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1.
The CO 2-exchange rate required to make full use of available N 2-fixation capacity, measured as acetylene reduction, was determined in soybean and alfalfa. Carbohydrates of root systems were depleted during a 40-hour dark treatment; then plants were exposed to a 24-hour light period during which different CO 2-exchange rates were maintained with various CO 2 concentrations. In three- and four-week-old soybeans and four-week-old alfalfa plants, acetylene-reduction capacity was used fully with CO 2-exchange rates as low as 10 milligrams CO 2 per plant per hour. In six-week-old alfalfa plants, however, acetylene reduction rates increased linearly, and apparent N 2-fixation capacity was not used fully when CO 2-exchange rates were higher than 40 milligrams CO 2 per plant per hour. Under the conditions established, the energy cost of N 2 fixation, measured as Δ(respiration of roots + nodules)/Δacetylene reduction over dark-treatment values, was 0.453 milligrams CO 2 per micromole C 2H 4 for all rates of acetylene reduction and for both ages of soybean and alfalfa plants. Thus, root-plus-nodule respiration was not promoted by higher rates of apparent photosynthesis after C 2H 2-reduction capacity became saturated, and all available capacity for apparent N 2 fixation had the same energy requirement. 相似文献
2.
The main goal of this study was to test the effect of [CO 2] on C and N management in different plant organs (shoots, roots and nodules) and its implication in the responsiveness of exclusively N 2-fixing and NO 3−-fed plants. For this purpose, exclusively N 2-fixing and NO 3−-fed (10 mM) pea ( Pisum sativum L.) plants were exposed to elevated [CO 2] (1000 μmol mol −1 versus 360 μmol mol −1 CO 2). Gas exchange analyses, together with carbohydrate, nitrogen, total soluble proteins and amino acids were determined in leaves, roots and nodules. The data obtained revealed that although exposure to elevated [CO 2] increased total dry mass (DM) in both N treatments, photosynthetic activity was down-regulated in NO 3−-fed plants, whereas N 2-fixing plants were capable of maintaining enhanced photosynthetic rates under elevated [CO 2]. In the case of N 2-fixing plants, the enhanced C sink strength of nodules enabled the avoidance of harmful leaf carbohydrate build up. On the other hand, in NO 3−-fed plants, elevated [CO 2] caused a large increase in sucrose and starch. The increase in root DM did not contribute to stimulation of C sinks in these plants. Although N 2 fixation matched plant N requirements with the consequent increase in photosynthetic rates, in NO 3−-fed plants, exposure to elevated [CO 2] negatively affected N assimilation with the consequent photosynthetic down-regulation. 相似文献
3.
Corn ( Zea mays L.) plants were assayed for nitrogenase activity (C 2H 2 reduction) during early ear development. Hybrid corn and inbred lines were grown separately at two experimental fields in New Jersey. Acetylene-dependent ethylene production was observed a few hours after harvest, from the field, on intact plants, root-soil cores, lower stem segments, and excised roots, all assayed under air and not preincubated previously. Incubation of excised roots at 1% O 2 resulted in lower rates of C 2H 2 reduction. The time course of C 2H 2 reduction by excised roots, assayed in air, was similar for all genotypes studied (two hybrids, eight inbreds, and a cross of corn × teosinte) and indicated that a long preincubation at reduced O 2 is not absolutely required for early detection of nitrogenase activity. Isolation of N 2-fixing bacteria from within the roots and stems, together with the diurnal fluctuation of nitrogenase activity in response to day/night cycles, were indicative of a close association with plant function. Collectively, the results provided strong evidence for the occurrence of nitrogenase activity associated with corn plants growing in a temperate climate and dependent upon indigenous N 2-fixing bacteria. 相似文献
4.
