M. Habte

Categories of vesicular-arbuscular mycorrhizal dependency of host species

SummaryBrassica nigra and selected species of Leucaena and Sesbania were used as indicator hosts in a greenhouse experiment designed to establish distinct categories of mycorrhizal dependence. The plants were grown in an oxisol with different concentrations of established soil solution P in the presence or absence of the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus aggregatum. The extent to which the plant species depended on the fungus for dry matter production diminished with increased concentrations of soil solution P, but the magnitude of this decrease varied from species to species. Five distinct mycorrhizal categories are proposed based on the differences observed, ranging from non-dependent to very highly dependent. The critical soil solution P concentrations that were useful for separating host species into distinct VAM-dependency groups were 0.02 and 0.2 mg/l. Species differing in their mycorrhizal dependency differed with respect to the soil solution P concentration required for the expression of maximum VAM effectiveness, the degree to which increasing concentrations of P depressed VAM infection and the pattern of immobile nutrient accumulation.

PLANT MECHANISMS AND MYCORRHIZAL SYMBIOSES TO INCREASE PHOSPHORUS UPTAKE EFFICIENCY

The tendency of phosphorus (P) to undergo adsorption and precipitation reactions in soil makes it one of the most immobile of the essential plant nutrients. As a result, the concentration of available P in soil solution is usually very low even though the total P content of soils is generally high. Consequently, P is second only to N in limiting crop productivity. In areas of the world such as the North America and Europe in which P fertilizers are applied liberally to agricultural fields, the loss of this nutrient through its transport in particulate and soluble forms from agricultural fields to lakes and rivers is of concern due to accelerated eutrophication. The need to provide crops with adequate P combined with the desire to minimize the adverse effect of P on the environment will require strategies to reduce P inputs and to maximize the efficiency of P uptake mechanisms in plants. Recent advances in molecular biology offer opportunities to manipulate plants to increase uptake efficiencies of P. Integration of arbuscular mycorrhizal fungi into cropping systems provides an alternative means of maintaining yields while reducing P inputs. To realize the potential of these new technologies, however, a thorough understanding of both soil and plant factors involved in P uptake, and the effects of cultural practices on arbuscular mycorrhizal associations are needed. A review of the literature was undertaken to summarize the relevant state of knowledge on P uptake processes in plants and the mechanisms by which P uptake efficiencies could be enhanced, with particular emphasis on arbuscular mycorrhizal fungi. *Journal Series No. 4468, College of Tropical Agriculture and Human Resources, University of Hawaii, Honolulu, HI.

Further evidence for the regulation of bacterial populations in soil by protozoa

After the addition to soil of large numbers of a cowpea Rhizobium strain, the population declined steadily until the numbers reached about 107/g, and the protozoa rose to about 104/g. When indigenous protozoa were suppressed by the addition of actidione to the soil, the density of the test rhizobium did not fall initially, but its abundance declined to about 107/g when actidione-resistant protozoa arose in significant numbers. The addition to actidione-treated soil of an antibiotic-resistant strain of Paramecium led to a rapid decrease in the population of the rhizobium, the density reaching essentially the same value as in soil receiving neither the drug nor the paramecia. The same changes occurred with Xanthomonas campestris as test prey except that its numbers fell to about 105/g of soil. These data provide further evidence for the key role of protozoa in controlling the abundance of populations of certain bacteria introduced into soil.

Synergistic Influence of an Arbuscular Mycorrhizal Fungus and a P Solubilizing Fungus on Growth and P Uptake of Leucaena leucocephala in an Oxisol

