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1.
Pisum sativum L. cv Alsweet (garden pea) and Lycopersicon esculentum Mill. flacca (mutant tomato) were chosen to evaluate the metabolic basis for plant injury from combinations of O 3 + SO 2. The plants were exposed under conditions reported to specifically alter O 3 or SO 2 toxicity; light versus dark exposures, and treatment with the fungal metabolite fusicoccin (FC), the O 3 injury inhibitor N-[2-(2-oxo-1-imidazolidiny) ethyl]- N′-phenylurea (EDU), and the SO 2 injury stimulator diethyldithiocarbamate (DDTC). Plants were grown in controlled environment chambers and exposed to combinations of O 3 (0.05-0.2 microliters per liter) and SO 2 (0.1-0.3 microliters per liter) for 2 hours. Peas treated with FC had the same or greater injury (quantified by visual rating) with O 3 + SO 2 exposures compared to plants not treated with FC. For plants with open stomata in the dark as well as light, i.e. FC-treated peas and tomatoes, there was no change or an increase in foliar necrosis with O 3 + SO 2 exposures in the dark versus light. Peas treated with EDU had an almost complete absence of O 3 injury, no change in SO 2 injury, and moderate decreases in injury from combinations of O 3 + SO 2 compared to plants not treated with EDU. Tomatoes treated with DDTC showed the same or less injury compared to plants not treated with DDTC and exposed to O 3 or SO 2. The plant responses to the experimental treatments and O 3 + SO 2 resembled O 3 responses more than SO 2 responses. The evidence for O 3-like responses are: no change or increase in injury in the light versus dark, and EDU-induced decreases in injury. Evidences for SO 2-like responses are: incomplete protection from injury with EDU, and no change or increased injury to FC-treated versus untreated plants. Thus, a metabolic mechanism affected by both pollutants may be associated with the combination injury, e.g. effects the plasma membrane. 相似文献
2.
An experimental chemical N-[2-(2-oxo-1-imidazolidinyl)ethyl]- N′-phenylurea (EDU), is an effective protectant against acute and chronic foliar injury due to ozone (0 3) when sprayed on intact leaves or supplied to the plants through soil application. An 0 3-sensitive snap bean cultivar ( Phaseolus vulgaris L. `Bush Blue Lake 290') was systemically treated with EDU (0, 25, 50, and 100 milligrams per 15-centimeter diameter pot) to determine if EDU-induced or activated protective oxyradical and peroxyl scavenging enzymes. EDU-enhanced tolerance to O 3 injury always correlated with increases in superoxide dismutase (SOD) and catalase activities in the leaves. Peroxidase levels correlated more closely with foliar injury. Greater SOD levels in young leves compared to older leaves were associated with lower ozone sensitivities in these tissues. 相似文献
3.
Ethylenediurea (EDU) was applied as soil drench in two Indian cultivars of black gram ( Vigna mungo L. cv. Azad-1 and BHU-1) and its ameliorating effect against ozone (O 3) stress was studied on selected growth, physiological, biochemical, and yield characteristics. The study site experienced
a high O 3 concentration of 41.3–59.9 ppb during the experimental period. It was found that growth parameters showed positive impact
on plants treated with EDU and yield attributes were also higher than that of non-EDU-treated ones. Significant increments
in ascorbic acid and protein contents were observed in EDU-treated plants as compared to control plants. Lipid peroxidation,
however, showed a reverse trend in both the cultivars. Photosynthetic efficiency increased by EDU treatment as depicted by
higher values for photosynthetic rate (Ps) and Fv/Fm ratio. EDU-treated plants had more efficient antioxidant enzyme defense
system with higher SOD and POX activities. Both the cultivars showed differential response against O 3 and cultivar BHU-1 proved to be resistant as compared to Azad-1. This investigation proves the usefulness of EDU as a biomonitoring
tool against O 3 for the remote areas having higher concentrations of O 3 and problem of frequent electricity failure. 相似文献
4.
