out from phloem tissue. 6. Membrane transport: Sugars
unloaded apoplastically are taken up in cells by a
transporter on the plasma membrane. 7. Metabolic
conversion: Translocation sugars unloaded in fruit by
the symplasmic or apoplastic pathway are converted to
various substances; first, invertase and sucrose synthase
metabolize sucrose, sorbitol dehydrogenase metabolizes
sorbitol, and then sucrose phosphate synthase synthesizes
sucrose. 8. Compartmentation: Sugars in cells are
mainly compartmented in vacuoles. High accumulation
of sugars in vacuoles produces a high osmotic pressure
that stimulates influx of water into vacuoles, leading to
a high turgor pressure that is the driving force for cellular
enlargement. Physiological steps for controlling the sink
strength of fruit are mainly unloading, membrane
transport, metabolic conversion, and compartmentation,
if sufficient photoassimilate is supplied. Among these
four major steps, the role of enzymes in metabolic
conversion seems to be the most important for producing
the sink strength of fruit because this step correlates
closely with unloading and compartmentation of sugar.
Thus, in this review, I want to describe mainly “metabolic
conversion”.