Observing plant roots in their environment: current imaging options and specific contribution of two-dimensional approaches

This paper provides an overview of current options for imaging roots in their environment. Our primary aim is to describe the potential and limitations of these techniques, rather than their physical principles. We primarily discuss two-dimensional imaging techniques which, while inherently limited by their 2-dimensional nature are technically simple, cheaper, more readily accessible, and offer larger field-of-view or better resolution for a similar field-of-view than three-dimensional imaging techniques. We show that techniques involving microscopic or high-resolution X-ray imaging of roots in undisturbed soil samples have a considerable value because they can provide, at a scale which is not accessible by any other means, essential information on complex interactions between roots and soil structure. We also illustrate how 2D imaging techniques, based on either visible light or X-ray attenuation through thin growing containers (rhizotrons), can be used to study whole root systems. We conclude that, while fully appropriate 3D imaging tools will ultimately become available, in the meantime, two-dimensional techniques should be considered as relevant options for observing roots in their environment.