volume.) A hypothetical model reconciles both the odd ultrastructural features (i.e., the ubiquitous pres-
ence of the symplastic links between the photosynthetic cells and the minor vein intermediary cells) and
the rather unusual carbohydrate biochemistry of type 1 plants [73].
The crux of this model (Figure 6B) is the hypothesis that the pore size of the plasmodesmata con-
necting the intermediary cells within the photosynthetic cells is wide enough to allow passage of disac-
charides such as sucrose and galactinol only, not their oligosaccharide products, the tri- and tetrasaccha-
rides raffinose and stachyose (Figure 6B). Thus, when stachyose and raffinose are synthesized within the
intermediary cells, they cannot move anywhere, except into the adjacent sieve tubes. This “polymeriza-
tion trap” model remains to be proved [74], and the details of compartmentation of the stachyose reac-
tions within the intermediary cell need to be elucidated, but the model does give a feasible explanation of
how symplastically linked cells might operate in establishing a solute gradient. In Cucumis meloL., the
sugar levels in individual cells are consistent with the operation of a polymer trap [75].
However, many plant species can be classified as having type 1 minor vein anatomy and yet do not
transport raffinose oligosaccharides. One example is parsley, which translocates only sucrose and an even
smaller molecular weight polyol, mannitol [29], and yet appears to have an “open” minor vein structure.
Similarly, willow (Salix babylonica) possesses a symplastically linked minor vein structure and yet
translocates only sucrose [76]. Plasmolysis and sugar distribution studies on willow leaves showed no
positive concentration gradient between the mesophyll and the minor veins of this species, and so it was
concluded that short-distance transport of photoassimilates must be entirely diffusional into the phloem.
In this case, long-distance transport would be reliant upon the capacity of sink tissues to remove solute
from phloem and maintain a positive pressure potential gradient. In contrast, Moing et al. [77] have sug-
gested, on the basis of PCMBS sensitivity, that peach leaves, which transport sucrose and sorbitol and
which have a symplastically open minor vein system, still apparently use an apoplastic pathway for
phloem loading.
C. Loading of Other Solutes
Because sugars are the predominant solutes translocated in the phloem, most of what is known about
phloem loading concerns the movement of sugars into the phloem. Relatively little is known of either the
pathways taken or the mechanisms used to load the other component solutes characteristically found in
phloem saps. There is evidence for the operation of proton–amino acid transporters in plant tissues [61],
but whether these are phloem tissue specific is not known. It is likely that active accumulation of the
potassium ion takes place in exchange for protons, but the carrier(s) involved has not yet been character-
ized. How other ions enter the phloem is not clear. It is quite likely that further studies will reveal that
phloem sap composition in crop plants is determined by means of a combination of apoplastic and sym-
plastic transport.
V. REGULATION OF PHLOEM TRANSPORT
Despite the great numbers of different sink tissues and organs that constitute a typical plant, most plants
tend to maintain a balanced ratio of shoot tissue to root tissue. This indicates that the plant has some means
of regulating the amount of photoassimilate that is delivered to developing roots and shoots and that some
metabolic control exists to control the direction of phloem transport. Because rate and direction of phloem
transport are dictated by the solute gradients between sources and sinks, the regulatory mechanisms can
exist either at the source end (where assimilates are loaded) or at the sink end (where assimilates are re-
moved from the phloem).
A. Regulation by Sources
Source leaves are the primary sites of photoassimilate production, but the plant faces a dilemma with re-
spect to allocation choices of photoassimilates. Because the photosynthetic period does not encompass
the entire diurnal period but the demand for assimilates does, source tissues must conserve part of the car-
bon fixed during photosynthesis for use during nonphotosynthetic periods. The role of the source leaf in
controlling phloem transport is therefore one of allocation, assigning fixed carbon to export or storage
pools in such a manner that export can be maintained at some “set point” level throughout the diurnal pe-
460 MIRANDA ET AL.