Drink quickly. Mycorrhizal roots deplete water faster from wet soil.

The aim of the experiment was to identify whether mycorrhizal (M) and non-mycorrhizal (NM) plants differed in their rates of water depletion from soil, which was initially very wet but became moderately dry over time. In a greenhouse pot experiment, dwarf-tomato plants (Solanum lycopersicum ‘Micro Tom’) were grown in a sterilized substrate. The M plants were inoculated with an isolate of the arbuscular mycorrhizal fungus Funneliformis mosseae ‘BEG95’. Moisture meters permanently installed in each pot recorded soil gravimetric water content (GWC) and allowed controlling soil moisture by pot-specific watering. Water depletion was quantified from these GWC data. We found that immediately after water application calculated to establish 17% GWC in each pot, M plants depleted water faster than did their NM counterparts, resulting in less residual substrate water in M as compared to NM pots. The difference in residual substrate GWC remained significant throughout the entire monitoring period of 44 h, with the mean value of the difference being 0.62% of GWC. The plants in the two treatments differed neither in shoot biomass nor leaf area nor root biomass. The M plants, however, had significantly shorter and thicker roots, whereas NM plants had longer, thinner roots. In line with the expectation that thicker roots exhibit higher hydraulic conductivity, we attribute the faster water depletion in M pots to enhanced ability of mycorrhizal roots to exploit wet substrates, where water availability did not constrain uptake. At later stages, both M and NM plants depleted water at similar rates.