Cluster root formation and function vary in two species with contrasting geographic ranges

Southern South American Proteaceae can occupy soils that are rich in total phosphorus (P) but poor in available P (for example volcanic soils) thanks to their cluster roots (CR), which mine soil P. However, some southern South American Proteaceae occur in a wide range of soil nutrition. We hypothesized that CR formation and function are more responsive to nutrient soil availability in the widely-distributed Embothrium coccineum than in the narrowly-distributed Orites myrtoidea, which exclusively occurs in recent volcanic depositions. Survival, growth rate, CR formation (number, biomass) and function (carboxylate exudation, phosphatase activity) were evaluated in seedlings of both species after five months of growth in either a volcanic or organic substrate. E. coccineum exhibited full survival in both substrates, but had significantly lower growth, higher CR formation, higher CR citrate and malate exudation, and higher phosphatase activity in the volcanic substrate. By contrast, O. myrtoidea had similar growth rate in both substrates but 73% lower survival and null CR formation in the organic compared to the volcanic substrate. Variation in soil nutrient availability caused variation in growth and CR formation and function in a southern South American Proteaceae species of wider distribution, but not in a narrowly-distributed counterpart.