Effects of external phosphorus supply on internal phosphorus concentration and the intitiation, growth and exudation of cluster roots in Hakea prostrata R.Br..

The response of internal phosphorus concentration, cluster-root initiation, and growth and carboxylate exudationto different external P supplies was investigated in Hakea prostrata R.Br. using a split-root design. After removalof most of the taproot, equal amounts of laterals were allowed to grow in two separate pots fastened together atthe top, so that the separate root halves could be exposed to different conditions. Plants were grown for 10 weeksin this system; one root half was supplied with 1 μM P while the other halves were supplied with 0, 1, 25 or 75μM P. Higher concentrations of P supplied to one root half significantly increased the P concentration of thoseroots and in the shoots. The P concentrations in root halves supplied with 1 μM P were invariably low, regardlessof the P concentration supplied to the other root half. Cluster root initiation was completely suppressed on roothalves supplied with 25 or 75 μM P, whereas it continued on the other halves supplied with 1 μM P indicating thatcluster-root initiation was regulated by local root P concentration. Cluster-root growth (dry mass increment) on roothalves supplied with 1 μM P was significantly reduced when the other half was either deprived of P or suppliedwith 25 or 75 μM P. Cluster-root growth was favoured by a low shoot P status at a root P supply that was adequatefor increased growth of roots and shoots without increased tissue P concentrations. The differences in clusterrootgrowth on root halves with the same P supply suggest that decreased cluster-root growth was systemicallyregulated. 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 theother half was deprived of P. Estimates of root P-uptake rates suggest a rather limited capacity for down-regulatingP uptake when phosphate was readily available.