Novel root clusters in the grasstree Kingia australis (Dasypogonaceae) increase the root surface:volume ratio by 70 times

Context Erosion of a riverbank in south-western Australia exposed previously unrecorded clusters of roots/rootlets produced by the grasstree, Kingia australis (Dasypogonaceae). Aims Our aim was to prepare an initial report on these roots and consider their possible functions. Methods Excavation of root clusters, and quantification of the morphological and histological characteristics, and distribution in the soil. Results There were 260 clusters per m3 of soil to a depth of 1.6 m, peaking at a depth of 50−70 cm where nutrients and water were accessible all year. Their length × width averaged 8.4 × 5.5 cm, with 520 roots/rootlets per cluster and 5140 rootlets per litre of rhizosphere soil. Clusters comprised a parent lateral (with aerenchyma), hundreds of secondary roots, 50 × 3 mm, and thousands of rootlets, 10 × 1 mm. Clusters are perennial, new roots occasionally emerging from the previous winter-growing-season cluster. We call these novel clusters kingioid roots. Total root length reached 70 m/litre of rhizosphere soil. All roots are covered in root hairs, with parent lateral hairs 250 μm long increasing to 700 μm for rootlets. They possess a ±150-μm thick mucigel layer. Fineness of the rootlets and the dense root-hair cover result in a 70-time increase in the surface:volume ratio compared with the parent roots. No endogenous fungal hyphae or (cyano) bacteria were evident. Conclusions Although perennial, these structures link with seasonal root-cluster types (proteoid/dauciform/capillaroid roots) via the abundance of extremely hairy rootlets and mucigel, implying that the structures enhance water and nutrient uptake rather than storage. Implications As these root clusters are distributionally/morphologically/anatomically distinct they deserve more detailed study.