Effect of root-inhabiting fungi on root growth potential of Uapaca kirkiana (Muell. Arg.) seedlings

Abstract

The influences of mycorrhizal fungi on the nutrient uptake and growth of host plants, as well as other advantages conferred on their hosts such as increasing resistance to drought, disease and heavy metal toxicities, have been very well documented. However, relatively less attention has been paid to the influence of pure and mixed cultures of these organisms on the ability of host seedlings to produce new roots rapidly, which is a key factor in seedling vigour and survival after planting. Therefore the objective of this study was to investigate this influence in Uapaca kirkiana (Muell. Arg.), a newly domesticated African fruit tree. To achieve this objective, seven pure cultures and a mixed culture of fungal root associates were isolated from Uapaca kirkiana seedlings dug out from a protected natural forest area near Kitwe (Zambia). The mixed culture was a suspension of mycelia and spores from roots cultured in distilled water. Pure cultures were characterized, and both pure and mixed cultures were later back-inoculated into fresh host seedlings whose white root tips had been removed. Root growth potential (RGP), defined as the total quantity of new white roots produced, was determined for each fungal inoculum after 28 days. A significant increase in new white root production compared with the uninoculated control ( P < 0.05) was observed in plants inoculated with the mixed culture and with inocula from six out of the seven pure isolates. However, the average number of new roots produced per plant in four out of those six isolates was significantly greater than those from plants receiving the general mixed culture. The number of surviving seedlings with new roots, RGP and inoculum source were all inter-related. Mortality was strongly related to a limitation in the plants' ability to produce new roots ( r= −1; P < 0.01), and all the plants that did not produce new roots died. The greatest seedling survival was 84%, which was found in plants inoculated with the mixed culture. The ability of some of the isolates and the mixture of isolates to significantly stimulate new root growth and enhance seedling survival compared with a sterile control is of practical importance to seedling regeneration and may be used as an index of seedling resilience and reforestation success in hostile environments.