Advances in remote sensing of plant stress

Since we first began to actively cultivate plants, we have been using remote sensing to assess the health and vigour of our crops and ornamentals. By looking at plants and observing changes in the angle of the leaves over time we can detect water stress, the colour of the leaves has informed us of nutrient limitations and imbalances, the patchiness of leaf colour and form often relates to pest and disease attack. Our ability to assess the health of plants and vegetation quickly and accurately simply by “looking” at them is being raised to new levels with the advent of new sensors and instruments that can “see” across a wider range of wavelengths than our eyes, and improved understanding of the physics and biochemistry underlying the relationships between vegetation status and its “appearance”. When light strikes a leaf, part of the light spectrum is reflected towards the observer. This reflectance is governed by leaf surface properties, internal structure and the concentration and distribution of biochemical components within the leaf (e.g. nitrogen, lignin, cellulose). Thus there is information in the reflected light that relates to the physical and biochemical properties of the leaf. At the canopy scale, factors such as leaf angle distribution, leaf area index, litter and soil properties and the view and illumination geometry all influence the reflectance properties of the scene. The interpretation of this complex radiation pattern is the major challenge of remote sensing. Vegetation reflectance can be detected using narrow-bandwidth spectroradiometers that measure in the visible and near-infrared parts of the spectrum. In the visible spectrum (400–700 nm), leaf reflectance is low because of absorption by photosynthetic pigments (mainly chlorophyll and carotenoids). In the near-infrared (700–1,300 nm), on the other hand, the reflectance is influenced by structural properties in the leaf. Variation in reflectance in the middle infrared region (1,300–3,000 nm) is related to absorption characteristics of water and other compounds. Both absolute and relative differences in the reflectance among these various wavebands have been used to derive indices that correlate with vegetation condition. For example, a change in the chlorophyll content will influence the reflectance in the red part of the spectrum but not the near-infra red; an index based on the ratio of these two wave bands (the simple ratio, NIR/R) has been shown to relate to green biomass Plant Soil (2012) 354:41–44 DOI 10.1007/s11104-011-1051-0