CLUSTER ROOT-FORMATION BY GYMNOSTOMA-PAPUANUM (CASUARINACEAE) IN RELATION TO AERATION AND MINERAL NUTRIENT AVAILABILITY IN WATER CULTURE

The term cluster root is used to refer to a dense cluster of determinate lateral roots (rootlets), in preference to the terms proteoid root and proteoid-like root used by other authors. Cluster roots are often formed by the actinorhizal plant Gymnostoma papuanum. In water culture, cluster root formation by G. papuanum was influenced by aeration, phosphorus level, and nitrogen source. Aeration was a critical factor, with nonaerated rooted cuttings having far fewer cluster roots than aerated ones. Phosphorus deficiency was the single nutrient deficiency that led to increased cluster root formation. Seedlings, grown under conditions of either low (0.8 mg L-1) or no phosphorus, responded by devoting a greater portion of root growth to the production of cluster roots, with no overall reduction in root growth for 6 weeks. The response to varying phosphorus level was modified by providing nitrogen in different forms. Supplying nitrogen as ammonium resulted in low levels of cluster root formation. Supplying nitrate to nodulated seedlings led to an increase in cluster root formation in comparison with plants that depended solely upon dinitrogen fixation by Frankia. Greatest cluster root formation occurred on plants grown in aerated water cultures supplied with nitrate and with little or no phosphorus.