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Volume 44 (2009): Issue 5 (Aug 2009) x
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Aaron E. Walworth and Ryan M. Warner

Freezing tolerance of many plant species increases after exposure to low, nonfreezing temperatures, a process termed cold acclimation. In some species, shortened photoperiods also bring about an increase in freezing tolerance. Within the plant family Solanaceae, species vary widely in cold acclimation ability. The objectives of this work were to examine the effects of low temperature and photoperiod on cold acclimation of Petunia hybrida (Hook.) Vilm. ‘Mitchell’ and to evaluate cold acclimation of several Petunia species by measuring freezing tolerance using an electrolyte leakage assay on leaf tissue discs. Temperature, but not photoperiod, influenced cold acclimation of P. hybrida. Whether grown under long days or short days, nonacclimated plants had an EL50 value (temperature at which 50% of cellular electrolytes are lost) of ≈–2 °C. Plants acclimated by gradual cooling at temperatures of 15 °C, 10 °C, and 3 °C for 7 days each reached an EL50 of ≈–5 °C regardless of photoperiod. Exposure to 3 °C under short days for 1 or 3 weeks resulted in EL50 temperatures of –3.9 and –4.9 °C, respectively. Freezing tolerance of petunia species P. exserta Stehmann, P. integrifolia (Hook.) Schinz & Thell., P. axillaris (Lam.) Britton et al. (USDA accessions 28546 and 28548), and P. hybrida ‘Mitchell’ was similar before cold acclimation, but varied from –5 °C for P. exserta to –8 °C for P. axillaris (accession 28548) after cold acclimation. Our results demonstrate the cold acclimation ability of Petunia spp. and identify wild germplasm sources with potential usefulness for improving freezing tolerance of cultivated petunia.

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Elisabeth M. Meyer, Thomas G. Ranney, Thomas A. Eaker and Kelly Ivors

Trees in the Theaceae tribe Gordonieae are valuable nursery crops, but some of these taxa are known to be highly susceptible to root rot caused by Phytophthora cinnamomi Rands. The objective of this study was to evaluate a collection of Gordonieae taxa for resistance to this pathogen. These taxa included Franklinia alatamaha Bart. Ex Marshall, Gordonia lasianthus (L.) Ellis, Schima wallichii Choisy, S. khasiana Dyer, ×Schimlinia floribunda Ranney & Fantz, and ×Gordlinia grandiflora Ranney & Fantz. Abies fraseri (Pursh) Poir. was also included in the study as a positive control. Container-grown trees were inoculated with three isolates of P. cinnamomi and symptoms were rated over an 84-day period during the summer of 2008. Disease symptom ratings from 1 (healthy) to 4 (dead) were collected twice weekly and area under the disease progress curve (AUDPC) values were calculated. None of the S. khasiana or S. wallichii exhibited any root rot symptoms or mortality, whereas the remaining species showed symptoms of infection at varying levels over time. Symptoms in F. alatamaha and A. fraseri were apparent before other taxa, and mortality for both species reached 100% by the end of the experiment. Comparison of AUDPC values indicated that F. alatamaha was the most susceptible followed by A. fraseri. There was no significant difference in AUDPC among the more resistant taxa, including G. lasianthus, both Schima species, and the intergeneric hybrids. Values for AUDPC in the hybrid taxa were similar to their more resistant parental genus, indicating that resistance to P. cinnamomi is a partially dominant trait in these plants. These results further suggest the potential to breed improved hybrids of Gordonieae trees with substantial resistance to P. cinnamomi.

