Abstract/Summary

Background and aims Ptilotus polystachyus occurs in phosphorus (P) – poor soils, but can hyperaccumulate P without toxicity. We examined, in P-poor soils, P accumulation and rhizosphere carboxylates for three Ptilotus species, and carboxylate adsorption and associated P release. Methods Ptilotus spp. macrocephalus, nobilis and polystachyus were grown in two soils at 40, 100 and 400 mg P kg−1 soil as KH2PO4. After 6 weeks, dry weight (DW), tissue nutrients and rhizosphere carboxylates were determined. Citrate, malate and oxalate adsorption, and associated P release, was investigated in three soils. Results For all species, shoot DW and carboxylate amount were little affected by increasing P, while green leaves reached ~ 45–60 mg P g−1 DW: tissue P and carbon were negatively correlated. Oxalate was the dominant carboxylate and a large effect of soil type differed with unit (μmol g−1 root DW or rhizosphere soil DW, μmol plant−1). Adsorption was highest for oxalate, but differed with soil type; Freundlich and Langmuir functions generally fitted well. Citrate was the most effective for P release, followed by oxalate. Conclusions Inability to down-regulate P uptake and rhizosphere carboxylates with increasing P may be characteristic of Ptilotus. The role of these traits in adaptation to P-poor soils merits further investigation.