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Salinization of vegetable soil became increasingly deteriorative, seriously affecting the yields and qualities of vegetables, reducing the economic benefits of vegetable production, hindering the rapid development of vegetable industry in China. With the rapid development of facility horticulture, watermelon cultivation area expand rapidly, but the secondary salinization of soil in facility has become one of the factors restricting the development of watermelon cultivation, so it has a great significance to take different measures to improve the salt tolerance of watermelon plants.Researches on calcium’s improving plant tolerance to stress have been made, particularly in salt tolerance. The increased availability of Ca2+(<10.0 mmol·L-1) within rhizosphere can alleviate the harmful effects of salinity on most plants. Under salt stress, increasing calcium could improve salt tolerance of plants and fruit quality, related researches have been done within physiology, growth, metabolism, gene expression to illustrate the mechanism by which calcium improve plant’s salt tolerance, but less salt tolerance mechanism have been researched during the whole growing period of plants. At home and abroad, more less studies have been done to investigate the mechanism calcium improve watermelon’s salt tolerance and its influence on fruit quality.In the experiment, salt-sensitive watermelon (Citrullus lanatus Mansfeld.) cultivar ’XiuLi’is used to study the effects of complementary calcium on watermelon plant growth and development, protein content and composition, photosynthetic performance, mineral elements, fruit quality under salt stress by supplementting calcium to nutrient solution via hydroponic way. Concrete results are as follows:The relative membrane permeability, malondialdehyde (MDA) and proline (Pro) content in leaves and roots, molal concentration of osmotic potential in leaf were significantly increased by salt stress. The dry biomass of seedlings and root activity decrease significantly, inhibiting the seedlings growth; Different concentrations of Ca2+ added to nutrient solution have a effect on watermelon seedlings growth under salt stress, but with a significant concentration effect, the 6 mmol·L-1 Ca2+ in salt nutrient solution plays a significant role in promoting watermelon seedlings growth under salt stress, the leaves and roots cell membrane permeability, MDA and Pro content were significantly decrease, while the net photosynthetic rate (Pn), PSII maximum photochemical efficiency (Fv/Fm), leaves osmotic potential, root activity, dry biomass increase significantly.Under salt stress, the soluble protein content in leaves and roots decrease significantly, and many expressions of the protein composition in leaves and roots change, some protein components in leaves express upward or downward, but many protein components express downward or disappear in roots, while dry seedlings biomass reduce significantly; the increasing of Ca2+concentration in nutrient solution from 4 mmol·L-1 to 6,8 mmol·L-1 can significantly improve the ratio of root to shoot in dry weight (R/T), and the dry biomass of watermelon seedlings under salt stress, to 78.2%,89.5% of control. Soluble protein content in leaves were increased to 92.4%,103.7% of control, the soluble protein content in roots were increased to 89.1%,104.3% of control, both soluble protein content are significantly higher than in plants under salt.28.5 kD protein components in leaves increase and 107.4 kD protein components decrease, while various protein components in roots express upward to the control.The increasing of Ca2+ concentration in nutrient solution could increase the content of Ca2+, K+, Mg2+ by different degrees, while reduce the content of Na+ and Cl- of plants under salt stress, but higher Ca2+ concentration is not conducive to increase the content of Mg2+ in upper leaves and growing point; salt stress reduces the content of Fe, Zn and Cu content in growing point, but the increasing of Ca2+ concentration in nutrient solution are conducive to transport Fe, Zn, and Cu from stem to growing point, thereby increasing Fe, Zn, and Cu content in growing point, balancing the redistribution of mineral elements in plants under stress. Microelements in other tissues change without regularity.The increasing of Ca2+ concentration in nutrient solution from 4 mmol·L-1 to 6mmol·L-1 can increase the Gs, Ci, Pn, Fv/Fm,ΦDPSII, rETR, qP, qN, Hill reaction activities of chloroplast and reduce the Ls and 1-qP/qN; While promote the second layer of palisade tissue formation in leaves under salt stress, increase Ca+ fluorescence intensity in protoplasts of leaf cells, and total leaf area, therefore promoting the photosynthetic performance; In addition, the leaf sucrose synthase (SS) activity decrease, promoting the transport of sucrose from stem to roots.As for fruit under salt stress, the increasing of Ca+ concentration in salt nutrient solution from 4 mmol·L-1 to 6 mmol·L-1 can increase the content of ascorbic acid (Vc), soluble solids, total soluble sugar, soluble protein, free amino acids and fruit weight, while reduce organic acid content, playing a effective role in improving fruit quality; Moreover, the content of Ca2+, Mg2+, Fe, Cu, Mn in watermelon fruit increase under salt stress, Na+ content reduce, balancing mineral ion in fruit; In addition, the increasing of Ca2+ concentration also promote the elongation of pollen tube under stress, which is conducive to the successful completion of fertilization and promote fruit growth and development.In conclusion, under salt stress, it could be suggested that the increasing concentration of Ca2+ appropriately in nutrient solution can reduce the harmful effects of salt stress on watermelon plants, and maintain the normal metabolism of watermelon plants, which is conducive to the growth of watermelon fruit and formation of fruit quality.
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