A Root trait accounting for the extreme phosphorus sensitivity of Hakea prostrata (Proteaceae)

Proteaceae are adapted to acquire P from nutrient-impoverishedsoils; many function at very low leaf P levels, butare killed by P fertilization. Phosphorus toxicity developsat a remarkably low external P concentration. Previousstudies have described P toxicity in Proteaceae, but thephysiological basis for it remained unclear. The aim of thepresent study was to elucidate the physiological basis of Ptoxicity inHakea prostrataR. Br. (Proteaceae).TriticumaestivumL. (Gramineae),Medicago truncatulaGaertn.,Lupinus albusL. (both Fabaceae) andHakea prostrataR.Br. were grown in solution at a range of P concentrations(0–1000 mmol P m-3), and determined net P-uptake ratesat 5 (all species) and 50 mmol P m-3(H. prostrataonly).With the exception ofH. prostrata, net P-uptake rates werefastest for plants grown without added P. Down-regulationoccurred forT. aestivum,M. truncatulaandL. albuswhenthe P concentration during growth was increased from 0 to0.8 mmol P m-3, whereas inH. prostratarates decreasedonly for plants grown at 10 mmol P m-3or more. The leaf[P] at which P toxicity occurred inH. prostrataexceeded10 mg g-1dry matter, similar to that for crop species. Thelow capacity to reduce P uptake in response to increasedsupply offers a physiological explanation for the extremesensitivity to P supply inH. prostrata, and possibly otherProteaceae.