Mycorrhizal promotion of host stomatal conductance in relation to irradiance and temperature.

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.