Effects of different cooling strategies on the transpiration rate and conductance of greenhouse sweet pepper crops

This paper presents some experimental results concerning the effects of three greenhouse cooling devices (fan-ventilation, fog-system and whitening) on the crop transpiration rate and on the conductance of sweet pepper plants growing in summer (planted in July) under a semi-arid climate (Almeria, South-Eastern Spain). The influence of the climatic conditions created by the cooling devices, and of the crop leaf area index (LAI), was studied and analysed. It was found that young sweet pepper plants have their own pattern of adaptation to a given cooling strategy, showing quite differentiated responses in transpiration rate and canopy conductance with respect to the prevailing greenhouse climate. The results indicated that, at the beginning of the growth cycle (LAI < 1), the transpiration rate was likely to be limited by the plant root capacity to take up water. The large differences in transpiration rate per unit of leaf area can probably be ascribed to differences in the root development and partitioning of assimilates between the root system and the aerial part of the plants. The differences in transpiration rate and conductance between treatments progressively decreased as the crop developed, indicating that the crop was taking more control over the climate with increasing LAI. For LAI > 2, the cooling devices appeared to have a rather limited influence on greenhouse climate and on crop gas exchanges.