Crassulacean acid metabolism in the epiphytic ferns <em>Drymoglossum piloselloides</em> and <em>Pyrrosia longifolia</em>: studies on responses to environmental signals.

The paper reports the results of the comprehensive study of crassulacean acid metabolism in two epiphytic tropical ferns, Drymoglossum piloselloides and Pyrrosia longifolia. The plants were investigated under different light, temperature and water status. It was found that both species are obligate CAM plants. The diurnal acidity rhythm is due to the fluctuation in malic acid concentration, which accounts for the change in titratable acidity. Besides malic acid, shikimate and oxalate are found to be present, but not contributing to the CAM acid rhythm. The diurnal rhythm of malic acid content results in a corresponding rhythm in leaf water relations. Both ψπ and ψtotal were lowest at the end of the night, i.e. when the level of malic acid was highest. The effects of temperature on CO2 exchange were inverse to those observed in other CAM plants. In both ferns studied, dark CO2 fixation increased when the night temperature was increased. Increase in day temperature reduced CO2 uptake during phase IV and during the following night. The observed responses of the ferns to temperature changes suggest that the in situ environmental conditions are optimal for their CAM performance. In weak light, the plants showed net CO2 output during the midday deacidification period. Increases in light intensity reduced such CO2 output. Under drought conditions, the CO2 exchange in the ferns was reduced to zero within 5-6d, indication that the ferns studied are more susceptible to water deficiency than other CAM plants. This could be due to a higher cuticular conductance for water. The results are discussed, in particular, in relation to CAM performance of epiphytes growing in the wet tropics. [ABSTRACT FROM AUTHOR]