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Storage of phosphorus (P) in stem tissue is important in Mediterranean Proteaceae, because proteoid root growth and P uptake is greatest during winter, whereas shoot growth occurs mostly in summer. This has prompted the present investigation of the P distribution amongst roots, stems, and leaves of Hakea prostrata R.Br. (Proteaceae) when grown in nutrient solutions at ten P‐supply rates. Glasshouse experiments were carried out during both winter and summer months. For plants grown in the low‐P range (0, 0.3, 1.2, 3.0, or 6.0 µmol d–1) the root [P] was > stem and leaf [P]. In contrast, leaf [P] > stem and root [P] for plants grown in the high‐P range (6.0, 30, 60, 150, or 300 µmol P d–1). At the highest P‐supply rates, the capacity for P storage in stems and roots appears to have been exceeded, and leaf [P] thereafter increased dramatically to approximately 10 mg P g–1 dry mass. This high leaf [P] was coincident with foliar symptoms of P toxicity which were similar to those described for many other species, including non‐Proteaceae. The published values (tissue [P]) at which P toxicity occurs in a range of species are summarized. X‐ray microanalysis of frozen, full‐hydrated leaves revealed that the [P] in vacuoles of epidermal, palisade and bundle‐sheath cells were in the mM range when plants were grown at low P‐supply, even though very low leaf [P] was measured in bulk leaf samples. At higher P‐supply rates, P accumulated in vacuoles of palisade cells which were associated with decreased photosynthetic rates. [ABSTRACT FROM PUBLISHER]

The response of internal phosphorus concentration, cluster-root initiation, and growth and carboxylate exudation to different external P supplies was investigated in Hakea prostrata R.Br. using a split-root design. After removal of most of the taproot, equal amounts of laterals were allowed to grow in two separate pots fastened together at the top, so that the separate root halves could be exposed to different conditions. Plants were grown for 10 weeks in this system; one root half was supplied with 1 μM P while the other halves were supplied with 0, 1, 25 or 75 μM P. Higher concentrations of P supplied to one root half significantly increased the P concentration of those roots and in the shoots. The P concentrations in root halves supplied with 1 μM P were invariably low, regardless of the P concentration supplied to the other root half. Cluster root initiation was completely suppressed on root halves supplied with 25 or 75 μM P, whereas it continued on the other halves supplied with 1 μM P indicating that cluster-root initiation was regulated by local root P concentration. Cluster-root growth (dry mass increment) on root halves supplied with 1 μM P was significantly reduced when the other half was either deprived of P or supplied with 25 or 75 μM P. Cluster-root growth was favoured by a low shoot P status at a root P supply that was adequate for increased growth of roots and shoots without increased tissue P concentrations. The differences in cluster-root growth on root halves with the same P supply suggest that decreased cluster-root growth was systemically regulated. Carboxylate-exudation rates from cluster roots on root halves supplied with 1 μM P were the same, whether the other root half was supplied with 1, 25 or 75 μM P, but were approximately 30 times faster when the other half was deprived of P. Estimates of root P-uptake rates suggest a rather limited capacity for down-regulating P uptake when phosphate was readily available. [ABSTRACT FROM AUTHOR]