Grafting With Salt-tolerant Rootstock Alleviates The Inhibitory Effects Of Salt Stress On Photosynthesis In Cucumber Plants

In recent years, with the develop of protected horticulture, the soil secondary salinity was severe year by year. It has become the main obstacle of limiting the developing of vegetable production under protected horticulture, and seriously hampered the sustainable development of protected cultivation. Cucumber (Cucumis satuvus L.) is a salinity-sensitive plant species that is cultivated in unheated greenhouses in many areas. However, some rootstocks used for grafted-cucumber, such as pumpkin (Cucurbita moschata Duchesne ex. Poir) and bottle gourd [Lagenaria siceraria (Molina) Standl.], are more tolerant to salt stress comparatively. Recently, the use of salt-tolerant rootstock was demonstrated to be a valid strategy in increasing the salt tolerance of cucumber plants. It was suggested that the improved salt tolerance of grafted plants was related with the mitigation of the toxic effect of the ion within the plant. However, data on the rootstock-grafting regulating photosynthesis in plants exposed to salinity were limited. In present study, the mechanism of salt-tolerant rootstock-grafting on regulating photosynthesis of cucumber plants under salt stress were evaluated, by investigating the respose of photosynthesis of grafted cucumber to salt stress in energy dissipation, damage and repair of PSII, and Rubisco activity. The main results in the present study are as folloows:1. The salt-sensitive cucumber cv. Jinchun No.2was grafted onto a salt-tolerant pumpkin cv. Chaojiquanwang. The non-grafted and self-grafted cucumber plants were used as controls. The plant growth, changes in the time course of chlorophyll content, gas-exchange parameters, chlorophyll fluorescence in the leaves of non-, self-, and rootstock-grafted cucumber plants under0and90mM NaCl stress for15d were studied. The results showed that NaCl stress inhibited the photosynthetic capacity in cucumber leaves by stomatal limitation during the early phase of salinity (1d,5d,10d), whereas the use of salt-tolerant rootstocks alleviated the inhibitory effect of salt stress on photosynthesis in cucumber leaves, to some extent, by regulating stomatal activity. During the later phase of salinity (15d), NaCl stress degradated the chlorophyll, and impaired the photosynthetic apparatus in cucumber leaves, while grafting cucumber plants onto salt-tolerant rootstock protected the chlorophyll against degradation by enhancing photochemistry quenching and energy dissipation in cucumber leaves, thereby improving the photosynthetic performance of cucumber leaves.2. The salt-sensitive cucumber ’Jinchun No.2’ was used as scion, and the salt-tolerant pumpkin ’Chaojiquanwang’ was selected as rootstock. Gas exchange, photosystem II (PSII) efficiency, xanthophyll cycle, and chloroplast ultrastructure of nongrafted, self-grafted, and pumpkin-grafted cucumber plants were investigated at day15after being treated with90mM NaCl. The results showed that the use of salt-tolerant rootstock alleviates salt stress in cucumber plants by delaying photo inhibit ion, probably due to a lower incidence of both stomatal and nonstomatal factors that limit photosynthesis. Furthermore, the similar responses of the nongrafted and self-grafted cucumber plants to salt stress suggest that the improved salt tolerance of the rootstock-grafted plants was more affected by the rootstock than the grafting itself3. A salt-sensitive cucumber cultivar Jinchun No.2was used. The cucumber leaf disks were treated with0,45mM, or90mM NaCl and lincomycin, and the oxygen-evolving activity of PSII was detected by a Clark-type oxygen electrode (Chlorolab-2). The results showed that, NaCl stress inhibited the oxygen-evolving activity of PSII of the cucumber leaf disks. And lincomycin markedly accelerated the decrease of PSII activity when the cucumber leaf disks were incubated in light at1000μmol m-2s-1. The inactivation observed in the presence of lincomycin was unaffected by NaCL Meanwhile, the result showed that NaCl stress significantly inhibited the recovery of PSII activity of the cucumber leaf disks by hindering of the repair of photodamaged PSII. In the presence of lincomycin, recovery was completely blocked, no matter treated with NaCl or not. These results demonstrated that, NaCl stress inhibited the repair of the photodamaged PSII, and did not accelerate damage to PSII directly.4. The salt-sensitive cucumber’Jinchun No.2’was grafted onto the salt-tolerant pumpkin ’Chaojiquanwang’. The self-grafted cucumber plants was used as control. Net photo synthetic rate, PSⅡ activity, D1protein content in the leaves, and Na+concentration in the roots and leaves of self-grafted and pumpkin-grafted cucumber plants were investigated at day15after being treated with90mM NaCl. The results showed that, the salt-tolerant rootstock decreased the shoot Na+concentrations by retaining and accumulating Na+within the pumpkin rootstock, which alleviated the inhibitory effect of Na+on the repair of D1protein in cucumber leaves, thereby protecting the photosynthetic apparatus against NaCl stress, and enhancing the photosynthetic performance of cucumber.5. The salt-sensitive cucumber’Jinchun No.2’was grafted onto two salt-tolerant rootstocks,’Chaojiquanwang’ and figleaf gourd (Cucurbita ficifolia Bouche). Changes in the plant growth, gas-exchange parameters, Rubisco activities and its related genes expression, N metabolism-related substances and enzyme acitivies were investigated in the leaves of self-grafted and rootstock-grafted cucumber plants under0and90mM NaCl stress for15d. The results demonstrated that grafting cucumber plants onto salt-tolerant rootstocks enhances N metabolism in the leaves by increasing the N metabolism-related enzyme activities under NaCl stress, thereby maintaining Rubisco expression and activity, and improving the photosynthetic performance of cucumber leaves via higher gas exchange capacities.In conclusion, the results suggest that salt stress inhibits the photosynthetic capacity in cucumber leaves by stomatal and/or non-stomatal limitation. The use of salt-tolerant rootstocks alleviate the inhibitory effect of salt stress on photosynthesis in cucumber leaves, probably due to a lower incidence of stomatal factor that limits photosynthesis during the early phase of salinity, to the thermal dissipation of excess energy through effectively regulating of xanthophyll cyle de-epoxidation that delays photo inhibition, to restricting Na+accumulation in the shoot that maintains the dynamic balance between the photodamage of PSⅡ and the repair of damaged PSⅡ, to improving Rubisco activity by effectively maintaining N metabolism.