Osmotic adjustment of tomato under mild soil salinity can enhance drought resistance

Osmotic adjustment is an important mechanism for plants to withstand osmotic stress. The accumulation of Na + can improve the osmotic adjustment of plants and enhance the ability of plants to withstand osmotic stress. However, the Na + accumulation, osmotic stress, ion toxicity and oxidative damage of plants have not been fully explored under combined water and salt stress. The objectives of this study were to explore the effect of soil salinity on the drought tolerance of tomato. We set up four soil salinity treatments and three water treatments in 2019 and four soil salinity treatments and four water treatments in 2020. The salt was evenly mixed into the soil before the soil was filled into the growing area, and water treatment was carried out during the reproductive growth stage. Both water deficit and soil salinity reduced the dry matter accumulation of plants, but the interaction effect of water deficit and soil salinity on dry matter accumulation was less than that of single water stress when salt was added to the soil at 3 g/Kg dry soil. When water deficit was conducted, soil salinity enhanced osmotic adjustment more than aggravates osmotic stress. When the Na + concentration in the leaves was lower than 100 mmol/L, the Na + had little effect on the K + concentration. Furthermore, mild and moderate water deficit did not lead to further increases malondialdehyde content in leaves under mild soil salinity (salt was added to the soil at 3 g/Kg dry soil). When leaf Na + concentration was lower than concentration threshold (100–120 mmol/L in this study), osmotic adjustment was significantly enhanced, but there was no further significant influence on ion balance and oxidative damage, which effectively improved the drought resistance of tomato. Therefore, mild soil salinity may improve tomato yield under water deficit.