Effect of misting on transpiration and conductances of a greenhouse rose canopy

The influence of greenhouse humidity control on the transpiration rate (lambdaE(c)). sensible heat flux (H-c) and bulk stomatal conductance (g(c)) of a soilless rose canopy (Rosa hybrida. cv. First Red) was studied in a greenhouse located in the coastal area of eastern Greece. Measurements were carried out during several days in the summer (i) without air humidity control and (ii) with a mist system operating when the relative humidity of the greenhouse air was lower than 75%. The diurnal course of g(c) was determined from the relation linking lambdaE(c) to canopy-to-air vapour pressure deficit (D-c) or from inversion of the Penman-Monteith equation. The two ways of estimating g(c) were in good agreement, showing a significant increase of g(c) under mist conditions. Covariation of radiation and humidity during the day caused diurnal hysteresis in lambdaE(c) and g(c). The hysteresis phenomena were less marked when the mist system was operating. Normalising g(c) by radiation removed most of the hysteresis and indicated a curvilinear stomatal response to vapour pressure deficit. The analysis of the energy partition at the canopy showed high negative values of the Bowen ratio (beta approximate to -0.7) in both conditions, indicating that canopy transpiration played a major role in cooling the greenhouse atmosphere. The contribution of the mist system to total evaporative cooling was estimated to be about 20%, with only 40-50% of the mist water being effectively used in cooling. Calculation of the crop water stress index confirmed that the crop was less stressed under misting conditions. It was concluded that the prediction of short-term variations of lambdaE(c) and g(c) in greenhouse environments must account for the magnitude and diurnal variation of air VPD. (C) 2001 Elsevier Science B.V. All rights reserved.