The use of natural abundance of nitrogen isotopes in plant physiology and ecology

Nitrogen stable isotope (¹⁵N, ¹⁴N) natural abundance has been much less used than carbon isotopes (¹³C, ¹²C) in plant physiology and ecology. Analytical problems, the lower fractional abundance of ¹⁵N than of ¹³C in the biosphere, the greater complexity of the N cycle relative to the C cycle, and smaller expressed discriminations in nature, are contributing factors. The major N pools, globally, have different isotope signatures: atmospheric N₂ is ¹⁵N-depleted relative to organic N (including sedimentary N), a situation resulting from a greater expressed discrimination in the organic N to N₂ (via denitrification) reaction than of diazotrophy during accumulation of the reduced N. Essentially all of the enzymes except nitrogenase which transform N compounds show discrimination against ¹⁵N, although for glutamine synthetase, and the amination of 2-oxoglutarate and pyruvate, this is only seen in terms of NH₄⁺ rather than the true substrate, NH₃. Discrimination is expressed in various N interconversions within plants, leading to substantial differences in δ¹⁵N (up to 12‰) among N compounds and macroscopic plant parts. N isotope fractionation during assimilation of exogenous combined N is often much lower than that expected from studies of isolated enzymes due to processes which show very little discrimination, such as limitation by transport through aqueous solution and membranes. Application of ¹⁵N/¹⁴N discrimination studies to plant ecology have concentrated largely on distinguishing diazotrophy from N supplied from combined N, based on the lower ¹⁵N/¹⁴N in diazotrophs due to the higher ¹⁵N/¹⁴N of combined N sources not being offset by fractionation during uptake. While potentially very useful, a number of pitfalls are discussed in its ecological use in both terrestrial and aquatic systems. N isotope discrimination is also useful in tracking N through food webs, and hence, back to combined N sources for plants.     

Correlation Between Electroencephalogram Isoelectric Time and Hippocampal Norepinephrine Levels, Measured by Microdialysis, During Ischemia in Rats

It is suggested that norepinephrine (NE) plays a role during transient forebrain ischemia. NE may have a protective action against neuronal cell death in the hippocampus, or it may be one of the causes of injurious ischemic effects. We used the microdialysis technique to study extracellular NE levels in the rat hippocampus before, during, and after 30 min of transient incomplete forebrain ischemia (induced by four-vessel occlusion) to describe the time course of NE in this condition. There was a maximal increase (fivefold) in extracellular NE after 10 min of reflow only when the electroencephalogram was isoelectric. NE levels returned to baseline 40 min after release of the carotid clamps and remained constant for the next 80 min. Thus there appears to be a transient NE overflow in the hippocampus during ischemia, closely related to the complete loss of brain electrical activity.     

The implications of naturally occurring levels of fumonisins in corn for human and animal health

Contamination of corn with the fungus Fusarium moniliforme and its secondary metabolites, the fumonisins, has been associated with several human and animal diseases. This paper summarizes present knowledge and presents new data on the levels of fumonisins present in foods and feeds associated with these diseases as well as in commercial corn and corn-based products. The doses of fumonisins to which humans and animals consuming these products would be exposed are compared with those doses known to produce LEM in horses and hepatocarcinogenesis in rats. It is concluded that the known naturally occurring levels of fumonisins present a potential threat to human and animal health and realistic tolerance levels need to be set.     

The effect of various culture conditions on the levels of ammonia assimilatory enzymes of Corynebacterium callunae

Corynebacterium callunae (NCIB 10338) grows faster on glutamate than ammonia when used as sole nitrogen sources. The levels of glutamine synthetase (GS; EC 6.3.1.2) and glutamate synthase (GOGAT; EC 1.4.1.13) of C. callunae were found to be influenced by the nitrogen source. Accordingly, the levels of GS and GOGAT activities were decreased markedly under conditions of ammonia excess and increased under low nitrogen conditions. In contrast, glutamate dehydrogenase (GDH; EC 1.4.1.4) activities were not significantly affected by the type or the concentration of the nitrogen source supplied. The carbon source in the growth medium could also affect GDH, GS and GOGAT levels. Of the carbon sources tested in the presence of 2 mM or 10 mM ammonium chloride as the nitrogen source pyruvate, acetate, fumarate and malate caused a decrease in the levels of all three enzymes as compared with glucose. GDH, GS and GOGAT levels were slightly influenced by aeration. Also, the enzyme levels varied with the growth phase. Methionine sulfoximine, an analogue of glutamine, markedly inhibited both the growth of C. callunae cells and the transferase activity of GS. The apparent K _m values of GDH for ammonia and glutamate were 17.2 mM and 69.1 mM, respectively. In the NADPH-dependent reaction of GOGAT, the apparent K _m values were 0.1 mM for -ketoglutarate and 0.22 mM for glutamine.Abbreviations GDH glutamate dehydrogenase - GS glutamine synthetase - GOGAT glutamate synthase     

