KINETIC-STUDIES ON THE XANTHOPHYLL CYCLE IN BARLEY LEAVES - INFLUENCEOF ANTENNA SIZE AND RELATIONS TO NONPHOTOCHEMICAL CHLOROPHYLL FLUORESCENCE QUENCHING

Xanthophyll-cycle kinetics as well as the relationship between the xan thophyll de-epoxidation state and Stern-Volmer type nonphotochemical c hlorophyll (Chl) fluorescence quenching (qN) were investigated in barl ey (Hordeum vulgare L.) leaves comprising a stepwise reduced antenna s ystem. For this purpose plants of the wild type (WT) and the Chi b-les s mutant chlorina 3613 were cultivated under either continuous (CL) or intermittent light (IML). Violaxanthin (V) availability varied from a bout 70% in the WT up to 97 to 98% in the mutant and IML-grown plants. In CL-grown mutant leaves, de-epoxidation rates were strongly acceler ated compared to the WT. This is ascribed to a different accessibility of V to the de-epoxidase due to the existence of two V pools: one bou nd to light-harvesting Chi a/b-binding complexes (LHC) and the other o ne not bound. Epoxidation rates (k) were decreased with reduction in L HC protein contents: k(WT) > k(mutant) much greater than k(IML plants) . This supports the idea that the epoxidase activity resides on certai n LHC proteins. Irrespective of huge zeaxanthin and antheraxanthin acc umulation, the capacity to develop qN was reduced stepwise with antenn a size. The qN level obtained in dithiothreitol-treated CL- and IML-gr own plants was almost identical with that in untreated IML-grown plant s. The findings provide evidence that structural changes within the LH C proteins, mediated by xanthophyll-cycle operation, render the basis for the development of a major proportion of qN.