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     

Effect of Temperature and Nitrogen Supply on the Growth of Perennial Ryegrass and White Clover. 2. A Comparison of Monocultures and Mixed Swards

White clover (Trifolium repens L.) and Perennial ryegrass (Loliumperenne L.) plants were grown, in Perlite, in simulated swardsas either monocultures or mixtures of equal plant numbers. Theywere supplied with a nutrient solution either high (220 µgg^–1) or low (40 µg g^–1) in ¹⁵N-labelled nitrateand grown to ceiling yield at either high (20°C day/15°Cnight) or low (10°C day/8°C night) temperature. Temperature had little effect on the maximum rates of grosscanopy photosynthesis which were similar in High-N grass andHigh-N and Low-N clover monocultures. However these maxima werereached more slowly in clover than grass, and more slowly atlow rather than high temperature. Nitrogen supply increasedphotosynthesis in grass but not in clover. Clover had higherN contents than grass in all four treatments, although in anygiven treatment its N content was lower, and contribution ofN₂-fixation relative to nitrate uptake higher, in mixture thanin monoculture. Conversely, grass had higher N contents in mixturethan monoculture, because more nitrate was available per plantand not because of transfer of biologically fixed N from clover. Under Low-N, clover outyielded grass in mixture, particularlyat high temperature. The grass plants in the Low-N mixtureshad higher N contents and higher SLA, LAR and shoot: root ratiosthan those in monoculture. It is proposed that competition forlight is the cause of the low relative yield and negative aggressivityof grass in these swards. Under High-N, grass outyielded cloverin monoculture and mixture, at both temperatures but particularlyat low temperature when grass had a high aggressivity. Nitrogenand yield component analyses shed no light on clover's apparentlylow competitive ability and evidence is drawn from the previouspaper to demonstrate that grass grew faster than clover onlyas spaced individuals during non-com petitive growth. The relativemerits of measures of competitive ability based on final harvestdata and physiological data taken over a growth period are discussed. Trifolium repens L., white clover, Lolium perenne, perennial ryegrass, competition, temperature, nitrogen     

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     

Radiation Interception, Partitioning and Use in Grass -Clover Mixtures

Mixed swards of perennial ryegrass /white clover were grownin competition under controlled environmental conditions, attwo temperatures and with different inorganic nitrogen supplies.The swards were studied after canopy closure, from 800 to 1200°C d cumulative temperatures. Clover contents did not varysignificantly during the period. A simulation model of lightinterception was used to calculate light partitioning coefficientsand radiation use efficiencies for both components of the mixturein this controlled environment experiment. Additionally, thissame radiative transfer model was applied to the field datafrom Woledge (1988) (Annals of Applied Biology112: 175 –186)and from Woledge, Davidson and Dennis (1992) (Grass and ForageScience47: 230 –238). The measured and simulated valuesof light transmission, at different depths in the mixed canopy,were highly correlated (P<0.001) with more than 80% of thetotal variance explained. The daily average of photosyntheticallyactive radiation (PAR) interception in a natural environmentwas estimated from simulations, for the field and controlledenvironment data. Under these conditions, white clover capturedsignificantly more light per unit leaf area than perennial ryegrassat low, but not at high, nitrogen supply. In the controlled environment experiment, the radiation useefficiency of the legume was lower than that of its companiongrass. For both species, radiation use efficiency was negativelycorrelated with the mean irradiance of the leaf. The role ofa compensation between light interception and light use forstabilizing the botanical composition of dense grass –cloverswards is discussed. Light interception; radiation transfer model; growth analysis; radiation use efficiency; white clover; perennial ryegrass; Trifolium repensL.; Lolium perenneL.; grassland     

Photosynthesis of White Clover Leaves as Influenced by Canopy Position, Leaf Age, and Temperature

