Abscisic acid and brassinolide combined application synergistically enhances drought tolerance and photosynthesis of tall fescue under water stress

Tall fescue (Festuca arundinacea Schreb.) is a widely used cool-season turfgrass, and its growth is mainly limited by water deficit. Abscisic acid (ABA) and brassinolide (BR) are two important stress hormones regulating plant physiological processes and growth under water deficit. To investigate effects of exogenous ABA and BR on physiological and photosynthetic performance of tall fescue under water stress conditions, ABA (10 and 20 mg L-1) and BR (0.4 and 0.8 mg L-1) were applied individually and in combination (0.4 mg L-1 BR and 10 mg L-1 ABA) under three soil water regimes [75 +/- 5% FC (HW), 50 +/- 5% FC (MW) and 25 +/- 5% FC (LW)] in the greenhouse. Results revealed that ABA and BR application markedly decreased the relative conductivity and malondialdehyde, notably increased leaf relative water content, antioxidant enzyme activity and proline content under water stress. ABA plus BR application was equally effective in improving the activities of antioxidant enzyme as ABA at 20 mg L-1. ABA application reduced stomatal conductance and decreased both transpiration and net photosynthetic rate (P-n), while BR application significantly increased P-n and water use efficiency (WUE) by enhancing chlorophyll content. The ABA and BR combination application showed higher P-n and WUE as well as BR single application. It indicated that ABA and BR combination application increased photosynthetic capacity, and reduced the effect on photosynthetic loss caused by ABA under water stress. All these confirmed that ABA plus BR application exhibited a synergistic interaction on enhancing drought tolerance and photosynthesis of tall fescue under water stress.