Nodulation of Soybeans as Affected by Half-root Infection with Heterodera glycines

A split-root technique was applied to soybean, Glycine max (L.) Merr. cv. Lee 68, to characterize the nature of the nodulation suppression by race 1 of the soybean cyst nematode (SCN), Heterodera glycines. Root-halves of each split-root plant were inoculated with Rhizobium japonicum, and one root-half only was inoculated with various numbers of SCN eggs. Nodulation (indicated by nodule number, nodule weights, and ratio of nodule weight to root weight) and nitrogen-fixing capacity (indicated by rate of acetylene reduction) were systemically and variously suppressed on both root-halves of the split-root plant 5 weeks after half-root inoculation with 12,500 SCN eggs. Inoculation with 500 eggs caused this suppression only on the SCN-infected (+NE) root-half; nodulation on the companion uninfected (-NE) root-half was stimulated slightly. The +NE root-halves inoculated with 5,000 eggs were excised at 2-week intervals; nodulation on the remaining -NE root-halves was not different from that of the noninoculated control when measured 6 weeks after the SCN inoculation. Thus, the systemic suppression of nodulation was reversible upon the removal of the SCN. Similarly, application of various levels of KNO₃ to the -NE root-halves of the split-root plant did not alleviate the suppressed nodulation on the companion +NE root-halves, even though plant growth was much improved at certain levels of nitrogen (125 μg N/g soil). This indicated that the localized suppression of nodulation by SCN was caused by factors in addition to poor plant growth.     

Effect of salinity on nodule formation by soybean

A split-root growth system was employed to evaluate the effect of NaCl on nodule formation by soybean (Glycine max L. Merr. cv Davis). By applying the salinity stress and rhizobial inoculum to only one-half the root system, the effects of salinity on shoot growth were eliminated in the nodulation process. Rhizobium colonization of inoculated root surfaces was not affected by the salt treatments (0.0, 26.6, 53.2, and 79.9 millimolar NaCl). While shoot dry weight remained unaffected by the treatments, total shoot N declined from 1.26 grams N per pot at 0.0 millimolar NaCl to 0.44 grams N per pot at 79.9 millimolar NaCl. The concentration of N in the shoot decreased from 3.75% N (0.0 millimolar NaCl) to 1.26% N at 79.9 millimolar NaCl. The decrease in shoot N was attributed to a sharp reduction in nodule number and dry weight. Nodule number and weight were reduced by approximately 50% at 26.6 millimolar NaCl, and by more than 90% at 53.2 and 79.9 millimolar NaCl. Nodule development, as evidenced by the average weight of a nodule, was not as greatly affected by salt as was nodule number. Total nitrogenase activity (C₂H₂ reduction) decreased proportionally in relation to nodule number and dry weight. Specific nitrogenase activity, however, was less affected by salinity and was not depressed significantly until 79.9 millimolar NaCl. In a second experiment, isolates of Rhizobium japonicum from nodules formed at 79.9 millimolar NaCl did not increase nodulation of roots under salt stress compared to nodule isolates from normal media (0.0 millimolar NaCl). Salt was applied (53.2 millimolar NaCl) to half root systems at 0, 4, 12, and 96 hours from inoculation in a third experiment. By delaying the application of salt for 12 hours, an increase in nodule number, nodule weight, and shoot N was observed. Nodule formation in the 12- and 96-hour treatments was, however, lower than the control. The early steps in nodule initiation are, therefore, extremely sensitive to even low concentrations of NaCl. The sensitivity is not related to rhizobial survival and is probably due to the salt sensitivity of root infection sites.     

Effect of different levels of mineral nitrogen on nodulation and N2 fixation of two cultivars of common bean (Phaseolus vulgaris L.)