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Shahrokh Khanizadeh, Yvon Groleau, Rong Tsao, Raymond Yang, Inteaz Alli, Robert Prange and Robert Demoy

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Valtcho D. Zheljazkov, Vasile Cerven, Charles L. Cantrell, Wayne M. Ebelhar and Thomas Horgan

Peppermint (Mentha ×piperita) is currently not a common crop in the southeastern United States. Recently, there has been an interest in peppermint as an essential oil crop for this region. In 2007, a field experiment was conducted in Mississippi to determine the effect of nitrogen (N) (0 and 80 kg·ha−1), location (Verona and Stoneville), and harvesting stage (bud formation and flowering) on peppermint productivity, oil content, and composition. Peppermint biomass yields were higher at Verona (8.12 t·ha−1) than at Stoneville (6.12 t·ha−1). However, the essential oil content was higher at Stoneville (1.1%) than at Verona (0.6%). Generally, N rate at 80 kg·ha−1 did not affect oil yield and composition compared with 0 kg·ha−1. The concentrations of the major essential oil constituents were: (-)-menthol at 26% to 30%, (-)-menthone at 14% to 21%, (+)-menthofuran at 5% to 11%, and eucalyptol at 3% to 4% of total essential oil (wt/wt) and were generally within the values previously reported. The concentrations of (-)-menthone and (+)-menthofuran were significantly higher at Stoneville, 21% and 11%, respectively, than at Verona (14% and 6%). The (-)-menthol, (-)-menthone, and (+)-menthofuran yields/ha were higher at Stoneville than at Verona. Overall oil content and yields were higher at bud formation than at flowering, 1.3 and 1.8 times, respectively. In addition, the concentration of (-)-menthone and eucalyptol and the yields of (-)-menthone, (+)-menthofuran, and eucalyptol were higher at bud formation than at flowering. The results from this study demonstrated peppermint can be successfully grown in Mississippi and provide essential oil yields and composition comparable to those in the northwestern United States, the traditional peppermint-growing region.

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Valeria Sigal Escalada and Douglas D. Archbold

Both aminoethoxyvinylglycine (AVG) and 1-methylcyclopropene (1-MCP) are useful tools for managing apple fruit ripening, and their impacts on apple volatile production have been independently assessed. In this work, their combined effect, as might occur in commercial production and postharvest storage, on ‘Royal Gala’ apple volatile production at harvest and after air storage was compared with the effect of each alone. An aqueous solution of AVG was applied to ‘Royal Gala’ apple trees 4 weeks before the normal harvest date (H1) at 124 g·ha−1/a.i. in 2004 and 2005. Control and AVG-treated fruit were treated at H1 or after harvest of AVG fruit 2 weeks later (H2) for 20 h at 30 °C with 1-MCP at a final headspace concentration of 1 μL·L−1. Fruit were ripened for 7 d at room temperature immediately after harvest and treatment or after treatment and then cold storage at 4 °C for 6 or 12 weeks. Peel and cortex tissue of control and AVG plus 1-MCP-treated fruit was provided with butanol or hexanol and ester production was quantified. The combination of AVG plus 1-MCP was more effective in reducing internal ethylene concentration than either alone. Both total volatile production and that of the major individual esters, including hexyl acetate, butyl acetate, and 2-methylbutylacetate, which are considered key constituents of ‘Gala’ aroma, were consistently repressed by the combination of AVG plus 1-MCP after harvest and up to 12 weeks of cold storage. The effects of AVG plus 1-MCP were evident even with H2 fruit when the effects of AVG alone on fruit ripening were at least partially lost. Because alcohol–acyl transferase activity was unaffected by AVG plus 1-MCP, AVG plus 1-MCP-treated peel and cortex samples had similar total ester production when they were provided butanol or hexanol. Total alcohols showed recovery in most treatments except AVG plus 1-MCP, so precursor availability was likely the major factor limiting ester production. The results indicated a sustained adverse effect of the AVG plus 1-MCP treatment on aroma volatile production that could impact consumer acceptability.

