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  • Author or Editor: William R. Graves x
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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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Attributes of Leitneria floridana Chapman have been recognized, but this North American shrub remains rare in commerce, and little information on propagation is available. We studied germination of seeds collected from several disjunct populations of L. floridana in 2002 and 2003. In 2002, ≤5% germination occurred when ripe drupes from Missouri and Florida were sown soon after collection. Effects of GA3 (750 mg·L-1 for 24 hours) were assessed on stored drupes leached with water and on seeds excised from stored drupes. Germination percentages were 21 and 32 for leached drupes and excised seeds from Florida, respectively, but ≤5% germination occurred among germplasm from Missouri and among untreated drupes from both provenances. Viability of ungerminated seeds among treatments ranged from 0% to 7%. In 2003, fleshy, apparently unripe drupes from Texas, which were scarified with H2SO4 and then treated with 1000 mg·L-1 GA3 showed 48% germination (germination value = 3.9). Up to 29% germination (germination value = 2.7) occurred when seeds were excised from unripe drupes from Arkansas and Missouri and then were treated for 24 hours with 750 or 1000 mg·L-1 GA3. We conclude that provenance, developmental stage of drupes when collected, storage, and pregermination treatments influence viability and germination of seeds of L. floridana. Barriers to germination may be avoided by collecting drupes when they are green and fleshy.

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Little is known about the reproductive biology of carolina buckthorn [Rhamnus caroliniana Walt. or Frangula caroliniana (Walt.) Gray], an attractive North American shrub or small tree that might merit increased use in managed landscapes. The fecundity and high germinability of seeds of the Eurasian common buckthorn (Rhamnus cathartica L.), however, have been characterized as factors contributing to its invasiveness. We compared seed germination of these species to ascertain how easily carolina buckthorn could be grown from seed in nurseries and to acquire data for predicting whether carolina buckthorn might be invasive if introduced into managed landscapes. Fruits of carolina buckthorn were collected from indigenous plants in central Missouri, southern Oklahoma, and southern Texas. Fruits of common buckthorn were collected from shrubs naturalized in central Iowa. Seeds of both species were stratified for up to 112 days in darkness at 4 °C; germination at 24 °C in the dark was then evaluated for 56 days. Quadratic functions best described how time of stratification influenced germination value and germination percentage of common buckthorn, whereas these measures of carolina buckthorn were best represented by exponential (value) or linear (percentage) functions. Stratification for 112 days maximized germination value and percentage for carolina buckthorn within the 56-day germination period, but shorter stratifications were sufficient to optimize germination of common buckthorn. While the overall mean germination of carolina buckthorn was 40%, results varied by provenance and ranged from 25% (Missouri) to 56% (Oklahoma). Mean germination of common buckthorn over times of stratification was 71%, and the overall mean daily germination of common buckthorn, 1.3, was 86% greater than that of carolina buckthorn, 0.7. We conclude that seeds of carolina buckthorn are more resistant to germination than seeds of common buckthorn. Our results suggest that plant propagators should cold-stratify seeds of carolina buckthorn for up to 112 days, and suggest that carolina buckthorn has a lower potential to be invasive than does common buckthorn.

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Carolina buckthorn (Rhamnus caroliniana Walt.) is ornamental and could be promoted as a stress-resistant shrub for horticultural landscapes. Its status as a relative of invasive species, including common buckthorn (Rhamnus cathartica L.), raises concerns regarding the environmental consequences of planting Carolina buckthorn outside of its natural habitat. To assess the ease of propagating Carolina buckthorn from seed, and to gather data relevant to assessments of invasiveness, we compared seed-germination characteristics between the two species. Seeds of Carolina buckthorn were collected from native populations in Missouri, Oklahoma, and Texas. Seeds of common buckthorn were collected from populations in Iowa. We stratified seeds of both species for up to 112 days at 4 °C. Germination at 20 °C then was evaluated for 56 days. Over stratification durations, 40% and 71% of seeds of Carolina buckthorn and common buckthorn germinated, respectively. Stratification for 112 days optimized germination value for Carolina buckthorn, but stratification for 42, 56, 84, and 112 days evoked similar germination percentages. Seeds of Carolina buckthorn from Oklahoma germinated at a higher percentage (56%) than did seeds from Missouri (25%). Neither germination value nor germination percentage of common buckthorn was influenced by stratification. We conclude that seeds of Carolina buckthorn are more recalcitrant than are seeds of common buckthorn. This suggests that Carolina buckthorn, particularly those from Missouri with low reproductive success, may be less invasive than their Eurasian kin. Horticulturists can optimize germination percentage of Carolina buckthorn by cold-stratifying seeds for as little as 42 days, but 112 days optimizes germination value.