An experiment was conducted under greenhouse conditions to evaluate the effect of mineral nitrogen on N₂ fixation of two cultivars of Phaseolus vulgaris L., Puebla 152 and Negro Argel. Nitrogen application was 0, 2.5, 12.5 and 25 mg N Kg^–1 of a vermiculite-sand-mixture at planting time. Shoot and root growth were elevated by nitrogen application at all growth stages. During vegetative growth (V 5) nodule dry weight and nitrogenase activity (acetylene reducing activity) per plant were reduced by nitrogen supply in both cultivars, but less in Negro Argel than in Puebla 152. At later stages nodulation in nitrogen-treated Puebla 152 did not differ from that in non-treated plants, whereas increased nodule number was found in Negro Argel at high nitrogen levels. The influence of mineral N on the total amount of nitrogen fixed in the two bean cultivars was only slightly different.     

Comparative study of nitrogen fixation and carbon metabolism in two chick-pea (Cicer arietinum L.) cultivars under salt stress

Two cultivars of Cicer arietinum with differential tolerance to salinity have been compared by analysing growth, photosynthesis, nodulation, nitrogenase activity, and carbon metabolism in the nodule cytosol. The aim was to help elucidate the relationships between, on the one hand, sucrose and malate metabolism in nodules and, on the other, the inhibition of nitrogen fixation under salt stress. Chick-pea cultivars Pedrosillano (sensitive) and ILC1919 (tolerant) inoculated with Mesorhizobium ciceri strain Ch-191 were grown in a controlled environmental chamber and were treated with salt (0, 50, 75, and 100 mM NaCl) from sowing to harvest time (28 d). Plant growth and photosynthesis were more affected by salt in Pedrosillano than in ILC1919. Also the effect of salt on nodulation and nitrogen fixation was much more pronounced in Pedrosillano. The increase in nodular mass in ILC1919 can partially counteract the inhibition of nitrogenase activity. The enzymes of sucrose breakdown were inhibited by NaCl, but in ILC1919 a rise in alkaline invertase was observed with salinity, which could compensate for the lack of the sucrose synthase hydrolytic activity. The activity of PEPC was stimulated by salt in ILC1919. Also, this cultivar showed higher malate concentrations in root nodules.     

Effect of plant nutrient supply on nodule effectiveness and rhizobium strain competition for nodulation ofLotus pedunculatus

Summary The effect of nutrient supply on nodule formation and competition between Rhizobium strains for nodulation ofLotus pedunculatus was studied. Limiting plant growth by decreasing the supply of nutrients in an otherwise nitrogen-free medium, increased the size but decreased the number and the nitrogenase activity of nodules formed by a fast-growing strain of Lotus Rhizobium (NZP2037). In contrast decreasing nutrient supply caused only a small decline in the size, number and nitrogenase activity of nodules formed by a slow-growing strain (CC814s). Providing small quantities of NH₄NO₃ (50 to 250 g N) to plants grown with a normal supply of other nutrients stimulated nodule development by both Rhizobium strains and increased the nitrogenase activity of the NZP2037 nodules. Differences in the level of effectiveness (nitrogen-fixing ability) of nodules formed by different Rhizobium strains on plants grown with a normal supply of nutrients were less apparent when the plants were grown with decreased nutrient supply or when the plants were supplied with low levels of inorganic N.Inter-strain competition for nodulation ofL. pedunculatus between the highly effective slow-growing strain CC814s and 7 other fast- and slow-growing strains, showed CC814s to form 42 to 100% of the nodules in all associations. The greater nodulating competitiveness of strain CC814s prevailed despite changes in the nutrient supply to the host plant. A tendency was observed for partially effective Lotus Rhizobium strains to become more competitive in nodule formation when plant growth was supplemented with low levels of inorganic nitrogen.     

