Genetic Variation For Drought Tolerance Research Articles

Weak local adaptation to drought in seedlings of a widespread conifer.

Tree seedlings from populations native to drier regions are often assumed to be more drought tolerant than those from wetter provenances. However, intraspecific variation in drought tolerance has not been well-characterized despite being critical for developing climate change mitigation and adaptation strategies, and for predicting the effects of drought on forests. We used a large-scale common garden drought-to-death experiment to assess range-wide variation in drought tolerance, measured by decline of photosynthetic efficiency, growth, and plastic responses to extreme summer drought in seedlings of 73 natural populations of the two main varieties of Douglas-fir (Pseudotsuga menziesii var. menziesii and var. glauca). Local adaptation to drought was weak in var. glauca and nearly absent in menziesii. Var. glauca showed higher tolerance to drought but slower growth than var. menziesii. Clinal variation in drought tolerance and growth species-wide was mainly associated with temperature rather than precipitation. A higher degree of plasticity for growth was observed in var. menziesii in response to extreme drought. Genetic variation for drought tolerance in seedlings within varieties is maintained primarily within populations. Selective breeding within populations may facilitate adaptation to drought more than assisted gene flow.

Genetic variation for effects of drought stress on yield formation traits among commercial soybean [Glycine max (L.) Merr.] cultivars adapted to Ontario, Canada.

Drought stress significantly limits soybean [Glycine max (L.) Merr.] yields in Ontario, Canada. Many studies of genetic variation for drought tolerance compare commercial lines with exotic, unadapted germplasm. We hypothesized that even current commercial cultivars adapted to Ontario would differ significantly for traits related to drought tolerance. In a greenhouse experiment, we grew fifteen soybean cultivars in field soil amended with sand in 1-m rooting columns, which allowed for simulation of field-like soil water profiles and rooting depths. Two watering treatments were imposed from the first flower until maturity by daily restoration of soil water to either 100% (control), or 50% (drought stress) of the maximum soil water holding capacity. Throughout the experiment, we measured volumetric soil water content at different depths in the soil profile, but found no evidence at any developmental stage that the cultivars differed for their ability to extract soil water from different depths. Drought stress reduced seed yield by 51% on average. Similar to the effects of drought in the field, pod number was the yield component most affected, with effects on seeds per pod and single-seed weight being comparatively minor. There were significant cultivar × treatment interactions for seed yield, pod number, shoot dry matter, and water use. We identified two drought-sensitive (Saska and OAC Drayton) and three drought-tolerant (OAC Lakeview, OAC Champion, and PRO 2715R) cultivars based on their ratios of seed yield under drought stress to seed yield under control conditions (seed yield ratio, SYR). Regression and principal component analyses revealed that drought-tolerant (high-SYR) cultivars were consistently those that maintained relatively high values for water use, biomass accumulation and pod number under drought stress; high water use efficiency under drought stress was also associated with a high SYR. One of the cultivars, OAC Lakeview, displayed a distinct mode of drought tolerance, maintaining a very high fraction of its control pod number under drought stress. This study helps define the physiological basis of soybean cultivar differences in drought tolerance, and provides direction for soybean breeders to select traits that could improve yield under drought stress.

Physiological and Yield Responses of Selected Mungbean Genotypes to Terminal Drought

