Breeding for resistance to Heterodera in white clover

Summary White clover resistant to cyst nematodes was selected as a contribution to the study of the importance of cyst nematodes in the disappearance of white clover from grassland. Resistance was obtained from different sources viz from clones of the Foundation for Agricultural Plant Breeding selected for persistence, from New Zealand white clover, from the Swedish variety Kiwi and from the Italian variety Lodigiano. Inheritance was found to be most likely determined by more than one gene.The results of tests with three clover populations and seven Dutch sources of nematodes did not show clear differences in biotypes among the eelworms. Biotypes will not so easily appear as with organisms reproducing sexually because reproduction is practically only mitotic parthenogenetic.Resistance to cyst nematodes was not correlated with high glucoside contents.     

Effects of inbreeding on nodal root system morphology and architecture of white clover (<Emphasis Type="Italic">Trifolium repens</Emphasis> L.)

Plants of white clover (Trifolium repens L.) cultivar Crau, a self-fertile Crau genotype, and nine generations of inbred progeny were raised in sand culture in a glasshouse experiment. Digital images of the root systems were made and root morphological characteristics were determined on all the plants. Root architectural parameters were measured on the Crau parent and the S₁, S₄, S₆, and S₉ inbred lines. The clover roots became shorter and thicker with inbreeding but the number of root tips per plant was unchanged. Root architecture (branching pattern) was largely unaffected by inbreeding. It is concluded that inbreeding white clover will lead to shorter, thicker roots, and reduced nutrient uptake efficiency compared with the parent clover. The degree to which these deleterious traits are overcome during the development of F₁ hybrids needs to be determined.     

The inheritance of the cold induction response with respect to floral initiation in white clover (Trifolium repens L.)

Summary The inheritance of the cold induction response in the UK cultivar of white clover Menna, was assessed in controlled environments.Only 50% of the population flowered without prior cold induction. The cold period increased the number of stolons which became reproductive.All families from the crosses between induction and non-induction requiring genotypes produced progeny lacking a cold requirement for flowering.Heritability estimates for the number of flowers per plant, reproductive buds per stolon and stolon number were high.The necessity of eliminating the cold induction requirement in potentially valuable white clover cultivars in relation to seed production at lower latitudes is discussed.     

Recurrent selection programs for white clover (Trifolium repens L.) using self-compatible plants. I. Selection of self-compatible plants and inheritance of a self-compatibility factor

Summary When white clover plants were self-pollinated, 75% of a 143-plant population did not set seed. Plants which did set small numbers of seed probably did so as a result of pseudo-self-compatibility. One highly self-compatible plant was selected from the breeding materials. The S₂ progeny of this plant was autogamous and genetically fixed for morphology and isozyme patterns. Genetic analyses of self-compatibility were conducted as far as the F₃ progeny. It was concluded from these data that self-compatibility was inherited in a simple Mendelian way and that plants homozygous for self-compatibility were obtained. There was no correlation between presence or absence of self-compatibility and growth vigor. The possibility of application of self-compatible plants to recurrent selection procedures is outlined and the utilization of self-compatible plants in white clover breeding programs is discussed.     

Nitrogen Fixation in Hybrids of White Clover (Trifolium repens L.) and Caucasian Clover (Trifolium ambiguum M. Bieb)

Hybrids between white clover (Trifolium repens L.) and Caucasian clover (or Kura clover, Trifolium ambiguum M. Bieb) are a potential route for the improvement of drought tolerance and persistence in white clover. However, to be agronomically viable they must show no significant reduction in their potential for nitrogen fixation relative to white clover. A comparative study of growth rate and nitrogen fixation was carried out in flowing solution culture without a supply of mineral nitrogen to the plants. The two parental species and two generations of backcross hybrids, with white clover as the recurrent parent, were assessed. The growth rate and N content of T. ambiguum were significantly lower than those of the other lines. However, dry matter production, nodule biomass per plant and rates of fixation were similar in second‐generation backcross plants and white clover. The results suggest that the agronomic potential of this novel germplasm is not compromised by limitations with respect to nitrogen fixation.     

