Economy of water, carbon, and nitrogen in the developing cowpea fruit

The nutritional economy of the fruit of cowpea (Vigna unguiculata (L.) Walp cv Vita 3) was assessed quantitatively from intake and utilization of carbon, nitrogen, and water. Fruits failed to make net gains of CO₂ from the atmosphere during daytime, although pod photosynthesis did play a role in the fruit's carbon economy by refixing a proportion of the fruit's respired CO₂. Of every 100 units by weight of carbon entering the fruit, 70.4 were finally incorporated into seeds, 10.3 remained as nonmobilizable material in pod walls, and the remaining 19.3 were lost in fruit respiration. Phloem supplied 97% of the fruit's carbon and 72% of its nitrogen. The xylem contribution of nitrogen occurred mainly in early growth. Ninety-six% of the fruit's nitrogen was incorporated into seeds, approximately 10% of this mobilized from the senescing pod. The mean transpiration ratio of the fruit was very low—8 milliliters water transpired per gram dry matter accumulated. Models of carbon, nitrogen, and water flow were constructed for the two consecutive 11 day periods of fruit development, and indicated a considerably greater entry of water through xylem and phloem than could be accounted for in changes in fruit tissue water and transpiration loss. This discrepancy was greater in the second half of fruit growth and was interpreted as evidence that a significant fraction of the water entering the fruit through phloem cycled back to the parent plant via the xylem.     

Partitioning of carbon and nitrogen and the nutrition of root and shoot apex in a nodulated legume

Empirically based models depicting exchanges of C, N, and H₂O in phloem and xylem among organs of nodulated white lupin (Lupinus albus cv Ultra) were constructed for the interval 51 to 58 days after sowing. Information was incorporated on the economy of C, N, and H₂O in plant parts, the solute composition of transport fluids collected at selected sites on the plant, and the photosynthetic inputs, transpirational losses, and translocatory activities of different age groups of leaflets and stem + petiole segments of the shoot. Partitioning of C and N showed preferential transfer of N to the shoot apex, which imported 13 milligrams C per milligram N, compared with 54 milligrams C per milligram N for the nodulated root. Leaves translocated assimilates at a C:N weight ratio of 43 to 59, and older leaves serving the roots produced the translocate most rich in N relative to C. The shoot apex was enriched with N, additional to its intake from leaves, by direct uptake of xylem fluid (C:N ratio, 2.4) and receipt of nitrogenous solutes transferred from xylem to upward-moving phloem streams in upper regions of the stem. The models for flow of N and H₂O indicated that xylem streams passing to leaves were substantially less rich in N than the adjacent stream moving through the body of the stem and that a progressive increase in concentration of N occurred within stem xylem elements from base to top of the shoot. This apparently resulted from an abstraction of N from xylem of departing leaf traces, possibly by xylem transfer cells, and a subsequent feedback of this N to xylem streams passing on up the shoot. Upper leaves and shoot apex, therefore, acquired more N from xylem per unit of H₂O transpired than lower parts of the shoot.     

Metabolism and translocation of fixed nitrogen in the nodulated legume

Summary Nitrogen (N₂) fixed by Rhizobium bacteroids in the legume nodule is excreted as ammonia to the surrounding host cell where it is efficiently assimilated into the amide group of glutamine. Generally glutamine is a minor exported solute of nitrogen, being further metabolised to asparagine in temperate species and to the ureides, allantoin and allantoic acid in tropical species. These solutes serve as the principal translocated forms of nitrogen in xylem. Compartmentalisation of the pathways of nitrogen metabolism and the role of ammonia in regulation of their activity is examined in nodules of both asparagine-forming (Lupinus albus L.) and ureide-forming (Vigna unguiculata L. Walp) symbioses.     

