Non-nodulated Plants Research Articles

Growth, ionic balance, proton excretion, and nitrate reductase activity in Alnus and Hippophaë supplied with different sources of nitrogen

Pre-cultivated, nodulated and non-nodulated plants of black alder (Alnus glutinosa) and sea buckthorn (Hippophae rhamnoides ssp.rhamnoides) were grown on different N sources, with and without acidity control. Dry matter yields were lowest when plants were supplied with only NO3− and were much greater when NH4+ was supplied either alone or in combination with NO3− as long as the external pH was controlled; the final yields of the N2-fixing plants were relatively low, especially withH. rhamnoides. Without acidity control, yields were greatly reduced in the presence of NH4+.

Natural15N Abundance of Field Grown Soybean Grains Harvested in Various Locations in Japan and Estimate of the Fractional Contribution of Nitrogen Fixation

Natural 15N concentrations (δ15N values by per mil relative to atmospheric nitrogen) of soybean grains and soils from various locations in Japan were analyzed by a ratio mass spectrometer. The δ15N values of nonnodulating plants showed considerable variations ranging from -3.34 to 11.56‰. The δ15N values of the nodulated plants also differed depending on the location. but mostly they were lower than the values of the nonnodulating plants in each location. The %N derived from atmospheric N2 in the nodulated plants based on the differences in the δ15N values was estimated at 50% as an average of 37 measurements. The δ15N values of the soils where soy beans were grown, showed small variations compared to those of soy bean grains. There was no special relation between the δ15N values of the soils and soybean grains.

Nitrogen fixation in soybeans as influenced by cultivar and Rhizobium strain differences

The effect of soybean variety and Rhizobium strain, as well as the level of nitrogen fertilizer, were tested in a field experiment in Greece. Three nodulating varieties were used and one non-nodulating variety as control, with two Rhizobium strains, one commercially available and one isolated from the soil of the experimental site. Nitrogen fertilizer labeled with 15N enabled the nitrogen derived from the atmosphere (Ndfa), from the soil (Ndfs), and from the fertilizer (Ndff) to be estimated. Statistical analyses showed that the combined variety-strain effect was responsible for most of the variation observed in all parameters, either measured directly or calculated, while the nitrogen level had no effect. The locally isolated strain generally performed equally as well as the commercial one. With the highest yielding variety in particular, the nitrogen-fixing efficiency of soybean nodules, expressed as plant total nitrogen in excess of that in the non-nodulated control in relation to nodule dry weight, was even superior. Taking into account the low fertilizer recovery by plants, it is suggested that adapted cultivars properly inoculated can fix most of the nitrogen they need for high yields. This is of great economic importance for crops with high nitrogen-fixing efficiency, like the soybeans used in this work, which can fix more than 200 kg N/ha, as 15N dilution and nitrogen difference evaluations have confirmed. The local cultivars in this connection were superior to the United States cultivar, while the difference between cultivars in symbiotic performance were not due to the ability to nodulate, but rather to a much higher nodule-specific efficiency. The commercially available strain was found to produce generally more nodules and the locally isolated strain produced less but larger and more efficient nodules. Finally, the grains from the nodulated plants contained significantly smaller quantities of Fe, Ni, Cu, Zn, and especially Mo compared with non-nodulated plants, as determined by X-ray fluorescence techniques.

Alternative Path Mediated ATP Synthesis in Roots of Pisum sativum upon Nitrogen Supply

