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James A. Schrader and William R. Graves*

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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William R. Graves and James A. Schrader

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James A. Schrader and William R. Graves

Genotypic variation and horticultural potential of Alnus maritima [Marsh.] Nutt. (seaside alder), a large shrub or small tree found naturally in only three small, disjunct populations, have not been studied. We examined effects of population of origin and environment on seed germination and the growth and morphology of seedlings. Our first germination experiment showed that 6 weeks of cold stratification applied to half-siblings from Oklahoma optimized germination at 73.2%. When this treatment was applied to multiple half-sib seed sources from all populations in a second experiment, seeds from Oklahoma had a higher germination percentage (55%) than seeds from both Georgia (31.4%) and the Delmarva Peninsula (14.7%). A third experiment showed that growth of seedlings increased with increasing irradiance intensity up to 258 μmol·m–2·s–1, and survival and growth of seedlings from Oklahoma varied with root media. In a fourth experiment, multiple groups of half-siblings from all three populations were grown in one environment to compare variation in growth and morphology within and among populations. Leaves of Oklahoma seedlings were longer (12.8 cm) and more narrow (2.15 length: width ratio) than leaves of seedlings from Georgia (12.0 cm long, ratio = 1.76) and the Delmarva Peninsula (11.6 cm long, ratio = 1.86). Seedlings from Oklahoma and Georgia had a higher growth rate (180.7 and 160.0 mg/day, respectively) than did seedlings from Delmarva (130.1 mg/day), while Oklahoma and Delmarva seedlings were more densely foliated (0.72 and 0.64 leaves and lateral shoots per cm of primary stem, respectively) than those from Georgia (0.46 per cm). These differences indicate both divergence among the three disjunct populations and potential to exploit genetic variation to select horticulturally superior A. maritima for use in managed landscapes.

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James A. Schrader and William R. Graves

Alnus maritima [Marsh.] Nutt. (seaside alder) is a rare, North American species with strong potential for use in managed landscapes. Information on the propagation and production of this species is not available. Our objective was to evaluate the potential for using softwood cuttings to propagate A. maritima, with emphasis on how IBA and plant provenance affect rooting success. Propagation trials were conducted with cuttings from seven trees native to the Delmarva Peninsula and seven trees from Oklahoma. Cuttings from both provenances were collected on 14 June and 23 Aug. Cuttings were wounded; treated with 0, 1, or 8 g/kg IBA; and placed under intermittent mist in a greenhouse for 9 weeks. The highest percentage of rooting (67.9) was achieved for the Oklahoma provenance by using 8 g/kg IBA in June. Across IBA treatments, rooting of cuttings from Oklahoma, 54.8% (June) and 12.4% (August), was higher than rooting of cuttings from Delmarva, 27.1% (June) and 3.1% (August). IBA at 8 g/kg caused a higher rooting percentage than the other IBA rates at both times of the season. More cuttings collected 14 June rooted (41%) than those collected 23 Aug. (7.7%) over IBA treatments. Another experiment involved cuttings from one juvenile, greenhouse-grown seedling from Oklahoma that showed 100% rooting with both 1 and 8 g/kg IBA. Shoot growth appeared more vigorous on rooted cuttings from these juvenile stems than on plants derived by rooting mature tissue collected in nature. We conclude that using softwood cuttings can be an effective way to multiply clones of A. maritima, particularly when stock plants are juvenile and cuttings are treated with IBA.

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James A. Schrader and William R. Graves

The genus Dirca L. (Thymelaeaceae) consists of three species of understory shrubs. Dirca palustris L. is sparsely distributed across eastern North America, D. occidentalis Gray is endemic near the San Francisco Bay, and D. mexicana Nesom & Mayfield is known only in one population in northeastern Mexico. Despite interest in the horticultural use of Dirca, plants seldom are marketed. Difficult propagation impedes production of Dirca. We sought to define protocols that promote uniform seed germination of all three Dirca spp. Endodormancy and paradormancy cause sporadic germination over several years under natural conditions, but endocarp removal, cold stratification, and treatment with GA3 increased germination percentage, speed, and uniformity. Dirca occidentalis was most responsive; up to 94% of seeds germinated after endocarp removal, 24 hours in GA3 at 50 mg·L–1, and stratification at 4 °C for 30 days. Treatments also were effective for D. palustris (up to 68% germination), but seeds of D. mexicana were unresponsive and germinated at 25% or less. Seed treatments should facilitate production of D. occidentalis and D. palustris, but further research is needed to define methods to propagate D. mexicana for horticultural use and for conserving this rare species in the wild.

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James A. Schrader and William R. Graves

Alnus maritima (Marsh.) Nutt. (seaside alder) is a rare, woody-plant species with potential for use in managed landscapes. Information on the propagation and production of this species is not available. Our objective was to evaluate the use of softwood cuttings to propagate A. maritima, with emphasis on how indole-3-butyric acid (IBA), plant provenance, and time of collection affect cutting survival, rooting percentage, the number of roots produced, and their length. Propagation trials were conducted with cuttings from seven trees on the Delmarva Peninsula (Eastern Shore of Maryland and southern Delaware) and seven trees in Oklahoma. Cuttings from mature plants in both provenances were collected on 14 June and 23 Aug. 1998; wounded; treated with IBA at 0, 1, or 8 g·kg-1; and placed under intermittent mist in a greenhouse for 9 weeks. Use of IBA at 8 g·kg-1 caused a greater rooting percentage (68%), root count (7.2), and root length (39.2 mm) than did the other IBA rates when applied to cuttings from Oklahoma in June, but IBA had little effect on cuttings from the Delmarva Peninsula. Across IBA treatments, rooting of cuttings from Oklahoma (55% in June and 12% in August) was greater than the rooting of cuttings from Delmarva (27% in June and 3% in August). Cuttings from Oklahoma had greater survival, callus development, root length, and root count than did cuttings from the Delmarva Peninsula during June and August trials. Averaged over IBA treatments and provenances, cuttings collected on 14 June rooted more frequently (41%) than did cuttings collected 23 Aug. (8%). We conclude that softwood cuttings from mature plants are an effective way to multiply clones of A. maritima, particularly when cuttings are collected early in the season and treated with IBA at 8 g·kg-1.

