Direct Somatic Embryoid Formation on Immature Embryos of Trifolium repens, T. pratense and Medicago sativa, and Rapid Clonal Propagation of T. repens

By direct somatic embryogenesis in vitro a clone of asepticplantlets can be raised from a single immature embryo of Trifoliumrepens (white clover) within about 6 weeks of pollination. Embryoidsare induced directly from intact zygotic embryonic tissue ona culture medium containing 0·025 or 0·05 mg 1^–1BAP and 1·0 g 1^–1 yeast extract. Similar directsomatic embryogenesis has also been achieved for Trifolium pratense(red clover) and Medicago sativa (lucerne). Applications ofembryo propagation by direct somatic embryogenesis are discussed,particularly in relation to multiple screening of host genotypesfor analysis of host/pathogen and legume/Rhizobium interactions. Trifolium repens L., Trifolium pratense L., Medicago sativa L., clover, lucerne, tissue culture, embryoid, somatic embryogenesis, legumes     

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     

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

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

Legume Nodule Morphology with Regard to Oxygen Diffusion and Nitrogen Fixation

Soybean (Gtycine max (L.) Merr. cv. Fiskeby V), white clover(Trifolium repens L. cv Blanca) and lucerne (Medicago sativaL.cv. Europe) nodules grown in fluid culture of Perlite wereexamined by cryo-scanning electron microscopy. The surfacesof all three species consisted of loosely packed cortical cells,collapsed areas covering intact cells and in soybean, troughswhich lacked the layers of loosely packed cells and were coveredby an amorphous matrix. The superficial cortical cells werehydrophobic and their surfaces were covered by a solvent-extractablestippling. This stippling was absent from the surfaces of underlyingcells. Air-filled intercellular spaces in fractured noduleswere seen throughout the cross-section. Nitrogen fixation, intercellular pathways, hydrophobicity, oxygen diffusion, cryo-SEM, soybean, Glycine max (L.) Merr. cv. Fiskeby V, clover, Trifolium repens L. cv. Blanca, lucerne, Medicago saliva L. cv. Europe.     

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     

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

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

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     

The Effect of Shade During Leaf Expansion on Photosynthesis by White Clover Leaves

In three experiments measurements of photosynthesis were madeon single leaves of white clover (Trifolium repens L.) on threecultivars grown in a controlled environment. Plants which had grown under an irradiance of 30 J m^–2s^–1, or in shade within a simulated mixed sward, producedleaves with photosynthetic capacities some 30 per cent lowerthan did plants grown at 120 J m^–2 s^–1 without shade.There were no differences between treatments either in photosynthesismeasured at 30 J m^–2 s^–1, or in respiration ratesper unit leaf dry weight. Respiration per unit leaf area washigher in the plants grown at 120 J m^–2 s^–1, reflectingthe lower specific leaf area of these leaves. There were nodifferences between the three cultivars examined. Leaves which were removed from the shade of a simulated swardshortly after becoming half expanded achieved photosyntheticcapacities as high as those which were in full light throughouttheir development. It is suggested that it is this characteristicwhich enables clover plants growing in an increasingly densemixed sward to produce a succession of leaves of high photosyntheticcapacity, even though each lamina only reaches the top of thesward at a relatively late stage in its development. Trifolium repens L., white clover, photosynthesis, leaf expansion, shade, specific leaf area, stomatal conductance     

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     

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     

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. 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     

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     

Influx and Efflux of Nitrate and Ammonium in Italian Ryegrass and White Clover Roots: Comparisons Between Effects of Darkness and Defoliation

