Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 3. Time-course Study on 13C Incorporation into Soluble Metabolites and Respiratory Evolution of 13CO2 from Roots and Nodules

Well-nodulated soya bean (Glycine max L.) plants were allowedto assimilate ¹³CO₂ for 10 h in the light, under steady-stateconditions in which CO₂ concentration and ¹³C abundance wereboth strictly controlled at constant levels. The respiratoryevolution of ¹³CO₂ from roots and nodules and ¹³C incorporationinto various metabolic fractions were measured during the ¹³CO₂feeding and subsequent 48 h chase period. CO₂ respired from nodules was much more rapidly labelled with¹³C than that from roots. The level of labelling (percentageof carbon currently assimilated during the ¹³COM₂ feeding period)of CO₂ respired from nodules reached a maximum of about 87 percent after 4 h of steady-state ^l3CO₂ assimilation and thereafterremained fairly constant. The absolute amount of labelled carbonevolved by the respiration of the nodules during the 10 h ¹³CO₂feeding period was 1·5-fold that of root respiration.These results demonstrated that the currently assimilated (labelled)carbon was preferentially used to support nodule respiration,while root respiration relied considerably on earlier (non-labelled)carbon reserved in the roots. Sucrose pools were mostly composed of currently assimilatedcarbon in all tissues of the plants, since the levels of labellingaccounted for 86–91 per cent at the end of the ¹³CO₂ feeding.In the nodules, the kinetics and levels of sucrose labellingwere in fairly good agreement with those of respired CO₂, whilein the roots, the level of labelling of respired CO₂ was significantlylower than that of sucrose. Succinate and malate were highly labelled in both roots andnodules but they were labelled much more slowly than sucroseand respired CO₂. The kinetics and levels of labelling of theseKrebs cycle intermediates resembled those of major amino acidswhich are derived directly from Krebs cycle intermediates. Itis suggested that large fractions of organic acids in noduleswere physically separate from the respiration site. Glycine max L., Soya bean, ¹³CO₂ assimilation, respiratory evolution of ¹³CO₂, carbon metabolism in root nodules     

Short-Term Nitrate Effects on Hydroponically-Grown Soybean cv. Bragg and its Supernodulating Mutant: II. DISTRIBUTION AND RESPIRATION OF RECENTLY-FIXED 13C-LABELLED PHOTOSYNTHATE

Fully symbiotic or nitrate treated (3 d, 4·0 mol m^–3)soybean (Glycine max [L.] Merr.) cv. Bragg and a nitrate tolerantsupernodulating soybean mutant nts 1007 were exposed to ¹³Cenriched CO₂ for a period of 10 h. During this period and forthe subsequent 24 h, continuous measurements of ¹³CO₂ and ¹²CO₂evolution of their root systems were undertaken. Three harvestsduring the experiment allowed determinations of the distributionof recently fixed carbon in different plant organs. These measurementsindicated higher dependence of N₂ fixation in nts 1007 on recentlyfixed carbon (RFC) by showing elevated RFC concentrations innodules as well as their augmented respiration. Root respirationof both genotypes was generally more reliant on stored carbon. Nitrate induced in all measured parameters a clear responsein the mutant analogous to the wild type, but quantitative differencesremained throughout. Nodule respiratory activity, the relativespecific activity (RSA), and the utilization of RFC were substantiallyreduced, but remained higher in nts 1007 than in Bragg, whilethe demand of roots for RFC increased in Bragg more than inthe supernodulator. The elevated carbon requirement of the nodulecomplement of the mutant and a high dependence on recently fixedcarbon could be attributed to higher nodule growth and maintenancecosts of the supernodulating genotype and were not associatedwith augmented nitrogen fixation activity. This less efficientutilization of carbon and the associated almost parasitic characterof the nodule complement of nts 1007 is considered to be thecause of reduced growth of the mutant. No evidence was foundfor a physiologically based nitrate tolerance in terms of nitrogenfixation. Key words: Glycine max, nitrate, N₂fixation, respiration, carbon partitioning, steady-state labelling     

Partitioning and Utilization of Photosynthate Produced at Different Growth Stages after Anthesis in Soybean (Glycine max L. Merr.): Analysis by Long-term 13C-Labelling Experiments

