Physiological response of 'Flame Seedless' table grape to three natural ventilation levels in a parral greenhouse

Out of season vegetable production in greenhouses represents the basis of economic development in large areas of the Mediterranean Basin. In South Eastern Spain most of this production is based in structures with little or no climate control. Early table grapes are an attractive, less hand-labour consuming alternative to vegetable production; however, the woody and perennial nature of Vitis viniferil creates important challenges for its cultivation under plastic. Previous experiences in cultivating 'Flame Seedless' in a parral type greenhouse have shown the possibility of advancing its harvest by more than a month, although the plants have also shown a progressive diminution in bud fertility, yield, and bunch and fruit size. With the aim of determining the critical climatic parameters responsible of such loss of productivity, we have used an automated greenhouse with two levels of ventilation and a control non-ventilated greenhouse. One ventilation level was the result of opening only the side vents whereas the other combined side ventilation with roof ventilation. The results showed that only the combination of roof and side ventilation provided acceptable cooling effects. Both the side ventilation and control treatments experienced temperature differences between inside and outside of the greenhouse above 6 degrees C during the central part of the day. As a consequence, the only treatment in which the leaves were able to cool themselves through transpiration was the combination of roof and side ventilation. In this treatment the temperature differences between the leaves and the greenhouse air were negative whereas in the other two treatments the temperature of the leaves was always above the ambient temperature (differences > 4 degrees C). However, stomatal conductance data showed the stomata clearly remained open for all the configurations, even in the closed greenhouse.