Optical tomography for 3D imaging of fine roots

Background/Question/Methods

Monitoring the root systems of plants is essential to improving models of crop productivity, soil carbon sequestration and biogeochemical cycling. Fine roots (considered to be smaller than 1-2 mm in diameter) are the dominant component of this system, with roots <0.2 mm in diameter often representing 50% to 95% of total root length. The plasticity and dynamism of fine root development must be characterized to accurately assess ecosystem biomass. This goal is most easily achieved using a high-throughput non-destructive evaluation tool by which repeated measurements of a given root system can track its evolution. We have developed new technology, using low coherence interferometric imaging, for rapid, in-situ, 3-dimensional imaging of fine root networks that overcomes many of the limitations associated with the collection and interpretation of conventional 2-dimensional minirhizotron imaging systems. We evaluated the capability of this new instrumentation, which we call a rhizotomographer, to image a variety of plant types (tomato, wheat, sawgrass, rice) grown in rhizoboxes filled with soil ranging from clay to sand to organic compost. A time-series study was also undertaken to determine the utility of the rhizotomographer for assessment of root dynamics.

Results/Conclusions

A new root imaging system based on optical tomography was constructed that was capable of providing high resolution (22.4 x 22.4 x 1.73 micrometers) over a field of view (11.2 mm x 5.6 mm) comparable to that of a minirhizotron.  Using near-infrared operating wavelengths, high contrast was achieved between all varieties of plant roots investigated and typical soil matrices expected to be encountered during field studies. The combination of high resolution and wide field of view of the rhizotomographer allowed boundaries of root hairs, primary roots, and higher order roots to be distinguished. Rhizotomographer imaging also captured linear elements within the root associated with its cellular structure, which may ultimately allow for the determination of root vitality and function. During the time series study, the instrument was able to image plant root networks with high repeatability, capturing growth and turnover of the root system. Root hairs (~100 micrometers in diameter) were clearly identified based on their uniform lengths and common lifetime. The high contrast of the resulting images facilitated post-processing and measurement of the root network relative to minirhizotron images collected simultaneously. Our approach offers new capabilities and advantages for studies of root network dynamics.