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91.
92.
Are some plant life forms more effective than others in screening out ultraviolet-B radiation? 总被引:18,自引:0,他引:18
Summary The unprecedented rate of depletion of the stratospheric ozone layer will likely lead to appreciable increases in the amount of ultraviolet-B radiation (UV-B, 280–320 nm) reaching the earth's surface. In plants, photosynthetic reactions and nucleic acids in the mesophyll of leaves are deleteriously affected by UV-B. We used a fiber-optic microprobe to make direct measurements of the amount of UV-B reaching these potential targets in the mesophyll of intact foliage. A comparison of foliage from a diverse group of Rocky Mountain plants enabled us to assess whether the foliage of some plant life forms appeared more effective at screening UV-B radiation. The leaf epidermis of herbaceous dicots was particularly ineffective at attenuating UV-B; epidermal transmittance ranged from 18–41% and UV-B reached 40–145 m into the mesophyll or photosynthetic tissue. In contrast to herbaceous dicots, the epidermis of 1-year old conifer needles attenuated essentially all incident UV-B and virtually none of this radiation reached the mesophyll. Although the epidermal layer was appreciably thinner in older needles (7 y) at high elevations (Krumholtz), essentially all incident UV-B was attenuated by the epidermis in these needles. The same epidermal screening effectiveness was observed after removal of epicuticular waxes with chloroform. Leaves of woody dicots and grasses appeared intermediate between herbaceous dicots and conifers in their UV-B screening abilities with 3–12% of the incident UV-B reaching the mesophyll. These large differences in UV-B screening effectiveness suggest that certain plant life forms may be more predisposed than others to meet the challenge of higher UV-B levels resulting from stratospheric ozone depletion. 相似文献
93.
Sulfate reduction and S-oxidation in a moorland pool sediment 总被引:3,自引:2,他引:1
In an oligotrophic moorland pool in The Netherlands, S cycling near the sediment/water boundary was investigated by measuring (1) SO4
2– reduction rates in the sediment, (2) depletion of SO4
2– in the overlying water column and (3) release of35S from the sediment into the water column. Two locations differing in sediment type (highly organic and sandy) were compared, with respect to reduction rates and depletion of SO4
2– in the overlying water.Sulfate reduction rates in sediments of an oligotrophic moorland pool were estimated by diagenetic modelling and whole core35SO4
2– injection. Rates of SO4
2– consumption in the overlying water were estimated by changes in SO4
2– concentration over time in in situ enclosures. Reduction rates ranged from 0.27–11.2 mmol m–2 d–1. Rates of SO4
2– uptake from the enclosed water column varied from –0.5, –0.3 mmol m–2 d–1 (November) to 0.43–1.81 mmol m–2 d–1 (July, August and April). Maximum rates of oxidation to SO4
2– in July 1990 estimated by combination of SO4
2– reduction rates and rates of in situ SO4
2– uptake in the enclosed water column were 10.3 and 10.5 mmol m–2 d–1 at an organic rich and at a sandy site respectively.Experiments with35S2– and35SO4
2– tracer suggested (1) a rapid formation of organically bound S from dissimilatory reduced SO4
2– and (2) the presence of mainly non SO4
2–-S derived from reduced S transported from the sediment into the overlying water. A35S2– tracer experiment showed that about 7% of35S2– injected at 1 cm depth in a sediment core was recovered in the overlying water column.Sulfate reduction rates in sediments with higher volumetric mass fraction of organic matter did not significantly differ from those in sediments with a lower mass fraction of organic matter.Corresponding author 相似文献
94.
Methane oxidation rates were measured in boreal forest soils using seven techniques that provide a range of information on soil CH4 oxidation. These include: (a) short-term static chamber experiments with a free-air (1.7 ppm CH4) headspace, (b) estimating CH4 oxidation rates from soil CH4 distributions and (c)222Rn-calibrated flux measurements, (d) day-long static chamber experiments with free-air and amended (+20 to 2000 PPM CH4) headspaces, (e) jar experiments on soil core sections using free-air and (f) amended (+500 ppm CH4) headspaces, and (g) jar experiments on core sections involving tracer additions of14CH4. Short-term unamended chamber measurements,222Rn-calibrated flux measurements, and soil CH4 distributions show independently that the soils are capable of oxidizing atmospheric CH4 at rates ranging to < 2 mg m–2 d–1. Jar experiments with free-air headspaces and soil CH4 profiles show that CH4 oxidation occurs to a soil depth of 60 cm and is maximum in the 10 to 20 cm zone. Jar experiments and chamber measurements with free-air headspaces show that CH4 oxidation occurs at low (< 0.9 ppm) thresholds. The14CH4-amended jar experiments show the distribution of end products of CH4 oxidation; 60% is transformed to CO2 and the remainder is incorporated in biomass. Chamber and jar experiments under amended atmospheres show that these soils have a high capacity for CH4 oxidation and indicate potential CH4 oxidation rates as high as 867 mg m–2 d–1. Methane oxidation in moist soils modulates CH4 emission and can serve as a negative feedback on atmospheric CH4 increases. 相似文献
95.
Wolfgang Schuhmann Roland Lammert Martin H mmerle Hanns-Ludwig Schmidt 《Biosensors & bioelectronics》1991,6(8):689-697
The electrocatalytic oxidation of NADH, ascorbate, urate, xanthine and H2O2 at different polypyrrole electrodes has been investigated. The conducting polymer was grown on platinum, glassy carbon, or graphite electrodes and modified by means of enclosed redox-active anions or other redox-active compounds covalently bound to either the N- or the β-position of the pyrrole. Copolymers of pyrrole and N-substituted pyrrole derivatives of chloranil or 2,3-dicholoro-1,4-naphthoquinone showed outstanding electrocatalytic properties for the oxidation of NADH. The application of these electrodes in amperometric steady-state measurements or flow-injection systems in combination with dehydrogenase reactions has been possible. 相似文献
96.
