Assimilation of 14CO2 by the Inflorescence of Poa annua L. and Lolium perenne L.

The relative assimilatory activity of the inflorescence, itsindividual components, and the leaves of flowering tillers ofPoa annua L. and Lolium perenneL. was determined over the periodfrom inflorescence emergence to seed shedding. The pattern of¹⁴CO₂ fixation was similar for both species and the inflorescencewas by far the most important assimilatory organ of the reproductivetiller, particularly over the latter period of seed developmentas leaf senescence progressed. With the exception of the seedsall parts of the inflorescence showed significant assimilatoryactivity and the lemmas and paleas accounted for 40–50per cent of the total ¹⁴C fixed by the inflorescence in bothspecies. The importance of the grass inflorescence as a photosyntheticstructure is discussed in relation to similar studies on cereals. Poa annua, Lolium perenne, carbon dioxide assimilation, inflorescence     

The Effect of Sulphur Dioxide on the Growth of Lolium perenne L., Lolium multiflorum Lam., Dactylis glomerata L., and Phleum pratense L.

Lockyer, D. R. 1985. The effect of sulphur dioxide on the growthof Lolium perenne L., Lolium multiflorum Lam., Dactylis glomerataL., and Phleum pratense L.?J. exp. Bot. 36: 1851-1859. Fouragriculturally important grasses, Lolium perenne L., Loliummultiflorum Lam., Dactylis glomerata L. and Phleum pratenseL. were exposed to sulphur dioxide (SO₂) in a system of exposurechambers. The plants were exposed for a total of 43 d to meanconcentrations of SO₂ in the air of 0,87 or 448 (µg m^–3and herbage was harvested twice. All four grasses showed chloroticlesions after exposure to the highest concentration of SO₂.The effect of SO₂ on the yield of herbage was statisticallysignificant only at the second harvest and at the highest concentration;the dry weights of shoots of D. glomerata and L. perenne werereduced by 33% and 16% respectively. Significant effects ofSO₂ were also found on the 'transpiration coefficients' measuredfor D. glomerata and P. pratense. The grasses differed in theiruptake of sulphur from the atmosphere but this was not relatedto their sensitivity to SO₂. Total–S concentration inthe shoots of L. perenne, L. multiflorum and D. glomerata increasedalmost linearly in response to increasing SO₂ concentration;with P. pratense only the highest SO₂ concentration raised total-Sabove the level in control plants. These increases were almostentirely due to the accumulation of sulphate–S. Key words: Sulphur diozide, Lolium perenne, Lolium multiflorum, Dactylis glomerata, Phleum pratense     

Studies on the Initial Biochemical Reactions in the Germination of Pollen Grains

Tritiated water (³HHO) has been used as the medium for the germinationof pollen grains of Pinus radiata D.Don (a gymnosperm), Ulexeuropaeus L., Salix caprea L. (dicotyledonous angiosperms),and Phormium tenax Forst. (a monocotyledonous angiosperm). ³H-labelledcompounds formed during the initial germination (egersis) periodhave been separated and identified to give information on themetabolism taking place.Since the earliest labelled compoundswere -aminobutyric acid, alanine, aspartic acid, and glutamicacid, it is concluded that these amino acids play an importantpart in the biochemical reactions during the early stages ofgermination. Citric acid, malic acid, and glutamine do not becomelabelled until later while carbohydrates, phosphate esters,and lipids do not appear to incorporate tritium within the firsthour of germination.The gross labelling pattern and the natureof the individually labelled metabolites, their intensity andsequence of labelling are similar for all the species investigatedexcept gorse, which showed a decrease in the intensity and thenumber of labelled metabolites with time.P. radiata pollen storedfor 3 years under vacuum or carbon dioxide has a labelling patternsimilar to freshly collected P. radiata pollen, except for aconsiderable increase in the amount of an unknown metabolite(Compound X).     

