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J. Ryan Stewart* and William R. Graves

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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William R. Graves and Sarah J. Gardner

Alnus maritima [Marsh.] Nutt. (seaside alder) is a rare species that occurs naturally only on soils that are frequently or constantly saturated with fresh water. The objective of our first experiment was to determine effects of drought and flooding treatments of differing severity on foliar gas exchange, water relations, and development of plants grown in containers in a greenhouse. In a second experiment we examined how the rate of water loss from soil during drought episodes affected the gas exchange and survival of leaves. In the first experiment, changes in soil moisture content, which ranged from saturation to 10% or less by volume across treatments, were associated with altered stem water potential and net photosynthesis. Analysis of the osmolarity of liquid extracted from leaves indicated that osmotic adjustment did not occur in response to drought. Shoot dry weight per plant ranged from over 7 g (only the lower portion of the soil profile kept saturated) to less than 3 g (entire soil profile constantly saturated). Episodes of drought of different severity led to plants with shoots that weighed between these two extremes, and exposure to soils with 10% water or less by volume did not elicit leaf desiccation or abscission. Results of the second experiment suggest that leaf desiccation can result from exposing plants to 10% water or less by volume if the drought develops rapidly in a small volume of soil. We conclude that, despite the niche it occupies in nature, seaside alder may have the potential to be used in managed landscapes with soils that vary in moisture content.

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Rolston St. Hilaire and William R. Graves

Differences in foliar morphology and anatomy of hard maples (Acer saccharum Marsh. and Acer nigrum Michx. f.) may explain contrasting responses to moisture stress of these species. We conducted a 2-year study to examine leaf morphology and anatomy of populations of hard maples indigenous near the 43°N latitude from 94°W longitude in Iowa to the 71°W longitude in Maine. Leaves were collected from shoots exposed to direct solar radiation on multiple trees at each of 24 sites in 1995, and at 36 sites in 1996. Samples collected in 1995 showed stomate frequency on the abaxial leaf surface ranged from 380 to 760 stomata/mm2. Mean guard cell pair width and length were 16 and 17 μm, respectively. Stomate frequency related quadratically to longitude, was greatest for leaves from Iowa, and was negatively correlated with mean annual precipitation of the sample site. Leaf thickness did not vary with longitude and averaged 96 μm. Palisade thickness showed a greater correlation than mesophyll thickness to total leaf thickness. Mesophyll thickness was more highly correlated than palisade thickness to specific leaf mass, which did not vary with longitude and averaged 5.2 mg·cm–2. Analysis of leaves collected over both years showed trichome frequency and lamina area were related quadratically to longitude; the largest and most pubescent laminae were from westerly sites. These studies are being coordinated with greenhouse experiments on responses of seedlings from selected populations to moisture deficits.

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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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J. Ryan Stewart and William R. Graves

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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Rolston St. Hilaire and William R. Graves

Selection of sugar maples (Acer saccharum Marsh.) and black maples (Acer saccharum Marsh. ssp. nigrum Desm. or Acer nigrum Michx. f.) that will be more resilient than existing cultivars in managed landscapes could be facilitated by defining relationships between geographic origin and foliar traits critical to leaf function. We examined variation in leaf morphology and anatomy of both taxa, known collectively as hard maples, near 43 °N latitude and tested for relationships between foliar traits and the longitude of origin from 70 ° to 94 °W longitude. Leaves exposed to direct solar radiation were sampled from up to 20 trees indigenous at each of 42 sites during 1995 and 1996. All leaves from east of 75.84 °W and from 92.73 °W and further west expressed morphological characters associated with sugar maple and black maple, respectively; leaves with intermediate traits were found between these two longitudes. Leaves from 90 ° to 94 °W had the highest surface area due to increases in the areas of middle and proximal portions of laminae. Up to 1162 trichomes/cm2 were present on the abaxial surface of laminae from west of 85 °W, while laminae from further east were glabrous or had ≤300 trichomes/cm2. Laminae from western habitats also had relatively high stomatal frequency, and stomatal apertures of laminae west of 91 °W were particularly narrow. Longitude did not affect specific weight and thickness of laminae, which averaged 5.5 mg·cm-2 and 90 μm, respectively. Principal component analysis of laminar traits showed existence of two clusters. A large group dominated by data from trees in New England also contained data from trees as far west as ≈93 °W longitude; data for trees further west were clustered separately. Although phenotypic continua were defined, laminae west of 93 °W were distinct, which suggests trees selected there may function differently in managed landscapes than trees selected from native populations further east.

