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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]

Cluster roots are an adaptation for nutrient acquisition from nutrient-poor soils. They develop on root systems of a range of species belonging to a number of different families (e.g., Proteaceae, Casuarinaceae, Fabaceae and Myricaceae) and are also found on root systems of some crop species (e.g., albus, Macadamia integrifoliaand Cucurbita pepo). Their morphology is variable but typically, large numbers of determinate branch roots develop over very short distances of main root axes. Root clusters are ephemeral, and continually replaced by extension of the main root axes. Carboxylates are released from cluster roots at very fast rates for only a few days during a brief developmental window termed an ‘exudative burst’. Most of the studies of cluster-root metabolism have been carried out using the crop plant L. albus, but results on native plants have provided important additional information on carbon metabolism and exudate composition. Cluster-root forming species are generally non-mycorrhizal, and rely upon their specialised roots for the acquisition of phosphorus and other scarcely available nutrients. Phosphorus is a key plant nutrient for altering cluster-root formation, but their formation is also influenced by N and Fe. The initiation and growth of cluster roots is enhanced when plants are grown at a very low phosphate supply (viz. ≤1 μM P), and cluster-root suppression occurs at relatively higher P supplies. An important feature of some Proteaceae is storage of phosphorus in stem tissues which is associated with the seasonality of cluster-root development and P uptake (winter) and shoot growth (summer), and also maintains low leaf [P]. Some species of Proteaceae develop symptoms of P toxicity at relatively low external P supply. Our findings with Hakea prostrata (Proteaceae) indicate that P-toxicity symptoms result after the capacity of tissues to store P is exceeded. P accumulation in H. prostrata is due to its strongly decreased capacity to down-regulate P uptake when the external P supply is supra-optimal. The present review investigates cluster-root functioning in (1) L. albus (white lupin), the model crop plant for cluster-root studies, and (2) native Proteaceae that have evolved in phosphate-impoverished environments. [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]