Bread wheat genotypes selection for delay planting adaptation in the north of Khuzestan using stress sensitivity index (SSI)

Introduction: Wheat production has always been a key focus in the pursuit of self-sufficiency; therefore, identifying biotic and abiotic constraints in the target environment is a priority. Temperature is one of the most important environmental factors, and any abnormal increase or decrease in temperature can lead to a reduction in yield. The overlap of rice, maize, and leafy and tuber vegetables harvest dates in Khuzestan province results in delays in the optimal wheat sowing date, ultimately decreasing yield. This delay exacerbates the damage caused by increased temperatures and heat stress at the end of the growing season.
Materials and methods: In this study, two international germplasm pools from the International Maize and Wheat Improvement Center (CIMMYT), consisting of 32 (6th WYCYT) and 50 (1st CWYT) genotypes, were evaluated in two replications. Each pool was tested with one replication sown on the optimal sowing date (December 5, 2018) and the other on the delayed sowing date (February 6, 2019) at the Safiabad Agricultural Research and Education Center in Khuzestan. The germplasm was arranged in an Alpha lattice design. Traits assessed for treatment comparisons include yield at both sowing dates (optimal and delayed) and the Stress Sensitivity Index (SSI). In addition, to examine the relationship and calculate the correlation coefficient of traits with SSI, the following traits were considered: days from sowing to heading, plant height, thousand seed weight, and days from sowing to physiological maturity at both sowing dates.
Results: The evaluation and selection of genotypes under delayed sowing conditions, compared to the optimal sowing date, led to the identification of six lines in the 1st CWYT pool that not only had a lower Stress Sensitivity Index than the control (Mehrgan) but also yielded 2 to 14% more than the control under delayed sowing. In the 6th WYCYT pool, none of the lines showed a significant difference in the SSI compared to the control, but 11 lines yielded approximately 1 to 9% more than the control, with a sensitivity to stress lower than 1. The results showed a significant negative correlation between yield and SSI under delayed sowing, while under optimal sowing, there was a significant positive correlation between SSI and yield. The most stable correlation coefficients between SSI and yield were observed for both sowing dates. Except for the genetic correlation coefficient of the 1st CWYT pool under the optimal sowing date, the other coefficients between yield and SSI were positive and highly significant. However, the relationship between SSI and yield was inversely significant under delayed sowing. In other words, genotypes that produced higher yields under the optimal sowing date due to their higher sensitivity to stress did not perform well under delayed sowing. On the other hand, under delayed sowing, different genotypes showed greater yield potential than the control and had higher grain yields and lower stress sensitivity indices. Consequently, Lines 604, 605, 611, 622, 623, 625, 629, and 630 in the 1st CWYT pool and lines 3, 7, 26, and 27 in the 6th WYCYT pool are suitable for delayed sowing in Khuzestan province and for combating late-season heat stress.
Conclusion: The use of the Stress Sensitivity Index (SSI) is strongly recommended for screening wheat genotypes for delayed sowing and stable selection in the national wheat improvement program in hot and dry regions.