Nitrogen:sulphur ratio alters competition between Trifolium repens and Lolium perenne under cutting: Production and competitive abilities

Grasslands species, like others crops, no longer benefit from high atmospheric sulphur deposition. This may cause sulphur limitation and in turn may induce a shift in plant communities by altering species performance and competitive abilities. To test this hypothesis, a greenhouse pot experiment was designed to investigate the interacting effects of sulphur (S) and nitrogen (N) availability on production, morphology and competitive abilities of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.). Plants were grown in monocultures and mixtures during 4 months. They were supplied with combinations of three levels of S and three levels of N and subjected to three defoliation events. Both S and N gradients altered plants performance and modulated competitive interactions. In L. perenne, above- and below-ground dry matter production, leaf and tiller number and contribution to mixture DM production were little affected by S, but highly by N. For T. repens, these traits were significantly increased by S, but only slightly by N. At establishment, T. repens was subjected to a significant interspecific competition at low N–low S and high N–high S. But at regrowth (cuts 2 and 3), T. repens was much more affected by intra- than interspecific competition. L. perenne was only sensitive to competition (intraspecific) when N supply was combined with no or moderate S supply. We conclude that N:S ratio of soil appears to drive species production, morphology and competitive abilities, which in turn affect leguminous–gramineous species ratio and grassland plant community structure. Since the growth of T. repens was significantly reduced in low S treatments, our results suggest that the drastic fall in atmospheric sulphur deposition could restrict leguminous species in high N soil conditions.     

Inheritance of downy mildew resistance at cotyledon and adult-plant stages in ‘Couve Algarvia’ (Brassica oleracea var. tronchuda)

Parasitic nematodes damage white clover (Trifolium repens) roots, negatively impacting forage yield and persistence. No single gene resistance to nematodes has been identified in white clover. Trifolium semipilosum (2n = 2x = 16) genotypes exhibiting either complete resistance or susceptibility to infection by the clover root-knot nematode (CRKN), Meloidogyne trifoliophila, were identified. F₁ progeny (n = 92) of a pair-cross between ‘TsR’, a plant heterozygous for the resistance phenotype and ‘TsS’, a plant homozygous for the susceptible phenotype, were challenged with infective CRKN juveniles and evaluated subsequently for root galling. Segregation analysis indicated the resistance phenotype may be conferred by a single dominant allele at a locus (designated TRKR, Trifolium Root-knot Resistance) subject to segregation distortion. TsR, TsS, and bulked resistant and bulked susceptible F₁ progeny (n = 12/bulk) were screened using T. repens microsatellite (SSR) markers. Three SSRs revealed polymorphism in TsR and the resistant bulk, of which prs090 and prs247 map to loci on T. repens linkage group D2. Progeny were genotyped with these three SSRs and 23 additional SSRs from T. repens groups D1 and D2. Linkage analysis in both TsR and TsS demonstrated macro-synteny between T. repens group D homoeologues and the T. semipilosum linkage group (designated D_Ts) containing the TRKR locus. Significant segregation distortion was detected in TsR, and recombination in the central region of the T. semipilosum linkage group was suppressed relative to T. repens in both parents. These data demonstrate the opportunities and challenges for comparative mapping in Trifolium and characterisation of loci conferring resistance to plant-parasitic nematodes.     

Variation for reproductive and agronomic traits among T. repens × T. nigrescens third generation backcross hybrids in the field

Interspecific hybridisation with Trifolium nigrescens Viv. is a possible strategy to improve the reproductive potential of white clover (Trifolium repens L.). Following the development of a fertile F1 hybrid, three generations of backcrossing have been carried out usingT. repens as the recurrent parent. Vegetative characteristics, stolon growth and seed yield components of the backcross (BC) 2 and 3 generations, as well as the parental species were measured on spaced plants grown in the field. Leaf size and plant spread of the BC2 and BC3 generations were less than T. repens but there was no difference in plant fresh weight. Numbers of inflorescences per plant and florets per inflorescence of the backcrosses were greater than T. repens however this was not reflected indifferences in seed yield per plant asT. repens had more seeds per floret and per plant than the backcrosses. Differences in stolon length, the proportion of flowering nodes and the pattern of axillary bud development were observed between T. repens and the backcrosses. Significant variation among the BC 3 generation for vegetative and reproductive traits was observed. Individual plants among the BC 3 generation were identified that combine high forage yield, substantial inflorescence production and good fertility, and these will form the basis of further selection. This revised version was published online in August 2006 with corrections to the Cover Date.     

