The Growth and Carbon Economy of Selection Lines of Lolium perenne cv. S23 with Differing Rates of Dark Respiration: 1. Grown as Simulated Swards During a Regrowth Period

Simulated swards of each of two selection lines of Lolium perennecv. S23 with ‘fast’ and ‘slow’ ratesof ‘mature tissue’ respiration were establishedin growth rooms at 20/15 °C day/night temperatures and studiedover four successive regrowth periods of 46, 30, 26 and 53 daysduration. The ‘slow’ line outyielded the ‘fast’,both in harvestable shoot (above a 5 cm cut) and in root andstubble. Its advantage increased over successive regrowth periodsto 23 per cent (total biomass). Gas analysis measurements onthe entire communities (including roots), during the final regrowthperiod, showed that the ‘slow’ line had a 22–34per cent lower rate of dark respiration per unit dry weight.This enabled it to maintain its greater mass of tissue for thesame cost in terms of CO₂ efflux per unit ground area. Halfthe extra dry weight produced by the ‘slow’ line,relative to the ‘fast’, could be attributed to itsmore economic use of carbon. The rest could be traced to a 25per cent greater tiller number which enabled the ‘slow’line to expand leaf area faster (though not at a greater rateper tiller), intercept more light and fix more carbon, earlyin the regrowth period. Lolium perenne L., ryegrass, respiration, maintenance respiration, tiller production, simulated swards, canopy photosynthesis, carbon economy     

Effects of Selection for Dark Respiration Rate on Enzyme Genotypes in Lolium perenne

Two lines of perennial ryegrass (Lolium perenne) that had previouslybeen selected for ‘fast’ and ‘slow’rates of leaf dark respiration were examined to determine whetherselection had differentiated allelic or genotypic frequenciesof five polymorphic enzyme loci, and determine whether respirationrates differed among genotypes. The lines were differentiatedfor genes coding for phosphoglucose isomerase (PGI) and UDP-glucosepyrophosphorylase (UDP). Respiration rates were heterogeneousamong UDP and 6-phosphogluconate dehydrogenase (6PGD) genotypes.Founder effects may have produced the genetic differentiationof lines at the PGI locus as we detected no differences amonggenotypes for respiration rates. The genetic differentiationamong lines for UDP was consistent with differences in darkrespiration among genotypes. Although we detected large differences(75%) in dark respiration rate among 6PGD genotypes in the fastrespiring line, selection did not differentiate the lines atthis locus. Lolium perenne L., perennial ryegrass, enzyme polymorphisms, dark respiration rate     

Leaf and Tiller Production of Prairie Grass (Bromus willdenowii Kunth) and Two Ryegrass (Lolium) Species

A detailed morphological study of three prairie grass cultivars(Bromus willdenowii Kunth) was conducted under ‘vegetative’and ‘reproductive’ growth conditions (short andlong photoperiods) and at different temperatures. Perennialryegrass (Lolium perenne L.) and Westerwolds ryegrass (Loliummuhiflorum Lam.) were compared during vegetative growth. Prairie grass had higher leaf appearance rates (leaves per tillerper day) and lower site filling (tillers per tiller per leafappearance interval) than the ryegrass species. Tillering rates(tillers per tiller per day) were also lower, except under vegetativeconditions at 4C. Low tiller number in prairie grass was notdue to lack of tiller sites but a result of poor filling ofthese sites. Lower site filling occurred because of increaseddelays in appearance of the youngest axillary tiller and lackof axillary tillers emerging from basal tiller buds. In prairiegrass, no tillers came from coleoptile buds while only occasionallydid prophyll buds develop tillers. Low tiller number in prairiegrass was compensated for by greater tiller weight. Prairiegrass had more live leaves per tiller, greater area per leafand a high leaf area per plant. Considerable variation between cultivars was found in prairiegrass. The cultivar ‘Bellegarde’ had high leaf appearance,large leaves and rapid reproductive development, but had lowlevels of site filling, tillering rate, final tiller numberand herbage quality during reproductive growth. ‘Primabel’tended to have the opposite levels for these parameters, while‘Grasslands Matua’ was intermediate and possiblyprovided the best balance of all plant parameters. prairie grass, Bromus willdenowii Kunth, perennial ryegrass, Lolium perenne L., Westerwolds ryegrass, Lolium multiflorum Lam., temperature, photoperiod, leaf appearance, leaf area, tillering, site filling, tillering sites, yield     

