The spectral-based photochemical reflectance index (PRI) and leaf surface temperature (T leaf ) derived from thermal imaging are two indicative metrics of plant functioning. The relationship of PRI with radiation-use efficiency (RUE) and T leaf with leaf transpiration could be leveraged to monitor crop photosynthesis and water use from space. Yet, it is unclear how such relationships will change under future high carbon dioxide concentrations ([CO 2 ]) and drought. Here we established an [CO 2 ] enrichment experiment in which three wheat genotypes were grown at ambient (400 ppm) and elevated (550 ppm) [CO 2 ] and exposed to well-watered and drought conditions in two glasshouse rooms in two replicates. Leaf transpiration (T r ) and latent heat flux (LE) were derived to assess evaporative cooling, and RUE was calculated from assimilation and radiation measurements on several dates along the season. Simultaneous hyperspectral and thermal images were taken at ~1.5 m from the plants to derive PRI and the temperature difference between the leaf and its surrounding air (∆T leaf−air ). We found significant PRI and RUE and ∆T leaf−air and T r correlations, with no significant differences among the genotypes. A PRI-RUE decoupling was observed under drought at ambient [CO 2 ] but not at elevated [CO 2 ], likely due to changes in photorespiration. For a LE range of 350 W m -2 , the ΔT leaf−air range was ~10°C at ambient [CO 2 ] and only ~4°C at elevated [CO 2 ]. Thicker leaves in plants grown at elevated [CO 2 ] suggest higher leaf water content and consequently more efficient thermoregulation at high [CO 2 ] conditions. In general, T leaf was maintained closer to the ambient temperature at elevated [CO 2 ], even under drought. PRI, RUE, ΔT leaf−air , and T r decreased linearly with canopy depth, displaying a single PRI-RUE and ΔT leaf−air T r model through the canopy layers. Our study shows the utility of these sensing metrics in detecting wheat responses to future environmental changes.

The combination of a future rise in atmospheric carbon dioxide concentration ([CO2]) and drought will significantly impact wheat production and quality. Genotype phenology is likely to play an essential role in such an effect. Yet, its response to elevated [CO2] and drought has not been studied before. Here we conducted a temperature‐controlled glasshouse [CO2] enrichment experiment in which two wheat cultivars with differing maturity timings and life cycle lengths were grown under ambient (aCO2 approximately 400 μmol mol–1) and elevated (eCO2 approximately 550 μmol mol–1) [CO2]. The two cultivars, bred under dry and warm Mediterranean conditions, were well‐watered or exposed to drought at 40% pot holding capacity. We aimed to explore water × [CO2] × genotype interaction in terms of phenology, physiology, and agronomic trait response. Our results show that eCO2 had a significant effect on plants grown under drought. eCO2 boosted the booting stage of the late‐maturing genotype (cv. Ruta), thereby prolonging its booting‐to‐anthesis period by approximately 3 days (p < 0.05) while unaffecting the phenological timing of the early‐maturing genotype (cv. Zahir). The prolonged period resulted in a much higher carbon assimilation rate, particularly during pre‐anthesis (+87% for Ruta vs. +22% for Zahir under eCO2). Surprisingly, there was no eCO2 effect on transpiration rate and grain protein content in both cultivars and under both water conditions. The higher photosynthesis (and transpiration efficiency) of Ruta was not translated into higher aboveground biomass or grain yield, whereas both cultivars showed a similar increase of approximately 20% in these two traits at eCO2 under drought. Overall, Zahir, the cultivar that responded the least to eCO2, had a more efficient source‐to‐sink balance with a lower sink limitation than Ruta. The complex water × [CO2] × genotype interaction found in this study implies that future projections should account for multifactor interactive effects in modeling wheat response to future climate.