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.     

O2 regulation and O2‐limitation of nitrogenase activity in root nodules of pea and lupin

The gas exchange characteristics of intact attached nodulated roots of pea (Pisum sativum cv. Finale X) and lupin (Lupinus albus cv. Ultra) were studied under a number of environmental conditions to determine whether or not the nodules regulate resistance to oxygen diffusion. Nitrogenase activity (H₂ evolution) in both species was inhibited by an increase in rhizosphere pO₂ from 20% to 30%, but recovered within 30 min without a significant increase in nodulated root respiration (CO₂ evolution). These data suggest that the nodules possess a variable barrier to O₂ diffusion. Also, nitrogenase activity in both species declined when the roots were either exposed to an atmosphere of Ar:O₂ or when the shoots of the plants were excised. These declines could be reversed by elevating rhizosphere pO₂, indicating that the inhibition of nitrogenase activity resulted from an increase in gas diffusion resistance and consequent O₂-limitation of nitrogenase-linked respiration. These results indicate that nodules of pea and lupin regulate their internal O₂ concentration in a manner similar to nodules of soybean, despite the distinct morphological and biochemical differences that exist between the nodules of the 3 species. Experiments in which total nitrogenase activity (TNA = H₂ production in Ar:O₂) in pea and lupin nodules was monitored while rhizosphere pO₂ was increased gradually to 100%, showed that the resistance of the nodules to O₂ diffusion maintains nitrogenase activity at about 80% of its potential activity (PNA) under normal atmospheric conditions. The O₂-limitation coefficient of nitrogenase (OLC_N= TNA/PNA) declined significantly with prolonged exposure to Ar:O₂ or with shoot excision. Together, these results indicate a significant degree of O₂-limitation of nitrogenase activity in pea and lupin nodules, and that yields may be increased by realizing full potential activity.     

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.     

The Glycine-Glomus-Bradyhizobium symbiosis. VIII. Phosphorususe efficiency of CO2 and N2 fixation in mycorrhizal soybean

Soybean [ Glycine max (L.) Merr. cv. Hobbit] plants nodulated by Bradyhizobium japonicum strain USDA 110 were grown in pot cultures in severely P- and N-deficient soil and either colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe or fertilized with a high (HP) or low (LP) level of KH₂PO₄ (0.6 or 0.3 m M , respectively), After 7 weeks of growth, nodule and chloroplast activities (C₂H₂ reduction and CO₂ exchange rate) were determined. Photosynthetic P-use efficiency of CO₂ fixation was significantly higher in VAM than in HP plants, while that of nitrogenase activity was lower. The LP plants were intermediate in both respects. The ratio of nodule to chloroplast activity [mol C₂H₂ reduced (mol CO₂ fixed)⁻¹] was highest in HP and lowest in VAM plants. Root colonization by the VAM fungus significantly increased nodule number and dry weight and reduced nodule specific mass and activity in comparison to HP plants. In spite of lower nodule activity, VAM plants were significantly larger and had higher N concentrations than the HP plants. The results suggest nonnutritional. VAM-elicited and host-mediated effects on the symbiotic functions of the legume association.     

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。     

Gaseous diffusive properties of soybean nodules cultured with non-ambient pO2

Measurements of the short-term response of nodulated roots of soybean ( Glycine max L. Merr, cv. Harosoy: Bradyrhizobium japonicum USDA 16) to rapid changes in surrounding pO₂ indicate that their ability to reversibly adjust gaseous diffusive resistance is retained whether plants are cultured in rhizospheres of very low (2.8%) or very high (61.2%) pO₂. Thus the capacity for reversible short-term diffusion adjustment is additional to structural changes in the fixed diffusional barriers of nodules which allow their continued fixation of N₂ in unfavourably high or low external pO₂. Anatomical evidence, involving quantitative measurement of intercellular spaces in the cortical tissues using electron microscopy of thin sections, indicates that the major fixed diffusional barrier is a boundary layer of cells in the inner cortex which may be as small as one cell thick in nodules from 2.8% O₂ to 5 or 6 cells thick, and almost completely devoid of intercellular spaces, in those from 61.2% O₂. The data are interpreted to indicate that the variable diffusion harrier is distinct from the boundary layer and is most likely to be a property of cells and/or intercellular spaces inside the boundary layer of the nodule cortex.     

