Isotopic Studies of Nitrogen Fixation in Non-Legume Root Nodules

Isotopic studies are presented on six of the eight recognizednon-legume nodule-forming genera of Angiosperms. Evidence hasbeen obtained of the occurrence of fixation of atmospheric nitrogenin the root nodules of Casuarina, Ceanothus, and Shepherdia,now examined isotopically for the first time. In further studiesof the nitrogen-fixing nodules of Hippopha and Alnus it is shownthat the fixation continues for a longer period after detachmentfrom the plant than is the case with legume nodules, and thatthe enrichment in ¹³N which they (and also detached Casuarinanodules) finally achieve after exposure to excess of the freeisotope considerably exceeds that shown by detached legume nodules.Fixation in detached Myrica nodules was not clearly affectedby reduction in oxygen supplied until the proportion of oxygenwas less than 5 per cent.     

Comparisons of the Respiratory Effluxes of Nodules and Roots in Six Temperate Legumes

The specific respiration rates of nodulated root systems, ofnodules and of roots were determined during active nitrogenfixation in soya bean, navy bean, pea, lucerne, red clover andwhite clover, by measurements on whole plants before and afterthe removal of nodule populations. Similar measurements weremade on comparable populations of the six legumes, lacking nodulesbut receiving abundant nitrate-nitrogen, to determine the specificrespiration of their roots. All plants were grown in a controlled-environmentclimate which fostered rapid growth. The specific respiration rates of nodulated root systems ofthe three grain and three forage legumes during a 7–14-dayperiod of vegetative growth varied between 10 and 17 mg CO₂g^–1 (dry weight) h^–1. This mean value consistedof two components: a specific root respiration rate of 6–9mg CO₂ g^–1 h^–1 and a specific nodule respirationrate of 22–46 mg CO₂ g^–1 h^–1. Nodule respirationaccounted for 42–70 per cent of nodulated root respiration;nodule weight accounted for 12–40 per cent of nodulatedroot weight. The specific respiration rates of roots lackingnodules and utilizing nitrate nitrogen were generally 20–30per cent greater than the equivalent rates of roots from nodulatedplants. The measured respiratory effluxes are discussed in thecontext of nitrogen nitrogen fixation, nitrate assimilation. Glycine max, Phaseolus vulgaris, Pisum sativum, Medicago sativa, Trifolium pratense, Trifolium repens, soya bean, navy bean, pea, lucerne, red clover, white clover, nodule respiration, root respiration, fixation, nitrate assimilation     

Growth and Photosynthesis in the First Leaf of Barley The Effect of Time of Application of Nitrogen

Using Proctor barley grown in sand under controlled-environmentconditions it was shown that when application of nitrogen, asnitrate, was delayed beyond day 4, at which time the first leafwas beginning to unfold, absolute and relative growth-ratesof seedlings were reduced so that the young plants were significantlysmaller. Delay in nitrate application led to reduction in length,breadth, area, and dry weight of the first leaf, and also toa lower photosynthetic activity on day 8, as measured by infra-redgas analysis. Measurement of the uptake of ¹⁴CO² by first leaves showed thatapplication of nitrate on days 2 or 4 led to high rates of fixationof carbon over the period days 8–12, whereas applicationon days 6 or 8 led to a substantially lower maximum rate offixation which was maintained for a shorter period. When nitratewas applied on day 8 total fixation of carbon over the perioddays 7–14 was only 60 per cent of that for leaves on plantsfor which nitrate was supplied on day 2. When amounts of carbon fixed were compared on a leaf dry-weightbasis, maximum values were found to be similar for all treatments,suggesting that the differences in fixation per leaf resultmainly from the effect of treatment on leaf area. For all treatmentsit was confirmed that a decline in photosynthetic activity occurredby day 14. This was not correlated with photosynthetic activityin the second leaf, nor with emergence of the third leaf. Analysis of different parts of the lamina of the first leafshowed all to be affected by the timing of the supply, bothin growth and in photosynthetic activity. Leaf dry weight didnot increase after day 8 for any treatment, yet when nitratewas applied on day 8 plants showed a fourfold increase in photosyntheticrate. The significance of this in relation to carboxylationand other resistances in photosynthesis is discussed.     

