Low inactivation of D1 protein of photosystem II in young canopy leaves of Anacardium excelsum under high-light stress

The response of young and mature canopy sun leaves of the tropical forest tree Anacardium excelsum to excess light was studied. Compared with the mature leaves, the young leaves, containing about half the amount of chlorophyll a+b per unit area, exhibited a higher proportion of total carotenoids as xanthophyll cycle pigments and had an increased ratio of total carotenoids to chlorophyll a+b. The capacity for thermal energy dissipation in high light, as expressed by non-photochemical (energy-dependent and photoinhibitory) chlorophyll fluorescence quenching in photosystem II, was higher in the young than in the mature leaves. Quantification of the photosystem II reaction center protein D1 following high-light exposure of leaves in the presence of streptomycin (an inhibitor of plastidic protein biosynthesis) showed pronounced differences between young and mature leaves. In the mature leaves, substantial degradation of D1 occurred as a result of photoinhibitory irradiation, but also in low light. In contrast, in young leaves, high light that caused a strong decline in potential photosystem II efficiency (indicated by the ratio of variable to maximum fluorescence emission, F-V/F-M) did not induce a significant D1 degradation. It is concluded that in the young leaves, which are highly acclimated to conditions of excess light, the D1 protein is stabilized by efficient photoprotective mechanisms.