Vegetation indices derived from digital images and stable carbon and nitrogen isotope signatures as indicators of date palm performance under salinity

Abstract Date palm is frequently irrigated with brackish water. Developing tools to monitor salinity effects at the single-tree level may assist agronomy and phenotyping. Sixteen elite varieties were grown for 15 years under irrigation with three levels of saline water (5, 10 and 15 dS m−1) at the International Center for Biosaline Agriculture (UAE). Trunk length and diameter, number of branches, and fruit yield per tree were recorded. Different vegetation indices were calculated from single tree-top images taken from the ground with an RGB (Red/Green/Blue) camera. These included indices derived from CIE (Commission Internationale de l’Eclairage) color space models; lightness, together with a* and b* dimensions (CIELab) and u* and v* coordinates (CIELuv); and the HSI color space, referring to the components Hue, Saturation and Intensity. Moreover, Green Area (GA) and the Greener Area (GGA) were also formulated. Also canopy temperature (CT) was measured as an indicator of canopy water status with an infrared thermometer. The carbon isotope composition (δ13C), as a time-integrated indicator of water status, and the nitrogen isotope composition (δ15N) and total nitrogen concentration (N), as nitrogen metabolism indicators, were analyzed in leaflet dry matter. Irrigation conditions and genotypes exhibited significant effects for biomass, fruit yield and all the remote sensing and stable isotope traits evaluated. Hue correlated positively, whereas most of the other RGB vegetation indices along with δ13C and CT correlated negatively with biomass and fruit yield across salinities. Leaf N concentration and δ15N did not correlate with biomass and fruit yield across salinities, but were the only traits correlated with genotypic variability in fruit yield within a given salinity level. Traits that describe canopy color characteristics represent affordable tools for monitoring palm growth and productivity under saline irrigation. However, the results do not support the direct use of RGB indices to phenotype genotypic variability.