Divergent functioning of Proteaceae species: the South American Embothrium coccineum displays a combination of adaptive traits to survive in high-phosphorus soils

Proteaceae species in south-western Australia thrive on phosphorus-impoverished soils, employing a phosphorus-mining strategy involving carboxylate-releasing cluster roots. Some develop symptoms of phosphorus toxicity at slightly elevated soil phosphorus concentrations, due to their low capacity to down-regulate phosphorus uptake. In contrast, Proteaceae species in Chile, e.g. Embothrium coccineum J.R. Forst. & G. Forst., occur on volcanic soils, which contain high levels of total phosphorus, but phosphorus availability is low. We hypothesised that the functioning of cluster roots of E.coccineum differs from that of south-western Australian Proteaceae species, in accordance with the difference in soil phosphorus status. With more phosphorus to be gained from the soil with high levels of total phosphorus, we expect less investment in biomass and more release of carboxylates. Furthermore, we hypothesised that E.coccineum regulates its phosphorus-uptake capacity, avoiding phosphorus toxicity when grown at elevated phosphorus levels. To test these hypotheses, E.coccineum seedlings were grown at a range of phosphorus supplies in nutrient solution. We show that E.coccineum allocated at least five times less biomass to cluster roots that released at least nine times more carboxylates per unit cluster root weight compared with south-western Australian species (e.g. Banksia, Hakea). The highest phosphorus supply caused a growth inhibition and high leaf phosphorus concentration, without symptoms of phosphorus toxicity. We accept our hypotheses on the functioning of cluster roots and the high capacity to reduce the net phosphorus uptake in plants grown at a high-phosphorus supply. This novel combination of traits indicates divergent functioning of Proteaceae species from southern South America, exposed to frequent phosphorus input due to volcanic activity, in contrast with the functioning of south-western Australian Proteaceae species that thrive on severely phosphorus-impoverished soils. These traits could explain the functioning of E.coccineum on soils that are rich in total phosphorus, but with a low concentration of available phosphorus.