DAILY XANTHOPHYLL.CYCLE PHOTOPROTECTION IN DEVELOPING LEAVES

There is widespread agreement that the xanthophyll cycle provides a major photoprotection system for photosynthesis in green leaves (l-8). Indeed this type of photoprotection seems to be ubiquitous for photosynthetic organisms. Photoprotection is provided via a rapid, near 1013 set, ability of zeaxanthin (Z) to dissipate excess light energy from photosynthesis because the energy of the excited singlet state of Z is lower than that of chlorophyll a (9). Details of historical and current understandings on the operation of the xanthophyll cycle in green tissues are readily available (3,6,9, A.M. Gilmore- these Proceedings). During initial research efforts to establish the xanthophyll cycle some developing photosynthetic tissues were appropriately used, e.g., to identify the pigments involved or to detect the enzymes required for epoxidation and deepoxidation. However these developing tissues were not subjected to natural plant diurnal growth conditions and this may have obscured cycle functions during the development of photosynthesis. For example; red kidney bean plants were dark grown for 4 days followed by giving I-msec flashes of Xenon light every 12 minute