Chronology and mechanisms of P uptake by mycorrhizal sweet potato plants

Abstract

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 difl^erent 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, W^e conclude that mycorrhiza mediated P uptake commenced vvith 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.

Arbuscular mycorrhiza and kinetic parameters of phosphorus absorption by bean plants

“…In addition, there was a positive and significant correlation (P < 0.007) between K m and C min (r = 0.61), suggesting that affinity to P and the ability to absorb it, when in low concentration in the solution, had the same tendency. Smith (1982) verified that the percentage of colonization in clover roots affected P influx, which occurs especially in young plants (Smith, 1990;O'Keefe & Sylvia, 1992), where the fraction of root colonization that is active is greater and shows abundant arbuscule development. In the present experiment, little colonization occurred in the younger plants (35 DAS) which, however, does not dismiss the possibility that it was more active, causing higher V max and P influx values.…”

Contribution by two arbuscular mycorrhizal fungi to P uptake by cucumber (Cucumis sativus L.) from32P-labelled organic matter during mineralization in soil

“…Phosphorus depletion zones around single roots extend 0.5-2.6 mm from the root surface (Hendriks et al, 1981;O'Keefe and Sylvia, 1992) due to uptake by root hairs and exudation of mobilizing agents. A measured average root diameter of 0.2 mm and a depletion radius between 0.93 and 1.13 mm would leave the whole soil volume in HRC efficiently exploited by roots at the present densities.…”