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1.
Here, we characterized nitrogen (N) uptake of beech ( Fagus sylvatica) and their associated ectomycorrhizal (EM) communities from NH 4+ and NO 3?. We hypothesized that a proportional fraction of ectomycorrhizal N uptake is transferred to the host, thereby resulting in the same uptake patterns of plants and their associated mycorrhizal communities. 15N uptake was studied under various field conditions after short‐term and long‐term exposure to a pulse of equimolar NH 4+ and NO 3? concentrations, where one compound was replaced by 15N. In native EM assemblages, long‐term and short‐term 15N uptake from NH 4+ was higher than that from NO 3?, regardless of season, water availability and site exposure, whereas in beech long‐term 15N uptake from NO 3? was higher than that from NH 4+. The transfer rates from the EM to beech were lower for 15N from NH 4+ than from NO 3?. 15N content in EM was correlated with 15N uptake of the host for 15NH 4+, but not for 15NO 3?‐derived N. These findings suggest stronger control of the EM assemblage on N provision to the host from NH 4+ than from NO 3?. Different host and EM accumulation patterns for inorganic N will result in complementary resource use, which might be advantageous in forest ecosystems with limited N availability. 相似文献
2.
Primary cultures of both mouse astrocytes and neurons accumulate more 125I – than 36Cl – from the medium. The average cell/medium ratio of 125I – of astrocytes (1.01) is greater than that of neurons (0.74), whereas the ratio of 36Cl – of neurons (0.47) is greater than that of astrocytes (0.25). The equilibrium potentials of both 125I – and 36Cl – calculated from the cell/medium ratios in astrocytes and neurons are significantly lower than their corresponding resting transmembrane potentials which suggest that both iodide and chloride are actively transported into both cell types. With respect to different transport inhibitors, thiocyanate is more effective in inhibiting 125I – uptake whereas furosemide is more effective in inhibiting 36Cl – uptake. Radioiodide uptake by mouse astrocytes was directly proportional to the [Na +] o but was not significantly affected by changes of [Cl –] o or [HCO
3
–
] o, except that it is low in bicarbonate-free medium. Radiochloride uptake by astrocytes was inversely related to [Cl –] o and [HCO
3
–
] o and was not affected [Na +] o, except that it was low in sodium-free medium. Radioiodide uptake by neurons was directly related to [Na +] o between 60 and 140 mM and inversely related to [HCO
3
–
] o between 10 and 40 mM, but it was not affected by [Cl –] o. Radiochloride uptake by neurons was directly related to [Cl –] o and to [Na +] o between 60 and 140 mM and was not affected by [HCO
3
–
] o. However, in sodium-free medium both 125I – and 36Cl – uptakes into neurons were higher than those in [Na +] o between 5 and 60 mM. These results indicate that uptake of 125I – and 36Cl – into astrocytes and neurons are different in their ion dependence and that they are under separate regulation.Special issue dedicated to Dr. Paola S. Timiras 相似文献
3.
Kinetics and inhibition of Na +/K +-ATPase and Mg 2+-ATPase activity from rat synaptic plasma membrane (SPM), by separate and simultaneous exposure to transition (Cu 2+, Zn 2+, Fe 2+ and.Co 2+) and heavy metals (Hg 2+and Pb 2+) ions were studied. All investigated metals produced a larger maximum inhibition of Na +/K +-ATPase than Mg 2+-ATPase activity. The free concentrations of the key species (inhibitor, MgATP 2 ? , MeATP 2 ? ) in the medium assay were calculated and discussed. Simultaneous exposure to the combinations Cu 2+/Fe 2+ or Hg 2+/Pb 2+caused additive inhibition, while Cu 2+/Zn 2+ or Fe 2+/Zn 2+ inhibited Na +/K +-ATPase activity synergistically (i.e., greater than the sum metal-induced inhibition assayed separately). Simultaneous exposure to Cu 2+/Fe 2+ or Cu 2+/Zn 2+ inhibited Mg 2+-ATPase activity synergistically, while Hg 2+/Pb 2+ or Fe 2+/Zn 2+ induced antagonistic inhibition of this enzyme. Kinetic analysis showed that all investigated metals inhibited Na +/K +-ATPase activity by reducing the maximum velocities (V max) rather than the apparent affinity (K m) for substrate MgATP 2-, implying the noncompetitive nature of the inhibition. The incomplete inhibition of Mg 2+-ATPase activity by Zn 2+, Fe 2+ and Co 2+ as well as kinetic analysis indicated two distinct Mg 2+-ATPase subtypes activated in the presence of low and high MgATP 2 ? concentration. EDTA, L-cysteine and gluthathione (GSH) prevented metal ion-induced inhibition of Na +/K +-ATPase with various potencies. Furthermore, these ligands also reversed Na +/K +-ATPase activity inhibited by transition metals in a concentration-dependent manner, but a recovery effect by any ligand on Hg 2+-induced inhibition was not obtained. 相似文献
4.
