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  • 1
    ISSN: 1573-5044
    Keywords: androgenesis ; chromosome doubling ; colchicine ; regeneration ; Triticum aestivum L
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract This study was conducted to determine the most suitable method of regeneration by comparing two approaches: transfer of anthers (with and without embryo-like structures) to regeneration conditions after a period of two to four weeks on induction medium (= anther-transfer treatment) and transfer of embryo-like structures to regeneration conditions after five to eight weeks on induction medium. The early transfer of anthers brought about a significant reduction in the number of embryos formed, but nevertheless significantly improved the frequency of plant regeneration. Combining an optimal date of anther transfer with the early addition of colchicine to the induction medium (100 mg l−1 for 1 and 3 days) led to an increase in the number of doubled haploid regenerants. The results indicate that transferring the anthers after 28 days and adding 100 mg l−1 colchicine to the induction medium on one day only caused a significant improvement in the ability of green plants to regenerate (7.0 compared to 0.50) as well as in chromosome doubling (success index: 4.0 compared to 0.33).
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  • 2
    ISSN: 1573-7357
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract At temperatures below the magnetic anisotropy energy, monodomain magnetic systems (small particles, nanomagnetic devices, etc.) must relax quantum mechanically-thermal activation is ineffective. The discrete nature of the spectrum is important. This quantum relaxation must be mediated by the coupling to both nuclear spins and phonons (and electrons if either particle or substrate is conducting). We analyze the effect of each of these couplings, and then combine them to get results for the physical relaxation of magnetic particles at low temperature and bias. This done for both conducting and insulating systems. The effect of electrons and phonons can be handled using “oscillator bath” representations; but the effect of environmental spins must be described using a “spin bath” representation of the environment, the theory of which was developed in previous papers. Conducting systems can be modelled by a “giant Kondo” Hamiltonian, with nuclear spins added in as well. At low temperatures, even microscopic particles on a conducting substrate will have their magnetisation frozen over millenia by a combination of electronic dissipation and the “degeneracyblocking” caused by nuclear spins. Raising the temperature leads to a sudden unblocking of the spin dynamics at a well defined temperature. We analyze in turn the 3 different cases of (a) conducting substrate, conducting particle (b) conducting substrate, insulating particle, and (c) conducting particle, insulating substrate. Insulating systems are quite different. The relaxation is strongly enhanced by the coupling to nuclear spins. At short times the magnetization of an ensemble of particles relaxes logarithmically in time, after an initial very fast decay-this relaxation proceeds entirely via the nuclear spins. At longer times phonons take over, but the decay rate is still governed by the temperature-dependent nuclear bias field acting on the particles-decay may be exponential or power-law depending on the temperature. Depending on the parameters of the particles and the environment, the crossover from nuclear spin-mediated to phonon-mediated relaxation can take place after a time ranging between fractions of a second up to months. The most surprising feature of the results is the pivotal role played by the nuclear spins. The results apply to any experiments on magnetic particles in which interparticle interactions are unimportant (we do not deal with the effect of interparticle interactions in this paper). They are also relevant to future magnetic device technology.
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  • 3
    ISSN: 1573-7357
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We consider the quantum dynamics of a magnetic domain wall at low temperatures, where dissipative couplings to magnons and electrons are very small. The wall motion is then determined by its coupling to phonons and nuclear spins, and any pinning potentials. In the absence of nuclear spins there is a dominant superOhmic l-phonon coupling to the wall velocity, plus a strongly T-dependent Ohmic coupling to pairs of phonons. There is also a T-independent Ohmic coupling between single phonons and the wall chirality, which suppresses “chirality tunneling”. We calculate the effect of these couplings on the T-dependent tunneling rate of a wall out of a pinning potential. Nuclear spins have a very strong and hitherto unsuspected influence on domain wall dynamics, coming from a hyperfine-mediated coupling to the domain wall position. For kBT »ω0 this coupling yields a spatially random potential, fluctuating at a rate governed by the nuclear T2. When kBT «ω0, the hyperfine potential fluctuates around a linear binding potential. The wall dynamics is influenced by the fluctuations of this potential, ie., by the nuclear spin dynamics. Wall tunneling can occur when fluctuations open an occasional “tunneling window”. This changes the crossover to tunneling and also causes a slow “wandering”, in time, of the energy levels associated with domain wall motion inside the pinning potential. This effect is fairly weak in Ni- and Fe-based magnets, and we give an approximate treatment of its effect on the tunneling dynamics, as well as a discussion of the relationship to recent domain wall tunneling experiments.
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  • 4
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    Electronic Resource
    Springer
    Journal of low temperature physics 113 (1998), S. 1147-1152 
    ISSN: 1573-7357
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Recent theory and experiment in crystals of molecular magnets suggest that fundamental tests of the decoherence mechanisms of macroscopic quantum phenomena may be feasible in these systems (which are also almost ideal quantum spin glasses). We review these, and suggest new experiments.
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Journal of low temperature physics 113 (1998), S. 1153-1158 
    ISSN: 1573-7357
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We calculate the magnetization relaxation in the short-time regime for an ensemble of nanornagnets in the presence of a low frequency external AC biasing field at temperatures lower than the magnetic anisotropy energy of the individual nanornagnets. It is found that the relaxation is strongly affected by AC fields with amplitude larger than that of the T 2 fluctuations in the nuclear field. This will allow experimental probing of the nuclear spin relaxation mechanism.
