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Robert D. Wright, Brian E. Jackson, Jake F. Browder, and Joyce G. Latimer

A pine tree substrate (PTS), produced by grinding loblolly pine trees (Pinus taeda), offers potential as a viable container substrate for greenhouse crops, but a better understanding of the fertilizer requirements for plant growth in PTS is needed. The purpose of this research was to determine the comparative fertilizer requirements for chrysanthemum (Chrysanthemum ×grandiflora ‘Baton Rouge’) grown in PTS or a commercial peat-lite (PL) substrate. The PTS was prepared by grinding coarse (1-inch × 1-inch × 0.5-inch) pine chips from debarked loblolly pine logs in a hammer mill fitted with 3/16-inch screen. The PL substrate composed of 45% peat, 15% perlite, 15% vermiculite, and 25% bark was used for comparative purposes. Rooted chrysanthemum cuttings were potted in each of the substrates on 15 Oct. 2005 and 12 Apr. 2006 and were glasshouse grown. Plants were fertilized with varying rates of a 20N–4.4P–16.6K-soluble fertilizer ranging from 50 to 400 mg·L−1 nitrogen (N) with each irrigation. Plant dry weights and extractable substrate nutrient levels were determined. In 2005 and 2006, it required about 100 mg·L−1 N more fertilizer for PTS compared to PL to obtain comparable growth. At any particular fertilizer level, substrate electrical conductivity and nutrient levels were higher for PL compared to PTS accounting for the higher fertilizer requirements for PTS. Possible reasons for the lower substrate nutrients levels with PTS are increased nutrient leaching in PTS due to PTS being more porous and having a lower cation exchange capacity than PL, and increased microbial immobilization of N in PTS compared to PL. This research demonstrates that PTS can be used to grow a traditional greenhouse crop if attention is given to fertilizer requirements.

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Amy N. Wright, Robert D. Wright, Jake F. Browder, and Brian E. Jackson

Posttransplant root growth is critical for landscape plant establishment. The Horhizotron provides a way to easily measure root growth in a wide range of rhizosphere conditions. Mountain laurel (Kalmia latifolia L.) plants were removed from their containers and planted in Horhizotrons in a greenhouse in Auburn, Ala., and outdoors in Blacksburg, Va. Each Horhizotron contained four glass quadrants extending away from the root ball, and each quadrant within a Horhizotron was filled with a different substrate (treatment): 1) 100% pine bark (Pinus taeda L., PB), 2) 100% soil, 3) a mixture of 50 PB: 50 soil (by volume), or 4) 100% soil along the bottom of the quadrant to a depth of 10 cm (4 inches) and 100% PB layered 10 cm (4 inches) deep on top of the soil. Root growth along the glass panes of each quadrant was measured biweekly in Auburn and weekly in Blacksburg. Roots were longer in all treatments containing pine bark than in 100% soil. When pine bark was layered on top of soil, roots grew into the pine bark but did not grow into the soil. Results suggest that amending soil backfill with pine bark can increase posttransplant root growth of container-grown mountain laurel.

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Lesley A. Judd, Brian E. Jackson, Ted C. Yap, and William C. Fonteno

An apparatus was developed that allows for a range of non-destructive measurements on root growth in containers (pot culture). The mini-Horhizotron was designed to measure root growth of small plant material such as seedlings, herbaceous plugs, or woody plant liners normally grown in containers less than 3.8 L. The mini-Horhizotron design has three chambers extending away from the center that could be filled with the same substrate or filled separately with different substrates/treatments to observe root growth response from a single plant. The objectives were: 1) to test the suitability of the mini-Horhizotron’s design and its effects on plant growth with several different species; 2) to test two different experimental designs on the mini-Horhizotrons for research purposes; and 3) to test the effect of wood-amended substrates on root length of a single species. Measurement included quantification of the longest roots growing away from the center (where the plug was transplanted). Herbaceous and woody plants grown in the mini-Horhizotrons included: Echinacea purpurea (L.) Moench ‘Prairie Splendor’, Chrysanthemum L. ‘Garden Alcala Red’, Rudbeckia hirta L. ‘Becky Yellow’, and Ilex crenata Thunb. ‘Steeds’. These plants produced root and shoot growth similar to plants grown in traditional greenhouse containers with approximately equal heights and volumes, allowing for root observations in the mini-Horhizotrons to be considered simulations of traditional container-grown crop production. Results from the initial root growth measurements provide evidence that the mini-Horhizotron may be used with a different substrate in each chamber, effectively altering a portion of the rhizosphere of one plant and reducing the number of mini-Horhizotrons needed for replications during scientific studies. Root growth was measured in three substrates containing by volume 70:30 peat:perlite (control), peat:pine-wood chips, or peat:shredded pine wood. For the species grown in pine-wood chips or shredded pine wood-amended substrates, root growth equaled or exceeded that observed in the control substrate at all time periods. The mini-Horhizotron was used to non-destructively measure treatment/substrate effects on root growth while providing full visual access to the root zone and developing root system.