K + and Na + fluxes and ion content have been studied in roots of Atriplex nummularia Lindl. and Avena sativa L. cv Goodfield grown in 3 millimolar K + with or without 3 or 50 millimolar NaCl. Compartmental analysis was carried out with entire root systems under steady-state conditions. Increasing ambient Na+ concentrations from 0 to 50 millimolar altered K+, in Atriplex, as follows: slightly decreased the cytoplasmic content (Qc), the vacuolar content (Qv), and the plasma membrane influx and efflux. Xylem transport for K+ decreased by 63% in Atriplex. For oat roots, similar increases in Na+ altered K+ parameters as follows: plasma membrane influx and efflux decreased by about 80%. Qc decreased by 65%, and xylem transport decreased by 91%. No change, however, was observed in Qv for K+. Increasing ambient Na+ resulted in higher (3 to 5-fold) Na+ fluxes across the plasma membrane and in Qc of both species. In Atriplex, Na+ fluxes across the tonoplast and Qv increased as external Na+ was increased. In oat, however, no significant change was observed in Na+ flux across the tonoplast or in Qv as external Na+ was increased. In oat roots, Na+ reduced K+ uptake markedly; in Atriplex, this was not as pronounced. However, even at high Na+ levels, the influx transport system at the plasma membrane of both species preferred K+ over Na+. Based upon the Ussing-Teorell equation, it was concluded that active inward transport of K+ occurred across the plasma membrane, and passive movement of K+ occurred across the tonoplast in both species. Na+, in oat roots, was actively pumped out of the cytoplasm to the exterior, whereas, in Atriplex, Na+ was passively distributed between the free space, cytoplasm, and vacuole. 相似文献
6.
N -substituted phenothiazines (PTs) and phenoxazines (POs) catalyzed by fungal Coprinus cinereus peroxidase and Polyporus pinsitus laccase were investigated at pH 4–10. In the case of peroxidase, an apparent bimolecular rate constant (expressed as k
cat/ K
m) varied from 1 ×10 7 M −1 s −1to 2.6×10 8 M −1 s −1 at pH 7.0. The constants for PO oxidation were higher in comparison to PT. pH dependence revealed two or three ionizable
groups with p K
a values of 4.9–5.7 and 7.7–9.7 that significantly affected the activity of peroxidase. Single-turnover experiments showed
that the limiting step of PT oxidation was reduction of compound II and second-order rate constants were obtained which were
consistent with the constants at steady-state conditions. Laccase-catalyzed PT and PO oxidation rates were lower; apparent
bimolecular rate constants varied from 1.8×10 5 M −1 s −1 to 2.0×10 7 M −1 s −1 at pH 5.3. PO constants were higher in comparison to PT, as was the case with peroxidase. The dependence of the apparent
bimolecular constants of compound II or copper type 1 reduction, in the case of peroxidase or laccase, respectively, was analyzed
in the framework of the Marcus outer-sphere electron-transfer theory. Peroxidase-catalyzed reactions with PT, as well as PO,
fitted the same hyperbolic dependence with a maximal oxidation rate of 1.6×10 8 M −1 s −1 and a reorganization energy of 0.30 eV. The respective parameters for laccase were 5.0×10 7 M −1 s −1 and 0.29 eV.
Received: 20 September 1999 / Accepted: 24 February 2000 相似文献
7.
