Differential sensitivity of nodulation to ethylene in soybean cv. Bragg and a supernodulating mutant

We previously found that the ethylene inhibitor Ag⁺ could overcome the inhibitory effect of nitrate on nodulation of soybean ( Glycine max ) cv. Bragg. The same treatment increased nodulation quantitatively under non-inhibitory conditions, strongly suggesting involvement of ethylene in the control of nodulation in this species. Supernodulation mutants that lack internal autoregulation of nodulation, however, had biosynthesis capacity similar to the wild type. In the present work, the effects of ethylene on nodulation of 'Bragg' and two separate, but allelic, supernodulating mutants ( nts382 and nts1007 ) were compared. The nodulation process appeared much more sensitive than plant growth and development to ethylene, which reduced the number of nodules per plant, but nearly twofold more in the wild type than in the supernodulation mutants. The cause–effect relationship is established by the counteracting effect of Ag⁺ and the fact that the stronger the inhibition by ethylene, the higher the recovery of nodulation ability with the ethylene antagonist. This higher tolerance of or lower sensitivity to ethylene in nts382 persists even under low inoculum dose, where nodule number and mass could be decreased to wild-type levels. Differences between the mutant and the wild type in the triple response test do not appear to support differences in ethylene perception on a whole-plant basis. The results suggest that sensitivity of nodulation to ethylene might have been affected in supernodulation mutants.     

Determination of the distribution of three nitrate reductase isoforms in soybean seedlings by chromatography and a simple method based on assay conditions

Blue Sepharose affinity chromatography was used to study the distribution of the constitutive NAD(P)H-nitrate reductase (EC 1.6.6.2: Cl-NR) and of the constitutive and inducible NADH-nitrate reductases (EC 1.6.6.1; C2-NR and i-NR, respectively), in the unifoliolate leaf (F0), the first and the second trifoliolate leaves (F1 and F2) and the roots of urea- and nitrate-grown soybean ( Glycine max [L.] Merr.) plants. The C1-NR eluted by NADPH is present in the F0 and F1 leaves and nearly absent in the F2 leaf. The activity pattern of this isoform is not modified by nitrate nutrition. The C2-NR eluted by NADH is high in the F0 leaf, low in the F1 leaf and nearly absent in the F2 leaf of urea-grown plants. The NADH elution from leaves of nitrate-grown plants is a mixture of C2-NR and i-NR, requiring careful interpretation of results. However, i-NR appears the principal isoform in the leaves especially in the F2 leaf. This i-NR is the only NR present in the roots.
The pH effect on the assay of the 3 partially purified isoforms was studied using LNR2 and LNR5 soybean mutants to remove the cross contamination. It appears that C1-NR and C2-NR activities are negligible at pH 8.5, which allows the assay of only the i-NR in a crude extract at this pH, even when C1-NR and C2-NR are present. It appears also that the assay of C1-NR activity at pH 6.5 with NADPH is free of interference by the i-NR. To estimate the C2-NR activity with NADH at pH 6.5 in a crude extract in the presence of C1-NR and i-NR, we propose a simple calculation using the coefficient from the pH responses. These calculations are used to compare the development of C1-NR, C2-NR and i-NR activities in the F0 and F1 leaves of plants previously grown on urea and transferred to nitrate. Only the activity of the inducible isoform is modified by the nitrogen treatment. Activity of the constitutive isofroms appear stable during the 48 h treatment, with only a slight decrease in C1-NR activity being observed with time.     

Evidence supporting a non-phloem source of water for export of solutes in the xylem of soybean root nodules

The vascular anatomy of soybean nodules [Glycine max (L.) Merr.] suggests that export of solutes in the xylem should be dependent on influx of water in the phloem. However, after severing of stem xylem and phloem by shoot decapitation, export of ureides from nodules continued at an approximately linear rate for 5h. This result was obtained with decapitated roots remaining in the sand medium, but when roots were disturbed by removal from the rooting medium prior to shoot decapitation, export of ureides from nodules was greatly reduced. Stem exudate could not be collected from disturbed roots, indicating that flow in the root xylem had ceased. Thus, ureide export from nodules appeared to be dependent on a continuation of flow in the root xylem. When seedlings were fed a mixture of ³H₂O and ¹⁴C-inulin for periods of 14–21 min, nodules had higher ³H/¹⁴C ratios than roots from which they were detached. The combined results are not consistent with the proposal that export of nitrogenous compounds from nodules is dependent on import of water via the phloem. The results do support the view that a portion of the water required for xylem export from soybean nodules is supplied via a symplastic route from root cortex to nodule cortex to the nodule vascular apoplast.     