Decomposition of soybean grown under elevated concentrations of CO2 and O3

A critical global climate change issue is how increasing concentrations of atmospheric CO₂ and ground‐level O₃ will affect agricultural productivity. This includes effects on decomposition of residues left in the field and availability of mineral nutrients to subsequent crops. To address questions about decomposition processes, a 2‐year experiment was conducted to determine the chemistry and decomposition rate of aboveground residues of soybean (Glycine max (L.) Merr.) grown under reciprocal combinations of low and high concentrations of CO₂ and O₃ in open‐top field chambers. The CO₂ treatments were ambient (370 μmol mol^?1) and elevated (714 μmol mol^?1) levels (daytime 12 h averages). Ozone treatments were charcoal‐filtered air (21 nmol mol^?1) and nonfiltered air plus 1.5 times ambient O₃ (74 nmol mol^?1) 12 h day^?1. Elevated CO₂ increased aboveground postharvest residue production by 28–56% while elevated O₃ suppressed it by 15–46%. In combination, inhibitory effects of added O₃ on biomass production were largely negated by elevated CO₂. Plant residue chemistry was generally unaffected by elevated CO₂, except for an increase in leaf residue lignin concentration. Leaf residues from the elevated O₃ treatments had lower concentrations of nonstructural carbohydrates, but higher N, fiber, and lignin levels. Chemical composition of petiole, stem, and pod husk residues was only marginally affected by the elevated gas treatments. Treatment effects on plant biomass production, however, influenced the content of chemical constituents on an areal basis. Elevated CO₂ increased the mass per square meter of nonstructural carbohydrates, phenolics, N, cellulose, and lignin by 24–46%. Elevated O₃ decreased the mass per square meter of these constituents by 30–48%, while elevated CO₂ largely ameliorated the added O₃ effect. Carbon mineralization rates of component residues from the elevated gas treatments were not significantly different from the control. However, N immobilization increased in soils containing petiole and stem residues from the elevated CO₂, O₃, and combined gas treatments. Mass loss of decomposing leaf residue from the added O₃ and combined gas treatments was 48% less than the control treatment after 20 weeks, while differences in decomposition of petiole, stem, and husk residues among treatments were minor. Decreased decomposition of leaf residues was correlated with lower starch and higher lignin levels. However, leaf residues only comprised about 20% of the total residue biomass assayed so treatment effects on mass loss of total aboveground residues were relatively small. The primary influence of elevated atmospheric CO₂ and O₃ concentrations on decomposition processes is apt to arise from effects on residue mass input, which is increased by elevated CO₂ and suppressed by O₃.     

The relationship between nodule adenylates and the regulation of nitrogenase activity by O2 in soybean

Nodulated soybeans (Glycine max L. Merr, cv. Maple Arrow) were exposed to various physiological and environmental treatments to determine the relationship between nodule adenylate pools and the degree of O₂ limitation of nitrogenase. Adenylate energy charge (AEC = [ATP + 0.5 ADP]/[ATP + ADP + AMP]) and ATP/ADP ratios declined under conditions of decreased (10%) external pO₂ but increased in nodules exposed to elevated (30%) external pO₂. Nitrogenase activity was inhibited by both pO₂ treatments, but recovered towards initial levels within 45 min. AEC also returned to initial levels during this period. To account for these and related data in the literature, it was hypothesized that 1) legume nodules regulate infected cell O₂ concentration (Oi) to maintain adenylate pools at levels which limit respiratory metabolism: 2) treatments which decrease Oi alter the adenylate pools and further limit nodule metabolism; 3) treatments which increase Oi to levels in excess of a narrow range alter the adenylate pools and activate biochemical pathways which are not conducive to nitrogenase activity. In a preliminary test of these hypotheses, changes in AEC and ATP/ADP ratio were studied in nodules in which nitrogenase activity was inhibited by stem girdling, nitrate fertilization and exposure to an Ar:O₂ atmosphere. All three treatments caused an increased O₂ limitation of nodule respiration and nitrogenase activity. However, decreases in AEC were observed only in the stem girdling and nitrate fertilization treatment: Ar:O₂ exposure had no effect on whole nodule AEC. While this result challenged the hypotheses suggesting a central role for adenylates in the regulation of O₂-limited metabolism, it was noted that the Ar:O₂ treatment would differ from the other treatments in that it would have a specific effect on the ATP demands for NH₃ assimilation in the plant fraction. Since AEC and ATP/ADP ratio would be affected by both the rate of ATP synthesis (potentially an O₂-limited process) and the demand for ATP, changes in these parameters in the whole nodule may not be a reliable indicator of adenylate-mediated O₂ limitation. Futher studies are needed to examine in vivo changes in adenylate pools in the plant and bacteroid fractions in nodules which vary in their degree of O₂-limited metabolism.     

