Direct nocturnal water transfer from oaks to their mycorrhizal symbionts during severe soil drying

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 seedlings 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 maintained. 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 potentially improve the nutrient status of deep-rooted plants during periods when the fertile upper soil is dry.