749 Use of Model-Artificial Leaves for Monitoring Aerodynamic Conductance in Greenhouses C. Kittas and N. Katsoulas University of Thessaly, School of Agricultural Sciences, Department of Agriculture, Crop Production and Rural Environment, Fytokou St., N. Ionia GR-38446, Magnisia Greece A. Baille Universidad Politécnica de Cartagena Department of Food and Agricultural Engineering Paseo Alfonso XIII E-30203 Cartagena Spain Keywords: climate control, ventilation, gas exchanges, coupling mechanisms, boundary layer conductance Abstract Improving the design, control and management of greenhouse ventilation systems requires a better comprehension of the coupling and feedback mechanisms between the crop, the internal climate and the outside climate. The objective of this work was to characterize the leaf boundary-layer conductance (g b ) under greenhouse conditions and to study its dependence on ventilation rate and outside conditions. For this purpose, continuous estimation of g b was carried out in a greenhouse cultivated with rose plants, using rose leaf replicas consisting of a pair of model artificial leaves, one being heated and the other unheated. The boundary layer conductance was derived from the energy balance of the pair of model leaves. Inside air velocity measurements were made by means of a 3-D sonic anemometer. The analysis of the data indicated that, for a given vent aperture and wind direction, there is a clear link between g b and the outside wind speed, and therefore between g b and the ventilation conductance. This relation can be described by a simple linear function in the condi- tions of our experiments. Using this function for expressing the global greenhouse conductance as a function of the leaf area index allowed getting a better insight on the coupling between the crop and the outside climate. The results also confirm that this model sensor (artificial leaves) might have potential applications in the evaluation of the ventilation performances, as well as in greenhouse climate control and manage- ment. INTRODUCTION Greenhouse climate and crop transpiration models were first developed and validated for climatic conditions prevailing in northern regions (Stanghellini, 1987; Yang et al., 1990; Jolliet and Bailey, 1992). In these northern conditions, the greenhouse is generally poorly ventilated during a large part of the growing season and the boundary layer conductance of the crops tends to be much smaller than would be expected for similar crops growing outdoors. The canopy is strongly decoupled from the outside atmosphere by the presence of the cover, and the heat and water released at crop surface accumulate inside the greenhouse. Consequently, the transpiration rate will adjust until it reaches a stable equilibrium transpiration rate dictated by the incoming net radiation (Jarvis, 1985). Climatic conditions are very distinct in the Mediterranean countries, where high outside radiation, temperature and vapour pressure deficit could create inside the green- house very harsh conditions, impeding crop production, unless adequate and efficient cooling systems be implemented (Baille, 2001). The occurrence of prolonged periods of high temperature and low humidity affects negatively the yield and quality of the production. One of the solutions would be to improve the design, control and manage- ment of the ventilation system. This requires a better insight and comprehension of the coupling and feedback mechanisms between the crop, the internal atmosphere and the outside atmosphere. How ventilation may affect and control these mechanisms is still Proc. IC on Greensys Eds.: G. van Straten et al. Acta Hort. 691, ISHS 2005