Testing the responses and interplay of leaf physiological and morphological traits at elevated CO2 levels in six common crop species

Eco-evolutionary optimality (EEO) states that plants adapt or acclimate to their environment, thereby eliminating uncompetitive plant strategies by natural selection. EEO has been proven successful for developing hypotheses and models of the terrestrial biosphere. On a plant leaf level, EEO theory is used to analyze and model plant processes including photosynthesis, gas exchange, and stomatal behavior. Plants regulate their gas exchange by dynamically adjusting their stomata on a short term time scale (opening and closing) and long term time scale (stomatal size and density), which also influences photosynthetic capacity. The operational stomatal conductance (Gop) is determined by the opening state of the stomata during typical growth conditions. The anatomical maximum stomatal conductance (Gsmax) results from the maximum stomatal aperture, stomatal density and pore depth. According to the work of McElwain et al. (2016), plants operate at the conservative  Gop:Gsmax ratio of ~0.25, which means that they utilize only a fraction of their anatomical potential. Yet, it is currently unknown whether conservation of Gop:Gsmax can be explained from EEO theory.

To further investigate this interesting coupling between leaf physiology and morphology in an EEO context, we conducted an experiment to gain insight into the differences in gas exchange, photosynthesis, morphology and Gop:Gsmax ratio resulting from acclimation to shifts in atmospheric CO₂ growth conditions. Plants of six common crop species were grown in ambient (400ppm) and elevated (1000ppm) CO₂ growth chambers. Species include four eudicots (including one woody species) and two monocots (one C3 and one C4 photosynthesis species), enabling an assessment of adaptation in species with different photosynthetic mechanisms and stomatal morphologies. For all species, a diurnal cycle, leaf mass per area, ACi response curves, light response curves, and Gop were measured. Additionally, imprints of the leaves were taken to derive Gsmax from microscope analysis.

Preliminary results show that exposure to elevated CO₂ leads to a decline in Gop, Gsmax and photosynthetic capacity, in-line with EEO theory. Results of one C3 eudicot showed the expected lower Vcmax, Jmax, and stomatal density at elevated atmospheric CO₂ concentrations. There was also a small decrease in Gop compared to the ambient group for this species. Overall, the Gop:Gsmax ratio of the elevated atmospheric CO₂ treatment was slightly higher than at ambient levels. Combining gas exchange and the ACi curves shows a shift of Gop towards the high sensitivity region where small changes in leaf internal CO2 concentration result in a relatively large change in net photosynthesis rate. Further analysis, including an assessment of adaption to atmospheric CO₂ in the other species, will reveal the overall responses of the small but diverse group of plants in this experiment, and potential differences in strategy between species with different photosynthetic mechanisms and stomatal traits. This will improve our understanding of EEO theory across different species and environmental conditions.