Can rhizotron tube studies predict deep rooting in the field? A comparison of root phenotyping methods

Roots have been neglected in crop research, and in particular deep roots which are more difficult to access. Yet they play a crucial role in water stress tolerance, especially in later developmental stages. Methods for phenotyping roots are needed in order to breed for deeper rooting. While field phenotyping methods are costly and laborious, smaller scale methods are often cheaper and more easily replicated, but do not necessarily represent field conditions. Existing studies have not found strong relationships between small-scale and field grown roots, especially in later developmental stages. This study aimed to investigate whether similar genotypic differences can be seen in deep rooting of winter wheat in field soil and in tube studies, and if tubes could therefore be used to predict deep rooting in the field. We used root imaging to compare deep rooting characteristics of eight modern Danish winter wheat cultivars using three different methods: field experiments assessing roots with minirhizotron tubes; the semi-field facility, RadiMax; and 1.5 m tall rhizotron tubes. While deep rooting genotypes showed mostly positive correlations across all methods, significant correlations between methods were observed only in one year, specifically between the tubes and semi-field. Furthermore, deep rooting exhibited significant correlations across years and months within the RadiMax method, suggesting consistent deep rooting patterns over time. The increase in variability as experiments became more field-like highlights the complexity of soil-root interactions. While this study suggests that under certain conditions, small-scale phenotyping methods can indicate deep rooting genotypes, the correlations were not consistent enough to be used to predict deep rooting in the field. This underscores the challenge of using small-scale experiments to extrapolate root measurements to the field. This study demonstrates the need for caution when interpreting small-scale root experiments, and underlines the need for continued developments in root research generally. Further studies are needed to improve the quality of methods, to evaluate the effects of different soil types and environmental conditions on root growth, and to relate these to field-grown roots. • Deep rooting wheat genotypes follow similar patterns across methods. • Significant correlations of deep roots occur only in one experiment. • Rooting varies in the field due to diverse soil influences and complex methods. • Small-scale experiments cannot consistently predict roots in the field.