Changes in the efficiency of root respiration were examined on intact plants of Pisum sativum L. cv Rondo after addition of nitrate or ammonium to the culture solutions. Nitrate was absorbed immediately after addition and elicited a respiratory rise (O(2)-uptake as well as CO(2)-production) to 160% at most. This occurred both in roots of plants fixing N(2) and in those of non-nodulated plants pregrown for 1 or 2 weeks on a nitrogen-free culture solution. In older plants, used after 2 weeks of N-free growth, the full capacity of the cytochrome path was engaged in root respiration. This was demonstrated by the absence of an effect of the uncoupler carbonylcyanide m-chlorophenylhydrazone in the presence of 25 millimolar salicylhydroxamate, an inhibitor of the alternative path. In these plants more than 90% of the nitrate-induced stimulation of root respiration was salicylhydroxamate-sensitive. In young plants, used after 1 week of N-free growth, the cytochrome path was not saturated. Its activity increased instantaneously at the expense of alternative path activity, which initially dropped to zero and subsequently increased to 160% of the control 7 hours after nitrate supply. The rate of photosynthesis rose to 120% of the control, but not before 1 hour after nitrate supply, suggesting that the stimulation of root respiration was not due to a higher rate of photosynthesis. Experiments with plants grown with a split-root system showed that respiration rate and alternative path activity only increased in the root halves exposed to nitrogen. Ammonium was equally effective as nitrate in stimulating root respiration. These results lead to the conclusion that alternative-path mediated root respiration contributes to synthesis of ATP during at least the first 24 hours following nitrogen supply.

Azospirillum–Rhizobium interaction leading to a plant growth stimulation without nodule formation

The effect of inoculation of white clover plants with mixed cultures of Rhizobium trifolii strain ANU870 and Azospirillum brasilense strain SP245 was examined. Ratios of Rhizobium–Azospirillum (R:A) of 1:200 to 1:2500 caused an inhibition of nodulation. However, these nonnodulated plants did not show nitrogen-deficiency symptoms when grown on nitrogenfree medium. When these plants were assayed for acetylene reduction activity a low level of ethylene production was detected. A significant increase in plant dry weights was also observed. Isolation of viable bacterial cells from surface-sterilized root segments of plants inoculated with an R:A ratio of 1:200 revealed that 80% of the bacterial population was made up of the Azospirillum strain. Under laboratory conditions transfer of the Rhizobium Sym(biosis) plasmid pBRIAN to strain SP245 was observed ex planta. However, the Sym plasmid was unstable in Azospirillum. A high frequency of Tn5 transfer from pBRIAN to strain SP245 occurred when strains ANU870 and SP245 were mixed in the rhizosphere and (or) in the root tissue. Tn5 transposed preferentially into the smallest indigenous plasmid of strain SP245 and was easily lost when this strain (SP245::Tn5) was not maintained on selective medium. This mutated Azospirillum strain caused plant growth stimulation when inoculated onto white clover plants.

Nitrogen Partitioning and Redistribution in Nonnodulating Peanut Related to Nitrogen Stress1

AbstractNitrogen is essential for vegetative and reproductive growth of peanut (Arachis hypogaeaL.), and hypothetically plants may become stressed for N especially during fruiting despite effective N2fixation. The objectives of this study were to determine the effects of N stress during specific fruiting phenophases on N partitioning and fruit growth. Reproductive growth was competitive with vegetative parts in peanut during periods of N stress. In a glasshouse study in 1982, N was completely withdrawn from nonnodulating (M4‐2) plants in pots containing silica sand supplied with nutrient solution. The duration of N removal from the medium was more critical than the specific fruiting phenophase in affecting root growth, pod yield, and N redistribution. Nitrogen was redistributed from vegetative parts to pods in the long N stress periods (> 81 days), whereas there was no detectable N redistribution in the control plants or those subjected to short N stress periods. The calculated N distribution index (NDI) showed that pods competed with vegetative parts for N, if the N stress period was long, but competition was not evident during shortstress periods (<30 days). The magnitude of redistribution (NDI) to seeds relative to source was leaves > stems > roots (in that order), the latter resisting redistribution except in stress periods of long duration

