P. Reddell et al., CLUSTER ROOTS AND MYCORRHIZAE IN CASUARINA-CUNNINGHAMIANA - THEIR OCCURRENCE AND FORMATION IN RELATION TO PHOSPHORUS SUPPLY, Australian Journal of Botany, 45(1), 1997, pp. 41-51
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.