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  • Author or Editor: L. C. Stephens x
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Several growth hormone combinations and silver nitrate concentrations were examined for their effect on regeneration of different pepper genotypes. Primary leaf explants from in vitro seedlings were cultured on a revised Murashige and Skoog medium supplemented with auxin, cytokinin and 1.6% glucose. Combinations of different concentrations of indole-3-acetic acid (IAA), 0-5 mg/l, and 6-benzylaminopurine (BAP), 0-5 mg/l, were tested to determine the most effective medium for shoot primordium formation. Experiments with IAA and BAP did not result in a specific growth hormone combination appropriate for regeneration of all genotypes tested. Of the silver nitrate concentrations tested, 10 mg/l resulted in the best shoot and leaf differentiation and reduced callus formation. Differences in organogenic response of individual genotypes were evaluated on a single regeneration medium. Whole plants were regenerated from 11 of 63 genotypes examined. Based on these experiments, a reproducible regeneration system for pepper was developed with a total of 500 plants regenerated to date.

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Several growth hormone combinations and silver nitrate concentrations were examined for their effect on regeneration of different pepper genotypes. Primary leaf explants from in vitro seedlings were cultured on a revised Murashige and Skoog medium supplemented with auxin, cytokinin and 1.6% glucose. Combinations of different concentrations of indole-3-acetic acid (IAA), 0-5 mg/l, and 6-benzylaminopurine (BAP), 0-5 mg/l, were tested to determine the most effective medium for shoot primordium formation. Experiments with IAA and BAP did not result in a specific growth hormone combination appropriate for regeneration of all genotypes tested. Of the silver nitrate concentrations tested, 10 mg/l resulted in the best shoot and leaf differentiation and reduced callus formation. Differences in organogenic response of individual genotypes were evaluated on a single regeneration medium. Whole plants were regenerated from 11 of 63 genotypes examined. Based on these experiments, a reproducible regeneration system for pepper was developed with a total of 500 plants regenerated to date.

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New Guinea Impatiens cultivars, I. hawkeri Bull., are susceptible to hot, windy conditions throughout much of the Midwest and Western U.S. Certain Indonesian Impatiens from Java (I. platypetala Lindl.) and Celebes (I. aurantiaca Teysm.) are much more heat-tolerant. Interspecific hybrids involving Java and Celebes Impatiens with the New Guinea species have been produced, but lack of fertility has been a persistent problem, unless amphidiploids are produced. Because selection is difficult in amphidiploid populations, other methods of recovering fertility have been investigated. Some interspecific hybrid fertility has been obtained from crossing Impatiens `Tangeglow' with a Java × New Guinea hybrid. Evidence will be presented on the role of the Celebes genome in female fertility, and the role of unreduced pollen in the Java × New Guinea hybrid. Approaches to understanding and overcoming sterility in Impatiens interspecific hybrids will be discussed.

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Forty deciduous azalea (Rhododendron sp.) cultivars from commercial sources were evaluated for powdery mildew (Microsphaera sp.) resistance. Plants were established in two duplicate field plantings in Ohio and Minnesota and evaluated in 2002 and 2003. Plants were scored using a disease symptom rating based on the percent of leaf area infected, evaluating both ab- and adaxial leaf surfaces. Highly significant differences were found for cultivar, location, year, cultivar × location and cultivar × year for disease severity. Calendulaceum × speciosum, `Fragrant Star', `Garden Party', `Late Lady', `Millennium', `Parade', and `Popsicle' showed no powdery mildew symptoms in both locations. Another group of plants with only minimal symptoms (<25% leaf area affected) included `Jane Abbott', `Magic', `Northern Hi-Lights' and `Snowbird'. The symptom-free cultivars exhibited glaucous foliage, suggesting a potential, common resistance mechanism. The mean scores for the abaxial and adaxial leaf surfaces were 2.34 and 1.64, respectively, although four cultivars had more disease symptoms on the adaxial surface. `Arneson Gem' showed nearly a two-point difference between abaxial and adaxial scores. Evaluations of azalea powdery mildew susceptibility should consider both leaf surfaces and use the highest score as the best estimate of host resistance.

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Forty-one deciduous azalea (Rhododendron subgen. Pentanthera G. Don) cultivars were assessed for powdery mildew (PM) resistance in a two-location, 3-year field trial. Disease severity (percent leaf area affected) on abaxial leaf surfaces was used to rate the level of field resistance. This measure was proportional to (r = 0.83) but higher than estimates from corresponding adaxial surfaces. Eleven of these cultivars (27%) appeared to be highly resistant under field conditions, i.e., evidence of PM on the leaves was zero or near zero. Twenty-three of the cultivars evaluated in the field experiment were also evaluated in a growth chamber experiment. In contrast to the field study, PM was more severe on the adaxial leaf surface in the growth chamber but still highly correlated with the abaxial response (r = 0.93). Based on adaxial disease scores, no cultivars in the growth chamber experiments were completely resistant. Growth chamber disease ratings based on either leaf surface were predictive of field performance (r 2 = 0.62), suggesting use of the chambers could serve as a low-cost, off-season, early selection component of a deciduous azalea PM resistance breeding program.

