The spectral-based photochemical reflectance index (PRI) and thermal imaging-derived leaf surface temperature (Tleaf) derived from thermal imaging are important metrics of plant functioning. The relationship between PRI and radiation use efficiency (RUE) and Tleaf and leaf transpiration could be used to monitor crop photosynthesis and water use. We conducted an [CO2] enrichment experiment in which three wheat genotypes were grown at ambient (400 ppm) and elevated (550 ppm) [CO2] under well-watered and drought conditions in two replicate glasshouses. Leaf transpiration (Tr) and latent heat flux (LE) were derived to assess evaporative cooling, and RUE was calculated from assimilation and radiation measurements on several days during the season. Simultaneous hyperspectral and thermal images were taken at 1.5 meters from the plants to derive PRI and the temperature difference between the leaf and its surrounding air (∆Tleaf-air). A PRI-RUE decoupling was observed under drought at ambient [CO2] but not at elevated [CO2], likely due to changes in photorespiration. For a LE range of 350 W m-2, the ∆Tleaf-air range was 10°C at ambient [CO2] and only 4°C under elevated [CO2]. Thicker leaves in plants grown at elevated [CO2] suggest higher leaf water content and consequently more efficient thermoregulation at high [CO2] conditions. PRI, RUE, ∆Tleaf-air, and Tr decreased linearly with canopy depth, displaying a single PRI-RUE and ∆Tleaf-air – Tr model through the canopy layers. Our study demonstrates the utility of these sensing metrics for detecting wheat responses to future climate and environmental changes.
