Rootstocks improve cucumber photosynthesis through nitrogen metabolism regulation under salt stress

We examined the growth, photosynthetic parameters, initial and total ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity, the relative expression of rbcL, rbcS, and rca gene, and nitrogen metabolism of cucumber (Cucumis sativus L. cv. Jinchun No.2, CS) plants grafted onto figleaf gourd (Cucurbita ficifolia Bouch,, CF) and pumpkin (Cucurbita moschata Duch. cv. Chaojiquanwang, CM) rootstocks. Growth inhibition under salt stress (90 mM NaCl) was characterized by the irreversible inhibition of CO2 assimilation in the cucumber plants grafted onto cucumber rootstocks (CS/CS). In contrast, this effect was significantly alleviated by grafting the cucumber plants onto the CF and CM roots (CS/CF, CS/CM). Under NaCl stress, the CS/CF and CS/CM plants exhibited higher photosynthetic activity, higher initial and total Rubisco activity, and higher Rubisco-related gene expression than the CS/CS plants. Salinity resulted in a lesser increase in nitrate content and decrease in free amino acid content in the CS/CF and the CS/CM plants compared with the CS/CS plants. Accordingly, the activity of nitrate reductase, glutamine synthetase, and glutamate synthase decreased significantly, especially in the CS/CS plants. These results suggest that grafting cucumber plants onto salt-tolerant rootstocks enhances Rubisco activity and the expression of Rubisco-related genes by effectively accelerating nitrate transformation into amino acids under NaCl stress, thereby improving the photosynthetic performance of cucumber leaves.