Effects of carbonyl sulfide and carbonic anhydrase on stomatal conductance

The potential use of carbonyl sulfide (COS) as tracer of CO₂ flux into the land biosphere stimulated research on COS interactions with leaves during gas exchange. We carried out leaf gas-exchange measurements of COS and CO₂ in 22 plant species representing deciduous and evergreen trees, grasses, and shrubs, under a range of light intensities, using mid-infrared laser spectroscopy. A narrow range in the normalized ratio of the net uptake rates of COS (A^s) and CO₂ (A^c), leaf relative uptake (A^s/A^c 3 [CO₂]/[COS]), was observed, with a mean value of 1.61 ± 0.26, which is advantageous to the use of COS in photosynthesis research. Notably, increasing COS concentrations between 250 and 2,800 pmol mol21 (enveloping atmospheric levels) enhanced stomatal conductance (g_s) to a variable extent in most plants examined (up to a normalized enhancement factor [f_e = (g_s-max - g_s-min)/gs-min] of 1). This enhancement was completely abolished in carbonic anhydrase (CA)-deficient antisense lines of both C3 and C4 plants. We suggest that the stomatal response is mediated by CA and may involve hydrogen sulfide formed in the reaction of COS and water with CA. In all species examined, the uptake rates of COS and CO₂ were highly correlated, but there was no relationship between the sensitivity of stomata to COS and the rate of COS uptake (or, by inference, hydrogen sulfide production). The basis for the observed stomatal sensitivity and its variations is still to be determined.