Lack of a direct nitrate-trifoliin A interaction in the Rhizobium-clover symbiosis

Summary Nitrate added at critical concentrations to plant growth medium inhibits the infection of legume roots by Rhizobium. The direct interaction, of nitrate and trifoliin A, a Rhizobium-recognition lection from white clover (Trifolium repens L.), was examined as a possible basis for this regulation. Selective molecular ultrafiltration studies to detect ligand-protein interactions showed that radioactive¹³NO₃ ⁻ did not bind directly to trifoliin A when incubated at two molar ratios. Immunoprecipitation of trifoliin A by its homologous antibody was unaffected by 15 mM NO₃ ⁻. In addition, there was no apparent reduction in attachment ofR. trifolii 0403 to root hairs of clover seedings during 1 h of incubation in the presence of 15 mM NO₃ ⁻. These results show that nitrate inhibition of these early steps of the infection process is not due to a direct interaction of nitrate with trifoliin A or its glycosylated receptors.     

Direct differentiation of shoot buds in leaf segments of white marigold (Tagetes erecta L.)

Summary A protocol has been developed for differentiation of shoot buds directly from leaf segments of white marigold (Tagetes erecta L.). Leaf segments were taken from in vitro-proliferated shoots of white marigold established in aseptic culture from shoot tips of field-grown plants. Gibberellic acid (GA) played a significant role in the induction of shoot buds as well as in suppressing callus formation. Shoot buds were induced directly in Murashige and Skoog's (MS) medium supplemented with 14.43 μM GA and 4.44 μM 6-benzyladenine in the absence of any auxin. In this medium two to five shoot buds differentiated from the margins as well as leaf lamina of the lower petiolar segment within 4 wk of incubation. Differentiated shoots grew well and proliferated in the MS medium having 1.1 μM BA and 29.41 μM AgNO₃, as it had a beneficial effect on the growth and proliferation of shoots. Shoots were excised, rooted in 0.27 μm α-naphthaleneacetic acid (NAA) and transplanted under glasshouse conditions, where they grew and flowered. Data on different morphological characters during flowering under field conditions were recorded for seed-grown control plants, tissue culture-raised primary regenerants (R₀) and first-generation (R₁) plants. It was found that all the economically desired characters of plant height, number and size of flowers per plant, number of viable seeds per flower, and days to full bloom, of the R₁ generation plants were significantly better than the control, thus increasing the commercial value of the tissue culture-raised plants in successive generations.     

EMS诱变西瓜突变体库的构建及表型分析

采用1.0%诱变剂甲基磺酸乙酯(EMS)处理西瓜品系W1-17种子9h,然后对M1和M2代群体单株在叶、花、茎、育性、分支习性等方面进行表型变异观察,同时选取M2代典型变异株系,利用23对西瓜SSR引物进行分析鉴定,构建西瓜突变体库。结果表明:(1)EMS诱变使M1代幼苗形态呈现出叶畸形、叶褶皱、部分黄化、花畸形、雄花不散粉、卷须畸形、矮小、生长缓慢、不育等特异性状,获得由1 252个单株组成的西瓜突变体M1群体,群体总变异频率为18.33%。(2)M2代共筛选到205个突变植株,40种表型变异,表现在子叶性状(黄化、扭曲不对称、折叠等)、叶和茎性状(叶黄化、变小、裂刻变深,茎变细,节间变短,分支少等)、花性状(花变大,花色变浅,两性花,花瓣皱缩、部分退化、数目突变,柱头畸形,雄蕊不成熟等)和其他性状(生长缓慢、不育等)等方面,总的表型突变率达到了19.59%。(3)针对M2代10个典型变异植株,通过SSR引物分析发现有9份材料在DNA水平上有变异。本研究初步构建了含有120个M1代家系及1 051株M2代植株、40种表型变异的西瓜突变体库。     

Nitrogen fixation by white clover when competing with grasses at moderately low temperatures

It was the aim of this study to determine the way in which low temperature modifies the effect of a competing grass on nitrogen fixation of a forage legume. White clover (Trifolium repens L.) was grown in monoculture or in different planting ratios with timothy (Phleum pratense L.) or perennial ryegress (Lolium perenne L.) in growth chambers at either 7.5/5°C (LoT) or 15/10°C (HiT) average day/night temperatures, and with 2.5 or 7.5 mM ¹⁵N-labelled nitrate in the nutrient solution.Competition with grass led to a marked increase in the proportion of clover nitrogen derived from symbiosis (% N_sym). This increase was slower at LoT where % N_sym was reduced considerably; it was closely related to the reduction in the amount of available nitrate as a result of its being utilized by the grass.Nitrogen concentration in white clover herbage and dry matter yield per clover plant were reduced, for the most part, when a competing grass was present. The amount of nitrogen fixed per plant of white clover decreased markedly with temperature. Low temperature consequently accentuated competition for nitrate. The capacity of white clover to compete successfully was limited by its slower growth and nitrogen accumulation.     

