Like minor vein phloem loading, sieve tube unloading into sinks can occur by either symplastic or
apoplastic routes (Figure 7B). Unloading into buffering sinks such as the sugar beet taproot [84] and sug-
arcane stem [85] occurs via an apoplastic route. Unloading of sugar into the apoplast from the sieve tubes
lowers the hydrostatic pressure and also promotes the flow of water out of the sieve element, thus allow-
ing bulk flow to occur from source to sink. Unloaded solutes are then taken up into the sink cell, where
compartmentation into the vacuole or conversion to insoluble starch (Figure 7A) can further dissipate the
hydrostatic pressure between the phloem and the sink organ. In some cases, sucrose is hydrolyzed prior
to uptake into the sink cell, and in other cases it may be taken up intact, then hydrolyzed in the vacuole
(Figure 7A).
- Terminal Sinks
Unlike the buffering sinks, terminal sinks, as the name implies, act as sinks only for assimilates. Carbon
partitioned to terminal sinks is unavailable for remobilization out of those sinks, usually because it is in-
corporated into structural, as opposed to storage, components. Prime examples of terminal sinks are re-
productive tissues, such as fruits and seeds, and rapidly growing meristems. Carbon partitioned to these
sinks cannot be reaccessed by the plant, even if the carbon is stored in a conventional storage form such
as starch. Sinks of these types, therefore, can exert a strong regulatory influence on phloem transport by
controlling the low end of the hydrostatic pressure gradient created in the sieve tubes.
Phloem unloading in terminal sinks occurs by either apoplastic or symplastic routes (Figure 7). In
rapidly growing meristematic organs such as developing roots [86] and leaves [87,88], unloading occurs
via the symplast. The conversion of imported assimilates to insoluble structural components, principally
cellulose, and other polymers (protein, nucleic acid, etc.) and their rapid utilization as respiratory sub-
462 MIRANDA ET AL.Figure 7 Pathways for phloem unloading in sink tissues: (A) apoplastic and (B) symplastic.