High vapour pressure de®cit in ̄uences growth, transpiration and quality of tomato fruits

Plants were grown in two glasshouse compartments under two vapour pressure de®cit (vpd) levels: low vpd was obtained by increasing air humidity with a fogging system, and high vpd was obtained during sunny hours in a greenhouse where air humidity was not controlled. The mean value, of the six driest hours of the day concerning the growing period of the fruits considered, was 1.6 kPa under low vpd and 2.2 kPa under high vpd conditions. Over the whole experimental period, the difference in mean hourly temperature never exceeded 0.88C. The oldest leaves were removed to different extents in different plots to analyse whether a different leaf/fruit ratio could modify the effect of high vpd. Fruit growth and transpiration rates greatly varied during daylight hours; these variations were enhanced under high vpd condition. The increase in vpd produced a signi®cant reduction in fruit fresh weight and in fruit water content, and an increase in soluble solids, while fruit dry weight was not affected. High vpd also enhanced the variability of fruit weight. When more leaves were removed the effect of vpd on soluble solids and water content was less important. The study shows that during summer, as vpd increases from 1.6 to 2.2 kPa, effects can be observed both on tomato growth and quality characteristics. Therefore, an improvement of the control of vpd for the optimisation of greenhouse tomato production is discussed. # 2000 Elsevier Science B.V. All rights reserved.