The influence of phosphorus deficiency on growth and nitrogen fixation of white clover plants

The effects of P deficiency on growth, N(2)-fixation and photosynthesis in white clover (Trifolium repens L.) plants were investigated using three contrasting relative addition rates of P, or following abrupt withdrawal of the P supply. Responses to a constant below-optimum P supply rate consisted of a decline in N(2)-fixation per unit root weight and a small reduction in the efficiency with which electrons were allocated to the reduction of N(2) in nodules. Abrupt removal of P arrested nodule growth and caused a substantial decline in nitrogenase activity per unit root weight, but not per unit nodule mass. Similarly, the rate of photosynthesis per unit leaf area was unaffected by abrupt P removal, whereas CO(2) acquisition for the plant as a whole decreased due to a decline in total leaf area, leaf area per unit leaf weight and utilization of incoming radiation. These changes followed the decline in tissue P concentrations. The ratio between CO(2)-fixation and N(2)-fixation was maintained under short-term P deprivation but increased under long-term low P supply, indicating a regulatory inhibition of nodule activity following morphological and growth adjustments. It is concluded that N(2)-fixation did not limit the growth of clover plants experiencing P deficiency. A low P status induced changes in the relative growth of roots, nodules and shoots rather than changes in N and/or C uptake rates per unit mass or area of these organs.     

The stimulating effect of ammonium on nodulation in Pisum sativum L. is not long lived once ammonium supply is discontinued

Although mineral N generally has a negative effect on legume-rhizobia symbioses, experiments in hydroponic culture in our laboratory (Waterer et al., 1992) have shown that low concentrations of NH⁺ ₄ can stimulate nodulation in pea (Pisum sativum L.). The objectives of the current study were to determine the immediate and residual effects of NH⁺ ₄ on nodulation and N₂ fixation in pea in sand culture. Peas (cv. Express) were exposed to 0.0, 0.5, 1.0, and 2.0 mM of ¹⁵N-labelled (NH₄)₂SO₄ for 28 days after inoculation (DAI). From 28 to 56 DAI the plants were grown on a NH⁺ ₄-free nutrient solution. Plants were harvested at 7, 14, 21, 28 and 56 DAI and nitrogenase activity was measured by gas exchange at 28 and 56 DAI. Root, shoot, and nodule dry weight (DW) and total N content were obtained, in addition to nodule counts and ¹⁵N enrichment of plant composites. The 1.0 and 2.0 mM NH⁺ ₄ treatments consistently resulted in higher total plant DW accumulation than the control (0.0 mM NH⁺ ₄). At 28 DAI, plants exposed to 1.0 and 2.0 mM NH⁺ ₄ had 1.8 to 2.8 times more nodules plant^-1, respectively, and plants exposed to 2.0 mM NH⁺ ₄ had 1.7 fold higher specific nodulation (nodule number g^-1 root DW). However, individual nodule DW was greater in control plants, such that there were no differences in nodule DW per plant among treatments. Ammonium treatment resulted in more nitrogen derived from the atmosphere (NDFA) in peas early in the experiment, but by 28 DAI there were no treatment effects on NDFA. Whole plant and nodule specific nitrogenase activity (µmol H₂ g^-1 nodule DW h^-1) was higher in control plants at 28 DAI. However, by 56 DAI, after an additional 4 weeks of NH⁺ ₄-free nutrition, no differences in nitrogenase activity nor whole plant or specific nodulation were detectable. This study indicates that nodulation in pea is stimulated in sand culture while exposed to NH⁺ ₄. However, once NH⁺ ₄ is removed, relative growth rate, nodulation and nitrogenase activity becomes similar to plants that were never exposed to NH⁺ ₄.     

