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1.
Low root temperature effects on vegetative growth of soybean (Harosoy 63 × Rhizobium japonicum USDA 16) were examined in 35 day old plants exposed to temperatures of 15°C (shoots at 25°C) for an 11 day period. Duing this period various aspects of C and N assimilation and partitioning were monitored including shoot night and nodulated root respiration, C and N partitioning to six plant parts, C 2H 2 reduction, H 2 evolution, leaf area, transpiration, net photosynthesis, and N 2 fixation. The low temperature treatment resulted in a decrease in the net rate of N 2 fixation but nitrogenase relative efficiency increased. In response, the plant retained N in the tissues of the nodulated root and decreased N partitioning to young shoot tissues, thereby inducing the remobilization of N from older leaves, and reducing leaf area development. The leaf area specific rate of net photosynthesis was not affected over the study period; however, shoot and nodulated root respiration declined. Consequently, C accumulated in mature leaves and stems, partly in the form of increased starch reserves. Three possibilities were considered for increasing low temperature tolerance in nodulated soybeans: (a) decrease in temperature optima for nitrogenase, (b) increased development of nodules and N 2 fixation capacity at low temperature, and (c) alterations in the pattern of C and N partitioning in response to low temperature conditions. 相似文献
2.
Summary Isotopic 15N 2 experiments confirmed nitrogen fixation in Parasponia parviflora. The conversion ratio C 2H 4/N 2 was 6.7 under the experimental conditions employed. Measurements of the δ 15N in leaves of Parasponia and Trema showed on the basis of these determinations that Parasponia parviflora possesses N 2-fixing capacity and can be distinguished in this respect from the non-nitrogen-fixing Trema cannabina tested by the same method. Therefore, δ 15N can be used to monitor N 2 fixation in natural ecosystems. Hydrogen evolution and the relative efficiency of N 2 fixation in this relation have been determined. Detached Parasponia parviflora root nodules grown in soil and tested in an argon/oxygen atmosphere produced appr. 4 μmol H 2.h −1.g −1 fresh weight root nodules. The relative efficiency of hydrogen utilization as measured in argon, air, and in the presence
of C 2H 2 10% (v/v) was for both equations
used for to express this efficiency 0.96 and 0.97, respectively. This indicates that Parasponia like the root nodules of
some actinorhizal symbioses (Alnus, Myrica, Elaeagnus) and some tropical legumes ( Vigna sinensis) has evolved mechanisms of minimizing net hydrogen production in air, thus increasing the efficiency of electron transfer
to nitrogen. The oxygen relation of nitrogen fixation (C 2H 2) in Parasponia parviflora root nodules was determined. The nitrogenase activity of Parasponia root nodules increased at increasing oxygen concentrations
up till c. 40% O 2. At oxygen levels above 40% O 2, the nitrogenase activity of the root nodules was nil or very erratic suggesting that at these oxygen levels the nitrogenase
is not longer protected against the harmful effect of oxygen. In this respect Parasponia root nodules differ from actinorhizal
root nodules in other nonlegumes, where optimal nitrogenase activity was observed in the range of 12–25% oxygen. Respiration
experiments with Parasponia root nodules showed that in the range 10, 20, and 40% oxygen, the respiration rate (CO 2 evolution) increased concomitantly with an increase of the acetylene reduction rate. The CO 2/C 2H 4 values obtained varied between 8.1 and 19.2, being therefore 2–3 times higher than similar estimations in some actinorhizal
and legume root nodules. The respiratory quotient (RQ) of detached Parasponia parviflora root nodules was in air initially approximately 2.0, but this value dropped to about 1.0 in a 3-hours period. 相似文献
3.