Trifolium ambiguum as a source of variation in the breeding of white clover

Summary Hybrids between T. ambiguum and T. repens were successfully established after ovule culture. The hybrids were male sterile but one of the hybrids produced a single seed when backcrossed to T. repens, while the other produced a significantly greater number of seeds. The majority of the backcross hybrids from the latter had 48 chromosomes, being produced from the fertilisation of unreduced gametes of the F1 hybrid by T. repens pollen. Regular meiosis in the BC1 hybrid resulted in the production of balanced gametes containing the full complement of T. repens chromosomes and the polyhaploid set of T. ambiguum chromosomes. There was no difficulty in establishing a substantial population of BC2 hybrids. The meiotic behaviour of the BC1 hybrid indicated that it should be possible to transfer genes from T. ambiguum into white clover T. repens. The significance of the results in terms of the use of alien variation in the improvement of white clover is discussed.     

Rhizobium specificity of Trifolium ambiguum M. Bieb × T. repens L. hybrid clovers

Growth of full-sib families of an F₁ interspecific hybrid between Trifolium ambiguum M.Bieb and T. repens L, and two generations of backcross hybrids (BC₁F₂ and BC₂F₁) with T. repens as the recurrent parent, were compared to their parental species. Plants were grown in a N-free medium and inoculated with Rhizobium leguminosarum biovar trifolii rhizobia effective on T. ambiguum or T. repens. Hybridisation produced progeny that nodulated with rhizobia from either T. ambiguum or T. repens, but plant growth varied. Mean weights of T. repens and hybrids, particularly F₁ and BC₁F₂, were higher when inoculated with a mixture of strains isolated from field grown T. repens than with the New Zealand inoculant strain for T. repens. When inoculated with the mix of rhizobia from T. repens, mean weights were 242, 189, 132, 125, and 100 mg/plant for T. repens, BC₂F₁, BC₁F₂, F₁ and T. ambiguum, respectively. However, although the mean weight of BC₂F₁ lines was significantly less than T. repens, there was considerable variation in individual full-sib families indicating the potential to select within BC₂F₁ hybrids for high plant growth/symbiotic nitrogen fixation. The weight of T. ambiguumplants inoculated with the New Zealand inoculant strain for hexaploid T. ambiguum was similar to T. repens inoculated with the mix of rhizobia from T. repens (253 and 242 mg/plant, respectively). Mean fresh weights of F₁ hybrid plants were similar when inoculated with rhizobia for T. ambiguum or T. repens (125 and 130 mg/plant, respectively). However, weight of T. repens, BC₁F₂ and BC₂F₁ hybrids inoculated with rhizobia for T. ambiguum were all less than 90 mg/plant. There was a significant relationship between plant fresh weight and ethylene production. The results indicate that measuring weights of inoculated plants growing in N-free media is a rapid initial method of screening a range of plant germplasm for plant growth/symbiotic nitrogen fixation rates. This revised version was published online in August 2006 with corrections to the Cover Date.     

