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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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Lorna C. Wilkins and William R. Graves

Development of half-sib Gleditsia triacanthos inermis Willd. (honey locust) seedlings was studied during exposure to osmotic and high root temperature stress. Seven days after seed scarification, seedlings of uniform fresh weight were transferred to static hydroponic culture vessels in a growth chamber. Three days later, vessel solutions were replaced with polyethylene glycol 8000-amended solutions with osmotic potentials (ψπ) of -0.05, -0.10, or -0.20 MPa at 23C. Within each ψπ treatment, root temperature was increased from ambient (23C) to 35C for 0, 6, 12, or 24 hr day-1 for 20 days. Root and shoot dry weights decreased with increasing exposure to 35C among seedlings in the -0.05 MPa solution and decreased for seedlings in -0.10 and -0.20 MPa solutions in all temperature regimes. Epicotyl expansion tended to decrease with decreasing ψπ and increasing exposure to 35C. However, for plants in the -0.20 MPa solution, epicotyl length was greatest when roots were exposed to 35C for 6 hr day-1.

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William R. Graves and Lorna C. Wilkins

A laboratory exercise for illustrating aspects of biological nitrogen fixation (BNP) to students in plant science courses is described. Surface-sterilized seeds of black locust (Robinia pseudoacacia L.) and soybean (Glycine max Merill) were sown together in plastic containers filled with a sterile, soilless medium. Containers were assigned randomly to treatments designed to show how inoculation with two strains of rhizobial bacteria and application of nitrate affect root nodulation and plant growth. Results demonstrated that BNF occurs in diverse legumes, that legumes vary in the strains of rhizobia with which they associate, that nodulation is inhibited by nitrate, and that dependency on BNP can reduce growth compared with plants provided nitrate.

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Lorna C. Wilkins, William R. Graves and Alden M. Townsend

Use of cultivars resistant to high soil temperature could improve the performance of urban trees. The objective of this project was to examine selections of red maple (Acer rubrum L. and A. x freemanii E. Murray) for genotypic differences in resistance to root-zone heat stress. Development of roots and shoots from rooted single-node cuttings of seven genotypes grown in solution culture was optimal at about 28C. Shoot extension stopped within 3 weeks and terminal buds formed on plants of all genotypes at 36C. In a second experiment, the influence of 34C root-zone temperature on development varied significantly among six genotypes. Formation of terminal buds at 34C was observed only on plants of cv. Morgan and cv. Red Sunset. The reduction in new dry matter at 34C compared to plants at 28C ranged from 21% for cv. Schlesinger to 69% for cv. Morgan. We conclude that genotypes of red maple differ in resistance to high root-zone temperature.

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Lorna C. Wilkins, William R. Graves and Alden M. Townsend

Six red maple (Acer rubrum L.) and four Freeman maple (A. ×freemanii E. Murray) cultivars were compared for rooting of single-node stem cuttings and subsequent development of rooted cuttings. Cuttings were taken in May 1990 and 1991 and treated with either 3 or 8 g IBA/kg. Rooting after 4 weeks differed among cultivars, ranging from 22% for `Karpick' to 100% for `Schlesinger' over both years. Rooting scores, based on root counts and lengths, were highest for `Schlesinger' and lowest for `Scarlet Sentinel' and `Karpick'. IBA at 8 g·kg–1 resulted in better rooting than at 3 g·kg–1. Mean length of shoots formed on potted rooted cuttings was 22.6 cm for `Franksred', which initiated shoots on 100% of the cuttings that rooted. In contrast, <50% of `Armstrong', `Jeffersred', `Karpick', `Northwood', and `Scarlet Sentinel' rooted cuttings initiated shoots, and mean length of new shoots was <4 cm for these cultivars. The amount of leaf desiccation that occurred after removing cuttings from the propagation bench varied among cultivars, and the percentage of viable leaf surface area correlated positively with final root or shoot dry mass for all cultivars. Chemical name used: indole-3-butyric acid (IBA).