Studies on Genetic Male-Sterile Soybeans

Soybean (Glycine max [L.] Merr.) germplasm, isogenic except for loci controlling male sterility (ms(1)) and nodulation (rj(1)), was used to investigate the effects of reproductive tissue development and source of nitrogen nutrition on accumulation, transport, and partitioning of nitrogen in a greenhouse experiment. Nodulated plants were supplied nitrogen-free nutrient solution, and nonnodulated plants were supplied nutrient solution containing 20 millimolar KNO(3). Plants were sampled from flowering until maturity (77 to 147 days after transplanting).Accumulation rates of nitrogen in whole plants during reproductive growth were not significantly different among the four plant types. Nitrogen accumulation in the sterile, nonnodulated plants, however, ceased 2 weeks earlier than in fertile, nonnodulated or fertile and sterile, nodulated plants. This early cessation in nitrogen accumulation resulted in sterile, nonnodulated plants accumulating significantly less whole plant nitrogen by 133 days after transplanting (DAT) than fertile, nonnodulated plants. Thus, changing the site of nitrogen assimilation from nodules (N(2)-fixing plants) to roots and leaves (NO(3)-fed plants) resulted in similar whole-plant nitrogen accumulation rates in fertile and sterile plants, despite the absence of seed in the latter.Leaflet and stem plus petiole tissues of both types of sterile plants had significantly higher nitrogen concentrations after 119 DAT than both types of fertile plants. Significantly higher concentrations and exudation rates of nonureide, reduced-nitrogen in xylem sap of sterile than of fertile plants after 105 DAT were observed. These latter results indicated possible cycling of nonureide, reduced-nitrogen from the downward phloem translocation stream to the upward xylem translocation stream in roots of sterile plants. Collectively, these results suggest a lack of sinks for nitrogen utilization in the shoots of sterile plants. Hence, comparison of nitrogen accumulation rates for sterile and fertile plants does not provide a definitive test of the hypothesis that reproductive tissue development limits photosynthate availability for support of N(2) fixation and nitrate assimilation in determinate soybeans.Nitrogen assimilation during reproductive growth met a larger proportion of the reproductive-tissue nitrogen requirement of nitrate-dependent plants (73%) than of N(2)-fixing plants (63%). Hence, vegetative-tissue nitrogen mobilization to reproductive tissue was a more prominent process in N(2)-fixing than in nitrate-dependent plants. N(2)-fixing plants partitioned nitrogen to reproductive tissue more efficiently than nitrate-dependent plants as the reproductive tissues of the former and latter contained 65 and 55%, respectively, of the whole-plant nitrogen at the time that nitrogen accumulation in reproductive parts had ceased (133 DAT).

The natural abundance of 15N in an irrigated soybean crop and its use for the calculation of nitrogen fixation

In a field experiment at Leeton, N.S.W., Chaffey soybeans were grown with irrigation at various plant spacings and with various inoc~ilation treatments and two pre-planting soil treatments. Uninoculated plants were almost completely non-nodulated. Measurements of the natural abundance of 15N (S15N) in the total nitrogen of the plants were made at all stages of growth and in the grain at harvest. The 6lSN in all nodulated treatments declined progressively with time in comparison with un-nodulated plants, due to the incorporation of atmospheric N2 of lower 15N concentration than the soil nitrogen. This enabled calculation of the proportions of plant-nitrogen obtained from the soil and by symbiotic N2-fixation. The main findings were as follows: There was a gradient of S15N in plant-available nitrogen across the experimental area. Therefore, treatments were compared by using the nearest non-nodulated plot for the estimate of S15N in plantavailable soil nitrogen. Despite large differences in plant size due to plant spacing, S15N in mature nonnodulated plants did not differ significantly, indicating that the natural abundance of 15N in plantavailable soil nitrogen was uniform in root zones of different sizes. In well-nodulated plants, the proportion (p) of shoot nitrogen derived from N2-fixation increased with time, reaching approximately 70% and 90% in previously fallowed and previously cropped soil respectively, during a period of rapid growth between 78 and 98 d& after planting. The fixed N, in the best-nodulated treatments at (114 days) was 143 and 244 kg N ha-1 respectively for previously fallowed and previously cropped soil. There were consistent trends for increased N2 fixation with increased inoculation rates. In non-nodulated plants, nitrogen recovered in the grain represented most of that present in the shoots at maturity. In well-nodulated treatments, grain nitrogen, although similar in S15N to that of shoots + fruits, represented only 47 and 59% of the 406 and 348 kg N ha-1 present at maturity in shoots + fruits from previously fallowed and previously cropped soils respectively. After harvesting more than 3 t ha-1 of grain, the nitrogen balance in the previously cropped soil, if all of the residual nitrogen in the soybeans could have been retained in the soil, was positive. In the previously fallowed soil there could have been a net depletion of soil nitrogen.

