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At least 991 records · Page 55Linked to original sources
PubMed1975

Potential for nitrogen fixation in maize genotypes in Brazil.

N(2) fixation in field-grown maize (Zea mays L.) plants was estimated by a nondestructive acetylene reduction method which permitted the plants to continue growing and produce seeds. Samples from six areas revealed mean nitrogenase activities of 74-2167 nmol of C(2)H(4)/(g of dry roots x hr) for 10 plants. Among 276 S(1) lines planted in two field experiments, 17 lines were selected for further nitrogenase activity assays after prescreening. Variability within lines was high but significant differences among lines were obtained in one experiment. The best lines showed mean nitrogenase activities of 2026, 2315, and 7124 nmol of C(2)H(4)/(g of dry roots x hr), whereas the original cultivar reduced only 313 nmol. The highest value approaches the nitrogenase activity of soybean. If the theoretical 3:1 (C(2)H(4)/N(2) reduced) conversion factor is used, a potential daily N(2) fixation of 2 kg of N(2)/hectare can be calculated. Periodic sampling within a brachytic maize cultivar revealed that maximum nitrogenase activity occurred at about the 75% silking stage. Soil effects also were pronounced. N(2)-fixing Spirillum sp. could be isolated from all active root pieces when they were surface sterilized. These organisms appear to be primarily responsible for root nitrogenase activity in maize.

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PubMed1983

Organization and expression of Rhizobium meliloti nitrogen fixation genes.

The boundaries of a nif gene cluster in Rhizobium meliloti were determined by Tn5 mutagenesis. These genes are clustered within a 14- to 15-kilobase (kb) region that includes the nitrogenase structural genes. Sequences within 10 kb on either side of this region are devoid of symbiotically essential gene function. RNA blot analysis identified a 5- to 6-kb transcript that corresponds to the nitrogenase structural gene operon. The 5' end of this transcript and its polarity were determined by nuclease S1 mapping. The 5' end of another nif transcript was also identified by nuclease S1 mapping. The promoter regions for these two nif transcripts control transcription in divergent directions and are separated by 1.9 kb of symbiotically unessential DNA. One Tn5 insertion within the nitrogenase operon did not create a polar mutation. The implications of this finding and the overall emerging picture of the genetic organization of this nif region are discussed.

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PubMed1993

Current Nitrogen Fixation Is Involved in the Regulation of Nitrogenase Activity in White Clover (Trifolium repens L.).

Previous studies have shown that nitrogenase activity decreases dramatically after defoliation, presumably because of an increase in the O2 diffusion resistance in the infected nodules. It is not known how this O2 diffusion resistance is regulated. The aim of this study was to test the hypothesis that current N2 fixation (ongoing flux of N2 through nitrogenase) is involved in the regulation of nitrogenase activity in white clover (Trifolium repens L. cv Ladino) nodules. We compared the nitrogenase activity of plants that were prevented from fixing N2 (by continuous exposure of their nodulated root system to an Ar:O2 [80:20] atmosphere) with that of plants allowed to fix N2 (those exposed to N2:O2, 80:20). Nitrogenase activity was determined as the amount of H2 evolved under Ar:O2. An open flow system was used. In experiment I, 6 h after complete defoliation and the continuous prevention of N2 fixation, nitrogenase activity was higher by a factor of 2 compared with that in plants allowed to fix N2 after leaf removal. This higher nitrogenase activity was associated with a lower O2 limitation (measured as the partial pressure of O2 required for highest nitrogenase activity). In experiment II, the nitrogenase activity of plants prevented from fixing N2 for 2 h before leaf removal showed no response to defoliation. The extent to which nitrogenase activity responded to defoliation was different in plants allowed to fix N2 and those that were prevented from doing so in both experiments. This leads to the conclusion that current N2 fixation is directly involved in the regulation of nitrogenase activity. It is suggested that an N feedback mechanism triggers such a response as a result of the loss of the plant's N sink strength after defoliation. This concept offers an alternative to other hypotheses (e.g. interruption of current photosynthesis, carbohydrate deprivation) that have been proposed to explain the immediate decrease in nitrogenase activity after defoliation.

