Thermal cameras can easily determine plant canopy temperature, and the resulting data can be used for irrigation scheduling in addition to other water management tools. This study aimed to develop a method to use thermal imaging for canopy temperature measurements in one-year-old citrus plants to assess citrus water status. We evaluated the influence of five water levels (25%, 50%, 75%, 100%, and 125%) based on the crop evapotranspiration replacement of two citrus species [‘Red Ruby’ grapefruit (Citrus paradisi) and ‘Valencia’ sweet orange (Citrus sinensis)] for 48 days in a greenhouse. To determine the irrigation requirements for the treatment 100%, we estimated the water loss from pots by calculating the difference in soil moisture between the day before and the day of the measurement. We irrigated the pots when the soil moisture was close to the maximum allowable water depletion, keeping the soil moisture between the field capacity and the maximum allowable depletion. A portable thermal camera was used to take images that were later analyzed using open-source software. We determined the canopy temperature, leaf photosynthesis and transpiration, and plant biomass. A positive relationship between the amount of water applied and the temperature response of plants exposed to different water levels was observed. Grapefruit and sweet orange plants that received less water presented water restrictions and reached 6 °C higher canopy temperatures than the air. The thermal images easily identified water-stressed plants. This study allowed quick measuring of the canopy temperature using readily available equipment and can be used as a tool to assess water status in citrus plants in greenhouses. An automated routine to process the thermal images in real-time and remove the background weeds to determine the canopy temperature can potentially allow using it for irrigation management.