The feedback mechanism of nitrate inhibition of nitrogenase activity in soybean may involve asparagine and/or products of its metabolism

A feedback mechanism which involves sensing of change in phloem N concentration has been proposed to control nodulation and dinitrogen fixation in the presence of external combined N. Whether this control is in response to a change in total N or in some specific signal compound(s) is not known. In the present study we reevaluated the hypothesis that control of nodulation and N₂ fixation involves sensing of change in tissue N composition and attempted to identify potential signal molecule(s) involved. Two soybean (Glycine max [L.] Merr.) genotypes (Williams 82 and NOD1-3) differing in nodule number and tolerance to nitrate were germinated in sand trays. Seven-day-old seedlings were inoculated with a solution of Bradyrhizobium japonicum and grown for 28 days in growth chambers, using a hydroponic system with limited N supply to promote nodulation. Half of 28-day-old plants were treated with 15 mM NO₃^?, then control and treated plants were sampled at the onset of nitrogenase inhibition (24 h following NO₃^?, treatment) for evaluation of nitrogenase activity and tissue concentration of total N and of each individual free amino acid. Phenylisothiocyanate-(PITC) amino acid derivatives were separated and quantified using HPLC. The decline in nitrogenase activity following the short-term nitrate treatment was associated with a dramatic asparagine concentration increase in the shoot and an increase in nodule aspartate and glutamate in both genotypes. Asparagine concentration in the shoot increased 35 times from a barely detectable level of 95 to 3 327 nmol g^?1 fresh weight in Williams 82, and more than tripled from 509 to 1 753 nmol g^?1 fresh weight in NOD1-3. Increase in levels of free Asn and in total free amino acids in the shoot following the short-term nitrate treatment was more pronounced in Williams 82 than in its partially nitrate-tolerant mutant NOD1-3. These results indicate that the feedback control of nodule activity may involve sensing changes in shoot asparagine levels and/or products of its metabolism (aspartate and glutamate) in the nodule. These results also indicate that partial-nitrate tolerance of nodulation in the hypernodulated NOD1-3 mutant is associated with a lesser change in tissue N following nitrate treatment.     

Inoculation and nitrate alter phytohormone levels in soybean roots: differences between a supernodulating mutant and the wild type

 The levels of different cytokinins, indole-3-acetic acid (IAA) and abscisic acid (ABA) in roots of Glycine max [L.] Merr. cv. Bragg and its supernodulating mutant nts382 were compared for the first time. Forty-eight hours after inoculation with Bradyrhizobium, quantitative and qualitative differences were found in the root's endogenous hormone status between cultivar Bragg and the mutant nts382. The six quantified cytokinins, ranking similarly in each genotype, were present at higher concentrations (30–196% on average for isopentenyl adenosine and dihydrozeatin riboside, respectively) in mutant roots. By contrast, the ABA content was 2-fold higher in Bragg, while the basal levels of IAA [0.53 μmol (g DW)⁻¹, on average] were similar in both genotypes. In 1 mM NO₃ ⁻-fed Bragg roots 48 h post-inoculation, IAA, ABA and the cytokinins isopentenyl adenine, and isopentenyl adenosine quantitatively increased with respect to uninoculated controls. However, only the two cytokinins increased in the mutant. High NO₃ ⁻ (8 mM) markedly reduced root auxin concentration, and neither genotypic differences nor the inoculation-induced increase in auxin concentration in Bragg was observed under these conditions. Cytokinins and ABA, on the other hand, were little affected by 8 mM NO₃ ⁻. Root IAA/cytokinin and ABA/cytokinin ratios were always higher in Bragg relative to the mutant, and responded to inoculation (mainly in Bragg) and nitrate (both genotypes). The overall results are consistent with the auxin-burst-control hypothesis for the explanation of autoregulation and supernodulation in soybean. However, they are still inconclusive with respect to the inhibitory effect of NO₃ ⁻. Received: 16 April 1999 / Accepted: 13 December 1999     

