In situStudies of Crassulacean Acid Metobolism inDrymoglossum piloselloides, an Epiphytic Fern of the Humid Tropics

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Abstract
This paper reports autecological field-studies in Singapore on Drymoglossum piloselloides (L.) Presl., an epiphytic fern of the humid tropics which is capable of performing Crassulacean acid metabolism (CAM). As indicated by the gas exchange patterns and by the occurrence of a diurnal malic acid rhythm, the plant also features CAM in situ at its natural sites. Both in well-watered and in naturally droughted plants external CO2 was taken up solely during the night. Water stress decreased nocturnal CO2uptake, but left the synthesis and storage of malic acid unaffected. This indicates that CO2 recycling of respiratory CO2 by CAM is ecophysiologically important at the high night temperatures typical of the tropical habitats of the fern. The plants showed a diel fluctuation of cell-sap osmotic pressure which paralleled that of malic acid, while the fluctuation of the xylem tension followed the curve of transpiration more closely than it followed that of the malic acid content. CAM in D. piloselloides was clearly not limited by natural access to mineral ions and nitrogen. It is concluded that the ecophysiological advantage of CAM for D. piloselloides lies in a better water use efficiency as compared with C3 ferns and in the salvaging of carbon by CO2 recycling.