Specialized 'dauciform' roots of Cyperaceae are structurally distinct, but functionally analogous with 'cluster' roots

When grown in nutrient solutions of extremely low [P](£1.0mM), the sedgeSchoenus unispiculatusBenth.(Cyperaceae) develops dauciform roots, which are shortand carrot shaped, and produce dense numbers of long roothairs. It has been suggested that dauciform roots of monocotyledonoussedges function to acquire P from nutrientpoor,P-fixing soils in a manner similar to that of cluster(proteoid) roots developed by some dicotyledonous species,but without evidence to substantiate this claim. Toelucidate the ecophysiological role of dauciform roots, weassessed carboxylate exudation, internal carboxylate and Pconcentrations and O2uptake rates during dauciform rootdevelopment. We showed that O2consumption was fastest[9 nmol O2g-1fresh mass (FM) s-1] and root [P] greatest(0.4 mg P g-1FM) when dauciform roots were young andrapidly developing. Citrate was the most abundant carboxylatein root tissues at all developmental stages, and wasmost concentrated (22.2mmol citrate g-1FM) in youngdauciform roots, decreasing by more than half in maturedauciform roots. Peak citrate-exudation rates (1.7 nmolcitrate g-1FM s-1) occurred from mature dauciform roots,and were approximately an order of magnitude faster thanthose from roots of species without root clusters, and similarto those of mature proteoid (cluster) roots of Proteaceae.Both developing and mature dauciform roots hadthe capacity to acidify (but not alkalinize) the rhizosphere.Anatomical studies showed that epidermal cells in dauciformroots were greatly elongated in the transverse plane;epidermal cells of parent roots were unmodified. Althoughstructurally distinct, the physiology of dauciform roots insedges appears to be analogous to that of proteoid roots ofProteaceae and Fabaceae, and hence, dauciform rootswould facilitate access to sorbed P and micronutrients fromsoils of low fertility.