The effect of drought on physiological responses of forage plants to salt stresses depends on occurring time

Drought and salinity are two common abiotic stresses that restrain plant growth and often co-occur, either simultaneously or sequentially. However, the effect of drought on physiological responses of plants to salinity remains unclear. Accordingly, we selected three forage species that differ in drought and salinity tolerance: ryegrass (Lolium perenne L.), alfalfa (Medicago sativa L.) and switchgrass (Panicum virgatum L.). Independent and combined (sequential and simultaneous pattern) drought and salinity were carried out in a pot experiment. Salinity severely injured the plants through decreasing leaf water content and increasing Na+ and Cl− concentrations. Compared to the individual salinity treatment, the sequential drought and salinity treatment remarkably enhanced the leaf water status and decreased toxic ion uptake, thereby decreasing levels of reactive oxygen species (ROS), malondialdehyde, and membrane leakage. In contrast, the oxidative damage that resulted from the simultaneous stress treatment was more pronounced than that produced by the independent salinity treatment, which can be attributed to decreased water content, increased toxic ion intake, or/and reduced antioxidant activity. Our results suggested that the physiological responses to combined drought and salinity remarkably depends on the temporal pattern of those stresses.