The screening of mutants and construction of mutant library for <Emphasis Type="Italic">Oryza sativa</Emphasis> cv. Nipponbare via ethyl methane sulphonate inducing

Following the sequencing of rice genome, the functional analysis of unidentified genes is gaining wide importance. Mutant isolation is one of the effective ways to isolate and clone the target genes and analyze their functions. To find the various mutants in the same genetic background, seeds of Oryza sativa cv. Nipponbare were treated with ethyl methane sulphonate (EMS). A total of 1056 mutants were screened for five categories in M₂ generation with the seedling frequency of 26.29‰ at three-leaf stage, but only 264 mutants were verified in M₃ generation with a frequency of 6.57‰. Among the mutants verified in M₃ generation, the frequency of leaf mutation was the highest (2.22‰), followed by seedling height (1.74‰) and the abiotic stress tolerance mutant (1.47‰). Nineteen characteristic mutations, including a big group of abiotic stress tolerant mutants such as herbicide resistant, salt tolerant and drought tolerant were identified at this stage. By observation of rice growth characteristics at different developmental stages, another 220 mutants have been isolated and verified in the M₃ generation with the mutant frequency of 53.9‰ covering about 28 mutant traits. Among those identified, the highest frequencies were obtained for appearance of brown rice mutant with 18.37‰, followed by panicle mutant with 13.47‰, and grain mutants with 9.06‰. All the mutants screened above were suitable for gene function analysis and for utilization in agronomy.     

Neighbourhood effect of genotypes of Rhizobium leguminosarum biovar trifolii, Trifolium repens and Lolium perenne

 Ryegrass, white clover and Rhizobium isolated from the corresponding clover nodules, were harvested from a natural pasture in the Massif Central mountains (France). The specificity between Lolium, Trifolium and Rhizobium, and the genetic diversity of Rhizobium were examined. This study showed that: 1) Natural neighbouring combinations of white clover and ryegrass, re-planted together in pots, accumulated a higher biomass than non-neighbouring ones. This increase of mass is higher in the presence of the native strain of Rhizobium. 2) When white clover was inoculated with a mixture of Rhizobium strains, nodules were more often formed by its native strain. 3) The genetical diversity of the Rhizobium leguminosarum biovar trifolii was very high, as revealed by electrophoresis of esterases on seven substrates. These results support the hypothesis that there is a co-adaptation between white clover, ryegrass and Rhizobium Received: 25 March 1996 / Accepted: 13 September 1996     

Animal treading during wet soil conditions reduces N2 fixation in mixed clover-grass pasture

A field experiment was carried out over 12 months to determine the effect of animal treading on N₂ fixation in a mixed white clover-ryegrass pasture. The experimental site was defoliated by mowing for the duration of the study. A single treading event of moderate or severe pugging intensity was initiated in plots during wet spring conditions by using dairy cows at varying stocking rates (4.5 cows 100 m⁻² for 1.5 or 2.5 h, respectively). Inputs of dung and urine onto the plots was avoided by overnight housing of the cows and interception of excreta during the pugging event. Soil air-filled porosity decreased from 21% in the non-pugged control to 15–16% in pugged treatments by day 3. Bulk density of soil was not significantly affected by pugging. Soil inorganic N concentration increased in pugged treatments, and was 4-fold greater on day 28 in severely pugged plots compared to non-pugged plots. White clover plant density and plant size was markedly lower in pugged treatments (up to 85% and 72% reduction, respectively under severe pugging). White clover growth was most affected during the first 156 days after pugging (up to 90% decrease under severe pugging), leading to an annual clover dry matter production loss of 9% and 52%, respectively. The proportion of clover N derived from atmospheric N₂ (%Ndfa; estimated by ¹⁵N dilution) was initially reduced (to a lower limit of 43%) by severe pugging (days 28–71) before recovery to control levels (90%) by day 91. Annual N₂ fixation in clover herbage decreased significantly from 76 kg N ha⁻¹ yr⁻¹ in the non-pugged control, to 66 and 36 kg N ha⁻¹ yr⁻¹ under moderate and severe pugging, respectively. Most of this difference was evident within the first 156 days after pugging. Our data indicates that the major loss in fixed N₂ input under pugging was due to reduced clover growth and production resulting from pugging damage and loss of residual white clover biomass by hoof action.     

