CAM plasticity in epiphytic tropical orchid species responding to environmental stress.

To counteract its dramatic species endangerment caused by extensive loss of habitat, Singapore is currently re-introducing into nature some of the native orchids to conserve and improve their germplasm. A main challenge of re-introduction is growing and establishing these plants under natural conditions, which are semi-arid with periodic drought. In this study, six native species were examined, of which three, Bulbophyllum vaginatum, Dendrobium leonis and Phalaenopsis cornu-cervi, are viewed as CAM species while the other three, Coelogyne rochussenii, Coelogyne mayeriana, and Bulbophyllum membranaceum are usually characterized as C₃ species. We aimed to compare their physiological responses to drought under two different light conditions: (1) moderate light (photosynthetic photon flux density, PPFD of 900 μmol m^-2 s^-1) and (2) low light (PPFD < 100 μmol m^-2 s^-1).

After 7 weeks of drought under moderate light (DRML), photosynthetic light utilization was reduced in all six species, and relative water content (RWC) in leaves decreased to < 50% in CAM orchids, compared to > 50% in C₃ species, while RWC in pseudobulbs (produced by 4 of the species) fell to < 50%. Both effects were reversed after 14 weeks of re-watering. Proline concentration in leaves increased in the CAM orchids and B. membranaceum (60-130 µmol g^-1 FW), and CAM acidity increased (0.2 to 0.8 mmol H⁺/g fresh weight) in leaves and pseudobulbs of most species including C₃ orchids after 7 weeks of DRML, but to lesser extent in B. membranaceum.

In the six native orchid species tested, osmoregulation by proline and CAM expression were adaptive responses to maintain photosynthesis under drought stress. Expression of CAM is a significant adaptive mechanism to drought in both C₃ and CAM orchids. For C₃ B. membranaceum, this CAM activity is best described as 'CAM-idling'. We propose that any future work in understanding adaptive responses in Singapore's native epiphytic orchids to periodic water deficit should also analyse the significance of CAM plasticity on water conservation within the plant and the regulation of CAM by prevailing water status and light intensity.