Relationships between carbonyl sulfide (COS) and CO2 during leaf gas exchange

P>Carbonyl sulfide (COS) exchange in C-3 leaves is linked to that of CO2, providing a basis for the use of COS as a powerful tracer of gross CO2 fluxes between plants and the atmosphere, a critical element in understanding the response of the land biosphere to global change. Here, we carried out controlled leaf-scale gas-exchange measurements of COS and CO2 in representative C-3 plants under a range of light intensities, relative humidities and temperatures, CO2 and COS concentrations, and following abscisic acid treatments. No 'respiration-like' emission of COS or detectable compensation point, and no cross-inhibition effects between COS and CO2 were observed. The mean ratio of COS to CO2 assimilation flux rates, As/Ac, was c. 1.4 pmol mu mol-1 and the leaf relative uptake (assimilation normalized to ambient concentrations, (As/Ac)(C(a)c/C(a)s)) was 1.6-1.7 across species and conditions, with significant deviations under certain conditions. Stomatal conductance was enhanced by increasing COS, which was possibly mediated by hydrogen sulfide (H2S) produced from COS hydrolysis, and a correlation was observed between As and leaf discrimination against C18OO. The results provide systematic and quantitative information necessary for the use of COS in photosynthesis and carbon-cycle research on the physiological to global scales.