Dynamics of the α-tocopherol pool as affected by external (environmental) and internal (leaf age) factors in Buxus sempervirens leaves

The pools of photoprotective molecules respond to changes in the environmental conditions and sometimes to leaf ageing. We asked to what extent both factors contribute to the contents of α-tocopherol and xanthophyll cycle [V + A + Z (VAZ)] pigments. To address this question, we used boxtree (Buxus sempervirens) as model species because its leaves are long-lived and evergreen and are subjected to a succession of different stress conditions during their lifespan. In three age classes of sun and shade leaves of this species, seasonal changes in photoprotective compounds were followed during 15 months and a leaf age interval of 40 months was covered. As could be expected, VAZ and α-tocopherol pools increased in parallel during stress periods (summer and winter), but only VAZ recovered to the initial pools once stress disappeared. As a result, the basal α-tocopherol level increased linearly in a time-dependent manner that was also higher in sun leaves of this species when compared with shade leaves, and in fact, the rate of tocopherol increase was directly proportional to irradiance in another evergreen (Laurus nobilis). To study whether light dependency of tocopherol accumulation is observed in other species, we performed a literature survey that revealed that this age-dependent tocopherol increase was significant in sun leaves from 65% of the species for which age-dependent tocopherol changes have been reported, and it was on average 2.2-fold higher in sun leaves as compared with shade leaves. We conclude that there are two components in the α-tocopherol pool, one dynamic that responds to environmental changes and one age-related which increases linearly with time in a light-dependent manner. The physiological meaning of the latter remains obscure.