The IJsselmeer and its phytoplankton--with special attention to the suitability of the lake as a habitat for Oscillatoria agardhii Gom

The IJsselmeer (surface area 1200 km², mean depth 4.5 m, residencetime 0.4 year, phosphorus load 7 g m^–2 year^–1) isa very important conservation area. Regular summer bloomingof Oscillatoria spp. can depreciate this value, so the boundsof possibility of this kind of blooming have been investigated.Therefore samples were taken along the shore and in the openwater from 1974 to 1982, continuous temperature profile measurementswere made in the same period in the middle of the lake and insitu primary production was measured in 1976 and 1977. The phytoplanktonconsists of green algae throughout the year, diatoms in spring,and blooms of Microcystis aeruginosa in summer. Blooming ofOscillatoria agardhii Gom occurs regularly in summer along theFrisian shore. In 1976, however, a heavy bloom of this algaoccurred in the whole lake. Comparing the IJsselmeer with shallowerOscillatoria-lakes in the Netherlands distinct differences arepresent concerning biomass, chlorophyll a content, relativevolume of the euphotic zone and light-dark cycle. Not only thelarger depth and extensiveness of the IJsselmeer are unfavourablefactors for Oscillatoria, but also the separation by land reclamationof many of the shallow littoral regions from the main body ofthe lake. High temperature and microstratification are neededto develop a bloom in the whole lake.     

Distribution in relation to life history in the directdeveloping gastropod Batillaria cumingi (Batillariidae) on two shores of contrasting substrata

Population structure and spatial distribution with growth ofthe direct-developing gastropod, Batillaria cumingi, were investigatedon two shores of differing substrata. Sand-mud shore and rockyshore populations differed in size structure; first-year snailswere ca. 7 mm in shell length (SL) in both populations, whereassecond-year snails, merging with older cohorts, measured 15–25mm SL in the sand-mud shore population and ca. 15 mm SL in therocky shore population. Egg distribution matched adult distributionin the sand-mud shore population, but was more restricted thanthat of adults in the rocky shore population. The distributionof newly-hatched juveniles (0–1 mm SL) was restricted inboth populations, but the growth stage at which snails extendedtheir distribution differed between the two populations; 1–2.5mm SL on the sand-mud shore and 5 mm SL on the rocky shore. Floatingachieved by early juveniles (ca. 2 mm SL), was commonly observedin the sand-mud population, but rarely in the rocky shore population.The sudden expansion in distribution of the 1–2.5 mm SLgrowth stage in the sand-mud shore population is consideredto have been caused by floating, while expansion of the distributionof older growth stages (>5 mm SL) in the rocky shore populationprobably occurs by crawling. (Received 13 May 1998; accepted 25 August 1998)     

The Effects of Nitrogen Supply on the Absorption and Distribution of Copper in Red Clover (Trifolium pratense L.) Grown in Flowing Solution Culture with a Low, Maintained Concentration of Copper

The absorption and distribution of Cu in red clover (Trifoliumpratense L.) were measured in plants grown in flowing solutionculture with Cu maintained throughout at 0.5 µg 1^–1and N supplied either as nitrate or through symbiotic fixation.Although there was a decrease in Cu absorption, both with time,and with a depleted nitrate supply, it increased to its formerrate when nitrate was adjusted to 10 mg N 1^–1 after aperiod of depletion. Differences in absorption between plantsdependent upon fixation and those supplied with nitrate wererelated to the slower initial growth of the plants fixing N.Considerable proportions (> 30 per cent) of the absorbedCu were retained by the roots. At the final harvest, and withthe exception of plants grown with nitrate adjusted to 0.1 mgN 1^–1 after a period of depletion, the proportion of theCu retained was related to the concentration of N in the roots.The different N treatments produced differences in Cu concentrationin the shoots, and the effects were greater in the youngestfully expanded leaves than in older leaves. Trifolium pratense L., red clover, absorption, copper, flowing solution culture, nitrogen     

