Consequences of arbuscular mycorrhizal fungi on plant drought responses and possible mechanisms

Kian G.M. Speck, College of Forestry and Conservation, University of Montana, Missoula, MT, Ylva Lekberg, Department of Ecosystem and Conservation Sciences, University of Montana and MPG Ranch, Missoula, MT and Anna Sala, Division of Biological Sciences, University of Montana, Missoula, MT

Background/Question/Methods
Conyza canadensis is a ruderal annual that thrives under drought despite it lacking obvious xeromorphic traits, such as succulent leaves or deep roots. This plant is also highly colonized by arbuscular mycorrhizal fungi (AMF) making it an ideal candidate to study how mycorrhiza affects plant drought tolerance. The overarching objectives of this research were to determine whether 1) mycorrhizal Conyza plants are more drought tolerant than nonmycorrhizal Conyza plants under drought stress, and, if so 2) to gain insight concerning potential underlying mechanisms.
The experiment used a 2x3 factorial design with two inoculation treatments (+AMF and -AMF) and three watering treatments (control, moderate, and severe drought). Each treatment was replicated eight times for a total of 48 plants. Conyza seedlings were grown for two months and drought was implemented using the wick method, which generated a constant difference in volumetric soil water content of 18%, 8% and 5% among drought treatments. We measured dry biomass, water content, leaf water potential, photosynthetic rate, stomatal conductance, and shoot nitrogen and phosphorus concentrations.

Results/Conclusions
All inoculated treatments were mycorrhizal and all control treatments were non-mycorrhizal. Shoot and root biomass declined with increasing drought stress and, overall, AMF suppressed shoot but not root biomass. Total biomass responses to AMF inoculations changed from parasitic to neutral with increasing stress, suggesting a potential shift in cost-benefit ratios and mycorrhizal function. Mycorrhizal plants had higher photosynthetic rates (P=0.05), but this upregulation was apparently insufficient to prevent reductions of shoot biomass when conditions were benign. Mycorrhizal plants also had higher stomatal conductance (P=0.01) and shoot water content (P=0.02), which is indicative of lower drought stress in general. Leaf water potential became increasingly negative with drought stress, especially in the most stressed non-mycorrhizal plants (PAMFxDrought<0.001) suggesting that AMF somehow protected the plants from the most severe stress. Both nitrogen and phosphorus concentrations were higher in AM plants under stress relative to non-AM plants suggesting that improvement of water status by AM could relate to nutrient availability, but other factors such as non-structural carbohydrates are currently being explored. Overall, we show that AMF can influence plant water relations in ways that may be related to nutritional responses.