Germination dimorphism and developmental flexibility in the ruderal weed Heterotheca grandiflora

Summary Heterotheca grandiflora Nutt. (Asteraceae, tribe Astereae) is one of the few native Californian plant species increasing its range as a weed. The production of dimorphic seed, together with flexible development in either the annual or biennial habit, may contribute to its range expansion. Well dispersed disc achenes germinate rapidly to high percentages while poorly dispersed ray achenes show considerable dormancy, germinating at a much lower rate to lower final percentages. Ray achenes appear more sensitive to environmental factors and have more specific germination requirements than do disc achenes. Thus, germination is distributed in space and time. Plants growing as annuals have one flowering period while those acting as biennials may flower up to three times, although seed production differs greatly among the stages. These factors aid in forming the general purpose genotype so frequently encountered in weedy species.     

Membrane adhesion in reconstituted proteoliposomes containing the light-harvesting chlorophyll ab-protein complex: The role of charged surface groups

The light-harvesting chlorophyll $\text{a}\text{b}$-protein complexes (LHCP) of spinach, pea, and barley thylakoids apparently contain four nonidentical polypeptide subunits of between 29,000 and 23,000 daltons on highly resolving sodium dodecyl sulfate-polyacrylamide gradient gels. Trypsin treatment of the spinach complex degraded at least the two major subunits by approximately 2000 daltons and resulted in a three-subunit pattern on gels. Proteoliposomes reconstituted with LHCP and the chloroplast diacyl lipids aggregated markedly in the presence of cations but vesicles containing LHCP prepared from trypsin-treated thylakoids did not. Amino acid analysis of native- and trypsin-treated LHCP indicated that the fragment(s) released by trypsin, which is essential for cation-induced stacking of thylakoids, contains lysine and arginine, but not aspartate or glutamate, and is thus cationic. Carboxyl groups on the surface of LHCP were charge neutralized using a water-soluble carbodiimide (1-ethyl-(3-dimethylaminopropyl)carbodiimide) plus [¹⁴C]glycine ethyl ester. Only two or three sites were labeled per 26,000-dalton polypeptide equivalent and only a minor fraction of this (22–24%) was located in the surface fragment(s) released by trypsin. Both LHCP and LHCP proteoliposomes, after carboxyl modification, aggregated avidly at low salt concentrations. The findings suggest that exposed anionic groups on the surface of LHCP contribute to an electrostatic repulsive force between membranes which must be attenuated, either by cation binding or chemical neutralization, before membrane-membrane adhesion can occur. In line with this the binding of Mn²⁺ by LHCP (approximately four Mn²⁺ bound/26,000-dalton polypeptide equivalent) was sharply decreased after carboxyl modification.     

Quantitative Microdialysis: Analysis of Transients and Application to Pharmacokinetics in Brain

The behavior of a microdialysis probe in vivo is mathematically described. A diffusion-reaction model is developed that not only accounts for transport of substances through tissues and probe membranes but also accounts for transport across the microvasculature and metabolism. Time-dependent equations are presented both for the effluent microdialysate concentration and for concentration profiles about the probe. The analysis applies either to measuring the tissue pharmacokinetics of drugs administered systemically, or for sampling of endogenously produced substances from tissue. In addition, an expression is developed for the transient concentration about the probe when it is used as an infusion device. All mathematical expressions are found to be a sum of an algebraic and an integral term. Theoretical prediction of time-dependent probe behavior in brain has been compared with experimental data for acetaminophen administered at 15 mg/kg to rats by intravenous bolus. Plasma and whole striatal tissue samples were used to describe plasma kinetics and to estimate a capillary permeability-area product of 0.07 min-1. Theoretical prediction of transient effluent dialysate concentrations exhibited close agreement with experimental data over 60 min. Terminal decline of the dialysate effluent concentration was slightly overestimated but theoretical concentrations still lay within the 95% confidence interval of the experimental data at 112 min. Microvasculature transport and metabolism play major roles in determining microdialysate transient responses. Extraction fraction (recovery) has been shown to be a declining function in time for five probe operating conditions. High rates of metabolism and/or capillary transport affect the time required to approach steady-state extraction, shortening the time as the rates increase. Conversely, for substances characterized by low permeabilities and negligible metabolism, experimental situations exist that are predicted to have very slow approaches to microdialysis steady state.     