Nitrogenase activity of two genotypes of Acacia mangium as affected by phosphorus nutrition

The specific nodulation, nitrogenase activity (acetylene reduction) and budgets of carbon allocation to respiration by nodulated roots were examined in two provenances of Acacia mangium Willd. grown in a glasshouse for 17 weeks to investigate the effects of soil phosphorus and genotypes of the host plant on symbiotic nitrogen fixation. Application of phosphorus (0–80 mg P kg^-1 soil) increased specific nodulation (g nodule dry weight g^-1 plant dry weight) of provenance Ma11 by two-fold and the percentage of nodulated root respiration allocated to nitrogenase by 50%, but had no effect on specific activity of nitrogenase or specific respiration coupled with nitrogenase activity. Improved phosphorus nutrition increased the specific nitrogenase activity of provenance Ma9 by 2-fold, the percentage of nodulated root respiration allocated to nitrogenase, and specific nitrogenase-linked respiration by 50%, respectively, but had no effect on the specific nodulation. The percentage of respiration coupled with nitrogenase activity in nodulated root respiration by provenance Ma9 was 60–70% higher than that in provenance Ma11, regardless of phosphorus levels applied. At the optimal level of phosphorus addition (10 mg P kg^-1 soil), provenance Ma9 had a lower dry mass than provenance Ma11. This was accompanied by a lower nodulated root respiration and a higher percentage of nodulated root respiration allocated to nitrogenase activity in provenance Ma9.     

Ammonium assimilation in root nodules of actinorhizal Discaria trinervis. Regulation of enzyme activities and protein levels by the availability of macronutrients (N,P and C)

Asparagine was found to be the main N compound exported from Discaria trinervis nodules. Aspartate (Asp), glutamate (Glu), alanine (Ala) and serine (Ser) were also detected in root xylem sap, but at lower concentrations. A comparable picture is found in nodulated alfalfa. We hypothesized that a similar set of enzymes for Asn synthesis was present in D. trinervis nodules. We demonstrate the expression of most of the enzymes involved in the synthesis of Asn from NH⁺ ₄ and oxoacids, in nodules – but not in roots – of fully symbiotic D. trinervis. By complementation of enzyme assays (^A) and immunodetection (^I) we detected glutamane-synthetase (GS^{A, I}), Asp-aminotransferase (AAT^A), malate-dehydrogenase (MDH^{A, I}, at least two isoforms), Glu-dehydrogenase (GDH^A), Glu-synthase (GOGAT^I) and Asn-synthetase (AS^I). PEP-carboxylase (PEPC) activity was not detected. We previously shown that N acts as a negative regulator of nodulation and nodule growth, while P is a strong stimulator for nodule growth. We present data on the regulation of nodule N metabolism by altering, during 4 weeks, the availability of N, P and light in symbiotic D. trinervis. NH₄NO₃ (2 mM) induced inactivation and degradation of nodule GS, MDH and AS, but activation of GDH and AAT; the amount of nitrogenase components was not affected. A 10-fold increase in P supply did not greatly affect activity and amount of enzymes, suggesting that N metabolism is not P-limited in nodules. On the other hand, suppression of P supply induced an important reduction of nodule GS, GOGAT, MDH and AS protein levels, although nitrogenase was not affected. GDH was the only measured activity that was stimulated by limiting P supply. Shading plants did result in complete degradation of nitrogenase and partial degradation of GS, AS and nodule-specific MDH isoform, but GDH and AAT were activated. These results are discussed in connection with the regulation of nodulation and nodule growth in D. trinervis.     

Effect of composted textile sludge on growth, nodulation and nitrogen fixation of soybean and cowpea

The effect of composted textile sludge on growth, nodulation and nitrogen fixation of soybean and cowpea was evaluated in a greenhouse experiment. The compost was incorporated into soil at 0, 9.5, 19 and 38 t ha(-1) (bases upon the N requirement of the crops, i.e., 0, 50, 100 and 200 kg available N ha(-1)). Growth, nodulation and shoot accumulation of nitrogen were evaluated 36 and 63 days after plant emergence. Nodule glutamine synthetase (GS) activity and leghemoglobin content were evaluated 63 days after emergence. Composted textile sludge did not show negative effects on nodule number and weight, nodule GS activity and leghemoglobin content. Nitrogen accumulation in shoot dry matter in soybean and cowpea was higher than other treatments with application of 19 t ha(-1) of compost. Composting can be an alternate technology for the management of solid textile mill sludge. This study verifies that the composted textile sludge was not harmful to growth, nodulation and nitrogen fixation of soybean and cowpea.     

