The water relations of young tomato fruit pericarp tissue were examine
d and related to tissue expansion, The relationship between bulk turgo
r pressure and tissue expansion (as change in fresh mass or length of
tissue) was determined in slices of pericarp cut from young, growing f
ruit by incubation in different osmotic concentrations of polyethylene
glycol 6000 or mannitol, The bulk turgor of this tissue was low (abou
t 0.2 MPa), even in fruit from plants that were otherwise fully turgid
, whether measured psychrometrically or by length change in osmotic so
lutions, The rate of tissue growth at maximum turgor was less than tha
t at moderate turgor unless calcium was added to the incubation medium
, However, added calcium also decreased the rate of growth at lower tu
rgor pressures, Yield turgor was < 0.1 MPa, but it was increased by th
e addition of calcium ions, Electrolyte leakage from tissue was greate
st at maximum turgor pressure but was decreased by the addition of cal
cium ions or osmoticum, Tissue growth was unaffected by a range of pla
nt growth regulators (IAA, abscisic acid, benzyladenine and GA(3)) but
was inhibited, particularly at high turgor, by low concentrations of
malic or citric acid. The low turgor pressure of pericarp tissue could
be due to the presence of apoplastic solutes within the pericarp, and
evidence for this is discussed, Thus, fruit tissue may be able to mai
ntain optimal expansion rates only at moderate turgor and low calcium
concentration.