The inheritance of floral characters in white clover (Trifolium repens)

A diallel cross was carried out among seven varieties of white clover to investigate the inheritance of some of those characters which affect potential seed yield. Peduncle length and number of seeds/floret have low narrow-sense heritabilities, while peduncle length displays the added complications of heterozygosity and non-allelic interactions. Consequently, improvements in these characters will be difficult. Seed number m^-2 is a composite character which apparently exhibits heterozygosity. Higher seed yields will therefore be obtained by selection for its component characters, particularly floret number and floral density, both of which are apparently inherited in a comparatively straightforward manner. These results are discussed in the context of improving the potential seed yield of white clover.     

Nodulation of white clover (Trifolium repens) in the absence ofRhizobium

Summary Spontaneous nodules developed on the roots of white clover (Trifolium repens cv. Ladino) in the absence ofRhizobium. A small subpopulation of uninoculated clover plants (0.2%) exhibited white, single-to-multilobed elongated structures on their root systems when grown without fixed nitrogen. Clonal propagation using aseptic stolons confirmed the genetic stability of the observation. Few if any viable bacteria of unknown origin were recovered from surfacesterilized structures. Nodule contents were incapable of eliciting nodulation. Histological observations showed that these structures possessed all the characteristic features of indeterminate nodules, such as active meristem, cortex, endodermal layer, vascular strands, and a central zone with parenchyma cells. Infection threads, intercellular or intracellular bacteria were absent. Instead, numerous starch grains were observed in the central zone, a feature absent in normal nitrogen-fixing nodules. Our observation broadens the concept of spontaneous nodulation, believed to be restricted to alfalfa (Medicago sativa), to other legumes, and suggests a degree of generality among indeterminately nodulated legumes displaying natural heterozygosity.     

Ovule abortion in white clover (Trifolium repens L.)

Plants of white clover ( Trifolium repens L.) cv. Olwen were grown in an open glasshouse maintained at a mean temperature of 20^oC and ovule growth and seed production measured. Differences in the rate of growth of ovules within ovaries were observed as early as 2 days after pollination. Ovules reached a maximum size after 8 days with the smallest only half the size of the largest. After 8 days, the smallest ovules became flaccid and shrivelled. Ovule position within the ovary had little effect on the frequency of seed set and although there was an apparently higher probability that central ovules produced a seed than those nearer the peduncle or style this was not statistically significant. Inflorescence position and floret position on the inflorescence had a significant effect on the number of seeds per floret and seed weight; the first formed inflorescences and the first florets to be pollinated on each inflorescence had more seeds per floret and heavier seeds and fewer florets with no seed than later pollinated florets. There were also differences between florets within the same whorl. The role of a number of factors which may influence floret site utilisation are discussed.     

The effect of canopy filtered light on the growth of white clover Trifolium repens

Summary Plants of white clover (Trifolium repens) were grown under canopies of clover leaves floating on shallow glass tanks of water and their growth was compared with that of plants under canopies of black polythene leaves. The experimental design allowed the growth of the clover plants in canopy filtered light to be compared with that in unfiltered light at different intensities of Photosynthetically Active Radiation (P.A.R.). The effect of canopy filtered light was to exaggerate the effects of reduced P.A.R. especially in promoting petiole extension and inhibiting stolon branching. Two clones of white clover differed in their responses to P.A.R. and to light quality and there were significant interactions between the effects of the intensity of P.A.R. and type of shade. It is argued that it may be important for both ecologists and plant breeders to recognise the role of radiation quality in regulating the dynamics of pasture.     

Inheritance of phosphorus response in white clover (Trifolium repens L.)

Genotypes of white clover that exhibited divergent responses to P were identified in a glasshouse pot trial. Six high P-responding genotypes were selected from previously identified high P-responding cultivars and 5 low P-responding genotypes were selected from previously identified low P-responding cultivars. These were crossed in a full diallel design without selfing and reciprocals were kept separate. The P-response of progeny lines was compared with parents. High P-response was dominant over low P-response with progeny from crosses between high and low P-response genotypes being similar to the high P-response parent. Reciprocal effects were not significant. The general combining abilities of high P-response genotypes were generally greater than that of the low P-response genotypes, although there were significant specific combining abilities. Narrow sense heritabilities for P response were moderate, 0.46 based on the linear coefficient and 0.33 based on the quadratic coefficient of the fitted response curves.The mode of inheritance, feasibility of manipulating differences in P response by breeding and future directions of this work are discussed.Deceased.Deceased.     

Ecotypic variation of in vitro plantlet formation in white clover (Trifolium repens)

Sixteen Trifolium repens (white clover) genotypes of diverse geographic origin were tested for differences in organogenic potential of petiole and root derived explants. Significant differences were observed between ecotypes and explant types with respect to callus initiation and plantlet regenerability. The observed differences are attributed to existing genotypic variability as well as origin of explant tissue.     

