Progress 09/15/04 to 09/14/07
Outputs
OUTPUTS: Acidified matrices consisting of caclined montmorillonite or crushed basalt were developed for evaluating the sensitivity of Acacia koa to Al or Mn toxicity in the presence or absence of microsymbionts. Several Arbuscular mycorrhizal (AM) fungal isolates were obtained from A. Koa roots in the field and acid soils collected from several sites on the Big Island, Maui, and Oahu. The extreme sensitivity of A. Koa to soil acidity when started from seed and small seedlings initially forced us to use other plant species as baits for isolating AM fungi from the soil samples. Subsequently, we discovered that relatively good growth of the legume could be obtained in acid soil by starting with large seedlings raised in nonacid medium. These were used to isolate more AM fungi. Evaluation of one of the latter isolates on its ability to protect A. Koa against aluminum toxicity in the calcined montmorillonite matrix described above yielded very promising results. In the presence of 600 mg
of Al per at pH 4.5, reduction in growth of A. Koa due to Al toxicity was reduced from 56% to 17% by AM fungal inoculation. In a study designed to evaluate the effectiveness AM fungi in protecting A. Koa against aluminum and manganese toxicity in soil, P uptake by A. Koa was stimulated significantly by inoculation, but there was no significant effect on dry matter yield, although plants grown in the presence of the inocula appeared healthier and greener than those that were not inoculated with the isolates. In a separate study, we had found that our elite isolate, Glomus aggregatum was as effective, or better effective than fungi that are indigenous to two acid soils. We evaluated the effectiveness of the fungus in protecting A. Koa against aluminum toxicity. The fungus enhanced P uptake by A. Koa and stimulated root and dry matter yield by the legume significantly in an acid aluminum rich soil, although the values observed were significantly lower than those observed if the soil was
limed to pH 6.4. Our findings suggest that AM fungi can reduce the adverse effect of Al toxicity on A. koa significantly. None of the Mn-rich acid soils we examined had pH values lower than 5.0, and A. koa was not particularly sensitive to Mn at these pHs. Twenty-two isolates of Bradyrhizobium were obtained from soils and nodules collected from the above mentioned sites. The isolates exhibited wide variations in nodulating A. Koa and in their symbiotic efficacy. So far, we have evaluated over half of the isolates for their ability to survive in aluminum-rich and manganese-rich acid soils, and none of them could survive in aluminum-rich soil, although recently we have identified two isolates that can survive in manganese-rich acid soil. The task of evaluating the effectiveness of AM fungi and bradyrhizobia together for their effectiveness in enhancing the establishment of A. Koa in acid soils has yet to be done because of this constraint. We will be able to do so within the next six
months.
PARTICIPANTS: The Co-PI, P. G. Scowcroft, played significant roles in the initial identification of sites for soil, seed, and nodule collection, and made the necessary arrangements for collecting soil, seeds, and nodule samples. However, shortly after the initiation of the project, his organization relocated from Oahu to the Big Island. His contribution to the project after his transfer to the Big Island was limited to seed collection and e-mail exchanges. Mr. Gaoussou, the graduate research assistant funded under the project, is in the process of writing his MS thesis. He has learned a lot about research techniques in general and about research revolving around plant-arbuscular mycorrhizal interaction in particular because of his participation in the project. Some of the techniques he picked up include analyzing soil and tissue samples for P, evaluating root samples for arbuscular mycorrhizal colonization and statistical analysis of research data.
TARGET AUDIENCES: Forestry researchers, tree growers, seedling producers, forestry extension specialists, forestry extension agents, mycorrhizal researchers, mycorrhizal inoculum producers, environmental restoration firms.
PROJECT MODIFICATIONS: Because of unavailability of sufficient seeds from different acid sites in Hawaii, we were not able to screen provenances of A. koa for sensitivity to acid soil toxicity. The long time required for equilibration of soil pH following acidification, has forced us to abandon the use of calcined Montmorillonite beyond the initial experiment, and instead has convinced us to rely on soils that had desired target pHs in their natural state.
Impacts
We now have numerous isolates of arbuscular mycorrhizal fungi and bradyrhizobia that will be useful to us and other researchers in the future. While AM fungi cannot completely protect A. koa against acid soil toxicity, we have found that they can reduce the adverse effects of acid soil toxicity significantly. Our findings that the tolerance to acid soil toxicity of A. koa can be enhanced through the management of seedling size suggests that combining large seedlings with appropriate AM fungi holds more promise for establishing A. koa on acid soils than the prospect of combing provenances of A. koa with appropriate AM fungi. We believe that the approach is transferable to the study of leguminous species other than A. koa. Our findings that nodules collected from roots of A. koa growing in acid soils in the field do not necessarily contain acid tolerant rhizobia and that the true tolerance to acid soil toxicity of bradyrhizobia is best ascertained by evaluating the
survival of the isolates in acid soils will save researchers in the field considerable amount of time and help them generate more reliable information on the tolerance of bradyrhizobia to acid soil toxicity.
