Mitiku Habte

Mycorrhiza-free and mycorrhizal seedlings of onion were transplanted in the field along rows of previously established plants of ginger in order to evaluate the possibility of enhancing arbuscular mycorrhizal (AM) colonization of ginger via a companion crop. Arbuscular mycorrhizal fungal colonization of roots determined 138 days after transplanting of onion showed that roots of all plants were colonized by the fungi. However, the level of AM fungal colonization of onion roots was significantly higher if onion seedlings were colonized by the fungi prior to transplanting than if they were not. Dry matter yield of onion was more than tripled by AM fungal inoculation. The level of AM fungal colonization of ginger roots was significantly enhanced in the presence of a mycorrhizal companion onion, not in the presence of a non-mycorrhizal one. However, the yield of ginger was unaffected by the presence of onion irrespective of mycorrhizal status. Our data raise the possibility that the mycorrhizal status of an established plant may be enhanced through the introduction of a compatible companion crop well colonized by AM fungi.

A series of experiments were conducted to evaluate the influence of charcoal on the development of arbuscular mycorrhiza (AM) on Leucaena leucocephala roots and the contribution of the symbiosis to the phosphorus (P) nutrition and growth of the legume. Arbuscular mycorrhizal fungal colonization of plants raised in Mansand (crushed basalt) in the first experiment was reduced if the medium was amended with fine charcoal and not with coarse charcoal. Charcoal amendment had no effect on AM fungal colonization, AM symbiotic effectiveness measured as pinnule (subleaflet) P content, or on growth of L. leucocephala in soil in the first experiment and in Mansand and in soil in subsequent experiments. However, AM fungal colonization of L. leucocephala roots, P content of pinnules, and growth of the legume were significantly enhanced (P < 0.05) by AM fungal inoculation in all experiments regardless of the growth medium used or charcoal amendment.

An investigation was undertaken to test the hypothesis that amending peat to increase its P buffer capacity and optimizing the P concentration of the amended medium for mycorrhizal activity will enhance its usefulness for raising mycorrhizal seedlings of tree species. The approaches entailed mixing a small quantity of soil of high P adsorbing capacity with peat and constructing a P sorption isotherm for the medium in order to establish solution P concentration near-optimal to optimal for mycorrhization of seedlings. A P sorption isotherm based on incubating the medium with graded amounts of P at 50% of available water-holding capacity was developed. Target solution P concentrations established using the approach enabled us to identify the optimal solution P concentration for mycorrhizal development on roots of our indicator plant Leucaena leucocephala grown in the medium. Arbuscular mycorrhizal colonization, host growth, and P status of L. leucocephala pinnules measured at target solution P concentrations ranging from 0.12 to 1.0 mg L -1 revealed that AM fungal activity and symbiotic effectiveness was maximum at solution P concentration of 0.2 mg P L -1 . Medium solution P concentrations in excess of 0.2 mg L -1 tended to depress AM fungal colonization, but colonization level did not decline below 43%.