Effect of Cutting on Production and Tillering in Prairie Grass (Bromus willdenowii Kunth) Compared With Two Ryegrass (Lolium) species. 2. Reproductive Plants

Effects of time of initial cut and subsequent cutting interval(1, 2 or 4 weeks) were examined in a glasshouse during summer1988 for reproductive plants of three prairie grass cultivars(Bromus willdenowii Kunth), Westerwolds ryegrass (Lolium multiflorumLam.) and perennial ryegrass (Lolium perenne L.). Measurementswere made of tiller and leaf numbers, sites of tillering, reproductivedevelopment, and herbage quality and yields. Effect of timeof initial cutting on regrowth appeared to be independent ofstage of reproductive development and unrelated to any of themeasured plant parameters. Characteristics for each cultivaras identified during undisturbed growth prior to the initialcuts, were modified by the subsequent cutting frequencies. Perennial ryegrass had the highest yields under frequent cuttingwith high herbage quality. Westenwolds ryegrass and the prairiegrass cultivars ‘Grasslands Matua’ and ‘Primabel’had the highest yields with infrequent cutting, but lower herbagequality than in perennial ryegrass. At each cut, tiller deathin prairie grass was determined by the number of reproductivetillers, and in the ryegrasses also by the numbers of elongatedvegetative tillers. Recovery of tiller numbers was rapid andprimarily from inhibited tiller buds at the base of reproductivetillers. In prairie grass, tiller numbers were relatively unaffectedby cutting frequency, but at frequent cutting, many axillarytillers originated from vegetative tillers rather than frominhibited tiller buds at the base of reproductive tillers. Prairie grass, Bromus willdenowii Kunth, Westerwolds ryegrass, Lolium multiflorum Lam., perennial ryegrass, Lolium perenne L., first (initial) cut, cutting frequency, regrowth, leaf appearance, tillering, yield, nitrogen, water-soluble carbohydrates, herbage quality     

Response to Selection for Dark Respiration Rate of Mature Leaves in Lolium perenne and its Effects on Growth of Young Plants and Simulated Swards

Selections for slow and for fast rate of dark respiration ofmature leaves were made from within Lolium perenne cv. S23.Selected parents were pair-crossed to provide 15 F₁ familieswith slow respiration and 15 with fast. Dark respiration was inherited and families with contrastingrates were subjected to sequentially harvested growth analysesfrom the third leaf stage to that of 95 per cent light interceptionin a growth room. Seven periods of regrowth of simulated swardsof the families were then recorded. During development of theprimary canopy, growth of the selections did not differ untilthe final harvest interval. At this stage slow respiration familieshad faster (P < 0.05) net assimilation rate and greater plantdry weight (P < 0.05) and leaf area index (P < 0.05) thanthe fast respiration group. Relative growth rate followed thesame trend. In the swards after each regrowth dry matter yieldof the slow respiration group was greater than that of the fast. In another experiment, simulated swards of six slow respirationfamilies yielded more than swards of six fast respiration familiesover sequential regrowth periods in a glasshouse from May toNovember: S23 was intermediate. Differences were most duringAugust and September. Crop growth rate at each harvest correlated(P < 0.05 or P < 0.01) with previously determined leafrespiration at 25 °C. Leaf protein levels in August weaklycorrelated (r = +0.57, P < 0.05) with respiration rate perunit dry weight but there was a significant residual negativecorrelation (r = –0.67, P < 0.05) between the rateper unit protein and growth at that time. Results are discussedin relation to the concept of ‘maintenance-relàted’respiration. Lolium perenne L., perennial ryegrass, respiration, maintenance respiration, relative growth rate, leaf area ratio     

