Chapter 3 - Long-distance Transport in the Xylem and Phloem

Abstract

This chapter focuses on the long-distance transport of solutes in the xylem and phloem, which plays important roles in shoot nutrition, redistribution of essential elements between tissues during ontogeny, maintenance of charge balance in leaves of nitrate-fed plants, removal of potentially toxic elements from leaf tissues, and the systemic signaling of plant nutritional status. Xylem transport is driven by the gradient in hydrostatic pressure (root pressure) and by the gradient in water potential, whereas long-distance transport in the phloem takes place in the living sieve tube cells. The direction of transport in phloem is bi-directional and it is determined primarily by the nutritional requirements of the various plant organs or tissues. Phloem transport is also an important component in cycling of nutrients between shoots and roots and for signaling the nutritional status of the shoots to the roots. During long-distance transport, elements and organic solutes are transferred between the xylem and phloem by extensive exchange processes. The xylem-to-phloem transfer of nutrients is of particular importance for the mineral nutrition of plants. The transfer of organic and inorganic solutes can take place along the entire pathway from roots to shoot, and here the stem plays an important role. The proportion of xylem-to-phloem transfer in the stem is influenced by the rate of xylem volume flow, with high rates resulting in reduced transfer to the phloem.