Salicylic acid delays leaf rolling by inducing antioxidant enzymes and modulating osmoprotectant content in Ctenanthe setosa under osmotic stress

The effects of salicylic acid (SA) on leaf rolling under osmotic stress in Ctenanthe setosa (Roscoe) Eichler were evaluated. The leaves were pretreated with SA, paclobutrazol (PAC), and antioxidant enzyme inhibitors for 15 h and then kept in polyethylene glycol (PEG) for 4 h to promote osmotic stress. The degree of leaf rolling was measured every hour during the 4-h PEG treatment. Antioxidant enzymes (superoxide dismutase, catalase, ascorbate peroxidase, and guaiacol peroxidase) and NADPH oxidase (NOX) activities, endogenous SA contents, hydrogen peroxide, proline, and soluble sugar were determined immediately after PEG treatment. SA treatment retarded leaf rolling and induced antioxidant enzyme activities under osmotic stress. Antioxidant enzyme inhibitors and PAC treatments caused reductions in all antioxidant enzyme activities and accelerated leaf rolling. The PAC treatment also decreased endogenous SA, hydrogen peroxide, proline, and soluble sugar contents and NOX activity. In conclusion, the delay in leaf rolling resulting from SA application was related to the antioxidant enzyme activities and also to osmoprotectants. These were supported by the increase in leaf rolling degree due to PAC, which led to a decrease in proline and sugar contents and antioxidant enzyme activities due to a decrease in hydrogen peroxide content followed by a reduction in NOX activity.