Evaluation of Drought Tolerance in Some Wheat Genotypes Based on Selection Indices

Wheat is a major crop among cereals and plays a vital role in the national economy of developing countries. Wheat (Triticum aestivum L.) is one of the most important crops in terms of acreage and production rates in the world. This crop has an important role in the food supply. According to the FAO (2010) statistics report, the average wheat yield in Iran was 2136 kg ha-1, while the worldwide average yield was 3009 kg ha-1. Iran, with an average annual rainfall of 250 mm, is located in the world desert belt. Yield loss due to drought stress is likely higher than other stresses. Therefore, introducing plants with high production under both drought stress and non-stress conditions is highly regarded. Stress tolerance indices are used for screening drought tolerant varieties. Tolerance (TOL), mean productivity (MP), stress susceptibility index (SSI), geometric mean productivity (GMP), stress tolerance index (STI) and modified STI (MSTI) have been employed under various conditions. Fischer and Maurer (1978) explained that cultivars with an SSI less than a unit are stress tolerant, since their yield reduction under stress conditions is smaller than the mean yield reduction of all cultivars (Bruckner and Frohberg, 1987). Mean productivity, GMP, harmonic mean (HM) and STI were reported as preferred criteria in selection of drought-tolerant barley genotypes by Baheri et al. (2003). Yield Index (YI) proposed by Gavuzzi et al. (1997), was significantly correlated with stress yield which ranks cultivars on the basis of their yield under stress. The genotypes with a high Yield Stability Index (YSI) are expected to have higher yield under both stress and non-stress conditions (Bouslama and Schapaugh, 1984). Mousavi et al (2008) introduced Stress Susceptibility Percentage Index (SSPI) as a powerful index to select extreme tolerant genotypes with yield stability. Fischer and Wood (1979) suggested that relative drought index (RDI) is a positive index for indicating stress tolerance. Lan (1998) defined a new drought resistance index (DI), which was commonly used to identify genotypes producing higher yield under both stress and non-stress conditions The objectives of this study were to evaluate the performance of different wheat cultivars under normal irrigation and drought stress conditions and to identify the most promising wheat genotypes for drought prone areas. Thirty-nine spring bread wheat genotypes were evaluated under two irrigation regimes, normal and moisture stress at grain filling period for a year. Under normal conditions soil was irrigated up to field capacity, while under stress conditions after the onset of flowering, irrigation was delayed until wilting point took place. The study was conducted at College of Aburaihan, University of Tehran, Iran, in Pakdasht. The experimental design was a randomized complete block design with three replications. Standard cultural practices were applied for all experiments. At harvest, grain yield (t ha-1) was calculated on the basis of plot area. Under normal irrigation, Pishtaz and Azadi (with 8.27 and 7.72 ton ha-1, respectively) and under stress conditions Moghan1 and Sistan (with 5.48 and 4.84 ton ha-1, respectively) had the highest grain yield. Based on regression analysis under normal and stress conditions, three variables entered the model and in normal conditions 70.8 percent and in stress conditions 64 percent of yield changes were explained. Based on grain yield, 15 susceptibility and stress tolerance indices were calculated. Results of correlation, principal component analysis and biplot display showed that GMP, STI, HARM, MP, YI, DI, MSTI and SNPI indices were the best criteria for genotype selection with high yield and stability in stress conditions. Cluster analysis (Ward method) was also used based on indices and grain yield in both normal and stress conditions to classify genotypes in similar classes. Dispersion of genotypes in the biplot, revealed genetic diversity among the genotypes under drought stress. Results showed that Moghan1, Sistan, Akbari, Bayat, Dez, Spring Roshan-BC, Mahdavi and Tabasi genotypes were identified as tolerant and Tajan, Navid, Shirodi, Zagros, Vee/Nak and Kohdasht genotypes as susceptible genotypes to terminal drought stress. These genotypes can be used for further cross and genetic analysis for drought tolerance through diallel or generation mean analysis designs.