Exogenous spermidine improves the sucrose metabolism of lettuce to resist high-temperature stress

Lettuce is a popular fresh vegetable, and high-temperature stress will reduce the yield of lettuce. Spermidine is an essential phytohormone in plant stress responses. However, under high-temperature stress, it is not clear whether spermidine treatment can change the yield of lettuce by regulating sucrose metabolism. Here, treatment with normal temperature was used as a control, and the effects of 1 mM exogenous spermidine on the morphology, photosynthetic characteristics, and sucrose metabolism of lettuce under high-temperature stress were studied. The results showed that the high-temperature treatment unbalanced the synthesis and decomposition of sucrose, caused excessive consumption of leaf starch and sucrose, and reduced the yield of lettuce. The high temperature spraying exogenous spermidine treatment increased the photosynthetic pigment (chlorophyll a, chlorophyll b, and carotenoids) contents of lettuce under high-temperature stress and increased the net photosynthetic rate. The high-temperature spraying exogenous spermidine treatment promoted the up-regulation of sucrose-metabolizing enzyme (sucrose synthase, sucrose phosphate synthase, acid invertase, and neutral invertase) activity and sucrose-metabolizing enzyme gene (SS-like, SPS1, NIA and NIE) expression of lettuce under high-temperature stress. It increased the accumulation of leaf starch and sucrose, thereby forming a new sucrose metabolism balance. The current results show that spermidine effectively resists high-temperature stress and increases yield by improving sucrose metabolism.