Intercellular location of glycoprotein in soybean nodules: effect of altered rhizosphere oxygen concentration

Abstract. A glycoprotein which occludes intercellular spaces in the inner cortex of legume nodules may be involved in controlling oxygen diffusion into rhizobial-infected cells. Here we investigated this possibility by localizing the glycoprotein using monoclonal antibodies and immunogold labelling in nodulated roots of soybean cv. Clarke inoculated with Bradyrhizobium japonicum strain RCR3442 exposed to atmospheres with either 10, 21 (control) or 40% oxygen for 28d. Infected cells showed evidence of premature senescence when grown in above or below ambient pO₂ particularly at 10% oxygen, although cortical cells appeared to be little altered by oxygen treatment. In the inner cortical cells, more glycoprotein was seen to be occluding intercellular spaces of those nodules subjected to 40% oxygen and less in those nodules exposed to 10% oxygen, when compared to controls. This observation, made at the light microscope level (using silver enhancement) was confirmed under the TEM using immunogold labelling. Therefore, it is suggested that intercellular space glycoprotein is one of the structural components of the diffusion resistance in the cortex of legume nodules.     

Grafting experiments on the nature of the decline in N2 fixation during fruit development in soybean

A decline in nitrogen fixation at the time of pod-filling is persistently seen in soybeans. This phenomenon was studied by grafting experiments. Young scions with 3 nodes were grafted near the base of fruiting stocks which had passed their peak of nitrogen fixation. These grafts produced a second peak of nitrogen fixation on the same root system, indicating that the decline is reversible. If the scions were grafted near the apex of the fruiting stocks then the second peak of nitrogen fixation was very small. Thus, the translocation system could have an important role in regulating the decline in nitrogen fixation. Grafting of a second shoot of the same age as the rootstock, after the decline in nitrogen fixation, did not reduce the rate of decline even when the fruits from the scion were removed. It appears that physiological changes in different components of the shoot jointly regulate the decline in the rate of nitrogen fixation in soybeans.     

Effects of elevated CO2 and temperature on photosynthesis and Rubisco in rice and soybean

Rice (Oryza sativa L. cv. IR-72) and soybean (Glycine max L. Merr. cv. Bragg), which have been reported to differ in acclimation to elevated CO₂, were grown for a season in sunlight at ambient and twice-ambient [CO₂], and under daytime temperature regimes ranging from 28 to 40°C. The objectives of the study were to test whether CO₂ enrichment could compensate for adverse effects of high growth temperatures on photosynthesis, and whether these two C₃ species differed in this regard. Leaf photosynthetic assimilation rates (A) of both species, when measured at the growth [CO₂], were increased by CO₂ enrichment, but decreased by supraoptimal temperatures. However, CO₂ enrichment more than compensated for the temperature-induced decline in A. For soybean, this CO₂ enhancement of A increased in a linear manner by 32–95% with increasing growth temperatures from 28 to 40°C, whereas with rice the degree of enhancement was relatively constant at about 60%, from 32 to 38°C. Both elevated CO₂ and temperature exerted coarse control on the Rubisco protein content, but the two species differed in the degree of responsiveness. CO₂ enrichment and high growth temperatures reduced the Rubisco content of rice by 22 and 23%, respectively, but only by 8 and 17% for soybean. The maximum degree of Rubisco down-regulation appeared to be limited, as in rice the substantial individual effects of these two variables, when combined, were less than additive. Fine control of Rubisco activation was also influenced by both elevated [CO₂] and temperature. In rice, total activity and activation were reduced, but in soybean only activation was lowered. The apparent catalytic turnover rate (K_cat) of rice Rubisco was unaffected by these variables, but in soybean elevated [CO₂] and temperature increased the apparent K_cat by 8 and 22%, respectively. Post-sunset declines in Rubisco activities were accelerated by elevated [CO₂] in rice, but by high temperature in soybean, suggesting that [CO₂] and growth temperature influenced the metabolism of 2-carboxyarabinitol-1-phosphate, and that the effects might be species-specific. The greater capacity of soybean for CO₂ enhancement of A at supraoptimal temperatures was probably not due to changes in stomatal conductance, but may be partially attributed to less down-regulation of Rubisco by elevated [CO₂] in soybean than in rice. However, unidentified species differences in the temperature optimum for photosynthesis also appeared to be important. The responses of photosynthesis and Rubisco in rice and soybean suggest that among C₃ plants species-specific differences will be encountered as a result of future increases in global [CO₂] and air temperatures.     