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     

Seasonal Changes in the Physiology of S24 Perennial Ryegrass (Lolium perenne L.). 1. Response of Leaf Extension to Temperature during the Transition from Vegetative to Reproductive Growth

The potential for leaf extension of plants of Lolium perennecv S24 growing in small artificial communities under naturalconditions was measured as the plants progressed from the vegetativeto the reproductive state In two consecutive years, 1975 and 1976, ‘simulated swards’were sown in autumn and overwintered in an open, unheated glasshouseIndividual swards from the batch sown in 1975 were brought into a growth room on two occasions in spring 1976 to measuretheir potential for leaf extension at a range of temperatures(5–20 °C) Individual swards from the batch sown inautumn 1976 were brought in to the growth room on 15 occasionsbetween November 1976 and May 1977 and their potential for leafextension was measured at a single temperature of 15 °CFrequent dissections were made in both years to describe changesin the developing apex. The potential for leaf extension at 15 °C decreased fromc 17 mm day^–1 in November to c 10 mm day^–1 in mid-winter.In January, the potential rapidly increased threefold to reach30mm day^–1 by mid February The increase began coincidentwith the earliest stages of floral initiation and was completedby the time of double-ridge formation Spring-grown vegetativeplants, however, showed potential rates of < 20 mm day^–1at 15 °C The results are discussed in relation to reproductive developmentand to changes in the carbohydrate strategy of the plants inearly spring Lolium perenne L perennial ryegrass, leaf extension, temperature response     

Assessment of the Response to Selection for Leaf Water Conductance in Lolium perenne

Measurements of leaf water conductance (LWC) were made in agrowth room on plants previously selected for either high orlow LWC as seedlings from two Lolium perenne L. populations.Differences in LWC were maintained between selection groupswhen measured as older plants, but there were considerable changesin LWC of individual selection groups on remeasurement. TheLWCs and the rankings of LWCs of individual genotypes were correlatedbetween experiments, thus demonstrating that selection of genotypeswas worthwhile. Thus the possibility of including LWC in selectionprogrammes was established. Investigations conducted to optimize sampling for LWC are described.It was concluded that little further reduction in the standarderror of LWC measurements was obtained by making more than eightmeasurements per genotype, and it was important to sample onat least four tillers. A new measure for the phenotypic stabilityof a selection group is proposed, named realized ‘selectivity’.The realized selectivity can be used to estimate the upper valuefor realized heritability and is about 0.5 for LWC in L. perenne.The values of phenotypic repeatability of LWC are compared withother similar measurements. Lolium perenne L, leaf water conductance, selection, components of variance, scale of measurement     

Variations in Tiller Dynamics and Morphology in Lolium multiflorum Lam.Vegetative and Reproductive Plants as affected by Differences in Red/Far-Red Irradiation

Lolium multiflorum Lam, plants were grown in a growth room undertwo light sources with red/far-red ratios of 1·62 and0·84 but similar photosynthetically active radiation.In both situations the capacity to produce new tillers and thelight available per tiller decreased with canopy growth. Tilleringwas further reduced by the low red/far-red ratio while lightinterception and plant dry weight were unaffected by this treatment.In both reproductive and vegetative plants under the lower red/far-redratio the time between leaf expansion and the appearance ofa tiller in its axil was increased and the proportion of ‘maturebuds’ that developed was reduced. Irradiation with lowred/far-red advanced the reproductive development and increasedthe number of fertile tillers per plant. It also caused longerleaf sheaths, blades and reproductive shoots. The results suggestthat as canopy density increases the lower light interceptionper tiller and the photomorphogenic effect of low red/far-redratios may reduce the capacity to produce new tillers. Lolium multiflorum, Lam., annual ryegrass, tillering, tiller growth, leaf growth, flowering, light quality.     

