Direct and indirect effects of VA mycorrhizal fungi and rhizosphere microorganisms on acquisition of mineral nutrients by maize (Zea mays L.) in a calcareous soil

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SUMMARY

Maize (Zea mays L.) was grown in a fertilized calcareous soil in pots which were separated by 30 μm nylon nets into three compartments, a central one for root growth and two outer ones for hyphal growth. The treatments comprised of sterilised soil, either inoculated with rhizosphere microorganisms together with Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe, with rhizosphere microorganisms alone, or non-inoculated (sterile control).

In the mycorrhizal treatment concentrations of P and Zn were increased in shoot and roots, as was uptake of P and Zn per unit root dry weight or per unit root length. Copper concentration in roots of mycorrhizal plants, and total uptake and uptake per unit root dry weight or per unit root length of Cu was also increased. The higher acquisition of P, Zn and Cu in the mycorrhizal plants is ascribed to VA mycorrhiza per se (hyphal uptake and translocation to the host). Besides these direct effects, VA mycorrhizal fungi and rhizosphere microorganisms affected the acquisition of other nutrients mostly through alteration of root morphology. In both mycorrhizal and sterile treatments, the decreased concentrations of Ca and Si in the shoot but increased concentrations in the roots seem to be related to restricted apoplasmic solute transport across the endodermal barrier due to decreased number of lateral roots.

The concentration of K in both shoot and roots, and Fe in shoot were lower in mycorrhizal plants due to reduced root length. Manganese concentration was also distinctly lower in both shoot and roots in mycorrhizal and sterile treatments, an effect which is attributed to the decrease in Mn reduction in the rhizosphere. Irrespective of nutrient, the uptake per unit root length was higher in mycorrhizal plants, probably due to hyphal contribution and increased supply of nutrients to the root surface by mass flow. Soil analysis indicates hyphal translocation of K but not Mg.

The results demonstrate that effects of VA mycorrhizal infection on root morphology and rhizosphere microorganisms have to be considered in interpretations of mycorrhizal effects on mineral nutrient acquisition.

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