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Christine L. Wiese, Amy L. Shober, Edward F. Gilman, Maria Paz, Kimberly A. Moore, Sloane M. Scheiber, Meghan M. Brennan and Sudeep Vyapari

Irrigation for establishing landscape plants is restricted to the first 60 days after planting by most water management districts in Florida, yet woody plants may require between 6 and 12 months to become established. Survival and growth of shrubs planted into landscapes depend on adequate irrigation until shrubs develop a root system capable of compensating for evapotranspiration losses. This study examined the effect of irrigation frequency on survival, quality, and growth of Ilex cornuta Lindl. & Paxt. ‘Burfordii Nana’ and Pittosporum tobira [Dryand] ‘Variegata’ planted in north (Citra, FL; USDA hardiness zone 8b) and central (Balm, FL; USDA hardiness zone 9b) Florida. Shrubs were planted into the landscape from 11.4-L (#3) containers at 3-month intervals for a total of eight planting dates over 2 years and irrigated every 2, 4, or 8 days with 3 L of water at each irrigation event. Scheduled irrigation was discontinued once roots grew to the canopy edge [12 to 22 weeks after planting (WAP)] and survival, quality, and growth were evaluated from that point through 104 WAP. Ilex cornuta ‘Burfordii Nana’ irrigated every 2 days had greater canopy growth index (52 through 88 WAP), canopy dry mass (52 and 104 WAP), and maximum root spread (20 through 64 and 88 WAP) when compared with shrubs irrigated every 8d in hardiness zone 8b. Pittosporum tobira ‘Variegata’ irrigated every 2 days had greater canopy growth index (12 through 104 WAP), maximum root spread (20 through 28 and 64 through 88 WAP), and canopy dry mass (52 and 104 WAP) when compared with shrubs irrigated every 8 days in hardiness zone 8b. However, there were no differences in shoot or root growth resulting from irrigation frequency for these shrubs planted in hardiness zone 9a. Irrigation frequency did not affect shrub survival or aesthetic quality at either location. Although more frequent irrigation (every 2 days) resulted in greater plant growth in zone 8b, the two shrub species tested survived and grew after planting in hardiness zones 8b and 9a on natural rainfall alone provided they were irrigated during establishment with 3 L every 4 to 8 days until roots reached the canopy edge. Subsequent supplemental irrigation was only needed in the following 18 months when plants showed visible signs of drought stress, which occurred when there was no measurable rainfall for 30 consecutive days.

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Qiansheng Li, Min Deng, Jianjun Chen and Richard J. Henny

Pachira aquatica Aubl. has recently been introduced as an ornamental foliage plant and is widely used for interiorscaping. Its growth and use under low light conditions, however, have two problems: leaf abscission and accelerated internode elongation. This study was undertaken to determine if production light intensity and foliar application of paclobutrazol [β-(4-chlorophenyl)methyl-α-(1,1-dimethylethyl)-1H- 1,2,4- triazole-1-ethanol] improved plant growth and subsequent interior performance. Two-year-old P. aquatica trunks were planted in 15-cm diameter plastic pots using a peat-based medium and were grown in a shaded greenhouse under three daily maximum photosynthetic photon flux densities (PPFD) of 285, 350, and 550 μmol·m−2·s−1. Plant canopy heights, average widths, and internode lengths were recorded monthly over a 1-year production period. Two months after planting, the plant canopy was sprayed once with paclobutrazol solutions at concentrations of 0, 50, and 150 mg·L−1, ≈15 mL per plant. Before the plants were placed indoors under a PPFD of 18 μmol·m−2·s−1 for 6 months, net photosynthetic rates, quantum yield, and light saturation and compensation points were determined. Results showed that lowering production light levels did not significantly affect canopy height, width, or internode length but affected the photosynthetic light response curve and reduced the light compensation point. Foliar application of paclobutrazol reduced internode length, thereby resulting in plants with reduced canopy height and width and more compact growth form. Paclobutrazol application also reduced the light compensation point of plants grown under 550 μmol·m−2·s−1. Plants with the compact growth form did not grow substantially, dropped fewer leaflets, and thus maintained their aesthetic appearance after placement indoors for 6 months. These results indicated that the ornamental value and interior performance of P. aquatica plants can be significantly improved by producing them under a PPFD range between 285 and 350 μmol·m−2·s−1 and foliar spraying of paclobutrazol once at a concentration between 50 and 150 mg·L−1.