Pasture restoration improvement methods for temperate degraded pastures and consequences of the climatic seasonality on soil–pasture complex

Short persistence of renovated pastures is a major problem for pastoral production systems. Pasture species establishment, species performance and persistency and their relationships with soil–water attributes following pasture improvement methods were evaluated over two years. Fertilised naturalised pasture (FNP), non-fertilised naturalised pasture (NFNP), Lolium perenne–Trifolium repens cultivated pasture (CP), L. perenne–T. repens direct-drilled pasture (DP), and Bromus valdivianus, Holcus lanatus, Dactylis glomerata, L. perenne and T. repens direct-drilled pasture (DDP) were established as a randomised complete block design. Pastures were limed, fertilised and sheep-grazed. Plant growth compensatory mechanisms, by decreasing tiller number and increasing tiller size, equalised pasture yield during the establishment. In both years, the improved pastures yield was greater than that of NFNP. In the second year, DDP and FNP reached the higher pasture growth rate, with a seasonality diminishment. During the second summer, soil matrix potential at 0–20 cm soil profile exceeded the permanent wilting point, resulting in the pasture growth rate of NFNP and the sown pastures ranging from 0.0 to 3.6 kg DM/ha per day, while for FNP, it was 17.8 kg DM/ha per day. Sown species, fast-growing species spontaneous colonisation and growth were all stimulated in the improved pastures over species that tolerate stress, improving agronomical and ecological pasture features.     

Introduction of a self-compatible gene of Lolium temulentum L. to perennial ryegrass (Lolium perenne L.) for the purpose of the production of inbred lines of perennial ryegrass

Interspecific hybrids between self-compatible species, Lolium temulentum L. and self-incompatible species, L. perenne L. were obtained using embryo rescue. Two cycles of backcrossing of interspecific hybrids with L. perenne were carried out. A 1: 1 segregation ratio of self comatibility and incompatibility was observed in backcross generations. These segregation data confirmed that self-compatibility of L. temulentum was controlled by a single gene. It suggested that the self-compatible gene of L. temulentum could be introduced to self-incompatible plants of L. perenne through interspecific hybridization. It appears that utilization of a self-compatible gene of L. temulentum would be useful for production of inbred lines of L. perenne. The possibility of breeding procedures of perennial ryegrass using a self-compatible gene was discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Allozyme differentiation of European populations and cultivars of Lolium perenne L., and the relation to ecogeographical factors

Summary Sixty Lolium perenne populations were screened for allozyme diversity at five loci. Objective was to determine whether allozyme diversity could be used as selection criterion for genebank accessions of L. perenne. Subsampling of locations was tested with allozyme analysis to determine whether genetically different populations could be collected at one location. Correlations between allelic frequencies and environmental factors and morphological data were established, to find ecogeographical patterns in the observed variation. Results indicated that with few examples each allelic variant could be observed in each population screened; very few unique alleles were found. Differences between populations were largely due to differences in allelic frequencies. Few correlations were found with environmental factors and morphological data. For some allele frequencies a north-south cline was observed. Generally, allozymic data of the five screened loci did not appear to be useful for the selection of accessions for genebank storage. Significant genotypic differences between populations collected at one location could be established. In general these results agreed with earlier results concerning phenotypic variation in the same populations.     

Genotype × environment interaction for resistance to the white potato cyst nematode (Globodera pallida,pathotype E) in Solanum vernei x S. Tuberosum hybrids

Summary The standard root-ball test for assessing quantitative resistance of Globodera pallida in host material derived from Solanum vernei has produced variable results. This study of two sets of clones shows that genotype x environment interactions are responsible and that linear regressions (b) of phenotypic means on environments may enable good predictions to be made of the effects of the interactions. A relationship between the regression value (b) and phenotypic mean is shown for this material. The implication of this for a screening procedure in a potato breeding programme aimed at increasing potato cyst nematode resistance in adapted cultivars of S. tuberosum is discussed.     

Yield components,harvest index and plant type in relation to yield differences in field pea genotypes

