Mulling over the mulla mullas: Revisiting phosphorus hyperaccumulation in the Australian plant genus Ptilotus (Amaranthaceae)

Species in the Australian genus Ptilotus (Amaranthaceae) grow well in soils with both very low and very high phosphorus (P) availability in the latter they hyperaccumulate P. However, it is not known whether this trait is common within Ptilotus, whether it is shared with other genera in the family, or whether it correlates with the wide array of morphologies and ecologies within Ptilotus. We therefore assessed P hyperaccumulation across the morphological, ecological and phylogenetic diversity of Ptilotus. Experiment 1 tested the response of 11 species to added P (0, 50 and 100 mg kg^-1), including six species of Ptilotus and the Australian amaranth Gomphrena canescens R.Br. Experiment 2 tested the response of five species-three Ptilotus spp., G. canescens and Kennedia prostrata R.Br.-to added P (5 and 150 mg kg^-1) and two pre-harvest P-pulse treatments (5 and 50 mg kg^-1). Ptilotus species hyperaccumulated P when grown in high-P soil, but curtailed uptake from a pulse. All Ptilotus species preferentially allocated P to leaves (reaching 73 mg g^-1) without development of P toxicity symptoms. Gomphrena canescens and K. prostrata preferentially allocated P to stems and roots, respectively, and suffered P toxicity. The lack of tolerance to high [P] in G. canescens suggests that the likely widespread, or universal, mechanisms for tolerance of high P by Ptilotus are not shared by amaranths. Further research will determine the mechanisms underlying the unusual P physiology of Ptilotus.