Abstract
SUMMARYAt several times during plant growth, we compared the effects of two irradiances (plants were either not shaded or were shaded to reduce 60–70% of the photon flux in the glasshouse) and two concentrations of soil phosphorus (P) (35 and 105 μg−1P g−1soil) on the formation of vesicular‐arbuscular (VA) mycorrhizas onTrifolium subterraneumL. byGigaspora calospora(Nicol. & Gerd.) Gerd. and byGlomus fasciculatum(ThaxtersensuGerd.) Gerd. & Trappe.By the fifth week from sowing, the proportion of root length infected by bothG. calosporaandG. fasciculatumwas reduced in plants grown at the greater concentration of P and in plants grown without shade. These effects were associated with decreased concentrations of soluble carbohydrates in roots at earlier harvests. Decreased concentrations of soluble carbohydrates in the roots of unshaded plants were not associated with increased concentrations of P in the plant.The percentage of root length infected byG, calosporawas decreased to a greater extent than that ofG. fasciculatumwith similar reductions in the concentrations of soluble carbohydrates in roots. This effect may partly explain why the percentage of root length infected byG. calosporais more sensitive than that ofG. fasciculatumto increasing P supply.G. calosporaandG. fasciculatummay also differ in the extent to which P may directly affect their ability to form mycorrhizas.The effects of P supply and shading on the percentage of root length infected byG. calosporaand byG. fasciculatumwere not apparent before the fourth week from sowing. These factors may have affected the ability of the mycorrhizal fungi to spread by secondary infections.
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