Nitric Oxide and Its Interaction with Hydrogen Peroxide Enhance Plant Tolerance to Low Temperatures by Improving the Efficiency of the Calvin Cycle and the Ascorbate–Glutathione Cycle in Cucumber Seedlings

The present study was aimed to assess the effect of nitric oxide (NO) and the interaction of NO with hydrogen peroxide (H 2 O 2 ) on plant tolerance to low temperatures in cucumber seedlings. Exogenous NO significantly increased the endogenous NO content, initial and total activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), RuBisCO carboxylation rate ( V c,max ), RuBP regeneration rate ( J max ) and the transcript levels of related genes in cucumber seedlings under low temperatures (11 °C/7 °C); however, the effect of NO was blocked by PTIO (NO scavenger). In addition, the SNP treatment significantly improved the contents of glucose, fructose, sucrose, starch, and the activities of sucrose phosphate synthase (SPS), acid invertase (AI), sucrose synthase (SS), as well as the expression levels of SUCROSE PHOSPHATE SYNTHASE 1&2 ( SPS1 , SPS2 ), SUCROSE TRANSPORTER 1&2 ( SUT2 , SUT4 ), β-starch hydrolase ( BAM ), and invertase gene ( INVERTASE ) in cucumber leaves under low temperatures, and the positive effect of NO was impaired by PTIO. Furthermore, we found that the H 2 O 2 , induced by NO, participated in NO-induced elevation of ascorbic acid (AsA), glutathione (GSH), and increased activities of related enzymes in the AsA-GSH cycle at low temperatures. However, the positive effect of NO was blocked by l -NAME (NOS inhibitor), PTIO, DPI (inhibitor of NADPH oxidase), and DMTU (reactive oxygen species scavenger). Taken together, our findings indicate that NO increased the low temperature tolerance of cucumber seedlings via H 2 O 2 by improving the efficiency of the Calvin cycle, which in turn increased the carbohydrates content and accelerated the AsA-GSH cycle to enhance ROS scavenging.