Background Drought, desertification and land degradation have become global environment problems for centuries, which lead to unsustainable development in agriculture. As tomato is one of the representative economic crops in northwest arid region, the study on its drought resistance has become a hot topic in recent years. Knowledge of physiological adaption of tomato to soil water deficit is well understood, but little is known about the effect of exogenous calcium (Ca²⁺) against drought at the seedling stage of greenhouse tomato. The study is aimed to investigate drought-induced physiological and morphology changes in tomato, as well as drought relief using exogenous Ca²⁺.

Methods The experiment set 3 water stress levels (W90: 90% FC; W60: 60% FC; and W30: 30% FC, FC refers to the field capacity) and 3 Ca²⁺ application levels (Ca0: 0; Ca1: 50 mg/kg and Ca2: 100 mg/kg). The changes of soil basic physicochemical properties and the physiological characteristics of greenhouse tomato were investigated, including stomatal characteristics, root morphogenesis, protective enzyme activities, and the osmotic regulatory substance content. The desert soil in arid regions and the tomato cultival "Fenbaoli" were taken as the experimental materials.

Results 1) The concentration of total salt, alkali-hydrolyzale nitrogen, available phosphorus and potassium in soil solutions of deficient irrigation were significantly higher than that of conventional irrigation (P < 0.05). 2) Deficient irrigation inhibited the growth of tomato seedlings, significantly reduced the plant moisture content, dry weight, total root length and root surface area (P < 0.05). Without additional Ca²⁺, the root shoot ratio (R/S) of tomato was negatively correlated with the irrigation quota. Compared with W90Ca0, R/S of W30Ca0 increased by 68.9%. Meanwhile, the antioxidative enzyme (catalase) activities and the osmotic substances (proline and soluble protein) contents of the deficient irrigation treatments were evidently higher. 3) Under mild deficient irrigation, 50 mg/kg Ca²⁺ increased leaf stomata opening degree, the number of starch grains in the mesophyll cells and the activities of antioxidant enzyme (superoxide dismutase) in the leaves. Furthermore, in terms of severe deficient irrigation, the mesophyll cell structure was severely damaged, chloroplast membrane collapsed, grana lamella disintegrated, and Ca²⁺ had no significant effect on the regulation of stoma and chloroplast structure.

Conclusions Ca²⁺ is involved in the defense regulation of membrane lipid peroxidation in tomato seedlings induced by drought stress. Therefore, it is possible to improve the drought tolerance for tomato in arid regions by the addition of Ca²⁺ during the seedling stage. The inhibition of mild drought stress for greenhouse tomato seedlings on the desert soil could be alleviated by foliar spray of 50 mg/kg Ca²⁺.