Arbuscular Mycorrhizal Associations of Metrosideros Excelsa and M. Robusta (Myrtaceae)

Abstract

Arbuscular mycorrhizal (AM) fungi inhabiting selected tree root zones of naturally occurring Metrosideros excelsa of Coromandel Peninsula and M. robusta of the Rimutaka Ranges were isolated from field and sorghum 'trap' culture samples and identified. Nineteen AM species were found in both study sites: Acaulospora dilatata, A. excavata, A. lacunosa, A. laevis, A. trappei, Acaulospora spl, Acaulospora sp2, Entrophospora infrequens, Glomus aggregatum, G. claroideum, G. fasciculatum, G. intraradices, G. invermaium, G. sinuosum, G. rubiforme, G. Tortuosum, G. versiforme, Glomus spl and Scutellospora calospora. Samples of Coromandel Peninsula did not harbour AM species such as A. excavata, A. laevis, Acaulospora spl, and G. tortuosum, while G. aggregatum, G. sinuosum and Glomus spl were not found in samples of Rimutaka Ranges. Previously known species include those of A. laevis, E. infrequens, G. fasciculatum, G. invermaium and G. rubiforme, while others were new records to New Zealand. Seedlings of Metrosideros excelsa and M. robusta were raised in these soil types in greenhouse pot cultures and sporulation of AM fungi was monitored. The following AM species sporulated in association with these two hosts: A. dilatata, A. lacunosa, A. trappei, E. infrequens, G. claroideum, G. fasciculatum, G. intraradices, G. invermaium, G. rubiforme, G. versiforme, and S. calospora. Spores of A. excavata, A. laevis, G. sinuosum and G. tortuosum, which were recovered only from field-collected soils, were not evident in pot cultures. Taxonomic details of all these AM species, including patterns of spore development of A. dilatata, A. lacunosa, A. trappei, Acaulospora spl, E. infrequens, G. claroideum, G. fasciculatum, G. intraradices, G. invermaium, G. rubiforme, G. versiforme, Glomus spl and S. calospora, which were raised in monospecific cultures, are documented. Seedlings of pot-grown Metrosideros excelsa and. M. robusta showed AM colonization by three months of growth. Mycorrhizal seedlings of M. excelsa raised in soils of Rimutaka Ranges and M. robusta of Coromandel Peninsula exhibited higher relative growth rates than their non-mycorrhizal counterparts. Mycorrhizal seedlings of both plant species which were subjected to intraspecific competition showed higher leaf chlorophylls : carotenoids ratios and root alkaline phosphatase activity than non-mycorrhizal seedlings, indicating that AM root colonization might be responsible for causing mycorrhizal seedlings to be more stress-tolerant. In another experiment, seedlings of both plant species were raised in an inoculum of known AM fungi and subjected to nutrient fluctuations: continuous application of a P-amended liquid nutrient solution for 12 weeks (nutrient enrichment) followed by a further period of 30 weeks without nutrient addition (nutrient depletion). Nutrient enrichment suppressed AM formation and lowered the relative growth rates of both plant species. Subsequent AM root colonization after l2-18 weeks of nutrient depletion resulted in 2-3 times higher relative growth rates in AM-colonized M. excelsa seedlings than the uncolonized counterparts, while such differences were not significant in M. robusta, suggesting that plant species such as M. excelsa are able to utilize AM associations and soil fertility more efficiently. Seven out of nine AM species of the original substrate sporulated, with Acaulospora dilatata dominating. Results suggest that majority of the AM fungi that are subjected to prolonged soil nutrient enrichment could remain viable without host root contacts and resume root colonization when plants begin to deplete the available soil nutrients.