- Top of page
- Materials and Methods
Sedges of the Cyperaceae are prevalent in many nutrient-impoverished ecosystems throughout the world (Grime, 2001). Colonizing a broad range of habitats, sedges are often found in phosphate-limited environments such as fens (Pèrez Corona et al., 1996), calcareous grasslands (Davies et al., 1973), wetlands (Jonasson & Shaver, 1999), arctic tundra (Aerts & Chapin, 2000) and highly weathered Australian soils (Lamont, 1981; Meney et al., 1993). Adaptations conferring low-phosphorus (P) tolerance in sedges are intriguing, as members of the Cyperaceae are nonmycorrhizal, or at best weakly mycorrhizal (Powell, 1975; Brundrett & Abbott, 1991; Meney et al., 1993; Miller, 2005). However, some species of the Cyperaceae form dauciform roots, which are thought to increase mobilization of scarcely available nutrients such as P (Shane et al., 2005a). These dauciform roots are structurally very different, but functionally possibly rather similar to cluster roots of the nonmycorrhizal Proteaceae and Lupinus albus, which are formed as an adaptation to low soil fertility (Lamont, 1982; Shane & Lambers, 2005).
One of the earliest reports on dauciform roots in members of the Cyperaceae was that by Selivanov & Utemova (1969), which was written in Russian. These roots were then further described by Davies et al. (1973) as swollen lateral roots with an abundance of dense, long root hairs, presumed to play a role in nutrient absorption based on their dominance in the organic surface layer of calcareous dune slacks very low in P. Considering the ‘striking resemblance’ (Davies et al., 1973) of swollen lateral roots of Cyperaceae to proteoid (or cluster roots) in the Proteaceae, Lamont (1974) conducted a comparative study, which revealed sufficient differences in growth and anatomy to merit a new classification as ‘dauciform’ (= carrot-shaped) roots. It was proposed that dauciform roots play an analogous role to that of cluster roots as an adaptation to low soil fertility (Lamont, 1982; Dinkelaker et al., 1995). It is now well established that cluster roots release large amounts of carboxylates (anions of organic acids) (Gardner et al., 1983; Dinkelaker et al., 1995), and subsequent studies have shown that many species that produce cluster roots also have increased extracellular acid phosphatase activity (Dinkelaker et al., 1997; Neumann et al., 1999; Grierson & Comerford, 2000), which confers the ability to hydrolyse poorly available organic phosphates (Duff et al., 1994). However, unlike the cluster roots of Proteaceae (Roelofs et al., 2001; Shane et al., 2004) or of the well-studied crop species L. albus (Keerthisinghe et al., 1998; Neumann et al., 2000), the biological role of dauciform roots remains largely undescribed.
Following the discovery of dauciform roots, the impact of varied nitrogen supply on dauciform root formation has been tested (Lamont, 1974), and their anatomy well documented (Davies et al., 1973; Lamont, 1974). However, there is a lack of information on the physiological aspects of dauciform root function and development. A recent study by Shane et al. (2005a) provides evidence that dauciform roots are widespread in members of the Cyperaceae occurring in south-west Australia. These authors found that the formation of dauciform roots is closely dependent on P supply, as these roots are most abundant when plants are grown in solution culture at low concentrations (0–1 µm) of solution P. The low-P induction of dauciform roots in many sedges (Bakker et al., 2005; Shane et al., 2005a) suggests that they may function to increase P mobilization under P deficiency. Previous research on dauciform root development of Schoenus unispiculatus Benth. (Cyperaceae) revealed high levels of citrate released in an exudative burst from mature dauciform roots (Shane et al., 2005b). This has particular relevance in the Australian context, where much of the naturally occurring phosphate in acid and alkaline soils is retained by iron (Fe) or aluminium (Al), and calcium (Ca) oxides, respectively, as a result of their high sorption energy for phosphate (Norrish & Rosser, 1983). The exudation of carboxylates, particularly citrate, by roots plays a crucial role in mobilization of P via ligand exchange from these largely fixed sources of soil P (Jones, 1998; Hinsinger, 2001; Lambers et al., 2002).
There are many Australian native sedges that thrive in the nutrient-poor conditions of Australian plant communities (Pate & Bell, 1999; Specht & Specht, 1999), and these provide an opportunity for discovery and elucidation of low-P adaptations in the Cyperaceae. The aim of the present study was to characterize the possible role of dauciform roots of the Australian sedge Caustis blakei (Cyperaceae) in the accumulation and exudation of carboxylates and/or acid phosphatases according to their developmental stage. The formation of dauciform roots at low solution P concentrations of ≤ 1 µm (Playsted et al., 2005) suggests that these specialized roots may be an adaptation for survival of C. blakei in low-P ecosystems (Johnston et al., 1996).