Nodulated and non-nodulated (not inoculated) soybeans ( Glycine max [L.] Merr. cv Wells) were grown in controlled environments with N 2 or nonlimiting levels of NO 3−, respectively, serving as sole source of nitrogen. The efficiency of the N 2-fixing plants was compared with that of the nitrate-supplied plants on the basis of both plant age and plant size. Efficiency evaluations of the plants were expressed as the ratio of moles of carbon respired by the whole plant to the moles of nitrogen incorporated into plant material. Continuous 24-hour CO2 exchange measurements on shoot and root systems made at the beginning of flowering (28 days after planting) indicated that N2-fixing plants respired 8.28 moles of carbon per mole of N, fixed from dinitrogen, while nitrate-supplied plants respired only 4.99 moles of carbon per mole of nitrate reduced. Twenty-one-day-old nitrate-supplied plants were even more efficient, respiring only 3.18 moles of carbon per mole of nitrate reduced. The decreased efficiency of the N2-fixing plants was not due to plant size since, on a dry weight basis, the 28-day-old N2-fixing plants were intermediate between the 28- and 21-day-old nitrate-supplied plants. The calculated efficiencies were predominantly a reflection of root-system respiration. N2-fixing plants lost 25% of their daily net photosynthetic input of carbon through root-system respiration, compared with 16% for 28-day-old nitrate-supplied plants and 12% for 21-day-old nitrate-supplied plants. Shoot dark respiration was similar for all three plant groups, varying between 7.9% and 9.0% of the apparent photosynthate. The increased respiratory loss by the roots of the N2-fixing plants was not compensated for by increased net photosynthetic effectiveness. Canopy photosynthesis expressed on a leaf area basis was similar for 28-day-old N2-fixing plants (15.5 milligrams CO2 square decimeter per hour) and 21-day-old nitrate-supplied plants (14.5 milligrams CO2 square decimeter per hour). Both were similar in total canopy leaf area. The larger nitrate-supplied plants (28-day-old) had lower photosynthetic rates (12.5 milligrams CO2 square decimeter per hour), presumably due to self-shading of the leaves. These data indicate that, during the early stages of plant development, dependence solely on N2-fixation is an expensive process compared to nitrate reduction in nitrate-supplied plants, since the N2-fixing plants retained 8% to 12% less of their photosynthate as dry matter. 相似文献
5.
A comparison was made of energy metabolism of nodulated N 2 fixing plants and non-nodulated NO 3-fed plants of Lupinus albus L. Growth, N-increment, root respiration (O 2 uptake and CO 2 production) and the contribution of a SHAM-sensitive oxidative pathway (the alternative pathway) in root respiration were measured. Both growth rate and the rate of N-increment were the same in both series of plants. The rate of root respiration, both O 2 uptake and CO 2 production, and the activity of the SHAM-sensitive pathway were higher in NO 3-fed plants than in N 2 fixing plants. The rate of ATP production in oxidative phosphorylation was computed also to be higher in NO 3-fed plants. It is concluded that both carbohydrate costings and ATP costings for synthesis + maintenance of root material were lower in N 2 fixing than in NO 3-fed plants. The respiratory quotient of root respiration was 1.6 in N 2-fixing plants and 1.4 in NO 3-fed plants. These values were slightly higher than the values calculated on the basis of CO 2 output due to N-assimilation and the experimental values of O 2 uptake, but showed the same trend: highest in N 2 fixing plants. Root respiration of NO 3-fed plants showed a diurnal pattern (both O 2 uptake, CO 2 production and the activity of the SHAM-sensitive pathway), whilst no diurnal variation in root respiration was found in N 2 fixing plants. However, C 2H 2 reduction did show a diurnal rhythm, which is suggested to be related to the diurnal variation in transpiration. Addition of NO 3 to N 2 fixing plants increased the rate of root respiration and the activity of the alternative pathway. This treatment did not decrease C 2H 2 reduction and H 2 evolution within 4 days. Withdrawal of NO 3-supply from NO 3-fed plants decreased the rate of root respiration but had no effect on the relative activity of the alternative pathway. It is suggested that the higher rate of root respiration and the higher activity of the SHAM-sensitive pathway in NO 3-fed plants is due to a larger supply of carbohydrates to the roots, partly due to a better photosynthetic performance of the shoots and partly due to a higher capacity of the roots to attract carbohydrates. 相似文献
6.
