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The influence of phosphorus (P) availability on growth and P uptake was investigated in South African Proteaceae: (1) Protea compacta R.Br., endemic on severely nutrient-impoverished colluvial sands; (2) Protea obtusifolia Bueck ex Meissner; and (3) Leucadendron meridianum I. J. Williams, the latter both endemic on comparatively fertile limestone-derived soils. Plants were grown hydroponically in 1000 L tanks at 0.01, 0.1 or 1.0 µm P for 14 weeks. Biomass accumulation was influenced by P availability, doubling as [P] increased from 0.1 to 1.0 µm. Total biomass was greatest for P. compacta, but L. meridianum and P. obtusifolia had two to four times greater relative biomass accumulation at 0.1 and 1.0 µm[P]. Proteoid root clusters developed at both 0.01 and 0.1 µm[P], but were suppressed at 1.0 µm[P]; this was a 10-fold lower [P] than previously reported to inhibit cluster root formation. Rates of net P uptake at 5 µm P decreased in response to increased P availability from 0.01 to 1.0 µm P. Significant between-species differences in rates of P uptake and capacity to down-regulate P uptake were observed: P. compacta < P. obtusifolia < L. meridianum. The species responses are discussed in terms of adaptation to mosaics in soil P availability and the high beta diversity in the natural habitat. [ABSTRACT FROM AUTHOR]

When grown at a low P supply, Hakea prostrata R.Br. (Proteaceae) develops dense clusters of determinate branch roots, termed ‘proteoid’ or ‘cluster’ roots and accumulates Mn in its leaves. The aim of this study was to vary the production of cluster roots and assess the relationship between Mn uptake and cluster-root mass. We collected native soil from a location inhabited by H. prostrata and amended this with ‘high’ and ‘low’ amounts of insoluble or soluble P. After 14 months, we measured the impact of the treatments on cluster-root development and the [P], [Mn], [Fe], [Zn] and [Cu] in young (expanding) and mature leaves. Dry mass and leaf area increased with increasing P availability in the soil, but growth decreased at the highest soluble [P], which caused symptoms of P toxicity. The [P] in young leaves (1.3–2.7 mg g−1 DM) exceeded that in older leaves (0.28–0.85 mg g−1 DM), except when plants were grown with soluble P (3.2–21 mg g−1 DM). Cluster-root formation was inhibited when leaf [P] increased; [P] in young leaves, rather than that in old leaves, appeared to be the factor that determined the proportion of the root mass invested in cluster roots. Old leaves of all treatments had [Mn] from 90 to 120 µg g−1 DM, except for plants grown at high levels of soluble P, when [Mn] decreased below 30 µg g−1 DM. The [Mn] and [Zn] in old leaves and the [Cu] in young leaves were positively correlated with the fraction of roots invested in cluster roots. These findings support our hypothesis that cluster roots play a significant role in micronutrient acquisition, and also provide an explanation for Mn accumulation in leaves of H. prostrata, and presumably Proteaceae in general. [ABSTRACT FROM AUTHOR]

• The incidence of species that develop specialised‘dauciform’ lateral roots, which are hypothesised to be important for phosphorus (P) acquisition, is uncertain. We investigated their occurrence in Australian reed, rush and sedge species, grown at low P concentration in nutrient solution, and studied the response ofSchoenus unispiculatus(Cyperaceae) to a range of P concentrations.• We assessed the fraction of root biomass invested in dauciform roots, their respiration and net P-uptake rate, and the P status of roots and leaves.• Dauciform-root development occurred only in particular genera of Cyperaceae when grown at low P supply. Increased P supply was associated with increased growth ofS. unispiculatusand increased leaf[P]. Dauciform-root growth was reduced by increased P supply, and reduced P uptake co-occurred with the complete suppression of dauciform roots.• The P-induced suppression of dauciform roots in Cyperaceae is similar to that observed for proteoid roots in members of Proteaceae andLupinus albus. The response of dauciform roots to altered P supply and their absence from root systems of some sedge species are discussed in terms of managed and natural systems.New Phytologist(2005)doi: 10.1111/j.1469-8137.2004.01283.x© New Phytologist(2005) [ABSTRACT FROM AUTHOR]

