Applying RGB- and Thermal-Based Vegetation Indices from UAVs for High-Throughput Field Phenotyping of Drought Tolerance in Forage Grasses

• Main Authors: Tom De Swaef, Wouter H. Maes, Jonas Aper, Joost Baert, Mathias Cougnon, Dirk Reheul, Kathy Steppe, Isabel Roldán-Ruiz, Peter Lootens
• Format: Article
• Language: English
• Published: MDPI AG 2021-01-01
• Series: Remote Sensing
• Subjects: UAV; RGB camera; thermal camera; drought tolerance; forage grass; HSV
• Online Access: https://www.mdpi.com/2072-4292/13/1/147

The persistence and productivity of forage grasses, important sources for feed production, are threatened by climate change-induced drought. Breeding programs are in search of new drought tolerant forage grass varieties, but those programs still rely on time-consuming and less consistent visual scoring by breeders. In this study, we evaluate whether Unmanned Aerial Vehicle (UAV) based remote sensing can complement or replace this visual breeder score. A field experiment was set up to test the drought tolerance of genotypes from three common forage types of two different species: <i>Festuca arundinacea</i>, diploid <i>Lolium perenne</i> and tetraploid <i>Lolium perenne</i>. Drought stress was imposed by using mobile rainout shelters. UAV flights with RGB and thermal sensors were conducted at five time points during the experiment. Visual-based indices from different colour spaces were selected that were closely correlated to the breeder score. Furthermore, several indices, in particular <i>H</i> and <i>NDLab</i>, from the HSV (Hue Saturation Value) and CIELab (Commission Internationale de l’éclairage) colour space, respectively, displayed a broad-sense heritability that was as high or higher than the visual breeder score, making these indices highly suited for high-throughput field phenotyping applications that can complement or even replace the breeder score. The thermal-based Crop Water Stress Index <i>CWSI</i> provided complementary information to visual-based indices, enabling the analysis of differences in ecophysiological mechanisms for coping with reduced water availability between species and ploidy levels. All species/types displayed variation in drought stress tolerance, which confirms that there is sufficient variation for selection within these groups of grasses. Our results confirmed the better drought tolerance potential of <i>Festuca arundinacea</i>, but also showed which <i>Lolium perenne</i> genotypes are more tolerant.