Grafting onto pumpkin rootstock is an efficient alternative to improve melon tolerance to NaCl stress

Abstract

The objective of this study was to determine whether salt tolerance of Cucumis melo L. ‘Baimi’ could be improved by grafting onto pumpkin rootstock Cucurbita maxima Duchesne × Cucurbita moschata Duchesne ‘Jingxin 3'. Both self-rooted and grafted plants were cultivated in a nutrient solution with or without 150 mM NaCl. Salt treatment significantly reduced melon plant growth and productivity, leading to a series of physiological and biochemical changes. Melon plants grafted onto pumpkin rootstock showed much less inhibition of growth and photosynthesis by salt stress compared with the self-rooted plants. Stress-induced activities of superoxide dismutase, peroxidase, and catalase were considerably higher (24.61%, 100.69%, and 26.27%) in the rootstock-grafted plants. Leaves of these plants also had higher concentrations of total soluble sugars (28.11%), higher activities of sucrose synthetase (16.77%) and sucrose phosphate synthase (90.70%). The pumpkin rootstock significantly mitigated the negative effects of salt stress by reducing Na uptake, enhancing K uptake, and maintaining a higher K/ Na ratio (19.01 for grafted plants, 8.77 for self-rooted plants) in the leaves and a lower K/Na ratio (0.42 for grafted plants, 1.44 for self-rooted plants) in the roots. Quantitative RT-PCR analysis showed that transcript levels of candidate genes, CmSOS1, CmNHX1 and CmGnT were significantly higher in the grafted plants than in the self-rooted plants under salt stress. These results demonstrated that the salt tolerance of melon plants could be significantly improved by using pumpkin as a rootstock.