Crassulacean acid metabolism in australian vascular epiphytes and some related species

Summary

The occurrence of crassulacean acid metabolism (CAM) among epiphytes and related plant species from tropical and subtropical rainforests in Eastern Australia was investigated. As judged from δ¹³C value and the absence of Kranz anatomy, indications of CAM were found in 66 species belonging to the families, Polypodiaceae (3), Orchidaceae (55), Asclepiadaceae (6) and Rubiaceae (2).

Two thirds of orchidaceous plants examined appeared to use CAM. Those species with thicker leaves generally had less negative δ¹³C values, as did those species growing on more exposed sites; leaves thicker than about 1 mm in most species yielded δ¹³C values indicative of pronounced CAM. Two leafless species, Chiloschista phyllorhiza and Taeniophyllum malianum, which depend on chloroplast-containing, stomata-less roots for photosynthesis also showed δ¹³C values typical of CAM. Pseudobulbs and swollen stems, a characteristic of many orchids, were usually somewhat enriched in ¹³C compared to corresponding leaves.

In Polypodiaceae CAM was found in the genus Pyrrosia. While δ¹³C values were generally less negative with increasing frond thickness, the leaf morphology was extremely variable within species. Pyrrosia confluens plants from shaded habitats had long, relatively thin and darkgreen fronds whereas specimens from sun-exposed sites were characterized by short, thick, bleached fronds. Both types showed the capacity for nocturnal accumulation of titratable acidity and exhibited continuous net CO₂ fixation during 12 h light/12 h dark cycles under laboratory conditions. Shade-fronds showed this capacity even when irradiance was lower than 2% of full sunlight during the 12 h light period.

In Asclepiadaceae CAM was found in species of two genera which usually have fleshy leaves, Hoya and Dischidia. In Rubiaceae CAM was recorded in two genera of epiphytic ant plants, Hydnophytum and Myrmecodia.

It is concluded that CAM is widespread in Australian epiphytes. It is most prevalent in species found in exposed microhabitats where the growing conditions are characterised by relatively high light intensities and short but frequent periods of water stress.