Breeding perennial ryegrass for agriculture

Summary Perennial ryegrass has become the most widely sown perennial forage grass in temperate regions due to its combination of high digestibility and tolerance of grazing. The primary objectives in breeding for agricultural use are to improve total and seasonal dry matter production over a range of fertiliser inputs, digestibility, persistency, freezing tolerance and drought or heat tolerance. Adequate seed production and resistance to a wide range of diseases and pests is also necessary. Improvements in productivity and persistency so far have been achieved mainly by hybridisation and recurrent selection using the polycross method and by the use of polyploidy. Further improvements in freezing tolerance, drought and heat tolerance and, for some localities, resistance to pests and snow moulds are required to extend the geographic range of the species. Modern cultivars show little improvement in leaf or stem digestibility but a high magnesium variety is now available. Perennial ryegrass breeding is at an early stage and prospects for further progress appear good.     

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.     

Aneuploids of perennial ryegrass (Lolium perenne)

Summary Aneuploid plants of perennial ryegrass (Lolium perenne L.) with 2n=15 to 30 chromosomes were obtained by crossing a near-triploid (2n=3x+1=22) with a diploid or on open-pollination with diploids and tetraploids. Aneuploids occurred with a frequency of 83% in near triploid × diploid progeny and 92% on open-pollination with diploid and tetraploid plants. Aneuploid plants with 15 to 18 chromosomes resembled diploids in morphology and those with 19 to 30 chromosomes were akin to tetraploids. Meiotic studies suggested that most aneuploid plants resulted from transmission of aneuploid egg cells (n=8 to 23). Aneuploid plants with 2n=27 to 30 chromosomes in the progeny of 22×14 cross originated from unreduced egg cells. Plants with 19 to 21 chromosomes were recovered only by immature seed culture. Aneuploid plants with 26 to 30 chromosomes and triploids (2n=21) had higher pollen fertility and bigger seeds than plants with 15 to 22 chromosomes.     

Mitochondrial DNA analysis reveals cytoplasmic variation within a single cultivar of perennial ryegrass (Lolium perenne L.)

Summary The extent of mitochondrial DNA restriction fragment length polymorphisms among individual plant samples of perennial ryegrass was determined. A total of 72 plants from three cultivars Yorktown II, S23 and Riikka were surveyed using three restriction enzymes (BamHI,EcoRI andHindIII) and three mitochondrial gene probes (coxI, coxIII andnad9). Polymorphisms were noted within each of the two cultivars Yorktown II and S23, whereas in Riikka no variation was detected. It seems most likely that the mitochondrial genome diversity within the same cultivar has resulted from non-homogeneous ancestor cytoplasms. The hybridization-based assay employed is simple to perform, gives unambiguous results, and may thus be used in mass screening of perennial ryegrass populations for breeding purpose.     

Testing perennial ryegrass (Lolium perenne L.) as spaced plants in swards

Summary A method of testing spaced plants in swards of another grass crop is described. The procedure provides information on sward performance at each stage of the breeding scheme. A large number of entries may be included, together with whatever cutting frequency.When used for perennial ryegrass, the method favours the selection of productive and persistent plants.     

A comprehensive multisite recurrent selection strategy in perennial ryegrass

Summary As the seed market for forage grasses in France is rather limited, proposed varieties should have a broad adaptation to a range of different environment. Therefore a comprehensive strategy of multisite recurrent selection has been devised for perennial reyegrass. It is based on an experimental cooperative network between National Institute for Agronomic Research (INRA) and private companies. A pragmatic approach has been retained that gives a compromise between theoretical and practical considerations. The method is illustrated by results from a breeding population consisting of 33 half-sib families derived from a local germplasm. Family selection with an intensity of 30% was applied to yield and persistence data from sward plot trials. The strategy was based on a multisite index where trait x location combinations are considered as different traits and given equal economic weights. Improvements of 10–15% were expected for summer-autumn production and persistence. For spaced plant nursery traits, a subset of locations was chosen according to a multiplicative model decomposition of G x E interaction, where a 10% mass selection was practised. This leads to interesting progress on crown rust tolerance, growth scores, leafiness and persistence. Moreover, the expected indirect responses from selection on spaced plant traits to sward traits and vice-versa are almost all favourable. As significant improvements are expected for most traits in all locations, this programme should allow to increase the general adaptation of ryegrass varieties for France.     