Effects of condensed tannins in white clover flowers on their digestion in vitro

Protein in white clover (Trifolium repens L.) is poorly utilised by ruminants because of its extensive degradation to ammonia in the rumen. However, white clover produces condensed tannins (CT) in its flowers, which can reduce rumen proteolysis. Effects of increasing proportions of clover dry matter (DM) as flowers (and therefore floral CT) on soluble protein, ammonia and volatile fatty acid (VFA) concentrations were determined with in vitro incubations. Minced mixtures of 0, 250, 500, 750 and 1000 g/kg of DM as white clover flower (F) with the remainder as white clover leaf, were incubated in vitro and sampled after 0, 2, 4, 8, 12 and 24 h. Treatments contained 0, 13, 26, 39 and 52 g CT/kg DM, respectively. A further treatment with 500 g/kg DM as flower and 500 g/kg DM as leaf had polyethylene glycol added to remove effects of CT. Increasing the proportion of white clover as flowers from 0 to 1000 g/kg DM reduced net conversion of plant N to ammonia N from 290 to 120 mM/M at least partly due to reduced solubility of the protein. Treatments with 750 g/kg DM or more as clover flowers reduced ammonia concentrations to levels likely to limit microbial growth. Total VFA production was not affected by flower content, although the proportion of acetate to propionate increased. The contribution of CT to treatment effects was small compared to effects attributed to difference in chemical composition between flowers and leaves.     

Dinitrogen fixation in white clover grown in pure stand and mixture with ryegrass estimated by the immobilized 15N isotope dilution method

Dinitrogen fixation in white clover (Trifolium repens L.) grown in pure stand and mixture with perennial ryegrass (Lolium perenne L.) was determined in the field using ¹⁵N isotope dilution and harvest of the shoots. The apparent transfer of clover N to perennial ryegrass was simultaneously assessed. The soil was labelled either by immobilizing ¹⁵N in organic matter prior to establishment of the sward or by using the conventional labelling procedure in which ¹⁵N fertilizer is added after sward establishment. Immobilization of ¹⁵N in the soil organic matter has not previously been used in studies of N₂ fixation in grass/clover pastures. However, this approach was a successful means of labelling, since the ¹⁵N enrichment only declined at a very slow rate during the experiment. After the second production year only 10–16% of the applied ¹⁵N was recovered in the harvested herbage. The two labelling methods gave, nonetheless, a similar estimate of the percentage of clover N derived from N₂ fixation. In pure stand clover, 75–94% of the N was derived from N₂ fixation and in the mixture 85–97%. The dry matter yield of the clover in mixture as percentage of total dry matter yield was relatively high and increased from 59% in the first to 65% in the second production year. The average daily N₂ fixation rate in the mixture-grown clover varied from less than 0.5 kg N ha⁻¹ day⁻¹ in autumn to more than 2.6 kg N ha⁻¹ day⁻¹ in June. For clover in pure stand the average N₂ fixation rate was greater and varied between 0.5 and 3.3 kg N ha⁻¹ day⁻¹, but with the same seasonal pattern as for clover in mixture. The amount of N fixed in the mixture was 23, 187 and 177 kg N ha⁻¹ in the seeding, first and second production year, respectively, whereas pure stand clover fixed 28, 262 and 211 kg N ha⁻¹ in the three years. The apparent transfer of clover N to grass was negligible in the seeding year, but clover N deposited in the rhizosphere or released by turnover of stolons, roots and nodules, contributed 19 and 28 kg N ha⁻¹ to the grass in the first and second production year, respectively. This revised version was published online in June 2006 with corrections to the Cover Date.     

Water deficit and plant competition effects on growth and water-use efficiency of white clover ( Trifolium repens,L.) and ryegrass (Lolium perenne,L.)

