Effect of shading on greenhouse energy balance and crop transpiration
The purpose of this study was to investigate the effect of greenhouse shading and irrigation water salinity on greenhouse microclimate and energy balance and on crop transpiration and leaf photosynthesis. Experiments were carried out on the experimental farm of the University of Thessaly in Velestino, in three similar, plastic covered greenhouses with hydroponic cucumber crop. One of the greenhouses was used as a control (without shading), the other two were shaded using two different shading nets (shading intensity of 35 and 50%). Two salinity levels were implemented in each greenhouse crop (normal-EC = 2.3 dS m-1 and high salinity EC = 6.3 dS m-1). Greenhouse microclimate parameters and crop photosynthesis and transpiration were measured. Climatic measurements were made during two growing seasons from April to June and from September to November 2008. The photosynthesis measurements were made during the autumn season. The results showed that shade did not significantly affect greenhouse air temperature, but improved crop stomatal conductance so that transpiration is was not adversely affected by shading. The analysis of the energy balance showed, however, that even the shade of 50% was not sufficient to cool the greenhouse during summer in Central Greece and that an additional cooling system is required. The measurements showed that shading reduced leaf photosynthesis and transpiration rate. Finally, no significant effects of salinity (or of salinity * shading interaction) were found on leaf photosynthesis and transpiration rate.