Selenium status of patients with Balkan endemic nephropathy

The selenium (Se) contents in common cereals in endemic and nonendemic areas in Serbia are very low. Plasma Se levels of both patients and healthy subjects, were also low, reflecting low Se intakes. Patients with Balkan endemic nephropathy (BEN) had significantly lower (p<0.05) plasma Se levels than healthy individuals, both from regions close to endemic areas, and from Belgrade. Mean plasma Se of BEN patients was slightly but insignificantly higher in samples taken immediately after dialysis than in those taken before, suggesting that very little of the Se present in plasma is dialyzable. Plasma SeGSH-Px activities before and after hemodialysis in both BEN and Nonendemic chronic renal failure (NCRF) patients were not significantly different, but BEN patients had lower enzyme activities than those with NCRF and healthy controls. In BEN patients, a significant correlation between plasma Se and SeGSH-Px activity was found. NCFR patients were with diagnoses: TBC of kidneys, chronic glomerulonephritis, chronic pyelonephritis, and polycystic kidneys.     

Oenanthe aquatica (L.)Poir.: Seed reproduction,population structure,habitat conditions and distribution in Czechoslovakia

The paper summarizes data concerning the biology and ecology ofOenanthe aquatica. The species is commonly distributed all over Czechoslovakia, from lowlands to the submontane belt, especially in fishpond and river basins.O. aquatica shows no special relationship to the chemical and physical soil properties, and is well adapted to habitats with a changing water level. The reproduction ofO. aquatica depends on emerging of the bottom. The most developed populations are formed by biennial plants and arise in the year following summer or autumn drainage.     

N-tosyl-L-phenylalanylchloromethane reacts with cysteine 81 in the molecule of elongation factor Tu from Escherichia coli

Elongation factor EF-Tu from Escherichia coli was labelled with N-[¹⁴C]tosyl-L-phenylalanylchloromethane, digested with trypsin and the peptides obtained separated by HPLC. The only radioactive peak recovered corresponded to tryptic peptide containing residues 75–98. Sequencing of the peptide by automated Edman degradation identified cysteine 81 as the site of N-tosyl-L-phenylalanylchloromethane modification. These results confirm the importance of this residue for the interaction with aminoacyl-tRNAs.     

The effect of feeding by mud snails, Ilyanassa obsoleta (Say), on the structure and metabolism of a laboratory benthic algal community

Eastern mud snails, Ilyanassa obsoleta (Say), in densities of 0, 80, and 160 snails · m^?2 were placed in flow-through laboratory microcosms containing 5 cm of frozen and sieved sediments. Other microcosms were raked once daily to a depth of 10 mm. All these containers were incubated for 5 wk and regularly sampled for plant pigments and light and dark transfer of oxygen and carbon dioxide. Feeding at the low density significantly increased chlorophyll standing stock. Respiration and gross photosynthesis increased by an even greater percentage compared to ungrazed controls. Standing stocks of algal pigments, respiration, and photosynthesis were depressed in microcosms which received the 160-snail or raking treatments.The dominant benthic algae in the containers were pennate diatoms. Grazed containers contained a larger percentage of non-motile as compared to motile forms.Sediments fertilized with ammonium at a rate equivalent to excretion of six snails, showed increased chlorophyll content equal to the 80-snail treatment. Daily raking inhibited this effect.We conclude that low densities of Ilyanassa obsoleta stimulate algal growth by accelerating nitrogen cycling and selectively removing specific components of the diatom community. At high snail densities these effects are overwhelmed by overgrazing and sediment stirring.     

COMPARATIVE KINETIC STUDIES OF NITRATE-LIMITED GROWTH AND NITRATE UPTAKE IN PHYTOPLANKTON IN CONTINUOUS CULTURE1

A comparative study of nitrate-limited growth and nitrate uptake was carried out in chemostat cultures of Ankistrodesmus falcatus (Corda) Ralfs., Asterionella formosa Hass., and Fragilaria crotonensis Kit. In each species growth rate (μ) was related to total cell nitrogen or cell quota (q) by the empirical Droop growth function. Nitrate uptake was a function of both external N concentration and q. The apparent maximum uptake rate (V_m') at a given μ was inversely related to q – q₀, where q₀ is the minimum quota. The apparent half-saturation constant for uptake, (K_m') appears to show a slight inverse trend with μ, although statistical analysis shows that this trend is inconclusive. When q approaches q₀, V_m' is several orders of magnitude greater than μq, the calculated steady-state uptake rate. As q increases, however, the difference between these two variables decreases sharply until q approaches q_m, the cell quota for nitrogen-rich cells. At this point the difference between μq and V_m' disappears. This behavior is explained by the feedback regulation of N uptake. The inverse relationship between V_m' and q – q₀ can be described by an empirical three-parameter equation.