Nitrogen fixation and heterocysts in the blue-green alga Anabaena cylindrica

The site of nitrogen fixation in the blue-green alga Anabaenacylindrica Lemra (Fogg strain) was investigated. Less than 4%of the total nitrogen fixed during a relatively short period(5-15 min) was recovered in heterocysts. When estimated on thecellular nitrogen basis, vegetative cells can fix molecularnitrogen at the same rate as do heterocysts. There was no positivecorrelation between nitrogen fixation and heterocyst formation.Results do not support the hypothesis that the heterocyst isthe main site for nitrogen fixation in blue-green algae. ¹ This work was supported by grant (No. 38814) from the Ministryof Education. (Received July 23, 1971; )     

Microclimate, Photosynthesis and Growth of Lucerne (Medicago sativa L.). II. Canopy Structure and Growth

The growth of lucerne var. Europe was examined in the fieldduring 1976. The annual dry matter production of unirrigatedlucerne during 1976, with no nitrogen fertilizer application,was 82.5 per cent greater than unirrigated S.24 perennial ryegrass,with a nitrogen application of 384 kg ha^–1. The mean aboveground growth rate of lucerne was 7.3 g DM m^–2 day^–1between March and early June, of which stem material contributeda maximum of 76.5 per cent. Significant losses of leaves andstems occurred from the end of April, indicating a loss of potentialforage material. Nitrogen analyses of the above ground crop suggested that in56 days lucerne yielded 10.7 per cent more nitrogen than didS.24 annually with a nitrogen fertilizer addition of 280 kgha^–1. Between 13 and 57 per cent of the daily photosynthate is translocatedbelow ground. Medicago sativaL, lucerne, dry matter production, canopy structure, nitrogen analyses     

Relationships between Acetylene Reduction Activity, Hydrogen Evolution and Nitrogen Fixation in Nodules of Acacia spp.: Experimental Background to Assaying Fixation by Acetylene Reduction under Field Conditions

Hansen, A. P., Pate, J. S. and Atkins, C. A. 1987. Relationshipsbetween acetylene reduction activity, hydrogen evolution andnitrogen fixation in nodules of Acacia spp.: Experimental backgroundto assaying fixation by acetylene reduction under field conditions.—J.exp. Bot. 38: 1–12 Glasshouse grown, symbiotically-dependent seedlings of Acaciaalata R.Br., .A. extensa Lindl., and A. pulchella R.Br. wereexamined for acetylene reduction in closed assay systems usingundisturbed potted plants, excavated whole plants, nodulatedroots or detached nodules. Nitrogenase activity declined sharplyover the first hour after exposure of detached nodules to acetylene(10% v/v in air), less steeply or not at all over a 3 h periodin assays involving attached nodules. Using detached nodules,rates of acetylene reduction, nitrogen (¹⁵N₂) fixation, andhydrogen evolution in air (¹⁵N₂) and acetylene-containing atmosphereswere measured in comparable 30 min assays. Total electron flowthrough nitrogenase in air was determined from rates of nitrogen(¹⁵N₂) fixation ( ? 3) plus hydrogen evolution, that in thepresence of acetylene from rates of acetylene reduction andhydrogen evolution in air: acetylene. Values for the ratio ofelectron flow in air: acetylene to that in air ranged from 0?43to 0?83 in A. pulcheila, from 0?44 to 0?66 in A. alala and from0?37 to 0?70 in A. extensa, indicating substantial inhibitionof electron flow through nitrogenase of detached nodules byacetylene. Relative efficiencies of nitrogenase functioningbased on hydrogen evolution and acetylene reduction were from0?15 to 0?79, those based on nitrogen (¹⁵N₂) fixation and hydrogenevolution from 0?53 to 0?87. Molar ratios of acetylene reducedto nitrogen (¹⁵N₂) fixed were 2?82 ? 0?24, 201 ? 0?15, and 1?91? 0?11 (?s.e.; n = 7) for A. pulcheila,A. extensa and A. alata respectively A standard 5–10 min acetylene reduction assay, conductedon freshly detached unwashed nodules in daytime (12.00–14.00h), was calibrated for field use by comparing total N accumulationof seedlings with estimated cumulative acetylene reduction overa 7-week period of glasshouse culture. Molar ratios for acetylenereduced: nitrogen fixed using this arbitrary method were 3?58for A. alata, 4?82 for A. extensa and 1?60 for A. pulchella.The significance of the data is discussed. Key words: Acacia spp, nitrogenase functioning     

