Water fluxes and growth of greenhouse tomato fruits under summer conditions

In summer conditions, the growth and water balance of tomato fruits are disturbed. The present study aimed at identifying the specific effect of different conditions of vapour pressure deficit (VPD) on the water content of fruits, 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 be explained by a lower accumulation of water while the accumulation of dry matter was not affected. These results are consistent with the fact that fruits growing under high VPD exhibited the same phloem flux but a higher transpiration rate and a lower xylem flux than the fruits in low VPD compartment. 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 was maximum at night. Low VPD conditions increased the difference of water potential between fruit and stem by maintaining the stem water potential around -0.28 MPa. Under high VPD conditions, the water potential of upper leaves could reach -0.98 MPa while leaves of the low VPD compartment remained around -0.65 MPa. These results prove the uncoupling of water and assimilate fluxes to the fruits and open the way to the modelling of the fruit growth and composition in relation to plant water status.