I. Rhodes

The relationship between productivity and some components of canopy structure in ryegrass ( Lolium spp.): II. Yield, canopy structure and light interception

Productivity and percentage conversion of light energy were measured in ( a )clonallypropagated swards of populations of genotypes selected from the variety S. 321 forcontrasting combinations of leaf rigidity and tiller angle, and ( b ) families from assortativecrosses between genotypes of this variety of differing canopy structure. A further experiment was carried out to examine the canopy structure and pattern of light interception of populations, families and genotypes of contrasting productivity. Under infrequent cutting (28–34 days) the tiller population selected for erect tillers and rigid leaves was more productive than those selected for prostrate tillers and rigid leaves, and prostrate tillers and lax leaves. Under frequent cutting (14–17 days), however, the populations with prostrate tillers were most productive. Considerable differences existed in the productivity of the S. 321 families, and their relative performance differed under the two cutting frequencies. Under infrequent cutting the highest yielding family was 17% more productive than the base population, whilst under frequent cutting the yield of the most productive family was 19% greater than that of the base population. The heritability of sward yield was 0·64 under infrequent cutting and 0·86 under frequent cutting. The most productive genotypes, populations and families under infrequent cutting had the highest leaf-area index (LAI) at complete light interception and the lowest extinction coefficients for visible radiation ( K vis ). By contrast, under frequent cutting the most productive types had high extinction coefficients and large LAI in the basal layers of the canopy. The physiological basis of differences in productivity is discussed in relation to canopy characters whioh may be used as selection criteria for increasing the efficiency of light utilization in herbage grasses.

Stolon characteristics related to winter survival in white clover

Changes in the stolon carbohydrate contents (water-soluble and total non-structural) of four white clover populations were measured at the beginning and end of winter. Three of the populations were of Swiss origin ― two of these were from high-altitude pastures, and the other from a valley location. Grasslands Huia was measured as a control variety. Levels of both types of carbohydrate declined during the winter in all populations. The Swiss populations from high altitudes contained the highest levels of both carbohydrate types. An artificial freezing test was carried out on stolon segments collected from a range of clover populations (including the two high-altitude Swiss clovers) growing in field plots in January, and values of «lethal dose 50» (LD 50 ) were calculated. The two Swiss populations had the lowest LD 50 values, indicating a greater intrinsic tolerance to freezing in those plants than in the other populations (of lowland temperate origin). One of the Swiss populations, Ac3785, was grown under two temperature regimes and short daylength to determine whether its tolerance to freezing increased with time. Significant increases in tolerance occurred after 8 days at 2°C, but plants grown at 8°C showed no change in tolerance even after 30 days. The results suggest that (i) stolon carbohydrate content is an important factor in the overwintering of white clover, (ii) there is a considerable amount of genetic variation in cold tolerance within white clover, (iii) artificial freezing tests can provide a method of predicting the survival of clover stolons during winter and (iv) the cold hardiness of white clover increases with time spent at temperatures near 0°C under short daylengths

Breeding white clover for tolerance to low temperature and grazing stress

SummaryLow 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.

Growth and reproductive characteristics in backcross hybrids derived from Trifolium repens L. × T. nigrescens Viv. interspecific crosses

Interspecific hybridisation with the close relative, Trifolium nigrescens (Ball clover) is a possible strategy to achieve increased reproductive potential of white clover (Trifolium repens). Fertile F1 plants have been used as the basis for two generations of backcrossing to T. repens as the recurrent parent. F1 and backcrossed plants were assessed in both glasshouse and field for a range of morphological traits, including inflorescence production, and the level of water soluble carbohydrates in the stolons. Plants resulting from two generations of backcrossing had an increased allocation of dry matter to inflorescence production in comparison with T. repens. Variation within these plants for agronomic traits (e.g. stolon length, dry weights etc.) suggests that selection for these traits is feasible and in combination with increased inflorescence production offers a potentially valuable approach to germplasm improvement in white clover.

Self-compatibility and heterosis in white clover (Trifolium repens L.)

Inbred lines of white clover (Trifolium repens L.) have been produced utilising the rare self-fertility (Sf) allele. Twenty-two lines of four distinct groups have been maintained through five generations of selfing by single seed descent. Fourteen lines were used in crosses to produce F1s. Both parents and hybrids were analysed for a range of morphological characters. Positive heterosis for dry matter production was observed in half the hybrids. No other trait showed significant heterosis. The degree of heterosis appears to be related to the extent of variation in morphological characters between the parental lines, some combinations of lines yielding heterotic hybrids in all cases, others in none. The majority of the F1 hybrids are superior in terms of dry matter production to eight control varieties.