Effect of chlorate, nitrogen source, and light on chlorate toxicity and nitrate reductase activity in soybean leaves

Growth chamber studies were conducted to assess the relationship between nitrate reductase (NR) activity and development of chlorate (KClO₃) toxicity symptoms in leaflets of soybeans [Glycine max (L.) Merr.]. Fourteen day-old soybean seedlings, grown in NO₃ - or urea-nutrient solutions, were exposed to various KClO₃ concentrations (0 to 2.0 mM) and light levels (100, 67, 33 and 0% of full light which was 750 μE m^?2s^?1) for 24 h. Visual KClO₃ toxicity symptoms were noted and NR activity was measured. Toxicity symptoms (interveinal chlorosis) were evident within 24 h following addition of 0.5 mM KClO₃ to the nutrient solution, regardless of N nutrition, and symptom severity increased with increased KClO₃ concentration (up to 2.0 mM). Leaflet NR activity was lower following 24 h KClO₃ treatments at concentrations of 0.5 mM and higher, indicating that ClO₃ - or some reduction product of ClO₃ - likely ClO₂ - was detrimental to enzyme functionality. The light study supported involvement of NR activity in KClO₃ toxicity in that comparison of control and KClO₃ treated plants exposed to decreased light levels revealed a decrease in NR activity of control plants parallel to a decrease in severity of KClO₃ toxicity symptoms of treated plants. Urea-grown plants, which have an apparent constitutive NR enzyme, were used to verify that the KClO₃ toxicity symptoms were not simply N starvation symptoms due to competition of ClO₃ - and NO₃ - for uptake and reduction. In vivo NR assays also ruled out that ClO₃ - was decreasing NR activity through competition with NO₃ - for reduction sites. The close relationship between KClO₃ toxicity symptoms and NR activity, in response to light treatments, suggested that KClO₃ toxicity symptoms were associated with reduction of ClO₃ - to ClO₂ - by the NR enzyme. However, the possibility that a more direct photochemical reaction occurred in the presence of KClO₃ to produce the toxicity symptoms could not be ruled out.     

Effects of norflurazon (San 9789) on light-increased extractable nitrate reductase activity in soybean [Glycine max (L.) Merr.]seedlings

Abstract. The effects of norflurazon (San 9789) on light-increased extractable NADH nitrate reductase activity (NRA) in soybean seedlings were studied. Continuous white light (W) increased NRA steadily in root and cotyledonary tissues over a 5 d period. Morflurazon, a pyridazinone herbicide which causes chlorophyll bleaching in W, reduced the initial NRA induction rate in roots and cotyledons. However, in cotyledons of norfiurazon-treated plants NRA increased at a more rapid rate than in the control after 24 h of W, with activity levels reaching three times those of control seedlings after 5 d. NRA induced by W in control and norflurazon-treated cotyledons was fluence-rate dependent. Continuous FR induced equal amounts of NRA in control and norflurazontreated tissues, suggesting that the superinduceable NRA of norflurazon-treated plants under W is not phytochrome induced. The FR-induced NRA of control and norflurazon-treated cotyledons had pH optima of 6.6, but during development under W the pH optimum of control cotyledons changed from 6.3 to between 6.6 and 7.1. The pH optimum of the norflurazon-induced NRA of the cotyledon under W was about 7.5. The NADH/NADPH NRA ratio after 4 d of W was 1.3 in control and 2.5 in norflurazontreated cotyledons. These data indicate that photosynthelic pigments are involved only secondarily in light-induction of NRA in this system.     

氨和谷氨酰胺对浑球红假单胞菌（Rhodobacter sphaeroides）固氮酶...