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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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William R. Graves and Anthony S. Aiello

Information on the heat resistance of silver maple (Acer saccharinum L.) could help develop stress-resistant Freeman maples (Acer ×freemanii E. Murray). Our first objective was to determine how 26, 30, 32, 34, and 36 °C in the root zone affect growth and water relations of plants from rooted cuttings of a silver maple clone indigenous to Mississippi (33.3 °N latitude). Fresh mass increased over time for plants at all temperatures and was highest for plants with root zones at 30 °C. Quadratic regression functions predicted maximal plant dry mass, leaf surface area, and stomatal conductance at 29, 29, and 28 °C, respectively. Stem xylem water potential (ψ) during the photoperiod decreased linearly with increasing root-zone temperature from -0.83 MPa at 26 °C to -1.05 MPa at 36 °C. Our second objective was to compare six clones of silver maple from the Mississippi location with six clones from 44.4 °N latitude in Minnesota for effects of 35 °C in the root zone on plant growth, stomatal conductance, and stem ψ. Provenance and temperature main effects were significant for most dependent variables, but there were no provenance × temperature interactions. Over both provenances, plant fresh and dry mass, leaf surface area, stomatal conductance, and stem ψ during the photoperiod were higher at 29 than 35 °C. Over both temperatures, plants from Minnesota clones had higher fresh and dry mass and more leaf surface area than plants from Mississippi clones. The lack of temperature × provenance interactions suggests that ecotypic or clinal variation in heat resistance is minimal and will not be useful for identifying superior genotypes for use in interspecific crosses with red maple (Acer rubrum L.).

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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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Carol M. Foster and William R. Graves

Desiccation damage in ornamental plant species is of particular concern to the nursery and landscape industry. Species in two genera, Acer and Alnus, display fundamental differences in how drought affects leaves. The same soil moisture content that causes foliar desiccation and abscission in Alnus maritima (Marsh.) Nutt. (seaside alder) causes neither response in Acer rubrum L. (red maple). Understanding molecular mechanisms associated with plant response to drought stress can be an im portant factor in developing strategies for improved sustainability in urban landscapes. Our objective was to characterize expression of drought-induced dehydrin genes in leaves of `Red Sunset' red maple (desiccation-resistant) and seaside alder (desiccation-sensitive) in response to dehydration and rehydration. Potted cuttings grown in a glasshouse were subjected to four cycles of drought and rehydration. Stomatal conductance and volumetric moisture content of rooting medium were used to determine when drought cycles ended. During the second and fourth cycles, leaves were sampled for RNA and protein extraction. Dehydrin probes were generated from genomic DNA of both species by using PCR with primers designed from conserved regions in dehydrin genes. Southern blot analyses revealed the presence of dehydrin genes in seaside alder and red maple genomes. Reverse transcriptase (RT)-PCR was used to isolate desiccation-induced dehydrin cDNAs from total RNA extracted from drought-stressed leaves. The cDNA clones show 61% to 66% identity at the nucleic acid level with dehydrin genes of soybean, sunflower, radish, and potato. Accumulation of dehydrin transcripts and proteins in leaves in response to dehydration and rehydration are being studied through northern and western blot analyses, respectively. Our results may lead to a rapid screening technique for seedlings with improved mechanisms of drought resistance.