Photosynthetic Acclimation and Photoinhibition of Terrestrial and Epiphytic CAM Tissues Growing in Full Sunlight and Deep Shade

Abstract

Light response curves of O2 exchange and fluorescence from photosystem II at 77K were examined in several species possessing crassulacean acid metabolism (CAM) growing under natural conditions of varying irradiance. All of the species exhibited considerable acclimation to low irradiance, but the epiphytes Dendrobium speciosum and Pyrrosia confluens were unable to acclimate fully to full sunlight. Although Opuntia stricta possessed a reduced quantum yield under full sunlight conditions, its photosynthetic capacity, unlike that of D. speciosum and P. confluens, was very high, and was probably due, at least in part, to the higher level of CAM activity present in this species. Shade tissues of O. stricta, however, were quite susceptible to photoinhibition by high light, as indicated by reductions in 77K fluorescence. Fronds of P. confluens growing in full sunlight were clearly photoinhibitedr However, the light environment in the epiphytic habitat is rarely static, and considerable recovery from photoinhibition in a sun population of P. confluens fronds was observed near the summer solstice when the canopy shielded this population from direct sunlight for most of the day. Such seasonal and diurnal changes in irradiance may be important to the survival of epiphytic species with a limited capacity for CAM activity.