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Mark H. Brand, Yiqin Ruan, and Richard Kiyomoto

To characterize the in vitro behavior of Rhododendron `Montego' with tissue proliferation (TP) to cytokinin and auxin, comparisons were made of normal [TP(–)], dwarf TP [TP(+) dwarf], and long TP [TP(+) long] shoot cultures. On basal medium TP(–) and TP(+), long shoots failed to multiply and had a low relative growth rate (RGR) of 0.1, whereas TP(+) dwarf shoots produced 31.8 shoots per tip, with most shoots being <5 mm long, and RGR was 0.3. Addition of 15 μm 2iP to basal medium induced the production of more than six shoots per TP(–) tip and doubled their RGR; TP(+) long shoots produced 16.8 shoots, most <5 mm long, and had an RGR of 0.3; TP(+) dwarf shoots produced only 16% as many shoots as on basal medium, but still exhibited an increase in RGR. Leaves from TP(–) and TP(+) sources failed to produce shoots on basal medium, but 74% of TP(–) leaves formed shoots when cultured on 1 μm IBA and 30 μm 2iP. TP(+) leaves were able to form shoot meristems on media containing only 5 μm 2iP (26% of explants), but these meristems failed to elongate into shoots. Calli from TP(–) leaves, TP(+) leaves, and TP(+) tumors grown on medium containing 10 μm NAA and 15 μm 2iP had higher RGRs than the same calli on basal medium during the first 8 weeks of culture. Over time, RGR decreased in both TP(–) and TP(+) leaf calli, but increased in TP(+) tumor callus. The increased RGR resulted from differentiation of shoot meristems on 85% of the calli between week 4 and week 8. Our results suggest that TP(+) tissues have altered hormone metabolism or sensitivity that leads to dramatic differences in in vitro behavior and probably contributes to tissue proliferation observed in whole plants. Chemical names used: 6-(γ,γ-dimethylallylamino) purine (2iP); indole-3-butyric acid (IBA); α-naphthaleneacetic acid (NAA).

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Ann C. Smigocki and Iris J. Honeczy

Lycopersicon esculentum cv. UC82b cotyledons were co-cultivated with A. tumefaciens carrying vectors with modified isopentenyl transferase (ipt) genes. The ipt gene was placed under the control of the RUBISCO promoter in both the sense and antisense orientation. Over 50 transformants were recovered on kanamycin-containing media. Seeds from RO plants were germinated on selective media and R1 plants transformed with the ipt gene identified by PCR and Southern blot hybridization. Phenotypes of the R1 plants, whether transformed with the ipt gene in the sense or antisense orientation, were comparable to the control plants transformed with an inactive cytokinin gene. Fruit weights from both were similar to those from control plants, however, yields were reduced and ripening delayed. Most fruit had no seeds or very few small seeds. Cytokinin levels are being determined in order to correlate them to the observed phenotypes.

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Zhaolong Wang, John Pote, and Bingru Huang

This study was designed to examine whether shoot injury induced by high root-zone temperature is associated with changes in shoot detoxifying metabolism and to determine the level and duration of high root-zone temperatures that would induce physiological changes in two cultivars of creeping bentgrass (Agrostis stolonifera var. palustris Huds) differing in heat tolerance. Plants of `Penn A-4' (heat tolerant) and `Putter' (heat susceptible) were grown in sand and exposed to root-zone temperatures of 20 (control), 21, 22, 23, 25, 27, 31, and 35 °C in water baths while air temperature was maintained at 20 °C in a growth chamber. Turf quality, leaf cytokinin content, and antioxidant enzyme activities declined at increased soil temperatures and the duration of treatment for both cultivars. A decline in turf quality occurred following 40 days of exposure to 35 °C for `Penn A-4' and 26 days of exposure to 31 °C for `Putter'. The root-zone temperature causing the decline of isopentenyl adenosine and zeatin cytokinins was 25 °C at 37 d for `Putter' and 27 °C at 47 days for `Penn A-4'. The temperature causing the decline of superoxide dismutase and catalase activities was 25 °C and 27 °C at 33 days for `Putter' and 27 °C and 31 °C at 43 days for Penn A-4, respectively. Malondialdehyde content increased at 27 °C for `Putter' and 31 °C for `Penn A-4' at 43 days of treatment. The decline in cytokinin content and antioxidant enzyme activity occurred at a lower soil temperature and earlier during the treatment than the decline in turf quality, possibly contributing to turf quality decline. The root-zone temperatures causing the decline in turf quality, cytokinin content, and oxidative damage were higher in the heat-tolerant cultivar than heat-susceptible cultivar.