2'', 3'', 5''-tri- O-acetyl- N6-(3-hydroxyphenyl) adenosine (also known as WS070117) is a new adenosine analog that displays anti-hyperlipidemic activity both in vitro and in vivo experiments as shown in many preliminary studies. Due to its new structure, little is known about the metabolism of WS070117. Hence, the in vivo metabolites of WS070117 in rat urine following oral administration were investigated. Identification of the metabolites was conducted using the combination of high-performance liquid chromatography (HPLC) coupled with diode array detector (DAD), ion trap electrospray ionization-mass spectrometry (ESI-MS), and off-line microprobe nuclear magnetic resonance (NMR) measurements. Seven metabolites were obtained as pure compounds at the sub-milligram to milligram levels. Results of structure elucidation unambiguously revealed that the phase I metabolite, N6-(3-hydroxyphenyl) adenosine (M8), was a hydrolysate of WS070117 by hydrolysis on the three ester groups. N6-(3-hydr-oxyphenyl) adenine (M7), also one of the phase I metabolites, was the derivative of M8 by the loss of ribofuranose. In addition to two phase I metabolites, there were five phase II metabolites of WS070117 found in rat urine. 8-hydroxy- N6-(3-hydroxy-phenyl) adenosine (M6) was the product of M7 by hydrolysis at position 8. The other four were elucidated to be N6-(3- O-β-D-glucuronyphenyl) adenine (M2), N8-hydroxy- N6-(3- O-sulfophenyl) adenine (M3), N6-(3- O-β-D-glucuronyphenyl) adenosine (M4), and N6-(3- O- sulfophenyl) adenosine (M5). Phase II metabolic pathways were proven to consist of hydroxylation, glucuronidation and sulfation. This study provides new and valuable information on the metabolism of WS070117, and also demonstrates the HPLC/MS/off-line microprobe NMR approach as a robust means for rapid identification of metabolites. 相似文献
5.
Background and aims Elevated atmospheric CO 2 ( eCO 2) and tropospheric O 3 ( eO 3) can alter soil microbial processes, including those underlying N 2O emissions, as an indirect result of changes in plant inputs. In this study, effects of eCO 2 and eO 3 on sources of N 2O in a soybean ( Glycine max (L.) Merr.) agroecosystem in Illinois (SoyFACE) were investigated. We hypothesized that increases in available C and anaerobic microhabitat under eCO 2 would stimulate N 2O emissions, with a proportionally larger increase in denitrification derived N 2O (N 2O D) compared to nitrification plus nitrifier denitrification derived N 2O (N 2O N+ND). We expected opposite effects under eO 3. Methods Isotopically labeled 15NH 4 14 NO 3 and 14NH 4 15 NO 3 were used to evaluate mineral N transformations, N 2O D, and N 2O N+ND in a 12-day incubation experiment. Results We observed minimal effects of eCO 2 and eO 3 on N 2O emissions, movement of 15?N through mineral N pools, soil moisture content and C availability. Possibly, altered C and N inputs by eCO 2 and eO 3 were small relative to the high soil organic C content and N-inputs via biological N 2-fixation, minimizing potential effects of eCO 2 and eO 3 on N-cycling. Conclusion We conclude that eCO 2 and eO 3 did not affect N 2O emissions in the short term. However, it remains to be tested whether N 2O emissions in SoyFACE will be unaltered by eCO 2 and eO 3 on a larger temporal scale under field conditions. 相似文献
6.
