Drought Effects on the Symbiosis Between Big Sagebrush Seedlings and Arbuscular Mycorrhizal Fungi

Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs that facilitate nutrient uptake in exchange for organic carbon. During severe drought, plants close their stomata, which limits carbon uptake. Under these conditions, AMF could become parasitic if the host plant cannot exclude the fungus. We investigated this possibility in Artemisia tridentata (big sagebrush) seedlings inoculated with a mixture of native AMF. AMF colonization of roots was analyzed in seedlings kept under well-watered conditions and seedlings exposed to drought. Drought was imposed by withholding watering until stomatal conductance was minimal and values of CO ₂ assimilation were negative. Drought did not decrease total AMF colonization, which was 58.8 (±6.1) and 48.1 (±6.7) % for well-watered and water-stressed plants, respectively ( p = 0.27). Similarly, the abundance of vesicles and intraradical spores was not affected by drought ( p = 0.5). Although not statistically significant, the major change was observed in arbuscular colonization, with was 37.4 (±7.84) and 18.0 (±8.5) % for well-watered and drought-stressed seedlings, respectively ( p = 0.1). These preliminary results suggest that the symbiosis could become parasitic under drought. To further analyze this possibility, we are currently investigating the activity of arbuscules, which are the sites of nutrient exchange.