5.5.1  Introduction

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Phloem loading plays a central role in determining productivity. Photoassimilates are loaded along the entire phloem transport pathway, from photosynthetic leaves to importing sinks. While most photoassimilate loading occurs in photosynthetically active leaves, root-produced metabolites, such as amino acids, move readily from xylem to phloem particularly at the stem nodes (see Case study 5.1). Phloem loading also occurs in storage organs during periods when reserves are remobilised and exported. Indeed, the membrane transport events contributing to phloem loading were first examined using export of sucrose remobilised from the endosperm of germinating castor bean seed as an experimental model (Kriedemann and Beevers 1967).

Events in leaves dominate this account of phloem loading. However, resources supplied from the xylem and during remobilisation of stored reserves will also be considered. An opening analysis of the cellular pathway for assimilate loading is followed by insights into mechanisms and controls of phloem loading.