Physiological effects of indigenous arbuscular mycorrhizal associations on the sclerophyll Agathosma betulina (Berg.) Pillans

[摘要] ENGLISH ABSTRACT: The Mountain Fynbos biome, a division of the Cape Floristic Region (CFR), ishome to round-leafed Buchu [Agathosma betulina (Berg.) Pillans], one of SouthAfrica's best-known endangered herbal medicinal plants. Agathosma betulina isrenowned as a traditional additive to brandy or tea, which is used for thetreatment of a myriad of ailments. In its natural habitat, A. betulina thrives onmountain slopes in acid and highly leached gravelly soils, with a low basesaturation and low concentrations of organic matter. To adapt to such adverseconditions, these plants have formed mutualistic symbioses with arbuscularmycorrhizal (AM) fungi. In this study, the effect of indigenous AM taxa on thephysiology of A. betulina is investigated. In addition, the AM taxa responsible forthese physiological responses in the plant were identified using morphologicaland molecular techniques.Agathosma betulina was grown under glasshouse conditions in its nativerhizosphere soil containing a mixed population of AM fungi. Control plants,grown in the absence of AM fungi, were included in the experimentation. In atime-course study, relative growth rate (RGR), phosphorus (P)-uptake, Putilization cost, and carbon (C)-economy of the AM symbiosis were calculated.The data showed that the initial stages of growth were characterized by aprogressive increase in AM colonization. This resulted in an enhanced P-uptakein relation to non-AM plants once the symbiosis was established. Consequently,the lower P utilization cost in AM plants indicated that these plants were more efficient in acquiring P than non-AM plants. When colonization levels peaked,AM plants had consistently higher growth respiration. This indicated that thesymbiosis was resulting in a C-cost to the host plant, characterized by a lowerRGR in AM plants compared to non-AM plants. Arbuscular mycorrhizalcolonization decreased with increasing plant age that coincided with a decline inP-uptake and growth respiration, along with increases in RGR to a level equal tonon-AM plants. Consequently, the AM benefit was only observed during theinitial stages of growth. In order to identify the AM fungi in planta, morphologicaland molecular techniques were employed, which indicated colonization by AMfungi belonging to the genera Acaulospora and Glomus. Phylogenetic analysesof a dataset containing aligned 5.8S ribosomal RNA gene sequences from allfamilies within the Glomeromycota, including sequences obtained during thestudy, supported the above mentioned identification.

[发布机构] Stellenbosch University