C-3 PHOTOSYNTHESIS IN THE GAMETOPHYTE OF THE EPIPHYTIC CAM FERN PYRROSIA-LONGIFOLIA (POLYPODIACEAE)

Sporophytes of some epiphytic species in the fern genus Pyrrosia exhib it Crassulacean acid metabolism (CAM), generally considered to be a de rived physiological response to xeric habitats. Because these species alternate between independent sporophytic and gametophytic generations yet only the sporophyte has been characterized physiologically, exper iments were conducted to determine the photosynthetic pathways present in mature sporophytes, immature sporophytes, and gametophytes of Pyrr osia longifolia. Diurnal CO2 exchange and malic acid fluctuations demo nstrated that although the mature sporophytes exhibited CAM, only C-3 photosynthesis occurred in the gametophytes and young sporophytes. Con sideration of the above results and those from previous studies, as we ll as the life cycle of ferns, indicates that the induction of CAM pro bably occurs at a certain developmental stage of the sporophyte and/or following exposure to stress. Elucidation of the precise mechanisms u nderlying this C-3-CAM transition awaits further research.