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Steven McArtney, Duane Greene, Terence Robinson, and James Wargo

. Application of gibberellins together with the synthetic cytokinin N-(2-chloro-4-pyridyl)-N-phenylurea (CPPU) had a positive synergistic effect on set of parthenocarpic fruit and fruit size in apple ( Bangerth and Schröeder, 1994 ; Watanabe et al., 2008

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F. A. Hammerschlag and A. C. Smigocki

Transgenic plants containing introduced phytohorm one genes have been shown to display altered growth and morphogenetic potential. Peach plants transformed with the ipt gene from Agrobacterium tumefaciens strain tms 328::T n5 and containing elevated levels of cytokinins were screened in vitro for compact growth habit on four different levels of 6-benzyladenine (BA). After nine weeks in vitro, the average number of axillary shoots per plant foe two of the transformants, 99-1 and 40-1, ranged from 1.5 to 6.6 times that for the controls on 0-30 uM of BA, whereas average fresh weight ranged from 1.1 to 3.6 times that for the controls. One of the transformants, 94-1, produced a greater number of axillary shoots only on 30 μM BA. Rooted plants derived through micropropagation from the original transformants were monitored for 30 months under greenhouse conditions. The average height of transformants 94-1 and 99-1 after six months in the greenhouee was 88 and 77% of controls, respectively and after 30 months was 90 and 75% of controls, respectively. In comparison to controls, transformants exhibited a greater number of branches per meter per plant after six weeks, but a lesser number after 30 months. These results suggest that the introduction of a cytokinin gene may be a useful approach to obtaining peach trees with a compact growth habit.

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Charleson R Poovaiah, Stephen C Weller, and Matthew A Jenks

An in vitro shoot regeneration procedure was developed for native spearmint (Mentha spicata L.) using internodal explants. Shoot regeneration from internodes was evaluated on Murashige and Skoog (MS) media supplemented with individual cytokinins thidiazuron (TDZ), benzylaminopurine (BA), kinetin (KT), or zeatin (ZT) or various pair wise combinations of these. The highest regeneration was achieved by the second internode on a medium containing MS basal salts, B5 vitamins, 10% coconut water, 1.0 mg·L–1 TDZ, 2.5 mg·L–1 ZT, and solidified with 0.2% phytagel. Unlike previous protocols this medium does not need sub culturing and produces elongated shoots in 4 weeks, rather than 6 weeks. Maximum number of shoots (36 per explant after 4 weeks) was observed when internodes from 2-week-old stock plants were used as explant source. The shoots were removed and roots were initiated on medium containing MS basal salts, 0.4 mg·L–1 thiamine-HCL, 100 mg·L–1 myo-inositol, 7.5 g·L–1 agar and 0.01 mg·L–1 ∝-napthaleneacetic acid (NAA) and then plants were transferred to the greenhouse 2 weeks after root initiation, where 100% of the plantlets developed into healthy plants.

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Akiko Ito, Hideaki Yaegaki, Hiroko Hayama, Shinnosuke Kusaba, Isomaro Yamaguchi, and Hirohito Yoshioka

Flower development of the lateral buds was accelerated in Japanese pear [Pyrus pyrifolia (Burm.) Nak.] when vertical shoots were bent at a 45° angle in late June. The indole-3-acetic acid (IAA) concentration in lateral buds on vertical (control) shoots increased in mid-July, while remaining nearly constant in bent shoots. The abscisic acid (ABA) concentration of buds in bent shoots rose between 4 July and 15 Aug., whereas control shoots exhibited an increase in concentration followed by a decline. Gibberellin4+7 (GA4+7) concentration was high on 16 June, and then declined by 4 July, with the decline being greatest in bent shoots. Gibberellin4+7 concentration was higher in the buds on vertical shoots than in those on bent shoots for much of July. The concentrations of zeatin-type cytokinins (CKs) in lateral buds were higher in bent shoots than in vertical shoots. Bending of pear shoots may weaken competition between buds and other organs through altering hormone levels in lateral buds, resulting in acceleration of flower development.

