Leaf chamber experiments on sunflowers indicate a temperature-dependent compensation point of carbonyl sulfide

Background: Carbonyl Sulfide (COS) is a potential tracer for estimating gross primary productivity (GPP), due to its co-uptake with CO ₂ in leaves and the assumed absence of re-emission. However, the effectiveness of COS as a GPP tracer depends on understanding the differential responses of COS and CO ₂ uptake to environmental factors such as temperature and humidity. Methods We conducted three sets of leaf gas exchange experiments on sunflower leaves. In each experiment, we varied only one environmental factor: COS mole fraction (at two temperatures), humidity, or temperature. During the experiments, COS and CO ₂ fluxes were measured, and the data were used to optimize a leaf conductance model. Results: We identified the existence of a COS compensation point, which increases with higher temperatures, suggesting potential emissions at higher temperatures when atmospheric COS concentrations are low. Our gas exchange measurements detected a COS compensation point of 58.9 ± 52.4 pmol mol ⁻¹ at 20°C and 139.9 ± 26.0 pmol mol ⁻¹ at 25°C. As vapor pressure deficit increased and stomatal conductance decreased, we observed that COS leaf uptake decreased more rapidly than CO ₂ assimilation. Consequently, the leaf relative uptake ratio (LRU) of COS to CO ₂ also decreased when stomatal conductance decreased. The optimized conductance model indicated that the optimum temperature for COS and CO ₂ enzymatic uptake was around 35°. However, the maximum net deposition velocity for COS lies between 20 and 25°, due to its temperature-dependent compensation point.