The carbon canopy economy of the association between cowpea and the parasitic angiosperm Striga gesnerioides

The association between the parasite Striga gesnerioides and cowpea (Vigna unguiculata) was investigated using measurements of growth and gas exchange together with calculations of the carbon budget of the association. Striga gesnerioides has a very low photosynthetic capacity coupled with high rates of respiration. Even at photosynthetic light saturation shoots exhibit no net carbon gain. Thus S. gesnerioides is highly dependent on its host for carbon as well as for water and inorganic solutes. It is estimated that 70% of the carbon transferred from host to parasite is used in parasite respiration. Infected cowpea had a lower photosynthetic capacity, at times less than half that of uninfected plants. Infection with S. gesnerioides reduced the growth of cowpea by 75%. Calculations indicate that the loss of carbon from the host by export to the parasite is more important than reduced photosynthetic capacity of the host in accounting for the observed growth reductions.     

Utilization of carbon and nitrogen sources and acid/alkali production by cowpea rhizobia

Summary Sixteen slow-growing strains of rhizobia (15 cowpea rhizobia and oneR. japonicum) were examined to determine the effects of carbon and nitrogen sources on acid/alkali production in culture media. We found that the pH changes of the medium were more influenced by nitrogen sources than carbon sources (with the exception of ribose). When ammonium sulphate was used as a nitrogen source, all the cowpea rhizobia strains produced acid. When yeast-extract was used as a nitrogen source, however, a heterogenous pattern for acid/alkali production was found. The majority of the strains produced alkali from nitrate, glutamate and urea irrespective of carbon sources and acid from ribose irrespective of nitrogen sources.     

Respiration and the energy requirement for nitrogen fixation in nodulated pea roots

Pisum sativum L. cv. Trapper plants were inoculated and grown in a controlled environment on N-free nutrient solution. After 4 weeks N was supplied to treatment plants as NH₄NO₃, KNO₃, or NH₄Cl and rates of C₂H₂ reduction, root + nodule respiration, and leaf photosynthesis were determined 1 week later. The increase in respiration per unit of C₂H₂ reduction was not affected by either the form of N added or the light conditions during growth, although the basal respiration rate with no C₂H₂ reduction increased with irradiance level. The mean regression coefficient from plots of respiration versus C₂H₂ reduction was 0.23 + 0.04 (P [unk] .01) mg of CO₂ (μmol of C₂H₂ reduced)⁻¹ which was very similar to the value for the coefficient of respiration associated with nitrogenase activity estimated by subtracting growth and maintenance respiration. Since the rate of N accumulation in N-free nutrient conditions was proportional to the rate of C₂H₂ reduction, it appears that the method gives a true estimate of the energy requirements for N fixation which for these conditions was equivalent to 17 grams of carbohydrate consumed per gram of N fixed.     

Carbon and nitrogen nutrition of nodulated roots of grain legumes

Abstract The spatial and temporal relationships between carbon (C) metabolism and nitrogen (N) nutrition in grain legumes are of great academic interest with the added bonus that any data for economically important species may ultimately prove useful to breeders and growers. So far there are no data which can be used to relate differences in carbon usage by any symbiotic association with differences in economic yield. Much research has focussed on the dependence of dinitrogen fixation on photosynthate supply, on the C–N relationships of nodulated roots and nodules and on diurnal and seasonal profiles of dinitrogen fixation. In all these aspects a plethora of responses have been described, often based on insufficiently proven measurement techniques; consequently unequivocal conclusions cannot be drawn. We know little about within-species differences due to cultivar, strain of Rhi-zobium or environment, or about the proportions of any heritable variations which might be sufficiently large to merit inclusion among the selection criteria of grain legume breeders.     

