Thermal phenotyping of stomatal sensitivity in spring barley

Thermometry and thermography are alternative methods used for measuring stomatal conductivity via transpirative cooling. However, the influence of mixed soil–plant information contained in thermal images compared to thermometric spot measurements on the measurement quality and relationships to agronomic traits remains unclear. To evaluate their respective influence, canopy temperature was measured simultaneously by two infrared thermometers (thermometry), which were oriented oblique to the plant canopy and mounted on a tractor, and a hand-held, nadir oriented thermal camera (thermography) in irrigated and drought-stressed spring barley cultivar trials in 2011. Canopy temperatures were separated from soil temperatures and extracted from the thermal images by matching thermal and RGB images. Thermometric measurements conducted at the beginning of shooting during a stable period of high radiation were more closely related to total plant biomass and straw yield at harvest than thermography under both irrigated and drought-stressed conditions. Taking into account the results of this evaluation, thermometry was used for assessing the agronomic importance of stomatal sensitivity, the earliness of stomatal closure, of spring barley cultivars subjected to different water supply in 2013. In this year, 16 spring barley cultivars were grown under mild drought stress and rainfed conditions. A stomatal sensitivity index was derived relating canopy temperatures of the cultivars grown under rainfed and drought-stressed conditions to each other. Under rainfed conditions, stomatal sensitivity was negatively related to grain protein yield with a coefficient of determination of R² =.43. Under increasing terminal drought stress, positive regression slopes of stomatal sensitivity to grain yield, biomass yield and culms/m² were observed with coefficients of determination amounting to R² =.22,.31 and.36, respectively. Stomatal sensitivity negatively impacts agricultural production under well-watered conditions, but maintains productivity under conditions of terminal drought.