Chlorophyll and carotenoid composition in leaves of Euonymus kiautschovicus acclimated to different degrees of light stress in the field

The response of carotenoid and chlorophyll composition to the actual degree of excess light experienced in the natural environment was examined in differently angled leaves of the sclerophyllous shrub Euonymus kiautschovicus. Increasing light stress caused a greater conversion of the xanthophyll cycle to zeaxanthin and antheraxanthin as well as thermal dissipation of a greater fraction of the absorbed light. Increasing light stress was also associated with increasing chlorophyll a/b ratios and increases in the pool size of the xanthophyll cycle. The response of all other carotenoids to light stress was less pronounced than that of the xanthophyll cycle pool. While the ratio of beta-carotene or lutein to chlorophyll increased with increasing light stress, the ratio of neoxanthin to chlorophyll remained constant. Only the (taxonomically restricted) carotenoids lactucaxanthin and alpha-carotene decreased relative to chlorophyll with increasing light stress. These findings are consistent with an increased emphasis on energy dissipation over light collection with increasing light stress, afforded presumably by a decreased ratio of major, peripheral (bulk chlorophyll-binding) to minor, proximal (xanthophyll cycle-rich) light-harvesting complexes of photosystem II. These responses to light stress within a single species could not be extrapolated to comparisons among different groups of species.