Heritability of Total and Fixed N Content of the Seed in Two Soybean Populations1

Improved seed yield in the soybean [Glycine max (L.) Merr.] may result from selection to increase the N2‐fixing ability of the crop. To investigate that possibility, broad‐sense heritabilities of total and fixed N in the seed were estimated for two F2 soybean populations. One population was derived from a cross of high (‘Ware’) and low (D66‐5566) N2‐fixing parents and was grown years. The second population, grown in 1 year only, was derived from a cross of intermediate N2‐fixing parents (‘Williams’ and ‘Calland’). Both populations and their parents were grown in 15N‐labeled soil in the field along with ‘Clark’ rj1rj1 nonnodulating plants as controls. The amount of fixed N in the mature seed at harvest was determined by the 15N‐dilution method for individual plants. Broad‐sense heritability estimates of fixed N content of the seed ranged from 0.53 to 0.60. Estimates for percentage of fixed seed N varied from 0.12 to 0.43. Estimates for total seed N [as a weight (from 0.49 to 0.61) and as a percentage (from 0.20 to 0.87)], for seed yield (from 0.52 to 0.59), and for plant weights (from 0.57 to 0.62) are also reported. strong positive association was noted between seed dry weight and fixed N content of the seed (r = 0.99). These results suggest that improving N2 fixation in the soybean is feasible through selection based on the mass of fixed N in the seed. Selection for high seed yield will probably likewise result in the identification of plants with a high rate of N2 fixation.

Soil-nitrogen accumulation in codulated and non-nodulated soybeans: A verification of the difference method by a 15N technique

The difference method of measuring nitrogen fixation in soybeans ( Glycine max (L.) Merr.) relies on the assumption that nodulated and nonnodulated plants utilize equal amounts of soil nitrogen (N). The validity of this assumption has been challenged by reports of differences in root system size for nodulated and nondulated and nondulated genotypes. This field study, using a 15N-tracer technique under conditions with reduced available soil N, was conducted to: (1) evaluate thee assumption that the nodulated and nonnodulated soybean genotypes utilize equal amounts of available soil N, (2) compare the root system mass in the nodulated and nonnodulated genotypes, and (3) determine the relationship between soil-N accumulation in the shoots and root dry-matter accumulation. In 1977 and 1978, one nonnodulated and six nodulated soybean genotypes were grown in replicated field plots labeled with 15N on an Aqualfic Normudult (silt loam) soil. Plantts were sampled at 49, 70, and 91 days after planting and at full maturity in 1977, and at 62 and 112 days after planting in 1978. Shoot and root dry-matter accumulation, seed yield (in 1977 only), total-N concentration, total-N accumulation, soil-derived N accumulation, and fixed-N accumulation were measured. Significant differences were present among genotypes both years for all variables except soil-derived N accumulation in the shoots. Shoot and root weights for the nonnodulated genotype could not be distinguished statistically from the nodulated genotype either year, although the seed yield for the nonoddulated was only 37% of the mean of yields for the nodulating genotype in 1977. Root dry weight was positively correlated with shoot dry weight ( r e = 0.90 for, 1977, r w = 0.86 for 1978), seed yield ( r w = 0.79 for 1977), shoot total-N accumulation ( r w = 0.82 for 1977, r w = 0.68for 1978), soil-derived N accumulation in the shoots ( r w = 0.71 for 1977), and accumulation of fixed-N in the shoots ( r w = 0.84 for 1977, r w = 0.85 for 1978). Results of this study support the assumption of equal utilization of soil-derived N by nodulated and nonnodulated soybean genotypes. The use of the difference method under conditions of low N fertility is appropriate for preliminary studies and in areas where 15N-tracer techniques are not available.

