Crop temperature measurements for crop water status identification in greenhouses

Proper irrigation management in hydroponics through efficient use of water and fertilizers is crucial to the production of high quality products. Low water supply rates may lead to crop water stress and reduction of production. The temperature of the plant has been identified as a good indication of plant water status and has been included in several crop water status related indices. However, measuring crop temperature is a complex task, as contact sensors must be small and can lose contact with the plant. Recently, remote sensing methods have offered a promising alternative for crop temperature measurements. This work was to assess the reliability of crop temperature measurements obtained by different infrared thermometers, either as single sensors (infrared thermograph and infrared thermocamera) or infrared thermometers connected to a wireless sensor network (WSN), and this work was to compare and evaluate different crop water status indices which are based on crop temperature. Experiments were conducted in a hydroponic cultivation system with tomato crops. The crop water status indexes used were: stress degree day (SDD), temperature stress day (TSD) and crop water stress index (CWSI). The indices were evaluated using plant physiological characteristics like crop transpiration, sap flow and crop photosynthesis, and were able to early detect crop water stress. The goal was to study the effective performance of the thermal indicators in detecting crop water stress on greenhouse conditions, in which, air temperature, humidity, vapor pressure deficit and solar radiation vary greatly. Measurements from all single sensors and the WSN were used for that purpose. It was concluded that crop temperature may be a proper indicator to detect water stress on a daily basis. On an hourly basis, different forms of stress cause an increase of crop temperature, therefore further analysis is required.