Microsatellite marker based genetic diversity analysis among cotton (<Emphasis Type="Italic">Gossypium hirsutum</Emphasis>) accessions differing for their response to drought stress

Genetic diversity is an essential input for any plant breeding programme. To assess the genetic divergence among the newly identified drought tolerant lines and elite cotton genotypes including popular varieties, a total of 51 distinctly polymorphic markers were identified after screening 142 genome-wide SSR markers. The identified polymorphic markers detected a total of 140 alleles with a mean of 2.75 alleles per loci and average polymorphism information content of 0.45. Jaccard coefficient based dissimilarity index between the genotypes ranged from 0.18 to 0.82 indicating existence of wide variation between and within the drought tolerant and susceptible genotypes at the DNA level. Cluster and factorial analyses have provided the structure of genetic diversity present and clearly distinguished the drought tolerant and susceptible cotton genotypes. Clustering pattern was in congruence with the source or pedigree of genotypes. The information generated in the present study on genetic divergence among genotypes having differential response to drought will help in selection of suitable lines as parents for developing drought tolerant cultivars in cotton. The polymorphic markers and diverse lines identified in the study will be of immense utility in molecular mapping and marker assisted breeding to achieve drought tolerance in cotton.     

Molecular mapping of QTLs for wheat flag leaf senescence under water-stress

A segregating population from the cross between drought sensitive (Variant-2) and drought tolerant (Cham-6) genotypes was made to identify molecular markers linked to wheat (Triticum aestivum L.) flag leaf senescence under water-stress. From 38 random amplified polymorphic DNA (RAPD) primers, 25 inter-simple sequence repeat (ISSR) primers and 46 simple sequence repeat (SRR) primers, tested for polymorphism among parental genotypes and F2 population. Quantitative trait locus (QTL) for flag leaf senescence was associated with 1 RAPD marker (Pr9), 4 ISSR markers (Pr8, AD5, AD2 and AD3), and 1 SSR marker (Xgwm382) and explained 44, 50, 35, 31, 22 and 73 % phenotypic variation, respectively. The genetic distance between flag leaf senescence gene and Pr9 was 10.0 cM (LOD score 22.9). The markers Pr8, AD5, AD2 and AD3 had genetic distances of 10.5, 14.6, 15.6 and 18.1 cM, respectively (LOD scores 22.6, 17.8, 17.5 and 14.6). The genetic distance between Xgwm382 was 3.9 cM (LOD score 33.8). Therefore, the RAPD, ISSR and SSR markers linked to the QTL for the drought-induced flag leaf senescence can be further used in breeding for drought tolerance in wheat.     

Molecular Assortment of Lens Species with Different Adaptations to Drought Conditions Using SSR Markers

The success of drought tolerance breeding programs can be enhanced through molecular assortment of germplasm. This study was designed to characterize molecular diversity within and between Lens species with different adaptations to drought stress conditions using SSR markers. Drought stress was applied at seedling stage to study the effects on morpho-physiological traits under controlled condition, where tolerant cultivars and wilds showed 12.8–27.6% and 9.5–23.2% reduction in seed yield per plant respectively. When juxtaposed to field conditions, the tolerant cultivars (PDL-1 and PDL-2) and wild (ILWL-314 and ILWL-436) accessions showed 10.5–26.5% and 7.5%–15.6% reduction in seed yield per plant, respectively under rain-fed conditions. The reductions in seed yield in the two tolerant cultivars and wilds under severe drought condition were 48–49% and 30.5–45.3% respectively. A set of 258 alleles were identified among 278 genotypes using 35 SSR markers. Genetic diversity and polymorphism information contents varied between 0.321–0.854 and 0.299–0.836, with mean value of 0.682 and 0.643, respectively. All the genotypes were clustered into 11 groups based on SSR markers. Tolerant genotypes were grouped in cluster 6 while sensitive ones were mainly grouped into cluster 7. Wild accessions were separated from cultivars on the basis of both population structure and cluster analysis. Cluster analysis has further grouped the wild accessions on the basis of species and sub-species into 5 clusters. Physiological and morphological characters under drought stress were significantly (P = 0.05) different among microsatellite clusters. These findings suggest that drought adaptation is variable among wild and cultivated genotypes. Also, genotypes from contrasting clusters can be selected for hybridization which could help in evolution of better segregants for improving drought tolerance in lentil.     

