Activated charcoal catalyses sucrose hydrolysis during autoclaving

Activated charcoal frequently promotes the in vitro growth and organogenesis of plant tissues by the absorption of compounds from the culture medium and/or from the container atmosphere.Our experiment demonstrates that sucrose hydrolysis, which normally reaches 10% during autoclaving, increases to 95% in presence of 1% of activated charcoal. This gives rise to the acidification of the solution due to a specific reaction of the formed fructose.Changing the available carbon source from the initial sucrose to a mixture of fructose, glucose and sucrose, causes a osmolarity increase, a drop in the agar gelling and the formation of furfural derivatives which are indirect consequences of the sucrose hydrolysis affecting the tissue culture media.     

Losses and biogeochemical cycling of soil organic nitrogen with prolonged arable cropping in the South African Highveld — evidence from d- and l-amino acids

We know little about the mechanisms that cause rapid losses in the soil organic N pool during cropping. As the analysis of amino acid enantiomers can provide insight into both the fate of microbial N and the ageing of cells in the environment, we used this technique as a tool to examine how the pool of protein-bound N in subtropical Plinthosols responds to increasing duration of arable cropping. The samples comprised bulk soils (0–20?cm) and clay fractions from each of three agro-ecosystems in semiarid South Africa; the sites have been cropped for periods varying from 0 to 98 years. The amino acid enantiomers contributed 34% to the total N content. With increasing number of years a piece of land has been cropped, the amino acid concentrations declined bi-exponentially to about 30% of their initial level in the native grasslands. Changes of the remaining soil protein-N pool were indicated by alterations in the -content of individual amino acids. As the years of arable cropping increased, the proportions of -alanine and -glutamic acid increased relative to the respective -enantiomers. This was attributed to an accumulation of N in residues of bacterial cell walls. In contrast, the /-ratios of leucine and aspartic acid declined in the long-term cultivated plots, probably reflecting losses of old amino acid-N reserves at the most degraded arable land.     

Fluid balance versus blood flow autoregulation in the elevated human limb: the role of venous collapse

This study evaluated the postural vascular adjustment in the human forearm which may be responsible for the recent observation that transcapillary fluid balance is maintained above the level of the heart while blood flow decreases in a linear fashion. In this study further evidence was provided that a posturally graded profile of collapsed veins holds for both an overall increase of resistance with height and compensation for hydrostatic effects on capillary pressure. This was achieved by manipulating peripheral venous profile/volume: a proximal outlet resistance (upper arm cuff) was used for re-opening of collapsed distal veins. In test (a), 12 healthy subjects underwent recordings of fluid reabsorption rate and blood flow in a 20-cm segment of their forearm horizontally placed at 36 cm above heart level (third intercostal space). Applying upper arm cuff pressures randomly between 0 and 25 mmHg (0–3.33 kPa) for 15 min led to maxima of blood flow and reabsorption rates at inflations of 5 or 10 mmHg (0.67 or 1.33 kPa). This was attributed to minima in postcapillary resistance facilitating flow and reducing capillary pressure. In test (b) the flow-maximizing outlet resistance found was studied for its effect in different forearm positions (–18, 0, 18, 36, 54 cm relative to heart level). Blood flow then showed a shift of its maximum from heart level to 36 cm above heart level, while the reabsorption rate increased above 18-cm height - in contrast to previous findings with a free circulation. It was therefore concluded that the venous profile in the forearm adjusts postcapillary resistance in such a way that local dehydration is confined at the cost of blood supply. Thicker and less collapsable veins may ensure better flow autoregulation during impaired fluid balance — as seen in the legs.