The Transport Phloem. Specifics of its Functioning

Abstract

Translocation of photoassimilates from the source leaves to the heterotrophic parts of the plant demands three different functions executed by the phloem system. The photosynthate is collected in the source leaves (phloem loading), translocated from source to sink (phloem transport), and delivered to the sink tissues (phloem unloading). It is expected that anatomy and physiology of the particular phloem sections reflect the specific tasks to be carried out. Investigantions over the past 10 years revealed a diversity of anatomical settings in sources and sinks which seem to correspond with a multiplicity of mechanisms. Several reviews and opinion papers were recently devoted to the phloem sections engaged in loading. The functioning of the transport phloem has gained much less attention, although some silent conceptual progress has been made over the past decade. Only aspects such as the photosythate unloading from the transport phloem and the relation between transport phloem and solute transfer through rays have been reviewed recently.