An Egyptian cultivar of potato ( Solanum tuberosum L. cv. Kara) was grown in the field at two locations in northern Egypt: a ‘rural’ and a ‘suburban’ site, from October 2000 and November 2002. The antiozonant ethylenediurea (EDU) and the fungicide chlorothalonil (1,3‐benzenedicarbonitrile‐2,4,5,6 tetrachloroisophthalnitrile) were applied as a foliar spray to plants at both sites. It was found that foliar injury symptoms were reduced greatly in plants treated with EDU and/or chlorothalonil, and the yield of treated plants was higher than that of the untreated ones, with the EDU having a greater protection than chlorothalonil. Antiozonant (EDU) and fungicide (chlorothalonil) combination sprays were even more effective in reducing O 3 injury. Moreover, the percentage of protection was higher in the rural area than in the suburban one, and this was associated with higher levels of O 3 recorded in the rural area. The response to O 3, EDU, and chlorothalonil of the leaf antioxidant scavenger system was examined. Antiozonant‐treated plants had the highest reduced glutathione/oxidised glutathione ratio. The results suggest that EDU and chlorothalonil do not act directly as antiozonant to inhibit O 3 injury but act through maintaining some antioxidant enzymes during O 3 exposure. To the best of knowledge, this is the first report demonstrating the marked enhancement of yield and plant oxidative enzymes by fungicides as a mechanism of protecting plants against noxious oxidative stress from the environment in the developing world. 相似文献
7.
The nonheterocystous filamentous cyanobacterial genus Lyngbya is a widespread and frequently dominant component of marine microbial mats. It is suspected of contributing to relatively high rates of N 2 fixation associated with mats. The ability to contemporaneously conduct O 2-sensitive N 2 fixation and oxygenic photosynthesis was investigated in Lyngbya aestuarii isolates from a North Carolina intertidal mat. Short-term (<4-h) additions of the photosystem II (O 2 evolution) inhibitor 3(3,4-dichlorophenyl)-1,1-dimethylurea stimulated light-mediated N 2 fixation (nitrogenase activity), indicating potential inhibition of N 2 fixation by O 2 production. However, some degree of light-mediated N 2 fixation in the absence of 3(3,4-dichlorophenyl)-1,1-dimethylurea was observed. Electron microscopic immunocytochemical localization of nitrogenase, coupled to microautoradiographic studies of 14CO 2 fixation and cellular deposition of the tetrazolium salt 2,4,5-triphenyltetrazolium chloride, revealed that (i) nitrogenase was widely distributed throughout individual filaments during illuminated and dark periods, (ii) 14CO 2 fixation was most active in intercalary regions, and (iii) daylight 2,4,5-triphenyltetrazolium chloride reduction (formazan deposition) was most intense in terminal regions. Results suggest lateral partitioning of photosynthesis and N 2 fixation during illumination, with N 2 fixation being confined to terminal regions. During darkness, a larger share of the filament appears capable of N 2 fixation. 相似文献
8.
The cytokinin-like activity of the growth regulating chemical EDU, N-[2-(2-oxo-1-imidazolidinyl)ethyl]-N'-phenylurea, was determined and compared with the actitivity of kinetin using the tobacco callus bioassay. EDU has a pronounced stimulatory effect on callus growth at concentrations of 5 × 10 −4 and 1 × 10 −3 M but was 5 000 times less potent than the synthetic cytokinin, kinetin. Senescence regulation and oxidant resistance induced by EDU and kinetin were also studied. EDU retarded the breakdown of chlorophyll, protein and RNA in 0 3-sensitive tobacco leaf discs during senescence. EDU was much more effective in arresting senescence and in protecting against 0 3 injury than kinetin. Results indicate the EDU-induced plant tolerance to 0 3 phytotoxicity may be indirect through enzyme induction regulation. 相似文献
9.
