Remote Sensing in Crops: What Can Be Sensed and From Where?.

With remote sensing it is becoming increasingly feasible to non-intrusively measure, predict and model a wide range of plant characteristics. Thus far, dedicated spectral indices have been used to estimate above-ground biomass, nutrient concentrations, photosynthetic and photo-protective pigments, as well as the water status of the canopy. Using chlorophyll fluorescence, photosynthetic performance can be measured including electron transport rate, down regulation, and photo-inhibition. Ground penetrating radar technology has been used to simulate 3D images of roots. High-throughput applications of remote sensing can also serve as a good indicator of plant stress (biotic and abiotic) and its impacts. For example, early ground cover (estimated with digital photography), cooler canopy temperatures (measured with IR sensors) and stay-green (estimated with i.e. NDVI) are generally associated with adaptation and/or stress tolerance. The degree of remoteness starts at the mm scale with ground based measurement (i.e. chl-fluorescence). Hand held instruments such as IR thermometers and NDVI sensors are typically performed at a distance of <1m from the crop. Thermal and spectral imaging devices mounted on airborne platforms operate at tens or hundreds of meters. Satellites can measure spectral indices at distances of several km from the field for measurement of crop agronomic characteristics. This increased remoteness without loss of precision will revolutionize phenotyping in breeding programs, as well as the automation of global stress monitoring.