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 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     

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     

Direct measurement of nitrogen fixation by Trifolium repens L. and Alnus glutinosa L. using 15N2

A closed-system flow-through enclosure apparatus was used tomeasure symbiotic nitrogen fixation directly. A legume-basedsystem comprising 6-week-old Trifolium repens L. (white clovercv. Blanca) growing with Lolium perenne L. (perennial ryegrasscv. Trani) in an agricultural soil was incubated for 19 d ina ¹⁵N-enriched atmosphere (mean value 3.663 atom%). An actinorhizal-basedsystem comprising 1 -year-old Alnus glutinosa L. (alder) saplingsgrowing with Festuca rubra L. (red fescue) in open-cast coalspoil was incubated for 21 d in a ¹⁵N-enriched atmosphere (meanvalue 3.265 atom%). Indirect estimates of N2 fixation were carriedout concurrently using N difference and ¹⁵N isotope dilutiontechniques. The theory underlying the three techniques and modificationswhich were adopted for comparative purposes are discussed. Thedirect measurements of N2 fixation were then compared with theindirect estimates using P_inc, the proportion of the N incrementduring the measurement period that was derived from fixation.The simple N difference method gave similar values for Pinc(0.94 and 0.97) as those derived from more complicated isotopemethodologies, both indirect (0.91) and direct (0.90). Valuesfor alder were far more variable, ranging from 0.16 to 0.92;this was due largely to variability within the trees and a verysmall N increment during the measurement period. Key words: N₂ fixation, ¹⁵N₂, white clover, alder, enclosure apparatus     

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     

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     

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     

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     

Oxygen Supply Limits Nitrogenase Activity of Clover Nodules After Defoliation

The aim of this study was to test the effect of oxygen partialpressure as a possible limiting factor of nitrogen fixationfollowing defoliation. The response of nitrogenase activity(C₂H₂-reduction) of defoliated and undefoliated white and redclover plants (Trifolium repens L. and Trifolium pratense L.)to either 19 kPa oxygen or 55 kPa oxygen was investigated. Priorto defoliation, white clover plants were grown for five weeksin a growth chamber, and red clover plants for 7 or 11 weeksin a glasshouse. The results included measurements of ¹⁶N₂-uptake. Increasing oxygen partial pressure from 19 to 55 kPa severelyrestricted nitrogenase activity of undefoliated white cloverplants; however, 2 h after complete defoliation, the same treatmentcaused a significant increase. A fivefold increase in nitrogenaseactivity upon exposure to the elevated oxygen partial pressurewas found at the end of a 24 h period. This beneficial effectdecreased gradually from 1 to 5 d after defoliation. The responseof recently defoliated red clover plants to 55 kPa oxygen partialpressure was similar to that of white clover, independentlyof plant age. The gradual recovery of nitrogenase activity duringthree weeks of regrowth was associated with a simultaneous changein the response to increased oxygen partial pressure, leadingagain to the response of undefoliated plants. These data suggested that lack of oxygen at the site of nitrogenfixation, resulting from a dramatic increase in oxygen-diffusionresistance, is the main factor limiting nitrogenase activityfollowing defoliation. Trifolium repens L., Trifolium pratense L., white clover, red clover, defoliation, regrowth, nodules, nitrogen fixation, nitrogenase activity, oxygen limitation     

Effects of soil mechanical impedance on root and shoot growth of Lolium perenne L., Agrostis capillaris and Trifolium repens L.

A method was used for applying a uniform mechanical impedanceto plant roots using sand packed at different bulk densitieswith depth within each growth cylinder. In a growth experiment,replicate cylinders were packed with sand to give the followingmechanical impedances: 0.25 MPa penetration resistance (negligibleimpedance), 1.40 MPa (moderate impedance) and 2.30 MPa (severeimpedance). Seedlings of Lolium perenne L., Trifolium repensL. and Agrostis capillaris were grown for 23 d in each impedancetreatment and effects on both roots and shoots were studied.Severe mechanical impedance affected both root and shoot growthrates for all three species resulting in smaller leaves andshorter roots. For the grasses, the root-to-shoot ratio at harvestwas the same for all the treatments, but a delay in the initiationof both shoots and roots was observed in the severe mechanicalimpedance treatment. The results are discussed in relation tothe possibility that roots penetrating the impeding treatmentsmay have caused signalling that kept shoot and root growth insynchrony. Differences observed in the response to mechanicalimpedance of T. repens compared to the other two species maybe a function of differences in the physiology between mono-and dicotyledonous species. Key words: Mechanical impedance, Lolium perenne, Trifolium repens, roots, shoots     