Genome-wide association mapping of yield components and drought tolerance-related traits in cotton

Drought causes serious yield losses in cotton production throughout the world. Association mapping allows identification and localization of the genes controlling drought-related traits which will be helpful in cotton breeding. In the present study, genetic diversity analysis and association mapping of yield and drought traits were performed on a panel of 99 upland cotton genotypes using 177 SSR (simple sequence repeat) markers. Yield parameters and drought tolerance-related traits were evaluated for two seasons under two watering regimes: water-stressed and well-watered. The traits included seed cotton yield (SCY), lint yield (LY), lint percentage (LP), water-use efficiency (WUE), yield potential (YP), yield reduction (YR), yield index (YI), drought sensitivity index (DSI), stress tolerance index (STI), harmonic mean (HM), and geometric mean productivity (GMP). The genotypes with the least change in seed cotton yield under drought stress were Zeta 2, Delcerro, Nazilli 87, and DAK 66/3 which were also the most water-use efficient cultivars. The average genetic diversity of the panel was 0.38. The linkage disequilibrium decayed relatively rapidly at 20–30 cM (r²?≥?0.5). We identified 30 different SSR markers associated with the traits. Fifteen and 23 SSR markers were linked to the traits under well-watered and water-stress conditions, respectively. To our knowledge, most of these quantitative yield and drought tolerance-associated loci were newly identified. The genetic diversity and association mapping results should facilitate the development of drought-tolerant cotton lines with high yield in molecular breeding programs.     

Mapping QTLs for pre-harvest sprouting tolerance on chromosome 2D in a synthetic hexaploid wheat×common wheat cross

Based on segregation distortion of simple sequence repeat (SSR) molecular markers, we detected a significant quantitative trait loci (QTL) for pre-harvest sprouting (PHS) tolerance on the short arm of chromosome 2D (2DS) in the extremely susceptible population of F₂ progeny generated from the cross of PHS tolerant synthetic hexaploid wheat cultivar ‘RSP’ and PHS susceptible bread wheat cultivar ‘88–1643’. To identify the QTL of PHS tolerance, we constructed two SSR-based genetic maps of 2DS in 2004 and 2005. One putative QTL associated with PHS tolerance, designatedQphs.sau-2D, was identified within the marker intervalsXgwm261-Xgwm484 in 2004 and in the next year, nearly in the same position, between markerswmc112 andXgwm484. Confidence intervals based on the LOD-drop-off method ranged from 9 cM to 15.4 cM and almost completely overlapped with marker intervalXgwm261-Xgwm484. Flanking markers near this QTL could be assigned to the C-2DS1-0.33 chromosome bin, suggesting that the gene(s) controlling PHS tolerance is located in that chromosome region. The phenotypic variation explained by this QTL was about 25.73–27.50%. Genotyping of 48 F₆ PHS tolerant plants derived from the cross between PHS tolerant wheat cultivar ‘RSP’ and PHS susceptible bread wheat cultivar ‘MY11’ showed that the allele ofQphs.sau-2D found in the ‘RSP’ genome may prove useful for the improvement of PHS tolerance.     

Photosynthetic characteristics and enzymatic antioxidant capacity of leaves from wheat cultivars exposed to drought

Two durum (Triticum durum L.), Barakatli-95 and Garagylchyg-2; and two bread (Triticum aestivum L.) wheat cultivars, Azamatli-95 and Giymatli-2/17 with different sensitivities to drought were grown in the field on a wide area under normal irrigation and severe water deficit. Drought caused a more pronounced inhibition in photosynthetic parameters in the more sensitive cvs Garagylchyg-2 and Giymatli-2/17 compared with the tolerant cvs Barakatli-95 and Azamatli-95. Upon dehydration, a decline in total chlorophyll and relative water content was evident in all cultivars, especially in later periods of ontogenesis. Potential quantum yield of PS II (F(v)/F(m) ratio) in cv Azamatli-95 was maximal during stalk emergency stage at the beginning of drought. This parameter increased in cv Garagylchyg-2, while in tolerant cultivar Barakatli-95 significant changes were not observed. Contrary to other wheat genotypes in Giymatli-2/17 drought caused a decrease in PS II quantum yield. Drought-tolerant cultivars showed a significant increase in CAT activity as compared to control plants. In durum wheat cultivars maximal activity of CAT was observed at the milk ripeness and in bread wheat cultivars at the end of flowering. APX activity also increased in drought-treated leaves: in tolerant wheat genotypes maximal activity occurred at the end of flowering, in sensitive ones at the end of ear formation. GR activity increased in the tolerant cultivars under drought stress at all stages of ontogenesis. SOD activity significantly decreased in sensitive cultivars and remained at the control level or increased in resistant ones. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.     

