Mycorrhizal Associations and Seedling Development in Australian Lobelioideae (Campanulaceae)

Abstract

The mycorrhizal associations of Australian Lobelioideae (Lobelia, Pratia and Isotoma) were found to be complex. Perennial species of these genera were solely VA mycorrhizal, whereas annual species were both VA and ectomycorrhizal. Lobelia gibbosa, L. simplicicaulis and L. rhomblfolia had unusual inter- cellular mycorrhizas formed with ectomycorrhizal ascomycetes.

While all annual Australian Lobelioideae formed associations with both VA and ectomycorrhizal fungi, five species of Lobelia from South Africa or North America were solely VA mycorrhizal, whether annual or perennial.

Seed of the Australian Lobelioideae examined required a ripening period of several months or longer before germination. Seed of L. gibbosa, L. simplicicaulis and L. rhombifolia on germination produced only a short radicle that did not develop further unless invaded by a mycorrhizal fungus. Seed of L. gibbosa and L. simplicicaulis placed beside growing mycorrhizas of certain ascomycetes with young plants of Melaleuca uncinata germinated, and the seedlings became mycorrhizal, grew, and flowered. The subterranean stage dependent on mycorrhizal association with a companion plant lasted 3-4 months.

Germinated seed of L. gibbosa on simple agar media failed to become mycorrhizal with appropriate ascomycetes or to develop further. In asymbiotic culture on media incorporating sucrose seedling growth was slight but, in the presence of sucrose and kinetin or N-benzylaminopurine, L. gibbosa developed shoots and often formed small plants that flowered. Root development, however, was scanty or absent even in the presence of the growth substances α-naphthaleneacetic acid or 2,4-Dichlorophenoxyacetic acid.

Young plants of L. gibbosa from the field transplanted to U.C. mix or sterilised forest soil failed to produce new roots. The plants grew slowly but remained small and produced few flowers, suggesting that mycorrhizal companion plants may also contribute to their growth besides being necessary in seedling development.