Effect of Nitrogen Fertilizer on Photosynthesis of Several Varieties of Winter Wheat

The rates of gross photosynthesis of the flag leaf and the nextleaf below (second leaf) in crops of winter wheat were estimatedfrom the ¹⁴C uptake of the leaves after exposure to short pulsesof ¹⁴CO₂. The photosynthetic rates of both leaves during thegrain-filling period decreased with increase in nitrogen fertilizerbecause the intensity of photosynthetically active radiationwas less at the surface of the leaves in the dense crops withadditional nitrogen. In addition, the rate of photosynthesisat saturating light intensity was slightly decreased by nitrogen.The effects of nitrogen, in decreasing the rate of photosynthesisper unit area of leaf and in increasing the leaf-area indexof the top two leaves, were such that the photosynthetic productivityper unit area of land of the flag leaf was increased by nitrogenbut the productivity of the second leaf was unaffected. Applying180 kg N ha^–1 increased the productivity of the top twoleaves by a factor of 2.3 but increased grain yield by only1.8. The photosynthetic productivity of the second leaf duringthe grain-filling period was about half that of the flag leaf. There was no difference in photosynthetic rate per unit areaof leaves of Cappelle-Desprez and Maris Huntsman which couldaccount for the larger yield of the latter cultivar. There wasa slight indication that the leaves of the semi-dwarf cultivarsMaris Fundin and Hobbit photosynthesized faster than those ofMaris Huntsman. Triticum aestivum L., winter wheat, photosynthesis, nitrogen fertilizer     

Nitrogen Fixation in the Canopy of Temperate Forest Trees: A Re-examination

¹⁵N₂ studies and acetylene reduction assays of leaves and shootsof Douglas fir and other forest trees do not confirm previousreports that extensive nitrogen fixation occurs on leaf surfacesand it is concluded that the importance of nitrogen fixationin the canopy of forest trees has been exaggerated. The presenceof nitrogen-fixing bacteria on the leaves of trees is confirmed,however, and they have been identified as Enterobacter agglomerans,Clostridium butyricum and Bacillus sp. Their distribution onleaves is fortuitous since dead oak leaves and artificial leavesbecome colonized to the same extent as living oak leaves. nitrogen fixation, acetylene reduction, Enterobacter agglomerans, Clostridium butyricum, Bacillus sp, Douglas fir, Pseudotsuga menziensii, larch, Larix x oak, Quercus petraea.     

Effect of Nitrogen on Growth, Yield and Photorespiratory Activity in Spring Wheat

Spring wheat plants growing in pots in controlled environmentrooms were given extra nitrogen after flag leaf emergence. Theeffect of nitrogen on growth, yield, the activity of ribulose1,5–bisphosphate carboxy–lase/oxygenase and thedistribution of¹⁴C in photorespiratory intermediates and indifferent parts of the plants was determined. Extra nitrogenincreased the movement of ¹⁴C to the ear and increased grainyield by 29 per cent, mainly because of an increase in grainnumber. Though extra nitrogen delayed senescence of the leaves,the growth of the ear in the later stages was not increasedin proportion to the extra green area. The relative inefficiencyof leaf area with extra nitrogen, which has also been foundin the field, was not due to a reduction in photosynthesis perunit leaf area. Nor was there evidence of an increase in photorespirationas reflected by a greater flow of carbon into the photorcspiratorymetabolites glycine and serine, or an increase in the activityof ribulose 1,5–bisphosphate oxygenase relative to thecarboxylase. We suggest that there may be an increase in theloss of carbon in dark respiration. Triticum aesttvum, nitrogen, growth, yield, photorespiration     