The carbon and nitrogen economies of a single cultivar of cowpea ( Vigna unguiculata (L.) Walp.cv Caloona) nodulated with either a high H 2-evolving strain (176A27) or a low H 2-evolving strain (CB756) of Rhizobium were compared. The two symbioses did not differ in total dry matter production, seed yield, nitrogen fixed, the spectrum of nitrogenous solutes produced by nodules for export, or the partitioning of net photosynthate within the plant throughout the growth cycle. Detailed examination of the carbon and nitrogen economy of the nodules, however, showed a significant difference between the symbioses. Nodules formed with CB756 lost less CO 2 in respiration compared to the higher H 2-evolving symbioses and this could have been largely responsible for a 36% better economy of carbon use in CB756 nodules during the period of maximum H 2 evolution (48-76 days) and over the whole growth period (20-90 days), a 16% economy. In terms of overall net photosynthate generated by the plant, these economies were equivalent to 5% and 2% of the carbon utilized in the two periods, respectively. From the differences in H 2 evolution and CO 2 production by nodules of the two symbioses, the cost of H 2 evolution was found to be 3.83±0.6 millimoles CO 2/millimoles H 2 for plants grown in sand culture and 1.69 ± 0.48 millimoles CO 2/millimoles H 2 for those in water culture. In both symbioses, the ratio of H 2 evolution to N 2 fixed varied markedly during ontogeny, indicating a significant variation in the relative efficiency and thus metabolic cost of N 2 fixation at different stages during development. 相似文献
4.
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
5.
In vivo CO 2 fixation and in vitro phosphoenolpyruvate (PEP) carboxylase levels have been measured in lupin ( Lupinus angustifolius L.) root nodules of various ages. Both activities were greater in nodule tissue than in either primary or secondary root tissue, and increased about 3-fold with the onset of N 2 fixation. PEP carboxylase activity was predominantly located in the bacteroid-containing zone of mature nodules, but purified bacteroids contained no activity. Partially purified PEP carboxylases from nodules, roots, and leaves were identical in a number of kinetic parameters. Both in vivo CO 2 fixation activity and in vitro PEP carboxylase activity were significantly correlated with nodule acetylene reduction activity during nodule development. The maximum rate of in vivo CO 2 fixation in mature nodules was 7.9 nmol hour −1 mg fresh weight −1, similar to rates of N 2 fixation and reported values for amino acid translocation. 相似文献
6.
Hydrogenase activity of root nodules in the symbiotic association between Pisum sativum L. and Rhizobium leguminosarum was determined by incubating unexcised nodules with tritiated H 2 and measuring tissue HTO. Hydrogenase activity saturated at 0.50 millimolar H 2 and was not inhibited by the presence of 0.10 atmosphere C 2H 2, which prevented H 2 evolution from nitrogenase. Total H 2 production from nitogenase was estimated as net H 2 evolution in air plus H 2 exchange in 0.10 atmosphere C 2H 2. Although such an estimate of nitrogenase function may not be quantitatively exact, due to uncertain relationships between H 2 exchange and H 2 uptake activity of hydrogenase, differences observed in H 2 exchange under various conditions represent an indication of changes in hydrogenase activity. Hydrogenase activity was lower in associations grown under higher photosynthetic photon flux densities and decreased relative to total H 2 production by nitrogenase. Total H 2 production and hydrogenase activity were maximum 28 days after planting. Thereafter, hydrogenase activity and H 2 production declined, but the potential proportion of nitrogenase-produced H 2 recovered by the uptake hydrogenase system increased. Of five R. leguminosarum strains tested two possessed hydrogenase activity. Strains which had the potential to reassimilate H 2 had significantly higher rates of N 2 reduction than those which did not exhibit hydrogenase activity. 相似文献
7.
The effect on nitrogen fixation of excising leaves or pods in pea ( Pisum sativum L. cv. Alaska) was determined over a 60-day period. Flower buds or their subtending leaves were removed, and C 2H 2 reduction, H 2 evolution and N accumulation were measured at weekly intervals. Highest percentage nitrogen content in all treatments coincided with time of maximal C 2H 2-reduction rates. Nitrogen fixation, calculated from C 2H 2 reduction and H 2- evolution data, was significantly lower in the partially defoliated and generally higher in the depodded plants than in the controls. Total N accumulation was greatest in the depodded plants and least in the defoliated ones. Percentage nitrogen content and N 2-fixation rates in the depodded plants were maximized approximately 10 days later than in the defoliated or control plants. The absolute rates of C 2H 2 reduction and H 2 evolution were significantly altered by plant organ removal, but the relative rates were proportional. As a result the ratios of H 2/C 2H 4 production and the related relative efficiency of N 2 fixation in the treatments were not significantly different from the controls. 相似文献
8.
