Paclobutrazol Application Improves Growth and Physiological Traits of Wheat Cultivars Under Heat Stress Imposed by Late Planting Date

Terminal heat stress caused by a late cultivation date, is one of the most important factors limiting the crop yield. This study was conducted to investigate the ameliorative effects of paclobutrazol (PBZ) on growth, physiological, and photosynthetic pigments in three wheat cultivars under various planting dates. This experiment was conducted in a split-factorial arrangement based on a randomized completely block design (RCBD) with three replicates over two years (2020–21 and 2021–22). Planting date was determined as main plots (including November 16, December 11, and January 5). Three wheat cultivars (Karim, Mehrgan, and Chamran-2) and application of PBZ at three concentrations (0 mg L −1 , 60 mg L −1 , and 120 mg L −1 ) were also established as two subplots. Our results revealed although late cultivation reduced grain yield, PBZ foliar spraying (120 mg L −1 ) improved grain yield of Chamran‑2 by 36%, Mehrgan by 32%, and Karim by 20%. Moreover, a decreasing trend in stem length was observed with increased PBZ concentration, which may be attributed to the ability of this growth modifier to remobilize assimilates on grains rather than on stems. Moreover, maximum specific weight, and assimilate remobilization increased with the application of PBZ. The reducing effect of terminal heat stress on photosynthetic pigments was also alleviated. Overall, our results showed that Chamran‑2 outperformed others especially in heat stress conditions. It can be concluded that PBZ foliar application can be an effective way to combat adverse effects of late cultivation wheat.