浑球红假单胞菌菌株601具有迅速对外源氨作出“关闭”固氮酶活性的反应。氨对固氮酶的抑制作用,可被谷氨酰胺合成酶(GS)抑制剂MSX所解除。反之,加入Glu代谢抑制剂DON,可延长氨抑制的持续时间。Gln对固氮酶也有抑制作用。在脱腺苷化GS的透性细胞中,加入Gln可抑制固氮酶活性,同时,GS腺苷化状态提高。然而,氨则对透性细胞的固氮酶活性和GS腺苷化状态没有影响。     

Biochemical characterization of nitrate and nitrite reduction in the wild-type and a nitrate reductase mutant of soybean

Enzyme activities involved in nitrate assimilation were analyzed from crude leaf extracts of wild-type (cv. Williams) and mutant ( nr₁ ) soybean [ Glycine max (L.) Merr.] plants lacking constitutive nitrate reductase (NR) activity. The nr₁ soybean mutant (formerly LNR-2), had decreased NADH-NR, FMNH₂-NR and cytochrome c reductase activities, all of which were associated with the loss of constitutive NR activity. Measurement of FMNH₂-NR activity, by nitrite determination, was accurate since nitrite reductase could not use FMNH₂ as a reductant source. Nitrite reductase activity was normal in the nr₁ plant type in the presence of reduced methyl viologen. Assuming that constitutive NR is similar in structure to nitrate reductases from other plants, presence of xanthine dehydrogenase activity and loss of cytochrome c reductase activity indicated that the apoprotein and not the molybdenum cofactor had been affected in the constitutive enzyme of the mutant. Constitutive NR from urea-grown wild-type plants had 1) greater ability to use FMNH₂ as an electron donor, 2) a lower pH optimum, and 3) decreased ability to distinguish between NO₃⁻ and HCO₃⁻, compared with inducible NR from NO₃⁻-grown nr₁ plants. The presence in soybean leaves of a nitrate reductase with a pH optimum of 7.5 is contrary to previous reports and indicates that soybean is not an exception among higher plants for this activity.     

Increase in nitrate uptake by soybean plants during interruption of the dark period with low intensity light

Diurnal patterns of net NO⁻₃ uptake by nonnodulated soybean [ Glycine max (L.) Merr. cv. Ransom] plants growing in flowing hydroponic culture at 26 and 16°C root temperatures were measured at hourly intervals during alternate days of a 12-day growth period. Ion chromatography was used to determine removal of NO⁻₃ from the culture solution. Day and night periods of 9 and 15 h were used during growth. The night period included two 6-h dark periods and an intervening 3-h period of night interruption by incandescent lamps to effect a long-day photoperiod and repress floral initiation. At both root temperatures, the average specific rates of NO⁻₃ uptake were twice as great during the night interruption period as during the day period; they were greater during the day period than during the dark periods; and they were greater during the dark period immediately following the day period than during the later dark period that followed the night interruption. While these average patterns were repetitious among days, measured rates of uptake varied hourly and included intervals of net efflux scattered through the day period and more frequently through the 2 dark periods. Root temperature did not affect the average daily specific rates of uptake or the qualitative relationships among day, dark and night interruption periods of the diurnal cycle.     

缺硼对大豆根瘤结构和功能的影响

在营养液培养条件下以普通结结瘤大豆Ｂｒａｇｇｃｖ．「Ｇｌｙｃｉｎｅｍａｘ（Ｌ．）Ｍｅｒｒ」及其超结瘤突变体ｎｔｓ３８２为实验材料,运用光学显微方法研究了硼对大豆根瘤结构的影响,并测定了根瘤固氮酶活性结果表明,缺硼使根瘤结构受到严重破坏,并使固氮酶活性显著下降,缺硼使根瘤结构受到破坏是导致固氮酶活性下降的可能原因。     

Regulation of nitrogenase activity in Bradyrhizobium japonicum/soybean symbiosis by plant N status as determined by shoot C:N ratio

Regulation of nitrogenase is not sufficiently understood to engineer symbioses that achieve a high N₂ fixation rate under high levels of soil N. In the present hydroponic growth chamber study we evaluated the hypothesis that nitrogenase activity and the extent of its inhibition by NO₃⁻ may be related to both N and carbohydrate levels in plant tissues. A wide range of C:N ratios in various plant tissues (8.5 to 41.0, 1.9 to 3.7, and 0.8 to 1.8, respectively, in shoots, roots, and nodules) was generated through a combination of light and CO₂ levels, using two soybean genotypes differing in C and N acquisition rates. For both genotypes, N concentration in shoots was negatively correlated to nitrogenase activity and positively correlated to the extent of nitrogenase inhibition by NO₃⁻. Furthermore, nitrogenase activity was positively correlated to total nonstructural carbohydrates (TNC) and C:N ratio in shoot and nodules for both genotypes. Nitrogenase inhibition by NO₃⁻ was negatively correlated to TNC and C:N ratio in shoots, but not in nodules for both genotypes. At the onset of nitrogenase inhibition by NO₃⁻, C:N ratio declined in shoots but not in nodules. These results indicate that both C and N levels in plant tissues are involved in regulation of nitrogenase activity. We suggest that the level of nitrogenase activity may be determined by (1) N needs (as determined by shoot C:N) and (2) availability of carbohydrates in nodules. Modulation of the nitrogenase activity may occur through sensing changes in plant N, i.e. changes in shoot C:N ratio, possibly through some phloem translocatable compound(s).     