Summary The effectiveness of yield components, harvest index and morphological characteristics as selection criteria among four field pea (Pisum sativum L.) genotypes was examined. Genotypes were grown at a wide range of plant populations (9 to 400 plants m^-2) to maximise environmental diversity.Both biological and seed yields approximately doubled from 9 to 100 plants m^-2. This response flattened from 100 to 400 plants m^-2. Differences among the genotypes were found only at 225 and 400 plants m^-2. Analysis of the yield components highlighted the plasticity and large genotype by environment interactions of field peas. The numbers of pods per plant and peas per pod were maximised when each genotype was grown as spaced plants, but the low plant numbers meant seed yields per unit area were at their lowest.Genotypic differences for plant harvest index (PHI) were also only found at 225 and 400 plants m^-2. Broad sense heritability estimates indicated that direct selection for PHI was not feasible. The inference from the yield component and PHI results was that alternative selection criteria such as physiological or morphological characteristics may be necessary for improved yield potential. Classification of each genotype indicated that low seedling vigour may be a positive attribute for crop plants of semi-leafless and conventionally leafed field peas. Selection based on any single plant attribute is unlikely to lead to dramatic improvements in the yield potential of field peas. Selection should be based on plant characteristics rather than on differences in yield components.     

Somatic hybridization as a tool for tomato breeding

Protoplast fusion can be used to produce somatic hybrids of species that cannot be obtained by sexual hybridization. The possibility to introgress genes from Solanum species into the cultivated tomato species Lycopersicon esculentum, and to obtain novel cytoplasm-nucleus combinations (cybrids) was considered as an important strategy to extend the genetic variation available for tomato breeding. Somatic hybrids between L. esculentum and other Lycopersicon species, as well as between L. esculentum and Solanum or Nicotiana species, have been produced. Specific mutants, genotypes with antibiotic resistances, and metabolic inhibition by iodoacetate or iodoacetamide and irradiation were used for the selection of hybrids. In addition, the improvement of protoplast culture techniques and the use of the favourable tissue culture traits derived from species such as L. peruvianum, which have been introduced into tomato by classical breeding, allowed the efficient recovery of somatic hybrids. However, the occurrence of somatic incongruity in fusion combinations of L. esculentum and Solanum and even more in L. esculentum and Nicotiana, did not allow the production of true cybrids and/or fertile hybrids, indicating the importance of both cytoplasm-nucleus and nucleus-nucleus interactions in somatic incongruity. Another problem with fusions between distantly related species is the strongly reduced fertility of the hybrids and the very limited homoeologous recombination between chromosomes of the parental species. Partial genome transfer from donor to recipient through microprotoplast (+) protoplast fusion, and the production of monosomic or disomic chromosome addition lines, light overcome some of these problems. In symmetric somatic hybrids between L. esculentum and S. tuberosum the occurrence of limited somatic and meiotic recombination was demonstrated. Fertile progeny plants could be obtained, though at a low frequency, when embryo rescue was performed on a large scale after backcrossing hexaploid somatic tomato (+) potato hybrids with a tetraploid potato genotype. The potential value of genomic in situ hybridization (GISH) and RFLPs for the analysis of the genome/chromosome composition of the hybrids has been demonstrated for intergeneric somatic hybrids between Lycopersicon and Solanum.Abbreviations cpDNA chloroplast DNA - mtDNA mitochondrial DNA     

Analysis of nucleolar organizer regions by silver staining in two Lolium species

Summary The nucleolar organizer activity of diploid and tetraploid Lolium perenne L. and diploid Lolium multiflorum Lam. varieties was analyzed by the silver staining procedure. Both species show six or twelve Ag-NORs (silver stained nucleolar organizer regions) at the diploid and tetraploid levels, respectively. A positive correlation between the size of secondary constrictions and Ag-NORs was observed.     

Pathotypes and NIP1 gene sequences of Finnish Rhynchosporium secalis isolates from barley, couch grass and rye

Rhynchosporium secalis (Oud.) J.J. Davis, causal agent of scald of barley (Hordeum vulgare L.), was isolated from barley, couch grass (Elymus repens L.) and rye (Secale cereale L.). Isolates were used to inoculate seedlings of a differential barley series containing several sources of major gene resistance to the disease. The series included Atlas46, (resistance gene Rrs1) and the isogenic line Atlas, which lacks the gene (rrs1). The necrosis-inducing peptide NIP1 has been suggested to be the product of the avirulence gene AvrRrs1 (NIP1) that with the barley resistance gene Rrs1 determines the incompatibility of the pathogen-host interaction. All R. secalis isolates were virulent only on the susceptible barley cultivars Arve and Chan, irrespective of spore concentration or original host species. There were no indications of redundant virulence among the R. secalis isolates. The NIP1 gene was sequenced from each isolate and there was no correlation with source of the isolate or sequence modification and virulence. Four isolates, from barley and couch grass, were characterised by a basic Type I NIP1sequence. The recorded NIP1 sequence changes are consistent withR. secalis populations not receiving selection through deployment of the Rrs1 gene in commercial barley cultivars in Finland. This revised version was published online in July 2006 with corrections to the Cover Date.     

