Limitation of vesicular-arbuscular mycorrhizal activity in Leucaena leucocephala by Ca insufficiency in an acid Mn-rich oxisol

Glomus aggregatum and Leucaena leucocephala were interacted in an acid Mn-rich oxisol unamended or amended with lime [Ca(OH)₂] or gypsum (CaSO₄) at soil P concentrations considered optimal for mycorrhizal host growth and sufficient for mycorrhiza-free growth. At 0.02 mg P 1^-1, both vesicular-arbuscular mycorrhizal fungal (VAMF) colonization and function were significantly curtailed if soil was not amended with gypsum or lime. The highest mycorrhizal effect was observed in the limed soil, followed by the soil treated with gypsum at the rate of 32 g Ca kg^-1 soil. Higher concentrations of gypsum deleteriously affected VAMF infectivity and effectivity. The first increment of gypsum compensated completely for the VAMF colonization and for part of the mycorrhizal effect that was lost due to low pH. The superiority of the limed soil to that amended with gypsum apparently lies in the fact that Ca supply was assured in the former and also that the adverse effects of toxic constituents such as H⁺ and Mn²⁺ were eliminated. We were unable to separate the effect of Ca on VAMF from its effect on the host because a P concentration sufficient for mycorrhiza-free growth was not attained due to interaction of some of the P with Ca to form insoluble phosphate.Contribution from the Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No. 4045     

Vesicular-arbuscular mycorrhizal effectiveness in an acid soil amended with fresh organic matter

A greenhouse investigation was undertaken to determine the influence of fresh organic matter on the formation and functioning of vesicular-arbuscular mycorrhizal symbiosis in Leucaena leucocephala grown in an acid aluminum-rich ultisol. In soil not amended with fresh organic matter or lime, plants failed to grow. Mycorrhizal infection level, mycorrhizal effectiveness measured in terms of pinnule P content of L. leucocephala leaves and dry matter yield of the legume increased with increase in fresh organic matter. Although VAM colonization level and dry matter yield of L. leucocephala were significantly higher if the test soil was limed (7.2 cmole OH^–) than if amended with fresh organic matter, the latter was as effective as lime in off-setting the detrimental effect of aluminum on mycorrhizal effectiveness. The lower mycorrhizal colonization level and the lower dry matter yield noted in the soil treated with fresh organic matter appears to be related to the inadequacy of Ca in the soil amended with fresh organic matter. These observations are supported by the low calcium status of soil and plant tissues in the absence of lime. It is concluded that while fresh organic matter, in appropriate amounts, could protect sensitive plants and VAM symbiosis against Al toxicity in acid soils, maximum mycorrhizal inoculation effects are not likely to be attained unless the soils are also amended with Ca.Contribution from Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No 3740.     

Mimosine produced by the tree-legume Leucaena provides growth advantages to some Rhizobium strains that utilize it as a source of carbon and nitrogen

Growth of most Rhizobium strains is inhibited by mimosine, a toxin found in large quantities in the seeds, foliage and roots of plants of the genera Leucaena and Mimosa. Some Leucaena-nodulating strains of Rhizobium can degrade mimosine (Mid⁺) and are less inhibited by mimosine in the growth medium than the mimosine-nondegrading (Mid^-) strains. Ten Mid⁺ strains were identified that did not degrade 3-hydroxy-4-pyridone (HP), a toxic intermediate of mimosine degradation. However, mimosine was completely degraded by these strains and HP was not accumulated in the cells when these strains were grown in a medium containing mimosine as the sole source of carbon and nitrogen. The mimosine-degrading ability of rhizobia is not essential for nodulation of Leucaena species, but it provides growth advantages to Rhizobium strains that can utilize mimosine, and it suppresses the growth of other strains that are sensitive to this toxin.     

Mimosine, a Toxin Present in Leguminous Trees (Leucaena spp.), Induces a Mimosine-Degrading Enzyme Activity in Some Rhizobium Strains

Thirty-seven Rhizobium isolates obtained from the nodules of leguminous trees (Leucaena spp.) were selected on the basis of their ability to catabolize mimosine, a toxin found in large quantities in the seeds, foliage, and roots of plants of the genera Leucaena and Mimosa. A new medium containing mimosine as the sole source of carbon and nitrogen was used for selection. The enzymes of the mimosine catabolic pathway were inducible and were present in the soluble fraction of the cell extract of induced cells. On the basis of a comparison of the growth rates of Rhizobium strains on general carbon and nitrogen sources versus mimosine, the toxin appears to be converted mostly to biomass and carbon dioxide. Most isolates able to grow on mimosine as a source of carbon and nitrogen are also able to utilize 3-hydroxy-4-pyridone, a toxic intermediate of mimosine degradation in other organisms.     

Mycorrhizal and nonmycorrhizal host growth in response to changes in pH and P concentration in a manganiferous oxisol

Glomus aggregatum and Leucaena leucocephala were allowed to interact in a manganese-rich oxisol at pH 4.3–6.0 and at soil P concentrations considered optimal for mycorrhizal host growth and sufficient for nonmycorrhizal host growth. At 0.02 mg P l^-1, vesicular-arbuscular mycorrhizal fungal (VAMF) colonization of roots increased as soil pH increased from 4.3 to 5.0. However, VAMF colonization of roots did not respond to further increases in pH. At pH 6.0, growth of mycorrhizal Leucaena observed at 0.02 mg P was comparable with that observed at 0.8 mg P l^-1. Increasing P concentration from 0.02 to 0.8 mg P 1^-1 increased target soil pH from 4.3 to 4.7 and reduced the concentration of available soil Mn from 15.1 to 1.9 mg 1^-1. Thus, the normal plant growth observed at the higher P concentration at pH<5 was mainly due to the alleviation of Mn toxicity as a result of its precipitation by excess P. VAMF colonization levels observed at pH 5.0–6.0 were similar, but maximal plant growth occurred at pH 6.0, suggesting that the optimal pH for mycorrhizal formation was substantially lower than for VAMF effectiveness. The poor growth of Leucaena at the lower P concentration in the unlimed soil was largely due to high concentrations of Mn²⁺ and H⁺ ions.Contribution from the Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No. 3910