An investigation was carried out to assess the role that P solubilizing micro-organisms play in the P nutrition of mycorrhizal and mycorrhiza-free Leucaena leucocephala (L am.). Soil microorganisms able to solubilize rock phosphate were isolated from the rhizosphere of L. leucocephala naturally growing on three different soils of Hawaii. The isolates were screened for their ability to solubilize rock phosphate in culture medium. The highest activity was observed with one of the fungal isolates, which was identified as Mortierella sp. It was multiplied and further evaluated with or without the mycorrhizal fungus Glomus aggregatum in a highly weathered soil for its effectiveness to enhance P uptake and growth of L. leucocephala. Phosphorus status of L. leucocephala pinnules monitored as a function of time revealed that plants colonized by both microorganisms had the highest P content followed by plants inoculated with the mycorrhizal fungus alone. Inoculation of soil with Mortierella sp. alone did not influence P content of plants measured at the time of harvest. However, Mortierella sp. increased the P content of mycorrhizal plants by 13% in the unfertilized soil and by 73% in the soil fertilized with rock phosphate. Shoot dry weight measurements showed that Mortierella sp. stimulated growth of nonmycorrhizal by 22%, while it stimulated the growth of myocorrhizal plants by 29% , regardless of P fertilization. The results suggest the existence of synergistic interaction between P solubilizing microorganisms and mycorrhizal fungi, although the degree of synergism was more pronounced in terms of P uptake than in terms of growth.

Development of vesicular-arbuscular mycorrhizal infection and the uptake of immobile nutrients inLeucaena leucocephala

The level of vesicular-arbuscular mycorrhizal (VAM) infection in the roots of Leucaena grown in a sand-soil mixture in the greenhouse increased rapidly with time and reached a peak value of 84% at 30 days from planting. The pattern of immobile nutrient uptake and accumulation closely paralleled that of the development of infection, particularly during the first 10–30 days after planting. Significant changes in dry matter yield were also observed only after a significant portion of the root length was colonized byGlomus aggregatum. The development of VAM infection was not accompanied by growth depression at any of the sampling periods. However, VAM roots had very high levels of Cu which was not translocated to shoots. It is hypothesized that such a diversion of Cu by the endophyte from the host could cause growth depression under conditions where the soil volume is supplied with sub-optimal levels of Cu.

Mechanisms of persistence of low numbers of bacteria preyed upon by protozoa

Abstract Tetrahymena pyriformis cultures were maintained when transferred serially in solutions containing 10 5 to 10 7 Klebsiella pneumoniae cells.ml −1 , bacterial numbers that were observed to persist in the presence of protozoa. The number of cells of one strain of K. pneumoniae surviving predation in solution was essentially the same in the absence of an alternative prey as in the presence of a second K. pneumoniae strain. Toxins deleterious to protozoa did not appear as the animal consumed the bacteria. T. pyriformis reduced the abundance of Escherichia coli from about 10 8 to 10 6 .ml −1 . but the latter number persisted for 15 days; however, in solutions containing chloramphenicol, the abundance of E. coli fell to 590 cells. ml −1 in 15 days. In solutions containing the antibiotic, T. pyriformis reduced the Rhizobium sp. population from more than 10 6 to less than 10 3 cells in 10 days and K. pneumoniae from more than 10 8 .ml −1 to zero in 18 days. An appreciable decline in abundance of these bacteria did not occur in the antibiotic-amended liquid free of protozoa. T. pyriformis did not greatly reduce Rhizobium sp. numbers when both were added to irradiated soil, but the predator caused the bacterial population to decline from 4 × 10 8 to fewer than 10 5 .g −1 in 16 days in chloramphenicol-treated soil. Colpoda sp. inoculated with Rhizobium sp. into soil sterilized by autoclaving only reduced the prey abundance from 10 9 to 10 8 .g −1 , but the protozoan caused the bacterial population to fall to about 100.g −1 in 15 days in the presence of the antibiotic. The population of Rhizobium sp. added to nonsterile soil dropped from in excess of 10 8 to 6 × 10 6 .g −1 in 29 days. but it declined to 550. g −1 in the same period when chloramphenicol was also introduced. It is concluded that the ability of these bacteria to maintain themselves in solution and in soil is governed by their capacity to reproduce and replace the cells consumed by predation.

