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Keywords:

  • VA mycorrhiza;
  • phosphate;
  • ammonium nutrition;
  • glutamine synthetase;
  • glutamate dehydrogenase

Summary

Activities of glutamate dehydrogenase and glutamine synthetase were determined using crude extracts of roots and shoots of mycorrhizal and non-mycorrhizal plants of Trifolium subterraneum L. and Allium cepa L., grown at different levels of fertilizer phosphate.

Glutamate dehydrogenase activity was low in all tissues [0.1 to 1.6 μmol NAD(P)H oxidized min−1 gFW−1 tissue] and there was no consistent effect of mycorrhizal infection or phosphate nutrition on this activity.

Glutamine synthetase (GS) activity (assayed by the transferase method) was in the range 1 to 40/iimol γ-glutamyl hydroxamate produced min−1 gFW−1. In general, activity of this enzyme was low in phosphate-deficient plants and was increased both by mycorrhizal infection and by improved phosphate supply. In T. subterraneum routine assays of GS were done on roots only. The effects of mycorrhizal infection in increasing enzyme activity in roots were similar whether natural soil inoculum (containing a mixture of several mycorrhizal fungi) or inoculum of Glomus mosseae Nichol. & Gerd. was used. Both increased phosphate supply and mycorrhizal infection increased nodulation of clover plants as well as GS activity, so that it was difficult to relate changes in GS activity to the interacting effects of mycorrhizal infection and phosphate nutrition.

Onions had low GS activity in uninfected roots, compared with shoots. Again improved phosphate supply resulted in increased enzyme activity in both roots and shoots. However, the patterns of interaction between phosphate supply, P concentration in tissues, mycorrhizal infection and enzyme activity were different in the two tissues. In shoots, as expected, the effects were consistent with an indirect effect of mycorrhizal infection on enzyme activity, via improved P nutrition. In roots there appeared to be a ‘fungal effect’ superimposed on the phosphate effect. This was investigated by manipulating the amount of fungal tissue in mycorrhizal roots via differences in propagule density of G. mosseae in soil. Results were again consistent with the hypothesis that the mycorrhizal fungi contributed GS activity to the symbiotic root system. Fungal structures were separated from roots following digestion in cellulase and pectinase. GS activity was high in fungal tissue from young roots (29 to 31 d), but low in older infections (55 d). The high activity could not have been caused by contamination of fungal tissue by root cells. The digestion technique reduced GS activity in uninfected and infected root segments, so that results obtained with separated fungi are not quantitatively comparable with those obtained from extracts of fresh tissues.

We conclude that vesicular-arbuscular mycorrhizal fungi are able to assimilate ammonium via GS. This ability would be important in increased uptake of nitrogen which is an inevitable prerequisite for increased growth following relief of phosphate stress. It is also consistent with the recent findings by others that hyphae of G. mosseae can absorb and translocate 15NH+4