Biomass production and nitrogen utilization by Alnus incana when grown on N2 or NH 4 + made available at the same rate

A single clone of Alnus incana (L.) Moench was grown in a controlled-environment chamber. The plants were either inoculated with Frankia and fixed atmospheric nitrogen or were left uninoculated but received ammonium at the same rate as the first group fixed their nitrogen. Nitrogen fixation was calculated from frequenct measurements of acetylene reduction and hydrogen evolution. The diurnal variation of acetylene reduction was also taken into account. The relative efficiency of nitrogenase could be used in the calculations of fixed nitrogen since the Frankia used did not show any detectable hydrogenase activity. Alders fixing nitrogen developed more biomass, longer shoots, larger leaf areas and contained more nitrogen than alders receiving ammonium. In one experiment, almost all ammonium given to the non-nodulated alders was taken up and 15% of the nitrogen taken up was excreted. In the other experiment, 34% of the ammonium was left in the nutrient solution and 8% of the nitrogen taken up was excreted. Alders inoculated with Frankia did not excrete any detectable amount of nitrogen. It seems that the energy demand for nitrogen fixation is not so high that biomass production in alders is retarded. The symbiotic system of A. incana and Frankia seems to be more efficient in utilizing its nitrogen than non-symbiotic A. incana receiving ammonium.     

Composition of Amino Compounds Transported in Xylem of Casuarina sp.

Seedlings (180-d-old) of Casuarina cunninghamianaM L., C. equisetifoliaMiq. and C. glauca Sieber inoculated with each of two differentsources of Frankia, were analysed for translocated nitrogenouscompounds in xylem sap. Analyses were also made on sap fromnodulated and non-nodulated plants of C. glauca grown with orwithout a range of levels of combined nitrogen. Xylem exudateswere collected from stems, roots, and individual nodules ofnodulated plants and from stems and roots of non-nodulated plants.While the proportional composition of solutes varied, the samerange of amino compounds was found in xylem sap from the threedifferent symbioses. In C. glauca asparagine was the major aminoacid in the root sap followed by proline, while in symbioticC. cunninghamiana arginine accounted for more than 25% of theamino compounds. Citrulline was the major translocated productfound in the stem exudate of symbiotic C. equisetifolia. Increasingconcentrations of ammonium nitrate in the nutrient solutionresulted in increasing levels of free ammonia and glutaminein xylem sap from stems of nodulated and non-nodulated C. glauca,but there was relatively little change in the prominent solutes,e.g. citrulline, proline, and arginine. The composition of nitrogenoussolutes in stem or root exudates of C. glauca was similar tothat of exudate collected from individual nodules and on thisbasis it was not possible to distinguish specific products ofcurrent N₂ fixation in xylem. The main differences in N solutecomposition between the symbioses were apparently due to hostplant effects rather than nodulation or the levels of combinedN. Also, the data indicate that the use of the proportion ofN in sap as citrulline (or indeed any other organic N solute)could not be used as an index of nitrogen fixation.     

Temporal variations in carbon isotope ratio of phytoplankton in a eutrophic lake

Temporal variations in carbon isotope ratio of phytoplanktonand dissolved inorganic carbon (DIC) in Lake Suwa were reported.In summer, blooming of Microcystis spp. resulted in low concentrationsof DIC and high pH, and HCO₃^– was the prominent speciesof DIC. Chlorophyll-specific rates of photosynthesis were relativelyconstant irrespective of the algal biomass during summer. Carboxylationin photosynthesis of Microcystis spp. was mainly catalyzed byribulose bisphosphate carboxylase (RuBPCase). Carbon isotopediscrimination between ¹³C of phytoplankton and DIC was considerablysmall in early summer and appeared to be negatively correlatedto DIC concentration. We concluded that carbon fixation by phytoplanktonin Lake Suwa is controlled not by the switch of photosyntheticpathways, but by low DIC concentration and high pH, suggestingthat photosynthesis of Microcystis spp. in Lake Suwa is governedby uptake kinetics other than the carboxylation step.     