Effects of Nitrogen Fertilizer on Growth and Yield of Spring Wheat

Nine amounts of nitrogen fertilizer, ranging from 0 to 200 kgN ha^–1, were applied to spring wheat cv. Kleiber in the3 years 1972-1974. In 1972 grain dry weight with 125 kg N ha^–1or more was 100 g m^–2 (23 per cent) greater than withoutnitrogen. Grain yield was unaffected by nitrogen in the otheryears. Leaf area at and after anthesis was increased throughoutthe range of nitrogen tested, most in 1972 and least in 1973.Consequently, the addition of 200 kg N ha^–1 decreasedthe amount of grain produced per unit of leaf area by approximately25 per cent in all years. The dry weight of leaves and stems at anthesis and maturitywas increased by nitrogen in all years, similarly to leaf area.However, the change in stem dry weight between anthesis andmaturity was not affected by nitrogen; stems increased in dryweight for about 20 days after anthesis and then decreased tovalues similar to those at anthesis. The uptake of CO₂ per unit area of flag leaf or second leaf(leaf below the flag leaf) was slightly decreased by nitrogenwhen the increase in leaf area caused by nitrogen appreciablydecreased the light intensity at the surface of these leaves.In spite of such decreases the CO₂ absorbed by flag and secondleaves per unit area of land was always increased by nitrogen,and relatively more than was grain yield. It is suggested that increases in respiratory loss of CO₂ withincreasing nitrogen fertilizer may explain why nitrogen increasedvegetative growth and leaf area relatively more than grain yield.     

Carbon Dioxide Fixation and Ribulose-1, 5-bisphosphate Carboxylase Activity in Intact Leaves of Sugar Beet Plants Exposed to Salinity and Water Stress

The influence of salinity in the growing media on ribulose-1,5-bisphosphate (RuBP) carboxylase and on CO₂ fixation by intactsugar beet (Beta vulgaris) leaves was investigated. RuBP carboxylase activity was mostly stimulated in young leavesafter exposure of plants for 1 week to 180 mM NaCl in the nutrientsolution. This stimulation was more effective at the higherNaHCO₂ concentrations in the reaction medium. Salinity also enhanced CO₂ fixation in intact leaves mostlyat rate-limiting light intensities. A 60 per cent stimulationin CO₂ fixation rate was obtained by salinity under 450 µEm^–2 s^–1. At quantum flux densities of 150 µEm^–2 s^–1 (400–700 nm) this stimulation was280 per cent. Under high light intensities no stimulation bysalinity was found. In contrast, water stress achieved by directleaf desiccation or by polyethylene glycol inhibited enzymeactivity up to fourfold at –1.2 MPa. Beta vulgaris, sugar beet, ribulose-1, 5-bisphosphate carboxylase, salt stress, water stress, carbon dixoide fixation, salinity     

Nitrogen Fixation in the Phyllosphere of Tropical Plants: Occurrence of Phyllosphere Nitrogen-Fixing Micro-organisms in Eastern India and their Utility for the Growth and Nitrogen Nutrition of Host Plants

Of the 560 leaf samples belonging to 259 species of green plantsexamined more than 50 per cent of the Angiosperms and 25 percent of the Pteridophytes and Gymnosperms revealed the presenceof N₂-fixing micro-organisms in their phyllosphere. Plants particularlyremarkable in this respect are orchids and several other epiphytes,Scindapsus officinalis, Ficus and cucurbits. Most of the isolatesappear to be biotypes of Klebsiella pneumoniae. The more activestrains fixed more than 5 mg N g^–1 glucose utilized andreduced more than 100 nmol C₂H₂ mg^–1 cell d. w h^–1. The efficacy of the phyllosphere N₂-fixing isolates for N-nutritionof host plants was studied by spraying suspensions of the culturesgrown on N-free media on rice and wheat seedlings. In IR-26rice or Sonalika and Janak wheat grown on soil in wooden flatsor earthenware pots, 22 per cent of the 161 cultures studiedcaused increased height and about three-quarters of the culturesenhanced dry weight by more than 50 per cent; chlorophyll andN-contents were enhanced more than 50 per cent by about halfand two-thirds of the cultures respectively. In N-free sandculture 26 of the 50 promising strains doubled N-content, and30 doubled dry weight of the tested plants. In some cases dryweight, number of grains per panicle, and 1000 grain weightwere increased by 300, 70–83 and 126–158 per centrespectively; N-content of straw and seed was increased three-or fourfold. In several cases the beneficial effects were foundto match closely the performance of plants receiving ammoniumsulphate. Nitrogen-fixing micro-organisms, nitrogen nutrition, phyllosphere, rice, tropical plants, wheat     

Distribution of Nitrogen during Growth of Sunflower (Helianthus annuus L.)

