Regulatory mechanism of NaCl stress on photosynthesis and antioxidant capacity mediated by transglutaminase in cucumber (Cucumis sativus L.) seedlings

In this study, we used two cucumber (Cucumis sativus L.) cultivars with different salt tolerances, Jinyou NO.4 (salt-sensitive) and Inbred line 9930 (salt-tolerant), to investigate the effects of salt stress (75 mM of NaCl) on photosynthesis and antioxidant capacity in cucumber leaves. The results showed that the plant height, fresh weight, and dry weight of the seedlings from the two cultivars were decreased under salt stress. The restrained degree of growth parameters induced by NaCl was more serious in Jinyou NO.4 than in Inbred line 9930. Salt stress caused significant decreases in photosynthetic pigment content, chl a/b, Pn, Gs and Tr in the leaves of the two cultivars. Ci content was increased in Jinyou NO.4, and it was decreased in Inbred line 9930. Moreover, Fv/Fm, Phi PSII, qP, NPQ, and the mRNA expression levels of RbcL, RbcS and RCA were decreased in the two cucumber leaves under salt stress, and the effects of salt stress on Jinyou NO.4 were more serious. Meanwhile, the activities and encoding gene expression levels of SOD, POD, CAT, and APX were increased before being decreased under salt stress. These expression levels were greater in Inbred line 9930 than in Jinyou NO.4. NaCl stress-induced TGase activity, gene expression and bound PAs contents were higher in Inbred line 9930. Salt stress increased the MDA content and O-2 generation rate in cucumber leaves, especially in Jinyou NO.4. These results suggested that salt-tolerant cultivars could enhance salinity tolerance by regulating photosynthetic efficiency and antioxidant capacity compared with the salt-sensitive cultivar. This could alleviate salt damage to cucumber seedlings.