Growth and photosynthetic response of nine tropical species with long-term exposure to elevated carbon dioxide.

Seedlings of nine tropical species varying in growth and carbon metabolism were exposed to twice the current atmospheric level of CO for a 3 month period on Barro Colorado Island, Panama. A doubling of the CO concentration resulted in increases in photosynthesis and greater water use efficiency (WUE) for all species possessing C metabolism, when compared to the ambient condition. No desensitization of photosynthesis to increased CO was observed during the 3 month period. Significant increases in total plant dry weight were also noted for 4 out of the 5 C species tested and in one CAM species, Aechmea magdalenae at high CO. In contrast, no significant increases in either photosynthesis or total plant dry weight were noted for the C grass, Paspallum conjugatum. Increases in the apparent quantum efficiency (AQE) for all C species suggest that elevated CO may increase photosynthetic rate relative to ambient CO over a wide range of light conditions. The response of CO assimilation to internal C suggested a reduction in either the RuBP and/or Pi regeneration limitation with long term exposure to elevated CO. This experiment suggests that: (1) a global rise in CO may have significant effects on photosynthesis and productivity in a wide variety of tropical species, and (2) increases in productivity and photosynthesis may be related to physiological adaptation(s) to increased CO.