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]

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]

Aims: To illustrate the morphology of sand-binding roots of Haemodoraceae, to conduct a comprehensive survey of the trait, spanning different climates across four continents, and to explore evolutionary hypotheses within a molecular phylogenetic framework. Methods: Sand-binding roots in Haemodoraceae were examined, measured and photographed in the field and on herbarium specimens. Photomicrographs were taken of southwest Australian species. The presence and absence of the sand-binding trait was mapped onto previously published phylogenies and an ancestral state reconstruction was performed. Results: Sand grains were very tightly bound to the root surface by persistent root hairs in Haemodoraceae. The majority of genera and species were found to possess sand-binding roots and only 2 of the 14 genera, Conostylis and Tribonanthes, had sister taxa with and without the trait. The trait was recorded in tropical, sub-tropical and wet temperate species, but mainly in semi-arid species. Sand-binding roots were likely to have been present in the ancestor of the family and both sub-families. Conclusions: The presence of sand-binding roots is the probable ancestral condition for Haemodoraceae, associated with a high degree of phylogenetic conservatism and some secondary loss, notably in Conostylis. Experimental studies are needed to understand the ecological and evolutionary forces at work. [ABSTRACT FROM AUTHOR]

Aims: Non-mycorrhizal species such as Banksia (Proteaceae) that depend on root exudates to acquire phosphorus (P) are prominent in south-western Australia, a biodiversity hotspot on severely P-impoverished soils. We investigated the consequences of an exudate-releasing P-mobilising strategy related to control of iron (Fe) acquisition in two Banksia species, B. attenuata R.Br. and B. laricina C. Gardner, that differ greatly in their geographical distribution and rarity. Methods: We undertook solution culture experiments to measure root-mediated Fe reduction (FeR) in non-cluster and cluster roots at four stages of cluster-root development, and whole root systems for plants grown at 2 to 300 μM Fe (as Fe-EDTA). As a positive control, we used Pisum sativum (cv. Dunn) to validate the FeR assay. Results: Unlike typical Strategy I species, both Banksia species showed no significant variation in FeR, for either cluster or non-cluster roots, when grown at a wide range of Fe supply. For roots of different developmental stages, we measured a range for B. attenuata cluster roots of 0.13 ± 0.03 to 1.29 ± 0.14 μmol Fe3+ reduced g−1 FW h−1 and 0.56 ± 0.11 to 1.10 ± 0.24 μmol Fe3+ reduced g−1 FW h−1 in non-cluster roots. Similarly, for B. laricina cluster-roots, FeR ranged from 0.22 ± 0.07 to 1.21 ± 0.37 μmol Fe3+ reduced g−1 FW h−1, and in non-cluster roots from 0.56 ± 0.11 to 0.71 ± 0.08 μmol Fe3+ reduced g−1 FW h−1. We also observed only minor differences for whole-root system FeR, and even though B. attenuata showed signs of leaf Fe deficiency in the 2 μM Fe treatment, its FeR was the lowest of both species across all treatments at 0.079 ± 0.009 μmol Fe3+ reduced g−1 FW h−1, compared with the fastest rate of 0.20 ± 0.014 μmol Fe3+ reduced g−1 FW h−1 for B. laricina in the 28 μM Fe treatment. Taking plants through a pulse from low to high Fe, then back to low Fe supply did not elucidate any significant response in FeR. Conclusions: Although Fe acquisition is tightly controlled in the investigated Banksia species, such control is not based on regulation of FeR, which challenges the model that is commonly accepted for Strategy I species. [ABSTRACT FROM AUTHOR]