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J. Ryan Stewart and William R. Graves

Rhamnus caroliniana Walt. (carolina buckthorn or indian cherry) is an attractive small tree or shrub found in diverse habitats in the United States. Because the species occurs in both mesic and xeric soils, we questioned whether selections of carolina buckthorn could be marketed as new nursery crops resistant to both drought and flooding. Our first objective was to characterize how soil water affects growth and gas exchange of carolina buckthorn. We studied potted plants subjected to soil moistures that ranged from complete submersion of the root zone to severe drought (7% soil water by volume). The maximal photosynthetic rate occurred at 27% soil water content, and complete submersion killed plants. Our second objective was to compare responses of carolina buckthorn to those of the invasive common buckthorn (Rhamnus cathartica L.) when potted plants were treated with partial flooding of root zones and drought. Carolina buckthorn resisted deleterious effects of partial flooding. In contrast, leaves of common buckthorn became epinastic, and rates of photosynthesis were low (2.14 μmol CO2/m2/s) after 17 days of treatment. Mean photosynthesis of common buckthorn increased to 5.52 μmol CO2/m2/s, a rate similar to that of carolina buckthorn, after 55 days of treatment. Drought reduced net photosynthesis by 52% and 68%, respectively, for carolina buckthorn and common buckthorn relative to rates of plants in the control treatment. We conclude that carolina buckthorn is capable of maintaining carbon fixation and growth over a wide range of soil water contents, and unlike common buckthorn, is not dependent upon morphological, anatomical, or physiological adjustments to optimize growth and net photosynthesis in extremely wet soil. Use of carolina buckthorn as an ornamental is warranted if invasiveness and other potential problems with the species are not identified.

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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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Anna J. Talcott and William R. Graves

Two species of North American shrubs in the genus Ptelea (Rutaceae), commonly known as eastern and western hoptree or wafer ash, have unfulfilled potential to increase the diversity of managed landscapes and support populations of pollinators and swallowtail butterflies. The white flowers of Ptelea are highly fragrant, and pistillate flowers give rise to clusters of distinctive samaras. The insufficiency of information about improving germination of seeds of Ptelea trifoliata and lack of recommendations for Ptelea crenulata prompted us to investigate effects of pericarp removal and cold (4 °C) stratification periods of 0, 4, 8, and 16 weeks on the germination of seeds of these two species. Samaras were collected from multiple plants of both species in the midwestern United States (P. trifoliata) and California (P. crenulata). The germination percentage of viable seeds, calculated after tests of viability with tetrazolium chloride of seeds that did not germinate, increased with longer stratification periods, with 100% germination for P. crenulata and 91% germination for P. trifoliata after 16 weeks of stratification. The germination value, a measure of the speed and uniformity of germination, and peak value also increased with longer stratification in both species. Pericarp removal increased the germination percentage of both species and increased the peak and germination value of P. crenulata. Propagators seeking to grow these species of Ptelea from seed should remove the pericarp and cold-stratify seeds for 16 weeks to improve germination success.

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Bryan J. Peterson and William R. Graves

Numerous genera of plants are distributed in both eastern North America and in portions of California with the dry summers of Mediterranean climates. We compared effects of flooding and drought on relative growth rate (RGR), photosynthesis, and biomass of seedlings of two genera, Sambucus L. and Ptelea L., with congeners in both regions. Ptelea crenulata Greene and Sambucus nigra ssp. cerulea (Raf.) R. Bolli from the San Francisco Bay area and Ptelea trifoliata L. and Sambucus nigra ssp. canadensis (L.) R. Bolli from mesic, deciduous forests in eastern North America were studied. Potted seedlings were subjected to six treatments, three extents of root-zone inundation and three severities of drought (irrigation when soil moisture by volume decreased to 5%, 10%, and 20%). After 5 weeks, deleterious effects of inundation were more pronounced among plants from the West than among their eastern congeners. For example, RGR of western and eastern Sambucus with completely inundated root zones was reduced 116% and 25%, respectively. All western and eastern Ptelea died when root zones were completely inundated, but inundating the lower half of the root zone killed all western plants but only reduced RGR among eastern plants. Photosynthesis of Sambucus from the West was lowest with complete inundation and was similar across the remaining treatments. In contrast, photosynthesis of eastern Sambucus was lowest during severe drought but otherwise similar. Photosynthesis of Ptelea was sensitive to both drought and flooding, and moderate root-zone water content led to the highest rates for both western and eastern plants. For both genera, maximal photosynthesis per unit leaf area was greater among western than eastern plants, but eastern plants had greater total leaf area and biomass. Root-to-shoot ratios of western Sambucus were greater than ratios of plants from the East after all treatments, whereas western Ptelea had greater root-to-shoot ratios than eastern Ptelea only under severe drought. Although comparative sensitivity to drought of plants from California and eastern North America varied in these genera, Mediterranean Sambucus and Ptelea both showed greater sensitivity to root-zone inundation than did their eastern congeners.

