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  • Author or Editor: Donglin Zhang x
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In the past, horticulture students at the University of Maine have been taught to irrigate plants using only hand irrigation. It is becoming increasingly important to irrigate and fertilize efficiently in commercial greenhouses in order to reduce water waste and nutrient leaching. In 2004 and 2006, greenhouse management or plant production students were exposed to alternate methods of irrigating Dendranthema ×morifolium (chrysanthemum) in greenhouses to train students more effectively in irrigation techniques. In 2004, students measured the quantity of water applied to chrysanthemums once they reached the permanent wilting point from 26 Sept. until 30 Oct. The irrigation frequency generally increased as crops grew, but, the quantity of water applied upon irrigation was not significantly different. This experience provided students with a tangible idea of how irrigation frequency and timing change as crops grow, which could be applied to irrigation timing decisions in the future. In 2006, students grew a crop of chrysanthemums using alternate methods of irrigation (hand watering vs. drip irrigation) and fertilization. Student surveys in 2006 indicated that only 25% of students with previous experience working in a greenhouse or nursery had grown crops using drip irrigation, but all students with prior experience had irrigated by hand. Expanding student experiences with irrigation in the greenhouse uses active learning to instill students with more knowledge of irrigation and provide them with practical skills for irrigating efficiently and conservatively in the future.

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Stewartias (Stewartia spp.) are prized for their camellia (Camellia japonica)-like flowers, intense fall color, and exfoliating bark. In spite of having outstanding ornamental value and features, these plants are not readily available for landscaping in the horticulture trade. The primary reason stated is the difficulty of its mass propagation and production. In the last two decades, considerable research has been conducted on various aspects of stewartia propagation such as seed germination, cutting type, light, rooting medium, rooting hormone, cold acclimation, and tissue culture. In this article, we discuss factors that directly influence propagation of stewartia and we highlight results of published studies to propagate stewartia. The evidence indicates success in adventitious rooting of cuttings but at the same time recognizes the continuing challenge associated with overwinter survival. Sexual propagation has also been studied, but its commercial application is limited. To date, there is lack of concrete information on why stewartia remains under-represented in our landscapes. It still remains unclear if it is the lack of consumer demand or existing propagation difficulties that is the cause of under utilization of stewartia. Given the information from most published studies, we suggest further research on the aspect of overwinter survival in addition to a survey of the nursery and greenhouse industry to accurately determine the cause behind the absence of stewartia in horticultural trade.

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Kalmia latifolia L. (mountain laurel), a member of Ericaceae, is a beautiful ornamental shrub native to the eastern United States. The plant is not common in the southeastern United States landscapes because of the limited heat tolerance of most commercial cultivars. Breeding of heat-tolerant cultivars can be achieved by cross hybridization, but is often challenged by low germination percentage, long germination time, and potential abortion of cross-hybridized seeds. We used in vitro seed germination to enhance germination and shorten germination time and investigated the appropriate collecting time, optimal basal medium, and pH for this approach. Collecting time affected in vitro seed germination, with more mature hybrid seeds [collected 4–5 months after pollination (MAP)] having higher germination rate (90% in 4 weeks) than the less mature seeds collected in 2 MAP (20% in 7 weeks). Seedlings from the mature seeds also produced two true leaves on average after 8 weeks of culture, whereas seedlings from the less mature seeds had no true leaves. Woody Plant Medium (WPM) better enhanced in vitro seed germination compared with Murashige and Skoog (MS) or Gamborg’s B5 (B5) medium. WPM yielded higher germination (98%) than MS (90%) and significantly greater total leaf area per seedling (67 mm3) than MS (50 mm3) and B5 (52 mm3) for seeds of ‘Firecracker’ × ‘Snowdrift’. Similar effects had been observed on seeds from ‘Little Linda’ × ‘Starburst’ and ‘Pristine’ × ‘Peppermint’. The pH ranging from 4.2 to 5.4 did not affect seed germination and seedling development of mountain laurel hybrids. Our protocol enabled early collection of mountain laurel hybrid seeds 1 month before their full maturation and permitted seeds to germinate in 4 weeks on WPM, which shortened the period from crossing to the seedling stage from up to 15 to 6 months and enhanced germination percentage from 30% to more than 90% compared with traditional seed germination. This protocol should be applied to promote the breeding and selection of new mountain laurel cultivars for the southeastern United States landscapes.

