Boron-induced improvement in physiological, biochemical and growth attributes in sunflower (Helianthus annuus L.) exposed to terminal drought stress

Terminal drought stress (drought at reproductive growth stage) has been considered a severe environmental threat under changing climatic scenarios and undoubtedly inhibits sunflower production. A field study was conducted to explore the potential role of foliar applied boron (B) (0, 15, 30, 45mg L-1) at late growth periods of sunflower in alleviating the adversities of terminal drought stress (75, 64, 53mm DI) grown from inflorescence emergence to maturity stages. The plant water relations such as leaf relative water content (RWC), water potential ((w)), osmotic potential ((s)), and turgor pressure ((p)) were increased significantly with B foliar sprays while exposed to terminal drought stress. Foliar B application considerably improved the nitrogen and B concentrations in leaf and seed tissues, and also chlorophyll a and b pigments under terminal drought stress conditions. Drought-induced proline accumulation prevented the damages caused by drought stress, nevertheless, B foliar spray increased its contents. Compared to well-watered conditions, terminal drought stress substantially declined the growth performance in terms of reduced leaf area index (LAI), crop growth rate (CGR), net assimilation rate (NAR), and total dry matter (TDM) production; however, foliar B supply (30mg L-1) might be helpful for improving drought tolerance in sunflower with reduced growth losses.