The effect of ozone exposure on the activities of reactive oxygen scavenging enzymes (SOD†, catalase, GSH-Px) in RBC of Japanese charr (Salvelinus leucomaenis) was examined. Ozone (0, 0.4 and 0.7 ppm as initial concentrations) was exposed to Japanese charr for 30 min, which definitely caused serious membrane damage to RBC of fish. Ozone exposure at 0.4 and 0.7 ppm decreased activities of both catalase and GSH-Px by 80 to 57+ of the control. On the other hand, the activities of SOD remained unaffected even by 0.7 ppm ozone exposure. A hypothesis on the RBC membrane damage and participation of SOD and heme-iron was proposed. 相似文献
97.
Dihydroxybenzoic acids (DBA), such as 3,4-DRA, 3,5-DBA, and 2,4-DBA—at all concentrations tested—inhibited the rate of DL-DOPA oxidation to dopachrome (λmax = 475 nm) by mushroom tyrosinase. 2,3-DBA and 2,5-DBA at relatively low concentration had a synergistic effect on the reaction, whereas at relatively high concentrations they inhibited the rate of DL-DOPA oxidation. The synergistic effect of 0.6-13.3 mM 2,3-DRA on the rate of DL-DOPA oxidation to dopachrome (λmax = 475 nm) was found to be due to the ability of 2,3-DBA-o-quinone (formed by the oxidation of 2,3-DBA by mushroom tyrosinase or by sodium periodate) to oxidize DL-DOPA to dopachrome (via dopaquinone) non-enzymatically. A similar explanation is likely to be valid for the synergism exerted by 2,5-DBA on the rate of DL-DOPA oxidation by mushroom tyrosinase. 相似文献
98.
The stoichiometric oxidation of uranous-to uranyl-uranium byThiobacllus ferrooxidans is demonstrated. Fixation of14CO2 and the effect of inhibitors demonstrate that energy is conserved during the oxidation and used for energy-dependent reverse electron flow and carbon dioxide fixation.Abbreviations HOQNO
2-heptyl-4-hydroxyquinoline-N-oxide
- 8-HQ
8-hydroxyquinoline
- TTFA
thenoyltrifluoroacetone 相似文献
99.
1. The role of tocopherols in tomato chloroplasts from fresh, cold and dark-stored as well as stored and illuminated leaves was studied.2. The cold and dark storage of leaves results in a loss of chloroplast α- and γ-tocopherols of about 30–40% accompanied by an increase in chloroplast δ-tocopherol of about 40%. On illumination of stored leaves, an elevation of α- and γ-tocopherol level to about 110 and 95% of the control, respectively, occurs, whilst δ-tocopherol content is not affected.3. Experiments performed with 2,2-diphenyl-1-picrylhydrazyl-treated chloroplasts show that only about 70% of total α-tocopherol is functionally active in the electron transport of Photosystem II between the diphenyl-carbazide (DPC) donation site and the inhibition site of DBMIB.4. A small amount of α-tocopherol quinone (about 10% of α-tocopherol content) is found in chloroplasts from fresh, fresh and illuminated as well as cold and dark-stored tomato leaves, whereas the illumination of the latter increases the chloroplast α-tocopherol quinone content 3-fold. Moreover, following the illumination of chloroplasts from cold and dark-stored as well as stored and illuminated leaves, the oxidation of exogenous α-tocopherol to α-tocopherol quinone is 2-fold faster then in chloroplasts from fresh leaves.5. The primary product (‘α-tocopheroxide’) formed during the α-tocopherol oxidation by illuminated chloroplasts was identified as 8a-hydroxy-α-tocopheron.6. Exogenous α-tocopherol inhibits the lipid photoperoxidation by about 40–50% in chloroplasts from all three kinds of tomato leaf.7. The results seem to suggest that chloroplast α-tocopherol is involved in both electron transport of PS II and antioxidizing system of chloroplasts. 相似文献
100.
The utilisation of substrates by Leishmania mexicana amastigotes and promastigotes differed significantly. The rates of uptake and catabolism of nonesterified fatty acids were up to 10-fold higher with amastigotes. Almost all the available exogenous fatty acids were consumed during amastigote transformation and by stationary phase of promastigote growth. The results suggest that fatty acids are important energy substrates for amastigotes, whereas promastigote utilisation may reflect the requirement for these substrates in anabolism. Glucose was utilised by amastigotes and promastigotes but the rate of catabolism was up to 10-fold higher in promastigotes. Uptake of glucose occurred throughout amastigote transformation and growth in vitro of promastigotes. High-subpassage promastigotes exhibited markedly lower glucose but higher amino acid utilisation than low-subpassage promastigotes. Asparagine, glutamine, glutamate, leucine, lysine, methionine, and threonine were consumed in large quantities by amastigotes and promastigotes, whereas alanine and glycine were excreted. Proline was catabolised to CO2 by amastigotes and promastigotes but only at a low rate, and it was excreted in large amounts throughout promastigote growth. The major end products of energy metabolism were found to be CO2 and succinate with both forms of the parasite and there was a secretion of up to 12 and 16% of the total protein synthesised by transforming amastigotes and growing promastigotes, respectively. Catabolism in amastigotes and promastigotes was found to be sensitive to cyanide and amytal, whereas 2-mercaptoacetate and 4-pentenoate primarily affected β-oxidation in the amastigote. 相似文献