Drought, salt, and combined stresses in plants: Effects, tolerance mechanisms, and strategies

Soil drought and salinity are stresses that seriously restrict plant productivity. Water and salt stress often occur simultaneously or sequentially in the heavily irrigated field in the arid and semi-arid areas. The impacts of drought and salinity on plants range from morphological adaptations to molecular responses. Instead of studying these twin abiotic stresses in isolation, there is a need to understand the interactions between water and salt stresses and the combined influence on plant growth, yield, and survival. Recent studies demonstrated that the responses of plants to twin stresses cannot be directly predicted from simply studying one of the stresses at a time. This paper reviews the effects of single and combined drought and salt stresses on photosynthetic, stomatal, mesophyll, and hydraulic traits of plant leaves and their coordination. Paper also discusses the drought and salt tolerances mechanisms involving morphological adaptation, hydraulic control, hormonal regulation, osmotic adjustment, ion homeostasis, and antioxidant defense. We propose some mitigation and management strategies, including screening tolerant varieties, inducing stress resistance/tolerance, adding bacteria and fungi, and optimizing field management, with an aimaim to improving water use efficiency and yield under combined stress.