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Ornamental peach (Prunus persica (L.) Batsch) is native to China. The ornamental value of peach is gaining popularity for its use in urban landscape and everyday gardens. However, the genetic relationship among ornamental peach cultivars is not clear, which limits the further studies of its molecular systematic. A sample of 51 cultivars of ornamental peach, originated from P. persica and Prunus davidiana, had been studied by using AFLPs. All samples were collected from China, Japan, and the US. A total of 275 useful markers between 75 to 500 base pairs were generated from 6 EcoRI/MseI AFLP primer combinations. Among them, 93% of bands were polymorphic markers. Total markers for each cultivar ranged form 90 to 140, and the average number of markers for each cultivar was 120. Two distinguished clad generated from PAUP-UPGMA tree. P. davidiana, as a species, was apparently an out-group to P. persica, which implied that P. davidiana was far away genetically from ornamental peach (P. persica). Within P. persica clad, 15 out of 17 upright ornamental peach cultivars in this study were grouped to one clad, which indicated cultivars that with upright growth habit had close genetic relationship. Five dwarf cultivars were grouped to one clad, with 81% bootstrap supported. The genetic relationships between these five dwarfs were much closer than any other cultivars, and showed that they probably derived from the similar gene pool. The results demonstrated that AFLP are powerful markers for revealing genetic relationships in ornamental peach. The genetic relationships among ornamental cultivars established in this study could help future ornamental peach germplasm identification, conservation, and new cultivars development.

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Nodal segments containing one axillary bud (1 to 1.5 cm) were disinfected using 10% bleach and were established on a Murashige and Skoog (MS) medium without hormones at 27 °C and with a 16-h photoperiod. The sprouted shoots (≈1.0 cm) were cultured on a MS medium supplemented with 6-benzylaminopurine (BAP), kinetin (KIN), or zeatin (ZT) at 2.3, 4.5, 9.1, or 18.2 μM. After 38 d, ZT and BAP significantly induced multiple shoot formation with multiplication rates of 4 to 6, whereas the multiplication rate of KIN was less than 2. Shoots cultured on ZT grew significantly taller than those on BAP and KIN. The height of the longest shoots treated with ZT was 4.6 cm, which was 1.6 to 2.2 times greater than those treated with BAP or KIN. To induce rooting, shoots (≈2 cm) were subcultured on one-fourth strength MS (1/4 MS) medium containing either 3-indolebutyric acid (IBA) or 1-naphthylacetic acid (NAA) at 2.6, 5.1, or 10.3 μM. Adventitious roots formed in vitro after 2 to 4 weeks. IBA at 10.3 μM produced the best rooting (100%) compared with other treatments after 38 d of culture. The average number of roots per shoot for IBA was ≈15, which was 1.6 to 3.1 times as many as that of other treatments. All rooted plantlets were then transplanted into a mix of peatmoss and perlite (1:1 v/v) and acclimatized in a mist system. Average plantlet survival was 73.6% after 35 d. After acclimatization, they were grown in a pot with Metro-mix under greenhouse conditions for 10 weeks where 95% of plants survived and grew up to 6.8 cm high. The micropropagation procedure, i.e., nodal segments containing one axillary bud proliferated on MS with 4.5 μM ZT followed by in vitro rooting on 1/4 MS plus 10.3 μM IBA, could be used for commercial mass production of new inkberry cultivars.

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The natural distribution and cultivated areas of Stewartia taxa are USDA cold hardiness zones 6 or warmer. One cold-tolerant clone, named Stewartia`UMaine' (UMaine Silk Camellia), has been growing well at the University of Maine Littlefield Ornamentals Trial Garden (USDA Zone 4). The plant also has brilliant red fall color and biennial flowering. Since cold hardiness field evaluation could not provide genetic information and no other taxa could grow in Zone 4, AFLP markers were employed to figure out its genetic relativeness with other 16 named Stewartia taxa. The three primer-pairs generated 360 useful markers with an average of 120 markers for each taxon. The genetic distance between S. sinensis and S. rostrata is only 0.031, which indicates that these two species are very similar and should not be treated as two species or cultivars, at least the plants in cultivation. The largest distance (0.533) occurs between S. pesudocamellia and S. malacodendron, two distinguished species accepted by all taxonomists. UMaine Silk Camellia is a distinguished taxon from all other 16 taxa and S. malacodendron `Delmarva' has the largest genetic distance of 0.453 to it. Although S. ×henryae`Skyrocket' has the smallest genetic distance of 0.183 to Stewartia`UMaine', UPGMA phenograms showed that they are not in a clad at all. AFLP data support that Stewartia`UMaine' is a new cultivar, which originated from a gene pool of S. pseudocamellia, S. sinensis, and S. koreana. These molecular results will also be used as guidance for future Stewartia breeding.