H 2 evolved by alfalfa root nodules during the process of N 2 fixation may be an important factor influencing the distribution of soil bacteria. To test this hypothesis under field conditions, over 700 bacterial isolates were obtained from fallow soil or from the 3-mm layer of soil surrounding alfalfa ( Medicago sativa L.) root nodules, alfalfa roots, or bindweed ( Convolvulus arvensis L.) roots. Bacteria were isolated under either aerobic or microaerophilic conditions and were tested for their capacity to metabolize H 2. Isolates showing net H 2 uptake and 3H 2 incorporation activity under laboratory conditions were assigned a Hup + phenotype, whereas organisms with significant H 2 output capacity were designated as a Hout + phenotype. Under aerobic isolation conditions two Hup + isolates were obtained, whereas under microaerophilic conditions five Hup + and two Hout + isolates were found. The nine isolates differed on the basis of 24 standard bacteriological characteristics or fatty acid composition. Five of the nine organisms were isolated from soil around root nodules, whereas the other four were found distributed among the other three soil environments. On the basis of the microaerophilic isolations, 4.8% of the total procaryotic isolates from soil around root nodules were capable of oxidizing H 2, and 1.2% could produce H 2. Two of the Hup + isolates were identified as Rhizobium meliloti by root nodulation tests, but the fact that none of the isolates reduced C 2H 2 under the assay conditions suggested that the H 2 metabolism traits were associated with various hydrogenase systems rather than with nitrogenase activity. Results from this study support the concept that H 2 evolution by alfalfa root nodules has a significant effect on the surrounding microenvironment and influences the number and diversity of bacteria occupying that region. 相似文献
9.
Hydrogen evolution from root nodules has been reported to make N 2 fixation by some legume- Rhizobium symbiotic systems inefficient. We have surveyed the extent of H 2 evolution and estimated relative efficiencies of nodules of Austrian winter peas formed by 15 strains of R. leguminosarum. Their rates of H 2 evolution in air were about 30% of the rates of H 2 evolution under an atmosphere in which N 2 was replaced by Ar. Relative efficiency values based on C 2H 2 reduction rates ranged from 0.55 to 0.80. With some of the strains, hydrogenase activities were demonstrated in intact nodules and in bacteroids, but the levels of activity were insufficient to recycle all the H 2 evolved by the nitrogenase system. In both intact nodules and bacteroids the hydrogenase is less sensitive to O 2 damage than the nitrogenase system, so H 2 uptake capacity was observed in intact nodules by suppressing the nitrogenase-dependent H 2 evolution with an atmosphere containing a high O 2 concentration, and in bacteroids by using aerobically prepared bacteroid suspensions. The hydrogenase activity of both was dependent on O 2 consumption. A K
mfor H 2 of near 4 M was determined in suspension of bacteroids from nodules formed by strains 128C53 and 128C56. 相似文献
10.
Dinitrogen (N 2) fixation and assimilation efficiency in a German and two Ethiopian varieties of Pisum sativum L. was studied in a pot experiment during vegetative and reproductive growth. The objective of the study was to assess whether genotypes having contrasting growth habits showed differences in physiological processes that affect the efficiency of N 2 fixation and assimilation. Dry matter formation, nodulation and nitrogen assimilation were compared between two treatments where one depended solely on N 2 fixation while the other was nourished with nitrate. Moreover, carbon (C) costs of N 2 fixation and the capacity of different respiratory chains in roots and nodules were determined at vegetative and reproductive growth. As compared to the Ethiopian cultivars, the German variety displayed a more rapid vegetative growth with intensive N 2 fixation and assimilation and highly efficient individual nodules. However, during reproductive growth, N 2 fixation in the German variety declined sharply, while continuing in the Ethiopian varieties. Lowest C costs of N 2 fixation coincided with most efficient individual nodules in both growth intervals. C costs of N 2 fixation were lower during reproductive growth in all varieties which was accompanied by a shift in root/nodule respiratory capacity towards more C efficient respiratory pathways. The results provide further evidence that unreliable nitrogen fixation rates during reproductive growth of pea can be connected with restricted C supply to nodules. One strategy of pea plants to adapt to critical C availability is an increase in capacity of more C efficient root/nodule respiration. 相似文献
11.