Extreme drought conditions have been a major limiting factor in legume production worldwide. The development of drought-resilient crops should help mitigate adverse effects of unfavorable environmental conditions such as drought; however, initial characterization is necessary to ensure that the germplasm has the essential genetic variation for drought tolerance. In this study, a field trial was conducted to characterize drought response and potential drought tolerance of 100 mungbean genotypes through the evaluation of several physiological parameters and seed yield. The genotypes were subjected to terminal drought stress before flowering (~ 30 d after planting), reducing soil moisture content (SMC) from 25.0–30.0% to 13.0–15.0% during treatment imposition. Significant differences were observed between mungbean genotypes for total chlorophyll content (Chl), total scavenging activity (SA), electrolyte leakage (EC), and chlorophyll fluorescence (Fv/Fm) during drought. Furthermore, by transforming each physiological parameter into principal components (PCs) via principal component analysis (PCA), five distinct clusters were generated. Based on their physiological response and seed yield, Clusters I and II were identified as potential drought susceptible group, Cluster III was moderately susceptible, and Clusters IV and V were identified as the potential drought-tolerant group. Correlation analysis and PCA also demonstrated the significant relationship among several key traits for drought tolerance such as chlorophyll content, electrolyte leakage, and antioxidant scavenging to chlorophyll fluorescence – suggesting the suitability of fluorescence parameter in mungbean mass screening for drought tolerance. Although weak correlation between physiological parameters to seed yield was present (0.3 <│r│< 0.5), correlations among each physiological parameter were moderately strong (0.5 <│r│< 0.7). The findings suggest that a physiological characterization during initial screening for drought tolerance was an effective approach in identifying possible sources of drought tolerance traits complementary with yield- related traits. The evaluation of chlorophyll fluorescence, scavenging activity, electrolyte leakage, and chlorophyll content was enough to differentiate and characterize the drought response of several mungbean genotypes and will be vital in the development of climate-resilient crops.

Effects of drought on agronomic and fiber quality in an introgressed backcross inbred line population of Upland cotton under field conditions

Upland cotton (Gossypium hirsutum) is the most important fiber crop and an important oilseed crop in the world. Developing drought tolerant cultivars is important to sustainable cotton production in arid and semi-arid regions. However, there is a lack of information on genetic variation for drought tolerance in breeding populations. In this study, a backcross inbred line (BIL) Upland population with germplasm introgression from a more drought-tolerant American Pima cotton (G. barbadense) parent was evaluated in multiple field environments under well-watered (WW) and water-limited (WL) conditions at the flowering stage. Drought at the flowering stage reduced seedcotton yield (SCY), lint yield (LY), boll weight (BW), fiber length (FL), and fiber strength (STR) but increased lint percentage (LP), micronaire (MIC), neps per size, nep per gram, seedcoat nep size, and seed coat fragment count by gram. Significant genetic differences in the BIL population were detected for most agronomic and fiber quality traits and drought susceptibility index (DSI) for these traits. DSI of SCY and LY ranged from 0.01 to 1.5 and 0.1–1.45, respectively, reflecting the tolerance level of the BILs under WL conditions. The broad-sense heritability estimates were moderate for most agronomic traits and moderate to high for most fiber quality traits. Results indicated that highly positive correlation was detected for agronomic and fiber quality traits between WW and WL conditions, with the highest correlations for SCY, LY, UHM, SCY, and MIC. Promising BILs with drought tolerance were identified. This study has provided a line of evidence that field drought tolerance was transferred from the more drought tolerant Pima line to Upland cotton.

Optimising the dosage of ethyl methanesulphonate mutagenesis in selected wheat genotypes

Narrow genetic variation limits the success of crop improvement programs. Mutagenesis using ethyl methane- sulphonate (EMS) provides an opportunity to increase genetic variation to enhance selection in wheat improvement. This study aimed at establishing the optimum dosage and treatment conditions for EMS to induce genetic variation for drought tolerance and enhanced carbon sequestration in wheat. Seeds of three genotypes (LM29, LM43 and LM75) were treated with four EMS dosages (0, 0.1, 0.4 and 0.7% v/v) at three temperatures (25, 30 and 35 °C) for three exposure periods (1, 1.5 and 2 h) using three replicates. The ideal treatment conditions for effective mutagenesis were 0.7% EMS for 2 h at 35 °C for genotypes LM29 and LM43 and 0.4% EMS for 2 h at 25 °C for LM75. The estimated EMS dosages for LM43, LM29 and LM75 were 0.32%, 1.07% and 1.81% v/v EMS, respectively. This information can be used for large-scale mutation induction, exploration of new genetic variation, evaluation for genetic improvement and selection of mutant individuals for drought tolerance, high root–shoot biomass and carbon sequestration.

Genome-wide analysis of maize OSCA family members and their involvement in drought stress.