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PubMed1999

Involvement of ureides in nitrogen fixation inhibition in soybean

The sensitivity of N2 fixation to drought stress in soybean (Glycine max Merr.) has been shown to be associated with high ureide accumulation in the shoots, which has led to the hypothesis that N2 fixation during drought is decreased by a feedback mechanism. The ureide feedback hypothesis was tested directly by measuring the effect of 10 mM ureide applied by stem infusion or in the nutrient solution of hydroponically grown plants on acetylene reduction activity (ARA). An almost complete inhibition of ARA was observed within 4 to 7 d after treatment, accompanied by an increase in ureide concentration in the shoot but not in the nodules. The inhibition of ARA resulting from ureide treatments was dependent on the concentration of applied ureide. Urea also inhibited ARA but asparagine resulted in the greatest inhibition of nodule activity. Because ureides did not accumulate in the nodule upon ureide treatment, it was concluded that they were not directly inhibitory to the nodules but that their influence mediated through a derivative compound, with asparagine being a potential candidate. Ureide treatment resulted in a continual decrease in nodule permeability to O2 simultaneous with the inhibition of nitrogenase activity during a 5-d treatment period, although it was not clear whether the latter phenomenon was a consequence or a cause of the decrease in the nodule permeability to O2.

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PubMed1999

Sucrose synthase in legume nodules is essential for nitrogen fixation

The role of sucrose synthase (SS) in the fixation of N was examined in the rug4 mutant of pea (Pisum sativum L.) plants in which SS activity was severely reduced. When dependent on nodules for their N supply, the mutant plants were not viable and appeared to be incapable of effective N fixation, although nodule formation was essentially normal. In fact, N and C resources invested in nodules were much greater in mutant plants than in the wild-type (WT) plants. Low SS activity in nodules (present at only 10% of WT levels) resulted in lower amounts of total soluble protein and leghemoglobin and lower activities of several enzymes compared with WT nodules. Alkaline invertase activity was not increased to compensate for reduced SS activity. Leghemoglobin was present at less than 20% of WT values, so O2 flux may have been compromised. The two components of nitrogenase were present at normal levels in mutant nodules. However, only a trace of nitrogenase activity was detected in intact plants and none was found in isolated bacteroids. The results are discussed in relation to the role of SS in the provision of C substrates for N fixation and in the development of functional nodules.

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PubMed1975

Acetylene reduction (nitrogen fixation) and metabolic activities of soybean having various leaf and nodule water potentials.

An apparatus was designed that permitted acetylene reduction (N(2) fixation) by root nodules to be measured in situ simultaneously with net photosynthesis, dark respiration, and transpiration of the shoot in soybean plants (Glycine max [L.] Merr. var. Beeson). Tests showed that acetylene reduction was linear with time for at least 5 hours, except for the first 30 to 60 minutes. Endogenous ethylene production did not affect the measurements. Successive determinations of acetylene reduction could be made without apparent aftereffects on the plant.This apparatus was used to investigate the effects of soil flooding and desiccation on acetylene reduction under conditions where soil, nodule, and leaf water potentials could be measured. No acetylene reduction was detectable in flooded soil or in soil desiccated to a water potential of -19.5 bars. Between these extremes, acetylene reduction displayed a sharp optimum. Removing the soil eliminated the inhibitory effects of flooding, suggesting that rates of gas exchange were restricted between the nodules and the atnosphere at soil water potentials above -2 bars.As the soil desiccated further, acetylene reduction decreased, and the decrease was correlated with decreases in photosynthesis and transpiration. Although dark respiration was inhibited, it was not affected to the extent that acetylene reduction, photosynthesis, or transpiration were. Consequently, it was concluded that photosynthesis, transpiration, or some direct effect on the nodules other than that caused by respiration were most likely to account for the inhibition of acetylene reduction at soil water potentials below -2 bars.

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PubMed1976

Adaptation of Nitrogen Fixation by Intact Soybean Nodules to Altered Rhizosphere pO(2).

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(2)s on N(2)[C(2)H(2)] 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)[C(2)H(2)] 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(2)s reduced N(2)[C(2)H(2)] 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)[C(2)H(2)]-fixing system of intact soybean plants is able to adapt to a wide range of external pO(2)s 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.

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PubMed1977

Ontogenetic Interactions between Photosynthesis and Symbiotic Nitrogen Fixation in Legumes.

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.

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