Salicylic Acid: A Novel Plant Growth Regulator – Role in Physiological Processes and Abiotic Stresses Under Changing Environments

The environment is greatly affected by climatic vagaries resulting in both biotic and abiotic stresses. Stresses occurring at the primary and secondary levels lead to abiotic stresses that consequently affect the yield of crop plants. Agriculture at the global level is unable to meet the rising demands due to the consistent increase in population. Since natural renewable resources have been exploited to the maximum, agriculture is under tremendous pressure. Increased human activity has affected the different ecosystems in a variety of different ways. One of these effects has led to degradation of habitats, including cultivated land. Furthermore, due to rapid industrialization, urbanization, and other developments, there is a constant threat to the environment and a depletion of natural resources. Disturbance of the natural habitats/environment results in the degradation of soil and pollution of the water resources. The situation will be further aggravated through climatic changes. The impact on agriculture could result in water scarcity, heat stress, drought, and new diseases, and one can expect to see more frequent flooding and more severe drought. Lack of irrigation water will hamper the fertility of agricultural soils [1]. The role of plant growth regulators/substances in mitigating the various type of stresses in plants is well known and documented [2]. In recent years, salicylic acid has been the focus of intense research due to its function as an endogenous signal mediating local and systemic plant defense responses against pathogens. Salicylic acid plays a role during plant responses to abiotic stresses such as drought, chilling, heavy metal toxicity, heat, salinity, UV radiation, and osmotic stresses, and helps in mitigating their deleterious effects to varying extents. In addition, salicylic acid is also involved in regulating physiological and biochemical processes during the entire life cycle of plants. Understanding the mechanism underlying the vital processes will pave the way in deciphering the signaling network of salicylic acid, and also ascertaining its role in disease resistance and plant health. The present chapter is focused on various intrinsic biosynthetic/metabolic pathways of salicylic acid, the interplay of salicylic acid and methyl-salicylic acid, its transport, and its actions as a signaling molecule. The effects of exogenous application of salicylic acid on seed germination, growth, photosynthesis, plant–water relations, various enzyme activities, nitrogen assimilation, productivity, and various biotic and abiotic stresses have also been analyzed under changing environmental conditions.