Abundance of root clusters in Kingia australis (Dasypogonaceae) highlights widespread non-symbiotic adaptations to nutrient-impoverished soils

Background and Aims: Recent erosion of a riverbank in southwestern Australia exposed previously unrecorded root clusters produced by the grasstree, Kingia australis (Dasypogonaceae). Our aim was to provide descriptions of these structures compared with currently known specialized roots. Methods: Excavation of root clusters, quantification of their distribution/morphology, and histology. Results: There were 260 clusters per m ³ of soil to a depth of 1.6 m and concentrated at 50-70 cm. They averaged 8.4 x 5.5 cm in length x width with 5360 roots/rootlets per L of rhizosphere soil. Clusters comprised a parent lateral (with cortical aerenchyma), secondary parent roots 50 x 3 mm in length x width, and rootlets 10 x 1 mm. Clusters are perennial, new roots sometimes emerging from the previous winter-growing-season’ cluster. We refer to these novel structures as staghorn-coralloid roots . Total root length reached 70 m per L of soil. The epidermal surface averaged 1600 cm ² per L of soil. All roots are covered in root hairs, with parent lateral hairs 250 µm long to 700 µm for rootlets. They must push through a ±150-µm thick mucigel layer to reach the soil. Adding root hairs increased the surface area exposed to the rhizosphere by 4.4 times for a mere 3.3% increase in root volume. Endogenous fungal hyphae or (cyano)bacteria were not evident. Conclusion: These root clusters link with proteoid, dauciform and capillaroid roots via their prolific production of extremely hairy rootlets with abundant mucigel, implying that they are functionally matched, but are otherwise morphologically and anatomically distinct.