The combined effects of soil water deficit and above and below ground interspecific plant competition on the growth, water-use efficiency (WUE), and measured carbon isotopic composition (δ¹³C) values of white clover and ryegrass were studied. White clover and ryegrass were grown in specially designed crates 1) individually; 2) in shoot competition; or 3) in shoot + root competition and either well-watered or at a moderate or severe soil water deficit. The effects of shoot + root competition on shoot dry matter growth were substantial and benefited both white clover and ryegrass when well-watered or at a moderate soil water deficit, while severely reducing white clover shoot dry matter growth at severe soil water deficit. Plant competition did not affect the WUE of white clover or ryegrass. As soil water deficit increased, the WUE of white clover did not change whereas the WUE of ryegrass increased and was greater than that of white clover. This was attributed to the lower leaf water conductance of ryegrass which conserved water and maintained growth longer compared to white clover. A stronger correlation existed between soil water deficit and measured δ¹³C values for ryegrass at each plant competition level (P<0.001) than existed for white clover (individual: P<0.01; shoot + root: P<0.001; shoot: P<0.10). Unlike white clover, the relationship between measured δ¹³C values and shoot dry matter growth indicated that C assimilation for ryegrass was dependent on type of plant competition. That WUE remained constant for white clover while measured δ¹³C values increased as soil water deficit increased, suggests that the role below ground respiration rate played in determining δ¹³C values increased. The WUE of white clover appears to be independent of the nature of the competition between plants and the soil water deficit level at which it is grown, whereas for ryegrass, the addition of root competition to shoot competition should lead to increases in its WUE. This revised version was published online in June 2006 with corrections to the Cover Date.     

Long-term effects of an increased CO2 concentration on terrestrial plants in model ecosystems. Morphology and reproduction ofTrifolium repens L.and Lolium perenne L.

Mixtures (ratio 11, 45 seeds·dm^–2) of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) were enclosed in environmentally controlled acrylic mini-glasshouses and supplied with additional CO₂ from September 1981 until September 1982 (547–600 ppm, control: 340 ppm CO₂). At growth optimum the single leaf area of white clover was 30% greater and the specific leaf weight 19% higher with additional CO₂; with the perennial ryegrass it was 18% resp. 29%. The length growth of the white clover petioles was enhanced by the additional CO₂: in the beginning 67% longer leafstalks, at growth optimum no effect and at the end of the experiment 100%. At growth optimum the specific petiole weight was approximately 52% higher. When the growing was terminated the blades of perennial ryegrass were 19% longer at high CO₂. The average volume of the flowerheads was about 53% greater and the adult ears 13% longer. The number of flowers per flower head and the number of earlets per ear did not increase significantly. On an average the white clover seeds were 28% heavier and the caryopses were 4% lighter.Supported by the Umweltbundesamt, Fed. Rep. of Germany, Contract BSM 104 02 620Presented at the 10th International Congress of Biometeorology in Tokyo, 1984.     

Variation for aluminium tolerance in white clover

Aluminium (Al) tolerance of fourteen white clover (Trifolium repens L.) cultivars from eleven countries was compared in the greenhouse in the Wainui silt loam (Typic Dystrochrept) to which Al had been added at nine levels (0, 2.5, 5, 20, 50, 150, 250, 500 and 750 mg kg⁻¹ of soil) as Al₂ (SO₄)₃ and incubated for 30 days. None of the white clover cultivars, including those either referred to as Al-tolerant, Dusi and Pathfinder, or from countries that have large areas of acid soils, El Lucero M.A.G., Bayucua, Bage and Zapican, showed greater Al-tolerance than ‘Grasslands Huia’ white clover. Subsequent screening for Al-tolerance can therefore be restricted to germplasm with wide agronomic adaptation.     

Influence of pH,exchangeable aluminium and 0.02M CaCl2-extractable aluminium on the growth and nitrogen-fixing activity of white clover (Trifolium repens) in some New Zealand soils

The effects of soil acidity on the growth and N₂-fixing activity of white clover in seven acid topsoils and subsoils of New Zealand were investigated using a glasshouse experiment.The application of phosphate (Ca(H₂PO₄)₂) to the soils resulted in very large increases in white clover growth on all soils. The application of phosphate, as well as increasing P supply, also decreased 0.02M CaCl₂-extractable Al levels, but had little effect on exchangeable Al levels.Where adequate phosphate was applied, increasing rates of lime (CaCO₃) resulted in increased plant growth on most soils. N₂[C₂H₂]-fixing activity was increased by the first level of lime for one soil, but generally remained approximately constant or declined slightly at higher rates of lime. Up to the point of maximum yield, white clover top weight was more highly correlated with 0.02M CaCl₂-extractable soil Al than with exchangeable Al or pH. At pH values greater than 5.5, plant yield declined on some soils, apparently because of Zn deficiency. The data suggest that white clover is unlikely to be affected by Al toxicity at 0.02M CaCl₂-extractable Al levels of less than about 3.3 g g^–1. However, there were differences between soils in apparent plant tolerance to 0.02M CaCl₂-extractable Al, which appeared to be caused by differing C levels in the 0.02M CaCl₂ extracts.     