Effects of water deficit and plant interaction on morphological growth parameters and yield of white clover (Trifolium repens L.) and ryegrass (Lolium perenne L.) mixtures

The effects of soil water deficit and interspecific plant interaction were studied on the dry matter (DM) yield of white clover and ryegrass and on the morphogenesis of white clover. Plants were grown either: (1) individually (no interaction); or in a mixture of equal plant numbers with either (2) just shoot interaction, or (3) both shoot+root interaction. Plants were subjected to soil water deficits corresponding to no (0 MPa), moderate (−0.5 MPa) or severe (−1.0 MPa) water deficit. Ryegrass had higher above- and below-ground DM yields than white clover. The above- and below-ground DM yield for ryegrass, and the above-ground DM yield, stolon growth and relative growth rate (RGR), and leaf appearance rate (LAR) for white clover decreased as soil water deficit increased. In shoot+root interaction at no and moderate soil water deficit levels, white clover had the highest proportion of above-ground growth in the leaf form (69%) and had, respectively, 11% and 32% more above-ground DM yield than when grown in just shoot interaction; ryegrass had, respectively, 20% and 25% more above-ground DM yield than ryegrass grown in just shoot interaction. In shoot+root interaction at severe soil water deficit, ryegrass had twice as much above-ground DM yield as white clover (3.50 g per plant versus 1.59 g per plant, respectively) and white clover had 60% less above-ground DM yield than when grown in just shoot interaction. In a soil column of restricted depth (30 cm) at no and moderate soil water deficit levels, remarkable increases in shoot biomass yield were observed for both white clover and ryegrass grown in shoot+root interaction. The increased shoot biomass yield of ryegrass can be attributed to benefits from white clover's N fixing ability, whereas the causes for increased white clover biomass yield need to be studied further. However, at severe soil water deficit, ryegrass had a competitive advantage over white clover when grown in shoot+root interaction. This was due to the larger root system of ryegrass and its ability to control transpirational losses more efficiently, thus prolonging its growth period compared with white clover.     

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.     

Breeding white clover for tolerance to low temperature and grazing stress

Summary Low temperature and grazing are the two major stresses limiting white clover yield in mixed swards grown in the marginal areas of the UK. White clover has traditionally been used to improve productivity in such areas but is increasingly being used to reduce input costs in more productive areas. Considerable genetic variation exists in characters associated with cold tolerance, low temperature growth and grazing tolerance. This paper describes recent progress in developing varieties tolerant to three major stresses and which give greater and more reliable production.     

果园种植三叶草对杂草群落及物种多样性的影响

为实现果园杂草生态控制,减少或避免使用化学除草剂。根据生态位先估和生态位竞争原理,设计间种豆科绿肥白三叶草和红三叶草两种杂草生态控制技术,同时设置清耕对照(CK),采用小样方多点取样连续两年调查果园杂草种群数量、生物量、平均株高等指标。结果表明：果园共有杂草13科28属34种;3个处理在4次调查时,对照杂草多度(Pi)高达1.001和1.000,间种白三叶草处理分别为0.755、0.664、1.001和0.945,间种红三叶草处理分别为0.986、0.889、0.982和0.992。种植白三叶处理果园在四次调查时期,杂草物种丰富度(S)和种群多样性(H)均大于对照,均匀度(J)在第一次调查时小于对照,优势集中性指数(C)均小于对照;间种红三叶草处理四次调查杂草总数量均低于对照。本研究表明间种白三叶草能改变果园杂草群落结构,有效控制杂草的生长。     

Quality characteristics of backcross hybrids between Trifolium repens and Trifolium ambiguum

Two generations of novel backcross hybrids between white clover (Trifolium repens L.) and the related species Trifolium ambiguum,with white clover as the recurrent parent, were analysed for forage quality characteristics. Water soluble carbohydrate (WSC) levels, nitrogen concentrations (%N), and dry matter digestibility (DMD) were recorded for the hybrids and parental species grown in deep soil bins with and without a companion grass. Half the experimental material was subjected to moisture stress by being left unwatered through the four-week period when measurements were made. Four such four -week cycles of drought were imposed. The first and second generation backcrosses had the highest values for WSC, but were lowest for N%. There was no difference between the different legume lines (parents and backcrosses) for DMD. No effects of either moisture stress or presence of a grass companion on quality traits were observed. The WSC and %N of the grass component differed significantly depending on which legume line it was grown with. The importance of these results for the potential use of these hybrids in agriculture is discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Introgression of reproductive traits from Trifolium nigrescens increases the seed yield of white clover (T. repens)

Interspecific hybrids between white clover and the annual, profuse flowering ball clover have been developed as a strategy to improve the potential seed yield of white clover. The seed yield and seed yield components of advanced white clover × ball clover hybrids were compared with white clover in replicated field experiments over two harvest years. The backcross 3 hybrid had more reproductive nodes per stolon than the control variety ‘AberHerald’ which was reflected in more inflorescences per unit area but inflorescences with fewer florets and seeds. The hybrid had a greater potential seed yield than the control variety in 2002 but not 2001. Backcross 3 hybrids have also been produced across the range of leaf size categories. Medium and large leaved hybrids produced significantly more inflorescences and a higher potential seed yield than control varieties of comparable leaf size in both harvest years. The small leaved hybrid was not significantly greater than the control variety ‘AberDale’ in either inflorescence production or seed yield.     