Abstract Atriplex amnicola was grown at 25, 200 or 400 mol m 3 NaCl. Root tissues at different stages of development were investigated for concentrations of K +, Na + and Mg 2+, and in some cases for Cl ?. Sugar and starch concentrations were measured for plants grown at 25 or 400 mol m 3 NaCl. In the ‘slightly vaeuolated’ root tips, Na + was only 40 mol m ?3 at an external concentration of 400 mol m ?3 NaCl. The concentrations of K + were not affected substantially by external NaCl between 25 mol m ?3 and 400 mol m ?3. The ‘highly vacuolated’ root tissues had substantially higher concentrations of K +, Na + and Cl ? in plants grown at 200 and 400 mol m 3 NaCl than in plants grown at 25 mol m ?3 NaCl. Concentrations of Cr and of the sum of the cations in recently expanded tissue were similar to those in the bulk of the roots, consisting mainly of old cells. However, the K +: Na + decreased with age; at 400 mol m ?3 external NaCl with a K +: Na + of 0.012, the K +: Na + in recently expanded 12 mm root tips was as high as 1.6, compared with 0.7 for the bulk of the roots. These ion data were used to estimate cytoplasmic and vacuolar concentrations of K + and Na +. Such calculations indicated that between 25 mol m 3 and 400 mol m ?3 external NaCl the concentration of the sum of (Na ++K +) in the cytoplasm was maintained at about 180–200 mol m ?3 (cell water basis). In contrast, the (Na ++ K +) concentration in the vacuole was 170 mol m ?3 for plants grown at 25 mol m ?3 NaCl and 420 mol 400 mol m ?3 NaCl. The expanding root (issues exhibited greatly decreased soluble sugars and starch between dusk and dawn. Ai both times, sugar and starch concentrations in these tissues were 2.5–4.0 times greater in plants grown at 400 mol m ?3 NaCl compared with plants grown at 25 mol m ?3 NaCl. In contrast, carbohydrate concentrations in expanded root tissues were very similar at 25 and 400 mol m ?3 and showed little diurnal fluctuation. This paper considers the causes for the slower growth of A. amnicola at 400 than at 25 mol m” 3 NaCl, using the data for the roots described here, and those for the shoots presented in the preceding paper (Aslam et al., 1986). There is no support for possible adverse effects by high internal ion concentrations. Instead, there may be deficiencies in supply of organic solutes for osmotic regulation; during part of the night a limited supply of such solutes may well restrict the rate of expansion of cells in plants growing at 400 mol m ?3 NaCl. There is insufficient evidence to decide whether this limitation in the expanding tissues is particularly prominent for the roots or for the shoots. 相似文献
8.
It has been pointed out that tea ( Camellia sinensis (L.) O. Kuntze) prefers ammonium (NH 4 + ) over nitrate (NO 3 ? ) as an inorganic nitrogen (N) source. 15N studies were conducted using hydroponically grown tea plants to clarify the characteristics of uptake and assimilation of NH 4 + and NO 3 ? by tea roots. The total 15N was detected, and kinetic parameters were calculated after feeding 15NH 4 + or 15NO 3 ? to tea plants. The process of N assimilation was studied by monitoring the dynamic 15N abundance in the free amino acids of tea plant roots by GC-MS. Tea plants supplied with 15NH 4 + absorbed significantly more 15N than those supplied with 15NO 3 ? . The kinetics of 15NH 4 + and 15NO 3 ? influx into tea plants followed a classic biphasic pattern, demonstrating the action of a high affinity transport system (HATS) and a low affinity transport system (LATS). The V max value for NH 4 + uptake was 54.5 nmol/(g dry wt min), which was higher than that observed for NO 3 ? (39.3 nmol/(g dry wt min)). K M estimates were approximately 0.06 mM for NH 4 + and 0.16 mM for NO 3 ? , indicating a higher rate of NH 4 + absorption by tea plant roots. Tea plants fed with 15NH 4 + accumulated larger amounts of assimilated N, especially glutamine (Gln), compared with those fed with 15NO 3 ? . Gln, Glu, theanine (Thea), Ser, and Asp were the main free amino acids that were labeled with 15N under both conditions. The rate of N assimilation into Thea in the roots of NO 3 ? -supplied tea plants was quicker than in NH 4 + -supplied tea plants. NO 3 ? uptake by roots, rather than reduction or transport within the plant, seems to be the main factor limiting the growth of tea plants supplied with NO 3 ? as the sole N source. The NH 4 + absorbed by tea plants directly, as well as that produced by NO 3 ? reduction, was assimilated through the glutamine synthetase-glutamine oxoglutarate aminotransferase pathway in tea plant roots. The 15N labeling experiments showed that there was no direct relationship between the Thea synthesis and the preference of tea plants for NH 4 + . 相似文献
9.