Plastics from household waste as a source of heavy metal pollution

An inventory study to the levels of cadmium in the plastic component of household waste was carried out utilizing INAA as the analytical technique. In a 2-h irradiation, 2-d decay, and 1-h measurement, protocol adequate sensitivities could be obtained for Cd, but also for a group of other metals: Cr, Co, Ni, Cu, Sr, Zn, As, Se, Mo, Sn, Sb, Ba, and Hg. Red-, orange-, and yellow-colored plastics either contain Cd at high levels (over 1000 mg/kg) or have relatively low Cd concentrations (<50 mg/kg). High concentrations were also occasionally found for Sr, Se, Ba, Sb, and Hg. INAA appeared very well to be routinely usable for such analysis because of the absence of a destruction step, adequate sensitivity, high accuracy, and multielement results.     

Ultrastructural changes in granulosa lutein cells and progesterone levels during preimplantation,implantation, and early placentation in the western spotted skunk

Summary The ultrastructure of corpora lutea obtained during the preimplantation, implantation and early postimplantation periods has been studied in 20 western spotted skunks. Fine structure of granulosa lutein cells was correlated with progesterone levels. The corpus luteum of the prolonged (7 month) preimplantation period contained undifferentiated small granulosa cells and differentiated large granulosa lutein cells. The former ranged in size between 12 and 20 and the latter between 20 and 45 . The ratio of small and large cells was about equal in an animal 2 days prior to nidation whereas only few small cells and numerous large cells were observed in an animal estimated to be 8 to 12 hours from nidation. Occasionally small cells were observed amidst large ones during the 24 hour nidation period, i.e. adhesion of trophoblast with the luminal uterine epithelium, but small cells were absent in animals after this period. Small cells had some smooth and rough endoplasmic reticulum, rod-shaped mitochondria with platelike cristae, small Golgi complex, and relatively smooth plasma membranes. Large lutein cells had abundant smooth endoplasmic reticulum, membranous whorls of smooth endoplasmic reticulum, usually round mitochondria with tubular and lamellar cristae, a well developed Golgi complex, variable amounts of lipid droplets, and highly plicated and ruffled plasma membranes. Peripheral plasma progesterone levels during the prolonged preimplantation period ranged between 1.1 and 7.9 ng/ml, but during implantation it was between 8 and 16.6 ng/ml. It is suggested that plasma progesterone levels fluctuate during the time of implantation and should not be regarded as a basis to predict actual nidation in the western spotted skunk.This research was supported in part by Grant Number HD06556 from the National Institute of Child Health and Human Development.     

Effect of light source for growth on photosynthetic reactions of Anabaena variabilis particles

Photosynthetic particles were prepared from the blue—green alga Anabaena variabilis grown under two different light conditions. Photosynthetic activity of particles from fluorescent-light-grown cells was lower than that of particles from tungsten-grown cells. The photosynthetic activities of particles from fluorescent-light-grown cells could be increased by moving these cultures to tungsten light. The extent of recovery depended on the ratio of fluorescent growth to the tungsten growth period. It was concluded that the difference in photosynthetic activities between tungsten and fluorescent grown cell-free preparations is more likely due to a lack of red light rather than to disturbance of pigment contents.     

A comparative study of metabolite levels in plant leaf material in the dark

Metabolite levels have been compared in the dark and during photosynthesis in leaves and protoplasts from spinach, pea, wheat and barley. In protoplasts the subcellular distribution was also studied. The levels of triose phosphates and sugar bisphosphates were high in the light and low in the dark. The hexose phosphates and 3-phosphoglycerate levels in the dark were very variable depending on the plant material. In most conditions, hexose phosphates and triose phosphates were mainly in the extrachloroplast compartment, while 3-phosphoglycerate and the sugar bisphosphates were mainly in the chloroplast compartment. Leaves always had a very low triose phosphate: 3-phosphoglycerate ratio in the dark, but in protoplasts this ratio was higher. Detailed studies with spinach showed that metabolite levels were very dependent on the availability of carbohydrate in the leaf, particularly starch. Starch mobilisation is not controlled just by the availability of inorganic phosphate and accumulation of phosphorylated intermediates. Hydrolysis of starch may provide precursors for sucrose synthesis while phosphorolysis leads to provision of substrates for respiration. Starch breakdown generates high enough levels of hexose phosphate to support substantial rates of sucrose synthesis in the dark. Respiration is not greatly increased when metabolite levels are high during starch mobilisation. Higher levels of metabolites shorten the length of the induction phase of photosynthesis.Abbreviations Chl chlorophyll - DHAP dihydroxyacetone phosphate - Fru2,6bisP fructose-2,6-bisphosphate - NMR nuclear magnetic resonance - PGA 3-phosphoglyceric acid - Pi inorganic phosphate - RuBP ribulose-1,5-bisphosphate - UDPGlc uridine-5-diphosphate glucose