Greenhouse Microclimate and Soilless Pepper Crop Production and Quality as Affected by a Fog Evaporative Cooling System

Abstract

The influence of greenhouse humidity control on greenhouse microclimate, crop transpiration rate, and yield and fruit quality of a soilless grown pepper crop was studied in a greenhouse located at the coastal area of western Greece. Measurements were carried out during summer and autumn in two distinct greenhouse compartments involving: (1) no air humidity control and (2) a fog system operating when the relative humidity of the greenhouse air was lower than 80%. Under fog conditions, the greenhouse air and the crop leaf temperature were about 3°C lower than those measured under no fog conditions. The fog system allowed maintenance of the greenhouse air temperature under 30°C, while a maximum value of about 35°C was reached under no fog conditions. In addition, under fog conditions, the air vapor pressure deficit was lower than 2 kPa, even during the warmest part of the day, while under no fog conditions it reached values near 4 kPa. However, the transpiration rate was not affected to such an extent by fog as the air vapor pressure deficit, and under fog conditions it decreased by about 26%. This is attributed to the values reached by the bulk stomatal conductance, which were about 1.5 times higher under fog than under no fog conditions. The crop leaf area index values observed after the middle of the experimental period and later, under fog conditions, were higher than that observed under no fog conditions. Finally, the fog system enhanced the mean fruit weight and the percentage of marketable fruits but reduced appreciably the total number of fruits per plant. The free (titratable) acidity and the total soluble solids in the pepper fruit sap were slightly reduced by the fog cooling system, while the fruit size was increased. The use of a fog cooling system proved to be beneficial for summer crops of pepper grown under Mediterranean climatic conditions due to a favorable impact of the reduced vapor pressure deficit on both the mean fruit weight and the quality in terms of fruits graded Class I. Nevertheless, the high air humidity imposed by a cooling fog system may reduce the number of fruits per plant, thereby offsetting the benefits from the increased mean fruit weight in terms of total yield.