Meta-Analysis of Effects of Melatonin Treatment on Plant Drought Stress Alleviation

Due to the increasing frequency of extreme drought events worldwide in recent years, improving the adaptability of plants to arid environments has become an important research topic. In particular, many studies have investigated the effects of melatonin on drought stress mitigation in plants. However, most of these studies were limited in terms of the number of sampling sites or regional scale, and thus we lack a comprehensive understanding of the effects of the exogenous application of melatonin on drought stress mitigation in plants on a global scale. In this study, we integrated previous research into the physiological and growth effects of melatonin application in arid environments worldwide and analyzed the response of plants to different melatonin concentrations, application methods, and different drought degrees in order to provide a scientific basis for promoting the use of melatonin in alleviating plant drought stress. The data used in this study were obtained from the “Web of Science” database, where the keywords “drought & melatonin” were used to search the relevant literature. In total, 61 valid publications with 140 data sets were retrieved. A meta-analysis was performed using the data with no melatonin treatment as the control group and melatonin treatment as the experimental group. Melatonin application significantly increased the plant biomass, chlorophyll content, and antioxidant enzyme activity to alleviate the damage caused by drought stress. The accumulated biomass and accumulation of chlorophyll in plants varied with the melatonin concentration. The threshold value range was identified as 80–120 μmol L −1 , and the effect of melatonin on the accumulation of biomass and chlorophyll decreased gradually above this range. In addition, the effects of various spraying methods on the mitigation of drought stress in plants differed significantly. Soil application had greater effects on reactive oxygen species scavengers in plants than foliar spraying. Moreover, the plant leaf membrane lipid peroxidation degree was relatively low, and the plant body chlorophyll content was higher under soil application than foliar spraying, and the cumulative biomass was lower than that with foliar spraying. The effects of melatonin on mitigating plant drought stress also varied under different drought levels when using the same melatonin concentration and application method. Soil irrigation is most effective if the main aim is to improve plant stress resistance and the below-ground root biomass, but foliar spraying is most effective for increasing photosynthesis and plant biomass.