The cellular pathway of postphloem sugar transport was elucidated in t
he outer pericarp of tomato (Lycopersicon esculentum Mill cv. Floradad
e) fruit at 13-14 and 23-25 days after anthesis (DAA). These developme
ntal stages are characterized by phloem-imported sugars being accumula
ted as starch and hexose, respectively. The symplasmic tracer, 5(6)-ca
rboxyfluorescein, loaded into the storage parenchyma cells of pericarp
discs, moved readily in the younger fruit but was immobile in fruit a
t 23-25 DAA. Symplasmic mobility of [C-14]glucose was found to be iden
tical to 5(6)-carboxyfluorescein. For the older fruit, the pericarp ap
oplasm was shown to be freely permeable to the apoplasmic tracer, tris
odium 3-hydroxy-5,8,1 0-pyrenetrisulfonate. Indeed, the transport capa
city of the pericarp apoplasm was such that the steady-state rate of i
n-vitro glucose uptake by pericarp discs accounted fully for the estim
ated rate of in-vivo glucose accumulation. For fruit at 23-25 DAA, the
inhibitory effects of the sulfhydryl group modifier, p-chloromercurib
enzenesulfonic acid (PCMBS), on [C-14]glucose and [C-14]fructose uptak
e by the pericarp discs depended on the osmolality of the external sol
ution. The inhibition was most pronounced for pericarp discs enriched
in storage parenchyma. Consistent with the PCMBS study, strong fluores
cent signals were exhibited by the storage parenchyma cells of pericar
p discs exposed to the membrane-impermeable thiol-binding fluorochrome
, monobromotrimethylammoniobimane. The fluorescent weak acid, sulphorh
odamine G, was accumulated preferentially by the storage parenchyma ce
lls. Accumulation of sulphorhodamine G was halted by the ATPase inhibi
tor erythrosin B, suggesting the presence of a plasma-membrane-bound H
+-ATPase. A linkage between the putative H+-ATPase activity and hexose
transport was demonstrated by an erythrosin-B inhibition of [C-14]glu
cose and [C-14]fructose uptake. In contrast, comparable evidence for a
n energy-coupled hexose porter could not be found in the pericarp of y
ounger fruit at 13-14 DAA. Overall, the data are interpreted to indica
te that: (i) The postphloem cellular pathway in the outer fruit perica
rp shifts from the symplasm during starch accumulation (13-14 DAA) to
the apoplasm for rapid hexose accumulation (23-25 DAA). (ii) An energy
-coupled plasma-membrane hexose carrier is expressed specifically in s
torage parenchyma cells at the latter stage of fruit development.