Cluster Roots and Mycorrhizae in Casuarina cunninghamiana: their Occurrence and Formation in Relation to Phosphorus Supply

The occurrence of mycorrhizae and cluster roots was assessed in natural populations of Casuarina cunninghamiana Miq. in north Queensland. Cluster roots were found at 90% of the sites surveyed. By contrast, arbuscular mycorrhizae occurred at only 45% of sites and the extent of colonisation was low. No ectomycorrhizae were found during this survey. Formation of arbuscular mycorrhizae and cluster roots in C. cunninghamiana seedlings in relation to external P supply was investigated in glasshouse experiments. Inoculation with the arbuscular mycorrhizal fungus, Glomus, significantly increased seedling dry weight at low P supply (10 mg P per kg soil), but had no effect in the absence of P or at P supply rates greater than 10 mg P per kg soil. Mycorrhizal colonisation of roots was highest at 0 and 10 mg P per kg soil and declined with further increases in P supply. No mycorrhizae formed at or above 100 mg P per kg soil. The number, dry weight and proportion of root biomass allocated to cluster roots in solution culture decreased with increasing P supply, and followed a generally similar pattern to that observed for arbuscular mycorrhizae in sand culture. No cluster roots formed at P supply of 100 µM P or higher, and the form of nitrogen available to the plant (either adequate supply of mineral nitrogen or dependency on symbiotic N2 fixation) had no effect on cluster root formation at any level of P supply. Casuarina cunninghamiana occurs naturally along stream and river banks and cluster roots may be advantageous at these sites by providing a reliable mechanism to enhance nutrient uptake in seasonally inundated environments that are unlikely to be favourable to mycorrhizal formation and activity.