SUMMARYThe 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 datum, 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.

SUMMARYVesicular-arbuscular (VA) mycorrhizal fungi typically produce spores either within roots of a host plant or in the soil. Experiments were conducted to examine the effects of plant nutrition, as measured by tissue N, P, K, and soluble sugar concentrations, upon colonization of roots of Paspalum notatum Flugge and sporulation within roots by Glomus intraradix Schenck & Smith and in the soil by Acaulospora longula Spain & Schenck and Gigaspora margarita Becker & Hall. Plants receiving a balanced nutrient solution without P consistently had the greatest percentage root length colonized by VA mycorrhizal fungi. Glomus intraradix produced more spores per root weight under conditions of nonmycotrophy, when plants received either water only or KHgPO^ only in the nutrient-poor soil used in these experiments. Gigaspora margarita and A. longula produced more spores in the soil when plants received the balanced nutrient solution without P. Similarly contrasting responses were seen when sporulation inside and outside the roots was correlated to plant nutrient and soluble sugar concentrations. The P-tolerant, intraradical-sporulating, VA mycorrhizal fungus G. intraradix sporulated heavily when N:P ratios of host tissue were imbalanced toward P. Acaulospora longula and G. margarita produced more spores in the soil when plant tissue N:P ratios were imbalanced toward N.

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