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  • Author or Editor: William R. Graves x
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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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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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Amur maackia (Maackia amurensis Rupr. & Maxim.) has potential as a more widely grown nursery crop, but little information is available on effects of media and nutrition on growth of containerized plants. We compared growth of seedlings in five media and determined growth responses to two fertility regimes. After 35 days, total dry mass of plants grown in 1 perlite: 1 vermiculite (by volume) or in 5 sphagnum peat: 3 perlite: 2 soil was 3.2 times the dry mass of plants grown in three soilless media that contained composted bark; and after 70 days, growth was greater in the medium with soil than in 1 perlite: 1 vermiculite. Plants grown in solution culture with N at 0.75 mm had 1.8 times the dry mass of those provided N at 3.75 mm. Form of N in solution did not affect dry mass, but N content of leaves of plants grown with >50% \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{+}\) \end{document} was 1.3 times as great as that of plants provided only \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{-}\) \end{document} . Plants in containers attained maximal dry mass when fertilized with solutions containing N at 10.8 mm from \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{-}\) \end{document} , \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{+}\) \end{document} , and urea or N at 7.5 mm with equal amounts of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{-}\) \end{document} and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{+}\) \end{document} . None of the soilless media used consistently evoked growth similar to growth of plants in the soil-based medium.

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A rapid, easy, and economical way to root softwood cuttings of red maple (Acer rubrum L. and A. × freemanii E. Murray) has been developed. Single-node cuttings were treated with 8 g IBA/kg and inserted in flats of perlite. Flats were placed in larger trays without drainage holes. Cuttings were subirrigated by adding a complete solution with 100 mg N/L to trays, saturating perlite at the bottom of the flat, below the cuttings. After 3 weeks, 94, 98, 100, and 100% of cuttings of `Indian Summer', `Autumn Flame', `Red Sunset', and `Autumn Blaze' had rooted, respectively. Leaves on cuttings remained turgid without mist or fog. In a subsequent study of `Red Sunset', 0, 50, and 100 mg N/L in the subirrigation solution resulted in 37, 100, and 100% rooting with 8 g IBA/kg and 0, 43, and 67% rooting without IBA. Rooting was fastest and chlorophyll in leaves was highest with both IBA and nutrients. Subirrigation can replace mist or fog when rooting cuttings of red maple.

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Horticulturists have not promoted use of Dirca palustris L. (eastern leatherwood) despite its suite of traits valued by gardeners and landscapers. Horticultural production of D. palustris may be hindered by slow shoot growth and sensitivity of plants to edaphic conditions. Because of discrepancies in reported tolerances of D. palustris to root-zone pH, we assessed whether pH of soils supporting indigenous populations in Florida, Maine, and North Dakota corresponded to responses of seedlings from the three provenances to root-zone pH of 4.5 to 7.3 in soilless media. Regression showed that root zones at pH 5.8 promoted maximum stem length of seedlings from Florida and North Dakota, whereas root zones at pH 4.5 led to maximum stem length of seedlings from Maine. Root-zone pH 5.6 and 5.5 fostered maximum root and shoot dry weight, respectively, for seedlings from Florida, whereas root zones at pH 4.5 promoted maximum root and shoot dry weights of seedlings from Maine and North Dakota. Averaged over provenance, relative leaf greenness decreased by 62%, and foliar nitrogen, iron, manganese, and zinc decreased by 49%, 70%, 95%, and 48%, respectively, as root-zone pH increased from 4.5 to 7.3. Foliar phosphorus decreased at both low and high pH. The pH of soils where seeds were collected did not predict optimal root-zone pH for stem length or biomass accrual in soilless media; genotypes from soils with a pH of 7.4 in North Dakota did not exhibit greater tolerance to high pH than genotypes from Maine or Florida, where pH of indigenous soil was 6.1 and 5.2, respectively. Averaged over pH treatments, seedlings from Florida showed the greatest stem length and formed the most shoot biomass, whereas seedlings from North Dakota had stouter stems, greater root biomass, and greater root-to-shoot ratios than did seedlings from Florida and Maine. Our results illustrate that acidic media facilitate horticultural production of D. palustris, that further evaluation of provenance differences could facilitate selection of genotypes for horticulture, and that tolerances of genotypes to root-zone pH do not strictly correspond to the pH of soils on which they were indigenous.

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The genus Nyssa L. includes several woody species with traits valued by horticulturists, but only black gum (Nyssa sylvatica Marsh.) is prevalent in the nursery trade. A congener, swamp tupelo (Nyssa biflora Walt.), might be a marketable shade tree, but little is known about propagating it from seeds. Because cold, moist stratification overcomes embryo dormancies of black gum, we compared germination of cleaned seeds (surrounding pulp of fruits removed) of swamp tupelo and black gum that were stratified at 5 °C for 0, 14, 28, 42, 56, 70, 84, and 112 days. Seeds of swamp tupelo within intact drupes were also stratified. Across all durations of stratification, 79% of cleaned seeds of swamp tupelo germinated, whereas 11% of seeds within drupes germinated. Germination value of cleaned seeds of swamp tupelo increased from 1.26 to 3.23 as duration of stratification increased. Although cleaned seeds of black gum responded similarly, the benefit of stratification was more pronounced, and the mean germination percentage was lower than for swamp tupelo (66% vs. 79%). In a second experiment, irrigation with low and high concentrations of an extract of fruit pulp of swamp tupelo reduced germination of seeds of basil (Ocimum basilicum L. ‘Superbo’), spinach (Spinacea oleracea L. ‘Bloomsdale’), zinnia (Zinnia ×marylandica Spooner, Stimart, and Boyle ‘Double Zahara Cherry’), and swamp tupelo by 25% to 63% (low concentration) and 40% to 70% (high concentration). Propagators should remove the surrounding pulp from seeds of swamp tupelo and cold stratify them at least for 4 weeks.

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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.

Open Access

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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Abstract

Most systems used for controlling rootzone temperature (RZT) involve grouping plants in each treatment together in one temperature-controlling apparatus (3, 5). The power of experiments using systems with grouped plants is limited because the groups constitute single experimental units during data analysis. Some systems have overcome this problem, but reports may lack fabrication details (2) or indicate a limited RZT range was used (1, 4). We designed a precise, inexpensive system capable of achieving a wide range of RZT in which individual plants are discrete experimental units.

Open Access