The effect of photosynthetic photon flux density (PPFD) on nitrogen utilization was determined in peas (Pisum sativum L. cv. Alaska) inoculated with Rhizobium leguminosarum and treated with nutrient solutions containing no combined nitrogen, 16 m M NO 3?, or 16 m M NH 4+. Plants were grown under controlled conditions at three PPFD values ranging from severely limiting to nearly saturating. Carboxylation efficiencies and CO 2-exchange rates were highest in the N 2-fixing plants and lowest in plants supplied with NH 4+, and they generally increased with increasing PPFD. Photoefficiencies increased with PPFD but did not differ appreciably with the form of nitrogen applied. Nitrogen fixation, calculated from C 2H 2-reduction and H 2-evolution data, was inhibited more by NH 4+ than by NO 3?application. Inhibition was counteracted by increasing PPFD. Percentage nitrogen decreased with increasing PPFD in plants treated with combined nitrogen and increased in the plants dependent on N 2 fixation. The data reveal that photosynthetic efficiency and the capacity to fix N 2 in peas are functions of PPFD and the availability of combined nitrogen and that these two factors are interrelated. 相似文献
7.
Soil-N (NO 3 ?) initiates as far as a threshold concentration is surpassed manifold physiological reactions on N 2-fixation. Organic N and ammonium oxidised to NO 3 ? means oxygen depletion. Plants suffering under O 2 or infection stress start to excrete ethylene (C 2H 4). C 2H 4 widens the root intercellulars that O 2-respiration will continue. Now microbes may more easily enter the plant interior by transforming the reached methionine into C 2H 4. Surplus nitrate and C 2H 4 inhibit nodulation of leguminous plants. Excess NO 3 ? in the nodulesphere could be diminished by N 2-fixing bacteria which in addition can denitrify or ammonify nitrate. Consequently, it was asked whether C 2H 4 interferes with the potential of N 2-fixing bacteria to reduce nitrate. The groundnut-nodule isolate TNAU 14, from which it was known that it denitrifies and ammonifies nitrate, served as inoculum of a KNO 3-mannitol-medium that was incubated under N 2-, 1% (v/v) N 2?C 2H 4-, and 1% (v/v) N 2?C 2H 2-atmosphere in the laboratory. C 2H 2 was included into the experiments because it is frequently used to quantify N 2-fixing potentials (acetylene reduction array, ARA). Gene-16S rDNA-sequencing and physiological tests revealed a high affiliation of strain TNAU 14 to Rhizobium radiobacter and Rhizobium tumefaciens. Strain TNAU 14 released N 2O into the bottle headspace in all treatments, surprisingly significantly less in presence of C 2H 2. Nitrate-ammonification was even completely blocked by C 2H 2. C 2H 4, in contrast rather stimulated growth, denitrification, and nitrate-ammonification of strain TNAU 14 which consumed the released NH 4 + during continuing incubation. 相似文献
8.