Yamagata, M., Kouchi, H. and Yoneyama, T. 1987. Partitioningand utilization of photosynthate produced at different growthstages after anthesis in soybean (Glycine max L. Merr.): Analysisby long term ¹³C-labelling experiments.—J. exp. Bot. 38:1247–1259. Soybean (Glycine max L. Merr. var. Akishirome) plants were allowedto assimilate ¹³CO₂ with a constant specific activity for 10h at different growth stages (a total of seven times at aboutone week intervals) after anthesis. The plants were harvestedperiodically until the time of full maturity and the partitioningof ¹³C into individual plant parts was investigated with anemphasis on the contribution of carbon assimilated at differentgrowth stages to the seed formation. Carbon assimilated at the middle to late seed-filling stagecontributed most to the seed production; one day contributionaccounted for 3–4% in total carbon of the seed at fullmaturity. Integrated contribution of carbon assimilated afteranthesis was estimated as 96% of the final seed carbon. An approximationbased on the temporal data of the incorporation of labelledcarbon into the seeds indicates that 77% of the final seed carboncame from direct transfer of current photosynthate from sourceleaves, which occurred within a few days after the photosyntheticfixation, while the rest originated from remobilization of carbonreserved mainly in leaves and stems plus petioles. In comparison with the total carbon accumulation in the seeds,protein carbon in the seeds was relatively more dependent onphotosynthate produced during the early period of reproductivegrowth stage, whereas lipid carbon was more dependent on photosynthateproduced during the later reproductive stage. Key words: Photosynthate partitioning, soybean (Glycine max L. Merr.), ¹³CO₂ assimilation, seed formation     

Short-Term Nitrate Effects on Hydroponically-Grown Soybean cv. Bragg and its Supernodulating Mutant: I. CARBON, NITROGEN AND MINERAL ELEMENT DISTRIBUTION, RESPIRATION AND THE EFFECT OF NITRATE ON NITROGENASE ACTIVITY

A comparison between two hydroponically-grown soybean genotypes(Glycine max [L.] Merr.) cv. Bragg and the supernodulating mutantnts 1007 was made in terms of dry matter accumulation, carbon,nitrogen, and mineral element distribution, ¹⁵N natural abundanceand the effect of short-term treatment with 4·0 mol m^–3KNO₃ on nitrogenase activity and respiration. Differences weremost pronounced in nodule dry weight and plant nitrogen content,both of which were recorded to be substantially elevated inthe mutant. Mineral element concentrations in different plantparts proved to be rather similar with the exception of Ca,found to be lower in leaves of the mutant, and Mn concentrationswhich were twice as high in roots of nts 1007. The values of¹⁵N natural abundance showed that both genotypes were equallydependent on nitrogen fixation when nitrate was absent. Theresults of the acetylene reduction assays indicated similarspecific nodule activity, while on a per plant basis nitrogenaseactivity of the mutant proved to be more than twice the amountof Bragg. This effect was also reflected in higher nodule respirationwhile root respiration remained below that of Bragg. Nitrate induced a substantial reduction in nitrogenase activitynot only in Bragg, but also in nts 1007. Nodule respiratoryactivity of Bragg was reduced by nitrate from 1·27 to0·34 mg C h^–1 plant^–1. In nts 1007 correspondingvalues were 2·70 to 1·52 mg C h^–1 plant^–1.Starch concentration in nodules was decreased in both genotypes,but nevertheless remained higher in nts 1007. Values for solublesugars in nodules even increased in the mutant in response tonitrate while the same treatment caused a reduction in Bragg.The data indicate that nitrogenase activities of Bragg and nts1007 are equally sensitive to short-term application of nitrate. Key words: Glycine max, C and N distribution, nitrate, root respiration, ¹⁵N natural abundance     

Respiratory Utilization of 13C-Labelled Photosynthate in Nodulated Root Systems of Soybean Plants

