Search

Grevillea crithmifolia R. Br. is a species of Proteaceae that is resistant to developing P-toxicity symptoms at phosphorus supplies in the root environment that induce P-toxicity symptoms in the closely related Hakea prostrata (Proteaceae). It was discovered previously that development of P-toxicity symptoms in H. prostrata is related to its low capacity to down-regulate net P-uptake rates (i.e. its low plasticity). The plasticity of net P-uptake rates and whole-plant growth responses in G. crithmifolia has now been assessed in two separate experiments: (i) a range of P, from 0 to 200 μmol P d−1, was supplied to whole root systems; (ii) using a split-root design, one root half was supplied with 0, 3, 75, or 225 μmol P d−1, while the other root half invariably received 3 μmol P d−1. Fresh mass was significantly greater in G. crithmifolia plants that had received a greater daily P supply during the pretreatments, but symptoms of P toxicity were never observed. Cluster-root growth decreased from about half the total root fresh mass when the leaf [P] was lowest (c. 0.1 mg P g−1 DM) to complete suppression of cluster-root growth when leaf [P] was 1–2 mg P g−1 DM. Split-root studies revealed that cluster-root initiation and growth, and net P-uptake rates by roots were regulated systemically, possibly by shoot P concentration. It is concluded that, in response to higher P supply, G. crithmifolia does not develop symptoms of P toxicity because of (i) greater plasticity of its net P-uptake capacity, and (ii) its greater plasticity for allocating P to growth and P storage in roots. This ecologically important difference in plasticity is most probably related to a slightly higher nutrient availability in the natural habitat of G. crithmifolia when compared with that of H. prostrata. [ABSTRACT FROM PUBLISHER]

Harsh hakea (Hakea prostrata R.Br.) is a member of the Proteaceae family, which is highly represented on the extremely nutrientimpoverished soils in southwest Australia. When phosphorus is limiting, harsh hakea develops proteoid or cluster roots that release carboxylates that mobilize sparingly soluble phosphate in the rhizosphere. To investigate the physiology underlying the synthesis and exudation of carboxylates from cluster roots in Proteaceae, we measured O2 consumption, CO2 release, internal carboxylate concentrations and carboxylate exudation, and the abundance of the enzymes phosphoenolpyruvate carboxylase and alternative oxidase (AOX) over a 3-week time course of cluster-root development. Peak rates of citrate and malate exudation were observed from 12- to 13-d-old cluster roots, preceded by a reduction in cluster-root total protein levels and a reduced rate of O2 consumption. In harsh hakea, phosphoenolpyruvate carboxylase expression was relatively constant in cluster roots, regardless of developmental stage. During cluster-root maturation, however, the expression of AOX protein increased prior to the time when citrate and malate exudation peaked. This increase in AOX protein levels is presumably needed to allow a greater flow of electrons through the mitochondrial electron transport chain in the absence of rapid ATP turnover. Citrate and isocitrate synthesis and accumulation contributed in a major way to the subsequent burst of citrate and malate exudation. Phosphorus accumulated by harsh hakea cluster roots was remobilized during senescence as part of their efficient P cycling strategy for growth on nutrient impoverished soils. [ABSTRACT FROM AUTHOR]