EFFECTS OF PHOSPHORUS ON THE FORMATION OF MYCORRHIZAS BY <i>GIGASPORA CALOSPORA</i> AND <i>GLOMUS FASCICULATUM IN RELATION TO ROOT CARBOHYDRATES</i>

Abstract

S ummary We compared the effects of phosphorus (P) supply on the formation of vesicular‐arbuscular (VA) mycorrhizas in Trifolium subterraneum L. by Gigaspora calospora (Nicol. &amp; Gerd.) Gerd. and Glomus fasciculatum (Thaxter sensu Gerd.) Gerd &amp; Trappe. For both fungi, increasing the supply of P decreased the percentage of root length infected. At low P, this decrease was associated with a reduction in the growth of the fungi inside the root. At higher P, the growth of the fungi outside the root was also reduced. These detrimental effects of P on the growth of both mycorrhizal fungi were correlated with reductions in the concentrations of soluble carbohydrates in root extracts and root exudates but were not correlated with the concentrations of free amino‐nitrogen in extracts or exudates. The percentage of root length infected by G. calospora was decreased to a greater extent than that of G. fasciculatum with increasing P supply. This could partially be explained by the lower concentrations of soluble carbohydrates in the roots of plants inoculated with G. calospora. Additionally, the percentage of root length infected by G. calospora appeared to be more sensitive to reductions in the supply of soluble carbohydrates than that of G. fasciculatum. Growth responses to inoculation were dependent on the species of mycorrhizal fungus and the P level. Enhancements and depressions in the growth of tops could be explained in terms of increased and decreased P uptake.