RESPONSES OF SIX SPECIES OF VESICULAR‐ARBUSCULAR MYCORRHIZAL FUNGI AND THEIR EFFECTS ON SOYBEAN AT FOUR SOIL TEMPERATURES

SUMMARY The effects of four soil temperatures on colonization of soybean roots, sporulation and enhancement of plant growth by six species of vesicular‐arbuscular (VA) mycorrhizal fungi were tested. The mean values for all fungus and plant variates in three tests were generally greatest at 30 °C and lowest at 18 °C. Four fungus species, Glomus claroideum, Glomus clarum, Gigaspora pellucida , and Gigaspora gregaria , produced the greatest number of spores per gramme of colonized root at 24 °C, while Glomus mosseae and Acaulospora laevis reached their maximum at 30 °C. Gigaspora gregaria had its maximum percentage of root colonization at 36 °C, Glomus mosseae reached its maximum at 24 °C, while all other species had maximum root colonization at 30 °C. Glomus claroideum and Acaulospora laevis were the only species which had maximum root colonization and sporulation occurring at the same temperature, 30 °C. Spore diameter was generally less at 30 than 24 °C, but this varied with fungus species and test. Soybean flower numbers, pod set and seed yield varied considerably in the three tests, but plant height was little affected by soil temperature or by fungus species. Generally, plants colonized with Glomus mosseae had an increased plant response and those colonized with Acaulospora laevis had a depressed plant response. There was considerable variation in plant response to various combinations of temperature and fungus species, with both growth stimulatory and growth repressive effects occurring.