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831.
Summary
Chaoborus crystallinus fourth-instar larvae were reared individually at 14°, 17° and 20° C under different food conditions. Daphnia magna of 1.25 mm average length served as prey. The following were measured: amount of prey ingested, larval weight gain, duration of fourth instar, body weight of the adults, and egg number per female. At a given temperature, the body weight, egg-number and developmental rate increased with food consumption. At a given food consumption, higher temperatures caused a decrease in body weight and egg number, and an increase in developmental rate. Gross production efficiencies for fourth-instar larvae were highest at temperatures around 17° C. The results clearly indicate that from an energetic point of view higher temperatures are disadvantageous. In C. crystallinus vertical migration is evidently a way of lowering the temperature to which the animals are exposed and hence optimizing food conversion into biomass and offspring production, especially if prey densities are below the saturation level. 相似文献
832.
The reduction of ferric leghemoglobin (Lb3+) from soybean (Glycine max (L.) Merr.) nodules by riboflavin, FMN and FAD in the presence of NAD(P)H was studied in vitro. The system NAD(P)H + flavin reduced Lb3+ to oxyferrous (Lb2+ · O2) or deoxyferrous (Lb2+) leghemoglobin in aerobic or anaerobic conditions, respectively. In the absence of O2 the reaction was faster and more effective (i.e. less NAD(P)H oxidized per mole Lb3+ reduced) than in the presence of O2; this phenomenon was probably because O2 competes with Lb3+ for reductant, thus generating activated O2 species. The flavin-mediated reduction of Lb3+ did not entail production of superoxide or peroxide, indicating that NAD(P)H-reduced flavins were able to reduce Lb3+ directly. The NAD(P)H + flavin system also reduced the complexes Lb3+ · nicotinate and Lb3+ · acetate to Lb2+ · O2, Lb2+ or Lb2+ · nicotinate, depending on the concentrations of ligands and of O2. In the presence of 200 M nitrite most Lb remained as Lb3+ in aerobic conditions but the nitrosyl complex (Lb2+ · NO) was generated in anaerobic conditions. The above-mentioned characteristics of the NAD(P)H + flavin system, coupled with its effectiveness in reducing Lb3+ at physiological levels of NAD(P)H and flavins in soybean nodules, indicate that this mechanism may be especially important for reducing Lb3+ in vivo.Abbreviations and Terminology FLbR
ferric leghemoglobin reductase
- Hb2+ /Hb3+
hemoglobin containing Fe2+ /Fe2+
- Lb2+ /Lb3+
leghemoglobin containing Fe2+ /Fe3+
- Lb3+ · nicotinate/acetate
Lb in which nicotinate or acetate are complexed to Lb3+
- Lb2+ · O2/CO/NO/nicotinate
Lb in which O2, CO, NO or nicotinate are complexed to Lb2+
- Rfl
riboflavin
- SOD
superoxide dismutase (EC 1.15.1.1)
Published as Paper No. 9237, Journal Series, Nebraska Agricultural Research DivisionWe thank M.B. Crusellas for his skillful drawings. M. Becana thanks the Spanish Ministry of Education and Science/Fulbright Commission for financial support. 相似文献
833.
Peptide substrates rapidly modulate expression of dipeptide and oligopeptide permeases in Candida albicans 总被引:2,自引:0,他引:2
Previous studies showed that peptide transport activity in Candida albicans was completely repressed by NH4+, and that growth on amino acids as sole nitrogen source stimulated transport to a basal level. Here we show that addition of peptide mixtures to culture media gives a further 5-fold increase in transport of dipeptides and oligopeptides; the effect is specific for peptide transport, amino acid uptake being unaffected. Presence of peptides but not amino acids overrides NH4+ repression of peptide transport. Step-up activation of transport activity, caused by addition of peptides to incubation media, and step-down inhibition that accompanies removal of peptides, occurs rapidly (within 30 min at 28 degrees C). Step-up is independent of de novo protein synthesis. This substrate-induced regulation is compatible with a rapid, reversible activation of plasma membrane-bound peptide permease(s), or a mechanism of endocytosis involving a cycle of insertion and retrieval of preformed permease components. These results are considered in relation to the expression of peptide permeases in vivo, and the development of synthetic anticandidal peptide carrier prodrugs designed to exploit these systems. 相似文献
834.
