Similar photosynthetic response to elevated carbon dioxide concentration in species with different phloem loading strategies

Species have different strategies for loading sugars into the phloem, which vary in the route that sugars take to enter the phloem and the energetics of sugar accumulation. Species with passive phloem loading are hypothesized to have less flexibility in response to changes in some environmental conditions because sucrose export from mesophyll cells is dependent on fixed anatomical plasmodesmatal connections. Passive phloem loaders also have high mesophyll sugar content, and may be less likely to exhibit sugar-mediated down-regulation of photosynthetic capacity at elevated CO₂ concentrations. To date, the effect of phloem loading strategy on the response of plant carbon metabolism to rising atmospheric CO₂ concentrations is unclear, despite the widespread impacts of rising CO₂ on plants. Over three field seasons, five species with apoplastic loading, passive loading, or polymer-trapping were grown at ambient and elevated CO₂ concentration in free air concentration enrichment plots. Light-saturated rate of photosynthesis, photosynthetic capacity, leaf carbohydrate content, and anatomy were measured and compared among the species. All five species showed significant stimulation in midday photosynthetic CO₂ uptake by elevated CO₂ even though the two passive loading species showed significant down-regulation of maximum Rubisco carboxylation capacity at elevated CO₂. There was a trend toward greater starch accumulation at elevated CO₂ in all species, and was most pronounced in passive loaders. From this study, we cannot conclude that phloem loading strategy is a key determinant of plant response to elevated CO₂, but compelling differences in response counter to our hypothesis were observed. A phylogenetically controlled experiment with more species may be needed to fully test the hypothesis.