SEASONAL DYNAMICS OF THE ASSOCIATION BETWEEN SWEET-POTATO AND VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGI

To better understand the behavior of selected vesicular-arbuscular mycorrhizal (VAM) isolates in the field, we documented the growth of roots, root hairs, and VAM colonization of inoculated and noninoculated sweet potato plants (Ipomea batatas (L.) Lam. cv White Star) over a growing season. We also determined the seasonal dynamics of P and Zn uptake, and shoot and storage-root growth. Shoot cuttings were inoculated with an isolate of either Glomus etunicatum Becker and Gerdemann or Acaulospora rugosa Mortan, or were not inoculated, and were harvested 2, 4, 8, 13, 20, and 27 weeks after planting (WAP). At each harvest, roots were sampled at 0 to 30, 30 to 60, and 60 to 90 cm depths and at 0, 23, 83, and 116 cm from the base of the shoot. At the end of the study, the roots of three non-inoculated plants were sampled by soil horizon. Inoculation had no affect on shoot growth or total shoot uptake of P and Zn; shoot dry mass and P and Z content increased rapidly up to 20 WAP, while shoot length continued to increase through 27 WAP. Shoot-P concentration of plants inoculated with A. rugosa at 2 and 8 WAP were higher than the noninoculated plants, while shoot-Zn concentration was not affected by inoculation. Storage-root yields of inoculated plants were higher than yields for noninoculated plants. Root length density, and percentage of root length with root hairs and VAM colonization were highest and most dynamic near the base of the plant. Percentage of root length colonization by VAM fungi was highest in the E2 horizon, intermediate in the Bh horizon, and lowest in the Ap horizon. Percentage of root length with root hairs had the opposite pattern. Intensive measurements of root characteristics close to the base of the plant, and shoot P-content and concentration during the period of rapid yield production, provided the most useful data for evaluating the activity of effective isolates.