[Hydrogen sulfide acted as a downstream signal was involved in the regulation of salicylic acid on photosynthesis of cucumber seedlings under low temperature and low light intensity].

Both salicylic acid (SA) and hydrogen sulfide (HS) play an important role in regulating plant growth and development and physiological metabolism under abiotic stresses. As signal molecules, the interaction between them in regulating cucumber photosynthesis under low temperature and low light is still unclear. Here, we examined the regulation and interaction of SA and HS on photosynthesis in cucumber seedlings under low temperature (8 ℃/5 ℃, day/night) and low light (100 μmol·m·s). Seedlings were foliar-sprayed with SA, sodium sulfide (NaHS, HS donor), and their scavenger or biosynthesis inhibitors, respectively. Seedlings treated with deionized water at suitable temperature and light condition were used as the control. The results showed that SA increased the L-/D-cysteine desulfhydrase (LCD, DCD) activities and relative mRNA expression, and consequently promoted the endogenous HS production. However, NaHS did not affect the activities and gene expressions of phenylalnine ammonialyase and isochorismate and endogenous SA level. Compared with the HO-treated seedlings under low temperature and low light, SA- and NaHS-treated seedlings showed an increase in the photosynthetic rate, stomatal conductance and transpiration rate, while a decrease in intercellular CO concentration. SA and NaHS increased the CO assimilation, which mainly attributed to the increases in the activities of the ribulose-1, 5-bisphosphate carboxylase, rubisco activase, sedoheptulose-1, 7-bisphosphatase and fructose-1, 6-bisphosphatase, as well as their mRNA expression. Meanwhile, SA and NaHS improved the actual photochemical efficiency and maximal photochemical efficiency of PSII, and therefore alleviated the damage in photosynthetic apparatus and negative effect on growth from low temperature and low light stress. The SA-induced higher photosynthesis and growth in stressed seedlings were suppressed by addition of HS scavenger hypotaurine. However, the HS-induced tolerance of photosynthetic apparatus to low temperature and low light was not affected by SA biosynthesis inhibitor paclobutrazol and 2-aminoindan-2-phosphonic acid. Our results suggested that HS, as a downstream signal of SA, was involved in regulating photosynthesis in cucumber seedlings under low temperature and low light.