Seasonal Changes in the Physiology of S24 Perennial Ryegrass (Lolium perenne L.): 3. Partition of Assimilates between Root and Shoot during the Transition from Vegetative to Reproductive Growth

The partition of ¹⁴C labelled current assimilates to root insimulated swards of Lolium perenne cv. S24 was measured duringthe transition from vegetative growth in autumn to reproductivegrowth in spring under close to natural conditions of lightand temperature. Assimilate partitioning was also measured in‘established’ swards cut three times during thegrowing season and in vegetative ‘seedling’ swardsgrowing in autumn and in spring. All measurements were madewhen the swards had achieved more than 90 per cent light interception,and all swards were abundantly supplied with water and mineralnutrients. During autumn there was a gradual decrease in the proportionof assimilates partitioned to the roots in both the ‘established’and the ‘seedling’ swards. In the established swards,partition to roots was low over winter, increased during earlyspring, but decreased dramatically, later in the spring, whenstem elongation began. In contrast, in the unvernalized vegatativeseedling swards in spring, partition to roots remained high. The seasonal pattern of assimilate partitioning is consideredin relation to changes in the natural environment and the rateat which the crop fixed carbon in photosynthesis. A decreasein the proportion of assimilates partitioned to roots duringlate spring was significant in increasing the production ofshoot at that time but seasonal differences in partition contributedvery little to the marked differences in shoot growth betweenthe spring and autumn crop. Lolium perenne L., perennial ryegrass, partition of assimilates, flowering     

Rates of Leaf and Tiller Production in Young Spaced Perennial Ryegrass Plants in Relation to Soil Temperature and Solar Radiation

In an experiment designed to investigate the rate of leaf appearanceand tiller production in young spaced plants of three clonesof perennial ryegrass grown in the field, it was found thatthe rate of leaf appearance per tiller increased linearly withmean soil temperature up to approx. 14 °C. The rate of productionof tillers in relation to the rate of leaf appearance (sitefilling) appeared to be virtually independent of weather conditions.In plants which were adequately established, but still relativelysmall, site filling was equal to or exceeded the theoreticalsteady state; all the tiller buds which were being formed weredeveloping into visible tillers. Thus the relative rate of tillerproduction was controlled by the rate of leaf appearance. Inlarger plants site filling was less complete, and site redundancieswere probably caused by within-plant competition for light atthe tiller bases. Lolium perenne L., perennial ryegrass, tillering, leaf production, solar radiation, soil temperature     

Effect of Temperature and Nitrogen Supply on the Growth of Perennial Ryegrass and White Clover. 1. Carbon and Nitrogen Economies of Mixed Swards at Low Temperature

Simulated mixed swards of perennial ryegrass (Lolium perenneL. cv. S23) and white clover (Trifolium repens L. cv. S100)were grown from seed under a constant 10°C day/8°C nighttemperature regime and their growth, and carbon and nitrogeneconomies examined. The swards received a nutrient solution,every second day, which contained either high (220 µgg^–1) or low (40 µg g^–1) nitrate N. The High-N swards had rates of canopy photosynthesis and drymatter production (over the linear phase of growth) similarto those previously shown by mixed swards at high temperature.The Low-N swards grew more slowly; canopy photosynthesis, ata given LAI, was similar to that at High-N but lower LAI's weresustained. Clover increased its contribution to total carbonuptake and total dry weight throughout the period in the Low-Ntreatment and, despite the fact that grass took up most of theavailable nitrate, clover maintained a consistently higher Ncontent by virtue of N₂-fixation. At High-N, grass dominated throughout the measurement period.Earlier, when plants grew as spaced individuals, clover grewless well than grass, but once the canopy was closed it hada similar relative growth rate and thus maintained a steadyproportion of total sward dry weight. It is proposed that earlyin the development of the crop, leaf area production is thelimiting factor for growth, and that in this respect cloveris adversely affected by low temperature relative to grass.Later, as the LAI of the crop builds up, and the canopy becomesfully light intercepting, net canopy photosynthesis plays amore dominant role and here the higher photosynthetic rate perunit leaf area of the clover is crucial. Trifolium repens, white clover, Lolium perenne, perennial ryegrass, low temperature, nitrogen, photosynthesis     

