Effect of arbuscular mycorrhizal (AM) fungal communities on growth of `Volkamer' lemon in continually moist or periodically dry soil

Citrus volkameriana Tan. and Pasq. (`Volkamer' lemon) seedlings were inoculated with ®ve different communities of arbuscular mycorrhizal (AM) fungi collected from citrus orchards in Mesa and Yuma, AZ, USA, and undisturbed North American Sonoran Desert and Chihuahuan Desert soils. Plants were then grown in a glasshouse for four months under continually moist or periodically dry conditions achieved by altering watering frequency so that before watering events container soil water tensions were approximately ÿ0.01 MPa (continually moist) or ÿ0.06 MPa (periodically dry) one half way down the container pro®le. Plants grown in continually moist soil had greater shoot growth than plants grown in periodically dry soil. Plant P status did not limit growth, and there was no interaction between watering frequency and AM fungal inoculum treatments. Plants inoculated with AM fungi from the Yuma orchard soil had signi®cantly less root dry weight and total root length, and lower photosynthetic ̄uxes than plants treated with inoculum from the other soils. Speci®c soil respiration and an estimated carbon cost to bene®t ratio were also higher for plants inoculated with AM fungi from the Yuma orchard soil than for plants treated with inoculum from the other soils. The Yuma orchard inoculum was distinctive in that > 80% of the total number of AM fungal spores were from a single species, Glomus occultum. These data showed that root growth suppression of plants treated with the Yuma inoculum, compared with plants treated with inoculum from all other sites, was substantial and greater in magnitude than the effect of periodic soil drying. Suppression of root growth might have resulted from increased AM fungal Scientia Horticulturae 84 (2000) 127±140 * Corresponding author. Fax: ‡1-480-965-5952. E-mail address: [email protected] (C.A. Martin). 1 Current address: Horticultural Science Department, University of Florida, PO Box 110690, Gainesville, FL 32611-0690, USA. 2 Current address: Yuma Mesa Agricultural Center, Rt 1 Box 40M, University of Arizona, Somerton, AZ 85350, USA. 0304-4238/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 4 2 3 8 ( 9 9 ) 0 0 1 1 2 0 activity resulting in higher carbon costs to the plant. # 2000 Elsevier Science B.V. All rights reserved.