Contrasting CO2 and temperature effects on leaf growth of perennial ryegrass in spring and summer

The effects of increased atmospheric carbon dioxide (CO₂) of700 µmol mol^–1 and increased air temperature of+ 4C were examined in Lolium perenne L. cv. Vigor, growingin semi-controlled greenhouses. Leaf growth, segmental elongationrates (SER), water relations, cell wall (tensiometric) extensibility(%P) and epidermal cell lengths (ECL) were measured in expandingleaves in spring and summer. In elevated CO₂, shoot dry weight (SDW) increased in mid-summer.In both seasons, SDW decreased in elevated air temperatureswith this reduction being greater in summer as compared to spring.Specific leaf area (SLA) decreased in elevated CO₂ and in CO₂ temperature in both seasons. In spring, increased leaf extensionand SER in elevated CO₂ were linked with increased ECL, %P andfinal leaf size whilst in summer all were reduced. In high temperature,leaf extension, SER, %P and final leaf size were reduced inboth seasons. In elevated CO₂ temperature, leaf extension,SER, %P, and ECL increased in spring, but final leaf size remainedunaltered, whilst in summer all decreased. Mid-morning waterpotential did not differ with CO₂ or temperature treatments.Leaf turgor pressure increased in elevated CO₂ in spring andremained similar to the control in summer whilst solute potentialdecreased in spring and increased in summer. Contrasting seasonalgrowth responses of L. perenne in response to elevated CO₂ andtemperature suggests pasture management may change in the future.The grazing season may be prolonged, but whole season productivitymay become more variable than today. Key words: Lolium perenne, ryegrass, CO₂ and temperature, leaf extension, cell wall rheology     

Increased Defoliation Frequency Depletes Remobilization of Nitrogen for Leaf Growth in Grasses

Plants ofLolium perenneandFestuca rubrawere grown in sand culturereceiving all nutrients as a complete nutrient solution containing1.5 mMNH₄NO₃, and subjected to one of three defoliation treatments:undefoliated, defoliated on one occasion, or defoliated weekly.¹⁵Nlabelling was used to determine the rate of N uptake, allowingthe amount of N remobilized from storage for the growth of thetwo youngest leaves (subsequently referred to as ‘newleaves’) growing over a 14 d period after defoliationto be calculated. The total plant N uptake by both species wasreduced, compared with undefoliated plants, by both a singleand repeated defoliation, although neither caused complete inhibitionof uptake. Regularly defoliatedL. perennehad a greater reductionin root mass, concomitant with a greater increase in N uptakeper g root than did regularly defoliatedF. rubra. In both species,the amount of N derived from uptake recovered in the new leaveswas unaffected by the frequency of defoliation. BothL. perenneandF.rubramobilized nitrogen to the new leaves after a single defoliation,mobilization being sufficient to supply 50 and 41%, respectively,of the total nitrogen requirement. In regularly defoliated plants,no significant nitrogen was mobilized to the new leaves inL.perenne, and only a small amount was mobilized inF. rubra. Plantsachieved greater leaf regrowth when only defoliated once. Weconclude that increasing the frequency of defoliation of bothL.perenneandF. rubrahad little effect on the uptake of nitrogenby roots which was subsequently supplied to new leaves, butdepleted their capacity for nitrogen remobilization, resultingin a reduction in the rate of growth of new leaves. Lolium perenne; Festuca rubra; defoliation frequency; mobilization; root uptake; nitrogen     

Quantitative Analysis of Cold Hardening and Dehardening in Lolium

The change in cold hardiness of three Lolium multiflorum Lam.varieties was followed in plants exposed to hardening or dehardeningconditions at a range of temperatures. Hardening and dehardeningwere analysed as quantitative processes dependent upon temperatureand time. Their time courses changed exponentially to an asymptotewhich was logistically related to temperature. Both componentsof the model were fitted simultaneously. Parameters of biologicalinterest, such as the initial rates of hardening and dehardeningfor a given temperature and the percentage of the process completedin a given time, were derived and compared for varieties ofcontrasting hardiness. The analysis demonstrated the importanceof dehardening in determining hardiness. Similar results wereobtained when the model was applied to hardening and dehardeningdata for Lolium perenne L. The potential of this quantitative analysis for distinguishingbetween possible mechanisms of cold hardiness is discussed andfurther experiments required to characterise the kinetics ofhardening and dehardening more fully are identified. Lolium multiflorum, Lolium perenne, cold hardening, cold dehardening, processes, model, varieties     