Response of Populations of Lolium perenne cv. S 23 with Contrasting Rates of Dark Respiration to Nitrogen Supply and Defoliation Regime: 1. Grown as Monocultures

Simulated swards of two populations of perennial ryegrass cv.S 23 selected for contrasting rates of mature leaf tissue respirationwere grown in a glasshouse. From establishment, the swards weresubject to three levels of nitrogen supply (14.5, 32 and 173.5ppm N) and from the first harvest 7 weeks after sowing, to threecutting frequencies (at intervals of 1, 3, and 6 weeks). Throughout the 18-week experiment, the herbage yield and themean tiller weight of the slow-respiring population were 10%greater on average than those of the fast-respiring population.Increasing nitrogen supply (from 14.5 to 173.5 ppm N) enhancedthe yield advantage of the slower-respiring population—moreso under infrequent cutting (from nothing to 22%) than underfrequent cutting (from 6 to 13%). Both maximum absolute yields,and the greatest yield advantage of the slow-respiring populationover the fast, were achieved when high nitrogen was combinedwith infrequent defoliation. Lolium perenne, perennial ryegrass cv. S23, respiration, nitrogen defoliation, simulated sward, dry-matter production, monoculture, population     

Effect of Selection for Dark Respiration Rate of Mature Leaves on Crop Yields of Lolium perenne cv. S23

Growth and nutritive quality of two polycross progeny (GL 83and GL 112) from F₁ plants of Lolium perenne ex cv. S23 showingslow dark respiration rates of mature leaves were compared inthe field with S23 in 3 years. Plots were cut either nine (simulatedgrazing) or five (conservation) times annually. In the firsttrial, sown in 1977, dry matter yields from plots of GL 83 werecompared with S23 during 1978 and 1979. A similar trial sownin 1979 compared GL 83, S23 and GL 112 during 1980. The slowrespiration populations displayed consistently greater (6–13per cent) annual dry matter yields than S23, mostly during midto late summer. In the first trial the difference in yield wasgreater in 1979 than 1978 and more under conservation than simulatedgrazing. In the second trial GL 83 and GL 112 yielded 11 and13 per cent more dry matter than S23 under simulated grazingand 5–6 per cent under conservation. There were no consistentdifferences in dry matter digestibility, water soluble carbohydratesor protein between the populations. Cellulose content of GL83 and to a much lesser extent GL 112, tended to be greaterthan that of S23 from July to September. Analysis of one harvestfrom another trial revealed no significant difference in energyvalue, as measured by adiabatic bomb calorimetry, between GL83 and S23. Thus, it appears that the greater dry matter productionof the progeny with slower mature leaf dark respiration ratesreflected a true increase in total energy yield not apparentlyassociated with any agronomically undesirable characteristics.The general significance of the results for crop improvementare discussed. Lolium perenne L., perennial ryegrass, yield of dry matter, energy value, dry matter digestibility     

Primary and Secondary Induction Requirements for Flowering of Contrasting European Varieties of Lolium perenne

