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

Abstract Drought is among the environmental stress factors that causes a decrease in plant productivity. A hot topic in abiotic stress physiology research is how to alleviate drought stress during plant growth. Exogenous substances have been observed to play a positive role in regulating plant responses to drought. Hibiscus (Hibiscus syriacus L.) has high ornamental and medicinal value. However, there is a shortage of reports focused on exogenous substances that can alleviate stress caused by environmental factors in hibiscus. This study used 1-year-old H. syriacus var. ‘elegantissimus’ seedlings as experimental materials. We investigated the roles and physiological mechanisms of melatonin (MT) and salicylic acid (SA) on hibiscus during drought stress by observing plant growth status and photosynthetic physiological parameters. The results showed that compared with the treatment of only drought stress, exogenous MT and SA increased the chlorophyll content of plants, enhanced photosynthesis, alleviated photoinhibition, and protected the photosystem. On the other hand, exogenous MT and SA increased the expression of antioxidant enzyme genes (Cu/Zn-SOD, POD-20, CAT, and APX-1) and improved their enzymatic activity. This reduced the ROS content and alleviated the drought-induced oxidative damage. In addition, MT and SA induced the expression of drought-responsive genes HsNCED and HsDBF1. In summary, Exogenous melatonin and salicylic acid can enhance drought resistance of hibiscus by regulating its photosynthetic system, antioxidant enzyme activity, and drought-related genes. The study results provide a crucial scientific basis and theoretical support for the cultivation management and subsequent care of hibiscus.