A database for a more efficient perennial ryegrass breeding programme

Summary The development and use of a computer database specifically designed to be used to store data from a plant breeding programme on perennial ryegrass (Lolium perenne) is described. Setting up a database requires data analysis in order to establish a conceptual dat model and a conceptual processing data. Every model has been written with MERISE method. With the models, a database has been built using ORACLE relational database management system. It is tailored to meet our own requirements which may be common to other database users.     

Breeding two base populations of perennial ryegrass

Summary A program designed to develop two base populations of perennial ryegrass was initiated in 1985 in Galicia. Multitrait mass selection by a base index was practised in two genetic resources groups of wild Galician populations. Plants selected were intercrossed and the two resulting base populations (F and G) were evaluated in two locations at the spaced plant level and in plot trials with the aim of estimating the genetic parameters and the possibilities for future breeding. The two populations did not show significant differences from commercial cultivars. Production characters had low heritabilities (0.01–0.32) in both locations. Seasonal yields had medium to high family heritabilities (0.36–0.73). For the economic traits chosen, the expected genetic advances were important in both populations for summer and autumn growth in Mabegondo and spring and summer-autumn dry matter yield (1^st and 3^rd year) in Puebla de Brollón. Expected genetic advance for early spring growth was 0.62 in population F. In the autumn of 1991 equal numbers of plants, selected at each location by combined family and mass selection from each of these populations, were planted in two polycrosses with the aim of improving the genetic basis and adaptation of these populations.     

Breakdown of self-incompatibility in perennial ryegrass at high temperature and its uses in breeding

Summary The maximum temperature (34°C) at which perennial ryegrass plants from one breeding population successfully shed pollen was determined and the seed set of ten clones taken from the same population and self-pollinated both in an unheated glasshouse and at a constant 34°C during anthesis was compared. Seed set by selfing of all ten clones was markedly increased by heat treatment, from a mean of 2.3% to 30.7%. Heat treatment of anthers, but not stigmas, greatly enhanced pollen tube growth on stigmas from the same genotype. Germination of progeny was not affected by the heat treatment. Heat treatment of ten clones from a breeding population with contrasting morphology and growth rate was not so successful. Only one clone shed pollen freely at 34°C and showed a marked improvement in subsequent seed set. The advantages to the breeder of routinely producing full-sib families of elite perennial ryegrass parents by selfing are outlined.     

Molecular characterization of genetic diversity in European germplasm of perennial ryegrass

Summary Perennial ryegrass (Lolium perenne L.) is the most important grass species for temperate grassland agriculture. The level and distribution of genetic variation in gene bank ecotype collections is still largely unknown but of great interest for the planning of breeding programs. The objectives of this study were to (i) assess the molecular diversity of Polish ecotypes of perennial ryegrass, and (ii) compare the relationship between this group and German ecotypes and European cultivars investigated previously. A total number of 166 polymorphic marker bands were detected among the 171 individual plants of the 9 Polish ecotypes. In a joint analysis with 9 Polish and 22 German ecotypes, and 22 European cultivars 172 polymorphic RAPD markers could be found. Genetic distance among the Polish ecotypes ranged from 0.31 to 0.51, while for all 53 populations a broader range was detected (0.25–0.67). An analysis of molecular variance (AMOVA) revealed a much larger variation within populations (71%) than among them (29%). The Polish ecotypes contained the highest within population variation (74%). The largest among group difference (15%) was found between the Polish ecotypes versus all other accessions. We conclude that the Polish ecotypes represent a valuable genetic resource for enlarging the genetic variation in the West European germplasm pool of perennial ryegrass.     