Increase in surface level of ozone (O 3) in last 30 years is one of the major problems for global agriculture. Field experiment was conducted using open top chambers on two Indian maize cultivars (Buland and Prakash) grown under ambient (AO) and elevated (EO) O 3 concentrations to evaluate the effect of an antiozonant ethylene diurea (EDU) given as soil drench. EDU application reduced the ROS production with concomitant decrease in lipid peroxidation. Inductions in activities of enzymatic antioxidants along with increased content of non-enzymatic antioxidants were observed in EDU-treated plants, though the response varied between the cultivars. Photosynthetic proteins (PEP carboxylase and RuBisCO large and small subunits) detected through SDS–PAGE analysis increased with EDU treatment. EDU also led to an increase in jasmonic acid and a decline in salicylic acid contents. The protective effect of EDU was further accompanied by increased pigments (chlorophyll and carotenoids), foliar carbohydrates (starch and total soluble sugars), enhanced biomass, and economic yield. Effectiveness of EDU was more evident at higher O 3 concentration and cultivar Prakash exhibited a more positive response with EDU as compared to Buland. 相似文献
10.
Tazarotene-induced protein 3 (TIG3) and HRAS-like suppressor family 2 (HRASLS2) exhibit tumor-suppressing activities and belong to the lecithin retinol acyltransferase (LRAT) protein family. Since Ca 2+-independent N-acyltransferase and H-rev107 (another tumor suppressor), both of which are members of the LRAT family, have been recently reported to possess catalytic activities related to phospholipid metabolism, we examined possible enzyme activities of human TIG3 and HRASLS2 together with human H-rev107. The purified recombinant proteins of TIG3, HRASLS2, and H-rev107 functioned as phospholipase (PL) A 1/2 in a Ca 2+-independent manner with maximal activities of 0.53, 0.67, and 2.57 μmol/min/mg of protein, respectively. The proteins were active with various phosphatidylcholines (PCs) and phosphatidylethanolamines (PEs), and for most of substrates the PLA 1 activity was much higher than the PLA 2 activity. In addition, HRASLS2 catalyzed N-acylation of PE to form N-acyl-PE and O-acylation of lyso PC to form PC. TIG3 and H-rev107 catalyzed the N-acylation and O-acylation at relatively low rates. Moreover, these three proteins showed different expression profiles in human tissues. These results suggest that the tumor suppressors TIG3, HRASLS2 and H-rev107 are involved in the phospholipid metabolism with different physiological roles. 相似文献
11.
The postillumination transient of CO 2 exchange and its relation to photorespiration has been examined in leaf discs from tobacco ( Nicotiana tabacum) and maize ( Zea mays). Studies of the transients observed by infrared gas analysis at 1, 21, and 43% O 2 in an open system were extended using the nonsteady state model described previously (Peterson and Ferrandino 1984 Plant Physiol 76: 976-978). Cumulative CO 2 exchange equivalents ( i.e. nanomoles CO 2) versus time were derived from the analyzer responses of individual transients. In tobacco (C 3), subtraction of the time course of cumulative CO 2 exchange under photorespiratory conditions (21 or 43% O 2) from that obtained under nonphotorespiratory conditions (1% O 2) revealed the presence of an O 2-dependent and CO 2-reversible component within the first 60 seconds following darkening. This component was absent in maize (C 4) and at low external O 2:CO 2 ratios ( i.e. <100) in tobacco. The size of the component in tobacco increased with net photosynthesis as irradiance was increased and was positively associated with inhibition of net photosynthesis by O 2. This relatively simple and rapid method of analysis of the transient is introduced to eliminate some uncertainties associated with estimation of photorespiration based on the maximal rate of postillumination CO 2 evolution. This method also provides a useful and complementary tool for detecting variation in photorespiration. 相似文献
12.
Factors affecting nitrate reduction by leaf discs of cotton ( Gossypium hirsutum L.) were investigated. When incubated in 30 m m nitrate, discs reduced nitrate much more slowly under air or O 2 than under N 2. Inhibition by O 2 did not occur at nitrate levels of 100 m m or greater. Treatment with arsenate had little effect under N 2 but stimulated nitrate reduction under air. Similarly, ammonium inhibited nitrate reduction, with the inhibition being partially relieved by arsenate. Uptake of nitrate was unaffected by ammonium. The NAD/NADH ratio increased in response to both oxygen and ammonium. The effects of these treatments on nitrate reduction can be explained by competition with nitrate for NADH generated by glycolysis. 相似文献
13.
