Influence of whitening on greenhouse microclimate and crop energy partitioning

The influence of whitening a greenhouse roof on microclimate and canopy behaviour was studied during summer in a greenhouse located in the coastal area of eastern Greece. Measurements of microclimate variables, intercepted net radiation, canopy temperature and canopy transpiration rate (lambdaE(c)) of a well-watered soilless rose canopy (Rosa hybrida, cv. First Red) were carried out over several days before and after roof whitening. Whitening reduced the average glasshouse transmission coefficient for solar radiation from 0.62 to 0.31. As a consequence, air temperature, vapour pressure deficit and canopy-to-air temperature difference experienced drastic changes while transpiration rate was not strongly affected, being slightly higher (about 18%) after whitening. The Bowen ratio beta was highly positive before whitening (beta approximate to 0.6) and negative (beta approximate to -0.5) after whitening. Calculations of the canopy stomatal conductance (g(c)) and crop water stress index clearly showed that the crop was significantly less water stressed after whitening. It was found that the values of g(c) doubled the day after whitening and tended to increase slightly afterwards. The latter may be ascribed to the recovery of the crop and the emergence of new and healthy leaves. Globally, glass whitening was inexpensive, presented positive effects on both microclimate and crop behaviour and can be considered as an efficient means for alleviating the large heat load during summer in warm countries. (C) 2001 Elsevier Science B.V. All rights reserved