Co-adaptation between neighbours? A case study with Lolium perenne genotypes

Summary Co-adaptation between neighbouring genotypes of Lolium perenne was examined in a pot experiment over an 18 month period. Competitive interactions in pure and mixed stands were examined using 9 genotype pairs (established as neighbours from electrophoresis data), 7 from a 10 year-old, and 2 from a 40+ year-old pasture. Combinations of each pair of neighbours grown together, and each grown with 5 randomly chosen alien (non-neighbour) genotypes sampled from the same pastures were also assessed. Seven harvests of green matter produced above 20 mm were made, and 50–135 kg N ha^-1 was administered after each harvest. Data for dry weight at each harvest for pure and mixed stands, and Relative Yield Totals (RYT) were obtained for neighbours and aliens in each treatment.In 6 of the 9 pairs of neighbours grown as pure stands, the yields of neighbours did not differ significantly at the final harvest. In 8 of the 9 pairs grown as mixtures of pairs of neighbours, the yield of one of the neighbours was significantly reduced and one neighbour had been eliminated. Over the last 5 harvests, yields from (alien + neighbour) mixtures had the greatest dry matter yields. No suppression of one component by the other was recorded for this (alien + neighbour) combination.Based upon RYT estimates, neighbours did not yield more when grown together than when grown separately with aliens. There was no relationship between original genotype size in their fields of origin and dry matter yield in this experiment.This experiment provides no evidence for he evolution of co-adaptation between surviving neighbours in the two pastures from which these individuals were sampled. The genotypic architecture of pasture would seem to be the product of random and/or competitive exclusion of individual genotypes, rather than the evolution of co-adapted groups of individuals through time. The consequences of extreme depletion of genetic variation in the first two years after establishment, the very low rates of seedling establishment in established pastures, and the impact of the grazing animal would suggest that there is little likelihood of or potential for, co-adaptation evolving between neighbours.     

Symbiotic variability in Vicia faba. 3. Genetic effects of host plant,rhizobium strain and of host × strain interaction

Summary Six Vicia faba populations were grown in all possible combinations with six Rhizobium leguminosarum strains in order to estimate the relative importance of the three genetic components of symbiotic variability. Additive genetic effects of host genotype and rhizobium genotype accounted for only 8.9% and 11.8% of the total phenotypic variation. Non-additive variation attriabutable to specific host genotype × rhizobium genotype interactions was by far the largest component of variation and accounted for 73.8% of phenotypic differences. Therefore the greatest improvement in symbiotic nitrogen fixation is likely to arise from simultaneous selection of both symbionts.     

Endosperm peroxidase electrophoresis patterns to distinguish tetraploid from hexaploid wheats

Summary A simple method is proposed to distinguish hexaploid (Triticum aestivum L.) from tetraploid (Triticum turgidum L., durum wheat) cultivated wheats on the basis of peroxidase isozymes coded by genome D. It can also be used as a first step to detect possible contamination by tetraploid genotype mixtures. The peroxidase patterns of endosperm and of embryo plus scutellum found among 349 entries of a durum wheat world basis collection are shown.     

