Observations of root hair patterning in soils: Insights from synchrotron-based X-ray computed microtomography

Background and Aims: Root hair emergence is affected by heterogeneities in water availability in the growth medium. Root hairs preferentially emerge into air, whereas their emergence into water is inhibited. Yet, these results were based either on destructive methods or on roots grown on an agar-air interface. Additionally, there is a lack of knowledge about the spatial distribution of root hairs as hairs elongate radially across the rhizosphere. Therefore, root hair growth in soils remains largely unexplored. Methods: Maize (Zea Mays L.) plants were grown in microcosms which were scanned with a synchrotron-based X-ray μ CT. The distribution of root hairs along the root epidermis and radially across the rhizosphere (i.e. as function of distance from the root epidermis) was analysed using spatial point pattern analysis. Results: While hairs emerged randomly in air-filled pores, their emergence was inhibited where the root was in contact with the soil matrix. As hairs elongated radially into the soil, they were preferentially located in the close proximity of soil particles. In maize, we rarely observed root hairs penetrating into soil aggregates. Conclusion: We conclude that in maize, root hairs grow in air-filled pores at the root-soil interface, where the flow of nutrients and water is impeded. Across the rhizosphere, hairs establish contact to the soil by growing in the proximity to soil particles. The effect of hairs on uptake processes, plant anchorage and rhizosheath formation might be limited (in maize) as they hardly penetrate into soil aggregates.