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Journal of low temperature physics 113 (1998), S. 1085-1090 
    ISSN: 1573-7357
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We investigate the motion of a domain wall in the presence of a dynamical hyperfine field. At temperature T high compared to the hyperfine coupling, the nuclear spins create a spatially random potential landscape, with dynamics dictated by the nuclear relaxation time T 2. The distribution of the domain wall relaxation times (both in the thermal and quantum regimes) can show a long tail, characteristic of stochastic processes where rare events are important. Here, these are due to occasional strong fluctuations in the nuclear spin polarisation.
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  • 7
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have searched for anomalous internal magnetic fields in highT c materials which are predicted to occur in anyon and flux phase models of superconductivity. The magnitude, anisotropy and temperature dependence of the observed fields inc-axis oriented samples of sintered YBa2CuO3O7 and of thick-film Bi2Sr2CaCu2O8 are consistent with a conventional nuclear dipolar origin. An upper limit of ≲0.08 mT is set for any anomalous magnetic fields along thec-axis atμ + sites in bulk CuO2 superconductors.
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  • 8
    ISSN: 1432-203X
    Keywords: Key wordsTriticum aestivum ; Anther culture ; Colchicine ; Chromosome-doubling ; Doubled haploids
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The aim of this study was to optimize the in vitro chromosome-doubling procedure in wheat anther culture. Colchicine, at concentrations of 100–5000 mg/l, was added to the induction medium for 1–5 days. Beneficial effects were obtained with concentrations of 100 and 1000 mg/l colchicine. With time, significant reductions in embryo–like structures as well as higher doubling indices were found. Similar results were obtained with the high- and low-responding genotypes. Colchicine (100 mg/l), added 5 and 20 days after inoculation for 1 and 3 days increased the induction response, but this value was reduced when colchicine was added 10 or 15 days after inoculation. The doubling effect was similar to the control, except for a significant increase with the 3-day application 20 days after inoculation. The highest success index was reached when colchicine was added to the culture medium after 20 days.
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  • 9
    ISSN: 1432-2242
    Keywords: Maize anther culture ; Chromosome doubling ; Cold treatment ; Colchicine ; Flow cytometer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Efficient methods of chromosome doubling are critical for the production of microspore-derived, doubled-haploid (=DH) plants, especially if, as in maize anther culture, spontaneous chromosome doubling occurs infrequently. In the present study, colchicine (5–1000 mg/l) was added to the induction medium and maize anthers were incubated in the colchicine-containing medium for different durations (1–7 days). In order to improve overall anther culture response, the culture temperature was adjusted to 14°C during the first 7 days. Colchicine applied at low concentration, i.e. 5 mg/l (7 days), or for short duration, i.e. 1–3 days (250 mg/l), showed beneficial effects on the formation of embryolike structures (=ES) and thus led to increased plant production, but was comparatively ineffective regarding chromosome doubling. Optimal doubling effects were observed when anthers had been exposed to culture medium containing 250 and 1000 mg/l of colchicine (7 days); in these treatments the doubling index (=DI), defined as the quotient of the number of DH plants and the number of totally regenerated plants in a specific treatment, rose to 0.56 and 0.53, respectively, compared to 0.20 in the untreated control. However, colchicine administered at concentrations higher than 250 mg/l seemed to be detrimental to general plant production; thus, in spite of a high DI, the overall DH plant production was even lower than in the control treatment. Maximum DH plant production for three different genotypes was accomplished with culture medium containing 250 mg/l of colchicine (7 days). With the best-responding genotype (ETH-M 36) a DH plant production of 9.9 DH plants/100 anthers was accomplished, i.e. a 7-fold increase compared to the non-treated anthers. This is the first report on efficient chromosome doubling in anther culture by subjecting anthers to colchicinecontaining induction medium during a post-plating cold treatment. Chromosome doubling as described here becomes an integral part of the maize anther culture protocol and thus represents a rapid and economical way to produce DH plants.
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  • 10
    ISSN: 1432-203X
    Keywords: Maize ; Anther culture ; Autoclaving ; Medium toxicity ; Activated charcoal
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Medium sterilization techniques (autoclaving, filter sterilization and separate sterilization of medium components), combined with preculture exposure to activated charcoal (AC) were evaluated for effects on maize anther culture response. The addition of AC to filter sterilized medium had no effect on the number of embryo-like-structures (ES) produced. For autoclaved medium, pre-culture AC treatment resulted in a 3-fold increase in ES yield over medium lacking AC. When AC was included, autoclaved medium was more productive than filter sterilized medium. Autoclaved media without AC gave lower response than filter sterilized medium. Separate sterilization of sucrose or FeEDTA was beneficial for media autoclaved in the absence of AC. However, when all components were autoclaved together in the presence of AC, there was no advantage to separate sterilization. The maximum ES frequency (224.6 ES/100 anthers) was obtained with the genotype ETH-M 52 cultured in autoclaved medium which had been exposed to AC (5 g/L) for 96 h prior to culture initiation. It is supposed that the higher ES frequencies observed with AC-treated, autoclaved media were due to the availability of glucose and fructose following heat-induced hydrolysis of sucrose and the AC-mediated adsorption of inhibitory compounds produced during autoclaving.
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