Seedlings of Italian ryegrass (Lolium multiflorum Lam. cv. RVP)and clonal stolon cuttings of white clover (Trifolium repensL. cv. Blanca) were grown for 19 d in flowing solution culture,with N supplied as either 250 mmol m^–3 NO₃^– or NH₃⁺.Rates of net uptake, influx and translocation of NO₃^–and NH₄⁺ were then determined using ¹⁵N and ¹³N labelling techniques:between 3–5 h into the photoperiod following 8 h darknessfor white clover (CL), and for ryegrass plants that were eitherentire (IL) or with shoots excised 90 min prior to ¹³N influx(IC); and 75 min into the photoperiod following 37–39h darkness for ryegrass (ID). Rates of net uptake, influx andefflux of NH₄⁺ exceeded those of NO₃^– in IL and IC ryegrassplants: the opposite occurred in white clover (CL). The decreasein net uptake following defoliation of ryegrass was greaterfor NH₄⁺ (62%) than NO₃^– (40%). For NH₄⁺ this was associatedwith a large decrease in influx from 110 to 6.0µmol h^–1g^–1 root fr. wt; but for NO₃^–, influx only decreasedfrom 42 to 37 µmol h^–1 g^–1. Prolonged exposureto darkness (ID plants) also lowered net uptake of NO₃^–and NH₄⁺ by, respectively, 86% and 95% of IL levels. For NH₄⁺this was characterized by a large decrease in influx and a smalldecrease in efflux; whilst for NO₃^– the effect of a largedecrease in influx was reinforced by a smaller increase in efflux. The data were used to estimate the translocatory fluxes of NO₃^–(03–20µmol h^–1 g^–1) and NH₄⁺ (003–0.4µmolh^–1 g^–1), assimilation in the roots of NO₃^–(02–26µmol h^–1 g^–1) and NH⁺⁴ (05–89 µmolh^–1 g^–1), and the concentrations of NO₃^– (9–15mol m^–3) in the cytoplasmic compartment of the roots.The relevance of variable influx and efflux to models for theregulation of N uptake is discussed. Key words: Lolium multiflorum, Trifolium repens, influx, efflux, nitrate, ammonium, ¹³N     

Evidence For Root Contraction In White Clover (Trifolium repens L.)

White clover (Trifolium repens L.) stolons become buried inthe field. It was observed that this also occurred in the greenhousewhere the accepted mechanisms of burial, treading by livestockand earthworm casting, did not occur. It was also observed thatthe crown of seedling T. repens plants become closely appressedto the soil. Experiments showed that, regardless of varietyof T. repens or depth of planting, all seedling hypocotyls firstlift the cotyledons clear of the soil, then ‘ contract’towards the soil until the cotyledons are in contact with orbelow the soil surface. Auxanometers were used to measure therate and extent of this contraction and were also attached tostolon nodes in experiments which showed that stolons move downwardsrelative to the soil surface and that the speed and extent ofthis duration varied with soil type. A further experiment showedthat only rooted nodes show this behaviour. The force exertedby the contraction of nodal roots was estimated experimentallyas 0.21 N g^-1fresh root. A mechanism for the root contraction,based on examination of root anatomy of seedling tap-roots andnodal roots, is suggested. These experiments provide evidencefor root contraction in T. repens which may lead to stolon burial.The importance of this to T. repens as a pasture species andas a means of further improving T. repens varieties is discussed.Copyright 1999 Annals of Botany Company White clover, Trifolium repens, L., stolon, seedling, burial, root, nodes, nodal, force, contractile, soil resistance, pasture, phloem, fibres.     

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     

Fe-deficiency stress response in Fe-deficiency resistant and susceptible subterranean clover: importance of induced H+ release

Plants can exhibit Fe-deficiency stress response when they areexposed to Fe-deficiency conditions. The relative importanceof the individual Fe-deficiency stress-response reactions, forexample, increased release of H⁺ from roots, enhanced root plasmamembrane-bound Fe³⁺ -reductase activity, and release of reductant,in Fe-deficiency resistance is not understood. To address thisproblem, the Fe-deficiency stress response of two cultivarsof subterranean clover (subclover), Koala (Trifolium brachycalycinumKatzn. and Morley) (Fe-deficiency resistant) and Karridale (T.subterraneum L.) (Fe-deficiency susceptible), were evaluated.The plants were cultured hydroponically at 0 (–Fe) and30 (+Fe) µM Fe³⁺ EDTA conditions. After 6 d Fe treatment,the –Fe Koala and Karridale decreased the pH of the nutrientsolution by 1.83 and 0.79 units, respectively, while the +Feplants increased the pH of the nutrient solution. The H⁺ -releaserate of the –Fe Koala determined 7 d after Fe treatmentinitiation was more than three times higher than that of the–Fe Karridale. The –Fe plants had a significantlyenhanced Fe³⁺ -reduction rate compared with the +Fe plants foreach cultivar, but the resistant cultivar did not exhibit ahigher root Fe³⁺ -reduction rate than the susceptible cultivarat each Fe treatment. Reductant release from the roots of subcloverwas negligible. These results indicate that Fe-deficiency-inducedH⁺ release may be the predominant factor influencing Fe-deficiencyresistance in subclover. Key words: Fe-deficiency, Fe³⁺ reduction, H⁺ release, stress response, Trifolium     