Neotyphodium lolii, a Fungal Leaf Endophyte, Reduces Fertility ofLolium perenneExposed to Elevated UV-B Radiation

Plants ofLolium perenne, grown with and without the balansoidfungal leaf endophyteNeotyphodium lolii, were exposed to threeultraviolet radiation treatments at an outdoor facility in theUK for 172 d. Plants were exposed to either (a) a 30% elevationabove the ambient erythemally-weighted level of UV-B (280–315nm) radiation under banks of cellulose diacetate filtered fluorescentlamps that also produce UV-A (315–400 nm) radiation (UV-B+A);(b) elevated UV-A radiation alone under banks of polyester filteredlamps; or (c) ambient levels of solar radiation under banksof unenergized lamps. The fertility of plants grown withN. loliiwassignificantly reduced by the elevated UV-B+A exposure. After172 d, these plants produced 70% fewer spikes, 75% fewer seeds,71% lower total weight of seed and 78% fewer seeds per g d.wt of plant tissue than plants colonized byN. loliiwhich wereexposed to ambient radiation. There was no discernible effectof elevated UV-B+A exposure on the fertility of endophyte-freeplants. Plants irradiated with UV-B+A developed 14% thickerleaves than those exposed to ambient radiation. Those whichwere irradiated with elevated UV-A alone produced seeds thatwere 20% heavier than plants exposed to ambient levels of radiation.Plants grown withN. loliihad 7% thicker leaves, 4% thicker stembases and 7% fewer tillers than those grown without it. Thefresh mass of tillers of plants grown withN. loliiwas 11% greaterthan those of endophyte-free plants, owing to their higher moisturecontents. These results suggest that the fertility ofL. perennecolonizedbyN. loliiin the natural environment could be deleteriouslyaffected by elevated fluxes of UV-B radiation associated withstratospheric ozone depletion and that this may affect the populationdynamics of the species.Copyright 1998 Annals of Botany Company Fungal leaf endophyte,Neotyphodium lolii, perennial ryegrass (Lolium perenne), stratospheric ozone depletion, UV-B radiation.     

Ethylene, Nitrate and Nitrite Interactions in the Promotion of Dark Germination of Common Purslane Seeds

Ethylene (10 µ1^–1) caused about one-third of highlydark-dormant seeds of common purslane (Portulaca oleracea L.)to germinate in the dark. Attempts were made to increase germinationin the dark with nitrate and ethylene combinations. When applieddirectly to the seeds, KNO₃ did not stimulate germination andKNO₃ plus ethylene did not increase germination above that ofethylene alone. Pre-incubation of seeds in KNO₃ for 4 to 7 dbefore the ethylene applications significantly increased germination.The effects of the KNO₃ pre-incubation were additive at eachof four ethylene concentrations (0.1–100 µ11^–1).Potassium nitrate was effective only when ethylene followedthe KNO₃ pre-incubation period. Potassium nitrite stimulatedabout 25 per cent of the seeds to germinate without a pre-incubationperiod and without ethylene. Also, ethylene plus KNO₂ enhancedgermination above that achieved by either stimulus alone. Silvernitrate did not block the ethylene promotion of germination,but reversed the typical ethylene inhibition of seedling growthfollowing germination. The results support the views that nitrateexerted its effect via conversion to nitrite within the seedand that the rate of nitrate conversion may be a limiting factorin the dark germination of common purslane seeds. Ethylene mayfacilitate nitrite activity by increasing seed sensitivity tothe stimulus. Common purslane, Portulaca oleracea L., ethylene, nitrate, nitrite, germination, dormancy     

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     

Seed Weight inLolium perenneas Affected by Interactions among Seeds within the Inflorescence

