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William R. Graves and Lorna C. Wilkins

Growth of honey locust (Gleditsia triacanthos var. inermis Willd.) seedlings was studied during exposure to reduced osmotic potential (ψπ) and high temperature in the root zone. Half-sib plants were cultured in solution. Root-zone temperature was increased from ambient (23C) to 35C for 0, 6, 12, or 24 hours·day -l. Within each temperature treatment, solution ψπ of -0.05, – 0.10, and – 0.20 MPa were maintained by additions of polyethylene glycol (PEG) 8000. Root and shoot dry weights decreased with increasing exposure to 35C among seedlings in -0.05-MPa solution and decreased for seedlings in - 0.10- and - 0.20-MPa solutions in all temperature regimes. Growth of epicotyls displayed similar trends, but epicotyls of plants in -0.20-MPa solution were longest with 6 hours·day-l at 35C. Significant interactions between effects of temperature and osmotic regimes indicated that water-stressed honey locust seedlings are relatively insensitive to elevated root-zone temperatures. However, related studies showed that PEG caused reductions in growth that could not be explained by decreases in ψπ and suggested that responses of honey locust to PEG differed from those when drought was imposed by withholding irrigation in an aggregate medium.

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William R. Graves, Robert J. Joly, and Michael N. Dana

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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Kimberly A. Klock, Henry G. Taber, and William R. Graves

Two root-zone temperatures (RZT) treatments, 21C and 34C were compared to evaluate their effects on growth and nutrient uptake for tomato (Lycopersicon esculatum Mill.), muskmelon (Cucumis melo L.), honey locust (Gleditsia triacanthos var. inermis Willd.), and geranium (Pelargonium hortorum L.H. Bailey). Plants were grown in a specialized hydroponic system with full strength Hoagland's No. 1 solution. RZT were initiated after a 7 day acclimation period and were held at the respective RZT continuously. Significant differences among the species were expected and noted for growth parameters of fresh wt., dry wt. of shoot and root, and elemental uptake. The 34C RZT, compared with 21C, reduced root length by 22, 51, and 57% for honey locust, tomato, and melon, respectively. P uptake rate dropped to 0 at 34C, as compared to 1.86 mg P/g root/day at 21C for melon. P uptake rate of the other crops was not affected by RZT.

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Alex B. Daniels, David M. Barnard, Phillip L. Chapman, and William L. Bauerle

), Betula nigra ‘Cully’ ( B. nigra ), Carpinus betula ‘Columnaris’ ( C. betula ), Cercis canadensis ( C. canadensis ), Gleditsia triacanthos var. inermis ‘Skycole’ ( G. triacanthos ), Magnolia stellata ‘Royal Star’ ( M. stellata ), Platanus

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Dewayne L. Ingram, John M. Ruter, and Chris A. Martin

between cultivars ( Wilkins et al., 1995 ). Gleditsia triacanthos var. inermis Willd. (honey locust) shoot extension, leaf area, root:shoot ratio, and root and shoot dry weights were less in a constant 21-d 34 °C RZT treatment compared with a 24 °C