Microswards of white clover (Trifolium repens L.) were grownin controlled environments at 10/7, 18/13 and 26/21 °C day/nighttemperatures. The vertical distribution of leaves of differentages and their rates of ¹⁴CO₂-uptake in situ were studied. Extending petioles carried the laminae of young leaves throughthe existing foliage. A final position was reached within 1/4to 1/3 of the time between unfolding and death. Newly unfoldedleaves had higher rates of ¹⁴CO₂-uptake per leaf area than olderones at the same height in the canopy. At higher temperatures,the decrease with age was faster. However, the light-photosynthesisresponse of leaves which were removed from different heightsin the canopy varied much less with leaf age than did the ratesof ¹⁴CO₂-uptake in situ. The comparison of the rates of ¹⁴CO₂-uptake in situ with thelight-photosynthesis response curves suggests that young leavesreceive more light than older ones at the same height in thecanopy. This would imply that young white clover leaves havethe ability to reach canopy positions having a favourable lightenvironment. This ability may improve the chances of survivalof white clover in competition with other species. Trifolium repens L., white clover, photosynthesis, canopy, leaf age, ¹⁴CO₂-uptake, ecotypes, temperature     

The Effect of Temperature on Photosynthesis of Ryegrass and White Clover Leaves

The rate of photosynthesis of leaves of perennial ryegrass (Loliumperenne L.) and white clover (Trifollum pratense L.) grown atdifferent temperatures was measured at a range of temperatures.There was a small effect of the temperature at which a leafhad grown on its photosynthetic rate, but a large effect ofmeasurement temperature, especially in bright light, where photosyntheticrates at 15°C were about twice those at 5°C. It appearsthat temperature could affect sward photosynthesis in the field.Ryegrass and clover had similar photosynthetic rates which respondedsimilarly to temperature. Lolium perenne L., ryegrass, Trifolium pratense L., white clover, photosynthesis, temperature, irradiance     

The Effect of Nitrogenous Fertilizer on the Photosynthesis and Growth of White Clover/Perennial Ryegrass Swards

The application of nitrogenous fertilizer in March to a whiteclover (cv. Blanca) and perennial ryegrass (cv. S23) sward resultedin a rapid suppression of the clover, relative to clover ina treatment given no added nitrogen. Thereafter, the cloverin both treatments grew more rapidly than the grass and itsproportion of the total leaf area in the mixture increased,as the leaf area index rose to 8. After a second applicationof N in early July, clover was not suppressed to the same extentas in the first growth period. Overall, the photosynthetic capacities of newly expanded cloverlaminae were similar in the two treatments. Clover laminae hadhigher photosynthetic capacities than grass, even in the grass-dominant+ N treatment. Lamina area, petiole length, and the number of live leaves perstolon were similar in the two treatments, indicating that thedifferences in total leaf area were due to the presence of fewerstolon growing points in the + N treatment. Trifolium repens L., white clover, Lolium perenne L., perennial ryegrass, nitrogen, leaf area index, photosynthesis, growth     

Photosynthesis by White Clover Leaves in Mixed Clover/Ryegrass Swards

Measurements of rates of net photosynthesis were made on singleBlanca white clover leaves on plants taken from a field-grown,mixed clover/perennial ryegrass sward during two regrowth periods. Net photosynthesis fell by 20 per cent in the first measurementperiod as leaf area index increased and the grass componentof the crop flowered, but did not change significantly in thesecond measurement period during which the grass remained vegetative. Leaves which had been artificially protected from shading inthe sward did not have significantly different photosyntheticcapacities from leaves in the undisturbed sward, even in thefirst measurement period. As leaf area index and sward height increased, successive cloverpetioles were longer, keeping the newly expanded leaves nearthe top of the sward where they received full light. It is suggestedthat it is this which allows successive clover leaves, unlikethose of vegetative grasses, to attain a high photosyntheticcapacity throughout a growth period. Trifolium repens, Lolium perenne, Photosynthetic capacity, shading, growth     

The Use of a Closed System Flow-Through Enclosure Apparatus for Studying the Effects of Partial Pressure of Dinitrogen in the Atmosphere on Growth of Trifolium repens L. and Lolium perenne L.