Regulation of nitrogenase activity in Bradyrhizobium japonicum/soybean symbiosis by plant N status as determined by shoot C:N ratio

Regulation of nitrogenase is not sufficiently understood to engineer symbioses that achieve a high N₂ fixation rate under high levels of soil N. In the present hydroponic growth chamber study we evaluated the hypothesis that nitrogenase activity and the extent of its inhibition by NO₃⁻ may be related to both N and carbohydrate levels in plant tissues. A wide range of C:N ratios in various plant tissues (8.5 to 41.0, 1.9 to 3.7, and 0.8 to 1.8, respectively, in shoots, roots, and nodules) was generated through a combination of light and CO₂ levels, using two soybean genotypes differing in C and N acquisition rates. For both genotypes, N concentration in shoots was negatively correlated to nitrogenase activity and positively correlated to the extent of nitrogenase inhibition by NO₃⁻. Furthermore, nitrogenase activity was positively correlated to total nonstructural carbohydrates (TNC) and C:N ratio in shoot and nodules for both genotypes. Nitrogenase inhibition by NO₃⁻ was negatively correlated to TNC and C:N ratio in shoots, but not in nodules for both genotypes. At the onset of nitrogenase inhibition by NO₃⁻, C:N ratio declined in shoots but not in nodules. These results indicate that both C and N levels in plant tissues are involved in regulation of nitrogenase activity. We suggest that the level of nitrogenase activity may be determined by (1) N needs (as determined by shoot C:N) and (2) availability of carbohydrates in nodules. Modulation of the nitrogenase activity may occur through sensing changes in plant N, i.e. changes in shoot C:N ratio, possibly through some phloem translocatable compound(s).     

Spectral balance and UV-B sensitivity of soybean: a field experiment

Soybean [Glycine max (L.) Merr.] cultivar Essex was grown and tested for sensitivity to UV-B radiation (280–320 nm) under different combinations of UV-A (320–400 nm) and PFD (400–700 nm) radiation in four simultaneous field experiments. The radiation conditions were effected with combinations of filtered solar radiation and UV-B and UV-A lamps electronically modulated to track ambient radiation. Significant UV-B-caused decreases in total aboveground production and growth were seen only when PFD and UV-A were reduced to less than half their flux in sunlight. When PFD was low, UV-A appeared to be particularly effective in mitigating UV-B damage. However, when PFD was high, substantial UV-A did not appear to be required for UV-B damage mitigation. Leaf chlorophyll fluorescence did not indicate photosynthetic damage under any radiation combination. With UV-B, leaves in all experiments exhibited increased UV-absorbing pigments and decreased whole-leaf UV transmittance. Results of these field experiments indicate difficulties in extrapolating from UV-B experiments conducted in glasshouse or growth cabinet conditions to plant UV-B sensitivity in the field. Implications for UV radiation weighting functions in evaluating atmospheric ozone reduction are also raised.     

Responses of soybean to ammonium and nitrate supplied in combination to the whole root system or separately in a split-root system

