Abstract

Embothrium coccineum (R. et J. Forst.) is a Proteaceae species from the southern part of South America. South-central Chilean soils are younger and contain more phosphorus (P) than soils in Australia and South Africa, where Proteaceae are common. Phosphorus deficiency is the main factor promoting cluster-root formation in Proteaceae. It is not known, however, whether this also applies to E. coccineum, which grows on soils with higher P content. Four-month-old seedlings were grown for 4 weeks in hydroponic cultures with 1 mu M P or 50 mu M P. The number of cluster roots, relative height increment, biomass distribution, cluster root/total root biomass ratio, foliar P concentration, root acid phosphatase activity and root carboxylate-exudation rates were determined. Seedlings growing at 50 mu M P showed a 10-, 1.3- and 3.3-fold greater increase in relative height, total dry mass and foliar P concentration, respectively, compared with those grown at1 mu M P. However, seedlings grown at 1 mu M P showed a 5-, 16-, 1.7- and 1.3-fold greater number of cluster roots, cluster root/total root biomass ratio, phosphatase activity and total carboxylate exudation, respectively, as compared with those grown at 50 mu M P. A low P supply promotes the initiation, growth and metabolic activity of cluster roots which is in accordance with reports on Proteaceae species occurring in ancient and highly weathered soils.