Agronomic traits and vegetation indices of two onion hybrids

• Two onion hybrids have differed significantly in their agronomic and spectroradiometric response.
• The more productive hybrid Cometa presented no significant correlations between yield and VIs.
• In the late growth stage, onion biomass and yield were highly correlated to nine VIs in Red Mech.

Environmental, genotypic and agronomic factors have an effect on the yield value of an onion crop, which is determined primarily by number, weight and size of bulbs. Spatial variability of soil properties affects crop yield. Remote sensed hyperspectral vegetation indices (VIs), calculated using crop reflectance at field scale can be used either as an index of the plant biophysical status, or as a tool to estimate crop variability. The aim of the study was to evaluate the relationships among traditional agronomic measurements of two irrigated onion hybrids (Cometa and Red Mech) with spectroradiometric measurements taken at field scale. The two hybrids differed significantly either for agronomic response (yield, yield components and distribution of yield classes) or for their spectral properties. Cometa showed significant higher yield and biomass than Red Mech, as well as significant higher VI values, although no correlations were found among agronomic parameters and spectroradiometric indices. On the contrary, Red Mech showed significant correlation among biomass, yield and bulb weight with VIs. Differences between the two onion hybrids in the spectroradiometric readings and agronomic traits underlined the importance of ground truth data verification when air-born images or satellite data are taken over onion crop field.