Experimental Evidence for Coordinated Leaf Trait Responses to Elevated CO2 in Five Common Crop Species

Understanding leaf trait responses to changing atmospheric CO2 concentrations ([CO2]) is essential for land- surface modelling. Eco-evolutionary optimality (EEO) theory predicts coordinated responses in leaf traits that maintain a balance of maximum carbon assimilation gain with minimal summed resource use costs. Although individual leaf trait responses to atmospheric [CO2] changes are relatively well understood, few experiments have documented coordination of responses across functional leaf trait categories related to photosynthesis, photosynthetic biochemistry, stomatal conductance (gs), and morphology. We examined leaf trait coordination in five common crop species, including buckwheat (C3), common bean (C3), maize (C4), soybean (C3), and wheat (C3). We specifically included traits relevant for testing EEO theory, including diurnal measurements of gs. Our results showed that among the four C3 species, gs and photosynthetic biochemistry traits were generally downregulated under elevated atmospheric [CO2], while photosynthetic rates increased, in line with P-model predictions. A principal component analysis (PCA) on the response ratios across C3 species revealed coordinated trait variation along two main axes: principal component (PC) 1 mainly described leaf economic traits and PC2 was mainly related to photosynthetic biochemistry and hydraulic capacity, indicating partial decoupling of leaf trait coordination. Intraspecific variation can arise from differences in photosynthetic pathways, as shown by the distinctive responses of the C4 species maize, with potential consequences for trait coordination. Our results provide support for EEO theory on predictions of leaf trait combinations and vegetation patterns under future climates.