Does Phloem Loading Strategy Alter Plant Response to Rising [CO2]?.

Atmospheric [CO₂] has risen significantly since the industrial revolution and will continue to rise this century. Species vary in response to rising atmospheric [CO₂] and this work tests the hypothesis that phloem loading strategy will be an important determinant of photosynthetic response in the future. There are three different strategies that plants use to generate pressure driven mass flow of assimilates in the phloem. The first, apoplastic phloem loading, is characterized by the use of proton motive force to accumulate sucrose in the phloem via proton-sucrose symporters. The resulting osmotic gradient drives water into the phloem which then generates high hydrostatic pressure that drives mass flow. The second strategy, polymer trapping, is characterized by the synthesis of raffinose and stachyose in companion cells to create a concentration difference between the phloem and mesophyll. As with apoplastic loading, this draws in water creating high turgor that then drives mass flow. The third strategy (passive loading) involves phloem translocation that is driven by high turgor in both mesophyll and phloem cells with no differential accumulation of solutes in the phloem. While much is known about plant responses to elevated [CO₂], the response of species with different phloem loading strategies to elevated [CO₂] has not been explicitly investigated. Plants with different phloem loading strategies have distinct foliar sugar signatures and this could interact with the sugar-mediated acclimation of photosynthetic capacity - a key element of plant responses to elevated [CO₂]. In this study we probe published literature for species photosynthetic responses to elevated [CO₂], comparing the response of species with different phloem loading strategies.