Emission characteristics of bulk aerosols excited by externally focused femtosecond filaments.

The bulk aerosol emissions excited by externally focused femtosecond laser filaments are characterized using time-resolved plasma imaging and spectroscopy. Images of N2 and N2+ plasma fluorescence are used to characterize the filament dimensions. Emission profiles from bulk Sr aerosols are studied, showing that several localized emission regions in the filament begin to develop for lower repetition rates and higher pulse energies. Plasma temperature and electron density profiles are determined using particle emissions along the length of short- and long-focused filaments, and results are compared for on-axis and side-collected spectra. The use of on-axis collection enables the sampling of light emitted over the entire length of the filament; however, the necessary back-propagation of light makes on-axis collection susceptible to self-absorption as the optical path is extended through the filament plasma column formed in bulk aerosols.

Putting pharmacogenetics into practice.

Genetics is slowly explaining variations in drug response, but applying this knowledge depends on implementation of a host of policies that provide long-term support to the field, from translational research and regulation to professional education.

Crosstalk between auxin, cytokinins, and sugars in the plant cell cycle.

Plant meristems are utilization sinks, in which cell division activity governs sink strength. However, the molecular mechanisms by which cell division activity and sink strength are adjusted to a plant's developmental program in its environmental setting are not well understood. Mitogenic hormonal as well as metabolic signals drive and modulate the cell cycle, but a coherent idea of how this is accomplished, is still missing. Auxin and cytokinins are known as endogenous mitogens whose concentrations and timing, however, can be externally affected. Although the sites and mechanisms of signal interaction in cell cycle control have not yet been unravelled, crosstalk of sugar and phytohormone signals could be localized to several biochemical levels. At the expression level of cell cycle control genes, like cyclins, Cdks, and others, synergistic but also antagonistic interactions could be demonstrated. Another level of crosstalk is that of signal generation or modulation. Cytokinins affect the activity of extracellular invertases and hexose-uptake carriers and thus impinge on an intracellular sugar signal. With tobacco BY-2 cells, a coordinated control of cell cycle activity at both regulatory levels could be shown. Comparison of the results obtained with the root cell-representing BY-2 cells with literature data from shoot tissues or green cell cultures of Arabidopsis and Chenopodium suggests opposed and tissue-specific regulatory patterns of mitogenic signals and signal crosstalk in root and shoot meristems.

Endogenous cytokinin oscillations control cell cycle progression of tobacco BY-2 cells.

The significance of cytokinins for the progression of the cell cycle is well known. Cytokinins contribute to the control of the expression of D-cyclins and other cell cycle genes, but knowledge as to how they affect the progression of the cell cycle is still limited. Highly synchronized tobacco BY-2 cells with clearly defined cell cycle stages were employed to determine cytokinin patterns in detail throughout the entire cycle. Concentrations of trans-zeatin, and of some other cytokinins, oscillated during the course of the cell cycle, increasing substantially at all four phase transitions and decreasing again to a minimum value during the course of each subsequent phase. Addition of exogenous cytokinins or inhibition of cytokinin biosynthesis promoted the progression of the cell cycle when the effects of these manipulations intensified the endogenous fluctuations, whereas the progression of the cycle was retarded when the amplitude of the fluctuations was decreased. The results show that the attainment of low concentrations of cytokinins is as important as the transient increases in concentration for a controlled progression from one phase of the cell cycle to the next. Cytokinin oxidase/dehydrogenase activity also showed fluctuations during the course of the cell cycle, the timing of which could at least partly explain oscillations of cytokinin levels. The activities of the enzyme were sufficient to account for the rates of cytokinin disappearance observed subsequent to a phase transition.

Adaptive femtosecond laser-induced breakdown spectroscopy of uranium.

Laser-induced breakdown spectroscopy (LIBS) is an established technique for material characterization applicable to a variety of problems in research, industry, environmental studies, and security. LIBS conducted with femtosecond laser pulses exhibits unique properties, arising from the characteristics of laser-matter interactions in this pulse width regime. The time evolution of the electric field of the pulse determines its interaction with sample materials. We present the design and performance of a femtosecond LIBS system developed to systematically optimize the technique for detection of uranium. Sample analysis can be performed in vacuum environment, and the spectral and temporal diagnostics are coupled through an adaptive feedback loop, which facilitates optimization of the signal-to-noise ratio by pulse shaping. Initial experimental results of LIBS on natural uranium are presented.