Comparison of Photosynthetic Responses of the Sympatric Tropical C3 Species Clusia multiflora H. B. K. and the C3-CAM Intermediate Species Clusia minor L. to Irradiance and Drought Stress in a Phytotron1

Abstract: Clusia multiflora H. B. K., an obligate C₃ species and Clusia minor L. a C₃/CAM intermediate species, are two physio-types of a similar morphotype. They can sympatrically occupy secondary savanna sites exposed to high insolation in the tropics. In C. multiflora severe stress, i.e., switching shade-grown plants to high light plus drought, resulted in leaves browning or yellowing and becoming necrotic. However, in long-term light stress C. multiflora was able to grow new leaves with their photosynthetic apparatus fit for high light conditions. Shade-grown C. minor readily overcame switching to high light conditions and drought, responding by a rapid change from C₃ photosynthesis to CAM. Decreasing _soil led to increased abscisic acid levels in the leaves of C. minor, however CAM induction was not directly related to this and was mainly determined by increased PPFD. Both species were capable of rapid accumulation of zea-xanthin for acute photoprotection following high PPFD exposure. The maximum capacity for zeaxanthin accumulation was larger in C. minor, but under steady high PPFD it only partially made use of this capacity, relying on high internal CO₂ concentrations of Phase Ill of CAM, in addition to zeaxanthin, for acute photo-protection. Thus, by different means the two species perform well under high light conditions. However, C. multiflora needs time for development of adapted leaves under such stress conditions while the more flexible C. minor can readily switch from low light to high light conditions.