Robert W. Griffith 《Environmental Biology of Fishes》1991,32(1-4):199-218
Synopsis The question of how (and why) the ureosmotic strategy, characteristic of Latimeria chalumnae and the chondrichthians evolved is addressed. There are three requirements for ureosmotic regulation: urea synthesis via the ornithine-urea cycle, urea tolerance involving biochemical and physiological adjustments, and urea retention that requires renal, branchial, metabolic and reproductive adaptations. Several examples of lower vertebrates in which urea plays a physiological role are considered to see whether they might provide insight into the origin of ureosmotic regulation. The guppy shows high urea synthesis and retention during embryonic development, and it is possible that a developmental role of urea is a general phenomenon in fishes. The toadfish, thought to be an enigma with high urea synthesis in the absence of an obvious physiological role of urea, is ureotelic under some conditions. Its urea excretion is likely related to renal function and/or parental care. In lungfish high ureogenesis is associated with estivation in periodically dry habitats. The resultant hyperuremia prevents ammonia toxicity, inhibits water loss and may repress metabolism. Latimeria is a classic marine ureosmotic regulator in which urea is used as an osmolyte that allows osmotic equilibrium with sea water while maintaining low ion levels. Adults of the frog, Rana cancrivora, are also ureosmotic regulators in brackish water. A scenario is proposed that suggests how ureosmotic regulation could have evolved in Latimeria and other fishes. The ornithine-urea cycle (composed of an arginine synthetic pathway and a second pathway that splits arginine into urea) occurred in fossil anadromous agnathans. Here the first pathway functioned in the ammocoete-like larvae for the generation of arginine to supplement a protein-deficient diet of algae, whereas the arginase pathway was important in the embryo for vitellin catabolism. Gnathostome evolution was associated with trends towards large eggs and prolonged development, requiring a complete ornithine-urea cycle for ammonia detoxification in embryos. Retention of a complete ornithine-urea cycle throughout adult life (via paedomorphosis) would preadapt any relatively large, sluggish, euryhaline fish for ureosmotic regulation when it was exposed to sea water. It is suggested that ureosmotic regulators evolved from freshwater or anadromous ancestors that entered the marine habitat. Once early ureosmotic regulators were established in the sea there would have been strong selection for internal fertilization and development, as is seen in Latimeria and many elasmobranchs. It is suggested that ureosmotic regulation was a common strategy in Paleozoic marine gnathostomes. 相似文献
835.
P. B. Cregan P. van Berkum 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1984,67(2-3):97-111
Summary It is necessary to increase protein productivity of grain crops to meet present and future world protein requirements. Conventional plant breeding methodology has been to select genotypes with enhanced yield or grain protein concentration. In addition to this determination of end product, we suggest measurements of a number of physiological and biochemical processes of nitrogen (N) metabolism which precede plant maturity as selection criteria for enhanced N metabolism and grain crop productivity. The measurement across the growing season of genotypic variation in components of N metabolism would constitute a physiological/biochemical selection program to be incorporated with the determination of harvestable end product. A properly designed physiological/biochemical selection program would integrate the effects of plant genotype, environment, and their interactions allowing identification of the factors limiting productivity of particular genotypes, and would also estimate end product. Our review of literature pertinent to whole plant N metabolism suggests that such a selection program initially include NO
3
-
uptake, N2 fixation, N accumulation, N remobilization, seed protein synthesis, and Nitrogen Harvest Index.Supported by USDA, Nitrogen Fixation and Soybean Genetics Laboratory, Beltsville, Md., and the Agronomy Department, University of Maryland, College Park, Md., under Cooperative Agreement 58-32U4-3-370, Scientific Article No. A-3400, Contribution No. 6473 of the Maryland Agricultural Experiment Station, Department of Agronomy, College Park, MD 20742, USA 相似文献
836.