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
12.
The interaction between the ATP-dependent evolution of H 2 catalyzed by nitrogenase and the oxidation of H 2 via a hydrogenase has been postulated to influence the efficiency of the N 2-fixing process in nodulated legumes. A comparative study using soybean ( Glycine max L. Merr.) cv. Anoka inoculated with either Rhizobium japonicum strain USDA 31 or USDA 110 and cowpea ( Vigna unguiculata L. Walp.) cv. Whippoorwill inoculated with Rhizobium strain 176A27 or 176A28 cultured on a N-free medium was conducted to address this question. Nodules from the Anoka cultivar inoculated with USDA 31 evolved H 2 in air and the H 2 produced accounted for about 30% of the energy transferred to the nitrogenase system during the period of active N 2 fixation. In contrast the same soybean cultivar inoculated with USDA 110 produced nodules with an active hydrogenase and consequently did not evolve H 2 in air. A comparison of Anoka soybeans inoculated with the two different strains of R. japonicum showed that mean rates of C 2H 2 reduction and O 2 consumption and mean mass of nodules taken at four times during vegetative growth were not significantly different. When compared to Anoka inoculated with USDA 31, the same cultivar inoculated with USDA 110 showed increases in total dry matter, per cent nitrogen, and total N2 fixed of 24, 7, and 31%, respectively. Cowpeas in symbiosis with the hydrogenase-producing strain 176A28 in comparison with the same cultivar inoculated with the H2-evolving strain 176A27 produced increases in plant dry weight and total N2 fixed of 11 and 15%, respectively. This apparent increase in the efficiency of N2 fixation for nodulated legumes capable of reutilizing the H2 evolved from nitrogenase is considered and it is concluded that provision of conclusive evidence of the role of the H2-recycling process in N2-fixing efficiency of legumes will require comparison of Rhizobium strains that are genetically identical with the exception of the presence of hydrogenase. 相似文献
13.
Soybean ( Glycine max [L.] Merr. cv Davis) was grown in a split-root growth system designed to maintain control of the root atmosphere. Two experiments were conducted to examine how 80% Ar:20% O 2 (Ar:O 2) and air (Air) atmospheres affected N assimilation (NH 4NO 3 and N 2 fixation) and the partitioning of photosynthate to roots and nodules. Application of NH 4NO 3 to nonnodulated half-root systems enhanced root growth and root respiration at the site of application. A second experiment applied Ar:O 2 or air to the two sides of nodulated soybean half-root systems for 11 days in the following combinations: (a) Air to both sides (Air/Air); (b) Air to one side, Ar:O 2 to the other (Air/Ar:O 2), and (c) Ar:O 2 to both sides (Ar:O 2/Ar:O 2). Results indicated that dry matter and current photosynthate ( 14C) were selectively partitioned to nodules and roots where N 2 was available. Both root and nodule growth on the Air side of Air/Ar:O 2 plants was significantly greater than the Ar:O 2 side. The relative partitioning of carbon and current photosynthate between roots and nodules on a half-root system was also affected by N 2 availability. The Ar:O 2 sides partitioned relatively more current photosynthate to roots (57%) than nodules (43%), while N 2-fixing root systems partitioned 36 and 64% of the carbon to roots and nodules, respectively. The Ar:O 2 atmosphere decreased root and nodule respiration by 80% and nitrogenase activity by 85% compared to half-root systems in Air while specific nitrogenase activity of nodules in Ar:O 2 was 50% of nodules supplied Air. Results indicated that nitrogen assimilation, whether from N 2 fixation or inorganic sources, had a localized effect on root development. Nodule development accounted for the major decrease in total photosynthate partitioning to non-N 2-fixing nodules. Soybean compensates for ineffective nodulation by controlling the flux of carbon to ineffective nodules and their associated roots. 相似文献
14.