The bumblebee Bombus ardens ardens (Hymenoptera: Apidae) visits white clover in orchards before Oriental persimmon blooms

Flowers on the ground of orchards can provide substantial resources for wild pollinators of orchard trees. Few studies, however, have examined the relative importance of groundcover flowers to orchard pollination by analyzing pollen on the body surface of pollinators. Oriental persimmon trees bloom within the longer blooming period of white clover, which is occasionally found as a flowering plant on the ground of persimmon orchards in Japan. The present study compared the insect species assemblage collected on persimmon flowers with that on clover. Before persimmon bloomed, Bombus ardens ardens and Apis cerana japonica were the major visitors of clover flowers. Once persimmon bloomed, the former was the most abundant bee that visited persimmon flowers over the flowering period. Apis mellifera was captured only on clover flowers. We found numerous clover pollen grains on the body surface of bumblebees captured on persimmon flowers, but far fewer persimmon pollen grains on bees that visited clover. These findings show that B. ardens ardens utilized the clover flowers under the orchards before persimmon bloomed.     

Symbiotically fixed nitrogen from field- grown white and red clover mixed with ryegrasses at low levels of15N-fertilization

A field study was carried out near Zürich (Switzerland) to determine the yield of symbiotically fixed nitrogen (¹⁵N dilution) from white clover (Trifolium repens L.) grown with perennial ryegrass (Lolium perenne L) and from red clover (Trifolium pratense L.) grown with Italian ryegrass (Lolium multiflorum Lam.). A zero N fertilizer treatment was compared to a 30 kg N/ha per cut regime (90 to 150 kg ha⁻¹ annually). The annual yield of clover N derived from symbiosis averaged 131 kg ha⁻¹ (49 to 227 kg) without N fertilization and 83 kg ha⁻¹ (21 to 173 kg) with 30 kg of fertilizer N ha⁻¹ per cut in the seeding year. Values for the first production year were 308 kg ha⁻¹ (268 to 373 kg) without N fertilization and 232 kg ha⁻¹ (165 to 305 kg) with 30 kg fertilizer N ha⁻¹ per cut. The variation between years was associated mainly with the proportion of clover in the mixtures. Apparent clover-to-grass transfer of fixed N contributed up to 52 kg N ha⁻¹ per year (17 kg N ha⁻¹ on average) to the N yield of the mixtures. Percentage N derived from symbiosis averaged 75% for white and 86% for red clover. These percentages were affected only slightly by supplemental nitrogen, but declined markedly during late summer for white clover. It is concluded that the annual yield of symbiotically fixed N from clover/grass mixtures can be very high, provided that the proportion of clover in the mixtures exceeds 50% of total dry mass yield.     

Genome mapping of white clover (<Emphasis Type="Italic">Trifolium repens</Emphasis> L.) and comparative analysis within the Trifolieae using cross-species SSR markers

Allotetraploid white clover (Trifolium repens L.), a cool-season perennial legume used extensively as forage for livestock, is an important target for marker-assisted breeding. A genetic linkage map of white clover was constructed using simple sequence repeat (SSR) markers based on sequences from several Trifolieae species, including white clover, red clover (T. pratense L.), Medicago truncatula (Gaertn.) and soybean (Glycine max L.). An F₁ population consisting of 179 individuals, from a cross between two highly heterozygous genotypes, GA43 and Southern Regional Virus Resistant, was used for genetic mapping. A total of 1,571 SSR markers were screened for amplification and polymorphism using DNA from two parents and 14 F₁s of the mapping population. The map consists of 415 loci amplified from 343 SSR primer pairs, including 83 from white clover, 181 from red clover, 77 from M. truncatula, and two from soybean. Linkage groups for all eight homoeologous chromosome pairs of allotetraploid white clover were detected. Map length was estimated at 1,877 cM with 87% genome coverage. Map density was approximately 5 cM per locus. Segregation distortion was detected in six segments of the genome (homoeologous groups A1, A2, B1, B2, C1, and D1). A comparison of map locations of markers originating from white clover, red clover, and alfalfa (M. sativa L.) revealed putative macro-colinearity between the three Trifolieae species. This map can be used to link quantitative trait loci with SSR markers, and accelerate the improvement of white clover by marker-assisted selection and breeding. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The effects of low concentrations of aluminium on the growth and uptake of nitrate-N by white clover

