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 natura l populations of Casuarina cunninghamiana Mig. in north Queensland. Cl uster roots were found at 90% of the sites surveyed. By contrast, arbu scular mycorrhizae occurred at only 45% of sites and the extent of col onisation was low. No ectomycorrhizae were found during this survey. F ormation of arbuscular mycorrhizae and cluster roots in C. cunninghami ana seedlings in relation to external P supply was investigated in gla sshouse experiments. inoculation with the arbuscular mycorrhizal fungu s, 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 colonisatio n of roots was highest at 0 and IO mg P per kg soil and declined with further increases in P supply. No mycorrhizae formed at or above 100 m g P per kg soil. The number, dry weight and proportion of root biomass allocated to cluster roots in solution culture decreased with increas ing P supply, and followed a generally similar pattern to that observe d for arbuscular mycorrhizae in sand culture. No cluster roots formed at P supply of 100 mu M P or higher, and the form of nitrogen availabl e to the plant (either adequate supply of mineral nitrogen or dependen cy on symbiotic N-2 fixation) had no effect on cluster root formation at any level of P supply. Casuarina cunninghamiana occurs naturally al ong stream and river banks and cluster roots may be advantageous at th ese sites by Providing a reliable mechanism to enhance nutrient uptake in seasonally inundated environments that are unlikely to be favourab le to mycorrhizal formation and activity.