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C. Olivella, C. Biel, R. Savé, and M. Vendrell

The effects of flooding (3 days) and recovery (3 days) on leaf gas exchange parameters (stomatal conductance, net photosynthetic assimilation rate, and leaf transpiration rate) were studied in five, 1-year-old gerbera (Gerbera jamesonii Bolus) cultivars under greenhouse conditions. Flooding reduced the measured parameters in all cultivars, which had not recovered 3 days after flooding ceased. A more detailed study was conducted with the `Beauty' cultivar, in which leaf water potential (Ψw); root hydraulic resistance (Rh); and hormonal concentrations of ABA, IAA, and CK were measured in leaves and roots. Plants were flooded for 2 days, then allowed to recover for 4 days. After 2 days of flooding, Ψw had decreased and Rh had increased. Leaf ABA content increased and IAA and CK content decreased from the start of flooding and did not change during recovery. In roots a transient increase in root ABA levels occurred during flooding and a sharp decrease was measured during recovery, which was related to root death. Similar patterns were observed in root CK concentrations. The IAA concentration in roots remained constant throughout the experiment. The results suggest that ABA and CK may act as signals of flooding stress. Also, the marked intolerance of gerbera to flooding could be a serious barrier to its culture under anaerobic conditions, and hence careful irrigation management is required. Chemical names used: abscisic acid (ABA); indoleacetic acid (IAA); cytokinin (CK).

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R. Savé, J. Peñuelas, I. Filella, and C. Olivella

One-year-old gerbera plants subjected to 1 night at 5C had reduced leaf water losses and chlorophyll content and increased root hydraulic resistance, but stomatal conductance and leaf water potential did not change. After 3 nights, leaf water potential had decreased and leaf reflectance in the visible and the near-infrared had increased. Similarly, abscisic acid (ABA) in leaves had increased and cytokinins (CK) in leaves and roots had decreased, but ABA levels in roots did not change. After 4 days at 20C, root hydraulic resistance, reflectance and leaf water loss returned to their initial values, but leaf water potential and chlorophyll content remained lower. Leaf ABA levels reached values lower than the initial, while root ABA and leaf CK levels retained the initial values. These data suggest that in the gerbera plants studied, 3 nights at 5C produced a reversible strain but otherwise plants remained uninjured, so this gerbera variety could be cultured with low energetic inputs under Mediterranean conditions. The results may indicate that ABA and CK were acting as synergistic signals of the chilling stress. Spectral reflectance signals seemed to be useful as plant chilling injury indicators at ground level.

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James M. Garner, Gary J. Keever, D. Joseph Eakes, and J. Raymond Kessler

A foliar spray of 0, 1250, 2500, or 3750 mg benzyladenine (BA)/L was applied to 10 Hosta Tratt. (Funkia K. Spreng; Niobe Salisb.) cultivars. Response to BA treatment was cultivar dependent, with BA promoting offset formation in half of the cultivars. Compared to the control, increase in offsets produced by cultivars treated with 3750 mg BA/L ranged from 116% in `Francee' to 3500% in `Francis Williams' at 30 days after treatment (DAT) and from 150% in `Royal Standard' to 2250% in `Francis Williams' at 60 DAT. Offset stage of development, as indicated by the number of unfurled leaves, was also cultivar- and BA-dependent. All cultivars treated with 3750 mg BA/L had an average of three or more unfurled leaves at 60 DAT, while among control plants, 40% of cultivars averaged fewer than three unfurled leaves. No phytotoxic symptoms were noted in any cultivar, and plant size was either increased or not affected by BA treatment. Chemical name used: N-(phenylmethyl)-1H-purin-6-amine (benzyladenine; BA).