Low R:FR light ratio enhances calcium nitrate resistance and stomatal movement in tomato seedlings by regulating H2O2 accumulation

Secondary salinization is a major environmental factor that causes a stress response and growth inhibition in plants in protected agriculture. The positive effect of a low red to far-red light ratio (R:FR) in salt tolerance through antioxidant defense has been reported, while the underlying model remains obscure. In this study, we used physiological and genetic approaches to investigate the relationship between H2O2 signaling and low R:FR-induced salt tolerance and antioxidant capacity in tomato seedlings. This study found that low R:FR treatment with calcium nitrate stress (SL treatment) enhanced the growth of plants and increased the net photosynthetic rate 5 days after stress compared with a higher R:FR ratio and calcium nitrate stress (S treatment). With transcriptomic analysis of tomato leaves at 5 d, compared with CK, most of glutaredoxin genes and antioxidant enzymes were upregulated by S treatment, which were upregulated further by SL treatment. Compared to the S treatment, within 5 days, the H2O2 level was increased faster before 24 h and it was slowed down after 24 h by SL treatment, with less H2O2 accumulation at 5 d than that of S treatment. The enhancement of gene expression of RBOH genes were also shown at 24 h under SL. It was found that stomatal conductance followed the dynamic change of H2O2, with a rapid closure of stomata and decrease at 3 h and an increase after 9 h in SL treatment compared to S treatment, respectively. There was same trend of stomata opening degrees of tomato leaves observed by optical microscope. However, the inhibitor of H2O2 production (DPI pretreatment) weakened the positive effect of low R:FR on the regulation of stomatal movement. In addition, SL treatment increased the antioxidant enzyme activities and proline content and decreased the MDA content as compared to the S treatment, while the enhancement of ROS homeostasis was reduced by the DPI pretreatment. In conclusion, low R:FR improved redox homeostasis and stomatal status under calcium nitrate stress through H2O2 signaling, improving the adaptation of tomato seedlings to soil salinization stress.