紫茎泽兰对两种牧草发芽的化感克异作用

研究了紫茎泽兰地上部分、根系及根际土壤不同浓度提取液对白三叶和多花黑麦草发芽的影响。研究结果表明：紫茎泽兰对白三叶和黑麦草种子萌芽存在化感克异现象，并且随浓度的增加，影响越大；相同条件下，对白三叶的影响强于多花黑麦草；低浓度对白三叶败育的影响较大，但对多花黑麦草几乎没有影响；紫茎泽兰根际土壤对两种牧草种子萌芽无明显的化感克异作用。     

Development of breeding populations from interspecific hybrids between Trifolium repens L. and T. occidentale Coombe

Trifolium occidentale is a diploid wild relative of T. repens (white clover) with adaptation to dry, saline coastal habitats. Transfer of drought and salt‐tolerant adaptive traits to white clover could be potentially valuable if interspecific hybridization can be achieved efficiently and leads to fertile hybrid populations. To achieve hybridization, 4x plants of T. occidentale were generated. Efficient techniques for generation of 4x plants and their identification using dry pollen shape are described. Interspecific 4x F₁ plants were achieved without embryo rescue. F₂ populations and first backcross hybrids to white clover were also efficiently achieved. Although male and female fertility were lower than in white clover, they were adequate to produce large amounts of seed from small numbers of inflorescences. Thus, early generation interspecific hybrid prebreeding populations can be readily developed, opening the way for transfer of traits from T. occidentale to white clover.     

Symbiotic Nitrogen Fixation of White Clover in a Mixed Sward is not Limited by Height of Repeated Cutting

The aim of this study was to investigate whether symbiotic nitrogen fixation in white clover nodules limits nitrogen supply and hence clover growth by repeated defoliation at two cutting heights. Other possible factors governing symbiotic nitrogen fixation in the field were also elucidated. Using ¹⁵N, a 2-year field experiment including white clover ( Trifolium repens L. cv. Ladino) and perennial ryegrass ( Loliumperenne L. cv. Bastion) in monocultures and in mixtures was conducted in Eschikon, Switzerland. The effect of two cutting heights (4 cm and 10 cm above ground level) on the performance of symbiotic nitrogen fixation of white clover in the different sward-types was investigated. After each harvest, the plots were fertilized with 3 g N m^-2(equivalent to 30 kg N ha^-1 cut^-1 or 210 kg N ha^-1 year^-1). In both years, white clover grown in a mixture with grass received a significantly higher percentage of nitrogen from symbiotic fixation compared with clover grown in monoculture. This phenomenon is attributed to the strong competitiveness of ryegrass in soil nitrogen uptake. Consequently, white clover in the clover-ryegrass mixture was more dependent on symbiotic nitrogen fixation than that grown in monoculture. The cutting height did not preferentially influence symbiotic nitrogen fixation, as opposed to the uptake of mineral nitrogen from the soil. From this finding it is suggested that symbiotic nitrogen fixation did not limit the supply of nitrogen to clover and hence its growth. It is proposed that symbiotic nitrogen fixation in white clover is regulated by the demand for nitrogen rather than by the availability of carbohydrate reserves in the stolons. Symbiotic nitrogen fixation should thus be looked upon as an integrated plant growth factor and not as an isolated phenomenon.     

Cytological analysis of plant chromosomes using rapid in situ hybridization

Summary A rapid fluorescent in situ hybridization protocol, which can be completed in one working day, has been utilised to visualize chromosome preparations of barley, chickpea, white clover, millet, rye, sugarcane, wheat, Aegilops spp. and Leymus spp. Both total genomic DNA and cloned nuclear rDNA were used as a probe. This protocol has the advantage of brevity and flexibility over more conventional approaches, and has made fluorescent in situ hybridization a routine cytological procedure.     

