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There is growing appreciation of arbuscular mycorrhizal effects on soil properties and their potential consequences on plant behavior. We examined the possibility that mycorrhizal soil may directly influence plant water relations. Using wild-type and noncolonizing bean mutants planted into soils previously produced using mycorrhizal or nonmycorrhizal sorghum plants, we partitioned mycorrhizal influence on stomatal conductance and drought resistance into soil and root components, testing whether effects of mycorrhizal fungi occurred mostly via mycorrhization of roots, mycorrhization of soil, or both. The mutation itself had no effect on any water relations parameter. Colonization by Gigaspora margarita Gerdemann & Trappe and Glomus intraradices Schenck & Smith had appreciable effects on leaf water potential at the lethal point and on osmotic adjustment, relative to nonmycorrhizal plants of comparable size. Mycorrhizal effects on drought resistance were attributable to an effect on the plant itself rather than to an effect of mycorrhizal soil. Mycorrhizal effects on stomatal conductance were attributable to mycorrhization of both roots and soil, as well as to mycorrhization of roots alone. Surprisingly, merely growing in a mycorrhizal soil resulted in promotion of stomatal conductance of nonmycorrhizal plants in both amply watered and droughted plants. Mycorrhizal effects on droughted plants did not appear to be related to altered soil water retention properties, as Gigaspora margarita and Glomus intraradices altered the soil's moisture characteristic curve only slightly.Résumé : On reconnaît de plus en plus les effets des mycorhizes arbusculaires sur les propriétés du sol et leurs consé-quences potentielles sur le comportement de la plante. Les auteurs ont examiné la possibilité que le sol mycorhizé puisse influencer directement les relations hydriques de la plante. À cette fin, ils ont utilisé une race sauvage de haricot ainsi qu'un mutant non colonisant, plantés dans des sols préalablement produits avec des plants de sorgho mycorhizés ou non mycorhizés. Ils ont ensuite réparti l'influence des mycorhizes sur la conductance stomatale et la résistance à la sécheresse entre les composantes sol et racines, la mycorhization du sol, ou les deux. La mutation elle-même est sans effet sur l'ensemble des paramètres hydriques. La colonisation par le Gigaspora margarita Gerdemann & Trappe et le Glomus intraradices Schenck & Smith a des effets manifestes sur le potentiel hydrique foliaire au point létal, et sur l'ajustement osmotique, par rapport à des plants non mycorhizés de grosseur comparable. On attribue les effets de la mycorhization sur la résistance à la sécheresse à une action sur la plante elle même plutôt qu'à un effet du sol mycorhizé. Dans le cas de la conductance stomatale, les effets des mycorhizes sont attribuables à la mycorhization à la fois des racines et du sol, aussi bien qu'à la mycorhization des racines prises séparément. Il est surprenant de constater que le seul fait de croître dans un sol mycorhizé a...

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Colonization of roots and soil by arbuscular mycorrhizal (AM) fungi sometimes promotes stomatal conductance (gs) of the host plant, but scientists have had difficulty predicting or manipulating the response. Our objective was to test whether the magnitude of AM influence on gs is related to environmental conditions: irradiance, air temperature or leaf temperature. Stomatal conductances of two groups of uncolonized sorghum plants were compared to gs of plants colonized by Glomus intraradices (Gi) or Gigaspora margarita (Gm) in 31 morning and afternoon periods under naturally varying greenhouse conditions. Stomatal conductance of Gi and Gm plants was often markedly higher than gs of similarly sized nonAM plants. AM promotion of gs was minimal at the lowest irradiances and lowest air and leaf temperatures, but was substantial at intermediate irradiance and temperatures. AM promotion was again low or absent at the highest irradiances and temperatures. Magnitude of AM promotion of gs was not a function of absolute gs. Promotion of gs by Gi and Gm was remarkably similar. Differing phosphorus fertilization did not affect gs.