Saline water irrigation induced differences in sodium ion distribution and tomato yield following drought hardening pre-treatment

Saline water irrigation is applied to address water scarcity due to climate change; however, it causes soil salinisation, which is harmful to soil quality and crop growth. Drought hardening can trigger resistance to additional stress events by activating various defence mechanisms. Herein, we examined the effect of drought hardening pre-treatment on soil sodium ion accumulation, sodium ion distribution in various organs, sodium ion excretion by plants and tomato yield change under saline water irrigation through a 2-year experiment. Different water and salinity treatments were designed, including four water levels (W1, W2, W3 and W4, indicating soil moisture contents of 75 %–85 %, 60 %–70 %, 55 %–65 % and 40 %–50 % of the field capacity, respectively) and three salt levels (S2, S4 and S6, indicating sodium chloride (NaCl) addition of 2, 4 and 6 g L⁻¹, respectively). The control treatment (CK) was treated without drought hardening and irrigated with tap water with no salt. The amount of soil sodium ions increased the most during the developmental stage, and sodium ion content in the whole-pot soil increased during the developmental stage by 420.18 %–1947.99 % in the 2-year experiment, compared with that during the seedling stage. Sodium ions were lowest in the middle-layer roots and highest in lower-layer leaves at the end of maturation stage. Sodium ions were lowest in the middle-layer roots decreased by 3.69 %–41.12 % in 2022, compared with that in upper- and lower-layer roots, and the decreased percentages were 14.06 %–57.89 % in 2023 at the end of maturation stage. Moreover, sodium ions in soil and various tomato organs increased with irrigation salinity, except for the W1S2 treatment in 2022 and the W4S4 treatment in 2023. Compared with sodium ions in soil and sodium ion excretion by plant, the sodium ion content in tomato organs was the lowest, at 1.98 %–12.35 %. The root is an important regulated organ that affects the distribution of sodium ions in both soil and plant organs. Mild drought hardening resulted in the lowest amount of sodium ions in soil under the S2 and S4 treatments and led to the highest sodium ion excretion. Yield increased with decreased irrigation salinity and gained the largest value under the mild drought hardening treatment. Yield under mild drought hardening treatment increased by 19.61 %–151.41 %, compared with other salt treatments. In conclusion, mild drought hardening could alleviate salinisation and improve tomato resistance to salt stress. This paper provides some reference for future research on the mechanism by which drought hardening enhances the salt tolerance in crops, and also offers some new ideas for applying salt water irrigation to solve the problem of water shortage.