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James A. Schrader and William R. Graves

Alnus maritima (Marsh.) Muhl. ex Nutt. is a rare woody plant species that exists as three subspecies found in widely disjunct locations in the United States. Although there is a growing interest in the phytogeography, ecology, conservation, and landscape potential of this species, the phylogeny of A. maritima has not yet been resolved by using molecular methods. We have combined a relatively new method of genome fingerprinting, ISSR-PCR, and the automated imaging capabilities of GeneScan technology to investigate the molecular systematics of A. maritima. Based on the molecular evidence from 108 ISSR loci, we confirm that the three disjunct populations of A. maritima have diverged sufficiently to be classified as subspecies. Our molecular phylogeny of the three subspecies of A. maritima agreed in topology with a phylogeny produced from morphological data and showed that subsp. oklahomensis is the most distinct of the three subspecies and was the first to diverge. The simultaneous analysis of molecular and morphological data provides a detailed and balanced phylogeny reconstruction for the three subspecies. Our results support the theory that A. maritima originated in Asia, migrated into North America across the Bering land bridge, and was established over a large range in the New World before being forced into its present meager disjunct distribution.

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Heidi A. Kratsch* and William R. Graves

Alnus maritima (Marsh.) Muhl. ex Nutt. is unique among alders in its degree of preference for low-oxygen soils of wetlands. An actinorhizal species with promise for use in sustainable horticulture, A. maritima develops a root-nodule symbiosis with nitrogen-fixing Frankia. Nodules of other actinorhizal species that are obligate wetland natives are adapted to low oxygen, and expression of hemoglobin is common to these taxa. Our objectives were to determine the range of oxygen tension under which Alnus maritima subsp. maritima fixes nitrogen and to investigate a potential role for hemoglobin in adaptation of nodules to low oxygen. Roots of plants, cultured aeroponically, were subjected to eight oxygen tensions from 0 to 32 kPa. After four weeks, plant dry weight, nodule fresh weight, nitrogenase activity, and photosynthetic rate were measured. In addition, nodules were assayed spectrophotometrically for the presence of hemoglobin. A quadratic function best described the influence of oxygen on plant dry weight, nodule fresh weight, nitrogenase activity, and photosynthetic rate with maximal values above 20 kPa. Alnus serrulata (Ait.) Willd. is sympatric with A. maritima subsp. maritima but is not an obligate inhabitant of wetlands. In a separate experiment, we found higher nitrogenase activity in A. maritima subsp. maritima than in A. serrulata (0.74 vs. 0.26 μmol/h per plant) at hypoxic oxygen tensions. Further, optical absorption spectra of nodule extracts confirmed hemoglobin within nodules of A. maritima subsp. maritima. Our data suggest that hemoglobin contributes to oxygen regulation in nodules of A. maritima subsp. maritima.

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Anthony S. Aiello and William R. Graves

Amur maackia (Maackia amurensis Rupr. & Maxim.) has potential for use in small, urban, or cold landscapes. Although Amur maackia is becoming increasingly popular, plants are currently grown from open-pollinated seed populations, and there has been no selection of cultivars. We have addressed the effects of climate on growth and have begun field trials for selection of horticulturally superior genotypes. In May 1995, a field trial near Ames was begun with 337 plants. These were selected from more than 2000 greenhouse-grown seedlings to represent 32 half-sibling seed groups from 16 arboreta across North America. After two growing seasons, the increase in stem length among seed groups ranged from 3% to 75%. Survival rate did not vary with seed group. In a related study, 30 plants from six half-sibling groups have been established at each of 10 sites in the U.S. and four in Canada to assess effects of location on survival and growth. The influence of seed group on survival after 1 year varied with the trial site location. Survival among combinations of half-sibling group and trial location ranged from 0% to 100% (mean = 54%). Half-sibling group and trial location affected growth without interaction. The greatest growth across locations, an 83% increase in stem length, was shown by seeds that originated from a tree at the Arnold Arboretum. At the 14 locations, changes in stem length over half-sibling groups varied from <0% in Ithaca, N.Y., to 179% in Puyallup, Wash.

