Minirhizotrons in modern root studies

In recent years, minirhizotrons have received increasing interest in field studies for characterising several biological processes, such as fine root production, root longevity, mycorrhization and parasitism, as collecting repeated video or digital images allows the fate of individual roots to be followed in time. This review summarises recent technical improvements in root observation and imaging, comparing results from various construction materials and installation angles. Both glass and transparent acrylic or polycarbonate tubes have shown a relatively low influence on root behaviour; tilted tubes are preferred to vertically oriented ones, in order to avoid artefacts, and to horizontal ones, which involve considerable soil disturbance during positioning in their surroundings in the open field. Precise camera positioning systems and new high-resolution (600 DPI and more) digital sensors are currently replacing external-tracked tubes and analog sensors, enhancing image capturing and quality. This allows for frequent root observation throughout the plant cycle and seasons, required for accurate estimation of root dynamics. Although manual and semi-automatic image analysis requires timing and tedious work, in the last few years minirhizotron systems have been increasingly used, thanks to fundamental advances in image analysis. The development of new algorithms for root detection and measurement in minirhizotron images has speeded up processing and research. The most interesting results in image analysis derive from the integration of luminance intensity- and geometry-based root vs. non-root classifiers. Discrepancies still remain between minirhizotron data and reference core sampling (root quantification) and carbon isotope methods (root turnover). Underestimation in root length mainly regards the top 0.3 m soil layer, and irreconcilable differences emerge between root life-span and root carbon residence time. A critical point in minirhizotron studies is exact identification of root activity, and most of the previous work was based on manual or semi-automatic visual evaluation (e.g., root appearance/disappearance, colour, shrinking, blotting, cortex degradation). Natural root fluorescence under UV light is only a reliable feature of root activity in some plant species, but interesting perspectives are expected from visible and near-infrared spectral images. Roots of GFP-engineered (green fluorescent protein) plants can also be distinguished in mixed plant stands by specific camera adaptation (light wavelengths, filters).