Effect of Nodulation on Assimilate Remobilization in Soybean

The objectives of this work were to determine the effect of nodulation on dry matter, reduced-N, and phosphorus accumulation and partitioning in above-ground vegetative parts and pods of field-grown soybean (Glycine max [L.] Merr. cv Harosoy).From comparison of nodulated and nonnodulated isolines, it was estimated that nodulated plants attained 81 and 71% of total-plant (above ground) N from uptake of soil N in 1981 and 1982, respectively. These data, along with visibly greener leaves of nodulated plants, led us to assume that nonnodulated plants were under a moderate N stress relative to nodulated plants. Nonnodulated plants accumulated less total-plant N and partitioned less dry matter and N to the pods, compared with nodulated plants. This occurred even though net photosynthesis, as estimated by rate and amount of dry matter accumulation, was the same for both nonnodulated and nodulated plants. Rate of dry matter and reduced-N accumulation in pods was less for nonnodulated than for nodulated plants while duration of podfill was similar for both isolines. From these data we concluded that moderate N stress affected partitioning of photosynthate rather than net photosynthesis, and that N played a role in translocation of photosynthate to the pods. Total plants (above-ground portion) and pods of both nodulated and nonnodulated plants accumulated similar amounts of phosphorus, which indicated that phosphorus and N accumulation were independent.Remobilization of nitrogen and phosphorus from vegetation to pods preceded dry matter remobilization. It appeared that either more nitrogen accumulation prior to podfill, or continued nitrogen assimilation during podfill would increase nitrogen and dry matter partitioning to pods, but that increasing photosynthesis without concomitantly increasing nitrogen input may not necessarily result in enhanced seed production.

Dinitrogen fixation by field grown soybeans: statistical analysis of variations in δ15N and proposed sampling procedureand proposed sampling procedure

This paper reports a field study for the assessment of the variation in15N natural abundance method to estimate nitrogen fixation. For two successive years (1979 and 1980) at the same site, distribution of the variable δ15N in populations of nodulating and non-nodulating soybeans (Glycine max) has been studied. In 1980, the population structure was studied in order to detect a neighbour effect which could explain bimodal trend observed in the distribution. There was no evidence for such an effect. A sampling procedure is proposed: with 15 to 30 nodulating plants chosen randomly, a confidence interval of ±0.5δ can be obtained, while less than 10 non-nodulating plants are sufficient to achieve a very good precision. Some other aspects are studied: effect of pooling plants which considerably reduce the experimental work; the use of a non leguminous reference plant is possible. All our results show that fixing capacity introduces major variability in the δ15N measured in the plants, while the absence of fixation leads to a homogenous population, so the percentage of fixed N can be evaluated with a rather good precision. The higher the reference value, the better the accuracy of this evaluation, for a given precision of a δ15N measurement.

Effects of Pod Removal on Metabolism and Senescence of Nodulating and Nonnodulating Soybean Isolines

The objectives of this work were to determine the effect of sink strength (presence or absence of pods) and nitrogen source (nodulating versus nonnodulating plants) on enzymic activities, chlorophyll concentration, and senescence of soybean (Glycine max [L.] Merr. cv Harosoy) isolines. A 2-year (1981-1982) field study was conducted.For both nodulated and nonnodulated plants, ribulose bisphosphate carboxylase (RuBPCase) activity of upper-canopy leaves was decreased by pod removal in both years, while chlorophyll concentration was decreased in 1981 only. Nonnodulated plants had lower RuBPCase activity in 1981 and lower chlorophyll concentration in both years compared with nodulated plants. In both years, and for all treatments, RuBPCase activity and chlorophyll began to decline at about the same time, but the rate of decline was less for depodded than for podded plants. Leaves in the middle and lower parts of the canopy had similar RuBPCase activity and chlorophyll concentration trends as upper-canopy leaves for all treatments.Profiles of nitrate reductase activity (NRA) were similar for all treatments in both 1981 and 1982. Acetylene reduction profiles were similar for nodulated-podded and nodulated-depodded plants. The peak and decline in NRA profiles preceded the peak and decline in acetylene reduction profiles. The rate of decline in acetylene reduction activity was less for depodded plants, especially in 1982, but activities reached zero by the final sampling time. Thus, nodule senescence was not prevented by pod removal.Based on seasonal profiles of RuBPCase activity, chlorophyll, NRA, and acetylene reduction activity, the initiation of senescence appeared to occur at the same approximate time for all treatments and, thus, did not depend on the presence or absence of pods or nodules. The hypothesis that nodules act as a nitrogen source and carbohydrate sink to delay senescence in the absence of pods was not correct.