BackgroundWorldwide cultivation of maize is often impacted negatively by drought stress. Hyperosmolality-gated calcium-permeable channels (OSCA) have been characterized as osmosensors in Arabidopsis. However, the involvement of members of the maize OSCA (ZmOSCA) gene family in response to drought stress is unknown. It is furthermore unclear which ZmOSCA gene plays a major role in genetic improvement of drought tolerance in Maize.MethodsWe predicted the protein domain structure and transmembrane regions by using the NCBI Conserved Domain Database database and TMHMM server separately. The phylogeny tree was built by Mega7. We used the mixed linear model in TASSEL to perform the family-based association analysis.ResultsIn this report, 12 ZmOSCA genes were uncovered in the maize genome by a genome-wide survey and analyzed systematically to reveal their synteny and phylogenetic relationship with the genomes of rice, maize, and sorghum. These analyses indicated a relatively conserved evolutionary history of the ZmOSCA gene family. Protein domain and transmembrane analysis indicated that most of the 12 ZmOSCAs shared similar structures with their homologs. The result of differential expression analysis under drought at various stages, as well as the expression profiles in 15 tissues, revealed a functional divergence of ZmOSCA genes. Notably, the expression level of ZmOSCA4.1 being up-regulated in both seedlings and adult leaves. Notably, the association analysis between genetic variations in these genes and drought tolerance was detected. Significant associations between genetic variation in ZmOSCA4.1 and drought tolerance were found at the seedling stage. Our report provides a detailed analysis of the ZmOSCAs in the maize genome. These findings will contribute to future studies on the functional characterization of ZmOSCA proteins in response to water deficit stress, as well as understanding the mechanism of genetic variation in drought tolerance in maize.

Intraspecific variation in drought susceptibility in Eucalyptus globulus is linked to differences in leaf vulnerability.

Understanding intraspecific variation in the vulnerability of the xylem to hydraulic failure during drought is critical in predicting the response of forest tree species to climate change. However, few studies have assessed intraspecific variation in this trait, and a likely limitation is the large number of measurements required to generate the standard 'vulnerability curve' used to assess hydraulic failure. Here we explore an alternative approach that requires fewer measurements, and assess within species variation in leaf xylem vulnerability in Eucalyptus globulus Labill., an ecologically and economically important species with known genetic variation in drought tolerance. Using this approach we demonstrate significant phenotypic differences and evidence of plasticity among two provenances with contrasting drought tolerance.

Genetic variation in drought tolerance at seedling stage and grain yield in low rainfall environments in wheat (Triticum aestivum L.)

Genetic architecture of seedling drought tolerance is complex and needs to be better understood. To address this challenge, we developed a protocol to identify the most promising drought-tolerant genotypes at the seedling stage in winter wheat. A population of 146 recombinant inbred lines (F9) derived from a cross between wheat cultivars, ‘Harry’ (seedling drought tolerant) and ‘Wesley’ (seedling drought susceptible) were used in this study. All genotypes were sown in three replications in a randomized complete block design under controlled conditions in a greenhouse. Seven traits were scored and grouped into tolerance traits; days to wilting, leaf wilting, and stay green and survival traits; days to regrowth, regrowth, drought survival rate, and recovery after irrigation. Three selection indices were calculated (1) tolerance index, (2) survival index, and (3) drought tolerance index (DTI). The same set of genotypes were also tested for grain yield in two low rainfall environments for two seasons. High genetic variation was found among all genotypes for all seedling traits scored in this study. Correlations between tolerance and survival traits were weak or did not exist. Heritability estimates ranged from 0.53 to 0.88. DTI had significant phenotypic and genotypic correlations with all seedling traits. Genotypes were identified with a high drought tolerance at the seedling stage combined with high grain yield in low rainfall. Breeding for tolerance and survival traits should be taken into account for improving winter wheat drought tolerance at seedling stage. The selected genotypes can be used for to further improve drought tolerance in high yielding wheat for Nebraska.