Proton density and apoplastic domains within soybean nodules in relation to the oxygen diffusion barrier

A spin-echo pulse sequence was used to obtain ¹H nuclear magnetic resonance microimages of soybean nodules, with resolution to 40 mm and 3D presentation. At an acquisition time (TR) of c. 270 ms and a short echo time (TE) of 8.3 ms, a high proton intensity was detected in infected tissue and vascular strands, relative to the cortex. A longer TE of 15.5 ms was associated with a relatively low proton intensity in infected tissue. Thus protons in infected tissue were characterized by a short T₂. Following detopping of the plant, or treatment of the root system with Ar:O₂, proton intensity (TE 5.9, TR 120 ms) decreased exponentially in a zone corresponding to the inner cortex-outer infected region. This result is interpreted as being caused by a loss of water mobility (i.e. increase in viscosity) or a filling of intercellular air spaces with fluid (i.e. loss of air-water interfaces). The membrane impermeant tracer lucifer yellow was observed to infiltrate from the rhizosphere through the nodule cortex into the infected region in intact nodules of detopped and Ar:O₂-treated, but not control, plants. This result is consistent with the filling of air spaces with fluid following the imposed treatments, allowing infiltration of aqueous tracers. Variation in intercellular air space volume within the inner cortex has been suggested as a mechanism to allow a variable rate of diffusion of O₂ into soybean nodules.     

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 role of temperature in determining the stimulation of CO2 assimilation at elevated carbon dioxide concentration in soybean seedlings

Soybean ( Glycine max cv. Clark) was grown at both ambient (ca 350 μmol mol⁻¹) and elevated (ca 700 μmol mol⁻¹) CO₂ concentration at 5 growth temperatures (constant day/night temperatures of 20, 25, 30, 35 and 40°C) for 17–22 days after sowing to determine the interaction between temperature and CO₂ concentration on photosynthesis (measured as A, the rate of CO₂ assimilation per unit leaf area) at both the single leaf and whole plant level. Single leaves of soybean demonstrated increasingly greater stimulation of A at elevated CO₂ as temperature increased from 25 to 35°C (i.e. optimal growth rates). At 40°C, primary leaves failed to develop and plants eventually died. In contrast, for both whole plant A and total biomass production, increasing temperature resulted in less stimulation by elevated CO₂ concentration. For whole plants, increased CO₂ stimulated leaf area more as growth temperature increased. Differences between the response of A to elevated CO₂ for single leaves and whole plants may be related to increased self-shading experienced by whole plants at elevated CO₂ as temperature increased. Results from the present study suggest that self-shading could limit the response of CO₂ assimilation rate and the growth response of soybean plants if temperature and CO₂ increase concurrently, and illustrate that light may be an important consideration in predicting the relative stimulation of photosynthesis by elevated CO₂ at the whole plant level.     

Indole-3-acetic acid oxidase and syringaldazine oxidase activities of peroxidase isozymes in soybean root nodules

Many isoperoxidases with indole-3-acetic acid oxidase (IAA) and syringaldazine oxidase activities were detected by polyacrylamide gel electrophoresis in soybean root nodules [ Glycine max (L.) Merrill, cv. Asgrow], detached at the onset of flowering. The kinetics of the two activities were studied with some of the isoperoxidases partially purified by ion exchange chromatography. IAA oxidase activity of the cationic isoforms showed a sigmoidal kinetic behaviour and a higher substrate affinity than the anionic ones, whereas typical saturation kinetics were found with an anionic fraction that contained leghemoglobins. So, nodule IAA oxidase activity may mainly be displayed by the cationic isoforms. These cationic isoperoxidases had high affinity towards syringaldazine and they also may be associated with cell wall rigidification.     

