Crassulacean acid metabolism in the shade. Studies on an epiphytic fern, Pyrrosia longifolia, and other rainforest species from Australia.

Crassulacean acid metabolism (CAM) was studied in a tropical epiphytic fern, Pyrrosia longifolia, from a fully sun-exposed and from a very shaded site in Northern Queensland, Australia. Measurements of instantaneous net CO2 exchange showed carbon gain via CO2 dark fixation with some net CO2 uptake also occuring during late afternoon, in both sun and shade fronds. Maximum rates of net CO2 uptake and the nocturnal increase in titratable acidity were lower in shade than in sun fronds. δ13C values of sun and shade fronds were not significantly different, and ranged between-14 and-15‰ suggesting that, in the long term, carbon gain was mainly via CO2 dark fixation. Sun fronds had a higher light compensation point of photosynthesis than shade fronds but the same quantum yield. Yet there was no acclimation of photosynthetic O2 evolution, (measured at 5% CO2) in sun and shade fronds and photosynthesis saturated at between 200 and 400 μmol quanta m-2 s-1. Use of higher light intensities for photosynthesis of sun fronds was probably precluded by low nutrient availability. Total nitrogen was less than 1% of dry weight in fully expanded sun and shade fronds. Exposure of shade fronds to full sunlight for 6 h led to a 60% decline in the quantum yield of photosynthesis and to a decline in variable fluorescence measured at room temperature. Photoinhibition by high light was also observed in Hoya nicholsoniae, a rainforest climber growing in deep shade. This species also exhibited CAM as demonstrated by nocturnal net CO2 uptake, nocturnal acidification and a δ13C value of-14‰. Photosynthetic O2 evolution in this species was saturated at 2.5% of full sunlight. Two species of Dendrobium (Orchidaceae) from sun-exposed sites, one species exhibiting CAM and the other one exhibiting net CO2 uptake exclusively during daytime via conventional C3 photosynthesis, showed similar light response curves and the same quantum yield for photosynthetic O2 evolution.