Mitiku Habte, Department of Tropical Soils and Plant Sciences, UH-CTAHR
Craig Elevitch, Agroforester
Paul Scowcroft, Institute of Pacific Islands Forestry, USDA Forest Service
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Contact Dr. Mitiku Habte (mitiku@hawaii.edu)
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OBJECTIVES
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- Evaluate the suitability of the different soilless media currently used by the local nursery industry with respect to the mycorrhization of seedlings.
- Determine the quantity of soil materials, P and other essential nutrients that must be added to an appropriate soilless medium in order to obtain maximum development of arbuscular mycorrhiza on seedlings.
- Determine the density of AM propagules that must be added to a suitable mycorrhization medium and the duration to which seedlings must be exposed to the fungi for rapid colonization of containerized seedlings and for best post-transplanting performance.
- Determine the influence of age of seedlings on the post-transplant performance of mycorrhizal and non-mycorrhizal plants.
- Determine the post-transplant performance of seedlings in the field.
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APPROACH
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A variety of soilless media will be screened for relative suitability for mycorrhiza development. The best soilless medium will be mixed with small quantity of an appropriate mineral soil and the nutrient content of the mixture will be optimized for maximal mycorrhization. Using the medium, the density of starter mycorrhizal inoculum, the age at which seedlings must be exposed to the inoculum and the duration to which seedlings must remain in contact with the inoculum will be standardized. The post-transplant performance of seedlings produced using the standardized procedure will be evaluated in the field. Based on the results, a protocol for mycorrhization of seedlings in containerized nursery will be developed.
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PROGRESS October 1999 to September 2000
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Having earlier identified the most suitable peat-based medium for producing mycorrhizal seedlings of tree species, we carried out investigations designed to determine the optimal levels of P and other essential nutrients that are compatible with the production of robust mycorrhizal seedlings. Based on the P sorption isotherm of the peat-based medium, a series of target soil solution P concentrations were evaluated for enhancing the development of mycorrhiza and nonmycorrhizal seedlings of Leucaena leucocephala . The results revealed that the optimum soil solution P concentration ranged between 0.2 and 0.4 mg P l-1. In order to determine the level of other nutrients necessary for mycorrhization of seedlings, different strengths of P-free Hoagland's solution were evaluated at solution P concentration of 0.2 mg l-1. The results showed that the lower two strengths of Hoagland's solutions were suitable for our purpose while higher strengths of the nutrient solution curtailed plant as well as mycorrhizal development, presumably because of salt effect. We also investigated the possibility of using slow release fertilizers in the production of healthy mycorrhizal seedlings. Optimal rates of the slow release micro- and macro nutrients were identified and found to be superior to highly soluble nutrient solutions in enhancing mycorrhizal development and seedling growth.
October 2000 to September 2001
Results of our earlier investigations have established that mycorrhizal seedlings could be produced in containers either using soluble nutrients or slow release fertilizers. In one of the studies we carried out during the current reporting period, the results suggested that some tree species neither developed acceptable level of AMF colonization nor grew well when a slow release fertilizer was used as the source of supplementary nutrients. This observation prompted us to explore the possibility of using soluble nutrient formulations which nursery growers can readily purchase from garden shops or plant food vendors. We have identified levels of a locally available nutrient mix that are compatible with the development of robust mycorrhizal seedlings using Leucaena leucocepahala as an indicator plant. As part of the same effort, we also observed that calcined motmorillonite can serve as a useful mineral additive to peat as well as the sole source of P for the production of mycorrhizal seedlings in containers. Our efforts to identify the critical arbuscular mycorrhizal fungal (AMF) propagule density associated with rapid development AMF colonization revealed that insufficiency of infective propagules can be a major source of variation in the production of mycorrhizal seedlings in containers. Our results show that as many as 1600 infective propagules may be required to produce seedlings having AMF colonization levels higher than 60%.
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IMPACTS
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Our investigations continue to yield useful information on the behavior of peat-based media which will impact tree and other seedling production enterprises in a significant way. One of the firms involved in the maintenance and propagation of native and endangered species of Hawaii is currently in the process of implementing the results of our studies. Our approaches will ultimately enable nursery growers to produce robust seedlings with much reduced levels of chemicals. And the seedlings will establish in the field faster and will grow at soil P concentrations much lower than would be required to grow mycorrhiza-free seedlings.
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