C₃ plants converge on a universal relationship between leaf maximum carboxylation rate and chlorophyll content

Abstract. The leaf maximum carboxylation rate (Vcmax) is one of the crucial parameters in determining the photosynthetic capacity of plants. Providing accurate estimates of leaf Vcmax₂₅ that cover large geographic areas and that incorporate plant seasonality is central to correctly predicting carbon fluxes within the terrestrial global carbon cycle. Chlorophyll, as the main photon-harvesting component in leaves, is closely linked to photosynthesis. However, how the nature of the relationship between the leaf maximum carboxylation rate scaled to 25 °C (Vcmax₂₅) and leaf chlorophyll content varies according to plant type is uncertain. In this study, we investigate whether a universal and stable relationship exists between leaf Vcmax₂₅ and leaf chlorophyll content across different C₃ plant types from a plant physiological perspective and verify it using field experiments. Measurements of leaf chlorophyll content (Chl) and CO₂ response curves were made on 283 crop, shrub, tree and vegetable leaves in China and the Borden Forest Research Station in southern Ontario, Canada. A strong relationship was found between the leaf Vcmax₂₅ and chlorophyll content across different C₃ plant types (R² = 0.65, P < 0.001). Validation showed that the model performs well, producing relatively low normalized root mean square errors (NRMSE) for crops (0.14), shrubs (0.17), trees (0.12) and vegetables (0.45). The results confirm that leaf chlorophyll content can be a reliable proxy for estimating Vcmax₂₅ across different C₃ plant types over space and time, opening the door to accurate spatially continuous estimates of Vcmax₂₅ at the global scale.