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Heidi A. Kratsch and William R. Graves

Although many species of Alnus Miller grow in wet soils, none is as closely associated with low-oxygen, waterlogged soils as Alnus maritima (Marsh.) Muhl. ex Nutt. (seaside alder). An actinorhizal species with promise for use in horticultural landscapes, land reclamation, and sustainable systems, A. maritima associates with Frankia Brunchorst, thereby forming root nodules in which gaseous nitrogen is fixed. Our objective was to determine how root-zone moisture conditions influence the occurrence, location, and anatomy of nodules on A. maritima. Plants of Alnus maritima subsp. maritima Schrader and Graves were established in root zones with compatible Frankia and subjected to four moisture regimens (daily watered/drained, partially flooded, totally flooded, and totally flooded with argon bubbled through the flood water) for 8 weeks. Oxygen content of the root zone, number and location of nodules on root systems, and dry weight and nitrogen content of shoots were determined. Root-zone oxygen content ranged from 17.3 kPa for daily watered/drained plants to 0.9 kPa for argon-treated plants. Across all treatments, 87% of the nodules were within the upper one-third (4 cm) of the root zone. Although shoot dry weights of daily watered/drained and partially flooded plants were not different, daily watered/drained plants had more nitrogen in their leaves (2.53 vs. 2.21 mg·g-1). Nodulation occurred in all treatments, but nodules on totally flooded roots (with or without argon) were limited to a single lobe; in contrast, multilobed nodules were prevalent on partially flooded and daily watered/drained plants. Frankia infection within submerged nodule lobes was limited to one or two layers of cortical cells. Submerged nodules developed large air spaces between cortical cells, and phenolic-containing cells appeared to inhibit Frankia expansion within the nodule. These data suggest that access to root-zone oxygen is critical to the Frankia-A. maritima subsp. maritima symbiosis, and that plants of this subspecies in the drained soils of managed landscapes may benefit more than plants in native wetland habitats from nodulation and nitrogen fixation.

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Brandon M. Miller and William R. Graves

A reputation for coarse root systems with dominant taproots, and for slow shoot development among seedlings, limits use of hickory species (Carya Nutt.) that could increase diversity in managed landscapes. We examined effects of root pruning and application of auxin on root and shoot development of seedlings of several species of hickory. Our hypothesis was that pruning the radicle shortly after seed germination and subsequent treatment with auxin would increase root branching without curtailing development of the shoot. Germinated seeds of Carya aquatica (F. Michx.) Nutt., Carya cordiformis (Wangenh.) K. Koch, Carya laciniosa (F. Michx.) Loudon, Carya ovata (Mill.) K. Koch, and Carya tomentosa (Lam.) Nutt. were treated by removing two-thirds of the length of the radicle with and without immediate application of 3000 mg·L−1 indole-3-butyric acid (IBA) via Hormex rooting powder #3 to the remaining one-third of radicle. Neither treatment altered stem height, stem caliper, or root dry weight. After 75 days, root-pruned seedlings of Carya ovata without auxin had 42% fewer fibrous first-order lateral roots than did unpruned controls. Root pruning plus auxin led to a 79% increase in the number of fibrous first-order lateral roots of C. laciniosa and an ≈50% increase in the shoot dry weight of C. aquatica. Both root pruning and root pruning plus auxin evoked formation of taproot branches for all species. Because species differed in responses of root and shoot systems to root pruning with and without auxin, the practice should be implemented cautiously based on the species.

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Lenore J. Nash and William R. Graves

Responses of five bottomland tree taxa to drought and flooding were studied to identify those adapted to urban environments. During one experiment, containerized `Franksred' red maple [Acer rubrum L. `Franksred' (trademark = Red Sunset)], sweetbay magnolia (Magnolia virginiana L.), black tupelo (Nyssa sylvatica Marsh.), bald cypress [Taxodium distichum (L.) Rich.], and pawpaw [Asimina triloba (L.) Dunal.] were treated with various irrigation regimes for up to 118 days. Net assimilation rate (NAR) and relative growth rate (RGR) were reduced more by flooding than by drought for plants of all taxa, except pawpaw, which showed similar NAR and RGR during flooding and drought. Only sweetbay magnolia and bald cypress maintained positive NAR and RGR during flooding, and sweetbay magnolia was the only taxon that did not produce significantly less leaf surface area, shoot dry mass, and root dry mass during flooding and drought. Apparent morphological mechanisms of stress resistance included an increase in specific mass of leaves (mg·cm-2) during drought for red maple and bald cypress and a 385% increase in the root: shoot mass ratio for droughted plants of pawpaw. Leaf water relations of drought- and flood-stressed `Franksred' red maple and sweetbay magnolia were determined in a second experiment. Predawn and mid-day leaf water potential (ψ) decreased with decreasing root-zone matric potential for both taxa, and transpiration rate was reduced by drought and flooding. Pressure-volume analysis showed that leaves of `Franksred' red maple responded to drought by shifting symplastic water to the apoplast. Leaves of drought-stressed sweetbay magnolia adjusted osmotically by reducing osmotic potential (ψπ) at full turgor by 0.26 MPa. Our results suggest that sweetbay magnolia and bald cypress will perform well at urban planting sites where episodes of drought and flooding regularly occur.