Physiological response and comprehensive evaluation of pumpkin rootstock resources to salt stress

Salt stress seriously threats the high-yield and high-quality cultivation of cucumber in protected environments. Screening salt-tolerance pumpkin rootstocks for grafting cucumber is an effective strategy to solve this problem. In this study, we investigated the morphological characteristics and growth performance of 22 pumpkin rootstock resources at the germination stage under 160 mmol/L NaCl stress, using five pumpkin commercial rootstock varieties as controls. Furthermore, principal component analysis (PCA), membership function and cluster analysis were employed to comprehensively assess the salt tolerance of these pumpkin materials. The results identified seven pumpkin rootstock resources with salt tolerance and ten with intermediate salt tolerance. Moreover, the morphological characteristics and physiological indexes of 11 pumpkin rootstock resources were examined at the seedling stage under NaCl stress, using five pumpkin commercial rootstock varieties as controls. The findings indicated that growth indexes of pumpkin seedlings under 160 mmol /L NaCl stress were all lower compared to the control group, with varying tolerance levels among different pumpkin materials. NaCl stress reduced chlorophyll content (Chl), net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr), while increased electrical conductivity (EC), malondialdehyde (MDA) content, and the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), the contents of proline (Pro), soluble sugar (SS) and soluble protein (SP) of pumpkin seedlings, compared to the control group. Furthermore, seven pumpkin rootstock resources with salt tolerance and three with intermediate salt tolerance were identified at the seedling stage using PCA, membership function and cluster analysis. Additionally, it was found that Z6, Z7 and Z12 exhibited good graft affinity with cucumber. J/Z7 and J/Z12 displayed higher yield per plant, total yield per hectare, vitamin C contents and the highest SP contents, while J/Z6 and J/Z7 showed higher total phenolics and flavonoid contents, which were significantly higher than those of self-rooted cucumber. Our findings not only provided a theoretical basis for screening the salt-tolerant pumpkin rootstocks, but also suggested that Z6, Z7 and Z12 were the potential candidate rootstocks for cucumber cultivation in saline soil.