Evaluation of tolerance to drought stress in bread wheat (Triticum aestivum L.) cultivars and landraces using tolerance indices

Introduction
Drought is one of the most destructive stresses affecting the growth and development of plants. The increase in global population and wheat demand as well as drought period has increased the importance of breeding for drought tolerance in bread wheat. The use of selection indices for drought tolerance is an effective strategy to screen and identify wheat tolerant genotypes in wheat breeding programs. The objective of the current study was to investigate the relationships between tolerance indices, to determine the efficiency of indices in separating genotypes, and finally to select drought tolerant genotypes in bread wheat cultivars and landraces.
Materials and methods
To investigate the effect of drought stress on grain yield of bread wheat, an experiment was conducted in a simple alpha lattice design under normal irrigation and drought stress conditions in the research field of Dryland Agricultural Research Institute, Maragheh, Iran, in two consecutive years, 2017-2018 and 2018-2019. The plant materials were 300 bread wheat genotypes, including 210 landraces and 90 commercial cultivars (15 dryland and 75 irrigated cultivars; 64 spring, 15 winter, and 11 facultative cultivars). To evaluate sensitivity or tolerance of genotypes to drought stress and identify drought-tolerant genotypes, various indices such as tolerance index (TOL), mean productivity (MP), stress sensitivity index (SSI), stress tolerance index (STI), geometric mean productivity (GMP), harmonic mean (HM), drought tolerance index (DI), abiotic tolerance index (ATI), and modified stress tolerance index (MSTI) were calculated using grain yield under normal and stress conditions for all genotypes. Also, to increase the selection efficiency and screen drought tolerant genotypes, the SIIG index was calculated based on the integration of 12 studied indices. The analysis of variance was performed using Agricolae package, heirarchical clustering of bread wheat genotypes using heatmap3, RColorBrewer, dendextend, gplots and colorspace packages, bi-plot diagrams using factoextra package, and heatmap diagram of correlation among grain yield under normal irrigation and drought stress conditions and stress tolerance indices using corrplot package in R4.2.2 software.
Research findings
The results of analysis of variance for grain yield showed that the environment, genotype, and genotype×environment interaction effects were significant (P<0.001). The significance of the effect of genotype indicates the appropriate diversity among the genotypes, and the significance of the interaction between genotype and environment indicates different responses of genotypes to changes in environmental conditions. Cluster analysis based on the data of each year and the average of two years classified the genotypes into three clusters. The results of principal component analysis (PCA) showed that cultivars Shanghai, Naz, Falat, Bam and landraces 626358, 623109 and 621908 in the first year, as well as cultivars Shanghai, Koohdasht, Karim, Pishgam, and landraces 628189, 627460, and 627616 in the second year were drought tolerant genotypes, while based on the average data of two years, cultivars Koohdasht, Karim, Pishgam, Adl, Naz, and landraces 628189, 627460, 624944, and 627299 were identified as drought tolerant genotypes. The estimation of correlation coefficients showed that grain yield had a negative and significant correlation with SSI and TOL indices and a positive and significant correlation with other indices, especially MP, GMP, HM, STI and K₂STI.  
Conclusion
The results of drought tolerance indices as well as the SIIG index, the old spring and irrigated cultivars Adl, Naz, and Shanghai and the new spring and dryland cultivars Karim and Koohdasht, as well as the landraces 626358 (originating from Isfahan) and 621908 (originating from Arak) were drought tolerant and high performance under two normal and drought stress conditions. The results obtained from this study provided valuable information on the selection of drought tolerant genotypes, which can be used to produce new cultivars with high yield potential in wheat breeding programs. The tolerant genotypes identified in this experiment can also be used as parents for genetic analysis, gene mapping, and improvement of drought stress tolerance in wheat.