Evaluation of Phenological Stages, Changes in GDD, and Grain Yield in Wheat Cultivars in Response to Changes in Planting Date and Application of Paclobutrazol

To evaluate the phenological stages and yield of wheat cultivars in response to the change of planting date and application of paclobutrazol, a field experiment was conducted in two years, 2020-2021 and 2021-2022, in Izeh city located in the northeast of Khuzestan, Southwest Iran. The experiment had three replications in a split factorial randomized complete block design. The main factor included planting date (November 16, December 11, and January 5) and the sub factors in the factorial form included cultivars (Karim, Mehrgan, and Chamran2) and the application of paclobutrazol (0, 60, 120 mg/L). In both years, the highest seeds/m2 (18847) belonged to Chmaran2 genotype when sown on 16 November and received 60 mg/L paclobutrazol. When the genotypes were sown in the delayed planting date of January 5, application of 60 and 120 mg/L paclobutrazol led to an increase in seed yield compared to the 0 paclobutrazol. With the application of 60 mg/L paclobutrazol, the seed yield of Chamran2 genotype was higher on the planting dates of November 16 (6941 kg/ha) and December 11 (6779 kg/ha), compared to other paclobutrazol levels. Chamran2 and Karim genotypes indicated the greatest and smallest number of days and growing degree days, respectively, in different stages of growth and development. However, these genotypes did not show significant difference in terms of biological yield, grain yield, and harvest index. When plants were sown on January 5, the length of the plant growth period from the planting to physiological maturity was reduced by 34 days compared to when sown on November 16. Overall, the effect of paclobutrazol on seed yield was positive, and the consumption of 120 mg/L paclobutrazol moderated the negative effects of the delayed planting of the examined wheat genotypes.