The flowering requirements of six European varieties of Loliumperenne L. were studied in controlled environments. In experimentson primary induction, flowering was recorded after transferto long days (LD) in a greenhouse at 12–24°C. In experimentson secondary induction, primary induction was first accomplishedat 6°C/10 h daylength for 12 weeks. When evaluated by the50% heading criterion, the requirement for duration of primaryinduction at 6°C/8 h daylength was <3 weeks in Mediterranean,5–6 weeks in Central European and 7–8 weeks in Scandinavianvarieties. While ‘Veyo’ (Italy) flowered profuselyregardless of temperature or daylength during primary induction,critical temperatures for primary induction in SD and LD were15 and 11°C in ‘Baca’ (Czech Republic) and 11and 7°C in ‘Falster’ (Denmark). The criticalphotoperiod for secondary induction at 15°C ranged from12 h in ‘Veyo’ and 14 h in ‘Baca’ to16.5 h in ‘Falster’ and 17.5 in ‘Kleppe’(Norway). The critical number of LD cycles varied correspondingly.While the Central and North European varieties required fewerLD cycles for 50% heading at 18 than at 12°C, ‘Veyo’showed the opposite response. It is concluded that the requirementsfor both primary and secondary induction of Lolium perenne increasewith increasing latitude of origin of the germplasm. In oneexperiment, 39–87% of the inflorescences came from tillersthat were not visible on transfer from primary to secondaryinduction, thus it is also concluded that there is no juvenilestage in tillers of Lolium perenne. Copyright 2000 Annals ofBotany Company Daylength, flowering, juvenility, perennial ryegrass (Lolium perenne L.), primary induction, secondary induction, temperature, varieties, vernalization     

Effect of Shading on the Regrowth of Lolium perenne Swards in the Field

Simulated swards of Lolium perenne cv. S.23 growing in the fieldwere shaded at various stages of regrowth during August andSeptember, when insolation was most likely to be limiting. Reductionof ambient illumination to 50 per cent for periods up to 24days had little effect on the current production of herbage(mainly leaf blade), which progressed at the expense of rootand stubble growth. After defoliation, however, previously shadedswards transferred to full light regrew relatively slowly. Leafarea and leaf extension were increased by shading, but leafappearance rates were only slightly affected. Shading decreasedtiller densi ties, and hence stubble yields, but did not affectmean tiller weights. The dangers of relating herbage yield tocurrent rates of insolation and photosynthesis, and the relevanceof these findings to other grass varieties are discussed.     

The Sensitivity of Net Photosynthesis in Several Plant Species to Short-term Fumigation with Sulphur Dioxide

The effects of exposure of up to 2 h with sulphur dioxide ona range of plant species was observed by measuring changes inthe rate of net photosynthesis under closely controlled environmentalconditions. Ryegrass, Lolium perenne ‘S23’ was thespecies most sensitive to SO₂; significant inhibition was detectedat 200 nl l^–1. Fumigations at 300 nl l^–1 also inhibitedphotosynthesis in field bean (Vicia faba cv. ‘Three FoldWhite’ and ‘Blaze’) and in barley (Hordeumvulgare cv. ‘Sonja’). No effect was detected inwheat (Triticum aestivum cv. ‘Virtue’) at concentrationsup to 600 nl l^–1 SO₂, or in oil-seed rape (Brassica napuscv. ‘Rafal’) except at 800 nl l^–1 SO₂). Recoverycommenced immediately after the fumigation was terminated andwas complete within 2 h when inhibition had not exceeded 20%during the SO₂ treatment. Key words: Sulphur dioxide, short-term fumigation, photosynthesis     

Effect of Elevated CO2 on the Photosynthesis, Respiration and Growth of Perennial Ryegrass

