No evidence of regulation in root-mediated iron reduction in two Strategy I cluster-rooted Banksia species (Proteaceae)

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.Gregory R. Cawthray, Matthew D. Denton, Michael A. Grusak, Michael W. Shane, Erik J. Veneklaas, Hans Lamber