Exploring the Functional Persistence of Mycorrhizal Associations in Water Uptake during Soil Drying Conditions

Arbuscular mycorrhizal fungi (AMF) are well-known to enhance plant resilience to water stress by improving water uptake from the soil. Few studies, however, address direct water transport to plant roots through hyphal networks, and it is virtually unknown how this function responds to soil drying. This study investigates AMF involvement in maize root water uptake across different soil water content conditions. Our primary objective is to explore how the direct contribution of AMF to root water uptake changes as soil dries. Zea mays (maize) plants were cultivated in experimental pots (4.1 L) filled with a 70% sand and 30% clay soil mixture. The pots were partitioned into two compartments using 31-μm nylon mesh, creating a 3.5-mm air gap to restrict root growth to the primary plant compartment and ensure exclusive access of mycorrhizal hyphae to a secondary soil compartment. Maize seeds inoculated with Glomus intraradices spores were cultivated for six weeks in the primary compartments under well-watered conditions before being subjected to one of three soil moisture regimes: well-watered (28-31% volumetric soil water content), moderate drought stress (14-17% volumetric soil water content), or severe drought stress (8-11% volumetric soil water content). When plants were eight weeks old, ²H-labelled water was added exclusively to the hyphae-only compartment, allowing for tracking and quantification of AMF-transported water. During and after labelling, plant shoots were enclosed in air-tight plastic bags connected to a stable isotope analyzer, which continuously monitored plant transpiration and ²H concentrations in transpired water vapor over a four-day period. Results from staining techniques demonstrate robust mycorrhizal colonization of roots and successful hyphal penetration across the air gap into the hyphae-only compartment. Results from the ²H labeling experiment indicate an early arrival of ²H in the transpired water of plants subjected to moderate drought stress (two days post-labelling) compared to well-watered conditions and severe drought stress (three days post-labelling). Additionally, plants exposed to drought stress exhibited higher ²H concentrations in their transpired water, suggesting an enhanced contribution of AMF to root water uptake under soil drying conditions. This investigation offers insights into the dynamics of AMF hyphal water transport under distinct soil moisture conditions. The outcomes will advance our understanding of mycorrhizal symbioses’ response to drought stress, potentially guiding strategies to optimize plant water acquisition in agricultural contexts facing escalating water scarcity challenges.