V-A mycorrhizas of winter cereals in S.E. England: Ecology, taxonomy and effects of agrochemicals

The association between vesicular-arbuscular mycorrhizal fung and winter wheat was investigated in the field, the greenhouse and the laboratory. Extensive early autumn fractional infection of commercially grown winter cereals was observed in two fields in Sussex over a three-year period. Fluctuations in seasonal infection levels were monitored over this period and two trends were noted in different seasons, an overwinter decline and continued infection spread through the winter months. The overwinter decline in infection may be correlated with low thermal time accumulation during the autumn period and therefore early sowing may be important. This effect, however, appeared to be influenced by other factors such as soil moisture status and consequently root growth dynamics. Three Glomus species were obtained in pure pot culture and their taxonomy investigated by means of morphology (light microscopy and electron microscopy) and polyacrylamide gel electrophoresis of spore esterases. Differences were noted between spores isolated from the field and their subsequent appearance in pure pot cultures. The effect of soil pesticides on the VAM symbiosis was investigated. In general, herbicide applications at equivalent field rates had negligible effects on VAM infection development in winter wheat grown in pots in the greenhouse, and on in vitro spore germination. Fungicide applications, also at equivalent field rates, covered the range of response from inhibitory to stimulatory on infection levels and spore germination depending on the nature of the active moiety of the agrochemical used. The indigenous VAM population from one of the Sussex field sites appeared less efficient at stimulating phosphorus uptake in winter wheat grown in partially sterile field soil compared with three introduced Glomus endophytes. Root and rhizosphere acid phosphatase activities differed between three VAM fungal infections of wheat grown in sterile sand culture. High activity did not correlate with increased shoot P uptake.