Assimilate Partitioning in Red and White Clover Either Dependent on N2 Fixation in Root Nodules or Utilizing Nitrate Nitrogen

During vegetative growth in controlled environments, the patternof distribution of ¹⁴C-labelled assimilates to shoot and root,and to the meristems of the shoot, was measured in red and whiteclover plants either wholly dependent on N₂ fixation in rootnodules or receiving abundant nitrate nitrogen but lacking nodules. In experiments where single leaves on the primary shoot wereexposed to ¹⁴CO₂, nodulated plants of both clovers generallyexported more of their labelled assimilates to root (+nodules),than equivalent plants utilizing nitrate nitrogen, and thiswas offset by reduced export to branches (red clover) or stolons(white clover). The intensity of these effects varied with experiment.The export of labelled assimilate to growing leaves at the terminalmeristem of the donor shoot was not influenced by source ofnitrogen. Internode elongation in the donor shoot utilized nolabelled assimilate. Whole plants of white clover exposed to ¹⁴CO₂ on seven occasionsover 32 days exhibited the same effect on export to root (+nodules),which increased slightly in intensity with increasing plantage. Nodulated plants had larger root: shoot ratios than theirequivalents utilizing nitrate nitrogen. Trifolium repens, Trifolium pratense, red clover, white clover, nitrogen fixation, nitrate utilization, assimilate partitioning     

Growth and 2, 4-Diaminobutyric Acid Composition of Flatpea (Lathyrus sylvestris L.) as Influenced by Mineral- and Symbiotically Fixed-Nitrogen

Flatpea (Lathyrus sylvestris L. cv. ‘Lathco’) plants,grown hydroponically, were inoculated with rhizobia (Rhizobiumleguminosarum strain 92F2), supplied with nitrate nitrogen (2meq 1^–1), or provided with both rhizobia and . Nodulated plants supplied with had the highest biomass and the highest level of free amino acids in leavesand roots. Nitrogen fixation was depressed 80% in inoculatedplants supplemented with . Compared to plants whose nitrogen requirement was satisfied either partially orfully by , plants that were totally dependent upon biological nitrogen fixation had fewer lateral shoots andlower concentrations of nitrogen, soluble protein, and totalfree amino acids in the shoots, especially in the leaves. Thesymbiotic association between plants and rhizobia for nitrogenfixation was not essential to the production and accumulationof 2,4-diaminobutyric acid (A₂bu) and other non-protein aminoacids in flatpea. Levels of A₂bu, and most of the other freeamino acids quantified, were significantly lower in the nodulatedplants grown in the absence of than in the inoculated or noninoculated plants supplied with . Arginine and glutamic acid were exceptions in that levels ofthese two amino acids were higher in roots bearing effectivenodules. Plant responses to the different treatments are attributedto the amount of nitrogen available to the plant, rather thanthe method or form of nitrogen supplied. Key words: 2,4-Diaminobutyric acid, Lathyrus, nitrogen     

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     

Influence of nitrogen on growth and photosynthesis of a C3 cereal, Oryza sativa, infected with the root hemiparasite Striga hermonthica

Striga hermonthica is a root hemiparasitic angiosperm nativeto the African semi-arid tropics. It is a major weed of C₄ cerealsbut locally it is also an important weed of the C₃ plant, rice[Oryza sativa). Infected rice plants produced 17% and 42% ofthe total biomass of uninfected plants when grown at two differentammonium nitrate concentrations, 1 and 3 mol m^–3, respectively.S. hermonthica prevented grain production at both concentrationsof nitrogen. At the lower concentration no heads were produced.At the higher concentration head weight was only 6% of uninfectedcontrols. S. hermonthica also altered the partitioning of drymatter between plant parts, such that shoot growth was reducedto a greater extent than root growth. As a consequence the root-to-shootratio of infected plants was approximately five times greaterthan that of uninfected control plants. Light saturated ratesof photosynthesis In infected plants were 56% and 70% of thoseof uninfected controls, at low and high nitrogen, respectively.Infection also led to lower values of stomatal conductance althoughthe substom-atal CO₂ concentration was unaffected. Analysisof the response of photosynthesis to substomatal CO₂ concentration(A/C_I curves) demonstrated that lower rates of photosynthesiscould not be solely attributed to lower stomatal conductances.Lower initial slopes and asymptotic rates suggest that bothcarboxylation and processes controlling regeneration of ribulose-1,5-bisphosphate are reduced by infection. The data are discussedwith respect to the influence of S. hermonthica on the growthand photosynthesis of C₄ hosts, where in contrast to the situationwith rice, nitrogen feeding results in a marked alleviationof the effects of the parasite on the host. Key words: Rice, Striga, growth, photosynthesis, nitrogen     

