Seasonal changes in non-structural carbohydrates and sucrose metabolism enzymes in two Sabina species

Evergreen plants need to store reserves to allow for their survival during the winter months and for new leaf growth in the following spring. In many of the tree species, these reserve functions are mainly carried out by starch, which is degraded to soluble carbohydrates during the dormant season to maintain active respiration and provide protection against freezing. In the present study, two evergreen woody plants, S. przewalskii Kom. (SP) and S. chinensis (Lin.) Ant.(SC), were used to investigate the patterns of seasonal variation in the concentrations of soluble sugars, sucrose, fructose and starch, and the activities of sucrose-phosphate synthase (SPS), sucrose synthase (SUSY), neutral invertase (NI) and soluble acid invertase (SAI). Foliar soluble sugar, sucrose, fructose, and starch concentrations were markedly higher in SC than in SP; moreover, the activities of SPS, SUSY, NI and SAI were also higher in SC than in SP. There were generally higher concentrations of soluble sugars, sucrose and fructose and SPS activities in the winter than in the summer for both Sabina trees, which was consistent with the necessity for protection against freezing; however, opposite results were found with regards to starch concentrations and the activities of SUSY, NI and SAI. In contrast with the activities of SUSY, NI and SAI, the negative correlation observed between SPS and air temperature was likely a reflection of its crucial role in the acquisition of freezing tolerance by sucrose metabolism in the winter. These results suggest that higher carbon reserves do not give S. chinensis (Lin.) Ant. a competitive advantage in tolerating cold temperatures and that only SPS, but not SUSY, NI and SAI, may play a positive role in freezing tolerance by increasing soluble sugar.