Effect of greenhouse ventilation on humidity of inside air and in leaf boundary-layer

After preliminary studies demonstrated that greenhouse air humidity increased considerably close to the lower leaf surface, particularly during daytime when crop transpiration is maximum, we have focused in this study on the consequences of greenhouse ventilation on the decoupling between outside and inside climate and between the latter and the climate in the neighbourhood of the leaves. The theory of moisture transfer within the leaf boundary-layer is first outlined and a model for the air temperature and humidity distribution reviewed. We have investigated also experimentally the ventilation dependence of inside air climate and of the climate in the leaf boundary-layer in two greenhouse tomato crop tunnels differing only in their ventilation conditions. We have demonstrated that wind speed and vent opening govern greenhouse ventilation flux and the resulting inside air speed, which determines inside air humidity, more particularly in the leaf boundary-layer and at leaf level. Inside air speed is also much higher in the area situated near the vent openings than in the centre of the tunnel and consequently inside climate and leaf boundary-layer climates near the vents or the open gable ends are more tightly coupled to outside conditions. Finally, implications of these findings are considered for biological control of whiteflies using myco-insecticides from the point of view of the tunnel ventilation, transpiration activity and crop growth systems. (C) 2004 Elsevier B.V. All rights reserved.