Phloem Loading As a Driver of Plant Photosynthetic Responses to Rising Atmospheric CO2.

Increased atmospheric [CO₂] stimulates photosynthesis of C₃ plants, leading to increased biomass and crop yield. At elevated [CO₂], however, sugars can accumulate in leaves and negatively feedback on photosynthesis. This phenomenon has only been tested in a narrow range of species, mostly those which use proton-sucrose symporters to accumulate sucrose in the phloem (apoplastic loading). Other species use passive diffusion along a sucrose gradient from leaf mesophyll cells to phloem (passive loading) and therefore usually require higher mesophyll sucrose concentrations than apoplastic loaders. The hypothesis for this experiment was that passive loaders, adapted to high mesophyll sucrose concentrations, would experience less sugar-mediated feedback of photosynthesis at elevated [CO₂] compared to apoplastic loaders. Both field experiments and meta-analysis of previous literature were used to test this hypothesis.  First, pea and beet (apoplastic phloem loaders) and strawberry and peony (passive phloem loaders) were grown at elevated [CO₂] in the field in 2013 and 2014. Decreases in photosynthetic capacity were observed in 2014, but not 2013, in all species. This corresponded to an increase in soluble sugar content in all species at elevated [CO₂] in 2014, but not in 2013. When photosynthesis measurements were taken at different CO₂ concentrations (A/c_i curves), the passive loading species transitioned from Rubisco- to electron transport-limited photosynthesis at much higher CO₂ concentrations than apoplastic loading species when grown at both ambient and elevated [CO₂]. A meta-analysis of previous literature was performed to determine whether the field results are consistent for broader range of species with different phloem loading strategies.