The purpose of the proposed investigations is to characterize the response of crassulacean acid metabolism (CAM) plants to high light, with a particular emphasis on the employment of a novel photoreceptive process that involves several leaf carotenoids. This process, which is associated with the formation of zeaxanthin from its precursors in the xanthophyll cycle, facilitates the safe removal of an excess of absorbed light in photosynthetic organs. The diurnal operation of the xanthophyll cycle will be characterized in several different types of plants that process CAM, including platyopuntias, rosette-forming yuccas, and two species which possess succulent leaves, Hoya australis and the fern Pyrrossia confluens. The acclimation of the carotenoid composition, and particularly the xanthophyll cycle, to different light environments will be examined through a comparison of leaves which have developed in deep shade versus those which have developed in full sunlight, In addition, this question will be addressed by examining the acclimation of the carotenoid composition to the gradient in the light environment that occurs within the leaves or cladodes. The operation of the xanthophyll cycle will furthermore be characterized in response to a combination of high light and chilling temperatures. The utilization of the xanthophyll cycle in photoreceptive energy dissipation in these CAM species which are found in habitats that experience high light will then be compared with that which occurs in C3 species.