Soybean plants ( Glycine max [L.] Merr. cv Williams), which were symbiotic with Bradyrhizobium japonicum, and which grew well upon reduced nitrogen supplied solely through N 2 fixation processes, often exhibited excess accumulation of starch and sucrose and diminished soluble protein in their source leaves. Nitrate and ammonia, when supplied to the nodulated roots of N 2-fixing plants, mediated a reduction of foliar starch accumulation and a corresponding increase in soluble protein in the source leaves. This provided an opportunity to examine the potential metabolic adjustments by which NO 3− and NH 4+ (N) sufficiency or deficiency exerted an influence upon soybean leaf starch synthesis. When compared with soybean plants supplied with N, elevated starch accumulation was focused in leaf palisade parenchyma tissue of N 2-fixing plants. Foliar activities of starch synthesis pathway enzymes including fructose-1,6-bisphosphate phosphatase, phosphohexoisomerase, phosphoglucomutase (PGM), as well as adenosine diphosphate glucose pyrophosphorylase (in some leaves) exhibited highest activities in leaf extracts of N 2-fixing plants when expressed on a leaf protein basis. This was interpreted to mean that there was an adaptation of these enzyme activities in the leaves of N 2-fixing plants, and this contributed to an increase in starch accumulation. Another major causal factor associated with increased starch accumulation was the elevation in foliar levels of fructose-6-phosphate, glucose-6-phosphate, and glucose-1-phosphate (G1P), which had risen to chloroplast concentrations considerably in excess of the Km values for their respective target enzymes associated with starch synthesis, e.g. elevated G1P with respect to adenosine diphosphate glucose pyrophosphorylase (ADPG-PPiase) binding sites. The cofactor glucose-1,6-bisphosphate (G1,6BP) was found to be obligate for maximal PGM activity in soybean leaf extracts of N 2-fixing as well as N-supplemented plants, and G1,6BP levels in N 2-fixing plant leaves was twice that of levels in N-supplied treatments. However the concentration of chloroplastic G1,6BP in illuminated leaves was computed to be saturating with respect to PGM in both N 2-fixing and N-supplemented plants. This suggested that the higher level of this cofactor in N 2-fixing plant leaves did not confer any higher PGM activation and was not a factor in higher starch synthesis rates. Relative to plants supplied with NO 3− and NH 4+, the source leaf glycerate-3-phosphate (3-PGA) and orthophosphate (Pi) concentrations in leaves of N 2-fixing plants were two to four times higher. Although Pi is a physiological competitive inhibitor of leaf chloroplast ADPG-PPiase, and hence, starch synthesis, elevated chloroplast 3-PGA levels in N 2-fixing plant leaves apparently prevented interference of Pi with ADPG-PPiase catalysis and starch synthesis. 相似文献
9.
The N 2-fixing legume nodule requires O 2 for ATP production; however, the O 2 sensitivity of nitrogenase dictates a requirement for a low pO 2 inside the nodule. The effects of long term exposures to various pO 2s on N 2[ C2H2] fixation were evaluated with intact soybean ( Glycine max [L.] Merr., var. Wye) plants. Continuous exposure of their rhizosphere to a pO 2 of 0.06 atmospheres initially reduced nitrogenase activity by 37 to 45% with restoration of original activity in 4 to 24 hours and with no further change in tests up to 95 hours; continuous exposure to 0.02 atmosphere of O 2 initially reduced nitrogenase activity 72%, with only partial recovery by 95 hours. Similar exposures to a pO 2 of 0.32 atmospheres had little effect on N 2[ C2H2] fixation; a pO 2 of 0.89 atmospheres initially reduced nitrogenase activity by 98% with restoration to only 14 to 24% of that of the ambient O 2 controls by 95 hours. Re-exposure to ambient pO 2 of plants adapted to nonambient pO 2s reduced N 2[ C2H2] fixation to similar magnitudes as the reductions which occurred upon initial exposure to variant pO 2 conditions, and a time period was required to readapt to ambient O 2. It is concluded that the N 2[ C2H2]-fixing system of intact soybean plants is able to adapt to a wide range of external pO 2s as probably occur in soil. We postulate that this occurs through an undefined mechanism which enables the nodule to maintain an internal pO 2 optimal for nitrogenase activity. 相似文献
10.
The effect of short- and long-term changes in shoot carbon-exchange rate (CER) on soybean ( Glycine max [L.] Merr.) root nodule activity was assessed to determine whether increases in photosynthate production produce a direct enhancement of symbiotic N 2 fixation. Shoot CER, root + nodule respiration, and apparent N 2 fixation (acetylene reduction) were measured on intact soybean plants grown at 700 microeinsteins per meter per second, with constant root temperature and a 14/10-hour light/dark cycle. There was no diurnal variation of root + nodule respiration or apparent N 2 fixation in plants assayed weekly from 14 to 43 days after planting. However, if plants remained in darkness following their normal dark period, a significant decline in apparent N 2 fixation was measured within 4 hours, and decreasing CO 2 concentration from 320 to 90 microliters CO 2 per liter produced diurnal changes in root nodule activity. Increasing shoot CER by 87, 84, and 76% in 2-, 3-, and 4-week-old plants, respectively, by raising the CO 2 concentration around the shoot from 320 to 1,000 microliters CO 2 per liter, had no effect on root + nodule respiration or acetylene-reduction rates during the first 10 hours of the increased CER treatment. When the CO 2-enrichment treatment was extended in 3-week-old plants, the only measured parameter that differed significantly after 3 days was shoot CER. After 5 days of continuous CO 2 enrichment, root + nodule respiration and acetylene reduction increased, but such changes reflected an increase in root nodule mass rather than greater specific root nodule activity. The results show that on a 24-hour basis the process of symbiotic N 2 fixation in soybean plants grown under controlled environmental conditions functioned at maximum capacity and was not limited by shoot CER. Whether N 2-fixation capacity was limited by photosynthate movement to root nodules or by saturation of metabolic processes in root nodules is not known. 相似文献
11.