The accumulation, distribution and redistribution of dry matterand nitrogen is described for Helianthus annuus L. cv. Hysun21 grown on 6 mM urea in glasshouse culture. Seed dry matterand nitrogen were transferred to seedlings with net efficienciesof 40 and 86 per cent respectively. At flowering, the stem hadmost of the plant's dry matter and the leaves most of its nitrogen.About 35 per cent of the plant's nitrogen accumulated afterthree-row anthesis. The amount of protein in vegetative parts,especially leaves, declined after flowering. Concentrationsof free amino compounds also decreased during growth. Matureseeds had 38 per cent of the total plant dry weight and 68 percent of the total nitrogen. Seeds acquired 33 per cent of theirdry matter and nitrogen from redistribution from above-groundplant parts. The stem was most important for storage of carbohydrate,leaves the most important for nitrogen. Over 50 per cent ofthe nitrogen in the stem and leaves was redistributed. Plantsthat received 6 mM nitrate accumulated more dry matter thanurea-grown plants. Seeds from nitrate-grown plants were heavier(58 mg) than those of urea-grown plants (46 mg), and their percentageoil was greater (50 and 41 respectively). The amount of nitrogenper seed was the same. Little or no urea was detected in xylem sap of plants suppliedwith 5 mM urea, but it was detected in sap of plants which received25 mM. Concentrations of urea and amino compounds in the sapdecreased up the stem. Plants supplied with nitrate had mostof the nitrogen in xylem sap as NO₂^–, suggesting littlenitrate reduction in roots. Plants grown on 6 mM nitrate andchanged to high levels of urea-nitrogen for 14 days still hadhigh levels of nitrate; little nitrate remained in plants receivinglow levels of urea. When urea is applied in irrigation waterto field-grown sunflower, the nitrogen is subsequently takenup as nitrate due to rapid nitrogen transformations in the soil. Helianthus annuus L., sunflower, urea, nitrate, nitrogen transport, xylem sap, nitrogen accumulation nitrogen distribution     

Effect of Nitrogen Supply on the Grass and Clover Components of Simulated Mixed Swards Grown under Favourable Environmental Conditions II. Nitrogen Fixation and Nitrate Uptake

Simulated mixed swards of Perennial Ryegrass (Lolium perenneL.) cv. S23 and White clover (Trifolium repens L.) cv. S100were grown from seed under a constant 20 °C day/15 °Cnight temperature regime and harvested at intervals over and88 d growht period. The swards received a nutrient solutiondaily, which was either High (220 mg l^–1) or Low (10 mgl^–1) in nitrate N. The nitrate was labelled with the ¹⁵Nisotope. An acetylene reduction assay was carried out on eachsward just prior to harvest. Rates of acetylene reduction agreed qualitatively with the ^l5Nanalyses but absolute values did not match (assuming a 4:1 C₂H₄:N₂ratio) and errors in the acetylene assay are discussed. In theLow-N swards clover relied almost entirely on symbioticallyfixed N₂, fixing more than ten times as much as the High-N cloverplants. In the Low-N treatment the grass was N-deficient despiteobtaining much more nitrate per unit root dry weight than clover.In the High-N swards, however, clover took up more nitrate perunit root weight than grass. The High-N clover plants also fixedsome N₂ and maintained a higher total-N content than grass throughoutthe period. There was no evidence of transfer of symbioticallyfixed N from the clover to the grass in either treatment. Trifolium repens, Lolium perenne, nitrate, nitrogen fixation, ¹⁵N, acetylene reduction     

Nitrogen Metabolism of Detached Kalanchoe Leaves in the Dark in Relation to Acidification, Deacidification, and Oxygen Uptake