Summer Drought Reduces the Growth of Invasive Tree-of-Heaven (Ailanthus altissima) Seedlings

Tree-of-heaven (Ailanthus altissima) is an invasive species in North America that is becoming widespread in the eastern part of the continent. In the drier, western interior of North America, however, the spread of tree-of-heaven appears to be limited to areas where supplemental water is available such as in urban areas or in riparian environments. To explore the effect of summer drought on tree-of-heaven seedlings and to look for genetic variation for drought tolerance among half-sibling families, we collected samaras from 12 open-pollinated mother trees growing in Utah County, Utah, and measured the samaras' area, maximum width, length, and weight. We then planted 10 samaras per mother tree into 10-cm pots in the greenhouse, measured the height of each resulting seedling, and randomly assigned them to a watering treatment (well-watered or droughted) that lasted for 20 wk. We examined the relationship between tree-of-heaven samara size parameters and seedling height before we started our drought treatment and for additional growth parameters after our drought treatment. Several growth parameters (relative growth, final height, final dry mass, final stem dry mass, final dry root mass, and root-to-shoot ratios) were reduced significantly by drought but differed among seedlings from different mothers only for relative growth. There was no genetic variation for drought tolerance observed for any of the growth parameters. These results suggest that tree-of-heaven seedlings from this part of western North America are uniformly and negatively affected by drought and may explain the relative dearth of tree-of-heaven in the interior West.

Leaf gas exchange and water-use efficiency of dry-land wheat genotypes under water stressed and non-stressed conditions

ABSTRACTThe objective of this study was to determine drought tolerance characteristics of dryland wheat genotypes based on leaf gas exchange and water-use efficiency in order to identify promising genotypes for drought tolerance breeding. Physiological responses of ten genetically diverse wheat genotypes were studied under non-stressed (NS) and water stressed (WS) conditions using a 2 × 10 factorial experiment replicated 3 times. A highly significant water condition × genotype interaction (P < 0.001) was observed for photosynthetic rate (A), ratio of photosynthetic rate and internal CO2 concentration (A/Ci), ratio of internal and atmospheric CO2 (Ci/Ca), intrinsic (WUEi) and instantaneous (WUEinst) water-use efficiencies suggesting genotypic variability among wheat genotypes under both test conditions. Principal component analysis (PCA) identified three principal components (PC’s) under both test conditions accounting for 84% and 89% of total variation, respectively. Bi-plot analysis identified G339 and G344 as drought tolerant genotypes with higher values of A, T, gs, A/Ci, WUEi and WUEinst under WS condition. The current study detected significant genetic variation for drought tolerance among the tested wheat genotypes using physiological parameters. Genotypes G339 and G344 were identified to be drought tolerant with efficient A, T, gs, A/Ci and water-use under water stressed condition.

Identification of drought stress tolerance in wild species germplasm of rice based on leaf and root morphology

AbstractDrought is the major abiotic constraint to the rice production in the rain-fed areas across Asia and sub-Saharan Africa. Wild species of Oryza offer a wide spectrum of adaptive traits and can serve as potential donors of biotic and abiotic stress tolerance. At the Punjab Agricultural University, we are maintaining an active collection of 1630 accessions of wild species germplasm (AA, CC, BBCC and CCDD) of rice. These accessions were screened to assess genetic variation for drought tolerance under field conditions. Severe water stress was imposed at the late vegetative stage by withholding water initially for 25 d and then extended further to 35 d during kharif season in the years 2013–14 and 2015–16. The tolerance score for drought stress was based on the extent of leaf rolling and leaf drying. Based on the 2 years’ data, seven accessions from Oryza rufipogon, four from Oryza longistaminata and one each from Oryza officinalis and Oryza latifolia were found tolerant to drought stress. These selected accessions were further phenotype for root morphology. The average root length among the selected accessions ranges between 36 and 80 cm and the number of primary roots vary from 30 to 87 cm. The O. rufipogon accession IRGC 106433, O. longistaminata accession IRGC 92656A, O. officinalis accession IRGC 101152 and O. latifolia accession IRGC 80769 showed approximately 2–2.5 times longer root length and number than the indica rice elite cultivar PR121. The results indicated potentiality of selected wild species germplasm for conferring drought tolerance to the elite cultivars.

