is not affected by PCMBS [12,64,65]. This is true despite the obvious sensitivity to inhibition by PCMBS
of the uptake of exogenously supplied sugars into leaf tissues [12,66]. This observation suggests that al-
though a PCMBS-sensitive proton symport mechanism may exist in type 1 plants, it is not utilized for
loading of assimilates into the phloem.
If an apoplastic step is not involved in phloem loading in type 1 plants, some alternative mechanism
must be invoked to create the high sieve element solute levels necessary for phloem transport. Plasmoly-
sis studies clearly show that high solute levels do exist in the phloem of type 1 plants [67]. The answer to
this dilemma may come from the observations that all type 1 plants export raffinose family oligosaccha-
rides such as stachyose in the phloem [6–8] and that synthesis of the oligosaccharides destined for export
most likely occurs within the intermediary cells characteristic of the type 1 morphology [68–71].
The biochemistry of raffinose sugar biosynthesis is somewhat anomalous compared with that of
other sugars in that the galactose donor is not a sugar nucleotide but a simple disaccharide, galactinol [72].
(For more details of this biochemistry, readers are referred to the chapter on carbohydrate synthesis in this
PHLOEM TRANSPORT OF SOLUTES IN CROP PLANTS 459
Figure 6 Diagrammatic representations of the processes involved in loading of the phloem via the apoplast
or symplast. (A) In apoplastic loading, sucrose leaves the photosynthetic cell and enters the cell wall space. It
is then taken up across the plasma membrane of the sieve element–companion cell (SE-CC) complex by a su-
crose carrier (solid circle), which transports sucrose in conjunction with a proton (H). The proton gradient is
established by proton extrusion via a plasma membrane ATPase (open circle). (B) In symplastic loading, dis-
accharides (sucrose, galactinol) are passed through the plasmodesmata from the photosynthetic cells to the in-
termediary cells, where the galactose residues () of galactinol are transferred to sucrose to form the trisac-
charide (raffinose) and tetrasaccharide (stachyose) with the release of myo-inositol (). The tri- and
tetrasaccharides then pass into the sieve element (SE) but are prevented from passing back into the photosyn-
thetic cell by the smaller diameter of the plasmodesmata connecting this cell to the intermediary cell.