To address the questions of whether allocation of carbohydrates to roots is influenced by ionic form of nitrogen absorbed and whether allocation of carbohydrates to roots in turn influences proportionality between NH₄⁺ and NO₃^? uptake from mixed sources, NH₄⁺ and NO₃^? were supplied separately to halves of a split-root hydroponic system and were supplied in combination to a whole-root system. Dry matter accumulation in the split-root system was 18% less in the NH₄⁺-fed axis than in the NO₃^?-fed axis. This, however, does not indicate that partitioning of carbohydrate between the two axes was different. Most of the reduction in dry matter accumulation in the NH₄⁺-fed axis can be accounted for by the retransport of CH₂O equivalents from the root back to the shoot with amino acids produced by NH₄⁺ assimilation. Uptake of NH₄⁺ or NO₃^? by the respective halves of the split-root system was proportional to the estimated allocation of carbohydrate to that half. When NH₄⁺ and NO₃^? were supplied to separate halves of the split-root system, the cumulative NH₄⁺ to NO₃^? uptake ratio was 0.81. When supplied in combination to the whole-root system, the cumulative NH₄⁺ to NO₃^? uptake ratio was 1.67. Thus, while the shoot may affect total nitrogen uptake through the export of carbohydrates to roots, the shoot (common for halves of the split-root system) apparently does not exert a direct effect on proportionality of NH₄⁺ and NO₃^? uptake by roots. For whole roots supplied with both NH₄⁺ and NO₃^?, the restriction in uptake of NO₃^? may involve a stimulation of NO₃^? efflux rather than an inhibition of NO₃^? influx. While only the net uptake of NH₄⁺ and NO₃^? was measured by ion chromatography, monitoring at approximately hourly intervals during the first 3 days of treatment revealed irregularly occurring intervals of both depletion (net influx) and enrichment (net efflux) in solutions. In the case of NH₄⁺, numbers of net efflux events were similar (21 to 24 out of 65 sequential sampling intervals) whether NH₄⁺ was supplied with NO₃^? to whole-root systems or separately to an axis of the split-root system. In the case of NO₃^?, however, the number of net efflux events increased from 8 when NO₃^? was supplied to a separate axis of the split-root system to between 19 and 24 when NO₃^? was supplied with NH₄⁺ to whole-root systems.     

Oxygen limitation of N2 fixation in stem-girdled and nitrate-treated soybean

The effects of increasing rhizosphere pO₂on nitrogenase activity and nodule resistance to O₂diffusion were investigated in soybean plants [Glycine max (L.) Merr. cv. Harosoy 63] in which nitrogenase (EC 1.7.99.2) activities were inhibited by (a) removal of the phloem tissue at the base of the stem (stem girdling), (b) exposure of roots to 10 mM NO₃over 5 days (NO₃-treated), or (c) partial inactivation of nitrogenase activity by an exposure of nodulated roots to 100 kPa O₂(O₂-inhibitcd). In control plants and in plants which had been treated with 100 kPa O₂, increasing rhizosphere O₂concentrations in 10 kPa increments from 20 to 70 kPa did not alter the steady-state nitrogenase activity. In contrast, in plants in which nitrogenase activities were depressed by stem girdling or by exposure to NO₃, increasing rhizosphere pO₂resulted in a recovery of 57 or 67%, respectively, of the initial, depressed rates of nitrogenase activity. This suggests that the nitrogenase activity of stem-girdled and NO₃-treated soybeans was O₂-limited. For each treatment, theoretical resistance values for O₂diffusion into nodules were estimated from measured rates of CO₂exchange, assuming a respiratory quotient of 1.1 and 0 kPa of O₂in the infected cells. At an external partial pressure of 20 kPa O₂, the stem-girdled and NO₃^--treated plants displayed resistance values which were 4 to 8.6 times higher than those in the nodules of the control plants. In control and O₂-inhibited plants, increases in pO₂from 20 to 70 kPa in 10 kPa increments resulted in a 2.5- to 3.9-fold increase in diffusion resistance to O₂, and had little effect on either respiration or nitrogenase activity. In contrast, in stem-girdled and NO₃^--treated plants, increases in external pO₂had little effect on diffusion resistance to O₂, but resulted in a 2.3- to 3.2-fold increase in nodule respiration and nitrogenase activity. These results are consistent with stem-girdling and NO₃^--inhibition treatments limiting phloem supply to nodules causing an increase in diffusion resistance to O₂at 20 kPa and an apparent insensitivity of diffusion resistance to increases in external pO₂.     

