INTERNAL AND EXTERNAL PHOTOPROTECTION IN DEVELOPING LEAVES OF THE CAMPLANT COTYLEDON ORBICULATA

Leaves of the CAM plant Cotyledon orbiculata produced a dense epiderma l wax which decreased the absorption of light, possibly functioning as an external photoprotective mechanism (Robinson et al. 1993). However , developing leaves did not accumulate wax until after 21 d with full wax coating not achieved until at least 35 d. In addition, young leave s had lower rates of electron transport than mature leaves. Leaf devel opment therefore occurs at higher incident PFD than that experienced b y the mature leaves, and, for young leaves, can lead to an increase in the proportion of light energy which is excess to requirements and mu st be dissipated non-photochemically. Changes in the photosynthetic ca pacity, PSII efficiency, rate of energy dissipation, and the content o f chlorophyll (Chl), carotenoids, wax and anthocyanins were followed i n developing leaves of C. orbiculata in an attempt to elucidate the re lative importance of the various photoprotective mechanisms during lea f ontogeny. The largest pools of xanthophyll cycle pigments (on a Chl basis) were found in the waxless, young leaves and were correlated wit h greater levels of energy dissipation activity. The importance of xan thophyll cycle-dependent energy dissipation in young C. orbiculata lea ves prior to development of a reflective wax covering, and full photos ynthetic capacity which for CAM plants includes appreciable nocturnal acid accumulation, is discussed, Also, we consider the possibility tha t anthocyanin pigments in the upper and lower epidermis may increase r eflectivity and act as external photoprotectants.