Influence of greenhouse ventilation regime on the microclimate and energy partitioning of a rose canopy during summer conditions

The influence of the greenhouse ventilation regime (natural or forced venti lation) on the energy partitioning of a well-watered rose canopy (Rosa hybr ida, cv. First Red) was investigated during several summer days in warm med iterranean conditions (Volos, Eastern Greece). Two types of ventilation sys tems were evaluated: (i) fans and roof openings (forced ventilation) and (i i) roof openings only (natural ventilation). The relevant climatic variable s (air temperature and vapour pressure deficit), average canopy temperature , transpiration rate and intercepted net radiation were continuously monito red. The data were processed in order to derive the canopy-to-air sensible heat flux, the bulk aerodynamic and stomatal conductance of the canopy, and the Bowen ratio. The results indicated that forced ventilation induced a m ore homogeneous vertical field for temperature and humidity, by providing a more intensive mixing of the inside atmosphere. It was found that the cano py-to-air sensible heat flux and the transpiration rate were not significan tly enhanced under fan-assisted ventilation and that the Bowen ratio was si milar under both regimes, ranging from - 0.5 to - 1.0. The canopy-to-air te mperature difference was significantly different, being less negative under forced ventilation. Calculated values of the crop water stress index were similar and relatively high (approximate to0.5) for both regimes. The value s of the bulk aerodynamic conductance were found to be dependent on the ven tilation regime, those for forced ventilation being roughly double than tho se for natural ventilation. The estimated values of the stomatal conductanc e of the canopy were slightly higher (about 10-15%) under natural ventilati on. Normalizing the stomatal conductance of the canopy by radiation reveale d a strong stomatal response to the humidity conditions prevailing at the s urface of the canopy. It appeared that forced ventilation increased signifi cantly the aerodynamic conductance, but did not influence significantly wat er consumption when compared to natural ventilation, because of the negativ e feedback between canopy-to-air vapour pressure deficit and stomatal condu ctance. (C) 2001 Silsoe Research Institute.