Acclimation of antioxidant pools to the light environment in a natural forest canopy

Leaf growth irradiance determines the pools of photoprotective molecules. We asked whether the potential for acclimation of antioxidant pool size to changes in the leaf light environment is affected by the position of the leaf within the canopy profile. The study was conducted in a mixed canopy formed by Tilia cordata at the lower level and Populus tremula at the upper level. Leaves were either exposed to extra light or enclosed in shade bags. Ascorbate, glutathione and alpha-tocopherol pools increased with growth irradiance. Only alpha-tocopherol increased in leaves of both species in response to extra light. The slope of tocopherol changes was positively correlated with growth irradiance in both species. It also correlated with the slope of xanthophyll cycle (VAZ, sum of violaxanthin, antheraxanthin and zeaxanthin) pool changes with cumulative extra light in T. cordata. We conclude that alpha-tocopherol is the key antioxidant altering tolerance to high light, and that it may cooperate with zeaxanthin. The pools of hydrophilic antioxidants either acclimate more slowly, or their pools are large enough not to limit the overall acclimation to altered light environment.