Summary The effects of aluminium (Al³⁺) at concentrations of 0, 25, 50 and 100 μM on the growth of white clover, dependent upon N supplied as NO ₃ ⁻ , were examined in flowing solution culture. Plants were established with a normal nutrient supply for 7 weeks and then grown with carefully controlled pH (at 4.5) and P concentrations, and with 0, 25, 50 or 100 μM Al³⁺ for a further three weeks. There were rapid visual effects (i.e. symptoms of P deficiency and reduction in root extension) and the dry weights of shoots and roots were reduced at 50 and 100 μM. Less than 10% of Al absorbed from solution was transported to the shoots. The uptake of P, and its transport between roots and shoots, were reduced in plants grown with Al. The uptake of NO ₃ ⁻ stopped immediately after the introduction of 50 or 100 μM Al, and was significantly reduced at 25 μM after three weeks. During a second phase of the experiment, plants previously grown at 0, 25, 50 and 100 μM Al, were grown for a further 2 weeks either with NO ₃ ⁻ (with and without 50 μM Al³⁺) or without NO ₃ ⁻ but with inoculation by Rhizobia (and with or without 50 μM Al³⁺). The effects of the previous treatments with Al on N uptake were small during the second phase, but uptake by all plants was restricted when Al was present. Inoculation did not result in nodulation in the second phase when Al³⁺ was present in the solution, but Al already in the plant from the first phase did not prevent nodulation in the absence of Al during the second phase.     

Hormonal stimulation of tyrosinase activity in human foreskin organ cultures

Summary A human foreskin organ culture system has been developed to study the response of human skin to hormonal stimulation. Foreskins are maintained in culture on floating plastic supports which allows the epidermal surface to be exposed to air while the dermis is bathed in nutrient medium. Both black and white human foreskins can be maintained in organ culture for at least 1 wk with no change in the tissue structure or cell viability as determined by histochemical staining and by dopa reaction staining. Tyrosinase activity in both black and white human foreskin cultures decays markedly during the first 2 d of culture to a new steady state level which remains stable throughout the culture period. Both black and white foreskin cultures consistently demonstrate 2- to 10-fold increases in tyrosinase activity when treated with theophylline (1 mM). Approximately 90% of all skin cultures examined showed an increase in enzyme activity when treated with this phosphodiesterase inhibitor. Dibutyryl cAMP (0.1 mM) and [Nle⁴, D-phe⁷]-alpha MSH (10⁻⁸ M), were also found to markedly stimulate tyrosinase activity in some skin cultures, whereas alpha-MSH and prostaglandin E₁ produced only an inconsistent and small increase in the activity of the enzyme. Histamine (1 μM), vitamin D₃ (1 μM), and retinoic acid (1μM) failed to stimulate tyrosinase activity in either white or black foreskin cultures. This hormone-responsive organ culture system can be utilized to characterize the molecular processes responsible for the regulation of tyrosinase and pigmentation in human skin. This work was supported by a research contract from the Oklahoma Center for the Advancement of Science and Technology (OCAST) and by a research grant from the Presbyterian Health Foundation.     

Consequences of including adapted white clover in northern European grassland: transfer and deposition of nitrogen

The interspecific transfer of nitrogen (N) between white clover (Trifolium repens) and smooth meadow grass (Poa pratensis) in legume-based grasslands was assessed under North European field conditions using ¹⁵N individual plant leaf labelling. On average 50% of N in the grass was transferred from the white clover and about 6% of N in white clover was transferred from the grass. This corresponds to 2.5 and 0.3 g N m⁻² being transferred over the growing season between the two species, respectively, and demonstrates that a significant part of the total N of the grass is coming through interspecific transfer. The majority of the ¹⁵N transferred was within a period of 20 days at relatively low soil temperatures. This implies that there is a need for a new focus on direct transfer pathways or exudation and transfer of organic N sources. Rhizodeposition in the top 10 cm of the soil was found to be 2.98 g N m⁻² on average over the growing season for the grass and white clover mixture. Inclusion of adapted white clover varieties in the low-input grassland systems of northern Europe will lead to a substantial contribution of N. Responsible Editor: Euan K. James.