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David G. Clark, Chris Dervinis, James E. Barrett, Harry Klee, and Michelle Jones

Cytokinins have been shown to delay the onset of leaf senescence. The focus of this project was to produce transgenic petunia (Petunia ×hybrida) plants that over-produced endogenous cytokinins in a senescence specific manner. This was achieved by transforming plants with the IPT (isopentenyl transferase) gene driven by the senescence-associated transcriptional promoter, PSAG12. Two independent transgenic events produced T1 and T2 generation seedling lines that demonstrated the desired nonsenescent phenotype in progeny trials. These lines were used to evaluate the horticultural performance of PSAG12-IPT petunia plants in terms of delayed senescence, rooting of vegetative cuttings, lateral branch growth, flower number, floral timing, and fruit set. Although both lines displayed a delayed senescence phenotype the two PSAG12-IPT transgenic lines differed from each other in regard to other horticultural traits. In addition to delayed leaf senescence, line I-1-7 also demonstrated a decrease in adventitious rooting and an increased number of branches during plant production. Line I-3-18 also demonstrated a delayed leaf senescence phenotype; however, plants of this line were not greatly altered in any other horticultural performance traits in comparison to wild-type `V26'. IPT transcript was detected in young fully expanded leaves of both lines, although mRNA levels were higher in I-1-7 plants. A greater than 50-fold increase in IPT transcript abundance was detected in leaves of transgenic plants following drought stress. These results demonstrate that it is possible to use PSAG12-IPT to produce transgenic plants with delayed leaf senescence but differences in plant morphology between transgenic lines exist, which may alter horticultural performance characteristics.

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David G. Clark, Christopher Dervinis, James E. Barrett, and Terril A. Nell

Experiments were conducted to determine if the seedling hypocotyl elongation and petal abscission assays could be used to identify differences in ethylene sensitivity among seedling geranium (Pelargonium ×hortorum) cultivars. When seedlings of six geranium cultivars were germinated and grown in the dark in the presence of the ethylene biosynthetic precursor 1-aminocyclopropane-1-carboxylic acid (ACC) at various concentrations, they exhibited the triple response (measured as reduced hypocotyl length). While seedlings from all six cultivars were sensitive to ACC, `Scarlet Elite' seedlings were most sensitive, and `Multibloom Lavender', `Elite White' and `Ringo 2000 Salmon' seedlings were the least sensitive when germinated and grown on 20 mm [2022 mg·L-1 (ppm)] ACC. Florets representing three developmental stages of each of the six cultivars were exposed to 1 μL·L-1 of exogenous ethylene for 0, 30, or 60 min to determine if differences in cultivar sensitivity could be determined for petal abscission. Of the six cultivars tested, `Ringo 2000 Salmon', `Multibloom Lavender' and `Elite White' were the least ethylene sensitive. Florets were also self-pollinated to test for cultivar differences in ethylene synthesis and subsequent petal abscission. Ethylene production and petal abscission were both promoted in self-pollinated florets compared to nonpollinated florets. `Ringo 2000 Salmon', `Multibloom Lavender' and `Elite White' florets produced similar amounts of ethylene as all other cultivars, but abscised fewer petals after pollination. Our results indicate that the seedling hypocotyls elongation assay may be used to identify geranium cultivars with reduced sensitivity to ethylene. The data also suggest that genetic variability exists among geraniums for both ethylene sensitivity and biosynthesis.

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Richard K. Schoellhorn, James E. Barrett, Carolyn A. Bartuska, and Terril Nell

Effects of heat stress on viable and nonviable axillary meristem development and subsequent lateral branching in 'Improved Mefo' chrysanthemum [Dendranthema ×grandiflorum Ramat. (Kitamura)] were studied. Plants grown at 33 °C day/27 °C night produced more nonviable buds than did plants grown at 23 °C day/18 °C night. A negative linear relationship {y = 28.7 + [-0.66 (x days)], r 2 = 0.70} between timing of exposure to high temperatures and the number of nonviable buds was observed. Histological examination 28 days after exposure to 33 °C/27 °C revealed that plants showed both normal and abnormal bud development. Abnormal bud development occurred as a consequence of premature differentiation of axillary meristematic tissue into nonmeristematic parenchyma tissue immediately after separation of axillary from apical meristems.

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Erika K. Gubrium, Donna J. Clevenger, David G. Clark, James E. Barrett, and Terril A. Nell

A series of experiments on ethylene-insensitive (EI) petunia plants (Petunia ×hybrida Hort. Vilm.-Andr.) generated in two genetic backgrounds were conducted to determine the involvement of ethylene in horticultural performance. Experiments examined various aspects of horticultural performance: days to flower, flower senescence after pollination and without pollination, fruit set and ripening, and adventitious root formation on vegetative stem cuttings. The development of EI plants was altered in several ways. Time from seed sowing to first flower anthesis was decreased by a week for EI plants grown at 26/21 °C. Flower senescence in nonpollinated and self-pollinated flowers was delayed in all EI plants compared to wild-type plants. Fruit set percentage on EI plants was slightly lower than on wild-type plants and fruit ripening on EI plants was delayed by up to 7 days. EI plants produced fewer commercially acceptable rooted cuttings than wild-type plants. There was a basic difference in the horticultural performance of the two EI lines examined due to a difference in the genetic backgrounds used to generate the lines. EI plants displayed better horticultural performance when grown with day/night temperatures of 26/21 °C than 30/24 °C. These results suggest that tissue-specific ethylene insensitivity as well as careful consideration of the genetic background used in transformation procedures and growth conditions of etr1-1 plants will be required to produce commercially viable transgenic floriculture crops. EI petunias provide an ideal model system for studying the role of ethylene in regulating various aspects of plant reproduction.

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G.H. Pemberton, Terril A. Nell, Ria T. Leonard, A.A. De Hertogh, Lena Gallitano, and James E. Barrett

Forced `Bumalda' and `Etna' Astilbe were evaluated for postproduction quality and longevity. Plants were sleeved, boxed and held at 9±2C for 3 days to simulate shipping at the following stages of floral development: tight bud (TB), 1-3 florets open, 25% florets open, 50% florets open, and 75% florets open. They were then placed at 21C and 14 μmol·m-2·s-1 (12h daylength) until flower senescence. Percent of inflorescences flowering increased from 34% at TB stage to 94% when shipped with 25 % of the florets open. `Etna' longevity increased from 3 days at TB stage to 12 days at 25% open stage. Optimum quality and longevity occurred when ≥ 25% of the florets were opened at shipping.

In a second experiment, `Bumalda' and `Etna' Astilbe were held at 18, 21 and 24C at irradiance levels of 7 or 14 μmol·m-2·s-1 when 25% of the florets were open. At 18C, longevity increased under 14 μmol·m-2·s-1 from 14 to 17 days. At 24C, longevity was only 10 days for both irradiance levels.