Little is known about interactions between seeds within theinflorescence of perennial ryegrass (Lolium perenneL.) and theireffect on final seed weight. These relations were investigatedin two glasshouse experiments using two genotypes, by reducingthe number of seeds in an inflorescence at anthesis. In thefirst experiment, entire spikelets were removed to study therelations between seeds in different spikelets. The effectson seed dry weight were not proportional to the number of spikeletsremoved. In one genotype, removal of two-thirds of the spikeletsincreased seed dry weight of the remaining seeds by 15%. Inthe other genotype, such treatment did not increase seed dryweight. In the second experiment, investigating seed interactionswithin a spikelet, either two proximal seeds or two centralseeds were maintained in a spikelet by removing the other ovulesin combination with no or 75% shading. Shading by 75% reducedseed dry weight by about 10%. In the unshaded treatment, seeddry weight was not affected by ovule removal. Under shading,the central seeds in a spikelet were about 12% heavier if theygrew alone, in contrast to the proximal seeds. The effects ofa reduction in seed number on seed dry weight were not relatedto the final nitrogen concentration of seeds. These experimentsshow that assimilate partitioning and the relationship betweenseeds in the inflorescence of perennial ryegrass are alreadylargely determined at anthesis; increasing seed yield by manipulationsafter anthesis is not feasible.Copyright 1998 Annals of BotanyCompany. Lolium perenne, perennial ryegrass, seed growth, seed interactions, shading, spikelet removal.     

The Effect of New Tiller Growth on Carbohydrates, Nitrogen and Seed Yield per Ear in Lolium perenne L.

To clarify whether new vegetative tillers that develop aroundanthesis in aLolium perenne seed crop can depress seed yields,the possible competition for carbohydrates or nitrogen betweenthe seeds and new tillers that develop after the onset of anthesiswas investigated. In two greenhouse experiments the number oftillers per plant was varied by a combination of cutting, nitrogensupply, light quality and light intensity treatments. Two genotypeswith different tillering rates were used. Seed yield per earwas largely independent of the number of tillers and regrowthof cut tillers after the onset of anthesis. It increased inone genotype, but only under low light and a reduced nutrientavailability, and no new tillers were produced. The amount ofwater-soluble carbohydrates in the reproductive tillers increasedin both clones under these conditions. Under more favourableconditions the increased tillering rate and regrowth of tillersafter cutting did not adversely affect seed yield per ear ineither clone, although carbohydrate reserves in the floweringtillers were sharply reduced. Tiller removal increased the concentrationand amount of nitrogen in the remaining flowering tillers, irrespectiveof the amount of regrowth. It is concluded that competitionfor carbohydrates or nitrogen between the seeds and new vegetativetillers that develop after the onset of anthesis, is not a majorcause of the low and variable seed yields inL. perenne seedcrops. Processes within the ear itself are probably limiting. Lolium perenne L.; perennial ryegrass; seed yield; seed set; tillering; carbohydrates; nitrogen; competition.     

The Effect of Ethylene on Quantitative and Qualitative Aspects of Respiration During the Breaking of Dormancy of Spergula arvensis L. Seeds

The effect of ethylene on quantitative and qualitative aspectsof respiration in Spergula arvensis L. seeds was studied. Oxygenconsumption and CO₂ evolution from seeds increased 60 h afteraddition of ethylene to the dry seed; these increases were consideredto be a function of the number of seeds that were germination,rather than a direct effect of ethylene. The respiratory quotient(RQ) on day 1 (0.5) was lower than on subsequent days, indicatingan absence of anaerobic processes in the early stages of germinationand the use of a different respiratory substrate to that oxidizedon days 2–4. Measurements of C₄/C₁ ratios showed thatethylene did not break dormancy by initiating a switch fromthe Embden-Meyerhof-Parnas pathway (EMPP) to the pentose phosphatepathway (PPP). On the contrary, some evidence was obtained fora switch from the PPP to the EMPP after 3–6 h in pre-imbibedand non pre-imbibed seeds treated with ethylene. Spergula arvensis L., seed germination, ethylene, respiration, Embden-Meyerhof-Parnas pathway, pentose phosphate pathway, respiratory quotient     