A closed system flow-through enclosure apparatus was constructedand used to enclose mixtures ofLolium perenne L. (perennialryegrass cv. Trani) and nodulatedTrifolium repens L. (whiteclover cv. Blanca) growing in soil in pots. There were no significantdifferences between the shoot growth, in terms of dry matteraccumulation and nitrogen content, of mixtures in the systemcompared to that of mixtures grown in a standard growth cabinet.This demonstrated that stable ambient conditions could be achievedby the closed system and its control circuits and that therewere no apparent side-effects of the recirculatory gases.Reducingthe partial pressure of dinitrogen in the atmosphere affectedwhite clover, but not perennial ryegrass. A fairly rapid effectwas observed 4 h after reduction in dinitrogen partial pressureas some of the clover leaves folded downwards along the petiole.These same effects were observed at two different partial pressuresof dinitrogen (22% and 39%) and with two different replacementgases (argon and helium). In the longer term (11 to 16 d) drymatter accumulation and nitrogen content of the clover shootswere significantly reduced. These effects of reduced partialpressure of dinitrogen were observed in both nodulated (NOD⁺)and nitrate-dependent (NOD^–) clover. Possible reasonsfor these effects were discussed with particular reference toimpurities in the gases used, stomatal responses and plant waterrelations. It was concluded that the closed system flow-throughapparatus provides a useful tool for studying whole plant-soilsystems and, in particular, the cycling of nitrogen. However,the use of a replacement gas to reduce the cost of labelleddinitrogen was obviously not a viable proposition. Key words: Dinitrogen, flow-through system, Lolium perenne, partial pressure, Trifolium repens     

Effects of Temperature on Growth and Nitrogen Fixation by Swards of Subterranean Clover

Subterranean clover (Trifolium subterraneum L.) cv. ‘Woogenellup’ swards were grown at 10, 15, 20 and 25 Cwith a 12 h photoperiod of 500 or 1000 µmol m^–2s^–1 [low and high photosynthetic photon flux density (PPFD)].Nitrogen-fixing swards received nutrient solution lacking combinednitrogen while control swards received a complete nutrient solution.Growth was measured by infra-red analysis of carbon dioxideexchange and by accumulation of dry matter. Swards were harvestedat intervals between 95 and 570 g d. wt m^–2 for estimationof nitrogenase activity by acetylene reduction and hydrogenevolution assays. Nitrogen fixation was also measured by increasein organic nitrogen. The growth rate was highest at 10 C at low PPFD, and at 10–15C at high PPFD. Nitrogen-fixing swards grew slower than thosereceiving combined nitrogen. Nitrogen fixation measured by increasein organic nitrogen responded similarly to the growth rate,as did acetylene reduction between 10 and 20 C. At 25 C therelationship between acetylene reduction and nitrogen fixationwas distrupted. The difference between the rates of acetylenereduction and hydrogen evolution, theoretically proportionalto nitrogen fixation, was not a reliable indicator of nitrogenfixation because hydrogen uptake developed. Trifolium subterraneum L, subterranean clover, growth, nitrogen fixation, temperature, acetylene reduction     

The Growth and Photosynthesis of Seedling Plants of White Clover at Low Temperature

The growth of white clover (Trifolium repens L.) in conditionstypical of April in Southern England (8 °C day/4 °Cnight, 12 h photoperiod of 90 J m^–2 s^–1 visibleradiation) was extremely slow, whether the plants were dependentfor nitrogen on fixation by their root nodules or were suppliedwith abundant nitrate; although growth was slower in the nodulatedplants. The reasons for slow growth were a large root: shootratio and a small leaf area, particularly in the nodulated plants,and a low photosynthetic rate in all plants. The probable effectsof these characteristics on the growth of white clover withgrasses in mixed pastures are discussed. Trifolium repens L, white clover, low temperature, leaf area, photosynthetic rate, nitrogen supply, growth     

Effect of Previous Defoliation Regime and Mineral Nitrogen on Regrowth in White Clover Swards: Photosynthesis, Respiration, Nitrogenase Activity and Growth