Treatment of methyl 4,6- O-benzylidene-α-D-mannopyranoside with dichloromethylenedimethylammonium chloride gave methyl 4,6- O-benzylidene-3-chloro-3-deoxy-2-( N, N-dimethylcarbamoyl)-α-D-altropyranoside and methyl 4,6- O-benzy]idene-2-chloro-2-deoxy-3-( N, N-dimethylcarbamoyl)-α-D-glucopyranoside. Methyl 4,6- O-benzylidene-α-D-allopyranoside gave under analogous conditions the corresponding 2-chloro-3-( N, N-dimethylcarbamoyl)-α-D-altrose and 3-chloro-2-( N, N-dimethylcarbamoyl)-α-D-glucose derivatives. Methyl 5- O-benzyl-α,β-D-ribofuranoside and methyl 5- O-methyl-β-D-ribofuranoside gave only the corresponding methyl 3-chloro-2-( N, N-dimethylcarbamoyl)-α-D-xylofuranoside derivatives. 相似文献
14.
O-α-d-Mannopyranosyl-(1→6)- O-(2-acetamido-2-deoxy-β-d-glucopyranosyl)-(1→4)-2-acetamido- N-(l-aspart-4-oyl)-2-deoxy-β-d-glucopyranosylamine ( 12), used in the synthesis of glycopeptides and as a reference compound in the structure elucidation of glycoproteins, was synthesized via condensation of 2,3,4,6-tetra- O-acetyl-α-d-mannopyranosyl bromide with 2-acetamido-4- O-(2-acetamido-3- O-acetyl-2-deoxy-β-d-glucopyranosyl)-3,6-di- O-acetyl-2-deoxy-β-d-glucopyranosyl azide ( 5) to give the intermediate, trisaccharide azide 7. [Compound 5 was obtained from the known 2-acetamido-4- O-(2-acetamido-3,4,6-tri- O-acetyl-2-deoxy-β-d-glucopyranosyl)-3,6-di- O-acetyl-2-deoxy-β-d-glucopyranosyl azide by de- O-acetylation, condensation with benzaldehyde, acetylation, and removal of the benzylidene group.] The trisaccharide azide 6 was then acetylated, and the acetate reduced in the presence of Adams' catalyst. The resulting amine was condensed with 1-benzyl N-(benzyloxycarbonyl)-l-aspartate, and the O-acetyl, N-(benzyloxycarbonyl), and benzyl protective groups were removed, to give the title compound. 相似文献
15.
Di- O-(6-azidohexanoyl)-xylan- graft-poly(l-lactide)s (XylC6N 3- g-PLLAs) were prepared by grafting propargyl-terminated poly(l-lactide) onto di- O-(6-azidohexanoyl)-xylan (XylC6N 3) via click chemistry. Di- O-(6-azidohexanoyl)-xylan (XylC6N 3) was prepared via two steps from xylan extracted from eucalyptus kraft pulp with aqueous sodium hydroxide solution. Propargyl-terminated poly(l-lactide)s (PLLA) with three different molecular weights were synthesized via ring-opening polymerization of l-lactide using propargyl alcohol as initiator and tin (II) octanoate (Sn(Oct) 2) as catalyst. XylC6N 3 and propargyl-terminated PLLAs were treated with N, N, N′, N′, N″-pentamethyldiethylenetriamine (PMDETA) and copper(I) bromide, and the graft copolymers XylC6N 3- g-PLLAs were obtained. DSC measurements revealed that the glass transition temperatures ( Tg) of the copolymers decreased compared to that of XylC6N 3, suggesting that the grafted PLLA side-chains act as an internal plasticizer for xylan. TGA measurements revealed that XylC6N 3- g-PLLAs had higher decomposition temperatures than those of XylC6N 3 or PLLA, and that the decomposition temperatures of the copolymers increased with decrease in the number of PLLA side-chains grafted to the xylan main-chain. 相似文献
16.