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Abstract

In vitro propagation of induced tetraploid Java (J) Impatiens platypetala Lindl. TR6-19, New Guinea (NG) Impatiens sp. ‘Skyrocket’, and 2 interspecific J x NG hybrids, T63-1 and T63-3, was achieved by culturing shoot tips on agar-solidified MS revised medium supplemented with levels of kinetin and NAA. The mean number of shoots per shoot tip increased linearly from 0.3 shoots at 1.39 µM to 8.5 shoots at 139.0 µM kinetin for T63-1, the most responsive genotype. The number of shoots produced by T63-1 and T63-3 were not significantly different, but both hybrids produced significantly more shoots than either ‘Skyrocket’ or TR6-19. Shoot tips produced in vitro rooted readily and were transferred successfully to soil. Chemical name used: 1-naphthaleneacetic acid (NAA).

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Abstract

Six-week treatment at a 13°C soil temperature initiated at the 4- and 6-leaf stages most effectively advanced flowering of Cyclamen persicum Mill. cvs. Mayer Reinweiss and Rosa von Zehlendorf-Tas. Stage of flower bud development immediately after treatment of 13°, 18.5, 24.0 and 29.5°C soil temperatures was more advanced, and mean flower bud number increased with each decrease of 5.5°C in soil temperature. At age 9 months, mean days to flower was lower and mean number of flowers per plant was higher with each decrease of 5.5°C in soil temperature. Plants treated at the 6-leaf stage flowered earlier and produced more flowers than those treated at the 4-leaf stage. The 9-month-old plants which had been treated at the 6-leaf stage were generally more advanced vegetatively than those treated at the 4-leaf stage, but the effect of soil temperature treatment on vegetative growth was negligible.

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Winter survival in woody plants is controlled by environmental and genetic factors that affect the plant's ability to cold-acclimate. A juvenile period in woody perennials raises the possibility of differences in cold-acclimating ability between juvenile vs. mature (flowering) phases. This study investigated the yearly cold hardiness (CH) changes of rhododendron populations and examined the relationship between leaf freezing tolerance (LFT) and physiological aging. Naturally acclimated leaves (January) from individual plants (parents-R. catawbiense and R. fortunei, F1, F2, and backcross) and F1 population generated from R. catawbiense and R. dichroanthum cross were subjected to controlled freeze-thaw regimes. LFT was assessed by measuring freeze-thaw-induced ion leakage from leaf discs frozen over a range of treatment temperatures. Data were then plotted with a sigmoidal (Gompertz) curve by SAS, to estimate Tmax—the temperature causing maximum rate of injury. Tmax for the 30- to 40-year-old parental plants (catawbiense, fortunei, and dichroanthum) and the F1 `Ceylon' (catawbiense × fortunei) were estimated to be about -52, -32, -16, and -43 °C, respectively. These values were consistent over the 3-year evaluation period. Data indicated the F2 (50 seedlings) and backcross (20 seedlings) populations exhibited significant, yearly Tmax increment (of ≈5-6 °C) from 1996 to 1998 as they aged from 3 to 5 years old. A similar yearly increase was observed in the 12 F1 progenies (compared 2 to 3 years old) of catawbiense × dichroanthum cross. The feasibility of identifying hardy phenotypes at juvenile period and research implications of age-dependent changes in CH will be discussed.

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Few genetic studies have been conducted on the inheritance of cold hardiness (CH) in woody plants. An understanding of the genetic control of CH can greatly assist the breeder in reducing winter injury. This study was initiated to evaluate the distribution of CH phenotypes in segregating populations of evergreen rhododendrons. Naturally acclimated leaves from individual plants (parents, F1 and 47 F2 progeny) were subjected to controlled freeze–thaw regimes. Using slow cooling rates, leaf discs were cooled over a range of treatment temperatures from –10°C to –52°C. Freezing injury of leaf tissue was assessed by measuring ion-leakage and non-linear regression analysis (data fitted to Gompertz functions) was used to estimate Tmax, the temperature causing the maximum rate of injury. Tmax for the parent plants (R. catawbiense & R. fortunei) and the F1 cultivar Ceylon, were estimated to be –51.6°C, –30.1°C, and –40.4°C, respectively. CH estimates among F2 progeny (Ceylon, selfed) were normally distributed from –14.8°C to –41.5°C, with mean of –27.6°C. Most F2 progeny were less cold-hardy than the tender parent, R. fortunei. The apparent reduction in F2 CH may be caused by the differences in age between the parents (20-year-old mature plants) and F2 progenies (3-year-old juvenile seedlings). Currently, we are testing age-dependent CH responses in rhododendrons, and are also characterizing CH distributions in a backcross population.

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Use of wild species for in vitro sweetpotato improvement has been limited, in part, by the lack of suitable regeneration systems for these species. Shoot regeneration in 4 closely related species, I. batatas, I. cordatotriloba, I. trifida and I. triloba, were evaluated. Callus was initiated using methods described by Otani and Shimada (1988). Calli were transferred to regeneration media containing 17.75 uM BAP and 0, 1, 10 and 100 uM PCIB. Organogenesis was enhanced by the presence of PCIB. With I. cordatotriloba calli grown on media with 10 uM PCIB, a 2-fold increase in the percentage of calli exhibiting shoot regeneration was observed as compared to calli grown on media with BAP alone. A significant increase in the average number of shoots per callus was also observed. The other species examined appeared to be less sensitive than I. cordatotriloba to the PCIB treatments.

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