Effects of Soil Nutrients and Moisture on Root/Shoot Ratios in Lolium perenne L. and Trifolium repens L

The dry weights of foliage and roots (excluding stembases) werecompared in a 3³x2² factorial experiment with Lolium perenneL. and Trifolium repens L. grown in pots with three nitrogen(0–300 ppm N), phosphorus (0–300 ppm P), and soilmoisture (pF 3.2, pF 2.4, and waterlogged) treatments, eachduplicated. Root/shoot ratios were influenced significantly (P < 0.001)by N, P, and W (soil moisture), but these main effects werequalified by highly significant first-order interactions andsecond-order interactions which included N and P. The lowestR/S ratios occurred where N, P, and W were all non-limiting,and the highest where there was an imbalance of these factors. The explanation suggested is that a decrease in the availabilityof N, P, or W causes an increase in the relative weight of roots,but in the presence of N, P, or W deficiency an increase inany one of these factors causes a further increase in the R/Sratio by increasing the relative efficiency of the foliage.     

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     

Uptake of 32P labelled phosphate by clover and ryegrass growing in mixed swards with different nitrogen treatments

Lolium perenne cv. S.23 and Trifolium repens cv. Olwen were sown together in 1975, fertilised then and in 1976, and finally given nitrogen doses of either 50, 100, 200 or 400 kg/ha (as N) combined with 0.64 times as much potassium (as K₂O) in 1977. As nitrogen increased, grass yield increased, but clover decreased. Grass roots absorbed more ³²P than clover roots, and nitrogen increased this difference. Grass roots bore more mycorrhiza than clover roots. The difference in ³²P uptake between grass and clover was less in June and July than in August. Clover roots took up most phosphate from the upper layers of soil, while grass absorbed ³²P rather uniformly down to 25 cm. It was concluded that optimum fertiliser placement for clover growth was a surface dressing in the early season.     

The effects of mixing Italian ryegrass (Lolium multiflorum) with perennial ryegrass (L. perenne) or red clover (Trifolium pratense) on the incidence of viruses

Mixing the ryegrass mosaic virus (RMV) resistant perennial ryegrass (Lolium perenne) cv. Endura with the susceptible Italian ryegrass (L. multiflorum) cv. RvP decreased infection of RvP wth RMV from 37% when grown alone to 22% when mixed. However, Endura yielded less than RvP and there was no yield benefit from mixing the two cultivars. Mixing red clover (Trifolium pratense) cv. Hungaropoly with RvP had no detectable effect on RMV incidence in RvP but did decrease the incidence of red clover necrotic mosaic virus in Hungaropoly from 9% to 1% and of white clover mosaic virus from 53-5% to 41%. The yield of the mixture was equal to that of RvP grown alone but given nitrogen fertiliser. The numbers of eriophyid mites, including Abacarus hystrix the vector of RMV, on ryegrass leaves were similar in pure and mixed swards. It is concluded that with herbage crops, the common practice of sowing mixtures of species may help control virus diseases.     

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.     

Distinct Influence of Root and Shoot Temperature on Nitrogen Fixation by White Clover

White clover (Trifolium repens L.) was grown in controlled environmentsto determine the distinct effects of root and shoot temperatureon the accumulation of total and fixed (¹⁵ N dilution) nitrogenat two levels of nitrate (10 and 75 mM). Nitrogen fixation(BNF) showed a positive response to higher shoot temperature(23 vs. 13 C day temperature), irrespective of whether or notroot temperature was increased in parallel. Low root temperature(5 C) caused a marked reduction in the accumulation of totalnitrogen at both nitrate levels, and led to a lower proportionof N derived from BNF. The temperature response of BNF was attributedfor the major part to an adaptation to the demand for fixedN. It is therefore concluded that BNF is not primarily responsiblefor the reduced clover growth at low temperatures. White clover, Trifolium repens L., temperature, nitrogen fixation, nitrate, root, shoot     

Measurement of biological dinitrogen fixation in grassland: Comparison of the enriched 15N dilution and the natural 15N abundance methods at different nitrogen application rates and defoliation frequencies