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Ju Ding, Kai Shi, Yan-Hong Zhou and Jing-Quan Yu

Root and foliar applications of 24-epibrassinolide (EBL), an immobile phytohormone with antistress activity, were evaluated for their effects on reducing fusarium wilt and their influence on antioxidant and phenolic metabolism in roots of cucumber plants (Cucumis sativus L. cv. Jinyan No. 4). EBL pretreatment significantly reduced disease severity together with improved plant growth and reduced losses in biomass regardless of application methods. EBL treatments significantly reduced pathogen-induced accumulation of reactive oxygen species (ROS), flavonoids, and phenolic compounds, activities of defense-related and ROS-scavenging enzymes. The enzymes included superoxide dismutase, ascorbate peroxidase, guaiacol peroxidase, catalase as well as phenylalanine ammonia-lyase and polyphenoloxidase. There was no apparent difference between two application methods used. EBL applications triggered a slight increase in H2O2 concentration followed by increases in the transcript levels of WRKY transcription factor and defense-related genes. This study demonstrated that EBL enhanced resistance to fusarium wilt by a novel mechanism that was not related to its active transport or increase in antioxidant system.

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ZhaoSen Xie, Charles F. Forney, WenPing Xu and ShiPing Wang

In this study, the ultrastructure of phloem and its surrounding parenchyma cells in the developing grape berry produced under root restriction or without (control) was for the first time systematically investigated through transmission electron microscopy during the entire developmental process of the berry. The results showed that root restriction increased the number of plasmodesmata between sieve elements (SE) and companion cells (CC) and between the SE/CC complex and phloem parenchyma cells. Sieve elements in fruit produced under root restriction were smaller in size than those from the control treatment, but CC were bigger than in the control treatment. During the first rapid growth phase of the grape berry, there was denser cytoplasm in the CC produced under root restriction having more abundant mitochondria, endoplasmic reticulum, multivesicular bodies, vesicles, and plastids than in control fruit. During the second rapid growth phase of the grape berry, CC under root restriction showed more serious plasmolysis. Cytoplasmic contents such as vesicles were fused into the vacuole of which the tonoplast nearly disappeared in the phloem parenchyma cells, and cytoplasmic contents in fruit cells produced under root restriction became denser than the control treatment. These results demonstrated that grape berry adapted to the root restriction stress through ultrastructure variation of the phloem, and this variation explained the increase of photosynthate accumulation in the grape berry observed under root restriction.

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Keith Yoder, Rongcai Yuan, Leon Combs, Ross Byers, Jim McFerson and Tory Schmidt

Effects of temperature and the combination of liquid lime sulfur (LLS) and fish oil (FO) applied during bloom on pollen germination and pollen tube growth in flowers and fruit set were examined in apples (Malus ×domestica Borkh.). Percent germination of pollen of ‘Manchurian’ crabapples and ‘Golden Delicious’ apple flowers on the stigmatic surface of ‘Golden Delicious’ pistils increased with increasing temperature from 13 to 29 °C in the first 24 and 48 h after pollination, respectively, but not thereafter. Pollen tube growth rate in the style increased quadratically with increasing temperature from 13 to 29 °C. ‘Manchurian’ was a more effective pollenizer of ‘Golden Delicious’ than was ‘Golden Delicious’ pollen. For example, at 24 or 29 °C, some ‘Manchurian’ pollen tubes grew to the base of ‘Golden Delicious’ styles by 24 h after pollination. On the other hand, no ‘Golden Delicious’ pollen tube grew to the base of a ‘Golden Delicious’ style regardless of temperature and time. Pollen tube growth rate in the style increased with increasing day/night temperature from 7/0 to 24/7 °C. The time required for pollen tubes to grow to the base of styles decreased with increasing day/night temperature from 13/2 to 24/7 °C. Only ≈36 h was required for pollen tubes to grow to the base of style at 24/7 °C, whereas pollen tubes grew very slowly and no pollen tubes grew to the base of style at 7/0 °C regardless of pollen source. LLS + FO, applied 4 or 24 h after pollination, inhibited pollen germination, pollen tube growth in the style, fertilization, and fruit set, but it had no effect when applied 48 h after pollination. These results suggest that LLS + FO applied at this bloom stage causes flower or fruit abscission most likely by inhibiting pollen germination, pollen tube growth in the style, and fertilization.