Is mesophyll conductance to CO2 in leaves of three Eucalyptus species sensitive to short-term changes of irradiance under ambient as well as low O2?

Mesophyll conductance to CO2 (g(m)) limits the diffusion of CO2 to the sites of carboxylation, and may respond rapidly (within minutes) to abiotic factors. Using three Eucalyptus species, we tested the rapid response of g(m) to irradiance under 21% and 1% O-2. We used simultaneous measurements of leaf gas exchange and discrimination against (CO2)-C-13 with a tuneable diode laser absorption spectrometer. Measurements under 1% O-2 were used to limit uncertainties due to C-13-C-12 fractionation occurring during photorespiration. Switching irradiance from 600 to 200 mu mol m(-2) s-1 led to a approximate to 60% decrease of g(m) within minutes in all species under both 21% O-2 and 1% O-2. The g(m) response to irradiance is unlikely to be a computation artefact since using different values for the parameters of the discrimination model changed the absolute values of g(m) but did not affect the relative response to irradiance. Simulations showed that possible rapid changes of any parameter were unable to explain the observed variations of g(m) with irradiance, except for C-13-C-12 fractionation during carboxylation (b), which, in turn, is dependent on the fraction of leaf C assimilated by phospho-enol pyruvate carboxylase (PEPc) (beta). g(m) apparently increased by approximate to 30% when O-2 was switched from 21% to 1% O-2. Again, possible changes of beta with O-2 could explain this apparent g(m) response to O-2. Nevertheless, large irradiance or O-2-induced changes in beta would be required to fully explain the observed changes in g(m), reinforcing the hypothesis that g(m) is responsive to irradiance and possibly also to O-2.