Relationship between autoregulation and nitrate inhibition of nodulation in soybeans

Ten of 11 supernodulating mutants of soybean [ Glycine max (L.) Merr.] cv. Bragg, in which nodulation was far in excess of that in the wild type, showed pronounced tolerance of nodulation to applied nitrate. Mutant nts (nitrate-tolerant symbiosis) 1116 had an intermediate nodulation response and also showed some inhibition by nitrate. Mutant 1029, a revertant of nts382 (an extreme supernodulator), showed a wild-type nodulation pattern and was equally sensitive to nitrate as cv. Bragg. Grafting experiments with cv. Bragg and nts382 indicated that both supernodulation and tolerance of nodulation to nitrate were dependent on shoot factors. Total leaf nitrate reductase (EC 1.6.6.1 and EC 1.6.6.2) activity of the supernodulating mutants was similar to that in cv. Bragg. We conclude from these results that the inhibitory effect of nitrate on nodule initiation and development in soybean depends on an interaction between nitrate and the autoregulation singal. In the supernodulating mutants, the autoregulation signal is either altered or absent and cosequently nodulation in these mutants is not sensitive to nitrate.     

A comparison of the effects of carbon dioxide concentration and temperature on respiration, translocation and nitrate reduction in darkened soybean leaves

BACKGROUND AND AIMS: Respiration of autotrophs is an important component of their carbon balance as well as the global carbon dioxide budget. How autotrophic respiration may respond to increasing carbon dioxide concentrations, [CO(2)], in the atmosphere remains uncertain. The existence of short-term responses of respiration rates of plant leaves to [CO(2)] is controversial. Short-term responses of respiration to temperature are not disputed. This work compared responses of dark respiration and two processes dependent on the energy and reductant supplied by dark respiration, translocation and nitrate reduction, to changes in [CO(2)] and temperature. METHODS: Mature soybean leaves were exposed for a single 8-h dark period to one of five combinations of air temperature and [CO(2)], and rates of respiration, translocation and nitrate reduction were determined for each treatment. KEY RESULTS: Low temperature and elevated [CO(2)] reduced rates of respiration, translocation and nitrate reduction, while increased temperature and low [CO(2)] increased rates of all three processes. A given change in the rate of respiration was accompanied by the same change in the rate of translocation or nitrate reduction, regardless of whether the altered respiration was caused by a change in temperature or by a change in [CO(2)]. CONCLUSIONS: These results make it highly unlikely that the observed responses of respiration rate to [CO(2)] were artefacts due to errors in the measurement of carbon dioxide exchange rates in this case, and indicate that elevated [CO(2)] at night can affect translocation and nitrate reduction through its effect on respiration.     

Salt stress induces a decrease in the oxygen uptake of soybean nodules and in their permeability to oxygen diffusion

The effects of short-term NaCl-salinity on nodules of soybean ( Glycine max L. cv. Kingsoy) were studied on hydroponically-grown plants. Both acetylene reducing activity (ARA) and nodule respiration (O₂ uptake and CO₂ evolution) were immediately inhibited, and the stimulation of them by rising the external partial pressure of O₂ (pO₂) was diminished by the application of 0.1 M NaCl in the nutrient solution. The permeability of the nodule to O₂ diffusion, estimated by O₂ consumption or CO₂ evolution, was significantly lower in the stressed nodules than in the cootrol ones. The respiratory quotient of intact nodules and the ethanol production of excised nodules were increased by low pO₂ and by salt stress. These data confirm that in salt-stressed soybean nodules, O₂ availability is reduced and fermentative pathways are stimulated.     

Effects of UV-B radiation on soybean yield and seed quality: a 6-year field study

Two soybean [ Glycine max (L.) Merr.] cultivars, Essex and Williams, were grown in the field for 6 consecutive seasons under ambient and supplemental levels of ultravio-Set-B radiation to determine the potential for alterations in yield or seed quality with a reduction in the stratospheric ozone column. The supplemental UV-B fluences simulated a 16 or 25% ozone depletion. The data presented here represent the first field experiment conducted over multiple seasons which assesses the effects of increased UV-B radiation on seed yield. Overall, the cultivar Essex was found to be sensitive to UV-B radiation (yield reductions of 20%) while the cultivar Williams was tolerant. However, the effectiveness of UV-B radiation in altering yield was strongly influenced by the seasonal microclimate, and the 2 cultivars responded differently to these changing factors. Yield was reduced most in Essex during seasons in which water availability was high and was reduced in Williams only when water was severely limiting. The results of these experiments demonstrate the necessity for multiple-year experiments and the need to increase our understanding of the interaction between UV-B radiation and other environmental stresses in order to assess the potential consequences of stratospheric ozone depletion.