Does mycorrhizal colonization of biofilter plants improve their water uptake or drought resistance?

Stormwater biofilter systems can provide at-source pollutant removal and runoff reduction. However, vegetation in biofilters systems can be prone to drought stress, reducing plant health and survival in between rainfall events. Mycorrhizal colonization has been shown to improve nutrient removal in stormwater biofilter systems, but it is not yet known whether mycorrhizal colonization of biofilter plants can improve their water use or resistance to drought stress. We used a pot dry-down experiment to determine whether mycorrhizal colonization differs among six common biofilter plant species and whether it improves their water use or changes their drought resistance. We measured evapotranspiration (ET) during a well-watered (WW) phase before commencing a water-deficit (WD) phase, where pots were no longer watered. Predawn and midday water leaf water potentials were measured when plants had effectively ceased ET. Water use during the WW phase ranged between 4.6 and 5.9 mm day-1 and there was no significant difference in evapotranspiration between inoculated and control treatments. Cumulative water use during the WD phase was also unaffected by mycorrhizal colonization. Species with greater biomass ceased ET earlier than those with less biomass. While greater colonization was correlated with greater biomass, there was no significant difference in biomass between control and inoculated plants. These results suggest that mycorrhizal inoculation does not improve plant water use or drought resistance in biofilters. However, as all plants were drought avoiders, they are unlikely to experience drought stress regularly, unless the period between rainfall events is substantial.