A mathematical model is described which allows the estimation of rates of O 2, CO 2, N 2, and H 2 exchange from legume nodules under steady state conditions of N 2 fixation. Calculated rates of gas exchange under defined conditions of nodule size, relative growth rate (RGR), specific total nitrogenase activity (TNA), nitrogenase electron allocation coefficient (EAC), uptake-hydrogenase activity (HUP) and nature of the N export product compared favorably with experimentally-obtained rates reported in the literature. Therefore the model was used to predict the effects of varying each of these nodule characteristics on the rates of gas exchange, and on the apparent respiratory cost (CO 2/NH 3) and sucrose cost (sucrose consumed/NH 3) of N 2 fixation.The model predicted that, all other characters being equal, ureide-producing nodules would consume 8% less sucrose per N fixed than asparagine-producing nodules, but would display an apparent respiratory cost which would be 5% higher than that in asparagine-producing nodules. In both ureide-producing and asparagine-producing nodules, the major factor affecting the apparent respiratory cost of N 2 fixation was predicted to be EAC, followed by TNA, nodule RGR and nodule size. The relative importance of HUP in improving the apparent respiratory cost of N 2 fixation was predicted to be largely dependent upon its potential role in the regulation of EAC.
Abbreviations: See Appendix 1. 相似文献
15.
Nitrogenase (EC 1.7.99.2) activity in pea ( Pisum savitum) nodules formed after infection with Rhizobium leguminosarum (lacking uptake hydrogenase) was measured as acetylene reduction, H 2 evolution in air and H 2 evolution in Ar:O 2. With detached roots the relative efficiency, calculated from acetylene reduction, showed a decrease (from 55 to below 0%) with increasing temperature. With excised nodules and isolated bacteroids similar results were obtained. However, the relative efficiency calculated from H 2 evolution in Ar:O 2 was unaffected by temperature. Measurements on both excised nodules and isolated bacteroids showed a marked difference between acetylene reduction and H 2 evolution in Ar:O 2 with increased temperature, indicating that either acetylene reduction or H 2 evolution in Ar:O 2 are inadequate measures of nitrogenase activity at higher temperature. 相似文献
16.
Shoot/root grafting studies showed organ and host cultivar effects on net H 2 evolution from Pisum sativum L. root nodules. Net H 2 evolution from those nodules represents the sum of H 2 formed by Rhizobium nitrogenase and H 2 oxidized by any uptake hydrogenase present in the bacteria. Grafts between pea cultivars `JI1205' or `Alaska' and `Feltham First' in symbioses with R. leguminosarum 128C53 showed that shoots of both JI1205 and Alaska increased H 2 uptake significantly (P ≤ 0.05) in Feltham First root nodules. The same plants also had less net H 2 evolution at similar rates of C 2H 2 reduction than plants formed by grafting Feltham First shoots on Feltham First roots. Although JI1205 and Alaska shoots increased H 2-uptake activity of Feltham First root nodules 28 days after the graft was made, intermediate to high levels of H 2 uptake activity were still present in nodules on roots of both JI1205 and Alaska grafted to Feltham First shoots. These results indicate the presence of a transmissible shoot factor(s) which can increase uptake hydrogenase activity in a Rhizobium symbiont and show that root genotype also can influence that parameter. Parallel grafting experiments using the same pea cultivars in symbioses with R. leguminosarum strain 300, which lacks uptake hydrogenase activity, suggested that a transmissible shoot factor(s) altered H2 formation from nitrogenase by changing the electron allocation coefficient of that enzyme complex. The root and shoot factor(s) detected in this study had no permanent effect on strain 128C53. Bacterial cells isolated from Feltham First nodules with low H2 uptake activity formed root nodules on JI1205 and Alaska with high H2 uptake activity. Bacteroids isolated from nodules on intact JI1205, Alaska, or Feltham First plants with high, medium, or low H2 uptake activity, respectively, maintained those phenotypes during in vitro assays. 相似文献
17.
Dihydrogen, a by-product of biological nitrogen fixation, is a competitive inhibitor of N 2 reduction by nitrogenase. To evaluate the significance of H 2 inhibition in vivo, we have measured the apparent inhibition constant for H 2 inhibition of N 2 reduction in Bradyrhizobium japonicum bacteroids isolated from soybean nodules. The rate of N 2 reduction was measured as ammonia production by bacteroids incubated in a buffer containing 200 micromolar leghemoglobin and 10 millimolar succinate under 0.02 atmosphere O 2 and various concentrations of N 2 and H 2. The apparent inhibition constant for H 2 under these conditions was determined to be approximately 0.03 atmosphere. This relatively low value strengthens the proposal that H 2 inhibition of N 2 reduction may be a significant factor in lowering the efficiency of nitrogen fixation in legume nodules. 相似文献
18.
