REGULAR ARTICLE The effect of phosphorus on cluster-root formation and functioning of Embothrium coccineum (R. et J. Forst.) M. Delgado & A. Zúñiga-Feest & M. Alvear & F. Borie Received: 20 September 2012 / Accepted: 2 July 2013 # Springer Science+Business Media Dordrecht 2013 Abstract Background and aims 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. Methods Four-month-old seedlings were grown for 4 weeks in hydroponic cultures with 1 μM P or 50 μM 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. Results Seedlings growing at 50 μM 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 μM P. However, seed- lings grown at 1 μM 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 μM P. Conclusions A low P supply promotes the initiation, growth and metabolic activity of cluster roots which is in accordance with reports on Proteaceae species oc- curring in ancient and highly weathered soils. Keywords Carboxylate exudation . Chilean Proteaceae . Phosphatase . Phosphorus Introduction Phosphorus (P) is an essential element in all living organisms and involved in many processes requiring energy (Marschner 2012). In soils this element is linked to Ca +2 (neutral or calcareous soils) and Fe +3 or Al +3 (acid soils), forming organic or inorganic com- pounds scarcely available to plants (Stevenson and Cole 1999). As a consequence of low levels of avail- able P, plants have developed root physiological strat- egies for P acquisition, including i) associations with symbiotic or free-living microorganisms, ii) the exuda- tion of carboxylates and phosphatases, which enable solubilisation and/or mineralisation of insoluble phos- phates; and, iii) the modification of root geometry and architecture to increase soil exploration (Richardson et al. 2011). Some plants can develop one or more of such strategies. This is the case of plants forming cluster (or proteoid) roots, which are dense clusters of fine rootlets around a main axis (Purnell 1960; Lamont 2003). The importance of cluster roots is that these structures chemically alter the rhizosphere, increasing plant’ s ability to acquire sparingly available nutrients Plant Soil DOI 10.1007/s11104-013-1829-3 Responsible Editor: Hans Lambers. M. Delgado : M. Alvear : F. Borie (*) Scientifical and Technological Bioresource Nucleus, BIOREN-UFRO, Universidad de La Frontera, Casilla 54-D, Temuco, Chile e-mail: fborie@ufro.cl A. Zúñiga-Feest Laboratorio de Fisiología Vegetal, Instituto de Ciencias Ambientales y Evolutivas, Facultad de Ciencias, Universidad Austral de Chile, Casilla 567, Valdivia, Chile