Space-time resolved simulation of femtosecond nonlinear light-matter interactions using a holistic quantum atomic model: application to near-threshold harmonics.

We introduce a new computational approach for femtosecond pulse propagation in the transparency region of gases that permits full resolution in three space dimensions plus time while fully incorporating quantum coherent effects such as high-harmonic generation and strong-field ionization in a holistic fashion. This is achieved by utilizing a one-dimensional model atom with a delta-function potential which allows for a closed-form solution for the nonlinear optical response due to ground-state to continuum transitions. It side-steps evaluation of the wave function, and offers more than one hundred-fold reduction in computation time in comparison to direct solution of the atomic Schrödinger equation. To illustrate the capability of our new computational approach, we apply it to the example of near-threshold harmonic generation in Xenon, and we also present a qualitative comparison between our model and results from an in-house experiment on extreme ultraviolet generation in a femtosecond enhancement cavity.

Intracavity ionization and pulse formation in femtosecond enhancement cavities.

We experimentally and numerically investigate the intracavity ionization of a dilute gas target by an ultrashort pulse inside a femtosecond enhancement cavity. Numerical simulations detail how the dynamic ionization of the gas target limits the achievable peak intensity of the evolving intracavity pulse beyond that of linear cavity losses, setting a constraint on the strength of the nonlinear interaction that can be sustained in such optical cavities. Experimental measurements combined with numerical simulations predict ionization levels in a femtosecond enhancement cavity for the first time. We demonstrate how the resonant response of the femtosecond enhancement cavity can itself be used as a sensitive probe of optical nonlinearities at high intensities.

Prospects for a millihertz-linewidth laser.

We propose a new light source based on having alkaline-earth atoms in an optical lattice collectively emit photons on an ultranarrow clock transition into the mode of a high Q resonator. The resultant optical radiation has an extremely narrow linewidth in the mHz range, even smaller than that of the clock transition itself due to collective effects. A power level of order 10;{-12} W is possible, sufficient for phase locking a slave optical local oscillator. Realizing this light source has the potential to improve the stability of the best clocks by 2 orders of magnitude.

Computer-assisted self-control of diabetes by adolescents.

This paper reports an experimental investigation of the microcomputer-based system, Diabetes in Self-Control (DISC), used to facilitate adolescents' self-management of insulin-dependent diabetes mellitus. Twenty 12- to 19-year-old subjects were matched into 10 pairs and randomly assigned to DISC or conventional education (CE). Following a no-treatment phase, information about diabetes and self-monitoring was presented during 7 weeks of phase 2 by means of computer-assisted instruction for the DISC group and printed materials for the CE group. During the 8 weeks of phase 3, DISC subjects received training on monitoring the relationships among variables, diabetes problem solving, and goal setting. Improvement that was both statistically and clinically significant was noted in the DISC group's prelunch and dinner glucose levels. The DISC group showed clinical improvement in frequency of blood glucose testing. Finally, DISC subjects reported more behavioral change as a function of what was learned.

The Physiological Basis for Cytokinin Induced Increases in Pod Set in IX93-100 Soybeans.

Previous investigations have shown the feasibility of increasing pod number on legumes by the application of 6-benzylaminopurine (BA) directly to the raceme. These investigations were designed to determine what reproductive parameter was affected by cytokinin application, and if these applications were overcoming a deficiency in root-produced cytokinins during late flowering. Five individual main stem racemes on greenhouse grown soybeans (Glycine max L. Merr.) were treated with 2 millimolar BA. A single application of BA when pods appeared at 25 to 50% of the proximal floral positions resulted in a 58% increase in pod set due primarily to a 33% reduction in floral abscission. Applications of BA at later intervals also resulted in significant reductions in total abscission. When three applications of BA were imposed on the upper five nodes of field grown soybeans, total pod number and seed weight were significantly increased in this section of the canopy by 27 and 18%, respectively. Throughout the flowering period, root pressure exudate was sampled for the subsequent separation and quantification of zeatin, dihydrozeatin, zeatin riboside, dihydrozeatin riboside, and isopentenyladenine. Total cytokinin flux peaked from 0 to 9 days after flowering began, and then dropped to one-half of this level by 15 days postanthesis. The probability that a flower would initiate a pod was directly related to the concentration of total cytokinins present in the exudate when the flower opened.

Soybean pod set enhancement with synthetic cytokinin analogs.

