Effect of brassinolide foliar application on some physiological and agronomic characteristics of sunflower (Helianthus annuus L.) under drought stress conditions

Sunflower (Helianthus annuus L.) is one of the most important oil plants, which is cultivated due to the high seed oil and adaptation to different climatic and soil conditions in a wide range of arid and semi-arid regions of the country. Due to the possibility of drought stress in these areas, it is essential to take action to cope with these stresses. One of the methods to reduce drought stress is the use of plant growth regulators. Considering that brassinolide is the plant growth regulating steroidal substance that can affect the increase of plant resistance against different stresses. Therefore, the aim of this study was to investigate the effects of brassinolide foliar application on some physiological and agronomic characteristics of sunflower in one of the semiarid areas of the country.

The experiment was conducted in the Noorabad area of Fars province using split-plot in a randomized complete blocks design with three replications and nine treatments in 2018. The main factor of the experiment included irrigation at three levels which consisted of without stress (100 percent of full irrigation), moderate stress (75 percent of full irrigation), and severe stress (50 percent of full irrigation) according to evaporation from class A pan, and sub-factor including the application of brassinolide hormone at three levels (0, 0.1 and 0.5 mg/L).

The results showed that yield, number of seeds per head and weight of 1000 sunflower seeds were affected by experimental treatments, using 0.5 mg/L brassinolide foliar application under severe drought stress conditions can significantly increase chlorophyll content and number of leaves per plant, therefore, the yield, number of seeds per head, 1000-seed weight and oil yield increased compared to control treatment. The highest grain yield (313.3 g/m2) was obtained in complete irrigation treatment and 0.5 mg/L brassinolide foliar application, so that 55% of the grain yield was higher than the control treatment. Also, the percentage and yield of oil were reduced under drought stress but the amount of these two traits was increased with brassinolide hormone application. The highest leaf proline content observed in 0.5 mg/L brassinolide foliar application and the lowest of this trait was obtained in the control treatment.

The results of this experiment showed that the use of brassinolide foliar application under severe drought stress conditions can increase drought tolerance by 63%. Therefore, under severe stress conditions, the use of brassinolide foliar application can be a solution to increase the production of sunflower .