The effect of ethylenediurea (EDU) was tested using the chlorophyll (Chl) a fluorescence transient analysis, performed with JIP-test, to assess ambient ozone (O 3) effects on photosynthesis of adult trees under natural conditions. Twelve adult European ash ( Fraxinus excelsior L.) trees, known to be sensitive or tolerant to O 3, determined by presence symptomatic (S) or absence asymptomatic (AS) trees of foliar symptoms in previous years, were treated
either with distilled water containing 450 g m −3 EDU or with distilled water. Once a month across the growing season [the accumulated exposure over a threshold of 40 nmol(O 3) mol −1 was 32.49 μmol mol −1 h −1], Chl a fluorescence transients were measured in vivo on dark-adapted leaves of 1-year-old labeled shoots, from the lower crown part. Twenty-five parameters were calculated. The
maximum quantum yield of primary photochemistry (ϕ Po or F v/F m) did not differentiate between S-and AS-trees, while increased Chl content and de-excitation rates suggested compensation
of O 3 injury in S-trees. Seasonal reductions in absorbing fluxes and increase in heat and fluorescence dissipation processes was
due to leaf ageing and drought, the latter suggesting water deficit influenced Chl a fluorescence stronger than ambient O 3 exposure. AS-trees showed elevated probability of connectivity among photosystem 2 units, a mechanism to stimulate energy
dissipation and reduce photo-oxidative injury. EDU prevented the inactivation of reaction centers. This slight effect does
not warrant EDU as a tool to assess O 3 effects on photosynthesis, while the JIP-test is suggested for a quantitative assessment in adult trees. 相似文献
17.
The ability of microorganisms in sediments from the Atlantic Coastal Plain to biodegrade methoxylated aromatic compounds was examined. O-demethylation activity was detected in deep (121- and 406-m) sediments, as well as in the surface soil. A syringate-demethylating consortium, containing at least three types of bacteria, was enriched from a deep-sediment sample in a medium containing syringate as the sole organic carbon source and with a N 2-CO 2 atmosphere. An isolate which demethylated syringate was obtained from the enrichment on an agar medium incubated under a H 2-CO 2 but not a N 2-CO 2 or N 2 atmosphere. O demethylation of syringate of this isolate was dependent on the presence of both H 2 and CO 2 in the gas phase. The metabolism of syringate occurred in a sequential manner: methylgallate accumulated transiently before it was converted to gallate. Mass balance analysis suggests that the stoichiometry of the reaction in this isolate proceeds in accordance with the following generalized equation: C 7H 3O 3(OCH 3) n- + nHCO 3- + nH 2 → C 7H 3O 3(OH) n- + nCH 3COO - + nH 2O. 相似文献
18.
The addition of glyoxylate to tobacco ( Nicotiana tabacum) leaf discs inhibited glycolate synthesis and photorespiration and increased net photosynthetic 14CO 2 fixation. This inhibition of photorespiration was investigated further by studying the effect of glyoxylate on the stimulation of photosynthesis that occurs when the atmospheric O 2 level was decreased from 21 to 3% (the Warburg effect). The Warburg effect is usually ascribed to the increased glycolate synthesis and metabolism that occurs at higher O 2 concentrations. Photosynthesis in control discs increased from 59.1 to 94.7 micromoles of CO 2 per gram fresh weight per hour (a 60% increase) when the O 2 level was lowered from 21 to 3%, while the rate for discs floated on 15 millimolar glyoxylate increased only from 82.0 to 99.7 micromoles of CO 2 per gram fresh weight per hour (a 22% increase). The decrease in the O 2 sensitivity of photosynthesis in the presence of glyoxylate was explained by changes in the rate of glycolate synthesis under the same conditions. The rate of metabolism of the added glyoxylate by tobacco leaf discs was about 1.35 micromoles per gram fresh weight per hour and was not dependent on the O2 concentration in the atmosphere. This rate of metabolism is about 10% the amount of stimulation in the rate of CO2 fixation caused by the glyoxylate treatment on a molar carbon basis. Glyoxylate (10 millimolar) had no effect on the carboxylase/oxygenase activity of isolated ribulose diphosphate carboxylase. Although the biochemical mechanism by which glyoxylate inhibits glycolate synthesis and photorespiration and thereby decreases the Warburg effect is still uncertain, these results show that cellular metabolites can regulate the extent of the Warburg effect. 相似文献
19.