Small swards of white clover (Trifolium repens L.) cv. Haifawere grown in solution culture in a controlled environment at24 °C day/18 °C night and receiving 500 µE m^-2S^–1 PAR during a 14-h photoperiod. The swards were cuteither frequently (10-d regrowth periods) or infrequently (40-dregrowth) over 40 d before being cut to 2 cm in height. Halfof the swards received high levels of nitrate (2–6 mMN in solution every 2 d) after defoliation while the othersreceived none. Changes in d. wt, leaf area and growing pointnumbers were recorded over the following 10 d. CO₂ exchangewas measured independently on shoots and roots and nitrogenase-linkedrespiration was estimated by measuring nodulated root respirationat 21% and 3% oxygen in the root atmosphere. There was a general pattern in all treatments consisting ofan initial d. wt loss from roots and stubble and reallocationto new leaves, followed by a period of total d. wt gain andrecovery, to a greater or lesser extent, of weight in non-photosyntheticparts. Frequently cut swards had a smaller proportion of theirshoot d. wt. removed by cutting and had a greater shoot d. wt,growing point number and leaf area at the start of the regrowthperiod. As a result of these differences, and also because ofdifferences in relative growth rates, frequently cut swardsmade more regrowth than infrequently cut. Initial photosyntheticrates were higher in frequently cut swards, although the laminaarea index was very low, and it was concluded that stolons andcut petioles made a significant contribution to carbon uptakeduring the first few d. Infrequently cut swards continued toallocate carbon to new and thinner leaves at the expense ofroots and stubble for longer than frequently cut swards andas a result achieved a similar lamina area index after 10 d. Nitrogenase-linked respiration was low in all treatments immediatelyafter cutting: frequently cut swards receiving no nitrate maintainedhigh nitrogenase activity, whereas recovery took at least 5d in infrequently cut swards. Swards which received nitrateafter cutting maintained only low rates of nitrogenase-linkedrespiration and their total nodulated root respiration overthe period was lower than those receiving no nitrogen: greaterregrowth in nitrate fed swards over the 10 d compared to N₂-fixingswards was in proportion to this lower respiratory burden. White clover (Trifolium repens L.), defoliation, regrowth, nitrogen, photosynthesis, respiration, nitrogenase-activity     

Influence of Elevated CO2 and Temperature on the Photosynthesis and Respiration of White Clover Dependent on N2 Fixation

Single clonal plants of white clover (Trifolium repens L) grownfrom explants in a Perlite rooting medium, and dependent fornitrogen on N₂ fixation in root nodules, were grown for severalweeks in controlled environments which provided two regimesof CO₂, and temperature 23/18 °C day/night temperaturesat 680 µmol mol^–1 CO₂, (C680), and 20/15 °Cday/night temperatures at 340 µmol mol^–1 CO₂ (C340)After 3–4 weeks of growth, when the plants were acclimatedto the environmental regimes, leaf and whole-plant photosynthesisand respiration were measured using conventional infra-red gasanalysis techniques Elevated CO₂ and temperature increased ratesof photosynthesis of young, fully expanded leaves at the growthirradiance by 17–29%, despite decreased stomatal conductancesand transpiration rates Water use efficiency (mol CO₂ mol H₂O^–1)was also significantly increased Plants acclimated to elevatedCO₂, and temperature exhibited rates of leaf photosynthesisvery similar to those of C340 leaves ‘instantaneously’exposed to the C680 regime However, leaves developed in theC680 regime photosynthesised less rapidly than C340 leaves whenboth were exposed to a normal CO₂, and temperature environmentIn measurements where irradiance was varied, the enhancementof photosynthesis in elevated CO₂ at 23 °C increased graduallyfrom approx 10 % at 100 µmol m^–1 s^–1 to >27 % at 1170 µmol m^–2 s^–1 In parallel, wateruse efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 In parallel,water use efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 to approx100 % at the highest irradiance Elevated CO₂, and temperatureincreased whole-plant photosynthesis by > 40 %, when expressedin terms of shoot surface area or shoot weight No effects ofelevated CO₂ and temperature on rate of tissue respiration,either during growth or measurement, were established for singleleaves or for whole plants Dependence on N₂, fixation in rootnodules appeared to have no detrimental effect on photosyntheticperformance in elevated CO₂, and temperature Trifolium repens, white clover, photosynthesis, respiration, elevated CO₂, elevated temperature, water use efficiency, N₂ fixation     

The Growth and Development of Simulated Swards of Perennial Ryegrass: II. Carbon Assimilation and Respiration in a Seedling Sward

