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 inocul ated with five different communities of arbuscular mycorrhizal (AM) fungi c ollected from citrus orchards in Mesa and Yuma, AZ, USA, and undisturbed No rth American Sonoran Desert and Chihuahuan Desert soils. Plants were then g rown in a glasshouse for four months under continually moist or periodicall y dry conditions achieved by altering watering frequency so that before wat ering events container soil water tensions were approximately -0.01 MPa (co ntinually moist) or -0.06 MPa (periodically dry) one half way down the cont ainer profile. Plants grown in continually moist soil had greater shoot gro wth than plants grown in periodically dry soil. Plant P status did not Limi t growth, and there was no interaction between watering frequency and AM fu ngal inoculum treatments. Plants inoculated with AM fungi from the Yuma orc hard soil had significantly less root dry weight and total root length, and lower photosynthetic fluxes than plants treated with inoculum from the oth er soils. Specific soil respiration and an estimated carbon cost to benefit ratio were also higher for plants inoculated with AM fungi from the Yuma o rchard 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 s howed that root growth suppression of plants treated with the Yuma inoculum , compared with plants treated with inoculum from all other sites, was subs tantial and greater in magnitude than the effect of periodic soil drying. S uppression of root growth might have resulted from increased AM fungalactiv ity resulting in higher carbon costs to the plant. (C) 2000 Elsevier Scienc e B.V. All rights reserved.