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Iris versicolor (blue-flag iris) is a native aquatic plant that grows from Maine to Virginia. It is an important species of wetland regeneration and restoration. Unfortunately, seed germination seldom occurs in the wild. To address this problem, seeds of Iris versicolor were soaked with gibberellin acid (0, 500, 1000, and 1500 ppm) for 24 h after 120 days of cold treatment at 4 °C and then were randomly assigned to three germination temperatures (constant 21 °C; 24 °C/18 °C; 27C/15 °C) and placed in darkness. Germination rates for the three temperature treatments were 54.4% (21 °C), 96.5% (24 °C/18 °C), and 96.0% (27C/15 °C). Oscillating temperature treatments had significantly greater germination rate than constant temperature. Gibberellin acid had significant influence on germination rate; only the constant 21 °C was not favorable for germination. The germination rate was higher at 1000 than at 500 ppm or 1500 ppm or more. Germination occurred within 10 days under germination temperature treatments. All seedlings in petri dishes were successfully transplanted into growing flats.

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To grow and sell fully charged hanging baskets to the customers is the future trend of retail markets. Impatiens ×hawkeri Bull. `Guadeloupe' (New Guinea Impatiens, Paradise KNG Series) was grown under liquid fertilizer culture until it reached marketable size on 4 June 1999. Then three different fertilizers (two slow-release and one organic) were applied at concentrations of 1.2, 2.4, and 3.6 g pure nitrogen and all hanging baskets were placed in four different family-owned nurseries. Overall performance, media pH, electrical conductivity (EC), and nutrients were measured every 3 weeks and tissue nutrients were analyzed every 6 weeks. New Guinea Impatiens requires low-fertility. Mature plants survived 3 months without extra fertilizer. The best performance was observed under the 3.6 g pure nitrogen treatment. Medium pH ranged from 6.0 to 7.5. No significant effect was observed among the different fertilizers and concentrations. EC increased in the first 6 weeks, then decreased to the beginning level at about 0.5 dS/m after 12 weeks under all fertilizer treatments. Compared with organic fertilizer, slow-release fertilizers had significantly lower EC, especially during the first 9 weeks. Macronutrients in media followed the same trend as EC, as did N and P levels in leaf tissues. K levels in leaf tissues decreased under all treatments and Ca and Mg levels showed an opposite trend compared with medium ones. The results indicate that both slow-release and organic fertilizers can be applied to charge New Guinea Impatiens hanging baskets for 3 months.

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Horticultural Science in the past quarter of a century has been shifting to increased emphasis on ornamental plants due to the growth of the modern green industry. Numerous species are being introduced into the exterior and interior landscapes. For popular species, the cultivar, as defined by the International Code of Nomenclature for Cultivated Plants (ICNCP), has become the basic taxon of cultivated plants. Named ornamental plant cultivars are rising at a rapid rate creating identification and segregation problems in the landscape industry, nurseries, botanic gardens, arboreta, and breeding programs. Government regulations and legal issues are beginning to infringe as solutions to the problems. There is a critical need existing for taxonomic research on ornamental cultivars utilizing classical morphological analysis supplemented with modern biotechnological techniques (e.g., anatomical, chemical, cytological, DNA, Sem analysis). Taxonomic research on existing and newer cultivars can provide quantitative botanical descriptions, keys of segregation, correct identification, determination of correct names and synonymy, improved cultivar documentation, and grouping of similar cultivars in large complexes. The taxonomic research is basic science that has immediate applied application within the horticultural society, and results should be published in the journals of ASHS.

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