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The objective of this paper is to investigate geometric and material parameters that affect the interaction of multiple pre-existing (initiated) cracks due to rolling contact fatigue (RCF) loading conditions. In particular, the behaviour of short cracks (head checks) in railway rail is studied. Parameters of interest are initial crack angle and spacing, distribution of initial crack lengths, width of the load/contact zone, and the material properties (in particular, the friction coefficient). Furthermore, the sensitivity of the finite element (FE)-mesh density is investigated. An open question of particular interest is the effect of crack interaction, e.g. shielding, on the prevailing crack spacing. In view of all uncertainties of the material model as well as the three-dimensional geometric complexity of the RCF problem, it is important to obtain a good understanding of the sensitivity of parameter changes. This is achieved in the presented investigation, although it is carried out using simplifying assumptions such as plane strain, linear elasticity, and Hertzian pressure distribution. [ABSTRACT FROM AUTHOR]

Synchronous cultures of Scenedesmus obtusiusculus Chod. were starved for phosphorus for 48 h. Such cells develop an efficient mechanism for phosphate binding which is very sensitive to metabolic inhibitions. Phosphate binding, fluctuations in the ATP pool during dark-light-dark transitions, and steady state levels of ATP, ADP, and AMP were studied. The experiments were carried out in a CO2-free N2 atmosphere. DCMU, phloridzin and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) were used as inhibitors of photophysphorylation. Light-induced phosphate uptake was inhibited to various extents by all the inhibitions. The dark-light-dark transition experiments show that neither the light-induced increment in ATP nor the decrease at darkening are affected by DCMU, but DBMIB and phloridzin inhibited both processes. DCMU seems to affect the regulation of the ATP pool size. The steady state levels of the adenylate pools were almost the same in the light as in the dark, and they were also little sensitive to the inhibitors. In unpoisoned cells in the light the steady state ATP/ADP ratio was 1.7 and the energy charge was 0.66. The rates of phosphate binding are not correlated to any of the adenylate parameters studied. This probably due to the diverse effects of the inhibitors on light-stimulated production of reducing equivalents, photophosphorylation and transfer of energy from the chloroplast to the cytoplasm. [ABSTRACT FROM AUTHOR]

The dependence of in vivo photophosphorylation on light intensity was studied in the unicellular green alga Scenedesmus obtusiusculus. By selective use of the inhibitor DCMU, phosphorylation in (I) the complete system, (II) the pseudocyclic system alone, and (III) the true cyclic system alone, were followed. When the total binding of phosphate was studied, all reaction types became light saturated in about the same manner. The effect of DCMU on the level of ATP varied according to light intensity. As for the specific systems of photophosphorylation, the following ATP data were found: (I) In the complete system the level of ATP decreases with light intensity. (II) Under pseudocyclic conditions light first increases and then decreases the ATP level. Under the atmospheric conditions used (i.e. CO2-free nitrogen) this indicates a regulation between photophosphorylation and glycolysis, for which possible explanations are discussed. (III) In the true cyclic conditions light has little effect on the ATP level. The possibility is indicated that there is a structural difference between the non-cyclic (site 1) and the pseudocyclic (site 2) sites of photophosphorylation on the one hand and the true cyclic site (3) on the other. [ABSTRACT FROM AUTHOR]

The effects of phosphorus starvation on morphology and intracellular structure and on reactions related to the energy metabolism of the unicellular green alga Scenedesmus obtusiusculus (Chod.) were studied over a period of 96 h by employing transmission electron microscopy and various methods for measurement of physiological reactions. Increase in cell size and shape and in cell wall thickness arc dominating features of phosphorus starvation. There is also an increase in number and size of starch granules and lipid globules and the internal structure of the cells appears successively disorganized. Shortage of phosphorus in the medium initially induces an increase of the adenylate pool whereas the energy charge value remains the same as for the controls. The photosynthetic and respiratory activities are high during incipient phosphorus starvation. Alter 24 h, as shortage of phosphorus becomes critical, the internal phosphorus reaches a low steady-state value, and this is also true for the adenylate energy charge. The total content of adenylates, however, peaks after 24 h of starvation and then decreases with increasing length of phosphorus starvation. Light-induced oxygen evolution appears not to be as much inhibited by a low phosphorus content in the cells as by the concomitant starch accumulation. The data indicate that the strategy for survival of the cells in a phosphorus-poor environment includes morphological and physiological changes that facilitate the transfer and adaption of the cells to environments with a more favourable supply of phosphorus. such as the often oxygen-poor hut phosphorus-rich bottom zones. [ABSTRACT FROM AUTHOR]

Hydroponically cultivated barley plants were exposed to nitrogen (N)-deficiency followed by N-resupply. Metabolic and genetic regulation of fructan accumulation in the leaves were investigated. Fructan accumulated in barley leaves under N-deficiency was mobilized during N-resupply. The enhanced total activity of fructan-synthesizing enzymes, sucrose:sucrose 1-fructosyltransferase (EC 2.4.1.99) and sucrose:fructan 6-fructosyltransferase (6-SFT; EC 2.4. 1.10) caused by N-deficiency decreased with the mobilization of fructan during N-resupply. The activity of the barley fructan-degrading enzyme, fructan exohydrolyase (EC 3.2.1.80) was less affected by the N status. The low level of foliar soluble acid invertase activity under N-deficiency conditions was maintained during the commencement of N-resupply but increased subsequently. Further analyses by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, western blot and northern blot demonstrated that the fructan accumulation and the total activity of fructan-synthesizing enzymes correlated with the 6-SFT mRNA level. We suggest that the changes in fructan levels under N stress are intimately connected with the regulation of fructan synthetic rate which is mostly controlled by 6-SFT.

9-Aminoacridine has been used to monitor the intrathylakoid pH of photosynthetically competent intact chloroplasts. Values obtained from 9-aminoacridine accumulation in the chloroplasts must be corrected for light-dependent binding of 9-aminoacridine to the thylakoid membranes. During nitrite reduction by intact chloroplasts, the intrathylakoid proton concentration increased. It decreased somewhat during CO2 reduction. However, low concentrations of uncoupling amines such as NH3 or cyclohexylamine, which rapidly penetrated the chloroplast envelope and decreased the intrathylakoid proton concentration, failed to reduce, and actually stimulated, rates of CO2-dependent oxygen evolution even under rate-limiting light. In contrast, low concentrations of carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) OR NIGERICIN, WHICH INHIBITED CO2 reduction, even appeared to increase the intrathylakoid proton concentration. As indicated by measurements of the 515 nm signal of the chloroplasts, the light-induced membrane potential was not much affected by low concentrations of the uncoupling amines, but was decreased by FCCP and by high concentrations of the amines. Even in the presence of high concentrations of NH4C1, ATP/ADP ratios of illuminated chloroplasts remained far above the ratios observed in the dark. In contrast, low concentrations of FCCP were sufficient to reduce ATP/ADP ratios to the dark value even under high intensity illumination. The observations are difficult to explain within the framework of the chemiosmotic hypothesis as presently discussed.