Tumor uptake rates, concentrations in mitochondrial fraction (containing lysosome) of liver and tumors, avid accumulations in connective tissue (especially inflammatory tissue) and binding substances in these tissues for 46Sc 3+ and 51Cr 3+ were essentially similar to those for 67Ga 3+, 111In 3+, 169Yb 3+, 167Tm 3+, 95Zr 4+ and 181Hf 4+. However, the main binding substance of 46Sc 3+ and 51Cr 3+ in tumor and liver was the acid mucopolysaccharide (as described concerning 95Zr and 181Hf) whose molecular weight exceeded 40,000, although the main binding substance of 67Ga 3+, 111In 3+, 169Yb 3+ and 167Tm 3+ was the acid mucopolysaccharide with a molecular weight of about 10,000. 相似文献
10.
13NO 3– and 13NH 4+ compartmental analyses were carried out in seedling roots of trembling aspen ( Populus tremuloides Michx.), lodgepole pine ( Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) and interior Douglas-fir ( Pseudotsuga menziesii var. glauca [Beissn.] Franco) at 0·1 and 1·5 mol m –3 external NO 3– or NH 4+ concentrations ([NO 3–] o or [NH 4+] o, respectively). At the lower [NO 3–] o, the capacities and efficiencies of acquisition and accumulation of NO 3–, based upon NO 3– fluxes and cytoplasmic NO 3– concentrations ([NO 3–] c), were in the order aspen >> Douglas-fir > pine. At 1·5 mol m –3[NO 3–] o, the NO 3– influx increased 18-fold in pine, four-fold in Douglas-fir and approximately 1·4-fold in aspen; in fact, at 1·5 mol m –3[NO 3–] o, the NO 3– influx in pine was higher than in aspen. However, at high [NO 3–] o, efflux also increased in the two conifers to a much greater extent than in aspen. In aspen, at both [NO 3–] o, approximately 30% of the 13N absorbed was translocated to the shoot during 57 min of 13N loading and elution, compared with less than 10% in the conifers. At 0·1 mol m –3[NH 4+] o, influx and net flux were in the order: aspen > pine > Douglas-fir but the differences were much less than in NO 3– fluxes. At 1·5 mol m –3[NH 4+] o, NH 4+ influx, efflux and [NH 4+] c greatly increased in aspen and Douglas-fir and, to a much lesser extent, in pine. In aspen, 29 and 12% of the 13N absorbed was translocated to the shoot at 0·1 and 1·5 mol m –3[NH 4+] o, respectively, compared with 5 to 7% in the conifers at either [NH 4+] o. These patterns of nitrogen (N) uptake, particularly in the case of NO 3–, and the observed concentration responses of NO 3– uptake, reflect the availability of N in the ecological niches, to which these species are adapted. 相似文献
11.
The effects of cadmium and lead on the internal concentrations of Ca 2+ and K +, as well as on the uptake and translocation of K( 86Rb +) were studied in winter wheat ( Triticum aestivum L. a. MV-8) grown hydroponically at 2 levels of K + (100 uM and 10 mM). Cd 2+ and Pb 2+ were applied in the nutrient solution in the range of 0.3 to 1000 u.M. Growth was more severely inhibited by Cd 2+ and in the high-K + plants as compared to Pb z+ and low-K + plants. Ions of both heavy metals accumulated in the roots and shoots, but the K + status influenced their levels. Ca 2+ accumulation was increased by low concentrations of Cd 2+ mainly in low-K + shoots, whereas it was less influenced by Pb 2+. The distribution of Cd 2+ and Ca 2+ in the plant and in the growth media indicated high selectivity for Cd 2+ in the root uptake, while Ca 2+ was preferred in the radial and/or xylem transport. Cd 2+ strongly inhibited net K + accumulation in high-K + plants but caused stimulation at low K + supply. In contrast, the metabolis-dependent influx of K +( 86Rb +) was inhibited in low-K + plants, while the passive influx in high-K + plants was stimulated. Translocation of K + from the roots to the shoots was inhibited by Cd 2+ but less influenced in Pb 2+-treated plants. It is concluded that the effects of heavy metals depend upon the K +-status of the plants. 相似文献
12.
The dependence of Ca uptake and translocation by intact roots of Phaseolus vulgaris on concurrent root metabolism was investigated using 45Ca-labeled Hoagland solutions at one-half and one-twentieth strength (2.5 and 0.25 mM Ca 2+). Adsorbed and absorbed 45Ca fractions in the roots were distinguished on the basis of the time course of exchange with the outer solution. Uptake of 42K, of which the characteristics are better known, was measured for comparison. The absorbed 45Ca fraction showed a markedly nonlinear increase with time in contrast to the near linear increase in 42K. Exposure of roots to cyanide, arsenate, 2,4-dinitrophenol, or low temperatures caused only slight reductions in 45Ca absorption by roots, but significant reductions of 42K. In all treatments involving inhibitors and low temperatures, the translocation to shoots of both 45Ca and 42K was strongly inhibited. The conclusion that much of the absorbed 45Ca fraction in the root tissue is taken up by processes which are not rate-limited by metabolism is discussed. 相似文献
13.