The genetic basis of spectral reflectance indices in drought-stressed wheat

Drought imposes a major constraint over the productivity of wheat, particularly in arid and semi-arid production zones. Here, the genetic basis of spectral reflectance indices was investigated in drought-stressed wheat by comparing, under two contrasting moisture regimes, the performance of an F₆ recombinant inbred line (RIL) population bred from a cross between the drought tolerant cultivar Pavon76 and the sensitive cultivar Yecora Rojo. The parents and RILs were genotyped with respect to both a set of microsatellite (SSR) loci and a number of known drought-responsive genes. In all, 28 quantitative trait loci (QTL) controlling dry weight per plant, water content of the above-ground biomass, leaf water potential, canopy temperature, and spectral reflectance indices traits were identified. The loci were distributed over 11 chromosomes, belonging to each of the three wheat sub-genomes. There were important location-flanking markers Barc109 and Barac4 on chromosome 5B relating to dry weight per plant accumulation under the limited irrigation regime. The same region-harbored QTL associated with leaf water potential, canopy temperature, and ratio index under the limited irrigation regime. Linkage between the known drought-responsive genes and aspects of the drought response was established. Some of QTL were of substantial enough effect for their linked markers to be likely usable for the marker-assisted breeding of drought tolerance in wheat.     

Molecular characterization and assessment of genetic diversity of inbred lines showing variability for drought tolerance in maize

A set of 24 genotypes bred at different centres in India as well as in CIMMYT showing variability for drought tolerance were selected for molecular and morpho-physiological characterization. A set of 35 SSR markers, having genome-wide coverage, was chosen for genotyping the inbreds. These markers generated a total of 111 polymorphic alleles with an average of 3.17 alleles per locus. The minimum and maximum PIC value was 0.27 and 0.77 with a mean of 0.5. A total of 13 unique alleles were found in the 24 inbred lines. The coefficient of genetic dissimilarity ranged from 0.192 to 0.803. NJ-based tree suggested the presence of three major clusters of which, two of them had subgroups. Phenotyping of inbreds by morpho-physiological traits revealed that there was a positive relationship among root length, chlorophyll content, relative water content while anthesis-silking interval was negative relationship with all these traits. Genotyping data complemented by morpho-physiological parameters were used to identify a number of pair-wise combinations for the development of mapping population segregating for drought tolerance and potential heterotic pairs for the development of drought tolerant hybrids.     

Diversity analysis of selected rice landraces from West Bengal and their linked molecular markers for salinity tolerance

Study of genetic diversity in crop plants is essential for the selection of appropriate germplasm for crop improvement. As salinity posses a serious environmental challenge to rice production globally and especially in India, it is imperative that the study of large collections of germplasms be undertaken to search for salt tolerant stocks. In the present study, 64 indica germplasms were collected from different agro-climatic zones of West Bengal, India, from the Himalayan foothills in the northern part down to the southern saline belt of the state keeping in view the soil characteristics and other edaphic factors prevailing in the region. Salt tolerance parameters were used to screen the large set of germplasms in terms of root-shoot length, fresh-dry weight, chlorophyll content, Na+/K+ ratio and germination potential in presence of salt. Standard evaluation score or SES was calculated to find out tolerant to sensitive cultivar. Twenty-one SSR markers, some associated with the Saltol QTL and others being candidate gene based SSR (cgSSR) were used to study the polymorphism of collected germplasm. A wide diversity was detected among the collected germplasms at the phenotypic as well as molecular level. Of the 21 SSR markers, 15 markers were found to be polymorphic with 88 alleles. Based on phenotypic and biochemical results, 21 genotypes were identified as salinity tolerant, whereas 40 genotypes turned out to be salt susceptible. The present study shows that apart from the established salt tolerant lines, several other landraces like Bonkanta, Morisal, Ghiosh, Patni may be the source of salt tolerant donor in future breeding programs.     

