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 in 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. Investigations 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 (Delrot 1987; Van Bel 1987, 1989, 1992; Gamalei 1989, 1990, 1991; Turgeon 1989; Turgeon and Beebe 1991; Van Bel and Gamalei 1991) and unloading (Murray 1987; Turgeon 1989; Patrick 1990; Oparka 1990; Wolswinkel 1990). 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 photosynthate unloading from the transport phloem (Patrick 1990) and the relation between transport phloem and solute transfer through rays (Van Bel 1990) have been reviewed recently.
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