Abstract
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 Helleborus viridis (Hv) and the sun plant Lupinus polyphyllus (Lp) 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 Hv and Lp, indicating the photoinhibition occurred in both species. However, Hv exhibited a much better photosynthetic stability to maintain Pn, Fv/Fm, and carboxylation efficiency (CE) than Lp during CA, suggesting that Hv had a greater photosynthetic resilience compared to Lp. Furthermore, Hv 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 Lp tended to divert and dissipate excess energy by thermal dissipation and cyclic electron flow during CA. Moreover, there were higher soluble sugar contents in Hv in comparison to Lp. These traits allowed Hv to recover photosynthetic efficiency and maintain cellular integrity better than Lp 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.