Long-distance transport in the xylem and phloem

Long-distance transport of solutes in the xylem and phloem is important for shoot and root nutrition, the redistribution of essential elements between tissues during ontogeny, the maintenance of charge balance in leaves of nitrate-fed plants, the removal of potentially toxic elements from leaf tissues, and the systemic signaling of plant nutritional status. This chapter describes the anatomy of the xylem and phloem, the composition of xylem and phloem saps, and the movement of (1) xylem sap from root to shoot in response to gradients of water potential generated by root pressure and transpiration and (2) phloem sap from source to sink tissues in response to osmotic gradients generated by differences in phloem sucrose concentration. Emphasis is placed on the pathways of solute movement within the plant and recent insight into the transport proteins catalyzing the loading and unloading of elements to and from the xylem and phloem.