Ectomycorrhizas increase apoplastic water transport and root hydraulic conductivity in Ulmus americana seedlings

The extent to which water channel transport is responsible for the observed increases in root water flow of ectomycorrhizal plants is reported here. To examine the contribution of water channel transport to root hydraulic conductance , temperatures in the range 4-20 degreesC and mercuric chloride (HgCl2) were used to study the kinetics of water transport in ectomycorrhizal and nonmycorrhizal roots of American elm (Ulmus americana) seedlings. Hydraulic conductance declined with decreasing temperatures in both mycorrhizal and nonmycorrhizal seedlings. However, hydraulic conductance! and conductivity were higher in the mycorrhizal than the nonmycorrhizal roots at all temperatures studied. Mercuric chloride had a relatively greater impact on root hydraulic conductance in nonmycorrhizal than mycorrhizal roots and activation energy for root hydraulic conductance was significantly higher in mycorrhizal than nonmycorrhizal plants. The results suggest that ectomycorrhizal hyphae increase hydraulic conductance of roots by decreasing water flow resistance! of the apoplast rather than by water channel-mediated transport. The high rates of hydraulic conductance at low root temperatures might be important to plants growing in cold soils and under other challenging environmental conditions that inhibit metabolism and limit water transport.