Effects of Pod Removal on Metabolism and Senescence of Nodulating and Nonnodulating Soybean Isolines

Field studies were conducted in 1981 and 1982 to ascertain the effects of pod removal on senescence of nodulating and nonnodulating isolines of soybean (Glycine max [L.] Merr. cv Harosoy) plants. Specifically, the test hypothesis was that nodules act as a nitrogen source and a carbohydrate sink which would in turn prevent or delay senescence in the absence of pods. Senescence was judged by changes in metabolite levels, in dry matter accumulation, and by visual observation.For both nodulated and nonnodulated plants, pod removal had no effect on the magnitude or rate of dry matter and reduced-N accumulation by whole plants. Phosphorus accumulation was significantly less in both nodulated- and nonnodulated-depodded plants, compared with respective control plants with pods. These data suggested a role for pods in phosphorus uptake. Accumulation of dry matter, reduced N, and phosphorus ceased at approximately the same time for all treatments.Pod removal did affect partitioning of plant constitments, with leaves and stems of depodded plants serving as a major alternate sink for accumulation of dry matter, reduced N, phosphorus, and nonstructural carbohydrates (primarily starch). While depodded plants eventually lost a significant amount of leaves, leaf drop was delayed relative to plants with pods; and depodded plants still retained some green leaves at 2 weeks past grain maturity of control (podded) plants.The results indicated that senescence patterns of soybean plants were the same for nodulated and nonnodulated plants, and that pods did not control the initiation of senescence, but rather altered the partitioning of plant constituents and the visual manifestations of senescence.

Spontaneous Phloem Bleeding from Cryopunctured Fruits of a Ureide-Producing Legume

The vasculature of the dorsal suture of cowpea (Vigna unguiculata [L.] Walp) fruits bled a sugar-rich exudate when punctured with a fine needle previously cooled in liquid N(2). Bleeding continued for many days at rates equivalent to 10% of the estimated current sugar intake of the fruit. A phloem origin for the exudate was suggested from its high levels (0.4-0.8 millimoles per milliliter) of sugar (98% of this as sucrose) and its high K(+) content and high ratio of Mg(2+) to Ca(2+). Fruit cryopuncture sap became labeled with (14)C following feeding of [(14)C]urea to leaves or adjacent walls of the fruit, of (14)CO(2) to the pod gas space, and of [(14)C] asparagine or [(14)C]allantoin to leaflets or cut shoots through the xylem. Rates of translocation of (14)C-assimilates from a fed leaf to the puncture site on a subtended fruit were 21 to 38 centimeters per hour. Analysis of (14)C distribution in phloem sap suggested that [(14)C]allantoin was metabolized to a greater extent in its passage to the fruit than was [(14)C] asparagine. Amino acid:ureide:nitrate ratios (nitrogen weight basis) of NO(3)-fed, non-nodulated plants were 20:2:78 in root bleeding xylem sap versus 90:10:0.1 for fruit phloem sap, suggesting that the shoot utilized NO(3)-nitrogen to synthesize amino acids prior to phloem transfer of nitrogen to the fruit. Feeding of (15)NO(3) to roots substantiated this conclusion. The amino acid:ureide ratio (nitrogen weight basis) of root xylem sap of symbiotic plants was 23:77 versus 89:11 for corresponding fruit phloem sap indicating intense metabolic transfer of ureide-nitrogen to amino acids by vegetative parts of the plant.