Publications
- No publications reported this period
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Progress 10/01/05 to 09/30/06
Outputs
Several Bradyrhizobial isolates were evaluated for survival ability in manganese-rich and aluminum-rich acid soils. None of the Bradyrhizobia isolated from Al-rich soils or Mn-rich soil survived in aluminum-rich acid soils irrespective of whether they were isolated from nodules collected from Acacia koa roots in the field or from soil from the rhizosphere of Acacia koa. Of the Bradyrhizobia isolated from Al-rich acid soils two of the isolates had excellent survival ability in Mn-rich soils, but none survived in Al-rich acid soils. Of the arbuscular mycorrhizal isolates we obtained, one isolated from Al-rich acid soil and another one obtained form Mn-rich acid soil exhibited relatively high inoculum potential. These were evaluated for their effect in enhancing nutrient uptake and growth of A. Koa in Mn-rich and Al-rich acid soils, respectively. Bradyrhizobia isolated from the Al-rich soil induced relatively high levels of AM fungal colonization on A. Koa roots at pH
4.5. The level of colonization was not only significantly higher than that observed in the acid soil not inoculated with the isolates but also was comparable to that observed at pH 5.7. Similarly, the isolate obtained from the Mn-rich acid soil increased AM fungal colonization of A. Koa roots significantly and the level of colonization observed was similar or better than that observed at pH 5.9. Leaf P status was significantly increased due to inoculation in both systems. However, the growth of A. koa was not stimulated significantly due to inoculation by any of the isolates in either of the soils. However, plants grown in the inoculated soils defoliated to a lesser extent and were appreciably greener than those grown in the non-inoculated soil. The results suggest that either the AM fungi did not have sufficient time to enhance the growth of A. koa or that some other factor was limiting the growth of the legume.
Impacts
With some more refinements, we will be on our way to developing a system that will enable us to establishing A.koa in acid soils containing toxic levels of manganese. We will be in a similar position to establishing A. koa in Al-rich acid soils as soon as we succeed in isolating Bradyrhizobia that are tolerant to Al-toxitiy.
Publications
- M. Habte. 2006. Contributions of arbuscular mycorrhizal fungi to soil and plant health in the tropics. A paper presented at the Permanent Seminar of the National University of Colombia at Medellin. Medellin, Colombia.
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Progress 10/01/04 to 09/30/05
Outputs
During the current fiscal period, we determined the concentration of H2SO4 necessary to adjust the pH of the matrices Calcined Montmorillonite (Turface) and crushed basalt (Mansand) to around 4.5-4.9. The pH of Turface was adjusted to an equilibrium value of 4.6 by incubating the material at maximal water holding capacity using 0.25% sulfuric acid solution for four weeks. Mansand was adjusted to 4.7 by using 2.5% sulfuric acid. These values remained stable for a year. The materials are needed for evaluating the tolerance limits of Acacia koa to Al and Mn toxicity in the presence or absence of tolerant arbuscular mycorrhizal fungi and Bradyrhizobia. We also characterized 26 soil samples collected for isolating Bradyrhizobia and arbuscular mycorrhizal fungi (AMF) for pH and contents of Al, Mn, and basic cations. We devoted the rest of the time to isolating and evaluating acid tolerant AMF and Bradyrhizobia. We tried to isolate Al- and Mn- tolerant arbuscular mycorrhizal
fungi (AMF) from the soils and from roots of A. Koa and a couple of other known plant species growing on Al-rich and Mn-rich soils collected from the Big Island, Oahu, and Maui. These attempts have yielded eighteen isolates of mixed populations. Some these isolates have been multiplied and are currently being evaluated for their effectiveness. Others are currently being multiplied. We had greater success in isolating AMF from soils than from roots. Leucaena leucocephala K636 and Trifolium repense were the nurse plants used; the former yielded isolates largely in the form of roots containing arbuscules and vesicles while the latter yielded isolates in the form of numerous spores attached to roots, with very little arbuscular and vesicular infection. We tried to isolate Al-tolerant and Mn-tolerant Bradyrhizobia directly from nodules of A. koa growing in acid soils in the Big Island, Oahu and Maui. We also tried to isolate Bradyrhizobia after growing A. koa on soils collected from the
sites. Of 29 isolates obtained, 23 have been evaluated for their nodulating ability and of these 7 were ineffectively nodulated and hence were considered unsatisfactory. Three isolates obtained from Umikoa Ranch on the Big Island, and the Pacific Palisades, and Koa Ridge on Oahu failed to survive beyond 48 h when introduced into sterilized Al-rich and Mn-rich acid soils obtained from the Waiawa Correctional Facility. Other isolates are in the process of being evaluated. Preliminary observation during testing of isolates for nodulation suggest that there are wide variations in symbiotic effectiveness among the Bradyrhizobial isolates.
Impacts
The unusual frequency with which infective nodules occurred among isolates obtained from the Big Island is of concern and our observation invites future research on the influence that such nodulation might have on the nitrogen nutrition of A. Koa. On the other hand, the fact that there were variations among bradyrhizobial isolates from acid soils effectively nodulating A. koa offers opportunity for selecting elite strains of Bradyrhizobia and to use them in combination with AMF in order to establish the legume on acid soils.
Publications
- No publications reported this period
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