Zeaxanthin and non-photochemical quenching in sun and shade leaves of C3 and C4 plants

The relationships between non-radiative energy dissipation and the carotenoid content, especially the xanthophyll cycle components, were studied in sun and shade leaves of several plants possessing C-3 (Hedera helix and Laurus nobilis) or C-4 (Zea mays and Sorghum bicolor) photosynthetic pathways. Sun-shade acclimation caused marked changes in the organisation and function of photosynthetic apparatus, including significant variation in carotenoid content and composition. The contents of zanthophyll cycle pigments were higher in sun than in shade leaves in all species, but this difference was considerably greater in C-3 than in C-4 plants. The proportion of photoconvertible violaxanthin, that is the amount of violaxanthin (V) which can actually be de-epoxidised to zeaxanthin, was much greater in sun than in shade leaves. The amount of photoconvertible V was always linearly dependent on the chlorophyll a/b ratio, although the slope of the relationship varied especially between C-3 and C-4 species. The leaf zeaxanthin and antheraxanthin contents were correlated with non-radiative energy dissipation in all species under different light environments. These relationships were curvilinear and variable between sun and shade leaves and between C-3 and C-4 species. Hence, the dissipation of excess energy does not appear to be univocally dependent on zeaxanthin content and other photoprotective mechanisms may be involved under high irradiance stress. Such mechanisms appear largely variable between C-3 and C-4 species according to their photosynthetic characteristics.