Effect of Foliar Application of Growth Regulators on Growth and Induction of Terminal Heat Tolerance in Wheat (Triticum aestivum L.)

Wheat is one of the most widely cultivated cereals in the world. Thermal stress is effective on photosynthesis, cellular and subcellular compounds, protein levels and antioxidant activity. The role of abscisic acid as an anti-stress hormone is easily proved. Abscisic acids cause stomatal closure and improve the plant's water relations. Salicylic acid, as a cell signal, increases the amount of defense compounds such as proline. Also, the use of salicylic acid increases the amount of polyphenols, spermidine and spermine in the plant, which can help stabilize the membrane under stress conditions. The present experiment was conducted to investigate the effect of foliar application with salicylic acid and abscisic acid on reducing the effects of terminal heat stress on the growth, yield and yield components of wheat (Fung and Chamran cultivars) in Ahvaz. This experiment was conducted in a split split plot based on a randomized complete block design with three replications at the experimental farm of Shahid Chamran University of Ahvaz in 2016-2017. The experiment was directed in a potted (in field conditions) and heat stress was considered based on the planting date according to the 30-year estimate of ambient temperature and it’s higher than the threshold of thermal tolerance of the wheat plant. In this experiment, three factors were investigated: foliar application (control (no foliar application), foliar application with abscisic acid (30 mg.l-1), and foliar application with salicylic acid (69 mg.l-1)), time of foliar application (15 days before flowering (A), flowering (B), 15 days after flowering (C) and 15 days before flowering, flowering and 15 days after flowering (ABC)) and wheat cultivars (Fung and Chamran). According to the results, with increasing number of spikelets per spike, the number of grains per spikelet increased and thus, the number of grains per spike increased. Foliar application increased the number of seeds per plant compared to control treatment, which increased the seed weight. The lowest plant height was obtained to treatment of foliar application of salicylic acid at flowering time on Fung cultivar. The highest and lowest spike length were observed in the control of Fong and foliar application of salicylic acid at flowering time and two weeks before and after than on Fung cultivar. The results of mean comparison of the traits showed that the highest mean spikelet number per spike belonged to foliar application of salicylic acid at two weeks after flowering in Chamran cultivar. The highest dry weight of shoot was observed in foliar application of salicylic acid at two weeks after flowering on Chamran cultivar. According to the results, for traits such as dry weight of shoot, grain weight per plant and 1000 grain weight, foliar application at flowering and two weeks before and after that (regardless of cultivars and type of solution) was better compared to the other foliar application times. Foliar application at flowering and two weeks before and after that, improved the photosynthesis potential after spike emergence, improvement of meiosis of pollen mothers cells and increasing the fertility of spikes, reducing the abortion of the claws and increasing the number of seeds per spike under heat stress conditions. Chamran cultivar was recognized as superior to shoot dry weight and number of seeds per plant, in comparison to Fung cultivar (regardless of the time of foliar application and solution type). Salicylic acid with the highest number of seeds per plant, seed weight per plant, 1000 seed weight, dry weight of shoot and harvest index were determined as the best solution (regardless of the foliar application time and cultivar). Therefore it seems that in order to reduce the effects of terminal heat stress, using the salicylic acid (30 mg L-1) at flowering and two weeks before and after that on Chamran cultivar is appropriate in Khuzestan province and regions with similar conditions.