Terminal Heat Tolerance in Bread Wheat Determined by Agronomical Traits and SSR Markers

Since global warming affects wheat cropping systems, more has yet to be indicated on the parameters, which control terminal heat tolerance, and severely influence wheat (Triticum aestivum L.) productivity. Identification of tolerant wheat genotypes by heat tolerance-linked molecular markers is a rapid and cost-effective screening tool in plant breeding. Accordingly, in a four-year field experiment (2015–2019), 44 wheat genotypes were selected out of 100 genotypes, and were examined in timely and late planting (mid-January resulting in heat stress). Stress decreased yield components, including 1000-kernel weight (TKW), grains per spike, and plants per square meter, and the physiological traits, including days to heading and days to maturity, grain filling duration, and greenness, and eventually decreased grain yield up to?~?28%. The early maturity genotypes resulted in higher yields under stress conditions by a stress-avoidance mechanism. Among 14 SSR markers, GWM577 was positively correlated with yield, and WMS3062, GWM261, and WMS1025 had positive correlations with longevity under stress. Accordingly, WMS3062 and GWM261 can be used to determine high yield and early maturity genotypes. Furthermore, GWM114 showed a positive correlation with TKW, indicating their usefulness for grouping wheat genotypes and for identifying heat-related markers. Since the crossing of the genetically distant genotypes can create more diverse populations, the results could be applied to plan breeding projects to establish more diverse populations for different chromosomal locations and traits under heat stress conditions. Moreover, our findings demonstrated that the morphological and molecular analyses could be useful for describing wheat genetic variation of heat tolerance.

     

Physiological characterization and allelic diversity of selected drought tolerant traditional rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) landraces of Koraput,India

Water-deficit stress tolerance in rice is important for maintaining stable yield, especially under rain-fed ecosystem. After a thorough drought-tolerance screening of more than 130 rice genotypes from various regions of Koraput in our previous study, six rice landraces were selected for drought tolerance capacity. These six rice landraces were further used for detailed physiological and molecular assessment under control and simulated drought stress conditions. After imposing various levels of drought stress, leaf photosynthetic rate (P_N), photochemical efficiency of photosystem II (Fv/Fm), SPAD chlorophyll index, membrane stability index and relative water content were found comparable with the drought-tolerant check variety (N22). Compared to the drought-susceptible variety IR64, significant positive attributes and varietal differences were observed for all the above physiological parameters in drought-tolerant landraces. Genetic diversity among the studied rice landraces was assessed using 19 previously reported drought tolerance trait linked SSR markers. A total of 50 alleles with an average of 2.6 per locus were detected at the loci of the 19 markers across studied rice genotypes. The Nei’s genetic diversity (He) and the polymorphism information content (PIC) ranged from 0.0 to 0.767 and 0.0 to 0.718, respectively. Seven SSR loci, such as RM324, RM19367, RM72, RM246, RM3549, RM566 and RM515, showed the highest PIC values and are thus, useful in assessing the genetic diversity of studied rice lines for drought tolerance. Based on the result, two rice landraces (Pandkagura and Mugudi) showed the highest similarity index with tolerant check variety. However, three rice landraces (Kalajeera, Machhakanta and Haldichudi) are more diverse and showed highest genetic distance with N22. These landraces can be considered as the potential genetic resources for drought breeding program.     

Identification and characterization of salt responsive miRNA-SSR markers in rice (Oryza sativa)

Salinity is an important abiotic stress that affects agricultural production and productivity. It is a complex trait that is regulated by different molecular mechanisms. miRNAs are non-coding RNAs which are highly conserved and regulate gene expression. Simple sequence repeats (SSRs) are robust molecular markers for studying genetic diversity. Although several SSR markers are available now, challenge remains to identify the trait-specific SSRs which can be used for marker assisted breeding. In order to understand the genetic diversity of salt responsive-miRNA genes in rice, SSR markers were mined from 130 members of salt-responsive miRNA genes of rice and validated among the contrasting panels of tolerant as well as susceptible rice genotypes, each with 12 genotypes. Although 12 miR-SSRs were found to be polymorphic, only miR172b-SSR was able to differentiate the tolerant and susceptible genotypes in 2 different groups. It had also been found that miRNA genes were more diverse in susceptible genotypes than the tolerant one (as indicated by polymorphic index content) which might interfere to form the stem-loop structure of premature miRNA and their subsequent synthesis in susceptible genotypes. Thus, we concluded that length variations of the repeats in salt responsive miRNA genes may be responsible for a possible sensitivity to salinity adaptation. This is the first report of characterization of trait specific miRNA derived SSRs in plants.     