Nitrogen fixation of nodulation mutants of soybean as affected by nitrate

It was previously reported that three soybean (Glycine max [L.] Merr.) nodulation mutants (NOD1-3, NOD2-4, and NOD3-7) were partially tolerant to nitrate when nitrate was supplied simultaneously with inoculation at the time of transplanting. The current study evaluated the effect of short-term nitrate treatment on nitrogenase activity (C₂H₂ reduction per plant and per nodule weight) and on relative abundance of ureides when nitrate application was delayed until plants were 3 weeks old and nodules were fully developed. Nitrogenase activity of the mutants was similar to that of Williams after an initial 3-week growth period, prior to nitrate treatment. Application of 5 millimolar nitrate resulted in greater inhibition of nitrogenase activity in Williams than in the three mutants. NOD1-3 was most tolerant of nitrate among the mutants tested and showed the highest relative abundance of ureides. Although C₂H₂ reduction activity per plant for NOD1-3 was higher than for Williams in the presence of nitrate, C₂H₂ reduction activity per gram of nodules was lower for NOD1-3 than for Williams in the presence and absence of nitrate. Compared to Williams, NOD1-3 had higher nodule ureide concentration and had similar glutamine synthetase activity in nodule tissue, indicating its nodules have normal nitrogen assimilation pathways. Nitrate application resulted in ureide accumulation in nodule tissue as well as in all plant parts assayed. Unexpectedly, nitrate treatment also increased the rate of ureide degradative capacity of leaves in both NOD1-3 and Williams. The data confirmed that nitrogenase activity of the selected nodulation mutants was more, but still only partially, tolerant of nitrate compared with the Williams parent.     

The effects of shading and defoliation on nodulation and nitrogen fixation by white clover

In a glasshouse experiment, single plants of ten-weeks old white clover (Trifolium repens L.) were subjected to two levels of shading and two levels of defoliation. Nodulation and nitrogen fixation parameters were measured at six sequential harvest over four weeks. Changes in nodule number and hence nodule dry weight per plant were due to nodule decay, sloughing off and non-production and were closely related to losses in root dry weight. Severe defoliation caused degradation of leghaemoglobin, an effect which was seen in less than three days from treatment. It led also to a temporary but marked decrease in the nitrogen fixation capacity of the nodules as measured by the acetylene reduction assay. Recovery of normal activity by the nitrogenase system took about ten days. The effects of shading and defoliation on the pattern of nodulation have been described briefly.     

Cinnamate toxicity expression on phenylalanine ammonia-lyase activity,germination and growth of cucumber (Cucumis sativis) seedlings

Low pH (5.2) decreased nodule number and acetylene reduction. Aluminium further depressed those parameters in theRhizobium leguminosarum-Pisum sativum associations examined. In the Al-treated plants nodule formation by strains 128C53 and 128C30 was not affected by 3 or 15 and 30 or 60 μM Al, respectively, as compared with the number of nodules on plants grown at pH 5.2 in the absence of Al. However, improved nodulation rates by those strains did not enhance plant dry weight or reduced nitrogen content. No differences in nitrogenase activity were found among strains of nodulating plants grown at the same aluminium level. These results suggest that Al-ions affected specifically nitrogenase activity and that this effect was primarily responsible for the reduction in plant growth.     

Effect of exogenous flavonoids on nodulation of pea (Pisum sativum L.)

