Freezing induced leaf movements and their potential implications to early spring carbon gain: Rhododendron maximum as exemplar

Thermonastic leaf movements (TLM) are induced by freezing and are correlated with freezing tolerance, and our goal was to determine the significance of TLM to photosynthesis during the winter and early spring. We performed field experiments in which we prevented TLM of Rhododendron maximum leaves and determined the consequences of that prevention to photoinhibition (measured by chlorophyll fluorescence) from fall to spring, photosynthesis (measured by gas exchange) in the winter, and recovery of photosynthesis in the spring. TLM significantly reduced photoinhibition in the winter for leaves on branches in the outer canopy of R. maximum plants, but not for leaves on inner canopy branches. During warm periods in the winter, TLM were associated with significantly lower photoinhibition, but TLM did not have a significant effect on photosynthesis during these times. In early spring, leaves with TLM recovered from photoinhibition more quickly than for leaves prevented from TLM. Photosynthesis in the early spring was higher at any stomatal conductance for leaves with TLM than for leaves prevented from TLM for outer canopy leaves only. Our results demonstrate that TLM during the winter prevent excessive photoinhibition and promote rapid recovery of photosynthesis in the early spring.