Impact of snow cover on photoinhibition and winter desiccation in evergreen Rhododendron ferrugineum leaves during subalpine winter

Effects of winter snow cover on photoinhibition and possible interactions with winter desiccation were investigated in situ in an evergreen subalpine woody species, Rhododendron ferrugineum L., at the alpine timberline (1950 m a.s.l.). Timing and duration of complete snow cover markedly influenced potential efficiency of photosystem II (PSII; F(v) /F(m)). Lack of snow cover led to severe but mostly reversible photoinhibition with F(v)/F(m) values as low as 0.05. Complete snow cover immediately stopped further reductions in PSII efficiency. Snow cover promoted recovery from photoinhibition, but only if, in addition to shading by snow, plants were exposed to nonfreezing temperatures close to 0 degrees C. The F(v)/F(m) ratio was closely related to minimum leaf temperatures because both photoinhibition and recovery from photoinhibition were strongly influenced by temperature. The period without major reductions in PSII efficiency lasted for only two months, reflecting the extremely short growing period in the subalpine environment. Compared with complete snow cover, incomplete snow cover led to significantly higher water losses as well as lower dehydration tolerance, because both osmotic adjustment and changes in turgor maintenance capacity were significantly reduced. Interactions between photoinhibition and winter desiccation were masked by the direct effects of freezing temperatures. However, both photoinhibition and winter desiccation are closely linked and occur together under field conditions in this evergreen subalpine woody species.