Protozoan Density and the Coexistence of Protozoan Predators and Bacterial Prey

Low numbers of Tetrahymena pyriformis provided with °2 x 109 Klebsiella pneumoniae or Rhizobium sp. cells/ml reduced the bacterial population only to 107/ml in solution. When 5—7 x 108 K. pneumoniae or 1 x 108 Rhizobium sp. were preyed upon, 106 bacteria/ml survived and coexisted with the ciliate. Low initial densities of the protozoan provided with 3 x 106 or 5 x 107 K. pneumoniae or 3—4 x 107 Rhizobium sp./ml reduced the prey population t 2—3 x 105/ml. However, when the initial number of T. pyriformis was high (2—34 x 104/ml) and the K. pneumonia or Rhizobium sp. density was °107/ml, the animal did not reduce the bacterial population size. The data suggest that the number of K. pneumoniae or Rhizobium sp. cells able to coexist in solution with T. pyriformis was governed by the number of protozoa initially present or developing as a result of predation. By contrast, 105 Xanthomonas citri cells/ml coexisted with the ciliate whether this bacterium was provided at 7 x 106 or 8 x 108 cells/ml. In soil treate...

Determining vesicular‐arbuscular mycorrh izal effectiveness by monitoring P status of subleaflets of an indicator plant

The development of rapid and reliable techniques for evaluating the effectiveness of vesicular‐arbuscular mycorrhizal (VAM) fungi is crucial in optimizing conditions for enhancing the benefits of the fungi in crop production. In this study we have proposed and tested such a technique. It is based on the monitoring of P status of subleaflets of indicator plant (Leucaena leucocephala) grown in sand‐soil mixture optimized for VAM activity. Individual subleaflets (pinnules) are regularly removed from the plant, dried and their P contents determined colorimetrically. P contents monitored by this technique were linearly related to shoot P contents (r = 0.97). The technique was useful for evaluating the influence of propagule density on VAM activity, for separating soils into categories of VAM effectiveness and for assessing the effects of soil biotic factors on VAM activity. The technique is a simple but powerful tool for selecting host‐endophyte combinations and for creating environmental conditions favorable ...

Response ofLeucaena leucocephala to vesicular-arbuscular mycorrhizal colonization and rock phosphate fertilization in an Oxisol

Response ofLeucaena leucocephala (Lam) de Wit to rock phosphate application and inoculation with the vesicular-arbuscular mycorrhizal (VAM) fungusGlomus aggregatum (Schenck and Smith emend Koske) was evaluated in a pot experiment. VAM colonization increased as rock phosphate application increased. Using phosphorus concentration in pinnules as an indicator of VAM activity, significant VAM activity occurred at 25 days after planting at the lower levels of rock phosphate application (0, 0.34 and 0.68 g P kg−1). The time required for significant VAM activity was shortened by 5 days at the higher P levels (1.36, 2.72 and 5.44 g P kg−1). The highest VAM activity was associated with the highest rate of rock phosphate application.Inoculation withG. aggregatum significantly increased the uptake of Cu, P and Zn and dry-matter yield at all levels of rock phosphate applied. Copper concentrations in roots of mycorrhizal Leucaena were significantly higher than that of shoots. The results indicated that Leucaena in symbiotic association with VAM fungi effectively utilized P from rock phosphate. However, high rates of rock phosphate are required to attain growth comparable to that obtained with the application of water-soluble phosphate.

Impact of simulated erosion on the abundance and activity of indigenous vesicular-arbuscular mycorrhizal endophytes in an Oxisol

SummaryThe influence of simulated erosion on the abundance and activity of indigenous vesiculararbuscular mycorrhizal (VAM) populations was evaluated in an Oxisol. Surface-soil losses in excess of 7.5 cm were generally associated with significant decreases in the numbers of total and active VAM propagules and in the symbiotic effectiveness of the active propagules. Surface-soil removal not exceeding 7.5 cm was associated with decreased propagule abundance without adverse effects on VAM colonization of roots and symbiotic effectiveness of the fungi. The extent of VAM colonization of roots and the degree of symbiotic effectiveness observed at this level of simulated erosion were significantly higher than those observed in the soil not subjected to simulated erosion. This stimulation is attributed to the removal of antagonistic biotic factors as the top 7.5 cm of soil was removed. It is concluded that propagules lost during erosional soil losses must be replaced before legumes grown on relatively highly weathered and severely eroded tropical soils could appreciably benefit from the VAM symbiosis.