Honey locust (Gleditsia triacanthos var. inermis Wind.) and tree-of-heaven Ailanthus altissima (Mill.) Swingle] sometimes are exposed to high root-zone temperatures in urban microclimates. The objective of this study was to test the hypothesis that seedlings of these species differ in how elevated root-zone temperature affects growth, leaf water relations, and root hydraulic properties. Shoot extension, leaf area, root: shoot ratio, and root and shoot dry weights were less for tree-of-heaven grown with the root zone at 34C than for those with root zones at 24C. Tree-of-heaven with roots at 34C had a lower mean transpiration rate (E) than those grown at 24C, but leaf water potential (ψ1) was similar at both temperatures. In contrast, shoot extension of seedlings of honey locust grown with roots at 34C was greater than honey locust at 24C, E was similar at both temperatures, and ψ1 was reduced at 34C. Hydraulic properties of root systems grown at both temperatures were determined during exposure to pressure in solution held at 24 or 34C. For each species at both solution temperatures, water flux through root systems (Jv) grown at 34C was less than for roots grown at 24C. Roots of tree-of-heaven grown at 34C had lower hydraulic conductivity coefficients (Lp) than those grown at 24C, but Lp of roots of honey locust grown at the two temperatures was similar.
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