Gas exchange of individual attached leaves of soybean, Glycine max (L.) Merr cv Davis, was monitored during exposure to exogenous ethylene (C 2H 4) to test the hypothesis that the effects of C 2H 4 on net photosynthesis ( PN) and stomatal conductance to H 2O vapor ( gs) are direct and not mediated by changes in leaf orientation to light. Leaflets were held perpendicular to incident light in a temperature-controlled cuvette throughout a 5.5 hour exposure to 10 microliters per liter C 2H 4. Declines in both PN and gs were evident within 2 hours and became more pronounced throughout the exposure period. In C 2H 4 treated plants, PN and gs decreased to 80 and 62%, respectively, of the rates in control plants. Because epinastic movement of the leaflets was prohibited by the cuvette, the observed declines in PN and gs were a direct effect of C 2H 4 rather than the result of reduced light interception caused by changing leaf angle. 相似文献
12.
Few studies have investigated the effects of elevated CO 2 on the physiology of symbiotic N 2-fixing trees. Tree species grown in low N soils at elevated CO 2 generally show a decline in photosynthetic capacity over time relative to ambient CO 2 controls. This negative adjustment may be due to a reallocation of leaf N away from the photosynthetic apparatus, allowing for more efficient use of limiting N. We investigated the effect of twice ambient CO 2 on net CO 2 assimilation (A), photosynthetic capacity, leaf dark respiration, and leaf N content of N2-fixing Alnus glutinosa (black alder) grown in field open top chambers in a low N soil for 160 d. At growth CO 2, A was always greater in elevated compared to ambient CO 2 plants. Late season A vs. internal leaf p(CO 2) response curves indicated no negative adjustment of photosynthesis in elevated CO 2 plants. Rather, elevated CO 2 plants had 16% greater maximum rate of CO 2 fixation by Rubisco. Leaf dark respiration was greater at elevated CO 2 on an area basis, but unaffected by CO 2 on a mass or N basis. In elevated CO 2 plants, leaf N content (μg N cm ?2) increased 50% between Julian Date 208 and 264. Leaf N content showed little seasonal change in ambient CO 2 plants. A single point acetylene reduction assay of detached, nodulated root segments indicated a 46% increase in specific nitrogenase activity in elevated compared to ambient CO 2 plants. Our results suggest that N 2-fixing trees will be able to maintain high A with minimal negative adjustment of photosynthetic capacity following prolonged exposure to elevated CO 2 on N-poor soils. 相似文献
13.
Two Clark-type polarographic electrodes were used to measure simultaneous H 2 and O 2 exchange from three species of the blue-green alga Anabaena. Maximum H 2 photoevolution from N 2-fixing cultures of Anabaena required only the removal of dissolved O 2 and N 2; no adaptation period was necessary. No correlation of H 2 photoproduction with photosynthetic O 2 evolution, beyond their mutual light requirement, was found. Hydrogen photoevolution has the following characteristics in common with N 2 fixation in these organisms: DCMU insensitivity; similar white light dependency with very low dark production rates; maximum efficiency in photosystem I light; inhibition by N 2, O 2 and acetylene; and an apparent requirement for the presence of heterocysts. Growth on nitrate medium reduces, and on ammonium medium obliterates, both reactions. Cultures grown under limiting CO 2 conditions have H 2 photoproduction rates proportional to their growth rates. Hydrogenase activity is inferred from H 2 uptake in the dark, but this activity apparently is independent of the photoevolution of H 2 which is ascribed strictly to the nitrogenase system. 相似文献
14.
