Cluster Roots in Casuarinaceae: Role and Relationship to Soil Nutrient Factors

N₂-fixing actinorhizal trees in the family Casuarinaceae areeconomically of great interest in tropical and sub-tropicalzones because they are used for many purposes including protectionagainst wind, stabilization of sand dunes and the productionof firewood and charcoal. They are usually able to grow in sandysoils with low fertility by virtue of their ability to fix N₂.The objective of this review is to discuss briefly the roleof mycorrhizas and, more extensively, that of cluster (proteoid)roots, developed by a number of species of Casuarinaceae toimprove the absorption of nutrients other than N from soil,especially those needed for N₂fixation and growth. After evaluatingthe actual relationships between mycorrhizas and the Casuarinaceae,we highlight the possible role of cluster roots as an effectivealternative to mycorrhizas, and as a means of improvement ofgrowth of the trees in nutrient-deficient soils. This raisesthe question of what triggers the formation of cluster rootsin the Casuarinaceae. In addition to phosphorus deficiency,iron deficiency seems to be a major factor inducing the formationof cluster roots in Casuarina glauca and C. cunninghamiana.The number of cluster roots and the precocity of their formationare directly related to plant chlorosis due to Fe deficiency,as expressed by the critical concentration of chlorophyll inthe shoot (0.60 mg g ^{- 1}shoot f.wt). The effect of the nitrogensource on cluster root formation is discussed in relation topH values in the plant culture solution. The number of clusterroots formed in nitrate-fed plants increases with pH in therange of 5 to 9. Experiments carried out with alkaline and acidicsoils show that cluster roots are only produced when they areneeded to overcome soil nutrient deficiency due to the immobilizationof nutrient elements (P and Fe) by soil alkalinity. The possibleinvolvement of ethylene in the initiation and/or the morphogenesisof cluster roots is discussed. Copyright 2000 Annals of BotanyCompany Casuarinaceae, Casuarina cunninghamiana, Casuarina glauca, cluster roots, ethylene, iron deficiency, phosphorus deficiency, proteoid roots