Responses of photosystem II and photosystem I function of Photinia× fraseri and Magnolia grandiflora to winter transient warming

<bold>Objective</bold> Under the background of global warming，winter transient warming events（daily maximum air temperature &gt; 15 ℃）frequently occur in subtropical regions.This study aimed to investigate the responses of photosystem II（PSII）and photosystem I（PSI）functions in <italic>Photinia × fraseri</italic> and <italic>Magnolia grandiflora</italic> to winter transient warming，analyze the differences in their overwintering photoprotective strategies，thus providing a theoretical basis for the introduction and domestication of evergreen plants. <bold>Method</bold> Using <italic>P. × fraseri</italic> and <italic>M. grandiflora</italic> cultivated in subtropical regions as experimental materials，the rapid light curves of chlorophyll fluorescence were measured for shade leaves and sun leaves.Measurements were conducted on the day before warming and on the 5th day of transient warming period in December 2023（5 consecutive days with daily maximum air temperature above 15 ℃）. <bold>Result</bold> The functions of PSII and PSI in both plant species，which were suppressed by winter low temperatures，partially recovered during the transient warming period，but exhibited species-specific responses.Winter low temperatures induced concurrent photoinhibition of PSII and PSI in sun leaves，whereas shade leaves only underwent PSI photoinhibition.Warming facilitated recovery of PSII photoinhibition in sun leaves of both plants：the maximum photochemical efficiency of photosystem II（<italic>F</italic>_v/<italic>F</italic>_m）in sun leaves increased from 0.758 to 0.843 in <italic>P.× fraseri</italic> and from 0.676 to 0.737 in <italic>M. grandiflora</italic>.However，warming had no significant effect on PSI photoinhibition recovery，as no notable changes in the maximum fluorescence signal of the P700（<italic>P</italic>_m）were observed in either shade or sun leaves of both species before and after warming.In <italic>P.× fraseri</italic>，warming increased the photochemical quantum yield of PSII（<italic>Y</italic>（II）），the photochemical quantum yield of PSI（<italic>Y</italic>（I）），and the photochemical quenching coefficient（<italic>q</italic>P），especially under medium-to-high light intensities，but decreased the non-photochemical quenching coefficient（NPQ），quantum yield of regulated energy dissipation in PSII（<italic>Y</italic>（NPQ）），and cyclic electron transport（CEF）.In <italic>M. grandiflora</italic>，warming promoted the recovery of <italic>Y</italic>（II），<italic>Y</italic>（I），and <italic>q</italic>P in sun leaves under low-to-medium light intensities，but had no significant impact on shade leaves.For <italic>M. grandiflora</italic>，warming decreased NPQ and <italic>Y</italic>（NPQ）in both shade and sun leaves under weak light intensities，however，shade leaves showed no significant changes compared with pre-warming levels，while sun leaves exhibited significant increases under medium-to-high light intensities.Meanwhile，warming also decreased CEF in both shade and sun leaves of <italic>M. grandiflora</italic>.Across both periods（pre-warming and during warming）<italic>M. grandiflora</italic> consistently showed significantly higher NPQ，<italic>Y</italic>（NPQ），and CEF in both shade and sun leaves compared to <italic>P.× fraseri</italic>. <bold>Conclusion</bold> Under low temperatures in winter，both plant species can defend against photoinhibition of PSII and PSIby down-regulating PSII function，enhancing photoprotective pathways such as heat dissipation and cyclic electron transport. <italic>P.× fraseri</italic> demonstrates strong tolerance to winter low temperature in both PSII and PSI，which functions can rapidly recover in response to warming.In contrast，<italic>M. grandiflora</italic> experiences partial damage to PSII and PSI under winter low temperature，with only partial functional recovery during transient warming.However，<italic>M. grandiflora</italic> possesses superior photoprotective capacities with energy dissipation and cyclic electron transport compared to <italic>P.× fraseri</italic>.The research results can provide a theoretical basis for the introduction，domestication，and landscape application of <italic>P.× fraseri</italic> and <italic>M. grandiflora</italic> in subtropical regions.