Arbuscular mycorrhizal contribution to plant water supply

Understanding the adaptations of terrestrial plants to water stress is crucial as climate change is already altering precipitation patterns. Mycorrhizal fungi enhance host water status through indirect mechanisms like nutrient uptake or plant osmoregulation. Direct water transport via fungal hyphae has also been demonstrated, but its exact contribution to total plant water uptake is still debated.

To demonstrate and quantify the direct transport of water from arbuscular mycorrhizal fungi (AMF) to its host plant, we utilized a plant mesocosm comprised of two compartments, separated by a porous membrane and an air gap. In the ‘plant-hyphae’ compartment, seedlings of microtomatoes were grown and inoculated with Rhizophagus irregularis. Hyphae, rather than plant roots, could cross the physical barrier of the porous membrane and the air gap to enter the ‘hyphae-only’ compartment. After several weeks of plant and hyphal growth, the ‘hyphae-only’ compartment was labelled with deuterated water (²H2O) and the isotopic composition of plant transpiration and soil water of both compartments were determined at different times after irrigation.

The presence of deuterated water in the plant transpiration stream confirmed that there was direct water transport via AMF hyphae to the plant. Previous studies have quantified the relative contribution of fungal-transported water by solving an isotope mass balance that includes the leaf transpired water and water extracted from soils of both ‘plant-hyphae’ and ‘hyphae-only’ compartments. This framework assumes that movement of deuterated water from the ‘hyphae-only’ to the ‘plant-hyphae’ compartment occurs only through fungal hyphae. However, we found that there was also diffusion of deuterated water vapour across the air gap separating the two compartments. This contamination led to overestimations of the relative contribution of AMF to total plant water uptake. After accounting for this contamination, the water contributed by AMF hyphae was quantified to 1% to 6% of total plant water uptake. Furthermore, using plant biomass as a weighing factor in the mixing model to account for differences in soil volume exploration by plant roots was critical for an accurate estimate of the contribution.