APPLE ROOTSTOCK RESPONSE TO VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGI INA HIGH PHOSPHORUS SOIL

A 12-week greenhouse experiment was undertaken to test the efficiency of inoculation of vesicular-arbuscular mycorrhizal fungi on four apple (Malus domestica Borkh) rootstock cultivars: M.26, Ottawa 3 (Ott.3), P.16, and P.22. The plants were grown in soil from an apple rootstock nursery, containing high levels of extractable P (644 kg Bray/1 ha-1). Inoculation treatments were Glomus aggregatum Shenck and Smith emend. Koske, G. intraradix Shenck and Smith, and two isolates of G. versifo rme (Karsten) Berch, one originally from California (CAL) and the othe r one from Oregon (OR). Mycorrhizal plants were taller, produced more biomass, and had a higher leaf P concentration than the uninoculated c ontrol plants. Mycorrhizal inoculation also significantly increased th e leaf surface area of 'M.26' and 'Ott.3' compared to the control. Glo mus versiforme(CAL)-inoculated plants generally had the best nutrient balance, the greatest final height and shoot biomass, and produced an extensive hyphal network. All the mycorrhizal plants had similar perce ntages of root colonization, but the size of the external hyphal netwo rk varied with fungal species. Glomus versiforme(OR) had a larger extr amatrical phase than G. aggregatum and G. intraradix. Mycorrhizal effi ciency was associated with a larger external hyphal network, but showe d no relation with internal colonization. Despite the high P fertility of the soil used, growth enhancement due to mycorrhizal inoculation w as attributed to improved P nutrition.