The α-tocopherol content of leaves of pedunculate oak (Quercus roburL.) - variation over the growing season and along the vertical light gradient in the canopy

This study was performed in order to investigate whether the actual requirement for defence against photo-oxidative stress is reflected by the α-tocopherol (α-Toco) content in leaves of pedunculate oak (Quercus robur L.). Antioxidants and pigments were quantified in leaves that were collected on six days between May and September 2000 in a mixed pine/oak forest at canopy positions differing in light environment. Pools of hydrophilic antioxidants and photo-protective xanthophyll cycle pigments (V+A+Z) reflected the anti-oxidative demand, as these pools increased with the average light intensity to which the leaves were acclimated. The photo-protective demand was not the determinant of the α-Toco content of oak leaves, as (1) foliage of a young oak, exposed to low light levels in the understorey, contained higher amounts of this lipophilic antioxidant than leaves sampled from semi-mature oaks at canopy positions with a similar light environment, and (2) a strong increase in the α-Toco content over the growing season was detected at each investigated crown position, whereas the V+A+Z pool did not show a concomitant accumulation during leaf ageing. The rate of α-Toco accumulation differed distinctly between samples taken at different canopy positions.