Four AM fungal isolates (Glomus sp.) from disparate edaphic conditions were screened for effects on leaf gas exchange of `Volkamer' lemon (Citrus volkameriana Ten. and Pasq.) plants of similar size under conditions of increased soil water deficit stress and recovery from stress. Mycorrhizal and non-mycorrhizal plants were grown in 8-L containers for 10 weeks under well-watered conditions in a glasshouse and then subjected to three consecutive soil-drying episodes of increased severity (mean soil water tension reached –0.02, –0.06, and –0.08 MPa, respectively). Gas exchange measurements were made on the last day of each soil-drying episode. Plants were irrigated after each soil-drying episode, and measurements were repeated on the following 2 recovery days, when soil remained moist. All measurements were made at mid-day with a LI-COR 6200 portable photosynthesis system. The effect of AM fungi on leaf gas exchange fluxes varied depending on the isolate and the intensity of soil water stress. Leaf gas exchange fluxes always were highest for plants colonized by Glomus mosseae (Nicol. & Gerde.) isolate 114C, except during the third soil-drying episode, when all mycorrhizal plants had similar, and lower, gas exchange fluxes compared with non-mycorrhizal plants. During recovery from the third soil-drying episode, Glomus mosseae isolate 51C had lower leaf gas exchange fluxes compared with all other plants. Our results show that AM fungi can alter leaf gas exchange fluxes of citrus, under conditions of optimal P nutrition, in an isolate-specific manner.
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