Canopy CO2 concentrations and Crassulacean acid metabolism in Hoya carnosa in a subtropical rain forest in Taiwan: consideration of CO2 availability and the evolution of CAM in epiphytes
The potential importance of CO 2 derived from host tree respiration at night as a substrate for night time CO 2 uptake during CAM was investigated in the subtropical and tropical epiphytic vine Hoya carnosa in a subtropical rainforest in north-eastern Taiwan. Individuals were examined within the canopies of host trees in open, exposed situations, as well as in dense forests. Although night time CO 2 concentrations were higher near the epiphytic vines at night, relative to those measured during
... se measured during the day, presumably the result of CO 2 added to the canopy air by the host tree, no evidence for substantial use of this CO 2 was found. In particular, stable carbon isotope ratios of H. carnosa were not substantially lower than those of many other CAM plants, as would be expected if host-respired CO 2 were an important source of CO 2 for these CAM epiphytes. Furthermore, laboratory measurements of diel CO 2 exchange revealed a substantial contribution of daytime CO 2 uptake in these vines, which should also result in lower carbon isotope values than those characteristic of a CAM plant lacking daytime CO 2 uptake. Overall, we found that host-respired CO 2 does not contribute substantially to the carbon budget of this epiphytic CAM plant. This finding does not support the hypothesis that CAM may have evolved in tropical epiphytes in response to diel changes in the CO 2 concentrations within the host tree canopy. Abbreviations: CAM -Crassulacean acid metabolism; PDB -Pee Dee belemnite; PPFD -photosynthetic photon flux density; WUE -water-use efficiency. Acknowledgments: Assistance in the field was gratiously provided by Ya-Hui Chang, Jih-horng Chunaung, Chao Hui-te, and Gene-Sheng Tung. Expert statistical advice was cheerfully offered by John Kelly. Special thanks go to Yi-Tzeng Her and Yau-tz Tang for measuring the in situ CO 2 concentrations and to Erin Wessely for assistance with gas exchange measurements in the laboratory. Financial assistance (project number NSC90-2621-B-018-001-A10) from the National Science Council (Taiwan) is gratefully acknowledged.