English

Sorghum is the fifth most important cereal crop and occupies the second position among the staple food grains in semi-arid tropics. The adaptation of grain sorghum to a wide range of environmental conditions has led to the evolution and existence of extensive genetic variation for drought tolerance. Accordingly, sorghum is expected to play an increasingly important role in agriculture and meeting world food demand in the face of climate change, land degradation and increasing water scarcity. The crop requires relatively less water than other important cereals such as maize and wheat. However, yield potential of the crop is significantly limited due to drought and heat stresses. Drought is one of the most important factors that affect crop production worldwide and continues to be a challenge to plant breeders, despite many decades of research. Underestimating the different mechanisms underlying drought tolerance is vital for the breeding to alleviate adverse effects of drought in order to boost productivity. In this literature review, the main effects of drought on crop growth and development, and yield are reported.

English

Sorghum is grown in semi-arid to arid regions of the world and serves as the staple food for about half a billion people in sub-Saharan Africa and Asia. The adaptation of grain sorghum to a wide range of environmental conditions has led to the evolution and existence of extensive genetic variation for drought tolerance. Consequently, sorghum is expected to play an increasingly important role in agriculture and meeting world food demand in the face of climate change, land degradation and increasing water scarcity. Drought is a complex phenomenon, and is considered one of the most significant factors limiting crop yields around the world and continues to be a challenge to plant breeders, despite many decades of research. Underestimating the genetics and the physiological mechanisms underlying drought tolerance is vital for the breeding to alleviate adverse effects of drought in order to boost productivity. In this literature review, research findings from the 1970s up to present are included. Most of the basic researches on the mechanism of drought tolerance were done in the early 1980s, and most of the current researches focus on verification and fine-tuning of methodologies. The paper outlines the main effects of drought on crop growth and development, and yield. It then examines the basic information on physiological mechanisms of drought in crops. Subsequent discussion is given on the genetic control of drought tolerance, and breeding methods in sorghum. &nbsp; Key words: Moisture stress, drought tolerance, breeding, genetics, mechanisms, Sorghum bicolor.

Developing drought-resilient crops for improving productivity of drought-prone ecologies

Limitation of water is major abiotic factor affecting crop productivity in different parts of world. Climate change is likely to make drought stress even more severe in the future, particularly in semi-arid and arid tropics. Therefore, development of crops with better adaptation to water stress is very critical to have sustainable crop production. An attempt is made here to assess different approaches undertaken to enhance crop performance under waterlimited conditions. Improving drought tolerance during reproductive phase is often a high priority in crop breeding programmes targeting drought-prone areas. Empirical approach followed in different crops to enhance drought tolerance has mainly addressed the issue of environment for selection and of criteria for improving drought adaptation. Using evaluation data from drought stressed and non-stressed environments, many studies highlighted importance of evaluation and selection in drought-prone locations. Most physiological research has concentrated on the identification of parameters that have found little place in regular breeding programmes owing to the lack of simple and easy techniques for selecting such characters on a large scale. Use of adapted germplasm has played a pivotal role in developing drought tolerant crops. The introgression of elite genetic material into drought-adapted germplasm has also been established as an effective approach to enhance performance under drought without compromising performance under stressfree environments. Molecular breeding is fast emerging as a supplement approach to enhance drought adaptation at a faster rate with greater precision. Efforts in this direction started with the development of a molecular marker-based genetic linkage maps in major food crops followed by identification of quantitative trait loci (QTL) determining yield under drought environments. Further success would depend upon development of repeatable, low-cost, high throughput phenotyping procedures that reliably characterize genetic variation for drought tolerance and its component traits.

