Due to the dry climate and water shortage in Iran, the use of substances that can increase the plant's tolerance to water stress is very important. In this study, the effects of exogenous application of salicylic acid (SA) and kaolin (KL) on drought stress tolerance of tomato plants was evaluated in field conditions. Experimental factors included foliar application (including control, 0.5 mM SA, 2.5% KL and simultaneous application of SA + KL) and irrigation intervals (at four levels of 2, 4, 6 and 8 days). The results showed that increasing the irrigation interval in tomato plant led to a decrease in growth and yield in this plant. As compared to the controls, irrigation intervals of 4, 6 and 8 days decreased total yield as much as 9, 11 and 21%, respectively. The relative water content, total chlorophyll, transpiration and photosynthesis decreased while malondialdehyde, proline, peroxidase and hydrogen peroxide increased with the increase of irrigation intervals. Both SA and KL were effective in reducing the adverse effects of drought stress on plant growth and productivity.  Application of SA and KL improved plant growth and yield by increasing chlorophyll, relative water content, peroxidase activity, photosynthesis rate and transpiration. The highest effect of foliar spraying was observed at irrigation intervals of 6 and 8 days. The results also showed that the simultaneous application of SA + KL did not have a synergistic effect on plant growth and productivity in comparison to their separate application. According to the obtained results, the use of KL or SA is recommended to reduce the effects of prolonged irrigation on the tomato plant.

Keywords: Drought, Foliar application, Malondialdehyde, Photosynthesis, Transpiration.

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