Root taxa identification in plant mixtures – current techniques and future challenges

Abstract

Background Studying root biomass, root system distribution and belowground interactions is essential for understanding the composition of plant communities, the impact of global change, and terrestrial biogeochemistry. Most soil samples and minirhizotron pictures hold roots of more than one species or plant individual. The identification of taxa by their roots would allow species-specific questions to be posed; information about root affiliation to plant individuals could be used to determine intra-specific competition. Scope Researchers need to be able to discern plant taxa by roots as well as to quantify abundances in mixed root samples. However, roots show less distinctive features that permit identification than aboveground organs. This review discusses the primary use of available methods, outlining applications, shortcomings and future developments. Conclusion Methods are either non-destructive, e.g. visual examination of root morphological criteria in situ, or require excavated and excised root samples. Among the destructive methods are anatomical keys, chemotaxonomic approaches and molecular markers. While some methods allow for discerning the root systems of individual plants, others can distinguish roots on the functional group or plant taxa level; methods such as IR spectroscopy and qPCR allow for quantifying the root biomass proportion of species without manual sorting.

Atlas of woody plant roots: morphology and anatomy with special emphasis on fine roots

“…In tree roots, the pattern of porosity may depend on the distance of the root from the stem. In the ring-porous species Ulmus glabra, roots in close proximity to the stem were semi-ring-porous, while more distal roots were diffuse-porous (Rewald et al 2012).…”

“…However, it requires time-consuming hand sorting and training of personnel (Rewald et al 2012). As morphological determination keys are lacking or limited to a couple of species on a local level, there is not a clear consensus on which morphological features are the most reliable.…”

“…According to Fitter (1987Fitter ( , 2002, the major morphological features in which differences can be discerned are root diameter, colour and surface texture. Root morphology may be subject to plasticity under changing environmental conditions and mycorrhizal symbiosis (Rewald et al 2012). Morphological criteria used to identify tree roots (e.g.…”

“…Hölscher et al 2002, Korn 2004) may have some information on geometry included as well, such as ramification characteristics. Rewald et al (2012) compiled morphological identification criteria for tree roots from different sources: root colour, odour, resilience to breakage, type of mycorrhiza, existence of root hairs, exodermal cell structure and periderm characteristics (furrows, dead periderm layers), root diameter, ramification pattern, root tip density, and morphotype of mycorrhizal root tips.…”

“…Colour as the sole criterion should therefore be avoided (Rewald et al 2012). The perception of root colour might also be obscured by the mineral particles adhering to the surface of the roots in samples obtained from mineral layers of soil and by the presence of fungi that grow on the periderm of roots, such as dark septate fungi.…”