Hebeloma crustuliniforme facilitates ammonium and nitrate assimilation in trembling aspen (Populus tremuloides) seedlings.

This study examined the role of ectomycorrhizal associations in nitrogen assimilation of Populus tremuloides seedlings. Seedlings were inoculated with Hebeloma crustuliniforme and compared with non-inoculated plants. Nitrogen-metabolizing enzymatic properties were also determined in H. crustuliniforme grown in sterile culture. The seedlings and fungal cultures were subjected to nitrogen treatments (including NO3⁻, NH4⁺ and a combination of NO3⁻ + NH4⁺) for 2 months to examine the effects on growth, nitrogen-assimilating enzyme activities and xylem sap concentrations of NH4⁺ and NO3⁻. Seedlings were also provided for 3 days with ¹5N-labeled NH4⁺ and NO3⁻, and leaf and root ¹5N content relative to total nitrogen was measured. Both NO3⁻ and NH4⁺ were effective in supporting seedling growth when either form was provided separately. When NO3⁻ and NH4⁺ were provided together, seedling growth decreased while enzymatic assimilation of NH4⁺ increased. Additionally, nitrogen assimilation in inoculated seedlings was less affected by the form of nitrogen compared with non-inoculated plants. Fungal ability to enzymatically respond to and assimilate NH4⁺ combined with aspen's enzymatic responsiveness to NO3⁻ was likely the reason for efficient assimilation of both nitrogen forms by mycorrhizal plants.

Root hydraulic properties and growth of balsam poplar (Populus balsamifera) mycorrhizal with Hebeloma crustuliniforme and Wilcoxina mikolae var. mikolae.

The effects of an E-strain fungus (Wilcoxina mikolae var. mikolae) and an ectomycorrhizal fungus (Hebeloma crustuliniforme) on growth and water relations of balsam poplar were examined and compared in the present study. Balsam poplar roots inoculated with W. mikolae var. mikolae (Wm) exhibited structures consistent with ectendomycorrhizal (EEM) associations, including a mantle surrounding the outside of the root and an extensive Hartig net that was located between cortical cells and extended to the vascular cylinder. Roots colonized with H. crustuliniforme (Hc) developed a mantle layer, indicative of an ectomycorrhizal (ECM) association, around the outer part of the root, but no distinct Hartig net was present. Wm-colonized balsam poplar also showed increased shoot growth, stomatal conductance (g(s)), and root volumes compared with non-inoculated and Hc-inoculated plants. However, Hc-inoculated plants had higher root hydraulic conductivity (L(pr)) compared with non-inoculated plants and Wm-inoculated plants. These results suggest that L(pr) was not a growth-limiting factor in balsam poplar and that hyphal penetration of the root cortex in itself may have little influence on root hydraulic properties.

Changes in root water flow properties of solution culture-grown trembling aspen (Populus tremuloides) seedlings under different intensities of water-deficit stress.

To study the effects of water-deficit stress on root water flow properties in trembling aspen (Populus tremuloides Michx.), seedlings were grown in solution culture and subjected to water-deficit stress by placing their roots in sealed high humidity chambers. After 17 h of stress treatment, seedlings showed mild stress (MS) symptoms with a decline in shoot water potentials. Within 20 h, shoot water potentials rapidly declined, and severe stress (SS) symptoms were present. Root hydraulic conductivity (L(pr)) increased more than two-fold and the relative concentration of apoplastic tracer dye trisodium 3-hydroxy-5, 8, 10-pyrenetrisulphonate (PTS(3)) in xylem exudate decreased by 73.6% in MS seedlings. Conversely, L(pr) decreased (55.3%) and PTS(3) increased (28.6%) in SS seedlings. Treatment of roots with 0.1 mM mercuric chloride decreased root volume flow density (J(v)) by about 29.0% in control and MS plants with no decrease measured in SS seedlings. Mercuric chloride also increased PTS(3) concentration in xylem exudate of control (59%) and MS (86%) seedlings with no change observed in SS plants. The results suggest that aquaporin-mediated transport is important in the regulation of root water flow under drought stress and that root water flow properties are strongly affected by the stress level. Regulation of root water flow may represent an important drought-stress resistance mechanism.