Foliar Application of Growth Regulators Mitigates Harmful Effects of Drought Stress and Improves Seed Yield and Oil Quality of Rapeseed (Brassica napus L.)

This study aimed to investigate the effect of plant growth regulators (PGRs) for improving the quantity and quality of rapeseed oil under different irrigation conditions. A 2-year experiment was conducted in a split-plot arrangement based on a randomized complete block design with three replications over 2 years (2015–16 and 2016–17) at Safiabad Agricultural and Natural Resources Research and Education Center. Main plots consisted of three irrigation regimes (optimal irrigation, restricted irrigation from flowering, and restricted irrigation from silique setting) and subplots included 10 levels of foliar application of PGRs (100, 200, and 300 mg l−1 ascorbic acid; 100, 200, and 300 µM salicylic acid; 10, 20, and 30% [w/v] methanol; and foliar application of distilled water as control). The studied cultivar was Hyola401. Restricted irrigation from flowering and silique-setting stages led to decreases in rapeseed seed yield by 38 and 15% as compared with the full irrigation regime, respectively. The interaction of irrigation regime × foliar application showed that spraying ascorbic acid (300 mg l−1) maximized the seed yield under optimal irrigation (4493.33 kg ha−1) and restricted irrigation from the silique-setting stage (3884.72 kg ha−1). Under restricted irrigation from the flowering stage, foliar spray of methanol (30%) produced the highest seed yield (2667.77 kg ha−1). Restricted irrigation from flowering and silique stages resulted in a decrease in oil content by 9.28 and 5.83%, respectively. Overall, foliar application of ascorbic acid, salicylic acid, and methanol improved the seed yield and oil quality of rapeseed under optimal water supply and drought-stress regimes.