Carbon isotope ratios are correlated with irradiance levels in the Panamanian orchid Catasetum viridiflavum

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Abstract
Carbon isotope composition (13C/12C) in leaves of the Panamanian epiphytic orchid Catasetum viridiflavum were measured on individuals growing in canopies over a water surface to distinguish the effects of a change in source CO2 and humidity from those of intercellular CO2 concentration in determining isotopic composition. Carbon isotope ratios were observed to vary by over 4 in response to changes in total daily photon flux (PFD, 400–700 nm). Changes in isotopic composition of source CO2 or changes in humidity were not likely to have played a role in determining leaf isotopic composition. Observed changed in carbon isotope discrimination (Δ) of leaves experiencing different light levels ranged from 17 to 21. Because leaf nitrogen contents were similar among all orchids, we suggest that the carbon isotope discrimination data indicated that stomatal limitation to photosynthesis increased with increasing irradiance.