Effect of cytokinin hormone on morphological and yield traits of sunflower under terminal heat stress

In order to study the effect of hormone cytokinin on morphological and yield traits of sunflower cultivars under terminal heat stress, a field experiment carried out in a split plot factoril in randomized complete block design with three replications at the research farm of Shahid Chamran University of Ahvaz in 2021. The main plots consisted of three sowing dates; 3 Feb, 18 Feb and 5 Mar, the sub-plot consisted of factorial arrangement of different concentrations of cytokinin (0, 25 and 50 and 75 μm l-1, Kinetin), and sunflower cultivars (Qasem and Oskar). Sowing date with desired temperature considered as control and delayed and late sowing dates considered as mild and severe heat stress so that the flowering and grain filling periods faced with terminal heat stress. Results showed that terminal heat stress in delayed and late sowing dates caused a decrease in plant height, grain number per head, head diameter, 1000-grain weight, biological yield, oil percentage, and oil and grain yield. The optimum hormone concentration needed to exhibit a considerable increase in studied traits of sunflower cultivars under heat stress was 50 μm l-1,particularly, in Late sowing date sunflower plants, foliar application of cytokinin (50 μm l-1) increased grain yield (by 17% and 15%), biological yield (by 5% and 8%), and oil yield (28 and 13%), in Oscar and Qasim cultivars, respectively. Results show that foliar application of cytokinin (50 μm l-1) can be considered as a strategy to enhance the grain yield of sunflower cultivars under under heat stress conditions in tropical regions such as Khuzestan.