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James A. Schrader and William R. Graves

Alnus maritima (Marsh.) Muhl. ex Nutt. is a large shrub or small tree with potential for use in managed landscapes. Because the three subspecies of A. maritima are indigenous only to areas with mild winter temperatures (USDA hardiness zones 7a and 7b), knowledge of their cold acclimation and cold hardiness is vital if they are to be used where winters are more harsh. Phenology and depth of cold hardiness were assessed by collecting stem samples seven times from 25 Sept. 2000 to 23 Apr. 2001, subjecting the samples to cold temperature ramping, and determining the lowest survival temperature (LST) via the tissue discoloration method. Samples were collected from indigenous plants of the three subspecies and from plants growing in a common garden near Ames, Iowa (USDA zone 5a). Results indicated that some plants from all three subspecies can survive midwinter extremes as low as -80 °C; that plants grown in Ames achieved a greater depth of cold hardiness for most of the winter and were more uniform in cold hardiness than plants growing in warmer native sites; and that the three subspecies did not differ in phenology or depth of cold acclimation. Results of field trials with plots of 150 plants each installed in three northern hardiness zones (USDA zones 5a, 4a, and 3a) supported these conclusions by showing survival of all 450 plants. We resolved differences among subspecies by rating the percentage of stem tissue survival for each plant in the field plots. Subspecies maritima, from the northernmost provenance (the Delmarva Peninsula), showed the least stem death across all three plots (3.9% tissue death), followed by subsp. georgiensis from northwestern Georgia (10% tissue death), and subsp. oklahomensis from southern Oklahoma (12.8% tissue death). Our results suggest that low temperatures should not limit the use of A. maritima in areas as harsh as USDA zone 3a. Selections based on cold hardiness may allow the use of A. maritima in areas with even colder winters.

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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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James A. Schrader and William R. Graves

Genotypic variation and horticultural potential of Alnus maritima [Marsh.] Nutt. (seaside alder), a large shrub or small tree found naturally in only three small, disjunct populations, have not been studied. We examined effects of population of origin and environment on seed germination and growth and morphology of seedlings. The first experiment showed that 6 weeks of cold stratification optimized germination of half-sibling seeds from Oklahoma at 73.2%. When this treatment was applied to multiple groups of half-siblings from all populations in a second experiment, seeds from Oklahoma had a higher germination percentage (55.0%) than seeds from Georgia (31.4%) and the Delmarva Peninsula (14.7%). In a third experiment, morphology and growth of multiple groups of half-siblings from all three populations were compared in one environment. Leaves of seedlings from Oklahoma were longer (12.8 cm) and more narrow (2.15 length to width ratio) than leaves of seedlings from Georgia (12.0 cm long; ratio = 1.76) and the Delmarva Peninsula (11.6 cm long; ratio = 1.86). Seedlings from Oklahoma and Georgia accumulated dry weight at higher rates (181 and 160 mg·d-1, respectively) than seedlings from Delmarva (130 mg·d-1), while seedlings from Oklahoma and Delmarva were more densely foliated (0.72 and 0.64 leaves and lateral shoots per centimeter of primary stem, respectively) than those from Georgia (0.46 per cm). These differences indicate genetic divergence among the three disjunct populations and the potential to exploit genetic variation to select horticulturally superior A. maritima for use in managed landscapes.

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Mark Kroggel, James A. Schrader and William R. Graves

Maackia amurensis Rupr. & Maxim. is a leguminous tree species possessing meritorious ornamental characteristics and is confirmed to associate with rhizobia that fix nitrogen, but few attempts to isolate symbiotically superior rhizobia have been made. Our goals were to isolate rhizobia from the root zones of indigenous trees of M. amurensis in two ecologically distinct forests in the Heilongjiang Province of China, characterize the rhizobia, and compare their effectiveness at causing nodulation of this host plant. Rhizobia were isolated and cultured from nodules that formed on seedlings grown in soils collected in May 1998, from the Maoershan (45°N, 127°E) and Liangshui (47°N, 128°E) Research Forests. Inoculants from each of the 160 isolates were applied to seedlings. A subset of 48 isolates that evoked the most nodules was partitioned by cluster analysis into 12 similarity groups based on measures of number of nodules (17.9 ± 6.5), the ratio of growth rate on two distinct media (2.26 ± 1.8), pH reaction as measured by absorption at 614 nm of bromthymol blue (0.98 ± 0.36), and tolerance to sodium chloride at 15 g/L (23 out of 48). By using single-isolate cultures of similar cellular concentration as inoculants, one isolate from each group and USDA 4349, an isolate obtained during previous research, are being compared for their capacity to infect and nodulate seedlings.