Analysis of competitive ability among genotypes of perennial ryegrass. I. Effect upon survival

Summary An analysis of the effects of competition upon the survival of three morphologically distinct genotypes of perennial ryegrass (Lolium perenne) in monocultures and the corresponding binary mixtures (duocultures), indicated that significant differences occurred between them in respect both of the pressures which they exerted and in their capacity to respond to these pressures. Response depended in this experiment upon cutting regime. This dependency is used to explain the rapid elimination of one of the genotypes from most of its duocultures under an infrequent but not under a frequent cutting regime. The implications of these results for survival in mixed pastures and for the ingress of weeds into such pastures are discussed briefly.     

Analysis of competitive ability among genotypes of perennial ryegrass. II. Effect upon dry matter production

Summary An analysis of the effects of competition upon the dry matter production per plant (after square root transformation) revealed no significant differences between the two genotypes of perennial ryegrass (Lolium perenne) used as regards the pressures which they exerted upon each other. Their response to these pressures differed significantly however. Using these estimates of the pressure and response parameters it was shown that the impact of intra-genotypic competition upon these two genotypes was considerably greater than that of inter-genotypic competition. In general this will be true whenever the pressure and response parameters take the same sign. Intensity of competition increased linearly with vigour of growth in both cutting regimes, though changes in vigour had a significantly greater effect upon the intensity of competition under frequent cutting.     

Assessment of perennial ryegrass (Lolium perenne L.) for breeding. II. Components of winter hardiness

Summary When breeding for good winter hardiness a broad range of adaptations must be considered. Such a multitrait approach requires some understanding of physiological and genetical relationships between relevant traits.Fourteen winter related characters were measured on field grown plants from 86 perennial ryegrass accessions originating from a wide range of climates. Genetic components of variance and covariance were extracted from the data and a principal components analysis carried out. The first three independent components accounted for 80% of the variance represented by the genetic correlations and were broadly identified as winter growth, winter hardiness and early spring activity factors. None of the accessions exhibited a combination of extreme winter growth and winter hardiness although several with excellent winter hardiness showed better than average winter growth. Amounts of water soluble carbohydrate maintained through the winter appeared important for good survival.It was concluded that a selection index approach may prove useful in future breeding programmes as further information concerning physiological relationships becomes available.     

Assessment of perennial ryegrass (Lolium perenne L.) for breeding. I Freezing tolerance

Summary Assessments of freezing tolerance, estimated in terms of temperature at which 50% mortality occurs (=LT₅₀), were obtained for 86 Lolium perenne accessions using a glycol bath test on naturally hardened field plants. Significant correlations were obtained between LT₅₀ and both heading dates of accessions and minimum temperature of the coldest month near their sites of origin. The effects of temperature and reproductive development on the hardening/dehardening of plants and their growth after freezing are discussed.     

Isozyme and quantitative traits polymorphisms in European provenances of perennial ryegrass (Lolium perenne L)