Kouchi, H., Akao, S. and Yoneyama, T. 1986. Respiratory utilizationof ¹³C-labelled photosynthate in nodulated root systems of soybeanplants.—J. exp. Bot. 37: 985–993. An improved method for the measurement of respiratory utilizationof current photosynthate in the nodulated root system of water-culturedsoybean (Glycine max L.) plants was developed using a steady-state¹³CO₂ labelling technique. Well-nodulated plants at the latevegetative stage were allowed to assimilate ¹³CO₂ for 10 h incontinuous light at a constant CO₂ concentration with a constant¹³C abundance. The respiratory evolution of ¹³CO₂ from rootsand nodules was measured continuously throughout the periodof ¹³CO₂ assimilation and during a subsequent 36 h chase periodby using a differential infrared ¹³CO₂ analyser. The plantswere grown with nitrogen-free or (15 mmol dm^–3)-containing culture solution for 3 d before¹³CO₂ assimilation. In plants grown without , nodule respiration averaged 69% of the total respiration of the undergroundparts over the full experimental period and the CO₂ respiredreached an apparent isotopic equilibrium at 80–85% labellingafter initiating ¹³CO₂ assimilation. By contrast, the CO₂ respiredfrom the roots did not reach an isotopic equilibrium and labellingwas only 56% at the end of exposure to ¹³CO₂ These findingsdemonstrated that nodule respiration is strongly dependent onrecently assimilated carbon compared with root respiration. Plants supplied with in the culture solution showed a decreased rate of nodule respirationand a slightly increased rate of root respiration. The extentsand time courses of labelling of respired CO₂ from both theroots and nodules were similar in the presence and absence of except that the maximum level of labelling of CO₂ derived from nodule respiration in plantswith was significantly higher (about 91%) than for plants growing without . Key words: Soybean (Glycine max L.), nodule respiration, ¹³CO₂, assimilation, carbon partitioning     

Compartmental Analysis of the Partitioning of Photo-assimilated Carbon in Nodulated Soybean Plants during the Light Period

Kouchi, H., Yoneyama, T. and Akao, S. 1986. Compartmental analysisof the partitioning of photo-assimilated carbon in nodulatedsoybean plants during the light period.—J. exp. Bot. 37:994–1005. Dynamics of the partitioning of photo-assimilated carbon invegetative nodulated soybean (Glycine max L.) plants in thelight period was investigated by compartmental analysis basedon data from steady-state ¹³CO₂ assimilation experiments. Themodel assumes a total of 18 compartments consisting of activeand temporary storage pools for soluble materials, starch andstructural materials in leaves, stems plus petioles, roots andnodules together with respired carbon from the roots and nodules.Carbon flow between compartments was described by 22 rate parameters.The rate parameters were evaluated by a non-linear least squaresearch method to optimize the fitness of the simulated resultswith the experimental tracer distribution. The compartment model was well applicable to interpret the carbonpartitioning in whole plants. The analysis showed that: (I)The largest carbon flux during the light period was to storagematerials (starch and temporary storage soluble pools) in theabove-ground parts. The total flux to storage pools was considerablylarger than the transporting flux to below-ground parts. (2)The main carbon flux to the nodules was via direct phloem pathwaysfrom the shoot and not via the compartment of root soluble materials.This flux was 72% of the total carbon flux from the shoot tothe nodulated root system. (3) A large amount of carbon wasreturned to the shoot from below-ground parts. The total returnof carbon flux to the shoot (85% from nodules) was equivalentto 54% of the total influx of carbon to below-ground parts.Direct carbon transfers between roots and nodules were relativelysmall. Key words: Compartmental analysis, carbon partitioning, root nodules, Glycine max L., ¹³CO₂, assimilation     

Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 2. The Incorporation of 13C into Carbohydrates, Organic Acids, Amino Acids and some Storage Compounds

Nodulated soya bean (Glycine max L.) plants at the early floweringstage were allowed to assimilate ¹³CO₂ under steady-state conditions,with a constant ¹³C abundance, for 8 h in the light. The plantswere either harvested immediately or 2 d after the end of the¹³CO₂ feeding, divided into young leaves (including flower buds),mature leaves, stems+petioles, roots and nodules; the ¹³C abundancein soluble carbohydrates, organic acids, amino acids, starchand poly-ß-hydroxybutyric acid was determined witha gas chromatography-mass spectrometry. The rapid turnover of ¹³C in the sucrose pools observed in allorgans of the plants showed that sucrose was the principal materialin the translocation stream of primary products of photosynthesis.At the end of the ¹³CO₂ exposure, sucrose in the mature leavesas the major source organs and in the stems+petioles was labelledwith currently assimilated carbon to about 75 per cent, whereasa much higher labelling of sucrose was found in the roots andin the nodules. This suggests the existence of two or more compartmentedpools of sucrose in mature leaves and also in stems+petioles. The relative labelling patterns of individual organic acidsand amino acids were similar in various plant organs. However,the rapid turnover of succinate and glycine was characteristicof nodules. Treatment with a high concentration of nitrate inthe nutrient media increased the turnover rate of amino acidcarbon in shoot organs and roots, while it markedly decreasedthe labelling of amino acids in nodules. The cyclitols, exceptfor D-pinitol, were significantly labelled with assimilated¹³C in mature leaves, but in nodules, the labelling was verymuch less. In the nodules, which were actively fixing atmospheric nitrogen,a large proportion (80–90 per cent) of currently assimilatedcarbon was found as sucrose and starch at the end of the ¹³CO₂feeding. This was also true of the roots. On the other hand,in young growing leaves, the distribution of currently assimilatedcarbon into sucrose, starch and other soluble compounds wasmuch less. This suggests that a large amount of carbon assimilatedby and translocated to young leaves was used to make up structuralmaterials, mainly protein and cell wall polymers synthesis,during the light period. Glycine max L., soya bean, ¹³CO₂ assimilation, carbon metabolism in nodules     

