Quantifying plant responses of a tomato crop under a thermal screen

Although thermal screens were already introduced more than twenty years ago in Western European glasshouse horticulture, growers were discouraged to install them in their glasshouse, since they were not able to quantify the crop's behaviour under a thermal screen. Nowadays, crop monitoring techniques have become commercially available, which might aid growers to support their decision to adapt their climate control set points once a thermal screen is installed. In this study, leaf temperature, stem diameter and sap flow measurements were performed on glasshouse tomato plants (Solanum lycopersicum L. 'Clothilde') grown inside two compartments in Beitem, Belgium during spring 2008. In one compartment, a movable and semi-transparent thermal screen was used during the night. Differences in leaf temperature between the two compartments were observed, which were mainly caused by differences in air temperature, although differences in radiative energy losses also had to be taken into account in the upper canopy layer in the case of cold nights. Based on these results, it is advised to include leaf temperature measurements in research concerning the effect of the use of thermal screens. Additionally, a reduced water uptake was observed in the morning when the screen was still closed, whereas during the evening hours no difference in water uptake could be revealed. However, when the thermal screen was closed in the evening, stem diameter measurements indicated that the internal water storage pools were restored more quickly under the screen. It is, however, acknowledged that these effects were only observed on sunny days. Consequently, in addition to leaf temperature measurements, sap flow and stem diameter measurements can provide growers and researchers useful feedback from the crop itself, when thermal screen application strategies are to be evaluated. As a general conclusion, it was stated that these crop monitoring techniques are imperative when novel techniques are to be introduced in glasshouse horticulture in a sustainable way