Search Results

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. versiforme (Karsten) Berch, one originally from California (CAL) and the other one from Oregon (OR). Mycorrhizal plants were taller, produced more biomass, and had a higher leaf P concentration than the uninoculated control plants. Mycorrhizal inoculation also significantly increased the leaf surface area of `M.26' and `Ott.3' compared to the control. Glomus 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 percentages of root colonization, but the size of the external hyphal network varied with fungal species. Glomus versiforme(OR) had a larger extramatrical phase than G. aggregatum and G. intraradix. Mycorrhizal efficiency was associated with a larger external hyphal network, but showed no relation with internal colonization. Despite the high P fertility of the soil used, growth enhancement due to mycorrhizal inoculation was attributed to improved P nutrition.

Herbicides are increasingly used in orchards. Since apple trees strongly depend on mycorrhizae, the effects of three commonly used herbicides on the host plant and endophyte were examined. Symbiosis between tissue-cultured P16 apple rootstocks and Glomus versiforme (Karsten) Berch was established under greenhouse conditions. Simazine (1, 2, 10, and 20 μg a.i./g), dichlobenil (1, 5, 10, and 25 μg a.i./g), paraquat (0.5, 1, 10, and 100 μg a.i./g), or water was applied to mycorrhizal and nonmycorrhizal plants as a soil drench. The response of mycorrhizal plants to herbicide was greater, and the relative elongation rate was more sharply reduced in mycorrhizal (76%) than in nonmycorrhizal plants (33%). Six weeks after herbicide application, dry mass reduction due to herbicides was similar (39% and 36%) for mycorrhizal and nonmycorrhizal plant shoots, respectively, while root dry mass reduction was larger for mycorrhizal (63%) than nonmycorrhizal plants (46%). None of the herbicide treatments affected root colonization. However, an in vitro hyphal elongation test with G. intraradices Schenck & Smith and herbicide-amended (0, 1, 10, 100, and 1000 μg a.i./g) gellan gum solidified water showed that either dichlobenil or paraquat, even at the lowest concentrations, could significantly reduce hyphal elongation. Simazine did not affect hyphal elongation in vitro, a result suggesting that improved absorption capacity of mycorrhizae explains, at least in part, the increased phytotoxicity of some herbicides. It was found that plant mortality was higher among mycorrhizal than nonmycorrhizal apple trees for all herbicide treatments. The increased CO₂ assimilation rates of dichlobenil-treated mycorrhizal plants contrasted with the decreased rates of control plants measured 1 week after dichlobenil treatment. This indicates a physiological interaction between mycorrhizal colonization and dichlobenil in the toxic response of apple plants. Chemical names used: 2-chloro-4,6-bis-ethylamino-s-triazine (simazine), 2,6-dichlorobenzonitrile (dichlobenil), 1,1'-dimethyl-4,4'bipyridinium (paraquat).