Genotypic variability for tuber yield, biomass, and drought tolerance in Jerusalem artichoke germplasm

Jerusalem artichoke could be an alternative feedstock for bioenergy during times when there are shortages of other raw materials for the ethanol industry. However, insufficient water under rainfed conditions is a major cause of Jerusalem artichoke losses. Genetic variation for drought tolerance is an essential prerequisite for the development of Jerusalem artichoke cultivars with improved drought tolerance. The objectives of this study were to determine the effects of drought stress on tuber dry weight and biomass and to investigate the genotypic variability in Jerusalem artichoke germplasm. The line-source sprinkler technique was used to compare moisture responses of a range of 40 Jerusalem artichoke genotypes grown using 3 water levels. Experiments were conducted on a Yasothon soil series in Northeast Thailand during 2010/11 and 2011/12 and included extended dry periods. Drought reduced tuber dry weight and biomass, and the reductions in tuber dry weight and biomass were greater under severe drought than moderate drought conditions. Over both seasons, CN 52867, HEL 53, HEL 231, HEL 335, JA 76, HEL 65, and JA 102 × JA 89 (8) had consistently high tuber dry weight (1.3 to 4.5 t ha -1 ) and HEL 53, HEL 61, HEL 231, HEL 335, JA 76, JA 15, JA 89, HEL 65, HEL 256, and JA 102 × JA 89 (8) had consistently high biomass (2.0 to 6.8 t ha -1 ). These Jerusalem artichoke genotypes are promising parents in breeding for drought tolerance.

Classification of Genetic Variation for Drought Tolerance in Tall Fescue using Physiological Traits and Molecular Markers

ABSTRACTDrought stress is a major factor limiting turfgrass growth. The objectives of the study were to determine genetic variation in major physiological traits associated with drought tolerance in a collection of tall fescue (Festuca arundinacea Schreb.) germplasm and to examine genetic diversity and relationships among germplasm varying in drought tolerance via molecular marker classification. Plants of 44 accessions from Africa, Europe, America, and Australia were exposed to well‐watered conditions or drought stress by withholding irrigation for 11 d in a trial conducted in June 2009 and for 13 d during a repeated trial conducted in September 2009 in a greenhouse. Visual evaluation of turf quality, leaf relative water content, electrolyte leakage, chlorophyll content, and evapotranspiration rate demonstrated wide phenotypic variation contributing to drought tolerance in this collection of tall fescue germplasm. A hierarchical cluster analysis was conducted based on physiological traits and among 44 tall fescue accessions, 12 accessions were classified as most tolerant, 17 as moderately tolerant, and 15 as susceptible. The genetic similarity matrix from molecular marker analysis demonstrated similarity indexes ranging from 0.51 to 0.83. Among the 44 accessions, SH‐3‐AM‐US from America and PI598949‐EU‐IT from Europe were genetically distinct, but both were classified as drought‐tolerant germplasm based on physiological data. Screening genetically diverse germplasm to identify superior drought tolerance through the combined approach of physiological and molecular marker analysis could prove useful in breeding programs for further improvement of drought tolerance in tall fescue turfgrass.

Characterization and expression of dehydrins in wild Egyptian pea (Pisum sativum L.)

Pea genotypes exhibited significant genetic variation in drought tolerance. Polymerase chain reaction (PCR) screening for dehydrins in different pea genotypes indicated the amplification of at least six dehydrin gene fragments varied in length from 500 to 1000 bp. In this study, a dehydrin gene, PsDhn1 (GenBank accession number JN182234), was isolated and characterized from wild Egyptian pea ( Pisum sativum L.) using specific PCR. It belongs to the Y 2 K 2 dehydrin subclass induced by drought stress. DNA sequence indicated an open reading frame which predicts a protein with 196 amino acids and molecular mass of about 21 kDa. Deduced amino acid sequence indicated the presence of the N-terminal consensus motif, Y-segment D(E/Q)YGNP, and the lysine rich domain, K-segment KIKEK(I/L)PG, common for all dehydrins, both of which are present in two copies. However, it lacks the track of serine residues (S-segment). Egyptian pea dehydrin (PsDHN1) contains two copies of a semi-conserved sequence of six amino acid residues in length (EYVREE), which lie in front of the lysine rich motif (K-segment), and common only for pea dehydrins. Northern blot analysis indicated two dehydrin mRNA transcripts detectable by PsDhn1 probe. Western blot analysis using anti-dehydrin polyclonal antibody detected two corresponding dehydrins (29 and 21 kDa) accumulated in developing pea cotyledons. Both mRNA transcripts and dehydrins showed different patterns of expression and accumulation. The amino acid composition of dehydrins with high content of charged and polar residues may promote their specific protective functions under conditions of cell dehydration. Key words : Dehydrins, late embryogenesis abundant (LEA) genes, drought tolerance, pea, desiccation.

