ENVIRONMENTAL EFFECT ON GROWTH AND ITS IMPLICATIONS FOR CLIMATE MANAGEMENT IN "MEDITERRANEAN" GREENHOUSES

A review is given of 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 growth and production of horticultural crops, mainly through their effect on assimilation and transpiration. The peculiarities of low-investment greenhouses in mild winter environments will be discussed, in order to analyze which factors might hamper productivity. Finally, the significance of recent research results for climate management in "Mediterranean" greenhouses will be considered.

In particular, the implications for climate control will be discussed of the recent awareness that the effect of ambient temperature on production is exerted through mean values on a few days basis. It seems possible, therefore, that strict control of temperature excursions-a known headache in Mediterranean regions, could be somehow pointless. A similar conclusion might be reached with respect to control of humidity: only prolonged high humidities have been proven to hamper growth, particularly with tomato.

On the other hand, it will be shown that CO₂ depletion, typical of poorly ventilated shelters subjected to high radiation regimes, are really devastating, for both the ensuing drop in assimilation as well as the increased sensitivity to water shortage. It would seem, therefore, that a system for CO₂ supply is mandatory rather than an extravagant luxury, undoubtedly if refuse CO₂ might be available. Another measure likely to have a good return, the substitution of white washing with movable screens to be applied only whenever a given radiation threshold is exceeded, will be evaluated.