In order to maximize crop productivity, investment in roots must be optimized to permit evapotranspiration (ET) rates that are none-limiting to photosynthesis, especially on favorable days when soil and atmospheric conditions permit high rates of carbon fixation. However, breeding pipelines usually neglect root traits in their selection process, driven mainly by the challenges associated with phenotyping below-ground structures. Despite the latest advances for root characterization under controlled environments, root phenotyping under field conditions is still limited and relies on direct observations of the root system, which largely rules out breeding scale root phenotyping, except perhaps for superficial roots using ‘shovelomics’ with restricted throughput.

To estimate root function throughout the soil profile, we propose the use of a root index derived from remote sensing of canopy temperature (CT) and the water spectral reflectance index (WI) to screen for genetic variation in root capacity in stressed and unstressed wheat plants. The development of such an index builds on previous findings by CIMMYT and collaborators, where CT is tightly related to root biomass and depth under drought and overall root capacity under heat conditions. Complementary measurements of WI provide a proxy for estimation of canopy biomass and water status, both important when evaluating the capacity of a given root mass. Strategic experimental conditions are imposed to the same plant population to maximize the association of CT and WI with the evapotranspiration, allowing the discrimination of different root capacities.

We set a bread wheat panel (CT root) consisting of 13 genotypes that were grown in three different environments: yield potential (i.e. optimal conditions), under an increasing water stress imposed from anthesis (i.e. after roots reached their maximum development), and under a high VPD environment (achieved via a late sowing). The yield potential and the water stress environments showed significant correlations for root biomass and root: shoot ratio, indicating that in both cases the maximum development of roots was achieved. Imposing a gradual water stress increased the ability of CT for to predict root biomass and root: shoot ratio, indicating that this environment can be used for predicting root capacity under yield potential. However, the high VPD environment showed the best relationship between CT and root biomass, especially for deeper roots. Considering the blind and limited (destructive) sampling procedures available for direct root phenotyping in the field, the Root Index may eventually give a truer estimate of functional root capacity since it is integrative of the whole root system.