Dispersed white roots in red beetroot influence the accuracy of root identification based on colours for intercropping studies

Abstract

Purpose Beetroot is a model crop for studying root competition in intercropping systems because its red-coloured roots facilitate non-destructive visual discrimination with other root systems of intercropped plants. However, beetroot also has white roots, which could alter how root competition is interpreted. Here we investigated the quantity of white versus red roots in beetroot to quantify the effect of this phenomenon. Methods Beetroot was mono-cropped or inter-cropped with white cabbage in a field trial. The distribution of beetroot roots was recorded to 2.5 m soil depth on three dates following the minirhizotron method. Roots in each 0.5 m soil layer were counted and categorised into groups based on colour (white roots, coloured roots, and white roots traced back to be coloured) to investigate the influence of white roots on accuracy of root registration. A pot experiment was conducted with three cultivars to verify if white roots are a general characteristic of beetroot. Results White roots in mono-cropped beetroot represented 2.5–4.8% of total roots, on average, across the rooted soil profile. However, white roots represented 6.9% and 11.6% of total roots in the deepest soil layer during August and October, respectively. White roots caused mono-cropped beetroot roots to be underestimated by 1–22% based on root colour discrimination. However, tracing white roots backwards and forwards to coloured parts of roots reduced underestimates to 0.5–15%. Intercropping did not influence the traceability of white roots compared to monocropping. The highest occurrence of white roots appeared during the early growth period and in the deepest soil layers, indicating a linkage to younger roots or higher root proliferation rates. Conclusion Beetroot represents a model crop for visual studies linking eco-physiology and root proliferation. The white roots of beetroot must be incorporated by studies of root competition in intercropping systems that use colour as a criterion.