The rates of net photosynthesis (P_n,c) in the light (85 W m^–2visible), and respiration in the dark, of a simulated swardof S24 ryegrass were measured for 12 weeks during its developmentfrom a collection of two-leaved seedlings to a closed canopywith an LAI of 23 (15 of green leaf laminae). By the sixth week light interception was complete (LAI = 10.6)and P_n,c had risen to 24 mg CO₂ dm^–2 h^–1, similarto rates recorded in the field. Photosynthetic functions (lightresponse curves) showed that the swards remained unsaturatedup to energy receipts of almost 400 W m^–2, whereas singleleaves were light saturated at about 130 W m^–2. Earlyin the development of the sward LAI had a greater effect onP_n,c than radiation receipt, later the reverse was true. Thegrowth habit of the sward ranged from moderately erect (an Svalue of 0.72) to moderately prostrate (‘S’ = 0.37),while the ability of the two youngest fully expanded leaveson a tiller to make use of light in photosynthesis declinedas the sward increased in density from values of A max of 20to 5 mg CO₂ dm^–2 h^–1. By varying the values of Sand A max fed into a model of canopy photosynthesis, withinthe above limits, it was demonstrated that, in practice, A maxis a greater determinant of canopy photosynthesis than S, exceptat low LAI where a prostrate sward has a marked advantage overan erect one. The rate of dark respiration rose as the swards increased inweight, although not in proportion to it, until the ninth weekwhen a ceiling yield of live plant tissue was reached. Respiratorylosses from the sward came almost equally from a component associatedwith maintenance (R_m) and one associated with growth (R_g). Therate of Rm was estimated to be about 0.014 g day^–1 pergram of plant tissue, and that of Ra about 0.25 g per gram ofnew tissue produced—both close to theoretical values.The measured dry matter production curve of the swards was comparedwith that estimated from the gas analysis data. Similarly therates of gross photosynthesis estimated from the gas analysisdata were compared with the predictions of the mathematicalmodel. In both cases the fit was reasonably good. A balancesheet was drawn up; of every 100 units of carbon fixed, 45 werelost in respiration and 16 as dead leaf, 5 ended up in the rootand 6 in the stubble; only 28 remained as harvestable live leaftissue.     

Environment-induced Modifications to Root Longevity in Lolium perenne and Trifolium repens

Understanding how environmental factors affect the longevityof roots is essential if root mortality linked nutrient cyclingprocesses within ecosystems are to be understood, and the impactof natural and anthropogenic climate change properly evaluated.In this study the longevity of roots at two geographically andclimatically distinct sites were compared to identify the scaleof change that can occur due to environmental differences. Minirhizotrontubes were inserted into swards sown with the same variety ofLolium perenne and Trifolium repens at sites in the UK and Italy.Roots were viewed using a video camera and digitized imagesof roots generated at intervals. From these images the lifehistory of individual roots was determined and compared. Therewere few differences in patterns of longevity between differentspecies at the same site. Major differences, however, were observedbetween roots of the same species at different sites. For example,73% of Trifolium repens roots failed to survive for 21 d inItaly compared to only 29% at the UK site. Similarly, over 84%of Lolium perenne roots failed to survive for more than 21 din Italy compared to 38% in the UK. These data suggest thatenvironmental factors can have a major impact on root longevity.Copyright 2000 Annals of Botany Company Climate change, Lolium perenne L., minirhizotron, perennial ryegrass, root longevity, Trifolium repens L., white clover     

Nitrogen Deficiency in Small Closed Communities of S24 Ryegrass. I. Photosynthesis, Respiration, Dry Matter Production and Partition

Small communities of S24 ryegrass were grown under supplementarylights in a glasshouse at 20°C, and abundantly suppliedwith a complete nutrient solution containing 300 p.p.m. of nitrogen,until they had a leaf area index of 5 and fully interceptedthe light. Half were then given a solution containing only 3p.p.m. of nitrogen (LN) while the rest were kept at 300 p.p.m.(HN). The LN plants had a rate of single leaf photosynthesis lowerthan that of the HN plants at all but the lowest light intensities(33 per cent lower at the saturating irradiance of 170 W m^–2).Similarly, the LN communities had rates of canopy gross photosynthesis(P_sc) markedly lower than those of the HN communities. A comparisonof the observed rates of P_sc with those predicted by a mathematicalmodel of canopy photosynthesis indicated that it was the effectof nitrogen on single leaf photosynthesis, rather than differencesbetween the communities in leaf area, which led to the observeddifferences in P_sc. The superiority of the HN communities in terms of P_sc was partlyoffset by a higher rate of respiration so that they only exceededthe LN communities in terms of canopy net photosynthesis atirradiances in excess of 180 W m_–2, and produced only15 per cent more total dry matter. Nevertheless, the HN plantsdirected less of that dry matter into root and more into topsso that they came to possess twice the weight of live laminae,and the HN communities twice the leaf area, of their nitrogendeficient counterparts. Lolium perenne, S24 ryegrass, photosynthesis, respiration, dry matter production and partition, nitrogen dekieacy     