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Long regarded as a genus of two species, Dirca L. was expanded to include a third North American shrub discovered in 1994 as one population in the Sierra Madre Oriental of Tamaulipas in northeastern Mexico. The designation of Dirca mexicana Nesom & Mayfield as a third species in the genus was based in part on geographical separation from Dirca palustris L. and Dirca occidentalis Gray, which occur farther north in eastern North America and in a small region of California, respectively. Morphologically, D. mexicana was regarded as more similar to D. occidentalis than to D. palustris. Our objectives were to obtain fruits of all species, germinate seeds, and compare the three species genetically through analyses of seedling DNA. Drupes of D. mexicana, D. palustris (from populations in Iowa), and D. occidentalis were collected as they abscised naturally from plants in native habitats in mid-May, late May to early June, and mid-June, respectively. Embryo extraction, gibberellin, and cold stratification were used to promote germination, and DNA was extracted from leaves of seedlings by using the fully automated Autogen Autogenprep 740 DNA extraction system. Genomic DNA templates were used to compare sequences of the internal transcribed spacers (ITS) and the 5.8S coding region of the nuclear ribosomal DNA repeat and to examine polymorphisms in inter-simple sequence repeats (ISSRs). These analyses reinforce the present morphologically based classification of the three Dirca species by confirming species-level divergence at the molecular level. ITS sequences and ISSR banding patterns also enabled us to reconstruct the phylogenetic relationship among the three extant species of Dirca.

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Some buckthorn species from other continents have proven invasive in North American landscapes. Carolina buckthorn (Rhamnus caroliniana Walt.) is an attractive, native species that would merit increased use in horticultural landscapes if concerns about its potential invasiveness are allayed. Invasiveness often is associated with efficient use of water and other resources. We tested for differences between Carolina buckthorn and common buckthorn (Rhamnus cathartica L.) in photosynthesis, aboveground dry matter accumulation, and water-use efficiency. Seedlings were grown in columns of field soil within insulated pots outdoors for 98 days. Net photosynthesis of Carolina buckthorn was 17% to 39% greater than that of common buckthorn through day 22. This difference between species was reversed through the end of the treatment period with a concomitant increase in leaf temperature of Carolina buckthorn. Final dry weight of aboveground tissues was similar for the two species, but a greater proportion of dry matter was partitioned to stems for common buckthorn compared to Carolina buckthorn. Although common buckthorn initially had higher water-use efficiency (110 mg·g-1 per day) than did Carolina buckthorn (60 mg·g-1 per day), the water-use efficiency of both species decreased to similar values for the remainder of the treatment period. We conclude that young plants of common buckthorn do not use water more efficiently than do young Carolina buckthorn under field conditions in central Iowa. Considering the possible species differences in the relationship between temperature and photosynthesis, comparative water-use efficiency should be tested further in other environments where Carolina buckthorn might be used for landscaping.

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Rhamnus alnifolia and Rhamnus lanceolata are shrubs of modest size with lustrous foliage. We evaluated seed germination of both species and propagated R. alnifolia by using softwood cuttings collected in early June. For R. alnifolia, cold stratification for up to 90 d resulted in 48% germination and a germination value of 1.9, whereas only 7% germination occurred among seeds stratified for 120 d. Seeds of R. alnifolia did not germinate if they were untreated or if scarified and stratified. Rhamnus lanceolata required 120 d of stratification to germinate, but percentages were low (≤ 5). Survival of germinants of both species was 90 to 100% regardless of prior seed treatment. Seedlings grew uniformly and had a mean leaf count of 11 and a mean height of 20 cm after 102 d. Application of 3000 and 8000 mg/L indole-3-butyric acid (IBA) in talc led to 85% rooting of R. alnifolia, whereas rooting was ≤ 15% after use of solutions with those IBA concentrations. While 75% of untreated cuttings rooted, fewer roots formed without IBA. More roots developed in 100% vermiculite than in 1 vermiculite: 1 perlite (by volume), which also diminished the number and apparent health of leaves on cuttings during the rooting period. We conclude that talc-based IBA and vermiculite should be used to root softwood cuttings of R. alnifolia, and that both species can be propagated from stratified seeds. Rhamnus lanceolata is more recalcitrant than is R. alnifolia and merits further study to optimize germination success.

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Symbiotic associations between Alnus maritima (Marsh.) Muhl. ex Nutt. (seaside alder) and the actinomycete Frankia result in root nodules in which atmospheric nitrogen (N) is fixed. This has led to interest in producing seaside alders with minimal N fertilizer and in using the species on low-N soils. Our objectives were to determine how applied N influences nodulation and to characterize how short-term changes in root-zone N affect the function of established nodules. Seaside alders native to the Delmarva Peninsula (Alnus maritima subsp. maritima) were grown in perlite inoculated with soil from roots of indigenous plants. Plants were treated with N-free Hoagland solution supplemented with ammonium nitrate at 0, 0.25, 0.5, 0.75, 1, 1.25, 1.5, 2, 4, and 8 mm for 10 weeks. Nodulation decreased as applied N increased. While plants treated with ammonium nitrate at 4 and 8 mm formed nearly no nodules, 0.5 mM resulted in vigorous, healthy plants that formed, on average, 70 nodules. In a second experiment, a nodulated population of seaside alders was established by treating seedlings with 0.5-mm ammonium nitrate in otherwise N-free Hoagland solution for 6 weeks. Plants then were provided ammonium nitrate at 0.5, 2, or 4 mm for two weeks. Acetylene-reduction assays showed that ammonium nitrate at 4 mm suppressed nodule activity. Daily irrigation with N-free solution subsequently led to a rapid depletion of root-zone N and a concomitant resurgence of nodule activity among plants previously provided 2- and 4-mm ammonium nitrate. These results provide useful information on how to manage fertility to optimize nodulation and show suppression of nodule activity caused by N fertilization can be temporary if excess N is leached from the root zone.

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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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