Effects Of Exogenous Ca（NO₃）₂on Physiology Metabolism Of Cucumber Seedlings Under Salt Stress

Salt stress is a severe problem which affects global agriculture production and ecology environment, and secondary salinization in soils is a universal problem in protected cultivation, which badly restricts the sustainable development of protected vegetable production. So enhancing crop’s salt tolerance is important for agriculture production. Calcium is a necessary mineral nutrient element for plants, which is important in maintaining the stability of cell wall, membrane and membrane bound protein and in regulating the transportation of inorganic ions. Moreover, Calcium is a second messenger of cellular physiological and biochemical reaction associating extracellular signals, which also play a important role in corp percption, transmission, adjusting and responsing of environment stress. Calcium nitriate not only can supply Ca2+but also NO3-which is lacking under salt stress. But the study of the effecting of exogenous Ca(NO3)2on plant physiology metabolism is less.In the present study, salt-tolerance cucumber (Cucumis sativus L.) cultivar’Xintai Mici’was chosen as experimental material. The cucumber was hydroponically cultured to study the effects of exogenous Ca(NO3)2(foliar spraying) on cucumber seedlings’growth, nitrogen metabolism, ions content, proton pump activity, photosynthesis chlorophyll fluorescence and reactive oxygen metabolism. The present study was to elucidate the physiological regulating function of Ca(NO3)2in enhancing crop’s salt tolerance. Main research results were as follows:1. NaCl treatment suppressed the growth of cucumber seedlings:dry and fresh weight,(net photosynthetic rate) Pn, stomatal conductance (Gs) decreased while MDA and electrolyte leakage increased significantly under salt stress. Foliar spraying proper concentration (0.5-3.0mmol·L-1) Ca(NO3)2could alleviate the inhibiton of cucumber seedlings’ growth under salinity:the plant height, biomass yield, Pn significantly increased while malondialdehyde (MDA) and electrolyte leakage decreased, and the2.0mmol·L-Ca(NO3)2is the best treatment.2. Under salt stress, nitrate-nitrogen (NO3--N) content and nitrate reductase (NR) activity decreased while NH4+-N and Pro content increased in the leaves and roots of cucumber seedlings. Exogenous Ca(NO3)2treatment significantly minished the range of the decreasing of NO3--N and NR activity, and the increasing of ammonia-nitrogen (NH4+-N) and proline (Pro) content. It can be concluded that Ca(NO3)2treatment can obviously alleviate the impact of salt stress on nitrogen metabolism.3. The root vigor, K+and Ca2+content, membrane and tonoplast H+-ATPase activity of cucumber seedlings were significantly decreased under salt stress, while the Na+and Cl-content and tonoplast H+-PPase activity were significantly increased. Exogenous Ca(NO3)2treatment significantly enhanced the cucumber seedlings’root vigor, K+and Ca2+content, membrane and tonoplast H+-ATPase, H+-PPase activity, meanwhile significantly decreased the Na+and Cl-content under salt stress. It demonstrates that exogenous Ca(NO3)2treatment maintains the power of ion selective absorbing and compartmentation by enhancing the proton pump activity, and then relieves the ion toxicity for plant of salinity.4. The cucumber seedlings’ leave Chl(a+b) content, Pn、Gs、 transpiration rate (Tr)、 intercellular CO2concentration (Ci), quantum efficiency of open PSII (Fv’/Fm’)、PSII actual fluorescence (ΦPSII)、photochemical quenching (qP) significantly decreased under salt stress, while the non-photochemical quenching (qN) significantly increased and the maximal quantum efficiency of PSII photochemistry (Fv/Fm) had no significant change. Application Ca(NO3)2under salt stress could reduce the range of the decreasing of leave’ Chl(a+b) content、Pn、Gs、Tr、ΦPSII、qP, but it had no significant change of Ci、Fv]/Fm’、 qN and Fv/Fm. These show that exogenous Ca(NO3)2treatment mitigates the plant’s photosynthesis inhibition, so enhances the plant’s salt tolerance by impacting stomatal limitation and PSII photochemical efficiency.5. Foliar spraying Ca(NO3)2treatment significantly decreased cucumber seedling leaves’ O2·-production rate and MDA content in salinity, while significantly increased the superoxide (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX)、 dehydroascorbate reductase (DHAR) and so on antioxidative enzyme activity, antioxidant substance AsA content, the ratio of AsA/DHA and GSH/GSSG. These results indicate that exogenous Ca(NO3)2treatment reduces the injury of ROS resulted in salinity on plant’s leaves by increasing the antioxidant enzyme activity and antioxidant substance content, then enhance plant’s antioxidant ability and salt tolerance.