Nitrogen fixation by nodulated soybean under tropical field conditions estimated by the 15N isotope dilution technique

The contribution of biological N 2 fixation to the N nutrition of nodulated soybean was estimated using the 15N isotope dilution technique and a non-nodulating soybean isoline as a non-fixing control plant. The plants were grown in the field in concrete cylinders (60 cm dia) and harvested at seven stages of plant growth. Labelled N was added to the soil either as labelled organic matter before planting or in seven small additions (2kg N ha −1) of (NH 4) 2SO 4 during the growing period. There was good agreement between isotope dilution estimates of nitrogen fixation for the two labelling methods. Acetylene reduction assays on intact root systems greatly underestimated N 2 fixing activity. The difference in total N between nodulated and non-nodulated plants generally gave higher estimates compared with the isotope technique. The data indicate that this was because nodulated plants recovered more N from the soil than the non-nodulated plants. After 92 days of growth, the soybean derived approximately 250kg N ha −1 from biological N 2 fixation.

Nitrate uptake by intact, non-nodulated bean plants: effect of ammonium and of light

Dual-phase kinetics were found for nitrate uptake at pH 5.5 by intact, non-nodulated bean plants. Apparent Km values for phase I (higher affinity for nitrate) were in the range between 0.2 and 0.4 mM and depended on the nitrogen nutrition of the plants prior to the experiment. Ammonium added to the medium at the same concentration as nitrate had no effect on nitrate uptake rates from nutrient Solutions containing 0.1-1.5 mM nitrate. Uptake rates for nitrate were considerably lower in plants preincubated on higher nitrate or ammonium concentrations than used for the experiment, compared to those preincubated on the same medium as used during the experiment. No effect of light on nitrate uptake from Solutions containing 0.1-1.5 mM nitrate was observed, although the transpiration rates were about 70% lower in darkness than in light. Nitrate uptake rates from Solutions containing more than 2 mM nitrate were slightly lower in darkness than in light.

Field Comparison of the Nitrogen‐15 and Difference Methods of Measuring Nitrogen Fixation1

AbstractFurther investigation of the methods of evaluating N2 fixation under field conditions is essential, since the accuracy and reproducibility of the method of measurement have profound effects on the results of an experiment; and, opinions differ concerning which method provides the best estimates. The amount of N2 fixed and the percentage of total plant N from biologically fixed N in field‐grown soybeans [Glycine max (L.) Merr.] measured by the difference method were compared with corresponding estimates from a 15N‐tracer technique in which 15N‐labeled soil organic matter is used as the tracer material. In 1977 and 1978, six N2‐fixing soybean genotypes were grown in rows alternating with rows of a nonnodulating genotype (Clark rj1 rj1) in replicated field plots on a Mattapex silt loam (fine‐silty, mixed, mesic Aquic Hapludult) where the 15N was incorporated into the soil organic fraction. The difference method compared total N contents of the N2‐fixing and adjacent nonnodulating plants. The 15N method determined the dilution in the N2‐fixing plants of soil‐derived 15N by symbiotically‐fixed N. Plants were sampled at 49, 70, and 91 days after planting and at full maturity in 1977, and at 62 and 112 days after planting in 1978. Sources of variation for the estimates were identified as the total N accumulation in the nonnodulating genotype for the difference method, and the 15N concentration in the nonnodulating genotype for the 15N method. Estimates from the difference and 15N methods for the amount of total N fixed were highly correlated (r = 0.89 for 1977, r = 0.92 for 1978). The percentage of total N fixed varied between the two methods (r = 0.32 for 1977, r = 0.69 for 1978) which was attributed to spatial variation of available soil N. The 15N method provided more accurate estimates for the percentage of total N fixed because the 15N concentration was less variable than the total N accumulation in the nonnodulating genotype.