Performance of Timothy-based Grass/Legume Mixtures in Cold Winter Region

This study sought to identify grass/legume mixtures that increase the yield and persistence of forage stands with improved nutritive quality in cold‐winter regions, compared with the standard mixture of timothy (Phleum pratense L.)/red clover (Trifolium pratense L.)/alsike clover (Trifolium hybridum L.). Timothy was mixed with either perennial ryegrass (Lolium perenne L.), meadow fescue (Festuca pratensis L.) or Kentucky bluegrass (Poa pratensis L.). The legumes in mixtures were red clover, alfalfa (Medicago sativa L.) or white clover (Trifolium repens L.). Averaged over three production years, the majority of mixtures had greater dry matter (DM) yields than the standard (8.35 t ha^?1). Timothy, grown alone and in three mixtures, outyielded the standard by 19–30 %. Yield reductions in mixtures over the 3‐year period were greatest with red clover, and least with bluegrass. Mixtures with alfalfa were highest in nitrogen (28.4 g kg^?1), while grasses grown alone (24.6 g kg^?1) and the standard mixture (25.1 g kg^?1) were the lowest in N. Mixtures with red clover or alfalfa had the least neutral detergent fibre (NDF), averaging 418 and 429 g kg^?1 respectively. Mixtures including white clover were initially low in NDF at 347 g kg^?1 in year 1 but increased to 550 g kg^?1 in year 3 as white clover composition declined in the sward.     

Aphid acceptance of Hordeum genotypes is affected by plant volatile exposure and is correlated with aphid growth

Nineteen genotypes of Hordeum vulgare (L.) were screened and characterized as partially resistant or susceptible regarding growth of the bird cherry—oat aphid (Rhopalosiphum padi L.). In a separate test, these same genotypes were treated with volatiles from undamaged plants of barley cultivar Alva. As a result of this treatment, aphid host acceptance (AHA) was significantly affected in seven genotypes, and the magnitude of the effect was positively correlated with aphid growth (AG) in the independent resistance screening test. Changes in AHA induced by volatiles from the same genotype as the volatile receiver were also positively correlated with AG. All the 19 genotypes were also tested as inducers with cultivar Kara as the receiver of volatiles. Five genotypes induced significant reductions in AHA of Kara. The results show that aphids are able to detect changes in responding plants induced by volatiles from another plant. Plant volatile interactions may thus be a component of induced resistance to aphids. These interactions could influence the results of experiments used to select for insect-resistant plants in plant breeding programmes, where normally plant genotypes are mixed at testing.     

The potential for breeding an improved lucerne-rhizobium symbiosis. 1. Assessment of genetic variation

Summary Six cultivars of lucerne (Medicago sativa L.) were grown in all possible combinations with eight strains of Rhizobium meliloti in order to assess genetic variation in symbiotic nitrogen fixation. Host genotype and Rhizobium genotype effects on nitrogen fixation were respectively 4.8% and 21.0% of total phenotypic variance. Genetic variation due to host cultivar × Rhizobium strain interactions accounted for a further 6.0% of phenotypic variance. The results indicated low heritability of general symbiotic effectiveness in the host with interaction effects being large enough to suggest that plant performance may be unpredictable with populations of R. meliloti in field soils. Joint regression analysis showed that about 50% of the interactive variation could be explained by generalized differences in the sensitivity of cultivars to alterations in the genotype of Rhizobium strains. One cultivar, Siriver, was relatively insentitive to changes in the Rhizobium genotype whilst still maintaining high average yield. The implications of the results for lucerne breeding are discussed.     

Heritability of NaCl tolerance at the seedling stage in seven grass species

Summary Narrow sense heritabilities of tolerance to NaCl were estimated in seven grass species using seedling root length after three weeks growth in solution culture containing NaCl. Heritability estimates were obtained from i) female parent-progeny regression, and from ii) selection response and selection differential (realized heritability).Estimates from parent-progeny regression; and realized heritability respectively for each species were as follows Lolium perence L. 0.44, 0.39; Dactylis glomerata L. 0.32, 0.24; Agrostis stolonifera L. 0.28, 0.27; A. castellana L. 0.26; 0.31; Holcus lanatus L. 0.19, 0.23; Festuca rubra L. 0.44, 0.36; Pucinnellia distans (L.) Parl. 0.72, 0.77. These data suggest that in all species except perhaps H. lanatus, improvement in seedling NaCl tolerance could be obtained through further selection and breeding.