Selected flavonoids that are known as inducers and a suppressor of nodulation (nod) genes of the symbiotic bacterium Rhizobium leguminosarum bv. viciae were tested for their effect on symbiosis formation with garden pea as the host. A solid substrate was omitted from the hydroponic growing system in order to prevent losses of flavonoids due to adsorption and degradation. The presumed interaction of the tested flavonoids with nod genes has been verified for the genetic background of strain 128C30. A stimulatory effect of a nod gene inducer naringenin on symbiotic nodule number formed per plant 14 d after inoculation was detected at concentrations of 0.1 and 1 micro g ml(-1) nutrient solution. At 10 micro g ml(-1), the highest concentration tested, naringenin was already inhibitory. By contrast, nodulation was negatively affected by a nod gene suppressor, quercetin, at concentrations above 1 micro g ml(-1), as well as by another tested nod gene inducer, hesperetin. The deleterious effect of hesperetin might be due to its toxicity or to the toxicity of its degradation product(s) as indicated by the inhibition of root growth. Both the stimulatory effect of naringenin and the inhibitory effect of quercetin on nodule number were more pronounced at earlier stages of nodule development as revealed with specific staining of initial nodules. The lessening of the flavonoid impact during nodule development was ascribed to the plant autoregulatory mechanisms. Feedback regulation of nodule metabolism might also be responsible for the fact that the naringenin-conditioned increase in nodule number was not accompanied by any increase in nitrogenase activity. By contrast, the inhibitory action of quercetin and hesperetin on nodule number was associated with decreases in total nitrogenase activity. Naringenin also stimulated root hair curling (RHC) as one of the earliest nodulation responses at concentrations of 1 and 10 microg ml(-1), however, the same effect was exerted by the nod gene suppressor, quercetin, suggesting that feedback regulatory mechanisms control RHC in the range of nodulation-inhibiting high flavonoid concentrations. The comparison of the effect of the tested flavonoids in planta with nod gene activity response showed a two orders of magnitude shift to higher concentrations. This shift is explained by the absorption and degradation of flavonoids by both the symbionts during 3 d intervals between hydroponic solution changes. The losses were 99, 96.4, and 90% of the initial concentration of 10 micro g ml(-1) for naringenin, hesperetin, and quercetin, respectively.     

Root-hair infection and nodulation of four grain legumes as affected by the form and the application time of nitrogen fertilizer

The effects of application of combined nitrogen fertilizer (ammonium nitrate or urea) on root-hair infection and nodulation of four grain legumes were studied. Young roots of each legume were inoculated with their compatible rhizobia. The application of the two forms of combined N either at the early stages of plant growth and/or at the time of nodule formation depressed root-hair curling, infection and nodulation. Infection of hairs on the primary roots was more sensitive to the N fertilizer than hair infection of secondary roots in bothVicia faba andPisum sativum. The nodule number and total fresh mass of the four legumes were drastically affected by fertilizer application. The combined N added both at early and at later stages significantly reduced the nodulation ofV. faba, Phaseolus vulgaris andVigna sinensis. The inhibitory effect of urea on nodulation ofP. sativum was only observed when the fertilizer was applied at the late stages of plant growth. It is concluded that, although the nodulation of the four legumes was suppressed by combined N, the initial events ofRhizobium-legume symbiosis (infection of roots and nodule initiation) are more sensitive to combined N than the stages after nodule formation.     

The regulation of nodulation,nitrogen fixation and ammonium assimilation under a carbohydrate shortage stress in the Discaria trinervis-Frankia symbiosis

N₂-fixation is sensitive to limitation in the availability of newly synthesised carbohydrates for the nodules. We decided to explore the response of the D. trinervis - Frankia symbiosis to a transient decrease in carbohydrate supply to nodules. Feedback inhibition of nodulation as well as nodule growth was not released by a 6-day dark stress in D. trinervis nodulated plants. However, nitrogen fixation and assimilation were affected by the imposed stress. Nitrogenase activity was totally inhibited after 4 days of darkness although high levels of nitrogenase components were still detected at this time. Degradation of FeMo and Fe nitrogenase subunits – both at similar rates – was observed after 6 days of dark stress, revealing the need for inactivation to precede enhancement of protein turnover. Glutamine synthetase (GS), malate dehydrogenase (MDH) and asparagine synthetase (AS) polypeptides were also degraded during the dark stress, although at a lower rate than nitrogenase. ARA and nitrogenase were totally recovered 8 days after resuming normal illumination. It seems that current nitrogenase activity and ammonium assimilation are not, or are only weakly linked with the feedback control of nodulation in D. trinervis. These observations give support to the persistence of an autoregulatory signal in mature nodules that is not sensitive to transient shortages of carbon supply and sustains the inhibition of nodulation in the transient absence of N₂ fixation.     