A Technique for Determining Quantitative Expressions of Dormancy in Seeds

A method of quantifying the heterogeneity of dormancy withinseed populations is described. The method compares germinationresponses of unstratified and stratified seeds of Pinus taedaL. (loblolly pine). Mathematical functions were fitted to cumulativegermination curves of unstratified and stratified seeds andfunctions were integrated to determine areas beneath respectivecurves. The area between respective curves was calculated bydifference and was considered to be a measure of dormancy withinseed lots. The method may be used to help identify various controlsof seed dormancy. seed germination, stratification, germination curves, Pinus taeda, loblolly pine, dormancy     

Changes in Dry Matter, Carbohydrate and Seed Yield Resulting from Lodging in Three Temperate Grass Species

The adverse effect of lodging on grass seed yield may be attributed,in part, to assimilate limitation during the seed filling period.This investigation examined plant dry matter assimilate partitioningand seed yield as affected by lodging in three species thatare closely related but phenotypically different: tall fescue(Festuca arundinacea Schreber.), Italian ryegrass (Lolium multiflorumLam.), and perennial ryegrass (L. perenne L.). Studies wereperformed in field plots at Corvallis, Oregon, USA. Seed yieldcomponents (seed number per inflorescence, seed yield per inflorescence,and single seed mass) and leaf, stem (lower, middle, and peduncle)and seed inflorescence dry mass were measured just prior toanthesis to seed maturity. Dry mass and water soluble carbohydrates(WSC) were determined for shoot components. The reduction indry mass and WSC in leaves and stem components following anthesiswas often greater in lodged plants compared to upright plants.The relatively low seed yield depression in lodged tall fescuesuggested a higher compensation potential for partitioning reserveassimilate from leaves and stems to support seed growth anddevelopment. This potential does not appear to be present tothe same degree in Italian ryegrass and to an even lesser extentin perennial ryegrass. These findings suggest that the potentialto compensate for reduced assimilate supply during the periodof high assimilate demand by seeds may be attributed, in part,to the total assimilate reserve accumulated prior to photoassimilatereduction caused by the lodged condition. Copyright 2000 Annalsof Botany Company Tall fescue, Festuca arundinacea Schreber., Italian ryegrass, Lolium multiflorum Lam., perennial ryegrass, L. perenne L., assimilate partitioning, source–sink     

A Quantitative Analysis of The Endosperm-Dependent Seedling Growth in Grasses

The growth of the organs of etiolated seedlings of Lolium perennecv. S24 during the period of endospermdependent growth is described.The lengths and the weights of the organs of L. perenne cv.S24 after 10 days growth in the dark and in daylight from heavyand light-weight caryopses are compared. The lengths of theetiolated seedling organs of six species of grasses are comparedand discussed in relation to seed weight.     

Sexual reproduction of Lolium perenne L. and Trifolium repens L. under free air CO2 enrichment (FACE) at two levels of nitrogen application

Field experiments in managed grassland have shown that the response of vegetative growth to elevated CO₂ is nitrogen‐dependent in grasses, but independent in N₂‐fixing legumes. In the present study, we tested whether this is also true for reproduction. We evaluated reproductive growth, flowering phenology, seed development, reproductive success and seed germination in the grass Lolium perenne L. and the legume Trifolium repens L., growing in monocultures in a free air carbon dioxide enrichment (FACE) system at ambient (35 Pa) and elevated (60 Pa) partial pressure of CO₂ and two levels of nitrogen fertilization (14 and 56 g N m^?2 a^?1). In both species, elevated CO₂ had no significant effect on sexual reproduction. In L. perenne, reproduction was mainly nitrogen‐dependent. The weak interactions between CO₂ and mineral N supply (13% more flowers and 8% more grains per spike at high N, 7% less flowers and 8% less grains at low N) were not significant. Under elevated CO₂, grain maturation was slightly enhanced and grain weight tended to decrease. No influence could be ascertained in the date of anthesis, the temporal pattern of grain growth, the rate of grain abortion and germination. Trifolium repens, grown under CO₂ enrichment at both levels of N fertilization, flowered 10 d earlier, tended to form more inflorescences per ground area and more flowers (8–12%) and seeds (>18%) per inflorescence than at ambient CO₂. The temporal pattern of seed growth was about the same in all treatments; embryo development, however, was accelerated in fumigated plants. The number of aborted seeds per pod, seed size, thousand‐seed weight and germinability did not show any influence of CO₂. Fumigated plants at high N were attacked slightly more frequently by seed‐eating weevils, which lowered the seed output per pod. In summary, the reproductive response of L. perenne and T. repens to CO₂ enrichment on the flower and inflorescence level was far weaker than expected from the results on vegetative growth.     

