Why does the leaf of Japanese Cypress in temperate region experience transient leaf reddening under winter excessive light stress

<p>The leaves of evergreen coniferous species in the temperate region sometimes were observed to change from green to red before or during winter and persist until next spring. This phenomenon is also called 'winter leaf reddening', which might be a photoprotection strategy for plant leaves to deal with excess light stress during winter and early spring. In gymnosperms, the xanthophyll cycle (VAZ cycle) and accumulation of red pigment (e. g. rhodoxanthin) can prevent excess light damage to the photosynthetic apparatus. However, the joint role of these two processes in corresponding with canopy photosynthesis under winter excess light stress has not been further studied. This study aimed to clarify: (1) whether the low temperature is the dominant factor affecting the winter leaf reddening phenomenon and ascertain the air temperature conditions when this phenomenon occurs; (2) whether rhodoxanthin and the VAZ cycle play a collaborating role in regulating low light-use efficiency (LUE) under low air temperature conditions during the winter season; (3) the difference between two leaf redness indicators obtained from phenological method and remote sensing method.</p><p>Canopy leaf redness indicators were obtained in two ways. The automated system with a digital camera was used to monitor the canopy phenological changes. The RGB channels of image data were extracted to calculate the Red-Green vegetation index (RGVI). Red index (RI) is obtained by spectral reflectance analysis to track rhodoxanthin variation patterns. The photochemical reflectance index (PRI) was utilized as a tool to reflect the VAZ cycle. The canopy CO₂ flux was measured with the eddy covariance method, which can be used to calculate LUE. Micrometeorological data were also monitored.</p><p>Our results suggest that low air temperatures in winter play a domain role in the occurrence of winter leaf reddening. The onset of winter leaf reddening was accompanied by a decrease in LUE and PRI and a corresponding increase in RGVI. This suggests that the accumulation of rhodopsin and the VAZ cycle may play a collaborative role in regulating LUE under the combined effect of chilling temperatures and high solar radiation conditions. There were temporal differences in the peak occurrence of RI and RGVI, but the change characteristics were largely consistent, which may indicate that RI can more sensitively monitor the timing of red appearance in the vegetation canopy.</p><p>Keywords: winter leaf reddening, Japanese cypress, photochemical reflectance index (PRI), Red index (RI), Red-Green vegetation index (RGVI), phenological analysis, digital camera, light-use efficiency (LUE)</p>