Photoinhibition of Photosynthesis in situ in Six Species of Eucalyptus

Abstract

Photoinhibition of photosynthesis was investigated in horizontally and vertically restrained leaves of Eucalyptus plants growing outdoors. This was done by measuring the quantum yield of PSII photochemistry, assayed by the ratio of variable to maximal fluorescence (F_V/F_M) after 15 min of dark adaptation, and the quantum yield of O₂ evolution. On clear days, a time-dependent depression in both parameters was observed. Though horizontal leaves showed stronger photoinhibition (max. 40%) than did vertical leaves (max. 30%), this difference was smaller than the threefold difference in light interception. Since the inherent susceptibility to photoinhibition was the same in vertical and horizontal leaves, a non-linear light dependence of photoinhibition was assumed. The time course of recovery from photoinhibition under low light followed the kinetics of a first order reaction, with 50% recovery after 45 min. When watering was restricted on a hot and clear day a more severe photoinhibition developed in horizontal leaves, which took several days of low light to overcome. This photoinhibition state was triggered by high leaf temperatures, resulting from decreased stomatal conductance and transpiration, which sensitised the leaf to light.