Triacontanol Foliar Spray Alleviated Drought Stress Effects by Maintaining Photosynthesis and Cellular Redox Balance in Sunflower Seedlings

Drought is one of the most dangerous threats to cultivated lands and agriculture in Egypt. Exploring strategies to enhance crop tolerance to drought is therefore imperative. This study aimed to examine whether triacontanol (TRIA) can mitigate the drought harmful effects on sunflower seedlings. Eleven-day-old sunflower (cv. Sakha 53) seedlings were exposed to two watering regimes: (1) well-watered and (2) drought-stressed for 5 days. Two concentrations of TRIA (10 and 25 μM) were applied as a foliar spray to drought-stressed seedlings. The effect of TRIA on growth attributes, physiology, and biochemistry of 16-day-old sunflower seedlings in the absence and presence of drought stress was evaluated. TRIA (25 μM) was relatively efficient in alleviating drought injurious effects on the growth of sunflower seedlings. TRIA also counteracted the drought-induced photosynthetic impairment by increasing the level of the photosynthetic pigments, the photosynthetic performance index (PI_abs), the photochemical efficiency of photosystem II (F_v/F_m), and the expression of the Rubisco small subunit (RBCS) gene. Also, TRIA treatment increased total soluble sugars (TSS), proline, antioxidant defense system (measured by DPPH scavenging capacity), ascorbic acid (ASA), total phenolics, enzyme activities of peroxidase (PX) and polyphenol oxidase (PPO), while decreased contents of abscisic acid (ABA) and malondialdehyde (MDA) as well as the activities of superoxide dismutase (SOD) and ascorbate oxidase (AO; particularly 10 μM) of drought-stressed seedlings. It is evident that TRIA is a potential candidate to enhance sunflower seedling tolerance to drought stress by maintaining photosynthetic activities and enhancing antioxidant defense systems.