Operation of the xanthophyll cycle in higher plants in response to diurnal changes in incident sunlight.

Changes in the carotenoid composition of leaves in response to diurnal changes in sunlight were determined in the crop species Helianthus annuus L. (sunflower), Cucurbita pepo L. (pumpkin), and Cucumls sativus L. (cucumber), in the diaheliotropic mesophyte Malva neglecta Wallr., and in the perennial shrub Euonymus kiautschovicus Loesner. Large daily changes were observed in the relative proportions of the components of the xanthophyll cycle, violaxanthin (V), antheraxanthin (A), and zeaxanthin (Z) in plants grown in full sunlight. In all leaves large amounts of Z were formed at peak irradiance, with the changes in Z content closely following changes in incident photon flux density (PFD) over the course of the day. All leaves also contained large total pools of the three xanthophyll-cycle components. However, the extent to which the V pool present at dawn became de-epoxidized during the day varied widely among leaves, from a 27% decrease in M. neglecta to a 90% decrease in E. kiautschovicus. The largest amounts of Z and the lowest amounts of V at peak irradiance (full sunlight) were observed in the species with the lower rates of photosynthesis (particularly in E. kiautschovicus and pumpkin), and smaller amounts of Z and a lesser decrease in V content were found at peak irradiance in those species with the higher rates of photosynthesis (particularly in M. neglecta and sunflower). In all species some Z was present in the leaves prior to sunrise. Furthermore, in individuals of sunflower, pumpkin, and cucumber grown at 85% of full sunlight and transferred to full sunlight, a further increase in the already large pool of the xanthophyll-cycle pigments occurred over the course of 1 d.