Relationships between Nitrate Reductase Activity, Plant Weight and Nitrogen Content in Seedlings of Triticum, Aegilops and Triticale

Seedlings of 12 genotypes were grown in pots and watered withnutrient solutions providing 0, 1, 6 and 20 mg equivalents ofnitrate per I. Increasing the external nitrate supply broughtabout increases in plant weight, nitrate, reduced nitrogen concentrationsand in vivo nitrate reductase activity. When given solutioncontaining 6 mg equivalents of nitrate per litre, the plantscontained approximately 0.1 per cent nitrate, a concentrationsimilarto that found in field-grown plantsat thesamestage of growth.At the 6 mg equivalent level nitrate supply, nitrate reductaseactivity was strongly positively correlated with the concentrationsof nitrate and reduced nitrogen and negatively correlated withplant weight. Similar, though weaker, correlations were foundat the lower and higher levels of nitrate supply. The two Triticalegenotypes however, had higher than average plant weights andnitrate reductase activities, while plants of the two Aegilopsspecies weighed much less, especially at the higher levels ofnitrate supply, than the average of all 12 genotypes and generallyhad correspondingly greater nitrate and reduced nitrogen concentrationsand nitrate reductase activities. For individual genotypes,plant weight at a given level of nitrate supply was stronglycorrelated with weight at all other levels. In a second experiment seedlings of 150 genotypes were grownin compost watered with 10 mM Ca(NO₃)₂ Nitrate and reduced nitrogenconcentrations were negatively correlated with plant weightbut there was no significant correlation between nitrate reductaseactivityand either plant weight, nitrate or reduced nitrogen concentration. The results are taken to indicate that genetic factors, otherthan those determining the supply of reduced nitrogen, werelimiting growth and that as a consequence small plants accumulatednitrate and reduced nitrogen compounds in greater concentrationsthan large ones. The greater nitrate concentrations in smallplants may have induced the increased nitrate reductase activityfound in these, as compared with larger plants. Because plantweight varied more than did reduced nitrogen concentration,variation in reduced nitrogen per plant was more highly correlatedwith plant weight than with per cent reduced nitrogen.     

The Function of Trace Metals in the Nitorgen Metabolism of Plants

In recent years trace metals have been found to be associatedwith flavo-proteins, a number of which are key enzymes in thenitrogen metabolism of plants. Thus Mo is a constituent of nitratereductase which mediates the reduction of nitrate to nitritein some bacteria, fungi, and higher plants. During the enzymicreduction of nitrate a one-electron change is involved, Mo⁶⁺Mo⁵⁺.Nitrite and hyponitrite reductases both require Fe and Cu andflavin for maximum activity and are inhibited by uncouplingreagents so that a phosphorylation takes place at these tworeductive steps. Hydroxylamine reductase, a flavoprotein thatis Mn-dependent, but, as with nitrate reductase, is unaffectedby uncoupling reagents. An aromatic route may also exist inplants for the reduction of organo-nitro compounds, but it ismore likely that the aliphatic route is the primary reductivepadiway. Mo, V, W, and Fe may all be involved in the process of N₂ fixationin free living micro-organisms and in the root nodule bacteria.The suggestion that the enzyme hydrogenase is coupled to N₂fixation is not borne out by the results of valency experimentswith Mo and V which show that an active hydrogenase does notnecessarily result in a vigorous fixation process. Although Mo has been shown to be a constituent of hydrogenasefrom Clostridium pasteurianum, experiments in this laboratoryshow that a deficiency of iron only reduced the enzyme in thisorganism and in Azotobacter. Both Mo and Fe, however, are essentialfor N₂ fixation. In some strains of Azotobacter, V or W canpartially replace Mo for N₂ fixation.     

