Categories of exogenous substances and their effect on alleviation of plant salt stress

Salt stress is one of the environmental factors that limit global agricultural productions and vegetative growths. For last several decades, plant scientists have been trying to fully explore the causes, types and responses of plants under salt stress and to overcome its associated problems. Many studies have recently focused on and identified various exogenous substances that demonstrated a potential use in alleviating plant salt stress. With this review, we have selected 118 articles for profound information of different groups of exogenous substances that have proven to be effective in enhancing plant salt tolerance. A total of 58 exogenous substances have been reported to be effective in alleviating salt stress in plants, which can be divided into 7 categories in: 1) regulating the ion balance and pH, 2) inducing and synthesizing osmotic adjustment substances, 3) inducing antioxidant enzymes, 4) regulating hormone production, 5) inducing gene expressions and signal transductions, 6) improving photochemical systems, and 7) pertaining to microbial regulating mechanisms. For each category of those substances, its research progresses and mechanisms involved were analyzed and summarized with some possible novel research frontiers that were proposed and discussed.