The reaction between Al 2iBu 4 and K results in the formation of K iBu and KAl 2iBu 3. The latter reacts with additional K to produce K 3Al 5iBu 10. The oxidation states of aluminum, associated with these reduced aluminum compounds, have been determined by oxidative hydrolyses. Spectroscopic characterization included infrared and NMR data; 1H, 13C, 27Al, and 39K. 相似文献
14.
Cobalt, nickel, copper and zinc coordination compounds of two thiosemicarbazones with general composition ML 2 (L: monodeprotonated ligand corresponding to 2-acetyl-γ-butyrolactone thiosemicarbazone, HL 1, and 2-furancarbaldehyde thiosemicarbazone, HL 2) and also complexes with general composition MCl 2(HL 2) were synthesized (except [NiCl 2(HL 2)] and [Co(L 2) 2]). The interaction of CuCl 2 with HL 2 gave [CuCl(HL 2)], a copper(I) complex. The ligands and metal complexes were characterized by IR, 1H and 13C NMR spectroscopy, and magnetic susceptibility measurements. The crystal structure of [Ni(L 2) 2] · 2dmso was determined and a trans-square planar coordination of the two κ 2-N,S chelate rings forming polymeric strips through H-bonds with dmso was observed. Actually, in all the reported complexes both ligands behaved as κ 2-N,S chelates, except in the case of [Co(L 1) 2] in which HL 1 is tridentate κ 3-N,S,O. The antimicrobial properties of all compounds were studied using a wide spectrum of bacterial and fungal strains. The copper complexes of HL 2 were the most active against all strains, including dermatophytes and phytopathogenic fungi. Most of the studied compounds, especially [Cu(L 1) 2], presented good activity against Haemophilus influenzae, a very harmful bacterium to humans. 相似文献
15.
Summary Intact chloroplasts isolated from spinach reduced NO 3
- and NO 2
- in the light without the addition of either co-factors or added enzymes. The maximum rate observed, however, for the reduction of NO 3
- was approximately 3Moles hr -1 mg -1 (chlorophyll) and for NO 2
- 6 Moles hr -1 mg -1 (chlorophyll). These rates were consistent with the enzyme content of whole chloroplasts, but much lower than those found in whole leaf extracts.The addition of both NO 3
- and NO 2
- in low concentrations resulted in transient increases in both O 2 evolution and CO 2 fixation. The increases in oxygen evolution were not consistent in amount and bore no relation to the amount of substrate reduced. Similar transients were observed in a number of experiments when NaCl or NH 4Cl were added.The addition of NO 2
- at concentrations of 10 -4 M and above resulted in marked inhibition of both O 2 evolution and CO 2 fixation. NO 2
- appears to inhibit by blocking the reduction of NADP. NO 3
- at similar concentrations had no such effect.An increase in the soluble amino nitrogen content of the chloroplasts was observed when NO 3
- or NO 2
- was reduced. There was, however, no increase in the incorporation of 14C from 14CO 2 into amino acids under these conditions. Even with the addition of ammonia the amount of 14C incorporated into the amino acids was not changed from less than 5% of the total 14C fixed. We conclude that while intact chloroplasts do have the ability to reduce both NO 3
- and NO 2
- at low rates, they do not synthesize appreciable amounts of amino acid directly, and this fact must be considered when formulating any pathways for nitrogen metabolism during photosynthesis.Supported in part by the National Research Council of Canada. 相似文献
17.
A set of three experiments is described which correlate aromatic resonances of histidine and tryptophan residues with amide
resonances in 13C/ 15N-labelled proteins. Provided that backbone 1H and 15N positions of the sequentially following residues are known, this results in sequence-specific assignment of histidine 1H δ2/ 13C δ2 and 1H ε1/ 13C ε1 as well as tryptophan 1H δ1/ 13C δ1, 1H ζ2/ 13C ζ 2, 1H η2/ 13C η2, 1H ε3/ 13C ε3, 1H ζ3/ 13C ζ3 and 1H ε1/ 15N ε1 chemical shifts. In the reverse situation, these residues can be located in the 1H– 15N correlation map to faciliate backbone assignments. It may be chosen between selective versions for either of the two amino
acid types or simultaneous detection of both with complete discrimination against phenylalanine or tyrosine residues in each
case. The linkages between δ-proton/carbon and the remaining aromatic as well as backbone resonances do not rely on through-space
interactions, which may be ambiguous, but exlusively employ one-bond scalar couplings for magnetization transfer instead.
