Effects of salicylic acid and brassinolide applications on salt tolerance in <i>Cyclocarya paliurus</i>: amelioration of oxidative stress

Soil salinity is a major constraint limiting plant growth globally. Cyclocarya paliurus (Batal.) Iljinsk, a valuable tree species, exhibits limited tolerance to salinity, hindering its cultivation in saline soils. This study investigates the effects of salicylic acid (SA) and brassinolide (BR) applications on improving salt tolerance in C. paliurus using physiological, cytological and molecular approaches. Results showed that the application of SA or BR significantly alleviated salt-induced growth inhibition and oxidative stress in C. paliurus, but the alleviating effects varied in their application doses. The applications of 0.5 mM SA or 1.0 mg L-1 BR enhanced seedling height by 89.7-97.4% and photosynthetic rate by 106.3-146.9% whereas reducing salt injury index by 36.0-38.0%, which is mainly via regulating the antioxidant enzyme activities, secondary metabolite accumulation and gene expressions associated with these processes. Visualization staining of H2O2, O2•- and cell viability also revealed that applications of 0.5 mM SA or 1.0 mg L-1 BR reduced the distributions of H2O2 and O2•- in leaves and invigorated cell viability under salt stress. Based on the analysis of reactive oxygen species metabolism and flavonoid biosynthesis pathways, we infer that the SA or BR applications could alleviate the salt-stress in C. paliurus mainly via regulating reactive oxygen species scavenging and the expression of genes related to antioxidant enzymes and secondary metabolite biosynthesis pathways. These findings suggest that proper exogenous applications of either SA or BR hold promise for improving the salt tolerance of C. paliurus.