Nitrogenase activity (acetylene reduction activity) was found to occur universally in the Cyperus papyrus swamp in Lake Naivasha. Low rates of acetylene reduction activity (0.9–104.9 nmol C 2H 4 g d.wt. roots -1 h -1) were associated with excised roots of C. papyrus but higher rates of activity (89.0–280.4 nmol C 2H 4 g d.wt. roots -1 h -1) were associated with intact root systems of the plant. It was estimated that nitrogen fixation associated with young roots
alone could supply about 26% of the nitrogen requirements of growing papyrus plants. Acetylene reduction activity in the lake
bottom sediments was generally low and associated with adjacent papyrus stands. Plate counts of putative aerobic and facultatively
anaerobic N 2-fixing bacteria associated with papyrus roots showed the presence of high numbers of diazotrophs (5.4 × 10 6 CFU g d.wt. roots -1). Fewer numbers of N 2-fixing bacteria were detected in the sediments (1.9 × 10 3-3.2 × 10 4 CFU g d.wt. sediment -1). 相似文献
15.
High rates of acetylene (C 2H 2) reduction (nitrogenase activity) were observed in woodroom effluent from a neutral sulfite semi-chemical mill under aerobic (up to 644 nmol of C 2H 4 produced per ml per h) and under anaerobic (up to 135 nmol of C 2H 4 produced per ml per h) conditions. Pasteurized effluent developed C 2H 2 reduction activity when incubated under anaerobic but not under aerobic conditions. Activities were increased by addition of 0.5 to 3.0% glucose or xylose. Enrichment and enumeration studies showed that N 2-fixing Azotobacter and Klebsiella were abundant, and N 2-fixing Bacillus was present. Of 129 isolates of Klebsiella from pulp mills, lakes, rivers, and drainage and sewage systems, 32% possessed nitrogen-fixing ability. 相似文献
16.
Nodulated cowpea ( Vigna unguiculata L. Walp. cv Vita 3: Bradyrhizobium CB 756) plants were cultured with their whole root system or crown root nodulation zone maintained for periods from 5 to 69 days after planting in atmospheres containing a range of pO 2 (1-80%, v/v) while the rest of the plant grew in normal air. Growth (dry matter yield) and N 2 fixation were largely unaffected by pO 2 from 10 to 40%. Decrease in fixation at pO 2 below 5% was due to lower nodulation and nodule mass and, at pO 2 above 60%, to a fall in specific N 2-fixing activity of nodules. Root:shoot ratios were significantly lower at pO 2 below 2.5%. The effect of pO 2 on nitrogenase activity (acetylene reduction), both of whole nodulated root systems and crown root nodulation zones, varied with plant age but was generally lower at supra- and subambient extremes of O 2. H 2 evolution showed a sharp optimum at 20% O 2 but was at most 4% of total nitrogenase activity. The ratio of CO 2 evolved to substrate (C 2H 2+H +) reduced by crown root nodulation zones was constant (6 moles CO 2 per mole substrate reduced) from 2.5 to 60% O 2 but at levels below 2.5 and above 80% O 2 reached values between 20 and 30 moles CO 2 per mole substrate reduced. Effects of long-term growth with nonambient pO 2 on adaptation and efficiency of functioning of nodules are discussed. 相似文献
17.
