Water motion and sugar translocation in leaves

We give an overview of the current understanding of the coupled water- and sugar flows in plants with special emphasis on the leaves. We introduce the Münch mechanism and discuss the particularities of osmotically driven flow in the phloem and the consequences for the allometry of the vasculature. This is first done in the context of the entire tree, where we discuss the optimum radius for the phloem tubes, and later for a single needle, where we give a more detailed solution of the osmotic flow profile, allowing us to understand the constraints on needle sizes. We then discuss recent results from microscopy of cross sections along the midvein of a birch leaf, allowing us to measure how the number and radius of the sieve elements depend on the distance from the petiole and compare this to the available area and the minor vein endings in the entire leaf. We finally discuss the pre-phloem water flow in the leaf, i.e. the coupled water/sugar transport from the mesophyll via the bundle sheath into the sieve tubes. We review the distinct sugar loading mechanisms with special emphasis on active symplasmic loading ('polymer trapping'), where one needs to compute water and sugar flow through extremely narrow channels.