Is the amount of water transported by arbuscular mycorrhizal fungal hyphae negligible? Insights from a compartmentalized experimental study

Arbuscular mycorrhizal fungi (AMF) play a crucial role in enhancing plant resistance to drought stress by improving the plant-water relationship. However, the precise mechanism of AMF-mediated water transport to host plant roots remains elusive.

In a compartmentalized experiment comprising both plant and non-plant compartments, we employed heavy-oxygen water (¹⁸O-labeled) to directly trace and quantify the transport of water by AMF hyphae to alfalfa under the condition of high soil moisture (70% of the maximum field water holding capacity) and low soil moisture (40% of the maximum field water holding capacity).

Our findings revealed that irrespective of soil moisture levels, hyphae entering the ¹⁸O-labeled compartment (AM treatment) significantly enriched ¹⁸O in alfalfa transpiration water compared to no hyphae entering the ¹⁸O-labeled compartment (NM treatment). We calculated the direct water transport by AMF using a standard isotope mixing model, demonstrating that in high and low- moisture soil substrates, AM fungi contributed 12.32% and 17.03% of the total transpiration water, respectively.

These results highlight that the direct transport of water from AMF hyphae to horticulture plants should not be underestimated in comparison to plant transpiration demand. Moreover, the water contribution of AM fungal hyphae to host plants is more significant in arid soil, especially in dry soil substrate. This underscores the critical role of mycelial water transport in supporting plant survival under water-limiting conditions.