Interrelationship of nitrogen fixation,hydrogen evolution and photoreduction in Rhodospirillum rubrum

Summary Rhodospirillum rubrum was grown: 1. photoheterotrophically on a medium containing dl-malate as the carbon source and ammonium chloride as the nitrogen source (medium No. 1); 2. phototrophically with N₂ and dl-malate (medium No. 2); 3. photoautotrophically with N₂, CO₂ and H₂ (medium No. 3).Resting cells derived from these cultures were tested for their ability to photoreduce CO₂, evolve H₂ and fix N₂. Only cells which were grown in medium No. 2 were able to perform all three gas exchanges. The activity pattern of gas exchanges altered in a characteristic way during the growth cycle of the bacterial culture. Cells newly transferred to medium No. 2 showed an enormous increase in the rate of H₂ evolution, which dropped sharply when all l-malate had been used up. The rate of photoreduction of CO₂ increased steadily and reached a maximum level after 120 h. The nitrogen fixing activity remained constant during the whole growth cycle.The yields of H₂ produced per mole of l-malate added were measured as a function of cell age. Only very young cultures gave appreciable yields, which dropped gradually with increasing age.The function of the carbon source is discussed as a regulating factor for photoreduction and hydrogen evolution.     

Significance of photosynthetic and respiratory exchanges in the carbon economy of the developing pea fruit

The nutritional economy of the developing fruit of Pisum sativum L. (cv. Greenfeast) was studied in terms of intake of translocate, incorporation of C and N into dry matter, transpiration, and CO₂ exchanges of the fruit with its external and internal atmospheres. The environmental conditions were 12-hr days (22 C, 850 μeinsteins m⁻² sec⁻¹ at fruit level); 12-hr nights of 15 C.     

Respiration and nitrogen fixation in nodulated roots of soya bean and pea

Summary Oxygen uptake, carbon dioxide evolution and nitrogenase activity, measured either as hydrogen evolution (under argon 80%, oxygen 20%) or as the reduction of acetylene to ethylene, were assayed over the same time period by a direct mass-spectrometric method. When carbon dioxide evolution was used to estimate carbohydrate consumption, the results agreed with other work on whole plants. The RQ values obtained in these experiments were always less than 1.0 and thus the carbohydrate consumption calculated from oxygen uptake suggests that previous estimates, using carbon dioxide evolution as a measure of the cost of nitrogen fixation may be underestimates. Lag periods observed in the reduction of acetylene to ethylene suggest that there is a resistance to diffusion of gases in the root nodules.     

Effects of applied nitrogen upon aspects of nitrogen metabolism and transport in developing nodulated winged bean plants

Nodulated winged bean [Psophocarpus tetragonolobus (L.) DC., cv. UPS 122] were grown under constant environmental conditions and supplied with mineral nutrient solution in which nitrogen was absent or was present as nitrate (12 mg N week^-1 plant^-1). Nitrate treatment dramatically promoted plant growth, increased fruit weight 1.6 fold, was necessary for tuberisation and enhanced nodulation. The in vitro accumulation of ¹⁴C into asparagine and aspartate components of excised nodules supplied with exogenous ¹⁴CO₂ and [¹⁴C]-D-glucose was greater for nitrate-treated plants, whilst accumulation into ureides was reduced by nitrate treatment. Levels of amino acids in xylem sap were greater for plants supplied with a complete nutrient solution, than those grown without applied nitrate, particularly for asparagine, glutamine and proline. Xylem ureide levels were greater for plants grown in the absence of supplementary nitrate. Nitrogen accumulated in leaf, stem and petiole, and root nodule tissues for utilisation during fruit development; peak nitrogen levels and time of anthesis were retarded for plants grown without applied nitrate. The shoot ureide content increased during fruiting, coincident with decreases in the total nitrogen content, indicating that ureide pools are not utilised during the early reproductive phase. However ureide reserves, particularly allantoin, were utilised during the later stages of pod fill. Enzyme activity which metabolised asparagine was found throughout the plant and was identified as K⁺-dependent asparaginase (EC 3.5.1.1) and an aminotransferase. Apart from temporal differences in developmental profiles of enzyme activity, the activity of these enzymes and of allantoinase (EC 3.5.2.5) in developing tissues were similar for both treatments. The main differences were greater asparaginase and asparagine:pyruvate aminotransferase activities in root tissues and fruit of nitrate-supplied plants; allantoinase activity in the primary roots of plants grown without nitrate decreased during development, whilst activity in developing tubers (nitrate-supplied plants) increased.     

