uations, unloading includes transfer of assimilates to a parasite or symbiont. Clearly, the pathway must
be apoplastic if the sink is a separate organism, for no plasmodesmata exist between parent and seed or
between a host and parasite.
Thorne [290] presented several mechanisms of phloem unloading:
- Sucrose is passed into sink cells via plasmodesmata. Once in sink cells, sucrose is degraded into
hexose in either the cytoplasm or vacuole of those cells. This type of unloading is reported to oc-
cur in sugar beet sink leaves, corn root tips, and bean endocarp. - Sucrose is unloaded into the apoplast, where it is degraded into hexoses. Hexoses are transferred
into sink cell cytoplasm, where sucrose is resynthesized. Sucrose is then accumulated in the vac-
uole. This mechanism has been reported for assimilate movement into corn and sorghum ker-
nels, sugarcane stalks, and into some parasites [130]. - Sucrose is unloaded from sieve tubes into the apoplast, transferred into sink cell cytoplasm, and
then accumulated in vacuoles. This has been reported for sugar beet tap root, legume seeds, and
wheat grain.
Eschrich [294] has added a fourth mechanism: carbohydrates follow any of these pathways, then
are incorporated into starch. It could be argued that any metabolic removal of sugar would be analo-
gous. One could also visualize various combinations of steps of listed processes to obtain even more
categories.
From a thermodynamic standpoint, no energy should be required for transfer of sugar across sieve
tube or sink cell plasmalemma or through the plasmodesmata [e.g., 295], for other processes maintain
appropriate gradients along the pathways such that diffusion can account for movement. Wang and
Fisher [296] presented evidence that metabolic energy is not used in movement of sucrose into the en-
dosperm cavity of developing wheat grains; however, metabolic energy may be a mechanistic require-
ment in some circumstances. A possible exception may exist for citrus fruit, for Koch and Avigne
[297] reported that the rate of sucrose imported into juice sacs peaked early in fruit development while
it was moving against its concentration gradient; then the rate fell as equillibrium was approached. It
then rose to a second higher peak after the concentration in vascular bundles had exceeded that in juice
sacs. This would imply a requirement for metabolic energy early in developmental, though the energy-
requiring step may involve accumulation of sucrose in vacuoles. This bimodal accumulation pattern
suggests that two separate mechanisms of sucrose transfer were involved. Furthermore, metabolic re-
quirements of corn root tips beyond the terminus of mature sieve tubes cannot be satisfied by diffusion
through plasmodesmata [298]. Therefore, it was proposed that solution flow through plasmodesmata
might be the mechanism of supply, although current ideas on structure of plasmodesmata would seem
to make that unlikely.
As stated earlier, unloading of assimilates into a developing embryo must involve transfer of as-
similates into the apoplast. Thorne and Rainbird [299] developed a technique for studying this process.
While leaving the seed coat attached to the parent plant, they removed the embryo. They demonstrated
that unloading of assimilate into agar in the seed coat proceeded at a rate comparable to that of un-
loading into an intact embryo. They also demonstrated that the nonpenetrating sulfhydryl reagent
PCMBS inhibited unloading into agar-filled seed coat cavity but not transfer of assimilates into the seed
coat, thus indicating that transfer of assimilates into seed coat cells was symplastic. However, NaF and
NaAsO 2 inhibited both processes, indicating an energy dependence of a portion of the unloading pro-
cess. Wang and Fisher [300] developed an analogous process for studying transfer of sucrose to wheat
endosperm cavities. The technique of embryo removal has been used extensively to study the unload-
ing process, and data indicate that assimilates are transferred from sieve tubes symplastically through
several cell layers before being deposited into the apoplast, either at the surface of the embryo/en-
dosperm or several cell diameters from that surface. Once at the embryo/endosperm surface, assimilates
are absorbed into developing cells [299,301]. Oparka [302] has referred to this as “apoplastic unload-
ing sink” rather than “apoplastic unloading.” He restricts the latter to unloading of assimilates into the
apoplast directly from sieve tubes.
A compilation from the literature by Fisher and Oparka [303] indicates that symplastic unloading of
the ST-CC is more common than apoplastic; however, eventual transfer to the apoplast must occur dur-
ing seed growth.
PRODUCTION-RELATED ASSIMILATE TRANSPORT 439