 50 mM, reduzindo o número de folhas, a altura das plantas, o diâmetro do caule e a massa seca e aumentando a relação raiz/parte aérea (R/Pa). O potencial hídrico foliar antes do amanhecer (Ypd) foi reduzido nas plantas dos tratamentos 75 e 100 mM de NaCl. A concentração de Na+ e Cl- nas folhas aumentou em função dos níveis de NaCl aplicados, mas, o teor de K+ não foi afetado. Nos caules e raízes, houve uma saturação na retenção de Na+ e Cl- nos tratamentos acima de 50 mM. Os resultados desta pesquisa permite inferir que existem diferenças fisiológicas e anatômicas entre os acessos de umbuzeiro estudados; que eles respondem de forma diferente à seca intermitente; que a manutenção da turgescência foliar está relacionada à reserva de água nos xilopódios associado ao mecanismo de fechamento estomático eficiente e não ao acúmulo de solutos osmoticamente ativos, tanto em situação de seca como de salinidade no meio; devido à grande variação encontrada, o acúmulo de solutos orgânicos não demonstrou ser um mecanismo fisiológico indicador de tolerância à seca e a salinidade nesta espécie; o umbuzeiro tolera níveis de salinidade de até 50 mM de NaCl sem apresentar alteraçõesfisiomorfológicas significativas na fase inicial do desenvolvimento.

Plusieurs études ont montré que la productivité des forêts et des cultures décroît sur l’ensemble de la planète en réponse au changement climatique, et que les risques de mortalité liée à la sécheresse s’accroissent. Ces mortalités massives ont été observées dans tous les écosystèmes forestiers et seraient dues à un dysfonctionnement hydraulique des plantes, causé par de l’embolie xylémiène. Le spectre interspécifique de résistance à l'embolie a été largement étudiée, mais peu d'attention a été accordée à la variabilité intra-spécifique de ce trait. C’est pourquoi, l’objectif de ce travail de thèse était d’améliorer notre compréhension de la variabilité intra-spécifique de la résistance à l’embolie chez une espèce cultivée(Helianthus annuus L.) et chez un conifère (Pinus pinaster). Nous avons déterminé la vulnérabilité à l’embolie de quatre variétés de tournesol, présentant des différences significatives de P50 (pression induisant 50% de perte de conductivité hydraulique), allant de -2.67 à -3.22 MPa. Un compromis entre sécurité hydraulique et croissance a été observé mais pas entre la sécurité hydraulique et la capacité de transport de l'eau du xylème. Seuls quelques caractères anatomiques, tels que la densité des vaisseaux et la surface du lumen, présentent une corrélation avec la résistance à l’embolie. Dans un second temps, nous avons phénotypé pour la résistance à l’embolie et la densité du bois 477 génotypes de P. pinaster, issus d’une F2 de pleins-frères correspondant à la 3ème génération consanguine obtenue par auto-fécondation d’un hybride inter-provenance “Corse × Landes. La variabilité de la P50 au sein de l’espèce était extrêmement faible (CVP=5.6%). D’autre part, nous avons mis en évidence l’absence de compromis entre sécurité hydraulique et densité du bois. La part de variance du caractère P50 expliquée par QTLs (PEV)correspondait à 23.6% de la variance phénotypique totale mais aucun QTL commun n’a pu être détecté entre P50 et densité du bois, suggérant qu’au sein de cette famille, la plus forte résistance à l’embolie de certains génotypes n’est pas liée à un bois plus dense. Une analyse multi-caractère du fonctionnement de la feuille et du xylème a été réalisée via l’étude de la dynamique saisonnière de la fluorescence du photosystème II et des traits hydrauliques de la tige, sur une famille de pleins-frères d’un hybride interprovenance “Maroc × Landes” de P. pinaster ; en réponse à une diminution du potentiel hydrique,l’efficience du photosystème II et la conductance stomatique ont tous deux diminué, induisant une décroissance rapide du taux d’assimilation. La résistance à l’embolie du xylème n’a pas montré de relation significative avec les traits foliaires. L’absence de compromis entre ces caractères permet donc d’envisager la sélection génétique de génotypes à la fois efficients et résistants
As a consequence of current global climate change, forest and crop ecosystems are decreasing inproductivity worldwide and are under increased threat of widespread drought-induced mortality due tohydraulic failure caused by xylem embolism. The range of embolism resistance across species has been intensively studied but little attention has been paid to intraspecific variability. This thesis aimed thereforeat improving our understanding of the intraspecific variation in embolism resistance in one crop species(Helianthus annuus L.) and one conifer species (Pinus pinaster). In the first place, vulnerability to embolism was determined across four sunflower accessions with significant variations in P50 (pressure inducing 50% loss of hydraulic conductivity) ranging between -2.67 and -3.22 MPa. A trade-off between hydraulic safety and growth was observed but not between hydraulic safety and efficiency. Only few anatomical traits, such as vessel density and vessel lumen area, were found related to embolism resistance.Secondly, we phenotyped 477 P. pinaster full-sib F2 genotypes corresponding to a three-generation inbred pedigree obtained by the self-pollination of an inter “Corsica × Landes” provenance hybrid, for embolismresistance and wood density. The variability for P50 within species was remarkably low (CVP=5.6%). Wealso found a lack of trade-off between hydraulic safety and wood density. The phenotypic variance explained by QTLs (PEV) detected for P50 accounted for 23.6% of the phenotypic variation but no QTL collocated between P50 and wood density, suggesting that embolism resistant genotypes in this family are not dependent on denser wood. A multi-trait analysis of leaf and xylem functioning was performed through seasonal measurements of photosystem II fluorescence and stem hydraulic traits on a full-sib family of aninter “Morocco × Landes” provenance hybrid of P. pinaster; it revealed that with decreasing water potential, the efficiency of both photosystem II and stomatal conductance decreased which resulted in rapid decrease in assimilation rate. Xylem resistance to embolism was found having no relationship with leaf functioning traits. The absence of trade-off between these traits makes it possible to consider selection for both efficient and resistant genotypes