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Jeb S. Fields, William C. Fonteno, Brian E. Jackson, Joshua L. Heitman, and James S. Owen Jr.

Pine tree substrates (PTSs) may provide growers with sustainable substrate component options. Improved processing of PTS components has provided new materials with little scientific evaluation or understanding of their hydrophysical behavior and properties. Moisture retention characteristics were developed for two PTSs and four traditional greenhouse components: sphagnum peat, coconut coir, perlite, pine bark, shredded-pine-wood (SPW), and pine-wood-chips (PWC). Mixtures of peat containing 10%, 20%, 30%, 40%, and 50% of perlite, SPW, or PWC were also characterized. Hydrophysical properties were measured, allowing for comparison of the PTS components to the more traditional substrate components (peat, coir, perlite, and pine bark). The SPW was constructed to retain water similarly to peat and pine bark, whereas the PWC was made to increase drainage like perlite. Shredded pine wood had higher total porosity and more easily available water than did PWC components. Total porosities of SPW and PWC were similar to pine bark and coir; air space and drainage were higher than peat and coir because of the lower percentage of fine particles in the PTS components. The two PTS components had a greater influence on water drainage and retention dynamics than did perlite when amended with peat as an aggregate. Water release patterns of SPW or PWC components at low tensions were lower than peat and greater than pine bark; drainage was similar to perlite at higher tensions. Equilibrium capacity variable models predicted similar physical properties (and trends) across multiple container sizes for peat mixes amended with perlite, SPW, or PWC. The impact of PWC on drainage and aeration was similar to perlite in all containers, but these effects were greater in smaller containers.

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Amy N. Wright, Robert D. Wright, Brian E. Jackson, and Jake A. Browder

Rhizosphere pH preferences vary for species and can dramatically influence root growth rates. Research was conducted to determine the effect of root zone pH on the root growth of BuxusmicrophyllaSieb. & Zucc. `Green Beauty' (boxwood) and KalmialatifoliaL. `Olympic Wedding' (mountain laurel). Boxwood plants removed from 3.8-L containers and mountain laurel plants removed from 19-L containers were situated in the center of separate Horhizotrons™. The key design feature of the Horhizotron is four wedge-shaped quadrants (filled with substrate) that extend away from the root ball. Each quadrant is constructed from glass panes that allow the measurement of roots along the glass as they grow out from the root ball into the substrate. For this experiment, each quadrant surrounding a plant was filled with a pine bark substrate amended per m3 (yd3) with 0.9 kg Micromax (Scotts-Sierra, Marysville, Ohio) and 0, 1.2, 2.4, or 3.6 kg dolomitic limestone. All plants received 50 g of 15N–3.9P–9.8K Osmocote Plus (Scotts-Sierra), distributed evenly over the surface of the root ball and all quadrants. Plants were grown from May to Aug. 2003 in a greenhouse. Root lengths were measured about once per week throughout the experiment. Root length increased linearly over time for all species in all substrates. Rate of root growth of boxwood was highest in pine bark amended with 3.6 kg·m3 lime and lowest in unamended pine bark. Rate of root growth of mountain laurel was lowest in pine bark amended with 3.6 kg·m3 lime. Results support the preference of mountain laurel and boxwood for acidic and alkaline soil pH environments, respectively.