A plant mixture of white clover (Trifolium repens L.), red clover (Trifolium pratense L.), and ryegrass (Lolium perenne L.) was established in the spring of 1991 under a cover-crop of barley. Treatments were two levels of nitrogen (400 and 20 kg N ha^-1) and two cutting intensities (3 and 6 cuts per season). Fixation of atmospheric derived nitrogen was estimated by two ¹⁵N dilution methods, one based on application of ¹⁵N to the soil, the other utilising small differences in natural abundance of ¹⁵N.Both methods showed that application of 400 kg N ha^-1 significantly reduced dinitrogen fixation, while cutting frequency had no effect. Atmospheric derived nitrogen constituted between 50 and 64% of harvested clover nitrogen in the high-N treatment, while between 73% and 96% of the harvested clover nitrogen was derived from the atmosphere in the low-N treatment. The amounts of fixed dinitrogen varied between 31–72 kg N ha^-1 and 118–161 kg N ha^-1 in the high-N and low-N treatment, respectively. The highest values for biological dinitrogen fixation were estimated by the enriched ¹⁵N dilution method.Estimates of transfer of atmospheric derived nitrogen from clover to grass obtained by the natural ¹⁵N abundance method were consistently higher than those obtained by the enriched ¹⁵N dilution method. Neither mineral nitrogen application nor defoliation frequency affected transfer of atmospheric derived nitrogen from clover to grass.Isotopic fractionation of ¹⁴N and ¹⁵N (B value) was estimated by comparing results for nitrogen fixation obtained by the enriched ¹⁵N dilution and the natural ¹⁵N abundance method, respectively. B was on average +1.20, which was in agreement with a B value determined by growing white clover in a nitrogen free media.     

Rates of Hydrogen Ion Efflux by Nodulated Legumes Grown in Flowing Solution Culture with Continuous pH Monitoring and Adjustment

The net efflux of H⁺ from lucerne (Medicago saliva L.), redclover (Trifolium pratense L.) and white clover (Trifolium repensL.) growing in flowing solution culture and dependent upon symbioticfixation of atmospheric N, was measured over a 75 d experimentalperiod. Considerable and rapid increases in acidity of the nutrientsolution of up to 1.45 pH units were recorded when the pH wasriot held constant over a 30 h period. There was little differencein H⁺ efflux when solution pH was held constant at 4.75, 5.75or 6.75, but there was an immediate cessation when it was adjustedto 3.75. Differences in the daily net efflux of H⁺ closely followedthe pattern of daily differences in incoming radiation, andthere was also evidence of a diurnal pattern of H⁺ efflux. Althoughthere were initially distinct differences between the speciesin the calculated rate of net H⁺ efflux (µg H⁺ g^–1dry shoot d^–), by day 75 these had diminished. In allspecies, however, the maximum rate of efflux per unit of shootsoccurred during the earlier rapid phases of growth. The measuredefflux of H⁺ was well equated with the plant content of excesscations (as measured by ash alkalinity) and, on average, theratio of acidity produced to N assimilated (expressed as anequivalent) was 0-24. Medicago sativa L., Trifolium pratense L., Trifolium repens L., lucerne, red clover, white clover, acidification, cation/anion balance, flowing solution culture, H⁺ efflux, nitrogen fixation     

The Influence of Volatile Substances from Incubated Litter of Pinus radiata on Seed Germination

Vapour from incubated Pinus radiata D. Don litter caused a depressionin the germination of Lolium perenne L. seeds and responsesvarying from stimulation to depression in germination of Trifoliumrepens L. seeds. These effects could be attributed to ethyleneand carbon dioxide. Experiments using ethylene and carbon dioxidesupported this conclusion. Pinus radiata, litter, Lolium perenne, Trifolium repens, seed germination, ethylene, carbon dioxide     

The Influence of Irradiance Distribution on the Growth of White Clover (Trifolium repens L.) in Differently Managed Canopies of Permanent Grassland

Growth of white clover was investigated in permanent grasslandcut three or five times per year. The influence of cutting frequencyand nitrogen fertilization on dry-matter yield, leaf-area distributionand the distribution of photosynthetically active radiationwithin the canopy were examined. In the five cut treatments, total dry-matter yield was nearlyequal, with and without nitrogen. However, nitrogen practicallyeliminated white clover. Leaf-area distribution showed characteristicpatterns for the different treatments. The small proportionof white clover in the treatment with nitrogen fertilizationwas thought to be due to the large leaf area of the other speciesat heights which white clover could not attain. This conclusionwas supported additionally by the radiation measurements withinthe canopy. The sunlit fractional area within canopy layers was measuredwith ‘quantum sensors’ and calculated from canopytransmission measured with tube solarimeters. The leaf areaindex of white clover was highly correlated (r² = 0.68) withthe sunlit fractional area above the canopy layers where whiteclover was present. This response of white clover leaf growth to the light regimeis discussed in relation to the potential petiole growth. White clover, Trifolium repens L., permanent grassland, irradiance distribution, sunlit fractional area, petiole extension, leaf area, dry matter, stratified clipping