The Effectiveness And Mechanism Of Improved Salt Tolerance Of Cucumber By Grafting Onto Salt Tolerant Rootstocks

Salinity is a world-wide environmental problem,7.26%of the world's total land area are salt affected. The protected cultivation of horticultural crops has expanded rapidly in recent years in China. Meanwhile, the secondary salinization of the protected horticulture is becoming increasingly severe, and has become an obstacle to limit vegetable production. Cucumber (Cucumis sativus L.) is one of the most popular vegetables under protected cultivation conditions, and is sensitive to salt stress. In the present study, the effectiveness of grafting to increase cucumber salt tolerance was evaluated, by investigating the responses of fruit yield and quality of grafted cucumber to NaCl stress, and the growth and physiological responses of grafted cucumber to salinity induced by major nutrients. In addition, using the techniques of gas exchange, chlorophyll fluorescence, X-ray microanalysis, by studying the osmotic solutes accumulation, photosynthesis, Na+ uptake and transport, the mechanism of grafting to increase cucumber salt tolerance was clarified, the main results are as follows:1. The fruit yield and fruit quality of salt sensitive cucumber'Jinchun No.2' (Cucumis sativus L.) were determined, either self-grafted or grafted onto the commercial salt tolerant rootstock'Figleaf Gourd'(Cucurbita ficifolia Bouche) and 'Chaofengkangshengwang'(Lagenaria siceraria Standl). Plants were treated with 0,30, and 60 mM NaCl. The results showed that plants grafted onto'Figleaf Gourd'and 'Chaofengkangshengwang'had higher fruit yield than those of self-grafted plants under 0, 30, and 60 mM NaCl. In comparison to the self-grafted plants, plants grafted onto 'Figleaf Gourd'and'Chaofengkangshengwang'had a lower fruit yield reduction, and an overall improved fruit quality under NaCl stress owing to an increase in contents of soluble sugar, titratable acidity, and vitamin C, as well as a decrease in the percentage of non-marketable fruit.2. The plant growth and leaf physiological responses of cucumber plants (cv. Jinchun No.2) were determined, either self-grafted or grafted onto the rootstock'Figleaf Gourd'and'Chaofengkangshengwang'. Plants were grown under saline conditions induced by major nutrients, making the electrical conductivity (EC) value as 1.9,5.7 and 9.8 dS m-1, respectively. Compared with the self-grafted plants, cucumber plants grafted onto'Figleaf Gourd'could significantly alleviate scion growth reduction, and maintain higher soluble sugar and manganese (Mn) contents, higher superoxide dismutase (SOD) and peroxidase (POD) activities, but lower EL (electrical leakage) and MDA (malondialdehyde) contents at 5.7 dS m-1. 3. Cucumber plants (cv. Jinchun No.2) were grafted onto the commercial salt tolerant rootstock'Chaofeng 8848'(Lagenaria siceraria Standl.). Grafted and ungrafted plants were exposed to 0 and 100 mM NaCl stress. The effects of NaCl stress on the organic solutes (soluble sugar and proline), and inorganic solutes (Na+, K+and Cl") contents in the leaves and roots were determined. The results showed that the grafted plants had higher accumulation of soluble sugar and proline, almost the same Na+and Cl', higher K+content in the roots compared with the ungrafted plants under NaCl stress. In the leaves, the grafted plants had higher accumulation of soluble sugar and proline, but lower contents of Na+and Cl-(especially Na+), and higher K+content in comparison to the ungrafted plants under NaCl stress.4. Grafting resulted in a higher accumulation of proline and salt tolerance in cucumber plants under NaCl stress, suggesting that there was a relationship between the enhanced salt tolerance and proline accumulation. To investigate the role of proline in alleviating NaCl stress (100 mM) in cucumber, a salt-sensitive cucumber cultivar 'Jinchun No.2'was used. Proline (10 mM) was applied as a foliar spray. The results showed exogenous application of proline significantly alleviated the growth inhibition of plants induced by NaCl, which was accompanied by higher leaf relative water content and POD activity, higher proline and Cl- contents, as well as lower MDA content. However, there was no significant difference in Na+ contents between proline treated and untreated plants under NaCl stress.5. The photosynthetic responses to NaCl stress of cucumber plants that have either been self-grafted or grafted onto two salt-tolerant pumpkin rootstocks'Chaojiquanwang' (Cucurbita moschata Duch), and'Figleaf Gourd', were investigated. The plants were grown hydroponically under 0,30,60, and 90 mM NaCl. Salinity induced a smaller decrease in plant net CO2 assimilation rate and gas stomatal conductance of the two rootstock-grafted plants, compared to the self-grafted plants. In addition, a significant decrease in the initial and total Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) activities, efficiency of excitation energy capture by open photosystem II reaction centers (Fv'/Fm'), maximal potential of photosystem II efficiency (Fv/Fm), were observed from the self-grafted plants under 90 mM NaCl treatment at Day 16. No obvious decrease in the initial and total Rubisco activities, Fv'/Fm', and Fv/Fm were observed in the two rootstock-grafted plants.6.'Jinchun No.2,'a salt-sensitive cucumber cultivar, and'Chaojiquanwang', a salt-tolerant pumpkin cultivar, were reciprocal grafting. Six types of plants were obtained (scion/rootstock):cucumber (ungrafted cucumber), cucumber/cucumber (self-grafted cucumber), cucumber/pumpkin, pumpkin (ungrafted pumpkin), pumpkin/pumpkin (self-grafted pumpkin), pumpkin/cucumber. The salt tolerance of different types of plants, Na+uptake and transport were investigated. The salt tolerance of cucumber was enhanced when pumpkin was used as rootstock. By contrast, using cucumber as rootstock resulted in a decreased salt tolerance of pumpkin. Grafting limited the transport of Na+from the salt-tolerant pumpkin (rootstock) to the salt-sensitive cucumber (scion), resulting in a higher accumulation of Na+in the rootstock part (salt tolerant), lower accumulation of Na+in the scion part (salt-sensitive). The relative Na+content in the cortex of root of pumpkin was higher, but the relative Na+content in the stele of root of pumpkin was lower than those of cucumber, thus avoiding excess Na+loading in the xylem, resulting in a less accumulation of Na+in the scion when pumpkin was used as rootstock.In conclusion, the use of salt tolerant rootstock is an effective way to increase cucumber tolerance to NaCl. In addition, grafting is also considered to be a useful approach to enhance cucumber tolerance to salinity induced by major nutrients. The scion has little effect on the Na+accumulation in the rootstock. The rootstock resulted in a lower accumulation of Na+in the scion (cucumber) through less loading of Na+to the xylem, which is the main mechanism of grafting to increase cucumber tolerance to NaCl.