Cluster roots in Casuarinaceae: Role and relationship to soil nutrient factors

N-2-fixing actinorhizal trees in the family Casuarinaceae are economically of great interest in tropical and sub-tropical zones because they are used for many purposes including protection against wind, stabilization of sand dunes and the production of firewood and charcoal. They are usually able to grow in sandy soils with low fertility by virtue of their ability to fix N-2. The objective of this review is to discuss briefly the role of mycorrhizas and, more extensively, that of cluster (proteoid) roots, developed by a number of species of Casuarinaceae to improve the absorption of nutrients other than N from soil, especially those needed for N-2 fixation and growth. After evaluating the actual relationships between mycorrhizas and the Casuarinaceae, we highlight the possible role of cluster roots as an effective alternative to mycorrhizas, and as a means of improvement of growth of the trees in nutrient-deficient soils. This raises the question of what triggers the formation of cluster roots in the Casuarinaceae. In addition to phosphorus deficiency, iron deficiency seems to be a major factor inducing the formation of cluster roots in Casuarina glauca and C. cunninghamiana. The number of cluster roots and the precocity of their formation are directly related to plant chlorosis due to Fe deficiency, as expressed by the critical concentration of chlorophyll in the shoot (0.60 mg g(-1) shoot f.wt). The effect of the nitrogen source on cluster root formation is discussed in relation to pH values in the plant culture solution. The number of cluster roots formed in nitrate-fed plants increases with pH in the range of 5 to 9. Experiments carried out with alkaline and acidic soils show that cluster roots are only produced when they are needed to overcome soil nutrient deficiency due to the immobilization of nutrient elements (P and Fe) by soil alkalinity. The possible involvement of ethylene in the initiation and/or the morphogenesis of cluster roots is discussed. (C) 2000 Annals of Botany Company.