Abstract

Cluster roots (CR) of Proteaceae improve P acquisition through carboxilates exudation in old impoverish soils. Embothrium coccineum and Orites myrtoidea are two South American Proteaceae species that grow on volcanic depositions and in young rocky soils in the Andes. Because (P) starvation is the key factor inducing cluster-root formation in other Proteaceae species, we carried out an experiment to determine the effect of soil P availability on cluster root formation and seedling performance. Seedlings of each species were grown in two types of soils (organic: 18 mg Olsen-P/Kg, and volcanic: 1,3 mg Olsen-P/Kg) during one growing season. Survival, growth rate, cluster-root number, root phosphatase activity and root acid exudation were evaluated. Embothrium coccineum had 100% survival in both soils, although it grew faster (200%) on organic than volcanic soil. Orites myrtoidea had 30% survival under the organic soil and had similar growth rates on both types of soils (70%). Both species had higher cluster-root number and relative mass on volcanic than on organic soil (15 and 35 clusters per plant, and 10 and 5% cluster root mass of total biomass, for E. coccineum and O. myrtoidea, respectively). In both species acid exudation was detected only from volcanic soils and root phosphatase activity was two times higher in CR from E. coccineum growing on volcanic soil than organic soil. Differences observed between species are in agreement with contrasting distribution and the success of Proteaceae on young soil.