EVAPOTRANSPIRATION IN GREENHOUSES WITH SPECIAL REFERENCE TO MEDITERRANEAN CONDITIONS

Present knowledge about the effect of the four major environmental factors (light availability, carbon dioxide content of the greenhouse air, its temperature and humidity) on transpiration of horticultural crops is reviewed. The peculiarities of low-investment greenhouses in mild winter environments are discussed, in order to analyze which factors might hamper productivity. Finally, the significance of recent research results for climate management in "Mediterranean" greenhouses is considered.

Production in greenhouses has a higher efficiency of water use, that might be improved further by a greater possibility of environmental management. Selective shading (which requires a movable screen), would probably be as effective as whitewash in preventing water stress, without being as detrimental to photosynthetic activity. In addition, some form of CO₂ enrichment, at least in order to prevent depletion, not only for the lesser assimilation but also for the increased sensitivity to water shortage, is another measure likely to have the prospect of a good return.

Besides prevention, detection of water deficiency is discussed. It is shown that, in a poorly ventilated ambient, stomatal closure might be lesser a defense against water loss than in the more natural environments where stomata evolved in the first place. On the other hand, plant temperature-based means of stress detection could be more timely in greenhouses than in open field.