Carbon Flow from Nodulated Roots to the Shoots of Soybean (Glycine max L. Merr.) Plants: An Estimation of the Contribution of Current Photosynthate to Ureides in the Xylem Stream

Kouchi, H. and Higuchi, T. 1988. Carbon flow from nodulatedroots to the shoots of soybean {Glycine max L. Merr.) plants:An estimation of the contribution of current photosynthate toureides in the xylem stream.–J. exp. Bot. 39: 1015–1023. Well-nodulated, water-cultured soybean plants were allowed toassimilate ¹³CO₂ at a constant specific activity for 10 h andthe ¹³C-labelling of total carbon and ureides in xylem sap wasinvestigated. Labelled carbon appeared very rapidly in the xylem stream. Percentageof labelled carbon (relative specific activity, RSA) in xylemsap was 18% at 2 h after the start of ¹³CO₂ assimilation andreached 53% at the end of the 10 h assimilation. The amountof labelled carbon exported from nodulated roots to the shootsvia the xylem during the 10 h labelling period accounted for33% of total labelled carbon imported into the nodulated roots.Ureides (allantoin and allantoic acid) in xylem sap were stronglydependent on currently assimilated carbon. The RSA of ureidesin xylem sap had reached 83% at the end of the assimilationperiod. Labelled carbon in ureides accounted for 51% of totallabelled carbon returned from nodulated roots to the shootsvia the xylem during the 10 h assimilation period. A treatmentwith 20 mol m^–3 nitrate in the culture medium for 2 ddecreased the ureide concentration in the xylem sap slightly,but greatly decreased the RSA of ureides. By comparing the data with the results of analysis of the xylemsap of nodule-detached plants, it was concluded that the majorityof labelled carbon exported to the xylem stream from noduleswas in ureide form. A considerable amount of carbon was alsoreturned from roots to shoots via the xylem stream but it wasmore dependent on (non-labelled) carbon reserved in the roottissues. Key words: Soybean(Glycine max L.), root nodule, carbon partitoning, ¹³CO₂ assimilation, xylem     

Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 1. Development and Application of 13CO2 Assimilation System at a Constant 13C Abundance

A long-term, steady-state ¹³CO₂ assimilation system at a constantCO₂ concentration with a constant ¹³C abundance was designedand applied to quantitative investigations on the allocationof photoassimilated carbon in nodulated soya bean (Glycine maxL.) plants. The CO₂ concentration in the assimilation chamberand its ¹³C abundance were maintained constant with relativevariances of less than ±0.5 per cent during an 8-h assimilationperiod. At the termination of 8-h ¹³CO₂ assimilation by plantsat early flowering stage, the currently assimilated carbon relativeto total tissue carbon (measured by the degree of isotopic saturation)were for young leaves (including flower buds), 13.9 per cent;mature leaves, 15.7 per cent; stems+petioles, 5.9 per cent;roots, 5.4 per cent and nodules, 6.9 per cent, 48 h after theend of the ¹³CO₂ assimilation period, they were 12.3, 7.5, 7.4,6.8 and 6.1 per cent, respectively. The treatment with a highconcentration of nitrate in the nutrient media significantlydecreased the allocation of ¹³C into nodules. Experiments on¹³CO₂ assimilation by plants at the pod-filling stage were alsoconducted. Labelling by ¹³C was weaker than at the early floweringstage, but an intense accumulation of ¹³C into reproductiveorgans was observed. Glycine max L., nodulated soya bean plants, ¹³CO₂ assimilation, carbon dynamics