Effects of soil and plant phosphorus concentrations on vesicular‐arbuscular mycorrhiza in sorghum plants

SUMMARY The effect of soil and plant phosphorus (P) on mycorrhiza was studied using a split‐root technique. In this experiment, sorghum (Sorghum bicolor (L.) Moench.) plants inoculated with the vesicular‐arbuscular (VA) mycorrhizal fungus Glomus macrocarpum (Tul. & Tul.), were subjected simultaneously to low and high phosphate concentrations applied to a phosphorus deficient Brazilian Dark Red Latosol. Phosphorus was translocated from one half of the root system to the other. When small additions of P were made to the soil, mycorrhizal infection. spore numbers and external hyphae increased compared with the unsupplemented soil. Translocation of phosphorus into roots growing in unsupplemented soil had little effect on percentage root infection, spore number or external hyphae in that compartment although infected root length increased slightly. In the treatment where soil P and plant P were high, percentage root infection and external hyphal growth were both similarly reduced. In the high phosphate treatments although percentage root infection was reduced, the total length of mycorrhizal root was not affected by phosphate fertilization.