The ability to recycle H 2 evolved by nitrogenase is thought to be of importance in increasing the efficiency of N 2 fixation and to be a factor in increasing plant yield in symbiotic systems. To determine whether this ability is a significant factor in the Rhizobium leguminosarum-Pisum sativum L. system, plants were inoculated with R. leguminosarum isolates which differed in their ability to oxidize H 2 and in their relative efficiency of N 2 fixation. These plants were grown at three levels of irradiance and harvested after 3, 4, and 5 weeks of growth for determination of C 2H 2 reduction, H 2 evolution and uptake, plant dry weight, and N content. Plants inoculated with uptake hydrogenase-positive (Hup +) isolates did not exhibit higher dry weight or N content than those inoculated with Hup − isolates under any of the growth conditions studied. The efficiency of the nitrogenase system of Hup − isolates increased at a low irradiance, a factor which may allow them to compete successfully with Hup + isolates. In some Hup +R. leguminosarum isolates, H 2 oxidation is coupled to ATP formation, whereas in others, it is not. There were no differences in plant dry weight and N content in plants inoculated with the two types and grown for 5 weeks at three irradiance levels. The addition of H 2 to Hup + nodules whose supply of photosynthate had been removed by stem excision did not increase C 2H 2 reduction in either coupled or uncoupled types. 相似文献
19.
Budgets for carbon and nitrogen in shoot, root, and nodulesof garden pea ( Pisum sativum L.) are drawn up for a 9-d intervalin the life cycle, from data on nitrogen fixation, carbon accumulationin dry matter, respiratory output of plant organs, and organicsolute exchange between shoot and nodulated root. Of the carbon gained photosynthetically by the shoot from theatmosphere 26 per cent is incorporated directly into its drymatter, 32 per cent translocated to the nodules, and 42 percent to the supporting root. Of the nodules share, 5per cent is consumed in growth, 12 per cent in respiration,and 15 per cent returned to the shoot via the xylem, as aminocompounds generated in nitrogen fixation. Growth and respirationof the root utilize, respectively, 7 and 35 per cent. The respiratory efficiency of a nodulated root in terms of nitrogenfixation (5.9mg C per mg N 2-N fixed) is found to be very similarto that of an uninoculated root assimilating nitrate (6.2 mgC per mg NO 3-N reduced). The nodules require in growth, respiration,and export 4.1 mg C ( 10.3 mg carbohydrate) for each mg N whichthey fix. The nodules consume 3 ml O 2 for every 1 ml N 2 utilized in fixation. In exporting a milligram of fixed nitrogen the nodules requireat least 0.35 ml of water. Almost half of this requirement mightbe met by mass flow into the nodules via the phloem. 相似文献
20.
Application of glyphosate ( N-[phosphonomethyl] glycine) to exporting leaves of sugar beet ( Beta vulgaris, L.) during the day lowered stomatal conductance and carbon fixation. Allocation of newly fixed carbon to foliar starch accumulation was nearly completely inhibited, being decreased by the same amount as net carbon fixation. In contrast, decreasing net carbon fixation in untreated leaves by lowering CO 2 concentration caused starch accumulation to decrease, but only in the same proportion as net carbon fixation. Shikimate level increased 50-fold in treated leaves but the elevated rate of carbon accumulation in shikimate was only 4% of the decrease in the rate of starch accumulation. Application of steady state labeling with 14CO 2 to exporting leaves confirmed the above changes in carbon metabolism, but revealed no other major daytime differences in the 14C-content of amino acids or other compounds between treated and control leaves. Less 14C accumulated in treated leaves because of decreased fixation, not increased export. The proportion of newly fixed carbon allocated to sucrose increased, maintaining export at the level in control leaves. Returning net carbon exchange to the rate before treatment restored starch accumulation fully and prevented a decrease in export during the subsequent dark period. 相似文献
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