Divergent Photosynthetic Strategies of and During Cold Acclimation and Freezing-Thaw Recovery.

Low temperatures can significantly affect the growth of ornamental plants, emphasizing the importance of improving their cold tolerance. However, comparative studies on the photosynthetic responses of sun and shade plants to low temperatures remain limited. In this study, gas exchange, chlorophyll fluorescence in Photosystem II (PSII) and Photosystem I (PSI), the antioxidant system, the osmoregulator substance, and lipid peroxidation were investigated in the shade plant () and the sun plant () during cold acclimation (CA) and the freezing-thaw recovery (FTR). The CA treatment significantly declined the net photosynthetic rate (Pn) and the maximum photochemical efficiency of PSII (Fv/Fm) in and , indicating the photoinhibition occurred in both species. However, exhibited a much better photosynthetic stability to maintain Pn, Fv/Fm, and carboxylation efficiency (CE) than during CA, suggesting that had a greater photosynthetic resilience compared to . Furthermore, preferred to maintain Pn, Fv/Fm, the actual photosynthetic efficiency of PSII (Y(II)), and the actual photosynthetic efficiency of PSI (Y(I)) to consistently provide the necessary energy for the carbon assimilation process, while tended to divert and dissipate excess energy by thermal dissipation and cyclic electron flow during CA. Moreover, there were higher soluble sugar contents in in comparison to . These traits allowed to recover photosynthetic efficiency and maintain cellular integrity better than after the freezing stress. In conclusion, CA significantly reduced the photosynthetic capacity and led to the divergent photosynthetic strategies of both species, which finally resulted in a different freezing tolerance after the freezing-thaw recovery. These findings provide insights into the divergent photoprotective strategies of sun and shade plants in response to cold temperatures.