Exogenous Melatonin and Salicylic Acid Enhance the Drought Tolerance of Hibiscus (Hibiscus syriacus L.) by Regulating Photosynthesis and Antioxidant System

Drought is a common environmental stress factor that causes a severe effect in plant growth, development, and productivity. Hibiscus (Hibiscus syriacus L.) has high ornamental and medicinal value. However, few reports about exogenous substances alleviating drought stress damage in hibiscus were reported. Here, we investigated the efficacy of exogenous melatonin (MT) and salicylic acid (SA) in conferring drought tolerance in hibiscus. MT (100 μmol L−1) and SA (500 μmol L−1) were applied as foliar spray to 1-year-old hibiscus. Plants were exposed to drought stress by withholding water for 21 days. Drought stress caused leaf wilting, weakened photosynthesis, and increased reactive oxygen species (ROS) content. Spraying with MT or SA conferred drought-stressed plants increased chlorophyll content, enhanced photosynthesis, and alleviated photoinhibition. Besides, compared to drought-stressed plants, antioxidant enzymatic activity (Cu/Zn-SOD, POD-20, CAT, and APX-1) and their encoding gene expression were increased, and the ROS content was reduced, and the drought-induced oxidative damage was alleviated after spraying MT or SA. In addition, MT and SA induced the expression of drought-related response genes HsNCED and HsDBF1. The efficacy of SA was higher than that of MT in promoting photosynthesis, antioxidant enzyme activity, and the expression of drought-related response genes. Exogenous application of MT or SA confer hibiscus drought tolerance by regulating photosynthetic system, antioxidant enzyme activity, and the expression of drought-related response genes. SA appeared to be more effective than MT.