The previously reported activity of benzyladenine and selected other cytokinin analogs to increase pod set in soybean (Glycine max [L.] Merr.) was further investigated to define the structure-activity relationship and evaluate the effects of the cytokinins on yield parameters. Enhancement of pod set was found to be greatest with N-6 saturated alkyl substituted analogs, and was only weakly associated with activity in a callus growth bioassay. The response of yield parameters to increasing pod load was evaluated by applying various cytokinin analogs having a range of pod set enhancement activity. The increased pod load at the treated nodes was not compensated by a reduction in pod number on the remainder of the plant. However, there was a compensatory decrease in seed size. Overall, a significant trend to greater total seed weight per plant was associated with the increased pod number. Initial evaluations indicated that foliar applications of select cytokinins could temporarily increase pod number. However, the increases in pod number obtained with foliar treatments were too small to be of practical utility and were not maintained to maturity.

Alteration of C-assimilate partitioning in leaves of soybeans having increased reproductive loads at one node.

The objectives of this study were to determine if the partitioning of recently fixed carbon between starch and water-soluble compounds could be altered by increasing the pod load in the leaf axil, and if the presence of source leaves acropetal to such a node would influence the partitioning of carbon within the subtending leaf. Soybeans (Glycine max L. Merr. cv Hodgson 78) were grown to full-bloom in a controlled environment chamber, and then deflowered at all nodes except the eighth. This treatment resulted in an 83% increase in the number of pods at the eighth node. At 24 days after flowering, one-half of the treated plants were girdled above the untreated node. Forty-two hours later, the eighth trifoliolate was pulsed with (14)CO(2) and sampled for radiolabeled starch and water-soluble compounds (WSC) at 0.5, 2, 4, 8, 12, and 24th after labeling.When no girdling was applied above the increased pod load at the eighth node more label was accumulated by the pod walls (+6.9%) and seeds (+6.3%) when compared to the controls. Starch accumulation was not altered in the labeled leaf of the nongirdled plants. When the stem was girdled above the eighth node, significantly less starch was retained in the labeled leaf. Girdling also resulted in an increase in label accumulation by the pod walls (+5.4%) and seeds (+6.6%). These data suggest that the plant will change the distribution patterns of assimilate to supply added sink demand before altering the partitioning of recently fixed carbon in the subtending leaf.

Effect of shortened photosynthetic period on C-assimilate translocation and partitioning in reproductive soyeans.

Starch accumulation rate in leaves of vegetative soybeans is inversely related to the length of the daily photosynthetic period. However, it is not known whether a similar response would be observed during reproductive growth. Soybeans (Glycine max L. Merr. cv Amsoy 71) were grown to three stages of reproductive growth (beginning seed, mid seed-fill, and late seed-fill) under 12-hour daylengths, and then shifted to 6-hour photosynthetic periods (12-hour photoperiods) for 4 days. One and 4 days after treatment, a mid-canopy leaf was pulsed with (14)CO(2), and sampled for radiolabeled starch and water-soluble compounds at 0.5, 1, 3, 9, and 21 hours after labeling.Plants exposed to the 6-hour photosynthetic periods at the beginning seed stage retained and incorporated significantly more label as starch than did those given 12-hour photosynthetic periods. However, plants exposed to the shortened photosynthetic periods at the late seed-fill stage partitioned less label into starch. Plants exposed at mid seed-fill gave a variable response.Shortened photosynthetic periods resulted in preferential partitioning of recently fixed carbon to the seed at the expense of the pod wall. The results of these experiments suggest that the increased sink demand present during late reproductive growth may be of greater importance in control of leaf starch accumulation than is the length of the daily photosynthetic period.

Ontogenetic variation of four cytokinins in soybean root pressure exudate.

Cytokinins exported from the root may be involved in the correlative control of plant development. To test this hypothesis in soybean ((Glycine max [L.] Merr. cv. McCall, cv Chippewa 64, and cv Hodgson 78), cytokinins were intercepted en route from the root to the shoot by collecting root pressure exudate from detopped roots. The quantities of four cytokinins in the exudate were studied throughout the development of plants grown in the field and in controlled environment chambers. Zeatin, zeatin riboside, and their dihydro derivatives, dihydrozeatin and dihydrozeatin riboside, were isolated and quantitated using high-performance liquid chromatography.Cytokinin fluxes (pmoles per plant per hour) were independent of exudate flux (grams per plant per hour). All fluxes are averages for a 6- or 8-h collection period. The ribosides accounted for the majority of the observed cytokinin transport. The fluxes of zeatin riboside and dihydrozeatin riboside increased from low levels during vegetative growth to maxima during late flowering or early pod formation. Before the seeds began rapid dry matter accumulation, zeatin riboside and dihydrozeatin riboside fluxes decreased and remained at low levels through maturation. The fluxes of zeatin and dihydrozeatin were low throughout development.No correlation was found between cytokinin fluxes and nodule dry weight or specific nodule activity (acetylene reduction).The timing of distinct peaks in zeatin riboside and dihydrozeatin riboside fluxes during flowering or pod formation suggests that cytokinins exported from the root may function in the regulation of reproductive growth in soybean.