Chronology and mechanisms of P uptake by mycorrhizal sweet potato plants



Vesicular-arbuscular (VA) mycorrhizal and non-mycorrhizal sweet potato plants [Ipomoea batatas (L.S) Lam. cv. White Star] were grown in a glasshouse in a phosphorus (P)-fixing soil in order to (i) establish the chronology of mycorrhiza-mediated P uptake, (ii) document the distribution of total and metabolically active external hyphae in relation to roots, root hairs and P-depletion zones at the time when the P-uptake response first becomes apparent, (iii) evaluate the pore-size distribution of the soil relative to the radii of roots, root hairs and hyphae of mycorrhizal fungi, and (iv) determine if sweet potato mycorrhiza alter the organic acid composition of the rhizosphere. Phosphorus inflow, VA mycorrhizal root colonization, and active hyphae increased during the first half of an 8 wk experiment. During the latter half of the experiment, however, colonization and active hyphae increased while P inflow decreased. Roots colonized by mycorrhizal fungi supported a higher proportion of active hyphae in the soil than non-colonized roots. In a second experiment, hyphal distribution around individual roots colonized by mycorrhizal fungi was not consistently different from hyphal distribution around non-colonized roots. Hyphae of mycorrhizal fungi were of a diameter that would allow them to penetrate pores that hold water at water contents less than field capacity while roots, and to a large extent root hairs, would be excluded from these pores. We found no evidence that sweet potato roots, whether or not they were colonized by mycorrhizal fungi, altered the organic acid composition of the rhizosphere. We conclude that mycorrhiza mediated P uptake commenced with the onset of colonization by the fungi and was due to a change in the ability of the root to come into contact with the soil solution.