Association between carbonyl sulfide uptake and ¹⁸Δ during gas exchange in C₃ and C₄ leaves

Carbonyl sulfide (COS) and C¹⁸OO exchange by leaves provide potentially powerful tracers of biosphere-atmosphere CO₂ exchange, and both are assumed to depend on carbonic anhydrase (CA) activity and conductance along the diffusive pathway in leaves. We investigated these links using C₃ and C₄ plants, hypothesizing that the rates of COS and C¹⁸OO exchange by leaves respond in parallel to environmental and biological drivers. Using CA-deficient antisense lines of C₄ and C₃ plants, COS uptake was essentially eliminated and discrimination against C¹⁸OO exchange (¹⁸Δ) greatly reduced, demonstrating CA's key role in both processes. ¹⁸Δ showed a positive linear correlation with leaf relative uptake (LRU; ratio of COS to CO₂ assimilation rates, A^s/A^c, normalized to their respective ambient concentrations), which reflected the effects of stomatal conductance on both COS and C¹⁸OO exchange. Unexpectedly, a decoupling between As and ¹⁸Δ was observed in comparing C₄ and C₃ plants, with a large decrease in ¹⁸Δ but no parallel reduction in As in the former. This could be explained by C₄ plants having higher COS concentrations at the CA site (maintaining high A^s with reduced CA) and a high phosphoenolpyruvate carboxylase/CA activity ratio (reducing ¹⁸O exchange efficiency between CO₂ and water, but not A^s). Similar A^s but higher A^c in C₄ versus C₃ plants resulted in lower LRU values in the former (1.16 ± 0.20 and 1.82 ± 0.18 for C₄ and C₃, respectively). LRU was, however, relatively constant in both plant types across a wide range of conditions, except low light (>191 μmol photon m^-2 s^-1).