A network model of water and sugar transport coupling in leaves

For every molecule of carbon dioxide a plant acquires for photosynthesis, hundreds of molecules of water are lost to the atmosphere. Yet, there is still uncertainty whether the movement of water between the vascular system and the sites of evaporation within the leaf occurs primarily through the cells or through the extracellular space. At the same time, sugars produced by photosynthesis must travel in the opposite direction of net water flow to be loaded into the vasculature and exported to the rest of the plant. To accomplish this, sugar transport must diffuse counter to advection that is generated by large negative water pressures. Here we present a network-based mathematical model accounting for pressure- and osmotically-driven flows to reconcile the two-way traffic of water and sugar in the leaf. In particular, we examine the parameter space in which the water and sugar currents move in opposite directions through the same physical channels. Furthermore, our model is generalizable to diverse anatomical configurations across plant genera, allowing us to also examine the physiology of the enigmatic, extravascular "transfusion tissue" characteristic of conifer needles.