Effects of Atmospheric NO2 on Azolla-Anabaena Symbiosis

Cultures of the water fern Azolla pinnata R, Br. exposed for1 week to atmospheric NO₂ (50, 100 or 200 nl l^-1) induced additionallevels of nitrate reductase (NaR) protein and nitrite reductase(NiR) activity. At low concentrations of NO₂ (50 nl l^-1), nitratederived from NO₂ provides an alternative N source for Azollabut does not affect rates of acetylene reduction. However, thesymbiotic relationship between Azolla and its endosymbiont,Anabaena azollae is only affected adversely by high concentrations(100 and 200 nl l^-1) of atmospheric NO₂. The resultant decreasesin rate of growth, nitrogen fixation, heterocyst formation,and overall nitrogen cycling are probably due to the additionalaccumulation of N products derived from higher levels of atmosphericNO₂. Parallel increases in levels of polyamines suggest thatAzolla partially alleviates these harmful effects by incorporatingsome of the extra NO₂-induced N into polyamines.Copyright 1994,1999 Academic Press Azolla-Anabaena symbiosis, nitrogen dioxide pollution, nitrogen metabolism, polyamines     

Carbon and Nitrogen Assimilation by Vicia faba L. at Low Temperature: the Importance of Concentration and Form of Applied-N

Low temperature (6 C) growth was examined in two cultivarsof Vicia faba L. supplied with 4 and 20 mol m^–3 N as nitrateor urea. Both cultivars showed similar growth responses to increasedapplied-N concentration regardless of N-form. Total leaf areaincreased, as did root, stem and leaf dry weight, total carboncontent and total nitrogen content. In contrast to findingsat higher growth temperatures, 20 mol m^–3 urea-N gavesubstantially greater growth (all parameters measured) than20 mol m^–3 nitrate-N. The increased carbon content per plant associated with increasedapplied nitrate or urea concentration, or with urea in comparisonto nitrate, was due to a greater leaf area per plant for CO₂uptake and not an increased CO₂, uptake per unit area, carbon,chlorophyll or dry weight, all of which either remained constantor decreased. Nitrate reductase activity was substantial inplants given nitrate but negligible in plants given urea. Neitherfree nitrate nor free urea contributed greatly to nitrogen levelsin plant tissues. It is concluded that there is no evidence for a restrictionin nitrate reduction at 6 C, and it is likely that urea givesgreater growth than nitrate because of greater rates of uptake. Vicia faba, broad bean, low temperature growth, carbon assimilation, nitrogen assimilation     

Possible causes of the physiological decline in soybean nitrogen fixation in the presence of nitrate

Nodulated soybean plants (Glycine max (L.) Merr. cv. Clarke)were supplied with 10 mol m^-3 nitrate at the vegetative stage.This treatment caused a rapid decline in nitrogen fixation (acetylenereduction) activity and a consequent decline in ureides in thexylem sap. However, there was virtually no effect on the nitrogenasecomplex, according to Western blots against components 1 and2. The effect on nitrogen fixation was matched by a decreasein nitrogenase-linked respiration and increases in nodule oxygendiffusion resistance and the carbon cost of nitrogen fixation.The addition of nitrate had little effect on protein contentfrom either nodule plant or bacteroid fractions. Activitiesof nitrate reductase (NR) and nitrite reductase (NiR) from eitherthe plant fraction or the bacteroids were affected in differentways during 8 d of supply. Nodule plant NR and bacteroid NiR were not affected. However,nodule plant NiR increased 5-fold within 2 d of supplying Bacteroid NR only increased after6 d. These results could be interpreted in terms of a restrictednitrate access into the infected region of nodules. However,denitrification was detected within 2 d of nitrate supply insoybean nodules. The results are discussed in relation to possiblecauses of the nitrate-induced decline in nitrogenase activity. Key words: Glycine max, nitrate, nitrogen fixation, nodules     

The Distribution of Nitrate Assimilation Between Root and Shoot in Vicia faba L.