Development of genome-wide SSR markers in horsegram and their use for genetic diversity and cross-transferability analysis

Horsegram [Macrotyloma uniflorum (Lam.) Verdc.] commonly known as kulthi or Madras gram is an important drought tolerant legume crop used as food and fodder in India and across the globe. Horsegram is tolerant to many biotic and abiotic stresses and considered a potential future food legume. Despite being a multiutility crop, insufficient genomic information is available in this species, which is otherwise required for genetic improvement. Hence, in the present work we used next-generation sequencing (NGS) technology for genome-wide development and characterization of novel simple sequence repeat (SSR) markers in horsegram. In all, 2458 SSR primer pairs were designed from NGS data and 117 SSRs were characterized in 48 diverse lines of horsegram. Cross-transferability of these markers was also checked in nine related legume species. The polymorphic SSRs revealed high diversity measures such as mean values of expected heterozygosity (He; 0.54), observed heterozygosity (Ho; 0.64), and polymorphism information content (PIC; 0.46). Analysis of molecular variance (AMOVA) revealed high degree of genetic variance within the populations. Dendrogram based on Jaccard’s similarity coefficient and principal component analysis (PCA) revealed two groups in the analyzed accessions. This observation was further confirmed by Bayesian genetic STRUCTURE analysis. The SSR markers developed herein can be used in diverse genetic analysis including association mapping in this crop and also in related legume crops with limited marker resources. Hence, this new SSR dataset can be useful for molecular breeding research in this underutilized pulse crop. In addition, genetic diversity estimates of analyzed germplasm can be important for devising future breeding programmes in horsegram.     

Genetic relationships among cherry species with transferability of simple sequence repeat loci

Sweet and sour cherries are two economically important species in the world. The capability to distinguish among cherry genotypes in breeding, cultivation and germplasm collection is extremely important for scientific as well as economic reasons. In the present research, sixteen simple sequences repeat (SSR) loci were used to estimate the relationships among sweet, sour, duke and wild cherries. All of the SSR markers showed high transferability across the studied species that allowed us to study genetic diversity in them. Totally 96 alleles were generated with SSR loci, of which 93 were found polymorphic with 97.57 % polymorphism. Values of genetic similarity between genotypes varied from 0.16 to 0.97 which indicated high level of genetic diversity. On the basis of their genetic similarities, SSR analysis allowed to group the genotypes into three main clusters according to their species. These results have an important implication for cherry germplasm characterization, improvement, and conservation.     

Evaluating Agronomic Performance and Investigating Molecular Structure of Drought and Heat Tolerant Wild Alfalfa (<Emphasis Type="Italic">Medicago sativa</Emphasis> L.) Collection from the Southeastern Turkey

Drought is a major stress factor for agricultural production including alfalfa production. One way to counterbalance the yield losses is the introgression of drought tolerant germplasm into breeding programs. As an effort to exploit such germplasm, 16 individual plants were selected from the Southeastern Turkey from their natural habitat and clonally propagated in field trials with an ultimate goal to use the germplasm as parents for releasing a synthetic cultivar. Forage yield and forage quality traits were evaluated and molecular genetic diversity among genotypes were determined using inter simple sequence repeat markers. Genotypes showed a variation from growth habit to yield and quality traits indicating sufficient phenotypic variation for diverse breeding efforts (for grazing or harvesting) and long term selection schemes. A large amount of genetic variation was observed even with a limited number of marker and genotypes. However, no pattern of spatial genetic structure was observed for the scale of the study when genetic variation is linked to the geographic origin. We conclude that ex situ natural variation provides a wealth of germplasm that could be incorporated into breeding programs aiming to improve drought tolerance. We also suggest an extensive collection of seeds/plant tissue from unique plants with desirable traits rather than putting more efforts to create a spatial germplasm sampling efforts in narrow regions.     

