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Mycorrhizal symbiosis, stomatal sensitivity and chemical signals of soil drying

Poster or talk, Monday afternoon session 1 or 3
AUGÉ, ROBERT1, XIANGRONG DUAN1, DAWN NEUMAN2, JANET REIBER2, CRAIG GREEN1
1Institute of Agriculture, OHLD, University of Tennessee, P.O. Box 1071, Knoxville, TN 37901-1071, USA
2Department of Biological Sciences, University of Nevada, Las Vegas, NV 89154-4004, USA
ICOM1 Abstract
AM symbiosis can alter stomatal behavior of host leaves but the mechanism is not always clear. We tested if colonization by Glomus intraradices would result in altered stomatal sensitivity to hormonal root-to-shoot signals or altered movement of hormones in xylem as a function of soil water. In whole Vigna unguiculata plants, AM symbiosis allowed transpiration (E), stomatal conductance (gs) and shoot water potential to be maintained to lower soil moisture than in comparative nonmycorrhizal plants. Fluxes and concentrations of ABA in xylem were lower in mycorrhizal than in nonmycorrhizal plants once soil drying was sufficient to begin closing stomates. Stomatal sensitivity of leaves to xylem sap pH or to fluxes or concentrations of ABA or zeatin riboside in xylem was not changed by AM colonization. Transpiration assays with detached leaves of V. unguiculata and Pelargonium hortorum suggest that there is generally no residual influence of the symbiosis in foliage; we almost always see mycorrhizal influence on gs only in plants having intact root systems (Rosa hybrida was an exception). Involvement of xylem sap pH, calcium or phosphate has been previously implicated in a chemical root-to-shoot signaling process; E of mycorrhizal and nonmycorrhizal leaves of V. unguiculata and P. hortorum remained similar when pH and these ions varied in the feeding solution. Higher gs of mycorrhizal plants in dry soil appeared primarily due to a greater hydraulic efficiency of the mycorrhizal root system.