Selection of drought tolerant bread wheat lines based on tolerance indexes and pedigree analysis

Introduction
Drought Stress is one of the most important and common environmental stresses that restricts agricultural production and reduces production efficiency in semi-arid and dryland areas (Sarmadnia, 1993). Information on yield stability of genotypes under both favorable and drought stress conditions is essential to identify drought-tolerant genotypes by plant breeders and genotypes with good performance in both environments are preferred (Uddin et al., 1992). In International Maize and Wheat Improvement Center (CIMMIT), the F3 and F4 generations are selected under limited irrigation conditions. The selected plant materials in the fifth and sixth generations are evaluated under normal conditions, and in seventh and eighth generations performance comparison and other evaluations are studied under water stress and natural conditions. This method of evaluation and selection under stress and non-stress conditions has been suggested and applied by many researchers on wheat and other crops (Ehdaie et al., 1988). The goal of this study was selection of drought tolerant bread wheat lines based on their tolerance indices and pedigree analysis.
Materials and Methods
In this experiment, 291 lines and cultivars of bread wheat along with four check cultivars including Parsi, Pishtaz, Sirvan, and Sivand were evaluated to identify tolerant genotypes for terminal drought using two identical experiments, one under non-stress condition and the other one under drought stress condition (irrigation cut-off after 50% of anthesis). Experiments were conducted using the observational method without replication. During the developmental stages, the traits including green percentage, days to heading, days to maturity, plant height, grain color, thousand-grain weight, lodging percentage, shattering and grain yield per plot were recorded. Drought susceptibility and tolerance indices such as STI, SSI, GMP, MP and TOL were calculated for all the investigated entries based on 1000-grain weight and grain yield. In order to investigate the genetic background of selected lines, the percentage of genetic contributions of each genotype was calculated based on the pedigree of the lines.
Results & Discussion
The results indicated that under both stress and non-stress conditions, the selected lines were relatively early mature. Comparison of arithmetic means (MP), geometric mean (GMP) and drought tolerance index (STI) of the genotypes revealed that selection based on these criteria leads to the selection of high yield genotypes under both stress and non-stress conditions. In terms of grain yield, genotypes 33, 2, 34, 23, 10 and 11 had the lowest SSI, which was associated with a higher yield of these lines under drought stress. Also, in terms of 1000-grain weight, genotypes 10, 11, 32, 33, 34, 8 and 5 had the lowest SSI indices where this trait was significantly correlated with high seed weight under drought stress. Lines 11, 14 and 34 had the highest STI index for grain yield and 1000-grain weight, which was significantly associated with high grain yield and 1000-grain weight under normal conditions. Finally, 34 superior drought tolerant lines were selected in two stages according to STI and SSI indices. Pedigree analysis of the selected lines showed that genotypes PASTOR, WBLL1, SKAUZ, and Y50E had contributions of 11.21%, 6.34%, 4.04%, and 4.04% in germplasm composition under drought condition, respectively, indicating their impact on drought tolerance. On the other hand, genotypes CHIBA, KACHU and 1-73-240 had 0.01%, 0.02%, and 0.02% contribution in germplasm composition under drought conditions, respectively, showing no influence in drought tolerance.
Conclusion
The results of this study showed that the genetic structure of some Iranian wheat cultivars such as Omid, Pishtaz, Parsi, Marvdasht, Alborz and Atrak, respectively, made contributions of 2.78%, 2.39%, 1.47%, 1.47% and 1.44% to the genome of the selected lines. Finally, it was suggested that, in addition to the above-mentioned Iranian cultivars, breeding lines derived from genotypes such as PASTOR, KAUZ, SKAUZ, WBLL1, PBW343, Y50E and MUNAL #1 can be used in breeding programs to produce suitable germplasm with early maturity and tolerance to terminal-drought stress for temperate climate of Iran.