Twenty populations of Lolium perenne originating from a range of habitats in Europe were compared for isoenzyme polymorphisms and agronomically important quantitative traits in order to establish relationships of the levels of diversity with the origin of each population and to assess their suitability to be included in the European core collection of Lolium germplasm. Forty genotypes from each of the twenty populations and each genotype represented by three clonal propagules, were field planted in a fully randomized spaced plant design and fifteen quantitative characters including yield, persistency, reproductive and disease resistence characters were evaluated over a period of two years. Seven putative isozyme loci were assayed to compare the allozyme divergence of populations. The results of the isozyme survey indicate that 71–100% of the loci were polymorphic, 2.3–3.0 alleles/locus and the gene diversity was varying from 0.234–0.410. Of the total allelic diversity 94% remained within populations (H_s) whilst only 6% was distributed among populations (D_st). The differences between populations were determined on the basis of allele frequencies and multivariate analyses of quantitative characters. Populations significantly differed in their allele frequencies at all loci analysed. Random mating was predominant in all populations at most of the loci. The study revealed that the German accession BA 10998 was clearly distinct from the rest both in quantitative characters and allele frequencies. German accession BA 11015 with the lowest gene diversity showed the highest genetic variation for quantitative characters. However, no strict relationship was found between the genetic distance and the geograpical distribution of the populations. Among the quantitative characters, flowering time showed a strong relationship with the type of management practised at the collection site than their place of origin. This revised version was published online in July 2006 with corrections to the Cover Date.     

Components of resistance to ryegrass mosaic virus in a clone of Lolium perenne and their strain-specificity

Summary The extreme resistance to ryegrass mosaic virus (RMV) of a clone of Lolium perenne L. was due to a combination of two distinct types of resistance: resistance to infection and resistance to multiplication and movement of virus within the plant. Resistance to infection was quantitatively inherited and highly effective against three strains of RMV, while resistance to multiplication and movement was controlled by two complementary recessive genes and was effective against only two of the three RMV strains.     

Association of agronomic traits with isozyme loci in perennial ryegrass (Lolium perenne L.)

Summary Inbred lines possessing different genotypes at three isozyme loci, PGI-2, GOT-3 and ACP-1, were produced by two generations of selfing from two contrasting cultivars of perennial ryegrass. Four separate F₂ populations were obtained by selfing or crossing F₁ plants from crosses between cultivar inbred lines. F₂ plants were scored for genotype at the three isozyme loci and for eight agronomic traits. The three isozyme loci showed independent segregation confirming that they belonged to separate linkage groups. A consistent association was observed between water soluble carbohydrate content and genotype at the PGI-2 locus. Some association was also detected between genotype at the ACP-1 locus and heading date. The relevance of these observations to ryegrass breeding programmes is discussed.     

Spaced-plant traits related to seed yield in plots of perennial ryegrass (Lolium perenne L.)

Summary Seed yield in perennial ryegrass is low and selection criteria for high seed production are lacking. Indirect selection in spaced plants would be efficient, but spaced-plant traits need to be identified that correlate with seed yield in drilled plots. Spaced plants were studied of nine perennial ryegrass cultivars with a different seed production when grown in plots. Genotypic variation within cultivars for seed yield components and other traits was assessed in four environments in replicated clonal experiments. Extensive genotypic variation for most traits was present within each cultivar. Based on cultivar means of 25 or 50 genotypes, significant differences among cultivars were found for most traits. Cultivar-year interactions were found for most traits, but no significant cultivar-location interactions were generally found. Spaced-plant traits in general showed poor correlation to corresponding traits in drilled plots. Cultivars with contrasting seed yield in plots could not be distinguished on the basis of their spaced-plant traits and therefore no criteria for indirect selection for seed production in drilled plots could be identified in spaced plants.     

A relationship between heading date and in vitro tillering propensity in perennial ryegrass (Lolium perenne L.)

Summary In vitro tillers of over 1000 genotypes from nine contrasting Lolium perenne cultivars were cultured on various media containing the synthetic cytokinin, 6-benzylaminopurine (BAP), in order to determine the effect on tillering. The cultures were assessed after seven weeks for survival, tiller production, size and vigour. Added auxins 2,4-dichlorophenoxyacetic acid (2,4-D) and -napthaleneacetic acid (NAA) had no consistent beneficial effects.A significant relationship was found between in vitro tiller production and cultivar heading date. Genotypes of early-heading, less persistent cultivars produced more tillers than genotypes from late heading more persistent cultivars. This was consistent with the generally higher in vitro tiller production in the biennial species Lolium multiflorum cultured on similar media.