Contribution of White Clover Varieties in High-Productivity Systems under Grazing and Cutting

Two medium leaf size varieties of white clover (Trifolium repens L.) were grown in mixture with a range of grass species and varieties in a field experiment conducted over 4 years. The clovers were grown singly or as a blend in swards with a tetraploid perennial ryegrass (Lolium perenne L.), a diploid Italian ryegrass (Lolium multiflorum L.) or tetraploid hybrids between these two species (Lolium x boucheanum Kunth). These grasses were also grown in monoculture. Swards were managed in a regime involving two periods of sheep grazing, two periods of cattle grazing and a silage cut in the same year for 4 years. The objectives of this experiment were 3‐fold: (1) to examine the yield and persistency of white clover in highly productive systems with modern perennial, hybrid and Italian ryegrasses; (2) to compare the performance of white clover varieties grown singly in mixture with grasses with their performance together as a blend, and (3) to examine the persistency of new Italian ryegrass varieties, exemplified by AberComo. The clover content of the sward remained high throughout the experiment, with a mean of 31 % overall. Differences in clover yield between plots were largely attributable to the effects of companion grasses. Total yields were very similar in plots containing different clover varieties and in plots where these varieties had been grown singly or as a blend. Grass yields in mixture were greater than those in monoculture but showed similar relative rankings. The diploid Italian ryegrass variety AberComo showed an unexpectedly high yield in the 4th year of the experiment. The implications of these results for strategies for productive, sustainable agriculture using modern white clover and ryegrass varieties are discussed.     

干旱胁迫对白三叶光合参数日变化的影响

通过白三叶(Trifolium repens cv.Haifa)的盆栽试验,研究了连续干旱对白三叶叶片相对含水量、叶绿素含量和光合参数的影响。结果表明,当植株遭受较长时间干旱胁迫时,白三叶叶片相对含水量显著下降;净光合速率、蒸腾速率下降明显,日变化由双峰曲线转变为单峰曲线;水分利用效率仅在早晨有小幅升高,但仍以下降为主。气孔导度显著降低,而胞间CO2浓度显著升高,说明干旱胁迫下白三叶的光合作用降低的原因主要是非气孔限制的结果。     

Bulked AFLP analysis for the assessment of genetic diversity in white clover (Trifolium repens L.)

The use of bulked leaf samples from individual plants for amplified fragment length polymorphism (AFLP) analysis was evaluated as a tool for assessment of genetic diversity in white clover (Trifolium repens L.). Bulking of leaf samples produced slightly simpler AFLP profiles compared to the combined profiles of individual plants from the same cultivar. Approximately 90% of bands which were present in individual plants were present in bulked samples of the same cultivar. The majority of those absent were rare bands, shared by less than 25% of individual plants. Replicate bulk samples gave almost identical banding patterns, demonstrating the robustness of the bulked AFLP technique. Cluster analysis of AFLP data derived from individual plants resulted in a phenogram similar to that produced from data derived from bulked samples of the same plants. AFLP analysis of bulked samples detected significant amounts of genetic variability among 52 cultivars and accessions with genetic similarity values ranging from 0.42 to 0.92. However, cluster analysis of AFLP data only partially reflected the geographic origin of cultivars and accessions and was not congruent with cluster analysis based on variation for morphophysiological characters. Bulked AFLP analysis provides a powerful tool for rapid assessment of genetic variability in white clover and may also be used for cultivar identification. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of Competition upon the Productivity of White Clover-Perennial Ryegrass Mixtures. Analysis of and Interrelations between Characters

A diallel crossing programme amongst tour contrasting white clover varieties was used to generate material for studying the interrelationships amongst those characters which affect the productivity of white clover- perennial ryegrass mixtures under a cutting-only management regime. Such a regime favoured clover growth. All the clover characters exhibited strong positive correlations with mixture yield. However, this combination of clover characters did not promote growth of the grass companion. For this a low growing variety with a dense stolon network appeared preferable. Path analysis indicated that differences existed between the two companion grass varieties used for this experiment in their upon clover characters.     

Effects of Competition upon the Productivity of White Clover-Perennial Ryegrass Mixtures. Genetic Analysis

A diallel cross amongst four contrasting white clover varieties was used to provide genetic information on those characters which affect the productivity of white clover-perennial ryegrass mixtures when grown under a cutting-only management regime. Generally speaking those characters recorded on the clover families and in mixtures exhibited highly significant additive and non-ad dative genetic effects, though for1 sortie characters the latter could not be ascribed solely to dominance effects. For two of the clover characters, stolon and leaf number, the genetic picture was radically altered in mixtures by the effects of inter-specific competition Pseudo-genetic effects were also detected for the grass characters, bur it was suggested that the two ryegrass varieties used here were exhibiting phenotypic plasticity. As a group the four intra-varietal crosses performed significantly better than the six intra-varietal crosses for all clover and mixture characters. Only one of the inter-varietal crosses had significantly positive specific combining ability effects for most characters and that was between the two long established varieties ‘S. 184’ and ‘S. 100’. The implications of these results for the breeding of white clover varieties for use in mixtures are briefly discussed.