Oecologia (2003) 134:55–64 DOI 10.1007/s00442-002-1078-2 ECOPHYSIOLOGY JosØ Ignacio Querejeta · Louise M. Egerton-Warburton · Michael F. Allen Direct nocturnal water transfer from oaks to their mycorrhizal symbionts during severe soil drying Received: 31 January 2002 / Accepted: 4 September 2002 / Published online: 18 October 2002  Springer-Verlag 2002 Abstract Symbiotic mycorrhizal fungi play an important role in the absorption of soil nutrients and water by most plants. It has been suggested that hydraulically lifted water might maintain the integrity of the external mycorrhizal mycelium during drought. We tested this hypothesis in the obligately mycorrhizal species, coast live oak (Quercus agrifolia), using a microcosm system that separated the effects of hydraulic lift in roots from those in the external mycelium. Mycorrhizal oak seed- lings were established in microcosms comprising three discrete compartments for (1) upper roots, (2) tap roots, and (3) external fungal mycelium. Eight months after planting, a drought treatment was initiated: irrigation to the upper root and fungal chambers was terminated and only irrigation to the taproot compartment was main- tained. After 3, 12, 30, 50, 70 and 80 days of drought, tracers were injected into the taproot compartment at dusk. At dawn the following morning, mycorrhizal hyphae (EM and AM) and spores (AM) in upper root and fungal compartments were extensively labeled with the tracers. In contrast, no labeling was observed when tracers were injected into the taproot compartment during daytime. Nocturnal water translocation from plant to mycorrhizal fungi occurred in association with hydraulic lift. Saprotrophic/parasitic fungi in the microcosms were not labeled, suggesting a direct water transfer from plants to their mycorrhizal mutualists and not to other fungi in the soil. Even after prolonged drought (70–80 days), mycorrhizal hyphae persisted in soils with water potential values as low as –20 MPa. Maintaining mycorrhizal activity through direct water translocation could poten- tially improve the nutrient status of deep-rooted plants during periods when the fertile upper soil is dry. Keywords Hydraulic lift · Mycorrhizae · Quercus agrifolia · Tracers Introduction Circadian cycles drive the daily rhythms of physiology in animals, plants and microorganisms (Millar et al. 1995; Dunlap 1999) and are entrained to the cycles of day and night by light and temperature signals at dawn and dusk respectively (Ouyang et al. 1998; Devlin and Kay 2000). Such fundamental properties are also reflected in “hy- draulic lift”, a mechanism that buffers plants against water stress during seasonal deficits (Richards and Caldwell 1987; Caldwell and Richards 1989). Hydraulic lift involves transport of water from deeper, wetter soil into drier, shallower surface layers by plant roots during the night when canopy transpiration is minimal (Dawson 1993; Caldwell et al 1998; Horton and Hart 1998; Millikin-Ishikawa and Bledsoe 2000). Water potential gradients in the soil profile provide the driving force for this process. Dawn signals the cessation of hydraulic lift and a return to water flow direction dominated by canopy transpiration. Mycorrhizal fungi play an important role in the uptake and transport of nutrients and water to their host plant since the fungal external mycelium functions as a physiological extension of the roots (Allen 1991; Smith and Read 1997). While it is commonly assumed that mycorrhizae senesce as the surface soil where they concentrate (0–40 cm) dries out (Nilsen et al. 1998; Swaty et al. 1998), some authors have hypothesized that hydraulically lifted water might maintain the integrity of the external mycorrhizal mycelium during prolonged drought (Caldwell et al. 1998; Horton and Hart 1998). We tested this hypothesis in the obligately mycorrhizal species, coast live oak (Quercus agrifolia Nee., Fagaceae) using a microcosm system that separated the effects of JosØ Ignacio Querejeta and Louise M. Egerton-Warburton contributed equally to this work J.I. Querejeta ( ) ) · L.M. Egerton-Warburton · M.F. Allen Center for Conservation Biology, The University of California, Riverside, CA 92521–0124, USA e-mail: nachoq@ucrac1.ucr.edu L.M. Egerton-Warburton Conservation Science Department, Chicago Botanic Gardens, 1000 Lake Cook Rd, Glencoe, IL 60022, USA