The relationship between productivity and some components of canopy structure in ryegrass ( Lolium spp.): IV. Canopy characters and their relationship with sward yields in some intra population selections

Studies were carried out to measure the productivity of selections for extreme expression of the various components of canopy structure from within six Lolium populations of contrasting morphology. Considerable variation existed for all canopy characters within each base population and this was reflected in the difference between the high and low selections. In the first harvest year productivity was closely positively correlated with leaf length. Although some significant differences in yield occurred between selections for other characters, no general pattern was evident as with leaf length. Within the base populations differences in yield occurred between selections of similar leaf length, emphasizing the role of other physiological factors in controlling yield. In general, dry-matter production was greater in the first harvest year than in the second harvest year. In the second year the long-leaved selections from L. multiflorum and L. multiflorum × L. perenne continued to produce greater yields than their respective short-leaved selections and base populations, but no such differences were evident in L. perenne S. 23 and S. 23 × Ba 6280. Whilst long-leaved selections from the former group were as persistent, or even more persistent than their base populations, the long-leaved selections from L. perenne S. 23 and S. 23 × Ba 6280 showed a lower persistency. Similarly, whilst erect tiller selections from L. multiflorum and L. multiflorum × L. perenne were as or more persistent than their base populations, the three L. perenne erect tiller selections exhibited reduced persistency. The plant breeding implications of these results are discussed.

Variation in cold tolerance and spring growth among Italian white clover populations

Seven populations collected at different altitudes in northern Italy, two Ladino breeding populations and two control cultivars (AberHerald and Grasslands Huia) of white clover (Trifolium repens L.) were included in a series of experiments analysing: (i) levels of cold tolerance using artificial and field-based methods; (ii) relationships between these measures of cold tolerance; (iii) components of spring yield, various physiological traits, and their relationships with cold tolerance. Rates of seedling and growing point mortality in the populations over winter, assessed in separate field experiments, were closely related (r = 0.85). Grasslands Huia showed the highest death rates, and material originating from high altitudes the lowest. The LT50 value, i.e., the temperature at which 50% of the growing points would die, estimated by an artificial freezing test, was significantly correlated with field-based measures of seedling (r = 0.64) and growing point (r = 0.84) mortality. The existence of these correlations is of potential interest for the development of indirect selection criteria for complex and expensive-to-evaluate traits such as winter survival in field plots. Besides being reliable, in this study the artificial assessment was also sensitive, providing a greater degree of separation of the populations means than field-based measures. Of the several physiological traits (water content,concentrations of water soluble and total non-structural carbohydrate, and water soluble protein content of stolons) measured at a mid-winter sampling date under field conditions, the only character showing significant variation between populations was soluble protein content. There was a slight trend for material with a higher protein content to exhibit greater field-based values of cold tolerance. High altitude populations tended to have low spring yields. The highest spring yield was found in one of the Ladino populations. The study identified two populations which combined, to differing extents, cold tolerance and spring yield characteristics that would be of potential use in breeding for specified agronomic/climatic zones.

Long-term performance of white clover varieties grown with perennial ryegrass under rotational grazing by sheep with different nitrogen applications

Williams, T. A., Evans, D. R., Rhodes, I., Abberton, M. T. (2003). Long-term performance of white clover varieties grown with perennial ryegrass under rotational grazing by sheep with different nitrogen applications. Journal of Agricultural Science, 140, (2), 151-159. Sponsorship: DEFRA

Response to divergent selection for stolon characters in white clover (Trifolium repens)

Stolon death, often caused by grazing or winter-kill, is a major factor determining the survival and persistence of white clover (Trifolium repens L.), the most important forage legume in UK agriculture. Since stolon morphology apparently affects stolon survival, this study was designed to assess the genetic variation for stolon characters within a white clover population from Switzerland and to assess the effects of two generations of selection for stolon characteristics on that population. Bidirectional selection was carried out simultaneously for stolon diameter (as the primary criterion of selection) and total stolon length i.e. the product of the length of the longest stolon and stolon number. Four selection lines were established: (a) plants with thick sparse stolons, (b) plants with thick profuse stolons, (c) plants with thin sparse stolons and (d) plants with thin profuse stolons. Realised heritabilities for stolon diameter, estimated in both directions and over both generations of selection, were found to lie within the range 0.28-0.44; significant shifts in population means for stolon diameter were demonstrated. Selection for thin profuse stolons and for thick sparse stolons was effective, but because of negative correlations between stolon diameter and both stolon length and number, selection for thin sparse stolons or thick profuse stolons was ineffective.

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.