Evaluation of the Effect of the Ppd-D1 Gene on Drought Stress Tolerance of Bread Wheat (Triticum aestivum L.) Isogenic Lines

Extended Abstract Background: One of the main challenges for wheat farmers and breeders worldwide is the limitation of growth, development, and yield in the face of environmental stresses. Dehydration is one of the most important stresses, leading to decreased efficiency and production of this agricultural product. As a classic adaptation mechanism, drought escape allows the plant to complete its life cycle before occurring drought stress. This issue is particularly important in areas where drought occurs at the end of the growing season. In addition to controlling the growth habit of wheat, photoperiod genes play a key role in the flowering time and earliness of wheat and are of interest in drought tolerance research. This research aimed to investigate drought stress tolerance in wheat isogenic lines using quantitative indices of stress tolerance. Methods: In previous research, earliness was transferred from an Australian early-heading variety, Excalibur, to Roshan and Kolhaidari using the backcrossing method to develop the BC3F2 generation. In the present research, the early-heading plants of this generation were crossed with the recurrent parents (Roshan and Kolhaydari) in 2017 to obtain the first generation from the fourth backcross. In this generation, parents and offspring were evaluated for photoperiod-controlling genes. During the first to fourth generations of the fifth backcross, only heterozygous plants (Ppd-D1a/Ppd-D1b) were selected using specific markers for the Ppd-D1 locus. Five generations of backcrossing and four generations of selfing were performed to have different alleles of Ppd-D1 in the same genetic background. Both homozygous genotypes (Ppd-D1a/Ppd-D1a and Ppd-D1b/Ppd-D1b) were selected in the fifth generation of the fifth backcross to generate isogenic lines for Ppd-D1. In this generation, homozygous lines were selected for final field trials. To investigate the effect of photoperiod genes on drought tolerance, four isogenic lines were created in experiments with a randomized complete block design and four replications in the crop year 2018-2019 in the rainfed conditions of Sepidan and in the well-watered conditions of Kerman in the crop year 2019-2020. The number of days to flowering, number of days to maturity, grain-filling period, number of spikes per square meter, number of seeds per spike, 1000-grain weight, and grain yield were evaluated in this research. The drought tolerance of isogenic lines was evaluated using eight indices, including productivity mean, yield index, stress tolerance index, geometric mean of production, stress sensitivity index, yield stability index, and stress tolerance score. Results: Unlike Ppd-D1b, which is a photoperiod-sensitive allele, Ppd-D1a as a photoperiod-insensitive allele, effectively improves early flowering under dryland and well-watered conditions. The Ppd-D1a allele reduced the number of days to flowering in the Roshan genetic background by 3.75 and 4.00 days, and in the Kolhaydari genetic background by 5.08 and 4.7 days in rainfed conditions of Sepidan and well-watered conditions of Kerman, respectively. These results were also reflected in the number of days to maturity, in which Ppd-D1a improved this trait by 7.04 and 8.02 days in the Roshan and Kalhaydari genetic backgrounds, respectively. Ppd-D1a improved earliness in both genetic backgrounds, with better performance in the Kalhaydari genetic background. These findings confirm the interaction between genetic background and the Ppd-D1 gene. Despite the positive effect of Ppd-D1a on earliness in both genetic backgrounds under all environments, there was an interaction between genetic backgrounds and Ppd-D1a alleles for earliness, implying that genetic backgrounds determine the extent of the response to selection. Among the studied isogenic lines under drought stress conditions, the Ppd-D1a allele significantly improved grain yield in the Roshan and Kalhaydari genetic backgrounds, respectively (96 kg/ha and 99 kg/ha). However, there were no significant differences between isogenic lines in both genetic backgrounds under well-watered conditions. These results highlight the importance of marker-assisted selection and backcrossing for Ppd-D1a in breeding programs for dryland conditions. According to yield correlation (0.952) in drought stress and normal environments, selection in both conditions can show high-yielding isogenic lines with good yield stability. In normal conditions, there was a high correlation between the geometric mean of production, stress tolerance index, yield index, stress sensitivity index, and grain yield. This correlation was high and significant for the geometric mean of productivity, stress tolerance index, and yield index under drought stress conditions. The stress tolerance score showed that Ppd-D1a isogenic lines in both Roshan and Kolhaydari genetic backgrounds, which had the photoperiod insensitivity allele, had the highest stress tolerance score. Principal component analysis showed that Ppd-D1a in the Roshan background was the most drought-tolerant isogenic line. Conclusion: The results showed that Ppd-D1a not only improved earliness in the isogenic lines but also improved drought tolerance using the drought escape mechanism.