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Lesley A. Judd, Brian E. Jackson, William C. Fonteno, and Jean-Christophe Domec

Root hydraulic conductance and conductivity are physiological traits describing the ease with which water can move through the belowground vascular system of a plant, and are used as indicators of plant performance and adaptability to a given environment. The ability to measure hydraulic conductance of container-grown herbaceous and semiwoody plants with soft conductive tissue was tested using a hydraulic conductance flow meter (HCFM). Although the HCFM is a hydraulic apparatus that has been used on woody plants to measure hydraulic conductance of intact roots, it has never been reportedly used on container-grown horticultural plants. Two herbaceous species, Chrysanthemum L. and Solenstemon scutellarioides Thonn., were grown in containers and hydraulic parameters were measured, including root conductance and root conductivity, as well as physical traits such as stem diameter and dry root mass. The HCFM was easily connected to intact roots even on herbaceous stems and was used to determine hydraulic conductance and conductivity directly on container-grown plants with minimal disturbance on the root system. Chrysanthemums, Buddleja davidii Franch., and Hibiscus moscheutos L. were grown in three different substrates, and both root mass and root hydraulic parameters were determined. Chrysanthemums showed a positive response with increasing root hydraulic conductance with increasing root mass. The substrates used in these studies only had an effect on root biomass of chrysanthemums, but substrates had no differential effect on root hydraulic conductivity.

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William G. Hembree, Thomas G. Ranney, Brian E. Jackson, and Mark Weathington

Camellia L., the most speciose member of the diverse tea family Theaceae, has a long and complex horticultural history. Extensive cultivation and hybridization have produced thousands of varieties of Camellia, including commercially important crops such as cultivated tea, oilseed, and iconic flowering shrubs. Cytogenetics of Camellia and related genera is complicated; chromosome number and ploidy can vary widely between species, and interspecific and interploid hybridization occurs. However, specific information regarding cytogenetics of many species, cultivars, and modern hybrids is lacking. The objectives of this study were to compile a consolidated literature review of the cytogenetics of Camellia and related genera and to determine chromosome numbers, ploidy, and genome sizes of specific accessions of selected species, cultivars, and interspecific and interploid hybrids. A review of the existing literature regarding Theaceae cytogenetics is presented as a consolidated reference comprising 362 taxa. Genome sizes were determined with flow cytometry using propidium iodide as a fluorochrome and Pisum sativum ‘Ctirad' and Magnolia virginiana ‘Jim Wilson’ as internal standards. Chromosome numbers of selected taxa were determined using traditional cytology and were used to calibrate genome sizes with ploidy level. Our results confirmed a base chromosome number of x = 15 for Theeae including Camellia, x = 17 for Stewartiae, and x = 18 for Gordoniae. Surveyed camellias ranged from 2n = 2x = 30 to 2n = 8x = 120, including diploids, triploids, tetraploids, pentaploids, hexaploids, and octoploids. Previously uncharacterized taxa such as Camellia azalea, C. amplexicaulis, C. chrysanthoides, C. cordifolia, C. cucphuongensis, C. flava, C. nanyongensis, and C. trichoclada were found to be diploid. Ploidy was also newly determined for Schima argentea, S. khasiana, S. remotiserrata, and S. sinensis (all diploids). Both diploid and triploid Stewartia ovata were found, and a ploidy series was discovered for Polyspora that ranged from diploid to octoploid. Ploidy determinations were used to confirm or challenge the validity of putative interploid hybrids. Monoploid genome sizes varied among subfamily and genera, with 1Cx values ranging from 0.80 pg for Franklinia to a mean of 3.13 pg for Camellia, demonstrating differential rates of genome expansion independent of ploidy. Within Camellia, monoploid genome sizes varied among subgenera, sections, and some species (range, 2.70–3.55 pg). This study provides a consolidated and expanded knowledgebase of ploidy, genome sizes, hybridity, and reproductive pathways for specific accessions of Camellia and related genera that will enhance opportunities and strategies for future breeding and improvement within Theaceae.

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William G. Hembree, Thomas G. Ranney, Nathan P. Lynch, and Brian E. Jackson

The genus Deutzia, in the Hydrangeaceae family, includes ≈60 species that range in ploidy from diploid (2x) to tetradecaploid (14x). There have been extensive breeding efforts for Deutzia, but this has been limited to a few parental species. Although there have been numerous studies of the cytogenetics of some species of Deutzia, the ploidy level of many species remains unknown, and there are few cytogenetic data available for Deutzia hybrids and cultivars. The purpose of this study was to validate the identification and determine the genome sizes and ploidy of a diverse collection of Deutzia species and hybrids using cytology and flow cytometry. Accessions were identified using the most current taxonomic key and voucher specimens were deposited for each at the North Carolina State University herbarium. Corrected and updated species names are provided for all cultivars and accessions studied. Traditional cytology was performed for roots of representative taxa to calibrate the genome size with the ploidy level. The genome size and estimated ploidy were determined for 43 accessions using flow cytometry. Ploidy levels were reported for the first time for three species of Deutzia including D. calycosa (2n = 4x = 52), D. paniculata (2n = 4x = 52), and D. glauca (2n = 12x = 156). The base and monoploid genome size (1Cx) were somewhat variable and ranged from 1.20 to 2.05 pg. No anisoploid hybrids were documented, suggesting the presence of an interploid block. The information produced from this study are beneficial to future curation, research, development, and improvement of this genus with corrected nomenclature and clone-specific data regarding cytogenetics.

