Radial-axial transport coordination enhances sugar translocation in the phloem vasculature of plants

Understanding the controls of mass transport of photosynthates in the phloem of plants is necessary for describing plant carbon allocation, productivity, and responses to water and thermal stress. While several hypotheses about optimization of phloem structure and function, and limitations of phloem transport under drought have been tested both with models and anatomical data, the true impact of radial water exchange of phloem conduits with their surroundings on mass transport of photosynthates has not been addressed. Here the physics of the Munch mechanism of sugar transport is re-evaluated to include local variations in viscosity resulting from the radial water exchange in two dimensions (axial and radial). Model results show that radial water exchange pushes sucrose away from conduit walls thereby reducing wall frictional stress due to a decrease in sap viscosity and increasing sugar concentration in the central region of the conduit. This leads to increased sugar front speed and axial mass transport for a wide range of phloem conduit lengths and allows sugar transport to operate more efficiently than predicted by previous models. A faster front speed leads to higher phloem resiliency under drought because more sugar can be transported with a smaller pressure gradient.