Single, seed-grown plants of ryegrass (Lolium perenne L. cv.Melle) were grown for 49 d from the early seedling stage ingrowth cabinets at a day/night temperature of 20/15 C, witha 12 h photoperiod, and a CO₂ concentration of either 340 or680µI 1^–1 CO₂. Following complete acclimation tothe environmental regimes, leaf and whole plant CO₂ effluxesand influxes were measured using infra-red gas analysis techniques.Elevated CO₂ increased rates of photosynthesis of young, fullyexpanded leaves by 35–46% and of whole plants by morethan 50%. For both leaves and whole plants acclimation to 680µI^–1 CO₂ reduced rates of photosynthesis in bothCO₂ regimes, compared with plants acclimated to 340µll^–1. There was no significant effect of CO₂ regime onrespiration rates of either leaves or whole plants, althoughleaves developed in elevated CO₂ exhibited generally lower ratesthan those developed in 340µI I^–1 CO₂. Initially the seedling plants in elevated CO₂ grew faster thantheir counterparts in 340µI I^–1 CO₂, but this effectquickly petered out and final plant weights differed by onlyc. 10%. Since the total area of expanded and unexpanded laminaewas unaffected by CO₂ regime, specific leaf area was persistently13–40% lower in elevated CO₂ while, similarly, root/shootratio was also reduced throughout the experiment. Elevated CO₂reduced tissue nitrogen contents of expanded leaves, but hadno effect on the nitrogen contents of unexpanded leaves, sheathsor roots. The lack of a pronounced effect of elevated CO₂ on plant growthwas primarily due to the fact that CO₂ concentration did notinfluence tiller (branch) numbers. In the absence of an effecton tiller numbers, any possible weight increment was restrictedto the c. 2.5 leaves of each tiller. The reason for the lackof an effect on tillering is not known. Key words: Lolium perenne, ryegrass, elevated CO₂, photosynthesis, respiration, growth, development     

The Growth and Development of Simulated Swards of Perennial Ryegrass: I. Leaf Growth and Dry Weight Change as Related to the Ceiling Yield of a Seedling Sward

The leaf growth, tiller production, light interception, anddry weight increase of a simulated sward of S24 perennial ryegrass(Lolium perenne) were followed during the development of thesward from a collection of two-leaved seedlings to a closedcanopy with an LAI of 23, of which 15 consisted of green leaflaminae. The dry weight of live shoots increased exponentiallyat first, but then entered a long linear phase of increase.This was equivalent to a crop growth rate of 200 Kg ha^–1day^–1 and a conversion efficiency of radiant energy (400–700nm) of 7.2 per cent. Towards the end of the growth period therate of increase of live shoots declined rapidly to zero anda ceiling yield was reached equivalent to 10 metric tons ha^–1.Leaf growth continued at a high rate, but was equalled by therate of leaf death, so that the weight of live leaf tissue remainedconstant. By this time the swards had achieved a stable tillerpopulation (about 1 cm^–1), each tiller bore a constantnumber of live leaves (about three), and the length of eachnewly expanded leaf equalled the length of the old leaf it replaced(about 70 cm). The swards were grown in Perlite so that in theabsence of soil fauna dead leaves accumulated at the base ofthe sward where, after 12 weeks, they accounted for 19 per centof the total weight of dry matter produced.     

Effect of Source of Nitrogen on the Growth of Fiskeby Soya Bean: the Carbon Economy of Whole Plants

Fiskeby V soya bean was grown from seed germination to seedmaturation with two contrasting patterns of nitrogen metabolism:either wholly dependent on dinitrogen fixation, or with an abundantsupply of nitrate nitrogen, but lacking root nodules. The carbonand nitrogen economies of the plants were assessed at frequentintervals by measurements of photosynthesis, shoot and rootrespiration, and organic and inorganic nitrogen contents. Plantsfixing atmospheric nitrogen assimilated only 25–30 percent as much nitrogen as equivalent plants given nitrate nitrogen:c. 40 per cent of the nitrogen of ‘nitrate’ plantswas assimilated after dinitrogen fixation had ceased in ‘nodulated’plants. The rates of photosynthesis and respiration of the shootsof soya bean were not markedly affected by source of nitrogen;in contrast, the roots of ‘nodulated’ plants respiredtwice as rapidly during intense dinitrogen fixation as thoseof ‘nitrate’ plants. The magnitude of this respiratoryburden was calculated to increase the daily whole-plant respiratory loss of assimilate by 10–15 per cent over thatof plants receiving abundant nitrate. It is concluded that ‘nodulated’plants grew more slowly than ‘nitrate’ plants inthese experiments for at least two reasons: firstly, the symbioticassociation fixed insufficient nitrogen for optimum growth and,secondly, the assimila tion of the nitrogen which was fixedin the root nodules was more energy-demanding in terms of assimilatethan that of plants which assimilated nitrogen by reducing nitratein their leaves.