PLANT GROWTH RESPONSES TO VESICULAR‐ARBUSCULAR MYCORRHIZA

Abstract

SUMMARY Interactions between vesicular‐arbuscular (VA) mycorrhiza, utilization of rock phosphate and nodulation were examined in three legumes (clover, Stylosanthes and Centrosema) and in onions, grown in eight P‐deficient soils ranging from pH 8.1 to 5.3. Irrespective of pH, inoculation with VA endophytes increased P uptake in all host plants in all the soils when the indigenous endophytes had been removed by irradiation, but appreciable increases in plant dry weight only occurred when P concentrations of the uninoculated plants were low, generally below 0.15%. In the acid soils adding rock phosphate generally improved growth of the non‐mycorrhizal plants and inoculation with VA endophytes greatly improved its utilization. The effects persisted when the soil was used a second time. In neutral and alkaline soils rock phosphate was unavailable to non‐mycorrhizal plants and remained so after inoculation with VA endophytes. Legumes inoculated with the appropriate Rhizobium strain only nodulated in the most P‐deficient soils when they were also mycorrhizal, and added rock phosphate greatly improved nodulation and nitrogen fixation of the mycorrhizal plants. Some pilot experiments in unsterile soils are also described and the bearing of these results on field inoculation with VA endophytes is discussed.