The significance of copper for nitrogen fixation in nodulated alnus and casuarina plants

Summary When supplied with copper, nodulated plants ofAlnus glutinosa in nitrogenfree culture solution showed significantly better growth than corresponding plants denied that element, the most marked effect being an increase in percent nitrogen in the leaf dry matter. Thus the nodular fixation of nitrogen appears to require an adequate level of copper, though the extent to which this is specific for the actual fixation process cannot be assessed from the data provided.     

Xylem and Phloem transport and the functional economy of carbon and nitrogen of a legume leaf

Exchanges of CO₂ and changes in content of C and N were studied over the life of a leaf of Lupinus albus L. These data were combined with measurements of C:N weight ratios of xylem (upper stem tracheal) and phloem (petiole) sap to determine net fluxes of C and N between leaf and plant. Phase 1 of leaf development (first 11 days, leaf to one-third area) showed increasing net import of C and N, with phloem contributing 61% of the imported C and 18% of the N. ¹⁴C feeding studies suggested the potential for simultaneous import and export through phloem over the period 9 to 12 days. Phase 2 (11-20 days, leaf attaining maximum area and net photosynthesis rate) exhibited net import through xylem and increasing export through phloem. Eighty-two% of xylem-delivered N was consumed in leaf growth, the remainder exported in phloem. Phase 3 (20-38 days) showed high but declining rates of photosynthesis, translocation, and net export of N. Phase 4 (38-66 days) exhibited substantial losses of N and declining photosynthesis and translocation of C. C:N ratio of xylem sap remained constant (2.3-2.6) during leaf life; petiole phloem sap C:N ratio varied from 25 to 135 over leaf development. The relationships between net photosynthesis and N import in xylem were: phase 1, 4.8 milligrams C per milligram N; phase 2, 24.7 milligrams C per milligram N; phase 3, 91.9 milligrams C per milligram N; and phase 4, 47.7 milligrams C per milligram N.     

Exploration of the nitrogen transport system of a nodulated legume using 15N

Summary Feeding experiments using ¹⁵N₂ or ¹⁵NO₃ are described investigating the transport of nitrogen in the field pea (Pisum arvense L.). Nitrogen assimilated by root or nodules moves preferentially upwards to the shoot through the xylem. Parts of the root below or distal to a region of assimilation can benefit from this nitrogen but do so to a much greater extent when the shoot is left attached than when it has been removed. A considerable proportion of the nitrogen received by a shoot from the root or nodules is apparently returned to the root in the translocation stream, this cycled nitrogen being especially important in the nutrition of outlying parts of nodulated roots growing in media lacking combined nitrogen.Nitrogen from nitrate fed to a mature leaf is exported in quantity to all parts of the plant except older regions of the shoot. Leaf and stem segments immediately above the fed leaf, and the root and its nodules receive large shares of this nitrogen, although the root's share declines noticeably as the plant ages.The root appears to be extremely inactive in transferring nitrogen from the downward translocation stream across to the stream of nitrogen leaving the root in the xylem. This may act as a major obstacle to the free circulation and mixing of nitrogen within the plant body.A scheme is proposed embracing the main quantitative features of the transport system for nitrogen in the species.     

A comparison of nitrogen-fixing ability of peanut,cowpea and siratro plants nodulated by different strains of Rhizobium

Summary Acetylene reduction activity and nitrogen accumulation in the plant top per unit nodule mass were compared among peanut, cowpea and siratro plants nodulated by six different strains of Rhizobium. Peanut was found to have several fold higher values than cowpea and siratro for both parameters for all strains of Rhizobium which nodulated it effectively, but the bacteroid content of the peanut nodules was similar to those of cowpea and siratro.     