Ontogenic variation in nitrogen assimilation,nodulation and nitrogen fixation in groundnut (Arachis hypogaea) genotypes*

Seasonal pattern of acetylene reduction (AR) and shoot nitrogen accumulation was studied in nine groundnut cultivars. Shoot N accumulation by all the cultivars was maintained until shortly before maturity and it occurred faster over the reproductíve growth phase than over the earlier phases. In all cultivars plant AR (PAR) did not reflect this pattern of N accumulation, being greater over the vegetative and pod initiation phases. This suggests that the commonly observed low PAR values for groundnut over the reproductive growth phase may be the result of factors other than sink competition. There were significant interactions of cultivar with stage of crop growth for PAR, nodule mass, and specific nitrogenase activity (SNA). Virginia types generally showed better nodulation, higher N₂-fixing capacity (both PAR and SNA) than valencias, and significant differences were observed between cultivars within a botanical type.     

一种高效研究大豆根瘤共生固氮的营养液栽培体系

为建立一种既可高效结瘤固氮, 又具有一定产量的大豆(Glycine max)营养液栽培系统, 设计并进行了2个试验。首先在不同供氮条件下, 研究了接种根瘤菌对大豆的结瘤状况、固氮能力、生物量及产量的影响。结果表明, 供氮过高或过低, 均影响大豆生长、产量形成及根瘤固氮; 并且植物生长所需的最适供氮水平远高于生物固氮所需的最适供氮水平。此外, 大豆生物固氮活性最高的时期在生殖期第一期(R1期)之前。由此推断, 大豆R1期前, 供应较低的氮, 有利于根瘤形成及固氮; 而从R1期起, 应提高供氮水平, 以促进植物生长及产量的形成。在此基础上开展第2个试验, 对供氮条件进行了优化处理(即R1期前低氮供应、R1期开始中氮供应)。结果表明, 与持续供应高氮相比, 优化供氮处理不仅可获得较多固氮酶活性较高的大根瘤, 还能保持较好的生长、获得更高的百粒重及维持80%左右的产量。研究结果不仅可为高效研究大豆根瘤共生固氮提供营养液配方, 还可为大豆高产高效栽培提供试验依据。     

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.     

The effect of phosphorus on nodule formation and function in the Casuarina-Frankia symbiosis

A study was conducted to investigate the effects of phosphorus on nodule formation and function in the Casuarina-Frankia symbiosis. The effects of P on growth and survival of Frankia in the rhizosphere was assessed by examing Frankia growth and survival in flasks of basal nutrient solution. There was no growth in the nutrient solution during the experimental period. However, the viability of Frankia in the nutrient solution without P supply was half that of the initial level, whereas, with P supply, there was only a minor decline during the first week. In a growth pouch experiment, supplying P increased plant and nodule growth, irrespective of P status of the inoculant Frankia culture. There were no effects of P status on any growth or nodulation parameters measured when the inoculants had been standardized on the basis of viability. In a split root experiment, Frankia inoculation and application of P together or separately did not cause any significant difference. This suggests that growth and nodulation respond only to total P supply. Increasing P from 0.1 to 10 M significantly increased plant growth but not N concentrations. Both nitrogen-fixation and nitrate supported growth were strongly increased as P increased from 0.1 to 1.0 M. This study indicates that P deficiency limits the growth of host plants more severely than nitrogen fixation processes and P deficiency on nodulation and symbiotic nitrogen fixation in Casuarina cunninghamiana operated indirectly via reducing host plant growth.