Do Plant Functional Groups Vary in Their Strategy for Sustained Energy Dissipation During Winter?

Evergreens require photoprotective strategies to cope with light absorption during subzero winter conditions. A key strategy involves sustained forms of thermal dissipation resulting in reductions in photochemical efficiency, often termed winter photoinhibition (WPI). WPI, although well studied in conifers and winter-tolerant crops, has been little studied in other plant functional groups, such as bryophytes and ferns. Our goal was to monitor a diversity of winter-tolerant plant functional groups (3-4 bryophytes, 1 fern, 3 conifers) in subzero conditions to determine the extent of WPI as well as the kinetics of recovery using chlorophyll fluorescence analysis. We also used HPLC pigment analysis to ask whether consistent seasonal changes in carotenoid pool sizes occur in plants from different functional groups. Results indicate all species showed pronounced reductions in F_v/F_m during subzero conditions, correlating with sustained retention of zeaxanthin. Functional groups differed in recovery kinetics; after 1 h of warming, bryophytes and ferns showed dramatic recovery of F_v/F_m, while conifers and angiosperms showed no or minimal recovery. Winter increases in the relative pool size of xanthophyll cycle pigments occurred in all functional groups except ferns, and winter increases in lutein occurred consistently in conifers and angiosperms. The data suggest bryophytes and ferns employ a flexible strategy for sustained dissipation during winter, induced directly by subzero temperatures, likely using mechanisms similar to those used during desiccation. In contrast, conifers and broad-leaved angiosperms in this study rely on mechanisms associated with sustained forms of zeaxanthin-dependent thermal dissipation and increased pools of the photoprotective carotenoids.