Knowledge of these aromatic chemical shifts is the prerequisite for the analysis of NOESY spectra, the study of protein–ligand
interactions involving histidine and tryptophan residues and the monitoring of imidazole protonation states during pH titrations.
The new methods are demonstrated with five different proteins with molecular weights ranging from 11 to 28 kDa. 相似文献
18.
Thorium-232 ( 232Th), a natural radionuclide from the actinide family, is abundantly present in monazite and other ores. It is used as one of the prime fuel materials in nuclear industry and may pose an exposure risk to nuclear workers and members of the public. Human erythrocytes, as a classical cellular membrane model, were coincubated with 232Th in order to elucidate whether this naturally occurring important radionuclide produced perturbations to cell membrane. Present study revealed that erythrocytes underwent aggregation or lysis depending on the ratio of 232Th to cell. Scanning electron micrographs showed that erythrocytes transformed into equinocytes and/or spherocytes after 232Th treatment. Further examination of erythrocyte by atomic force microscopy suggested significant increase in surface roughness after 232Th treatment. Experiments on neuraminidase treated and/or anti-GpA antibody blocked erythrocytes suggested significant role of membrane sialic acid and glycophorin A (GpA) protein in aggregation or hemolytic effects of 232Th. Further results showed that 232Th caused hemolysis by colloid osmotic mechanism, as evidenced by potassium efflux, osmotic protection and osmotic fragility studies. Osmoprotection experiments indicated that hemolysis get elicited through the formation of membrane pores of ∼2.0 nm in size. Hemolysis studies in presence of inhibitors (TEA, bumetanide, DIDS and amiloride) revealed the role of K + channel, Na +/K +/2Cl − channel, Cl −/HCO 3− anion exchanger and Na +/H + antiporter in 232Th induced erythrolysis. Presence of non-diffusible cation (N-methyl d-glucasamine) or anion (gluconate) in erythrocyte suspending medium further confirm the role of Na + and Cl − influx in hemolytic effect of 232Th. These findings provide significant insight in structural, biochemical and osmotic toxic effects of 232Th on human erythrocytes. 相似文献
19.
- 1.1. The study was carried out on 22 species of insects from 5 orders. The osmolality of their hemolymph varied from 319 to 421 mOsm/kg H2O, concentration of Na+ 4.6 to 118 mM/l, K+ 6.3 to 73mM/l, Ca2+ 3.6 to 12.9 mM/l, Mg2+ 2.3 to 76 mM/l. The most abundant cation in the hemolymph of insects from higher orders is either K+ or Mg2+.
- 2.2. In the muscles of lower and higher insects K+ is usually within 80–120 mM/kg wet wt.
- 3.3. Most Ca2+ and Mg2+ in hemolymph is bound with protein and low molecular anions, concentration of free Ca2+ is 0.9-2.1mM/l Mg2+ 3.7–8.0 mM/l.
- 4.4. It is concluded that, in insects, potassium hemolymph, cell volume regulation and accumulation of ions in the cell, are ensured by an increased osmolality of hemolymph due to a high percentage contribution of low molecular organic substances which are retained in the hemolymph due to the absence of filtration apparatus in the Malpighian tubules.
相似文献
20.
Summary Concentration dependence of net ammonium and nitrate uptake was monitored for a cultivar of tomato, Lycopersicon esculentum, and two accessions of a neotropical wild relative, L. hirsutum. The kinetics of net NH
4
+
uptake differed among these taxa and were not dependent on the ionic composition of the nutrient solution. The kinetics of net NO
3
-
uptake were dependent on the composition of the nutrient solution; the presence of NH
4
+
or Cl - enhanced net NO
3
-
uptake for the cultivated species and for a highland accession of the wild species. The capacity for net NO
3
-
uptake was greater than the capacity for net NH
4
+
uptake in all three taxa; the proportion of NO
3
-
to NH
4
+
absorbed was much greater for the wild taxa. Our data suggest that NO
3
-
may be a more important source of mineral nitrogen than NH
4
+
for these tropical taxa. 相似文献
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