Yield increases observed among eight genotypes of tomato ( Lycopersicon esculentum Mill.) grown at ambient CO 2 (about 350) or 1000 microliters per liter CO 2 were not due to carbon exchange rate increases. Yield varied among genotypes while carbon exchange rate did not. Yield increases were due to a change in partitioning from root to fruit. Tomatoes grown with CO 2 enrichment exhibited nonepinastic foliar deformation similar to nutrient deficiency symptoms. Foliar deformation varied among genotypes, increased throughout the season, and became most severe at elevated CO 2. Foliar deformation was positively related to fruit yield. Foliage from the lower canopy was sampled throughout the growing season and analysed for starch, K, P, Ca, Mg, Fe, and Mn concentrations. Foliar K and Mn concentrations were the only elements correlated with deformation severity. Foliar K decreased while deformation increased. In another study, foliage of half the plants of one genotype received foliar applications of 7 millimolar KH 2PO 4. Untreated foliage showed significantly greater deformation than treated foliage. Reduced foliar K concentration may cause CO 2-enhanced foliar deformation. Reduced K may occur following decreased nutrient uptake resulting from reduced root mass due to the change in partitioning from root to fruit. 相似文献
18.
The aquatic filamentous cyanobacteria Anabaena oscillarioides and Trichodesmium sp. reveal specific cellular regions of tetrazolium salt reduction. The effects of localized reduction of five tetrazolium salts on N 2 fixation (acetylene reduction), 14CO 2 fixation, and 3H 2 utilization were examined. During short-term (within 30 min) exposures in A. oscillarioides, salt reduction in heterocysts occurred simultaneously with inhibition of acetylene reduction. Conversely, when salts failed to either penetrate or be reduced in heterocysts, no inhibition of acetylene reduction occurred. When salts were rapidly reduced in vegetative cells, 14CO 2 fixation and 3H 2 utilization rates decreased, whereas salts exclusively reduced in heterocysts were not linked to blockage of these processes. In the nonheterocystous genus Trichodesmium, the deposition of reduced 2,3,5-triphenyl-2-tetrazolium chloride (TTC) in the internal cores of trichomes occurs simultaneously with a lowering of acetylene reduction rates. Since TTC deposition in heterocysts of A. oscillarioides occurs contemporaneously with inhibition of acetylene reduction, we conclude that the cellular reduction of this salt is of use in locating potential N 2-fixing sites in cyanobacteria. The possible applications and problems associated with interpreting localized reduction of tetrazolium salts in cyanobacteria are presented. 相似文献
19.
Photosynthetic data collected from Pisum sativum L. and Phaseolus vulgaris L. plants at different stages of development were related to symbiotic N 2 fixation in the root nodules. The net carbon exchange rate of each leaf varied directly with carboxylation efficiency and inversely with the CO 2 compensation point. Net carbon exchange of the lowest leaves reputed to supply fixed carbon to root nodules declined in parallel with H 2 evolution from root nodules. The decrease in H 2 evolution also coincided with the onset of flowering but preceded the peak in N 2 fixation activity measured by acetylene-dependent ethylene production. A result of these changes was that the relative efficiency of N 2 fixation in peas increased to 0.7 from an initial value of 0.4. The data reveal that attempts to identify photosynthetic contributions of leaves to root nodules will require careful timing and suggest that the relative efficiency of N 2 fixation may be influenced by source-sink relationships. 相似文献
20.
Photosynthetic efficiency, primary productivity, and N 2 reduction were determined in peas ( Pisum sativum L. var. Alaska) grown at light intensities ranging from severely limiting to saturating. Plants grown under higher light intensities showed greater carboxylation and light capture potential and higher rates of net C exchange. Uptake of N 2, computed from measured C 2H 2 reduction and H 2 evolution rates, also increased with growth light intensity, while the previously proposed relative efficiency of N 2 fixation, based on these same parameters, declined. The plot of N/C ratios (total nitrogen content/plant dry weight) increased hyperbolically with light intensity, and the plot of N 2/CO 2 uptake ratios (N 2 uptake rate/net CO 2 uptake rate) increased linearly. Both plots extrapolated to the light compensation point. The data indicate that the relative efficiency of N 2 fixation is not necessarily correlated with maximum plant productivity and that evaluation of a plant's capacity to reduce N 2 is related directly to concurrent CO 2 reduction. A measure of whole plant N 2 fixation efficiency based on the N 2/CO 2 uptake ratio is proposed. 相似文献
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