Nitrate assimilation was examined in two cultivars (Banner Winterand Herz Freya) of Vicia faba L. supplied with a range of nitrateconcentrations. The distribution between root and shoot wasassessed. The cultivars showed responses to increased applied nitrateconcentration. Total plant dry weight and carbon content remainedconstant while shoot: root dry weight ratio, total plant nitrogen,total plant leaf area and specific leaf area (SLA) all increased.The proportion of total plant nitrate and nitrate reductase(NR) activity found in the shoot of both cultivars increasedwith applied nitrate concentrations as did NO₃^–: Kjeldahl-Nratios of xylem sap. The cultivars differed in that a greaterproportion of total plant NR activity occurred in the shootof cv. Herz Freya at all applied nitrate concentrations, andits xylem sap NO₃^–: Kjeldahl-N ratio and SLA were consistentlygreater. It is concluded that the distribution of nitrate assimilationbetween root and shoot of V. faba varies both with cultivarand with external nitrate concentration. Vicia faba L., field bean, nitrate assimilation, nitrate reductase, xylem sap analysis     

The Carbon Balance of a Legume and the Functional Economy of its Root Nodules

Budgets for carbon and nitrogen in shoot, root, and nodulesof garden pea (Pisum sativum L.) are drawn up for a 9-d intervalin the life cycle, from data on nitrogen fixation, carbon accumulationin dry matter, respiratory output of plant organs, and organicsolute exchange between shoot and nodulated root. Of the carbon gained photosynthetically by the shoot from theatmosphere 26 per cent is incorporated directly into its drymatter, 32 per cent translocated to the nodules, and 42 percent to the supporting root. Of the nodules’ share, 5per cent is consumed in growth, 12 per cent in respiration,and 15 per cent returned to the shoot via the xylem, as aminocompounds generated in nitrogen fixation. Growth and respirationof the root utilize, respectively, 7 and 35 per cent. The respiratory efficiency of a nodulated root in terms of nitrogenfixation (5.9mg C per mg N₂-N fixed) is found to be very similarto that of an uninoculated root assimilating nitrate (6.2 mgC per mg NO₃-N reduced). The nodules require in growth, respiration,and export 4.1 mg C ( 10.3 mg carbohydrate) for each mg N whichthey fix. The nodules consume 3 ml O₂ for every 1 ml N₂ utilized in fixation. In exporting a milligram of fixed nitrogen the nodules requireat least 0.35 ml of water. Almost half of this requirement mightbe met by mass flow into the nodules via the phloem.     

White Clover N2-Fixation in Response to Root Temperature and Nitrate: II. N2-FIXATION, RESPIRATION, AND NITRATE REDUCTASE ACTIVITY

Established, nodulated white clover plants were transferredto eight tanks of a flowing culture apparatus with solutiontemperatures of 5, 11, 17, and 25 ?C (two tanks per temperature).Shoot temperature and light environment were common to all plants.After 7 d, (10 mmol m^–3) was continuouslysupplied to one tank at each temperature while in the remainingfour tanks (one at each temperature) the plants were completelydependent on nodule N₂-fixation. Plants were randomly selected at intervals during the following14 d period in order to measure root and nodule respirationand acetylene reduction activity (ARA) in a flow-through systemset at the adapted root temperature. Additional plants wereassayed for in vitro nitrate reductase activity in leaves, roots,and nodules. Apparent nitrogenase activity (ARA) and respiration associatedwith it were each markedly affected by temperature in two ways;(1) Activity per unit weight of nodule was reduced at lowertemperatures; (2) Development of the plant, and thus also nodulemass, was restricted at lower temperatures which, in turn, restrictedtotal nodule activity per plant. The presence of nitrate significantly reduced ARA of nodules,particularly at higher temperatures. However, significant discrepancieswere found when N₂-fixation rates, estimated from the acetylenereduction assay, were compared with N₂-fixation rates calculatedfrom curves fitted to N accumulation data (minus the rate of uptake in the case of nitrate-treated plants). Carbon use efficiency (CO² respired per C₂H₄ produced) was notsignificantly affected by temperature or the presence of nitrate. Nitrate reductase activity (NRA) developed in all plant partsat the three highest temperatures, but not at 5 ?C. We calculatethat leaf NRA may account for 82, 75, and 68% of total nitratereduction at 11, 17, and 25 ?C respectively. Key words: Trifolium repens, white clover, N₂ fixation, root temperature, acetylene reduction assay, nitrate, nitrate reductase     

The Effect of Nitrogen Source on Transport and Metabolism of Nitrogen in Fruiting Plants of Cowpea (Vigna unguiculata (L.) Walp.)