The role of hydrogen peroxide-producing and hydrogen peroxide-consuming peroxidases in the leaf apoplast of cowpea in manganese tolerance

The apoplast is considered the leaf compartment decisive for manganese (Mn) toxicity and tolerance in cowpea (Vigna unguiculata). Particularly apoplastic peroxidases (PODs) were proposed to be key enzymes in Mn toxicity-induced processes. The presented work focuses on the characterization of the role of hydrogen peroxide (H2O2)-producing (NADH peroxidase) and H2O2-consuming peroxidase (guaiacol POD) in the apoplastic washing fluid (AWF) of leaves for early stages of Mn toxicity and genotypic differences in Mn tolerance of cowpea. Leaf AWF of the Mn-sensitive cultivar (cv) TVu 91 but not of the Mn-tolerant cv 1987 showed an increase of guaiacol-POD and NADH-peroxidase activities at elevated AWF Mn concentrations. two-dimensional resolutions of AWF proteins revealed that cv TVu 91 expressed more and additional proteins at high Mn treatment, whereas Mn-tolerant cv TVu 1987 remained nearly unaffected. In both cultivars, NADH-peroxidase activity and accompanied H2O2 formation rate in vitro were significantly affected by Mn2+, p-coumaric acid, and metabolites occurring in the AWF. The total phenol concentration in the AWF was indicative of advanced stages of Mn toxicity but was rather unrelated to early stages of Mn toxicity and genotypic differences in Mn tolerance. The NADH oxidation by AWF PODs was significantly delayed or enhanced in the presence of the protein-free AWF from cv TVu 1987 or cv TVu 91, respectively. High-performance liquid chromatography analysis of AWF indicates the presence of phenols in cv TVu 1987 not observed in cv TVu 91. We conclude from our studies that the H2O2-producing NADH peroxidase and its modulation by stimulating or inhibiting phenolic compounds in the leaf apoplast play a major role for Mn toxicity and Mn tolerance in cowpea.     

The effects of nitrate on the enzymes involved in nitrogen assimilation in nodulated pea roots

Pea Plants ( Pisum sativaum L. ev. Little Marvel) were grown in N-free medium and when well nodulated (28 days) were supplied for 8 days with nitrate or ammonium. Over the 8 days of nitrate treatment, total amino and amide N in sap declined, and the proportion of aspartate relative to the other amino acids increased. After 8 days of treatment, nitrogenase (EC 1.18.2.1) activity in nitrate-treated plants declined to about 30% of the activity in controls even though nodules were not directly in contact with nutrient solution. Nitrogenase activity was also decreased by the addition of ammonium chloride (10 m M ). With addition of nitrate or ammonium. clear signs of senescence began to show in the nodules after 4 days. Nitrate reductase (EC 1.6.6.1) activity was induced in roots by nitrate, but decreased sharply in nodules. In response to nitrate addition, newly formed root tissues showed 3- to 5-times higher glutamine synthetase (GS. EC 6.3.1.4) activity than newly formed tissues of control plants, expressed on a protein or weight basis. In complementary experiments, when ammonium salts were used instead of nitrates, the increase in GS activity was significantly lower. GS activity decreased in nodules of treated plants and total extractable protein was 3 times lower in nodules of nitrate-treated plants than in controls at day 8 of treatment.     

Partitioning of biologically fixed nitrogen in cowpea during pod development

Two days after exposure of roots to¹⁵N labeled N₂, partitioning of biologically fixed N into leaves, stems, peduncles, pods, roots and nodules was measured in the early pod development stage of cowpea (Vigna unguiculata (L.). The experimental objective was to determine the quantity of biologically fixed N that is incorporated into vegetative tissue before being mobilized to pods. For the three varieties of cowpea included in the experiment a maximum of 50% of the N, biologically fixed two days earlier, was contained in the pods. The remaining N was distributed throughout the vegetative portion of the plant with at least 30% in stems and leaves which indicates that much of the newly fixed N must cycle through a N pool in these tissues before reaching the pods.