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

Proteaceae are adapted to acquire P from nutrient-impoverished soils; many function at very low leaf P levels, but are killed by P fertilization. Phosphorus toxicity develops at a remarkably low external P concentration. Previous studies have described P toxicity in Proteaceae, but the physiological basis for it remained unclear. The aim of the present study was to elucidate the physiological basis of P toxicity in Hakea prostrata R. Br. (Proteaceae). Triticum aestivum L. (Gramineae), Medicago truncatula Gaertn., Lupinus albus L. (both Fabaceae) and Hakea prostrata R.Br. were grown in solution at a range of P concentrations (0-1000 mmol P m(-3)), and determined net P-uptake rates at 5 (all species) and 50 mmol P m-3 (H. prostrata only). With the exception of H. prostrata, net P-uptake rates were fastest for plants grown without added P. Down-regulation occurred for T aestivum, M. truncatula and L. albus when the P concentration during growth was increased from 0 to 0.8 mmol P m(-3), whereas in H. prostrata rates decreased only for plants grown at 10 mmol P m(-3) or more. The leaf [P] at which P toxicity occurred in H. prostrata exceeded 10 mg g(-1) dry matter, similar to that for crop species. The low capacity to reduce P uptake in response to increased supply offers a physiological explanation for the extreme sensitivity to P supply in H. prostrata, and possibly other Proteaceae.