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Jeb S. Fields, James S. Owen Jr., James E. Altland, Marc W. van Iersel, and Brian E. Jackson

Water-efficient soilless substrates need to be engineered to address diminishing water resources. Therefore, we investigated soilless substrates with varying hydrologies to determine their influence on crop growth and plant water status. Aged loblolly pine (Pinus taeda) bark was graded into four particle size fractions. The coarsest fraction was also blended with either sphagnum peat or coir at rates that mimic static physical properties of the unfractionated bark or conventional substrate used by specialty crop producers within the eastern United States. Hibiscus rosa-sinensis ‘Fort Myers’ plugs were established in each of the seven substrates and maintained at optimal substrate water potentials (−50 to −100 hPa). After a salable crop was produced 93 days after transplanting, substrate was allowed to dry until plants completely wilted. Crop morphology and water use was affected by substrate hydrology. Increased substrate unsaturated hydraulic conductivity (K) allowed for plants to access higher proportions of water and therefore increased crop growth. Maintaining optimal substrate water potential allowed plants to be produced with <18 L water. Measurements of plant water availability showed that the substrate water potential at which the crop ceases to withdraw water varied among substrates. Pore uniformity and connectivity could be increased by both fibrous additions and particle fractionation, which resulted in increased substrate hydraulic conductivity (K s). Plants grown in substrates with higher hydraulic conductivities were able to use more water. Soilless substrate hydrology can be modified and used in concert with more efficient irrigation systems to provide more water sustainability in container crop systems.

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Jacob H. Shreckhise, James S. Owen Jr., Matthew J. Eick, Alexander X. Niemiera, James E. Altland, and Brian E. Jackson

Soilless substrates are routinely amended with dolomite and sulfate-based micronutrients to improve fertility, but the effect of these amendments on phosphorous (P) in substrate pore-water during containerized crop production is poorly understood. The objectives of this research were as follows: compare the effects of dolomite and sulfate-based micronutrient amendments on total P (TP), total dissolved P (TDP), orthophosphate P (OP), and particulate P (PP; TP − TDP) concentrations in pour-through extracts; to model saturated solid phases in substrate pore-water using Visual MINTEQ; and to assess the effects of dolomite and micronutrient amendments on growth and subsequent P uptake efficiency (PUE) of Lagerstroemia L. ‘Natchez’ (crape myrtle) potted in pine bark. Containerized crape myrtle were grown in a greenhouse for 93 days in a 100% pine bark substrate containing a polymer-coated 19N–2.6P–10.8K controlled-release fertilizer (CRF) and one of four substrate amendment treatments: no dolomite or micronutrients (control), 2.97 kg·m−3 dolomite (FL); 0.89 kg·m−3 micronutrients (FM); or both dolomite and micronutrients (FLM). Pour-through extracts were collected approximately weekly and fractioned to measure pore-water TP, TDP, and OP and to calculate PP. Particulate P concentrations in pour-through extracts were generally unaffected by amendments. Relative to the control, amending pine bark with FLM reduced water-extractable OP, TDP, and TP concentrations by ≈56%, had no effect on P uptake efficiency, and resulted in 34% higher total dry weight (TDW) of crape myrtle. The FM substrate had effects similar to those of FLM on plant TDW and PUE, and FM reduced pore-water OP, TDP, and TP concentrations by 32% to 36% compared with the control. Crape myrtle grown in FL had 28% lower TDW but pour-through OP, TDP, and TP concentrations were similar to those of the control. Chemical conditions in FLM were favorable for precipitation of manganese hydrogen phosphate (MnHPO4), which may have contributed to lower water-extractable P concentrations in this treatment. This research suggests that amending pine bark substrate with dolomite and a sulfate-based micronutrient fertilizer should be considered a best management practice for nursery crop production.