Host-fungus competition for carbon as a cause of growth depressions in vesicular-arbuscular mycorrhizal ryegrass

The effect of Gigaspora margarita an endomycorrhiza—forming fungus, on the growth and chemical composition of perennial ryegrass plants grown at three concentrations and two sources of N were studied. The yield of plant tops was significantly depressed by mycorrhizal infection. Mycorrhizal plants had higher concentrations of free NH 4 + (and, consequently, higher cation excess concentrations), suggesting an impairment of protein synthesis possibly resulting from an inhibited supply of C. Total oxidizable C, soluble sugar content and C/N ratios were lower in mycorrhizal plants, suggesting that the mycorrhizal fungi were competing with the host for photosynthetically-derived C, consequently causing growth depressions. A possible mechanism of this competition is discussed.