Nitrogen metabolism and transport were studied during reproductivedevelopment of cowpea (Vigna unguiculata (L.) Walp. cv. Vita3) under three contrasting nitrogen regimes: (1) nitrate suppliedcontinuously (plants non-nodulated), (2) symbiotic N₂ fixation(no combined nitrogen), (3) nitrogenstarvation post-anthesisof previously N₂-fixing plants. The last treatment involveddaily flushing of the root systems with 100% oxygen which suppressedpost-anthesis N₂-fixation by 76–79%, thereby making fruitgrowth almost entirely reliant upon mobilization of previouslyaccumulated nitrogen. The bulk of the xylem nitrogen (root bleedingsap or peduncle tracheal sap) of nitrate-fed plants was nitrateand amide, that of symbiotic and O₂-treated plants largely ureide.The composition of fruit cryopuncture phloem sap, however, wasclosely similar in all treatments, with most nitrogen as amidesand amino acids. The evidence suggested intense metabolic transferof root derived nitrate-N or ureide-N to amino acids by vegetativeplant parts prior to translocation to fruits. All tissues offruits showed patterns of development of enzymic activitiesconsistent with release of nitrogen from both ureides and amidesand re-assimilation of ammonia to form amino acids. Althoughthe levels of enzyme activities varied between treatments thedifferences could not be readily associated with individualpatterns of nitrogen transport in the treatments. Nitrogen sufficiencyin the NO₃-fed plants was marked by elevated vegetative biomassand low harvest indices for dry matter and nitrogen, while nitrogendeficiency of the O₂-treated plants was associated with seedabortion, small seed size and low seed nitrogen concentration,and efficient mobilization of nitrogen from vegetative partsto fruits. Key words: Nitrogen, Translocation, Cowpea     

Changes in the Morphology of Roots and Leaves of Carob Seedlings Induced by Nitrogen Source and Atmospheric Carbon Dioxide

Carob seedlings were grown hydroponically for 9 weeks under360 and 800 µl l^-1CO₂. One of two nitrogen sources, nitrateor ammonium, was added to the nutrient medium at concentrationsof 3 mol m^-3. Root systems of the developing plants suppliedwith nitrate compared to those supplied with ammonium were characterizedby:(a)more biomass on the lower part of the root;(b)fewer lateralroots of first and second order;(c)longer roots;(d)higher specificroot length;(e)a smaller root diameter. The morphology of theroot systems of nitrate-fed plants changed in the presence ofelevated carbon dioxide concentrations, resembling, more closely,that of ammonium-fed plants. Total leaf area was higher in ammonium-than in nitrate-fed plants. Nitrate-fed plants had greater totalleaf area in the presence of high carbon dioxide than in normalCO₂, due to an increase in epidermal cell size that led to developmentof larger leaflets with lower stomatal frequency. The observedchanges in the morphology of roots and shoots agreed with theresults observed for total biomass production. Nitrate-fed plantsincreased their biomass production by 100% in the presence ofelevated CO₂compared to 15% in ammonium-fed plants, indicatingthat the response of carob to high CO₂concentrations is verydependent on the nitrogen source. Under elevated CO₂, nitrate-grownplants had a larger content of sucrose in both roots and shoots,while no significant difference was observed in the contentof sucrose in ammonium-grown plants, whether in ambient or enrichedcarbon dioxide. Hence, the differences in soluble carbohydratecontents can, at least partly, account for differences in rootand shoot morphology.Copyright 1997 Annals of Botany Company Ceratonia siliquaL.; carob; ammonium; carbohydrate; carbon dioxide; nitrate; morphology; sucrose     

ADAPTIVE FORMATION OF NITRATE REDUCING SYSTEM IN ANABAENA CYLINDRICA

1. Changes in capacities for reducing nitrate, nitrite and hydroxylaminecaused by provision or depletion of various nitrogen sourceswere investigated with a nitrogen fixing blue-green alga, Anabaenacylindrica, and adaptive nature of these reducing system wasdemonstrated.
2. It was found that, under light-aerobic conditions,nitrate-and nitrite- reducing systems were induced by nitrateor nitritebut not by N₂ ammonia and glutamate. On the otherhand, theactivity of enzymes pertaining to hydroxylamine reductionwasstimulated equally by nitrate, nitrite and N₂. The latteractivitywas suppressed markedly in the presence of ammoniaor glutamate.
3. Adaptive formation of nitrite reducing systemis completelyinhibited by chloramphenicol, a potent inhibitorof proteinsynthesis. No formation was also observed under theanaerobiccondition or in the dark.
4. On the basis of thesefindings, a tentative scheme for pathwaysof nitrate reductionand nitrogen fixation in Analaena cylindricawas proposed.

