Zeaxanthin and non-photochemical quenching in sun and shade leaves of C-3 and C-4 plants

The relationships between non-radiative energy dissipation and the caroteno id content, especially the xanthophyll cycle components, were studied in su n and shade leaves of several plants possessing C-3 (Hedera helix and Lauru s nobilis) or C-4 (Zea mays and Sorghum bicolor) photosynthetic pathways. S un-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 h igher in sun than in shade leaves in all species, but this difference was c onsiderably greater in C-3 than in C-4 plants. The proportion of photoconve rtible violaxanthin, that is the amount of violaxanthin (V) which can actua lly be de-epoxidised to zeaxanthin, was much greater in sun than in shade l eaves. The amount of photoconvertible V was always linearly dependent on th e chlorophyll a/b ratio, although the slope of the relationship varied espe cially between C-3 and C-4 species. The leaf zeaxanthin and antheraxanthin contents were correlated with non-radiative energy dissipation in all speci es 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 d ependent on zeaxanthin content and other photoprotective mechanisms may be involved under high irradiance stress. Such mechanisms appear largely varia ble between C-3 and C-4 species according to their photosynthetic character istics.