Drought-tolerance indices in a tall fescue population and its polycross progenies

Development of drought-tolerant cultivars is hampered by a lack of effective selection criteria. In this research, drought tolerance of 75 genotypes of tall fescue in three sets (25 parental, 25 early, 25 late-flowering progenies) was evaluated under no soil moisture stress and soil moisture stress in the field during 2009 and 2010. Five drought-tolerance indices were calculated: stress tolerance (TOL), mean productivity (MP), geometric mean productivity (GMP), stress susceptibility index (SSI), and stress tolerance index (STI). These calculations were based on forage yield (dry matter basis) under drought (Ys) and non-drought (Yp) conditions. Soil moisture stress caused significant reduction in forage yield. Considerable genetic variation for drought tolerance was found among genotypes. A moderately high relationship was found between Yp and Ys using regression analysis, with a clear relationship in the second year. Indices GMP and STI were found to be valuable aids in the selection of drought-tolerant, high-yielding genotypes. Plots of the first and second principal components identified drought-tolerant genotypes in each set. Results indicated that selection for drought-tolerant genotypes should be planned separately for first year (establishment stage) and second year (productive stage) in tall fescue.

Detection of Epistasis and Estimation of Additive and Dominance Components of Genetic Variation for Drought Tolerance in Durum Wheat

Detection of Epistasis and Estimation of Additive and Dominance Components of Genetic Variation for Drought Tolerance in Durum Wheat

Evolution of drought tolerance and defense: dependence of tradeoffs on mechanism, environment and defense switching

Plants evolve defenses against herbivores and pathogens in stressful environments; however, plants that evolve tolerances to other environmental stressors may have compromised defenses. Such tradeoffs involving defenses may depend on limited resources or otherwise stressful environments; however, the effect of stressful environments on defense expression might be different for different genotypes (G×E). To test these predictions, we studied genetic variation and co‐variation of drought stress tolerance and defenses at two levels of genetic variation: between and within closely related species. We did this across an experimental drought stress gradient in a growth room for species for which genetic variation in drought tolerance was likely. In apparent contrast to predictions, the species Boechera holboellii (Brassicaceae) from lower and dryer elevations had slower inherent growth rates and correspondingly higher total defensive glucosinolate concentrations than the closely related species B. stricta from higher elevations. Thus, B. holboellii was both drought tolerant and defended; however, optimality theory does predict tradeoffs between defense and growth. Differences between species in the direct effect of water deficiency on glucosinolate production did not obscure the grow‐or‐defend tradeoff. B. holboellii may also have been more resistant to the specialist herbivore Plutella xylostella; a trend that was less clear because it depended on plant development and water deficient conditions. At finer scales of genetic variation, there was significant variation among families and naturally occurring inbred lines of B. stricta in drought tolerance measured as inherent growth, the reaction norm of growth across drought treatments, shoot water potential, and transpiration rates. Evidence for tradeoffs was also found within B. stricta in genetic correlations between resistance and transpiration rates, or glucosinolates and growth rates. No G×E was detected at these finer scales of genetic variation, although sometimes the tradeoff was dependent on drought conditions. Direct effects of drought stress resulted in an apparent plastic switch between resistance and tolerance to damage, which might be a cost avoidance mechanism because tradeoffs never involved tolerance to damage. Thus, when drought tolerance is manifest as slow inherent growth rates, plants may also have relatively high defense levels, especially in stressful environments. Otherwise, defenses may be compromised by drought‐coping mechanisms, although plastic switches to less costly defenses may alleviate constraints in stressful environments.