A tetranuclear copper(II) complex [Cu 4L 2(CH 3COO) 2(OH) 2]·6H 2O, in which L stands for the dianion of N-(3-carboxylsalicylidene)-4-(2-iminoethyl)morpholine, was synthesized and characterized by elemental analysis, IR, UV-Vis, TGA and X-ray single crystal diffraction. The crystal structure shows that the coordination unit is centrosymmetric with all the Cu(II) ions in square pyramidal coordination geometry. The coordination unit consists of two equivalent parts [Cu 2L(CH 3COO)(OH)], each containing two Cu(II) ions, a tetradentate N 2O 2 Schiff base dianion L 2−, a CH 3COO −, and a OH − anion. In [Cu 2L(CH 3COO)(OH)], the six coordination atoms (N 2O 4) are nearly coplanar, with Cu(1) and Cu(2) enchased in between; the phenolate oxygen and the OH − oxygen as bridging atoms bind the two Cu(II) ions in close proximity; both O 4 around Cu(1) and N 2O 2 around Cu(2) form the basal plane of the coordination square pyramids. The two parts are connected by sharing two μ3-OH − oxygens and two μ2-CH 3COO − oxygens from each other, forming four edge-sharing coordination square pyramids around the four Cu(II) ions. A 3D network is formed through hydrogen bonding along a and c axis, and π- π interaction along b axis. 相似文献
20.
Nitrogenase (EC 1.7.99.2) activity (acetylene reduction) and nitrogen fixation ( 15N 2 fixation) were measured in cyanobacteria freshly isolated from the coralloid roots of Macrozamia riedlei (Fisch. ex Gaud.) Gardn. Light and gas phase oxygen concentration had marked interactive effects on activity, with higher (up to 100-fold) rates of acetylene reduction and 15N 2 fixation in light. The relationship between ethylene formation and N 2-fixation varied in the freshly isolated cyanobacteria from 4 to 7 nanomoles of C 2H 4 per nanomole 15N 2. Intact coralloid roots, incubated in darkness and ambient air, showed a value of 4.3. Maximum rates of nitrogenase activity occurred at about 0.6% O 2 in light, while in darkness there was a broad optimum around 5 to 8% O 2. Inhibition of nitrogenase, in light, by pO 2 above 0.6% was irreversible. Measurements of light-dependent O 2 evolution and 14CO 2 fixation indicated negligible photosynthetic electron transport involving photosystem II and, on the basis of inhibitor studies, the stimulatory effect of light was attributed to cyclic photophos-phorylation. Nitrogenase activity of free-living culture of an isolate from Macrozamia ( Nostoc PCC 73102) was only slightly inhibited by O 2 levels above 6% O 2 and the inhibition was reversible. These cells showed rates of light-dependent O 2 evolution and 14CO 2 fixation which were 100- to 200-fold higher than those by the freshly isolated symbiont. Furthermore, nitrogenase activity was dependent on both photosynthetic electron transport and photophosphorylation. These data indicate that cyanobacteria within cycad coralloid roots are differentiated specifically for symbiotic functioning in a microaerobic environment. Specializations include a high heterocyst frequency, enhanced permeability to O 2, and a direct dependence on the cycad for substrates to support nitrogenase activity. 相似文献
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