Effect of Salinity and Humidity on δ13C Value of Halophytes: Evidence for Diffusional Isotope Fractionation Determined by the Ratio of Intercellular/Atmospheric Partial Pressure of CO₂ under Different Environmental Conditions

Seedlings of two mangrove species, Avicennia marina and Aegiceras corniculatum, were grown in a range of salinities and humidities in controlled environment chambers, and Phaseolus vulgaris plants were grown in the glasshouse. The fractionation of carbon isotopes in the three species was correlated with the ratio of intercellular and ambient partial pressures of CO₂. The results are consistent with fractionation being due both to diffusion in air and to carboxylation in the leaf. It was concluded that the latter process discriminates against13CO2relative to^{12}\text{CO}{}_{2}$by about 27‰.