(Received August 22, 1962; )     

Verification of 14C and O2 derived primary organic production measurements using an enclosed ecosystem

The stimulus for these experiments came from a recent seriesof papers which have suggested that the ¹⁴C technique may underestimateprimary production by as much as 10-fold. We evolved the followingstrategy to attempt to verify the ¹⁴C technique in nearshorewaters: (i) to examine the validity of in vitro (i.e., bottleincubation) measurements by comparing observed in situ oxygenchanges in a large enclosed natural ecosystem against thosedetermined in vitro and if no evidence of containment effectswere indicated, (ii) compare ¹⁴C and oxygen measurements insimultaneous in vitro incubations. The first step essentiallytests the containment problem, the second the physiologicaland calibration problems of the ¹⁴C technique. The first experimentwas run with nitrate as the main source of inorganic nitrogen,the second with ammonia. PQs for converting the ¹⁴C measurementsto oxygen values were calculated from the equation PQ = PQ_c+ 2/(C/NO₃) where PQ_c is the ‘carbon PQ’ (takenas 1.25) and (C/NO₃) is the molar carbon to nitrate assimilationratio. Although there appear to be some minor residual problemsin the interpretation of the data when nitrate was the dominantnitrogen source, the overriding conclusions were: first, thatthe close agreement between the changes in in situ and in vitrodissolved oxygen concentration during the photoperiod gave noevidence for any notable containment effect upon photosynthesis.Secondly, the in vitro rates of ¹⁴CO₂-determined photosyntheticproduction and gross photosynthetic oxygen production agreed,within the precision of the two techniques. The experiment furtherdemonstrated the need to determine soluble as well as particulateorganic production and to pay attention to the potential effectof the nitrogen nutrient upon the PQ. Thus it was concludedthat our data give no evidence for marked errors in the ¹⁴C-techniqueof measuring primary organic production for coastal waters. ^{Present address: Department of Marine Microbiology, Instituteof Botany, University of Gothenburg, Carl Skottsbergs Gata 22,S-413 19 Gothenburg, Sweden.}     

The photosynthetic response of nutrient-depleted dilute cultures of Skeletonema costatum to pulses of ammonium and nitrate; the importance of phosphate

Experimental data on changes in carbon fixation rate causedby nutrient pulses in dilute cultures of nutrient-depleted Skeletonemacostatum are reported. Pulses contained single nutrients ormixtures of a nitrogen source and phosphate, concentrationsranging from 0 to 5 µM for ammonium and nitrate and from0 to 1 µM for phosphate. The cultures were incubated overnightafter pulsing to allow the rapid stage of nutrient uptake tocome to completion before measurement of carbon fixation thenext day. Increments in N:C ratios due to nitrogenous nutrientuptake depended upon the P:C ratio in the cells as well as theconcentration of the pulse. When P:C ratios were low (<0.005),increases in nitrogen repressed photosynthesis. Phosphate uptakewas independent of the absence or presence of a nitrogen sourceand had only a small stimulatory effect upon carbon fixation.When added jointly with ammonium or nitrate, however, largeincreases in the rate of photosynthesis resulted. These weredue mainly to rises in chlorophyll a concentrations resultingfrom higher N:C ratios in the cells. Chlorophyll-specific carbonfixation rates were hyperbolic functions of P:C ratios but exhibiteda C-shaped relationship to N:C ratios. The stimulatory and repressiveeffects of the nutrient pulses are shown to be consistent withthe view that the rate of photosynthesis at constant illuminationand carbon dioxide partial pressure is mainly controlled bythe chlorophyll a concentration and phosphate availability inthe phytoplankton.