Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/6/2016
Publication Date: 8/31/2016
Citation: Zobel, R.W. 2016. Fine Roots – functional definition expanded to crop species. New Phytologist. 212:310-312.

Interpretive Summary: There is a need for a greater understanding of the developmental processes and physiological costs of the finest roots of plants. An improvement in the assessment of just what classifies as a fine root has been proposed, but this applies only to long lived plants, for example, trees. This paper demonstrates that the concept can be extended to crop plants. Extending the concept to crop plant species should result in a much greater understanding of the intricacies of plant root system function.

Technical Abstract: A recent review (McCormack et al., 2015) proposes to split fine roots (roots less than 2 mm diameter) into two different functional groups: Absorptive and Transport. This is a significant step forward to account for some of the previous comments by Pregitzer (2002) and Zobel (2003) on the situation with fine roots. The historic designation of fine roots as roots less than 2 mm diameter was arbitrary (i.e. different species might, rationally, have had different cut off points) and originated with research on long lived perennials; i.e. trees (Bohm, 1979). In many crop species (both annual and perennial) 2 mm diameter roots are often considered large to very large. The McCormack et al. (2015) review only addresses relatively long lived perennials and focuses primarily on issues related to global net primary production (NPP). In the bottom footnote of table 4, they state that: “(for cultivated annuals) all roots (are) classified as absorptive, assuming that no roots in cultivated annual systems persist through multiple years”. Of course roots of annuals do not persist in a functional fashion over years. It does not mean, however, that the differentiation of annual fine roots into two functional classes is not an equally appropriate approach.