Synchronized somatic embryo development in embryogenic suspensions of grapevine Muscadinia rotundifolia (Michx.) Small

High-frequency, synchronous embryogenic systems in liquid culture facilitate plant regeneration and can be used as an essential model for performing functional genomics studies and understanding molecular aspect of the ontogenesis of higher plants. In the present study, synchronous somatic embryogenic cultures were developed for Muscadinia rotundifolia cv. Darlene and Vitis vinifera cv. Velika. High cell density and presence of 2,4-dichlorophenoxyacetic acid (2,4-D) proved to be essential for the establishment of the suspension cultures. Low cell density and continuous availability of auxin (2,4-D) was crucial for maintenance of suspension cultures. High cell density and withdrawal of 2,4-D is sufficient to advance somatic embryo development toward embryo differentiation and plantlets regeneration. Cells and cell clusters fractionation by density gradient centrifugation in Ficoll solution demonstrated to be a suitable method for separation of subpopulations with various potential for embryo development. The high frequency of synchronous development and differentiation of somatic embryos was attained essentially for the heaviest (at 16-18 % and >18 % Ficoll layer) cell population

Spin-driven Phase Transitions in ZnCr2_2Se4_4 and ZnCr2_2S4_4 Probed by High Resolution Synchrotron X-ray and Neutron Powder Diffraction

The crystal and magnetic structures of the spinel compounds ZnCr$_2$S$_4$ and ZnCr$_2$Se$_4$ were investigated by high resolution powder synchrotron and neutron diffraction. ZnCr$_2$Se$_4$ exhibits a first order phase transition at $T_N=21$ K into an incommensurate helical magnetic structure. Magnetic fluctuations above $T_N$ are coupled to the crystal lattice as manifested by negative thermal expansion. Both, the complex magnetic structure and the anomalous structural behavior can be related to magnetic frustration. Application of an external magnetic field shifts the ordering temperature and the regime of negative thermal expansion towards lower temperatures. Thereby, the spin ordering changes into a conical structure. ZnCr$_2$S$_4$ shows two magnetic transitions at $T_{N1}=15$ K and $T_{N2}=8$ K that are accompanied by structural phase transitions. The crystal structure transforms from the cubic spinel-type (space group $Fd$\={3}$m$) at high temperatures in the paramagnetic state, via a tetragonally distorted intermediate phase (space group $I4_1$ / $amd$) for $T_{N2} < T < T_{N1}$ into a low temperature orthorhombic phase (space group $I m m a$) for $T < T_{N2}$. The cooperative displacement of sulfur ions by exchange striction is the origin of these structural phase transitions. The low temperature structure of ZnCr$_2$S$_4$ is identical to the orthorhombic structure of magnetite below the Verwey transition. When applying a magnetic field of 5 T the system shows an induced negative thermal expansion in the intermediate magnetic phase as observed in ZnCr$_2$Se$_4$.Comment: 11 pages, 13 figures, to be published in PR

Morphological variability of cashews from the Brazilian Cerrado.

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Glass Transition in the Polaron Dynamics of CMR Manganites

Neutron scattering measurements on a bilayer manganite near optimal doping show that the short-range polarons correlations are completely dynamic at high T, but then freeze upon cooling to a temperature T* 310 K. This glass transition suggests that the paramagnetic/insulating state arises from an inherent orbital frustration that inhibits the formation of a long range orbital- and charge-ordered state. Upon further cooling into the ferromagnetic-metallic state (Tc=114 K), where the polarons melt, the diffuse scattering quickly develops into a propagating, transverse optic phonon.Comment: 4 pages, 4 figures. Physical Review Letters (in Press