Predicting transpiration of shaded and non-shaded tomato fruits under greenhouse environments

Fruit water balance is determined by entry of sap through xylem and phloem, and losses due to back̄ow from fruits to other organs, and to transpiration. The latter, which may vary according to growing and climatic conditions, plays a signi®cant role in fruit water balance. To better understand and predict tomato fruit transpiration, measurements of the transpiration rate of shaded and nonshaded fruits were carried out under a wide range of climatic conditions in two greenhouse compartments with different levels of air vapour pressure de®cit (VPDair). Linear models relating transpiration and either air VPD (VPDair) or fruit-to-air VPD (VPDfr-air) were proposed and their parameters were identi®ed. The best ®t was always obtained when using the explicative variable VPDfr-air. The model using VPDair as variable ®ts moderately well in the case of shaded fruits, but is not adequate for non-shaded fruits. Values of cuticular conductance, gc deduced from our measurements appeared to depend (i) on the growth-VPD regime and (ii) on the prevailing values of VPDfr-air. Our results suggest that, as for leaf transpiration, VPD between the evaporating surface and the air is the variable that drives the fruit transpiration rate, and that more realistic models could be based on the hypothesis of a variation of gc vs.VPD. # 2000 Elsevier Science B.V. All rights reserved.