Physiological Mechanisms Of Salt Tolerance Improved By Grafting In Cucumber (Cucumis Sativus L.)

Grafting is an important agricultural technique. Grafting is mainly used to limit the effects of soil diseases. Grafting has been widely used in such crops as watermelon, melon, cucumber, tomato and eggplant, et al.. Grafting plays an important role in the production of horticulture. In recent years, with the cultivation expanded greatly in the greenhouse, the secondary salinization of the soil is becoming worse and has become a leading factor of limiting the sustainable development of vegetable production. Cucumber (Cucumis sativus L.) is an important vegetables and is widely planted in open field and greenhouse. Therefore, the research of salt tolerance in cucumber is of theoretic and practical significance to direct the production of cucumber and improve the use efficiency of greenhouse. Recently, it has been reported that grafting improved the salt tolerance of watermelon and tomato. However, information is only a little regarding the effects of grafting on salt tolerance of cucumber. The effects and mechanisms of grafting on cucumber salt-tolerance are not clear. Therefore, CuCurbita ficifolia serving as the stock, the effects of grafting on the physiological and biochemical mechanisms in hydroponic cucumber seedlings under salt stress were investigated, in order to clarify the possible mechanisms of enhancing salt tolerance in plants. The main results were presented as follows:1. Under salt stress, grafting treatment significantly increased plant height and fresh weight. 90 mM NaCl stress significantly decreased photosynthetic rate (Pn), stomatal conductance (Gs), quantum yield of PSII (OPSII) and photochemical quenching coefficient (qP) in leaves of cucumber seedlings, and the grafted seedlings were significantly higher than nongrafted seedlings under 90 mM NaCl. 90 mM NaCl stress significantly decreased transpiration rate (Tr) and electron transport rate (ETR), but the reduction degree of grafted seedlings was lower than nongrafted seedlings. Under salt stress, intercellular CO₂ concentration (Ci) significantly increased and the concentrations of grafted seedlings were higher than nongrafted seedlings. The water use efficiency (WUE) significantly increased in the same NaCl concentration. Under 90 mM NaCl salt stress, the reduction degree of Fv/Fm of nongrafted seedlings is higher than grafted ones. Salt stress increased non-photochemical quenching of fluorescence (NPQ), and the grafted seedlings were significantly higher thannongrafted ones. Grafting significantly increased the contents of proline and soluble sugar in cucumber leaves under salt stress. The results showed that grafting could improve the capture and conversion of light energy and improve the osmotic adjustment ability of cucumber seedlings under salt stress and boost plant growth.2. Under salt stress, plasma membrane permeability, MDA and H2O2 content significantly increased and the increased extent of grafted seedlings was lower than nongrafted ones. Grafting increased root vigor under salt stress. Under salt stress, the activities of SOD and APX significantly increased while the activities of GPX and CAT significantly decreased in cucumber leaves. But the enzymatic activities of grafted seedlings were higher than nongrafted ones. It could be concluded that higher activities of SOD, GPX, CAT, APX, lower plasma membrane permeability and less contents of H2O2 and MDA in salt-stressed leaves induced by grafting may protect the plant tissues from membrane oxidative damage under salt stress, thus mitigating salt toxicity and improving the growth of cucumber plants.3. Salt stress significantly decreased the absorption of potassium and increased the absorption of sodium. The increased extent of sodium in the root of grafted seedlings was higher than nongrafted ones, which indicated that grafting greatly improved the activities of avoiding sodium. Under Salt stress, the content of K and the value of K/Na decreased, but grafted seedlings were higher than nongrafted ones, which indicated that grafting greatly improved the activities of selective absorbing potassium. In a word, roots were stronger and the activities of selective absorbing potassium were higher after grafting. The results showed that grafting improved cucumber salt-resistance.