Effects of forest fire ash on germination and early growth of four pinus species

Fire modifies the germination of seeds of numerous species. One of the fire factors that cause these modifications is the ash. This study analysed the germination of seeds of Pinus sylvestris L., Pinus nigra Arn. Pinus radiata D. Don and Pinus pinaster Aiton, subjected to different ash treatments, and the development of seedlings that grew in these conditions for 14 weeks. We obtained the ash by completely burning leaves and small twigs from the most abundant woody species in populations of pines. The ash treatments applied were as follows: Control (without ash), Low (half of the amount registered in a fire), Medium (equal to the amount registered in a fire) and High (double the amount registered in a fire). Germination took place on paper inside Petri dishes and in soil, and growth was only analysed for the plants that grew in the soil. For all species, and especially P. nigra, germination rate decreased as the amount of ash applied increased. The negative effects of the ash were more apparent following treatments in Petri dishes than in the soil. In the dishes, the average germination time varies little between the four species of pine. In soil, the average germination time is more prolonged, and only some differences were observed between treatments in P. sylvestris and P. nigra. In these species the high treatment significantly increased the average germination time. We found a greater sensitivity of P. sylvestris and P. nigra to the addition of ash, showing a higher seedling mortality rate. However, it appears that seedling development (length and weight) was not affected in any of the species through the addition of ash. These tests allow us to deduce that, in P. sylvestris, P. nigra, P. radiata and P. pinaster, the ash produced by forest fires has an inhibiting effect on germination and little effect on the development of seedlings in the first months of life. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of Elevated CO2 on Stomatal Size and Distribution in Perennial Ryegrass

Plants of ryegrass (Lolium perenne L. cv. Melle) were grownfrom the early seedling stage in growth cabinets at a day/nighttemperature of 20/15 °C, with a 12-h photoperiod, and aCO₂ concentration of either 340 or 680 ± 15 µl1^–1 CO₂. Young, fully-expanded, acclimated leaves fromprimary branches were sampled for length of stomata, and ofepidermal cells between stomata, numbers of stomata and epidermalcells per unit length of stomatal row, numbers of stomatal rowsacross the leaf and numbers of stomatal rows between adjacentvein ridges. Elevated CO₂ had no significant effect on any ofthe measured parameters. Elevated CO₂, Lolium perenne, ryegrass, stomatal distribution, stomatal size     

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     

An analysis of the temperature response curves of CO2 exchange in the leaves of two temperate and one tropical grass species

Summary The temperature dependence of carbon dioxide exchange in two temperate grass species, Lolium perenne (S.23) and Lolium multiflorum (S.22), and one tropical grass species, Cenchrus ciliaris, have been examined. It has been shown that the decline in the net CO₂ exchange rate above the optimum temperature is caused, in the case of C. ciliaris, by denaturation, and, in the case of L. perenne and L. multiforum by the greater response to temperature of respiration compared with photosynthesis.Activation energies have been obtained for both photosynthetic and respiratory systems. For both systems the values fall into discrete groups, and it is postulated that each group may be characteristic of a particular rate-limiting process. The distribution of values changes with changes in experimental conditions, there being marked differences in the distribution of values obtained for light-limited and CO₂-limited photosynthesis.