Gregory G. Brown Jong Seob Lee Normand Brisson Desh Pal S. Verma 《Journal of molecular evolution》1984,21(1):19-32
Summary We have analyzed the sequences of soybean leghemoglobin genes as an initial step toward understanding their mode of evolution. Alignment of the sequences of plant globin genes with those of animals reveals that (i) based on the proportion of nucleotide substitutions that have occurred at the first, second, and third codon positions, the time of divergence of plant and animal globin gene families appears to be extremely remote (between 900 million and 1.4 billion years ago, if one assumes constancy of evolutionary rate in both the plant and animal lineages) and (ii) in addition to the normal regulatory sequences on the 5 end, an approximately 30-base-pair sequence, specific to globin genes, that surrounds the cap site is conserved between the plant and animal globin genes. Comparison of the leghemoglobin sequences with one another shows that (i) the relative amount of sequence divergence in various coding and noncoding regions is roughly similar to that found for animal globin genes and (ii) as in animal globin genes, the positions of insertions and deletions in the intervening sequences often coincide with the locations of direct repeats. Thus, the mode of evolution of the plant globin genes appears to resemble, in many ways, that of their animal counterparts. We contrast the overall intergenic organization of the plant globin genes with that of animal genes, and discuss the possibility of the concerted evolution of the leghemoglobin genes. 相似文献
837.
Seven tree species from three different light environments in the wet lowland forests of Costa Rica were grown under controlled environment conditions to assess light related photosynthetic potentials. Light saturated photosynthesis rates were clearly related to light levels of the field environments. Mean saturated, net photosynthetic rates ranged from 6.8 to 11.3 to 27.7 mol m–2 sec–1 for plants from heavy shade, canopy light gaps and man-made clearings respectively. Light saturation of plants from clearings occurred at photosynthetic photon flux densities greater than 1000 mol m–2 sec–1 whereas plants from heavy shade environments became light saturated near 500 mol m–2 sec–1. Plants that normally occur in intermediate light environments were intermediate in light saturation levels. Mean maximum stomatal conductances ranged from 1.0 to 7.3 mm sec–1 and followed a pattern similar to photosynthetic rates. 相似文献
838.
Localization of nitrogenase in vesicles of Frankia sp. Cc1.17 by immunogoldlabelling on ultrathin cryosections 总被引:1,自引:0,他引:1
T. M. Meesters 《Archives of microbiology》1987,146(4):327-331
Immunogoldlabelling on ultrathin cryosections of Frankia sp. Cc1.17 showed specific labelling of nitrogenase in the spherical cells called vesicles. No label was found in the hyphae in any cells grown on a medium with combined nitrogen, nor in those to which no specific antiserum was added. Similar results were obtained with cultures grown under high (20%) and low (2%) oxygen tension in the gas phase.Abbreviations BSA
Bovine serum albumin fraction V (Sigma)
- PBS
phosphate buffered saline. Phosphate buffer 0.1 M, 8 g NaCl/1, 0.2 g KCl/1
- PIPES
Piperazine-1,4-bis(ethane sulphonic acid) 相似文献
839.
本文报道生长在不同海拔生境条件下(云南省元江、玉溪、昆明、大理、丽江)的水稻叶片及茎鞘氮素含量的变化。所获得的主要结果如下: 1.自然生境条件下,水稻叶片和茎鞘的氮素含量随着生育期的进展而变化,有一个由高到低的下降趋势。而前期低海拔地区的含量比高海拔地区的高,成熟期则是海拔越高其叶片含氮量也越高,茎鞘含氮量的情形与叶片相反。 2.穗肥施用时期不同,对叶片和茎鞘氮素含量的影响不同,在低和较低海拔地区施用时期越晚,成熟期残存于叶片及茎鞘的氮素就越多;而且海拔高的地区的含量也高。在冷凉的高海拔地区丽江以颖花分化期追肥比幼穗分化和减数分裂期施用则黄熟期保持较高的含氮量。 相似文献
840.
Nitrogen fixation by cyanobacteria in a moss community on East Ongul Island (69°00'S 39°35'E), Antarctica was investigated
using the acetylene reduction method.
The mean acetylene reduction rate at 10°C and 200 μE·m−2·s−1 photosynthetically active radiation was 7.12 nmol C2H4 per square centimeter of moss community per hour. The effects of temperature, radiation, desiccation and rehydration on the
acetylene reduction rates were examined.
A simple predictive model was constructed in order to estimate the amount of nitrogen fixed in the field. Using this model,
the daily amount of nitrogen fixation was calculated from microclimatic data (temperature and radiation) measured in the experimental
field at Syowa Station on East Ongul Island between 1983 and 1984. The cumulative amount of nitrogen fixation in the growing
season during this period was estimated to be 329 mg N per square meter of moss community.
It is suggested that nitrogen fixation by cyanobacteria in the moss community is important as a nitrogen source for the community
growth on East Ongul Island. 相似文献