The simultaneous use of 14CO2 and 15N2 labelling techniques to study the carbon and nitrogen economy of legumes grown under natural conditions

A method based on simultaneous short-term exposure to ¹⁴CO₂ and ¹⁵N₂ is described for studying nitrogen fixation and distribution in legumes relative to carbon assimilation and use. Equipment designed to accomodate experiments under natural conditions with very little disturbance of the N₂ fixing association is used. It permits continuous measurement and regulation of variables such as air temperature, humidity and CO₂ concentration as well as soil aeration. Measurements of distribution and use of assimilates, respiration of nodulated roots, quantitative N₂ fixation and the distribution and fate of fixed N as a function of time lead to a precise estimation of C and N budgets for each labelling period. When experiments are done at several phenological stages they give a new insight into the complex C and N interrelations in legume symbiosis.
A series of trials throughout the growth period of Glycine max (L.) Merr. cv. Hodgson demonstrated the sensitivity of the method. The development of the plants from vegetative to reproductive stages was accompanied by a complete change in the distribution patterns of current assimilates and products of nitrogen fixation. Maximum sink strength moved from the leaves to the pods and seeds which ended up receiving 70% of the incoming C and 35% of the fixed N. The fact that up to 85% of fixed N in the plants was in the reproductive organs at maturity can be accounted for by remobilisation from vegetative parts.
The respiration of nodulated roots utilized 33% of carbon translocated to below-ground plant parts before nitrogen fixation started, but as much as 50% during the period of optimal fixation. The advantages and limitations of the isotopic method described are critically discussed as a prelude to future investigations.     

Effects of drought on nitrogen fixation in soybean root nodules

Soybean plants [Glycine max (L.) Merr.] were grown in silica sand and were drought stressed for a 4 week period during reproductive development and without any mineral N supply in order to maximize demand for fixed nitrogen. A strain of Bradyrhizobium japonicum that forms large quantities of polysaccharide in nodules was used to determine whether or not the supply of reduced carbon to bacteroids limits nitrogenase activity. A depression of 30–40% in nitrogen content in leaves and pods of stressed plants indicated a marked decline in nitrogen fixation activity during the drought period. A 50% increase in the accumulation of bacterial polysaccharide in nodules accompanied this major decrease in nitrogen fixation activity and this result indicates that the negative impact of drought on nodules was not due to a depression of carbon supply to bacteroids. The drought treatment resulted in a statistically significant increase in N concentration in leaves and pods. Because N concentration and chlorophyll concentration in leaves were not depressed, there was no evidence of nitrogen deficiency in drought‐stressed plants, and this result indicates that the negative impact of drought on nodule function was not the cause of the depression of shoot growth. At the end of the drought period, the concentration of carbohydrates, amino nitrogen, and ureides was significantly increased in nodules on drought‐stressed plants. The overall results support the view that, under drought conditions, nitrogen fixation activity in nodules was depressed because demand for fixed N to support growth was lower.     

Comparison of photosynthetic acclimation to elevated CO2 and limited nitrogen supply in soybean

Plants grown at elevated CO₂ often acclimate such that their photosynthetic capacities are reduced relative to ambient CO₂-grown plants. Reductions in synthesis of photosynthetic enzymes could result either from reduced photosynthetic gene expression or from reduced availability of nitrogen-containing substrates for enzyme synthesis. Increased carbohydrate concentrations resulting from increased photosynthetic carbon fixation at elevated CO₂ concentrations have been suggested to reduce the expression of photosynthetic genes. However, recent studies have also suggested that nitrogen uptake may be depressed by elevated CO₂, or at least that it is not increased enough to keep pace with increased carbohydrate production. This response could induce a nitrogen limitation in elevated-CO₂ plants that might account for the reduction in photosynthetic enzyme synthesis. If CO₂ acclimation were a response to limited nitrogen uptake, the effects of elevated CO₂ and limiting nitrogen supply on photosynthesis and nitrogen allocation should be similar. To test this hypothesis we grew non-nodulating soybeans at two levels each of nitrogen and CO₂ concentration and measured leaf nitrogen contents, photosynthetic capacities and Rubisco contents. Both low nitrogen and elevated CO₂ reduced nitrogen as a percentage of total leaf dry mass but only low nitrogen supply produced significant decreases in nitrogen as a percentage of leaf structural dry mass. The primary effect of elevated CO₂ was to increase non-structural carbohydrate storage rather than to decrease nitrogen content. Both low nitrogen supply and elevated CO₂ also decreased carboxylation capacity (V_cmax) and Rubisco content per unit leaf area. However, when V_cmax and Rubisco content were expressed per unit nitrogen, low nitrogen supply generally caused them to increase whereas elevated CO₂ generally caused them to decrease. Finally, elevated CO₂ significantly increased the ratio of RuBP regeneration capacity to V_cmax whereas neither nitrogen supply nor plant age had a significant effect on this parameter. We conclude that reductions in photosynthetic enzyme synthesis in elevated CO₂ appear not to result from limited nitrogen supply but instead may result from feedback inhibition by increased carbohydrate contents.