Introduction
Terminal drought stress represents a major threat to wheat production in Iran. Developing cultivars with enhanced drought tolerance is therefore a key objective of many wheat breeding programs. In most regions of Iran, bread wheat cultivation is subjected to drought stress during the grain-filling period, one of the crop’s most sensitive developmental stages. To assess crop responces, various indices have been developed, including the the tolerance index (TOL), mean productivity (MP), geometric mean productivity (GMP), harmonic mean (HM), stress tolerance index (STI), stress sensitivity index (SSI), yield index (YI), yield stability index (YSI) and relative stress index (RSI). The effectiveness of each index depends on the specific breeding objectives and the characteristics of the target environment.
Materials and Methods
This study was conducted over two consecutive cropping seasons (2022-2023 and 2023-2024) at the Gonbad Agricultural Research Station. To evaluate thirty bread wheat lines and cultivars under terminal drought stress, two separate experiments were conducted in format of split plots design within a randomized complete block framwork, each with three replications. Two levels of irrigations [normal moisture and terminal drought stress] were assigned to the main plots, while the thirty genotypes were randomized in the subplots. Drought stress was imposed by withholding irrigation beginning at the full heading stage. Based on grain yield under normal (Yp) and stress (Ys) conditions, the following indices were calculated: mean productivity (MP), geometric mean productivity (GMP), harmonic mean (HM), stress tolerance index (STI), stress sensitivity index (SSI), yield index (YI), tolerance index (TOL), relative stress index (RSI) and yield stability index (YSI). Data analysis was performed using iPASTIC and NTSYS software packages.
Results and Discussion
The results of this study showed that the mean grain yield across all genotypes was 5.219 t ha⁻¹ under optimal irrigation and 4.129 t ha⁻¹ under terminal drought stress, corresponding to an approximately 21% reduction under stress conditions. Correlation analysis of drought tolerance indices identified mean productivity (MP), geometric mean productivity (GMP), harmonic mean (HM), and the stress tolerance index (STI) as the most reliable indicators for identifying drought-tolerant genotypes. Principal component analysis (PCA) showed that the first and second principal components accounted for 63.64% and 36.24% of the total variation, respectively. Furthermore, biplot analysis demonstrated that the majority of drought-tolerant genotypes were closely associated with the vectors representing these key tolerance indices. The combined use of a three-dimensional index plot and hierarchical cluster analysis classified genotypes 9, 11, 17, 18, 19, and 20 as tolerant to terminal drought stress, whereas genotypes 1, 6, 8, 12, 23, 25, 27, and 30 were identified as highly sensitive. These drought-tolerant genotypes represent valuable genetic resources for wheat breeding programs aimed at improving drought resilience. Their incorporation into future breeding efforts is expected to facilitate the development and eventual release of high-yielding, drought-tolerant cultivars for cultivation in terminal drought-prone environments.
Conclusions
Correlation analysis of drought tolerance indices revealed that mean productivity (MP), geometric mean productivity (GMP), harmonic mean (HM), and stress tolerance index (STI) were the most suitable metrics for selecting drought-tolerant genotypes. Based on three-dimensional plotting, principal component analysis, and cluster analysis, genotypes 9, 11, 17, 18, 19, and 20 were identified as the most tolerant to terminal drought stress, whereas genotypes 1, 6, 8, 12, 23, 25, 27, and 30 were classified as the most sensitive.