Biochemical Basis for Partitioning of Photosynthetically Fixed Carbon between Starch and Sucrose in Soybean (Glycine max Merr.) Leaves

The control of photosynthetic starch/sucrose formation in leaves of soybean (Glycine max L. Merr.) cultivars was studied in relation to stage of plant development, photosynthetic photoperiod, and nitrogen source. At each sampling, leaf tissue was analyzed for starch content, activities of sucrose-metabolizing enzymes, and labeling of starch and sucrose (by (14)CO(2) assimilation) in isolated cells. In three of the four varieties tested, nodulated plants had lower leaf starch levels and higher activities of sucrose phosphate synthetase (SPS), and isolated mesophyll cells incorporated more carbon (percentage of total (14)CO(2) fixed) into sucrose and less into starch as compared to nonnodulated (nitrate-dependent) plants. The variation among cultivars and nitrogen treatments observed in the activity of SPS in leaf extracts was positively correlated with labeling of sucrose in isolated cells (r = 0.81) and negatively correlated with whole leaf starch content (r = -0.66). The results suggested that increased demand for assimilates by nodulated roots may be accommodated by greater partitioning of carbon into sucrose in the mesophyll cells. We have also confirmed the earlier report (Chatterton, Silvius 1979 Plant Physiol 64: 749-753) that photoperiod affects partitioning of fixed carbon into starch. Within two days of transfer of nodulated soybean Ransom plants from a 14-hour to a 7-hour photoperiod, leaf starch accumulation rates doubled, and this effect was associated with increased labeling of starch and decreased labeling of sucrose in isolated cells. Concurrently, activities of SPS, sucrose synthase, and uridine diphosphatase in leaves were decreased.Four nodulated soybean cultivars were grown to maturity in a greenhouse. Fully expanded leaves at the top of the canopy were sampled during vegetative growth (45 days), at flowering (79 days), and at mid-podfill (120 days). In general, activities of SPS and uridine-5'-diphosphatase were highest during vegetative growth, and they decreased during reproductive development, whereas activity of sucrose synthase and leaf starch content tended to increase. Leaf starch was negatively correlated with levels of SPS (r = -0.71). The results support the postulate that sucrose-P synthetase is a key control point regulating the photosynthetic formation of sucrose, and, hence, starch.

Economy of Carbon and Nitrogen in Nodulated and Nonnodulated (NO3-grown) Cowpea [Vigna unguiculata (L.) Walp.

The response of non-nodulated cowpea (Vigna unguiculata (L.) Walp. cv Caloona) to a wide range of NO(3) levels in the rooting medium was studied 40 days after sowing by in vitro assays of plant organs for NO(3) reductase (EC 1.6.6.1) and analyses of root bleeding (xylem) sap for nitrogenous solutes. Plants fed 1, 5, 10, 20, and 40 millimolar NO(3) showed, respectively, 64, 92, 94, and 91% of their total reductase activity in shoots and 34, 30, 66, 62, and 58% of the total N of their xylem sap as NO(3). These data, and the absence in the plants of significant pools of stored NO(3), indicated that shoots were major organs of NO(3) assimilation, especially at levels of NO(3) (10 to 40 millimolar) that maintained plant growth at near maximum rates. Partitioning and utilization of C and N were studied in nodulated, minus NO(3) plants and non-nodulated plants fed 10 or 20 millimolar NO(3), the levels of NO(3) which gave rates of growth and N assimilation closest to those of the symbiotic plants. The conversion of the C of net photosynthate to dry matter was similar in nodulated plants (67%) and NO(3)-grown plants (64%), but greater proportions of photosynthate were translocated to below ground parts of nodulated plants (37%) than of NO(3)-fed plants (23 to 26%). Greater photosynthate consumption by nodulated roots was associated with proportionately greater root growth and respiration and 2-fold greater export of C in xylem than in the NO(3)-fed plants. Theoretical considerations suggest that the elevated CO(2) output of nodulated roots was due not only to CO(2) loss associated with nodule function, but also to a much greater nonassimilatory component of respiration in the supporting root of the nodulated plant compared to roots of the NO(3)-fed plants. Data are compared with previously published information from other legumes.