Detached Kalancho leaves were placed in the dark and changesin the amounts of total soluble N, asparagine, glutamine, aminoN, ammonia, and titratable acid were followed during the periodsof acidification and deacidification. Abundant starch was presentthroughout the experiments. The amount of acid which accumulatedin the initial period was increased by placing the leaves in5 per cent. CO₂ and also by decreasing the temperature from25 to 10°. In neither case were the nitrogen fractions affectedin a similar way to the acid levels. No consistent relationshipcould be found between the amount or change in amount of acidand the amounts of the nitrogen fractions. Over prolonged periodsthe drifts in the amounts of the nitrogen fractions were verysimilar to those known in non-succulent leaves. It is concludedthat, despite the large amounts of free acid and starch present,the nitrogen metabolism of detached Kalancho leaves in the darkis similar to that of non-succulent leaves. A possible explanationis suggested for the incorporation of ¹⁴C from ¹⁴CO₂ into nitrogenoussubstances in Kalancho leaves during dark acidification. Changesin the amounts of glutamine, which seemed to be related to oxygenuptake rather than to acid fluctuations, were measured in comparableleaf samples showing different rates of oxygen uptake. The rateof oxygen uptake of leaves and leaf disks in the dark was controlledby placing them in an atmosphere of nitrogen, or 5, 20, or 100per cent, oxygen, by the addition of 10^–3 M. cyanide,or by varying the temperature. At the end of a given periodthe amount of glutamine was always greater in the samples whichhad shown the higher rate of oxygen uptake during this perod.This correlation was found irrespective of whether the amountof glutamine had increased or decreased. With the assumptionsthat glutamine production and consumption were proceeding simultaneouslyand that the rate of consumption was unaffected by the rateof oxygen uptake, the experimental values are consistent withthe hypothesis that the rate of glutamine production and therate of oxygen uptake were directly related. The possible significanceof these findings is discussed.     

Effect of Nitrogen Supply on the Grass and Clover Components of Simulated Mixed Swards Grown under Favourable Environmental Conditions I. Carbon Assimilation and Utilization

Simulated mixed swards of Perennial Ryegrass (Lolium perenneL.) cv. S23 and White clover (Trifolium repens L.) cv. S100were grown from seed under a constant 20 °C day/15 °Cnight temperature regime and their growth and carbon economyexamined. The swards received a nutrient solution daily, whichcontained either High (220 mg l^–1) or Low (10 mg l^–1)nitrate N. Rates of canopy photosynthesis and respiration, and final drymatter yields were similar in the two treatments although theproportions of grass and clover differed greatly. The Low-Nswards were made up largely of clover. The grass plants in theseswards had high root: shoot ratios and low relative photosyntheticrates – both signs of N deficiency – and were clearlyunable to compete with the vigorously growing Low-N clover plants.These had higher relative growth rates and dry matter yieldsthan their High-N counterparts. In the High-N swards clovercontributed around 50 per cent to the sward dry weight throughoutthe measurement period despite having a smaller proportion ofits dry weight in photosynthetic tissue (laminae) than grassover much of it. The latter was compensated for, initially bya higher specific leaf area than grass, and later by a higherphotosynthetic rate per unit leaf weight. The results are discussedin relation to observed declines in the clover content of swardsafter the addition of nitrogen fertilizer in the field. Trifolium repens, white clover, Lolium perenne, perennial ryegrass, nitrogen, photosynthesis, carbon balance     

Microclimate, Canopy Structure and Photosynthesis in Canopies of Three Contrasting Temperate Forage Grasses: I. Canopy Structure and Growth

The crop growth rates and structures of three temperate foragegrasses Lolium perenne cv. S24, L. perenne cv. Reveille andFestuca arundinacea cv. S170, were examined in the field duringa summer growth period. The growth rates of the varieties wereremarkably similar at 7 g DM m^–2 day^–1. The angularstructures of the varieties were different and they varied duringthe experiment. However, these differences did not seem to affectcrop growth rates. Nevertheless, a decrease in the efficiencyof light energy conversion of approximately 24 per cent wasobserved after a change to a more prostrate form of canopy dueto lodging. There appeared to be an inverse relationship betweenthe number of tillers per unit ground area and the weight ofan individual stem. There were large numbers of relatively lighttillers in S24 whereas S1 70 had fewer but heavier tillers.Furthermore, S24 had many small leaves per unit ground areacompared with SI70 which had fewer longer leaves per groundarea and a slower rate of leaf appearance. There were diurnalchanges in the rates of leaf extension for all the varieties.The mean daily extension rates declined as the canopies developed.