Sitona weevils (Coleoptera: Curculionidae) as agents for rapid transfer of nitrogen from white clover (Trifolium repens L.) to perennial ryegrass (Lolium perenne L.)

The effects of root feeding by larvae of Sitona hispidulus (F.) (a common weevil pest of white clover) on the rate of transfer of nitrogen between plants of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) were investigated using a nutrient slant board technique. Clover plants, labelled with ¹⁵N were grown adjacent to ryegrass plants and were either infested with Sitona larvae or not infested. Ryegrass plants associated with the infested clover plants had a significantly higher dry matter yield and nitrogen content (75% and 74% respectively) than the uninvested plants, after 33 days exposure to insect herbivory. It was concluded that root feeding insects could play an important role in the cycling of nitrogen in grass/clover swards.     

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     

Long- and short-term effects of crop residues on aluminum toxicity,phosphorus availability and growth of pearl millet in an acid sandy soil

Pasture swards containing perennial ryegrass (Lolium perenne L.) alone or with one of five different white clover (Trifolium repens L.) cultivars were examined for production and transfer of fixed nitrogen (N) to grass under dairy cow grazing. Grass-only swards produced 21% less than mixed clover-grass swards during the second year after sowing. Production from grass-only plots under a mowing and clipping removal regime was 44% less than from grass-only plots under grazing. Much of this difference could be attributed to N transfer. In swards without clover, the ryegrass component also decreased in favour of other grasses.The average amount of fixed N in herbage from all clover cultivars was 269 kg N ha^–1 yr^–1. Above-ground transfer of fixed N to grasses (via cow excreta) was estimated at 60 kg N ha^–1 yr^–1. Below-ground transfer of fixed N to grasses was estimated at 70 kg N ha^–1 yr^–1 by ¹⁵N dilution and was similar for all clover cultivars. Thus, about 50% of grass N was met by transfer of fixed N from white clover during the measurement year. Short-term measurements using a ¹⁵N foliar-labelling method indicated that below-ground N transfer was largest during dry summer conditions.     

Effects of CO2 enrichment and intraspecific competition on biomass partitioning, nitrogen content and microbial biomass carbon in soil of perennial ryegrass and white clover

Seedlings of perennial ryegrass (Lolium perenne L. cv. Parcour)and white clover (Trifolium repens L. cv. Karina) grown at fivedifferent plant densities were exposed to ambient (390 ppm)and elevated (690 ppm) CO₂ concentrations. After 43 d the effectsof CO₂ enrichment and plant density on growth of shoot and root,nitrogen concentration of tissue, and microbial biomass carbon(C_mic) in soil were determined. CO₂ enrichment of Lolium perenneincreased shoot growth on average by 17% independent of plantdensity, while effects on root biomass ranged between -4% and+ 107% due to an interaction with plant density. Since tilernumber per plant was unaffected by elevated CO₂, the small responseof shoot growth to CO₂ enrichment was atributed to low sinkstrength. A significant correlation between nitrogen concentrationof total plant biomass and root fraction of total plant drymatter, which was not changed by CO₂ enrichment, indicates thatnitrogen status of the plant controls biomass partitioning andthe effect of CO₂ enrichment on root growth. Effects of elevatedCO₂ and plant density on shoot and root growth of Trifoliumrepens were not significantly interacting and mean CO₂-relatedincrease amounted to 29% and 66%, respectively. However, growthenhancement due to elevated CO₂ was strongest when leaf areaindex was lowest. Total amounts of nitrogen in shoots and rootswere bigger at 690 ppm than at 390 ppm CO₂. There was a significantincrease in C_mic in experiments with both species whereas plantdensity had no substantial effect. Key words: CO₂ enrichment, intraspecific competition, biomass partitioning, Lolium perenne, Trifolium repens, grassland