In summer conditions, the growth and water balance of tomato fruits are dis
turbed. The present study aimed at identifying the specific effect of diffe
rent conditions of vapour pressure deficit (VPD) on the water content of fr
uits, on the water influx to and efflux from the fruit and on the gradients
of water potential in the plant. Thus a greenhouse compartment was misted
to maintain VPD under 1.5 kPa while a control compartment experienced high
variations in VPD.
High VPD conditions reduced the fresh yield of tomato fruits, which could b
e explained by a lower accumulation of water while the accumulation of dry
matter was not affected. These results are consistent with the fact that fr
uits growing under high VPD exhibited the same phloem flux but a higher tra
nspiration rate and a lower xylem flux than the fruits in low VPD compartme
nt. The fruit water potential (similar to 0.4 MPa) was not sensitive to VPD
whereas the stem-fruit water potential gradient was reduced under high VPD
conditions around midday. Thus the apoplasmic water flow into the fruit wa
s maximum at night. Low VPD conditions increased the difference of water po
tential between fruit and stem by maintaining the stem water potential arou
nd -0.28 MPa. Under high VPD conditions, the water potential of upper leave
s could reach -0.98 MPa while leaves of the low VPD compartment remained ar
ound -0.65 MPa.
These results prove the uncoupling of water and assimilate fluxes to the fr
uits and open the way to the modelling of the fruit growth and composition
in relation to plant water status.