Genetic Variation and Association Analysis of the SSR Markers Linked to the Major Drought-Yield QTLs of Rice

Drought is one of the major abiotic stresses, which hampers the production of rice worldwide. Informative molecular markers are valuable tools for improving the drought tolerance in various varieties of rice. The present study was conducted to evaluate the informative simple sequence repeat (SSR) markers in a diverse set of rice genotypes. The genetic diversity analyses of the 83 studied rice genotypes were performed using 34 SSR markers closely linked to the major quantitative trait loci (QTLs) of grain yield under drought stress (qDTYs). In general, our results indicated high levels of polymorphism. In addition, we screened these rice genotypes at the reproductive stage under both drought stress and nonstressful conditions. The results of the regression analysis demonstrated a significant relationship between 11 SSR marker alleles and the plant paddy weight under stressful conditions. Under the nonstressful conditions, 16 SSR marker alleles showed a significant correlation with the plant paddy weight. Finally, four markers (RM279, RM231, RM166, and RM231) demonstrated a significant association with the plant paddy weight under both stressful and nonstressful conditions. These informative-associated alleles may be useful for improving the crop yield under both drought stress and nonstressful conditions in breeding programs.     

Proteomics uncovers a role for redox in drought tolerance in wheat

Proteomic analysis offers a new approach to identify a broad spectrum of genes that are expressed in living systems. We applied a proteomic approach to study changes in wheat grain in response to drought, a major environmental parameter adversely affecting development and crop yield. Three wheat genotypes differing in genetic background were cultivated in field under well-watered and drought conditions by following a randomized complete block design with four replications. The overall effect of drought was highly significant as determined by grain yield and total dry matter. About 650 spots were reproducibly detected and analyzed on 2-DE gels. Of these, 121 proteins showed significant change under drought condition in at least one of the genotypes. Mass spectrometry analysis using MALDI-TOF/TOF led to the identification of 57 proteins. Two-thirds of identified proteins were thioredoxin (Trx) targets, in accordance with the link between drought and oxidative stress. Further, because of contrasting changes in the tolerant and susceptible genotypes studied, several proteins emerge as key participants in the drought response. In addition to providing new information on the response to water deprivation, the present study offers opportunities to pursue the breeding of wheat with enhanced drought tolerance using identified candidate genetic markers. The 2-DE database of wheat seed proteins is available for public access at http://www.proteome.ir.     

Genetic analysis and molecular mapping of seedling survival drought tolerance gene in lentil (<Emphasis Type="Italic">Lens culinaris</Emphasis> Medikus)

Lentil populations were developed from crosses between ‘JL-3’ (sensitive to drought stress) and ‘PDL-1’ and ‘FLIP-96-51’ (tolerant to drought stress), to study the inheritance of drought tolerance and to identify the markers associated with it. The parental types, F₁, F₂, F₃, and backcross (BC) generations were screened for drought tolerance using seedling survivability and drought scores. The F₁ hybrids responded similar to the drought-tolerant parent, indicating dominance of seedling drought tolerance over sensitivity. Segregation for seedling survival drought tolerance versus sensitivity in F₂ generation was in complete agreement with monogenic 3:1 ratio. The F₃ families and backcross data additionally confirmed monogenic tolerance based on seedling survival under drought. Out of 51 SSR markers screened, thirteen markers were polymorphic between the parental types. Seven markers among them were found to be associated with seedling survival drought tolerance through bulk segregant analysis. Association of these markers with seedling survival drought tolerance was further confirmed through their screening on 10 drought-tolerant and drought-sensitive genotypes. These seven markers were screened in F₂ mapping population (JL-3 × PDL-1) of 101 individuals to map their position in relation to the gene for seedling survival drought tolerance. Linkage analysis mapped the seven markers within a map distance of 133.2 cM. A single major gene Sdt was identified with a LOD value of 19.9 and phenotypic variation (R ²) of 69.7 %. The Sdt locus was obtained in the marker interval of PLC_105–PBA_LC_1480 spanning 24.9 cM with the closest marker PLC_105 at a distance of 9.0 cM on the obtained linkage group. This is the first report on genetic control and linkage of SSR markers for drought tolerance in lentil. These linked markers can be used in molecular breeding programmes for introgression of seedling survival drought tolerance gene in high-yielding cultivars.