Ion Transport and the Effects of Acetic Acid in White Clover: II. POTASSIUM ABSORPTION

Dunlop, J., Knighton, M. V. and White, D. W. R. 1988. Ion transportand the effects of acetic acid in white clover. II. Potassiumabsorption.—J. exp. Bot. 39: 89–96. Acetic acid stimulated K⁺ influx into cells of white cloverin suspension culture and net K⁺ influx by roots of intact whiteclover plants. At concentrations of acetic acid up to 2·0mol m^–3 the stimulation continued unabated for at least2 h. However, at higher concentrations the rate of absorptiondeclined to near zero values after 2 h. When acetic acid wasremoved from the solution there was a net efflux of K⁺. Thestimulation was pH dependent with a maximum at pH 5·0.There was maximum stimulation at an acetic acid concentrationof 2·0 mol m^–3 Net H⁺ efflux was reduced by 1·0 mol m^–3 aceticacid. When Rb⁺ uptake and H⁺ efflux were measured at a rangeof RbCl concentrations Rb⁺ uptake increased with concentrationwhereas H⁺ efflux was maximum in the absence of Rb⁺ (and K⁺)and decreased as RbCl concentration was increased Acetic acid caused a hyperpolarization of the membrane electricalpotential difference (E) of about 25 mV. In 1·0 mol m^–3acetic acid the hyperpolarization persisted for at least 2 hwhereas at 10 mol m^–3 d E subsequently depolarized tovalues around –80 mV. With a slight lag, the time courseof the stimulation of the rate of K⁺ absorption followed thepolarization and depolarization of E. These results imply thatthe linkage between K⁺ and H⁺ movements is probably throughE. Key words: Proton efflux, membrane electrical potential difference, Trifolium repens     

Host range and overwintering sources of bean leaf roll and pea enation mosaic viruses in England

Bean leaf roll virus (BLRV) and pea enation mosaic virus (PEMV) were each transmitted by Acyrthosiphon pisum (Harris) to fifteen of thirty species of legumes in the glasshouse; eleven species were susceptible to both viruses. The only biennial or perennial species infected by BLRV were hop trefoil (Medicago lupulina L.), lucerne (M. sativa L.) and red clover (Trifolium pratense L.), but naturally infected sainfoin (Onobrychis viciifolia Scop.) and white clover (T. repens L.) were found. The only perennial species infected with PEMV in the glasshouse was kidney vetch (Anthyllis vulneraria L.). Eggs of A. pisum, which seems to be the main vector of BLRV and PEMV in England, were found in winter on several species of cultivated perennial legumes, most on lucerne, fewest on white clover. In spring, more viviparae of A. pisum were found on lucerne than on other perennial legumes, and many on lucerne, but few on red or white clover, were infective with BLRV. Lucerne is probably the main overwintering source of BLRV in areas where lucerne is common, but elsewhere red and white clovers are probably as important. No aphid collected from perennial legumes between 1965 and 1968 was infective with PEMV, but this virus can overwinter in common vetch (Vicia sativa L.). Lucerne infected with BLRV was usually symptomless or showed only transient mild yellowing of young leaves. Lucerne plants showing vein-yellowing, similar to that previously reported as a symptom of BLRV, were possibly infected with an aberrant strain of BLRV but more probably with BLRV and another aphid-transmitted agent. Inoculations from lucerne with vein-yellowing symptoms sometimes caused vein-yellowing, and sometimes typical BLRV-symptoms, in crimson clover (Trifolium incarnatum L.).