Loss of root-soil contact due to root and root hair shrinkage

Due to global warming, future agriculture will have to face increasing temperatures, more frequent and extreme drought events and consequently water and nutrient scarcity. Thus, it is necessary to improve our understanding of how plants deal with dry conditions. Since there is still a lack of knowledge concerning below ground feedbacks of plants to drought, we are particularly interested in the response of below ground organs to soil drying.

Hence, the objective of our study was to determine morphological responses of roots and root hairs to soil drying in situ.
For this purpose, we have grown maize plants (Zea maize wildtype) in seedling holder microcosms for 8 days before harvesting and performing high-resolution synchrotron X-ray CT in order to visualize root compartements as well as the elongated root hairs (Koebernick et al. 2017). The segmented images served as basis for the quantification of our observations.

The results revealed that not only roots (Carminati et al. 2012) but also root hairs lose turgidity under dry soil conditions. This shrinkage of hairs occurs at high soil water potentials and reduces the surface and soil contact area of roots tremendously. Root hair shrinkage is the first step in a sequence of responses to progressive soil drying. The follow up processes within this sequence are the formation of cortical lacunae and root shrinkage resulting in air filled gaps at the root-soil interface. Severe cavitation within the xylem was not observed at the corresponding soil water potentials meaning that xylem embolism occurs at even lower potentials. This leads to the conclusion that there is a severe loss of root-soil contact and consequently of hydraulic conductivity at the root-soil interface before xylem cavitates and reduces water as well as nutrient fluxes in the radial root direction.
As not only roots but also root hairs take up nutrients and release exudates (Holz et al. 2017), they are assumed to be an important trait of the rhizosphere for both nutrient uptake and microbial activity. Furthermore, they increase the radial extent of the rhizosphere and although it is not yet clear if shrunk root hairs are inactive in exudation and nutrient uptake, their enormous shrinkage due to soil drying might limit rhizosphere processes.

In summary, losses of root-soil contact due to root and particularly root hair shrinkage are profound and occur at high water potentials.

 

References

• Carminati, A., Vetterlein, D., Koebernick, N., Blaser, S., Weller, U., & Vogel, H.-J. (2012). Do roots mind the gap? Plant and Soil, 367(1–2), 651–661. https://doi.org/10.1007/s11104-012-1496-9
• Holz, M., Zarebanadkouki, M., Kuzyakov, Y., Pausch, J., & Carminati, A. (2017). Root hairs increase rhizosphere extension and carbon input to soil. Annals of Botany, 121(1), 61–69. https://doi.org/10.1093/aob/mcx127
• Koebernick, N., Daly, K. R., Keyes, S. D., George, T. S., Brown, L. K., Raffan, A., Cooper, L. J., Naveed, M., Bengough, A. G., Sinclair, I., Hallett, P. D., & Roose, T. (2017). High-resolution synchrotron imaging shows that root hairs influence rhizosphere soil structure formation. New Phytologist, 216(1), 124–135. https://doi.org/10.1111/nph.14705