Contribution of High-Resolution Multispectral and Thermal-Infrared Airborne Imagery to Assess the Behavior of Fruit Trees Facing Water Stress: Proof of the Concept and First Results in an Apple Variety Field Trial

A combined airborne thermal and multispectral imagery campaign was performed by UAV in two apple variety trials (flights at 40m elevation) located in South France. Both experimental orchards were dedicated to the assessment of various tree traits, including the response to water deficit. Subplots of each variety were submitted either to normal irrigation (WW), scheduled on the basis of soil hydric potential, or to water restriction (WS) during summer period (July 2015). In planta measurements performed in these 'genotype by environment' experiments comprised the monitoring of fruit diameter increase, the assessment of tree water status by stem or predawn leaf hydric potential, while local microclimatic data were also collected. Thanks to remotely-sensed thermal imagery, significantly higher canopy temperatures were found in WS trees compared to WW ones (average of 3 trees), and also significant differences in narrow-range spectral wavebands (multispectral images), and the vegetation indices calculated (NDVI, GNDVI and PRI). Differences were linked to the severity of water deficit reached, assessed by hydric potential, which varied significantly according to the genotype. Two varieties -more water spending- experienced a characteristic water stress while two others -more water saving- were less affected by water deficit, as revealed by image- and tree-based variables. The experiment thus demonstrated the potential of multispectral and thermal high resolution imagery to non-invasive phenotyping of fruit trees cover