Response to soil aluminium of two white clover (Trifolium repens L.) genotypes

An aluminium (Al) tolerant genotype of white clover was compared with an Al susceptible genotype in artificial soil profiles in which exchangeable Al increased with depth. The tolerant genotype had a greater proportion of its root mass deeper in the soil than the susceptible genotype. Nitrogenase activity showed a similar pattern. Shoot Al concentration did not vary between the genotypes but root Al in the susceptible line was twice that in the tolerant genotype. Plant potassium content in the susceptible line was relatively less, probably in response to higher aluminium content.     

Detection of selection in populations of white clover (Trifolium repens L.)

Seed populations of white clover polymorphic for the presence/absence of both ovariogenic glucosides and the hydrolysing enzyme linamarase, were introduced into three natural populations. Over the first six months of life a significant increase in the frequency of linamarase containing individuals occurred. Estimated selection coefficients against plants lacking linamarase were in the region of 0.3. This result may have been due to selection at the enzyme locus alone, or to selection favouring cyanogenic individuals which possess both cyanogenic glucosides and enzyme.     

Spectrophotometer-aided evaluation of cyanogenic potential in white clover (Trifolium repens L.)

In comparison with ordinary methods of colorimetric evaluation of cyanogenic potential based on visual evaluation of the alkaline picrate reaction, a spectrophotometer-aided method could be more accurate (since it determines the exact amount of hydrogen cyanide released by the plant material), and less time-consuming as it can be performed on bulk material rather than on a number of individual plants. Ten white clover populations were evaluated by a spectrophotometer-aided method and by two visual evaluation criteria. All methods indicated the presence of large variation between populations. Visual methods gave almost identical results and allowed only for the distinction between cyanogenic and substantially acyanogenic populations. The results were only moderately consistent with those obtained by the spectrophotometer-aided method, which could detect the presence of variation also between cyanogenic populations. The effects of various incubation times (from 4 to 48 h) and of the addition of β-glucosidase on hydrogen cyanide release were also investigated. Comparable results for ranking of populations could be obtained over a range of incubation times, but at least 24 h were needed for a reliable estimation of the hydrogen cyanide produced by plants. The addition of enzyme did not increase the released cyanide. The effect of season and/or conditions of evaluation was marked on mean cyanogenic potential but limited on ranking of populations. Copyright © 2000 John Wiley & Sons, Ltd.     

Effects of temperature and photoperiod on vegetative growth of white clover (Trifolium repens) ecotypes

Effects of photoperiod and temperature on vegetative growth of seedling populations and clones of white clover ( Trifolium repens L.) originating from various latitudes (58°48'–69°54'N) and altitudes (up to 1100 m above sea level) were studied in a phytotron. Dry matter production, stolon elongation, petiole elongation and leaf lamina size were enhanced by long photoperiod. The requirement for long photoperiod increased with decreasing temperature. At 6°C the maximum growth was recorded under 24-h daylength. At 18°C already an increase in photoperiod from 12 to 15 h significantly enhanced growth, and maximum growth was obtained at 21-h photoperiod. The studied populations responded similarly to daylength, and the results did not indicate photoperiodic ecotypes in the material. The southernmost clones and populations generally had the highest dry matter production at all temperature treatments (6–18°C). Variation between clones within one location was, however, significant, and rapidly growing clones were found also in high-latitude locations. Dry matter production was poorly correlated with the morphological characters observed, but in some cases significant correlation with leaf lamina size was found.     

Effects of temperature and photoperiod on frost resistance of white clover (Trifolium repens) ecotypes

Effects of photoperiod and temperature on frost resistance of seedling populations and clones of white clover (Trifolium repens L.) originating from various latitudes (58°48'–69°54'N) and altitudes (up to 1100 m above sea level) were studied in a phytotron. Low-temperature-induced cold acclimation in all populations was significantly stimulated by short photoperiod and low temperature. The highest levels of frost resistance were found in the northernmost seedling populations and in clones from northern coastal locations.     

Aridity shapes cyanogenesis cline evolution in white clover (Trifolium repens L.)

Adaptive differentiation between populations is often proposed to be the product of multiple interacting selective pressures, although empirical support for this is scarce. In white clover, populations show adaptive differentiation in frequencies of cyanogenesis, the ability to produce hydrogen cyanide after tissue damage. This polymorphism arises through independently segregating polymorphisms for the presence/absence of two required cyanogenic components, cyanogenic glucosides and their hydrolysing enzyme. White clover populations worldwide have evolved a series of recurrent, climate‐associated clines, with higher frequencies of cyanogenic plants in warmer locations. These clines have traditionally been hypothesized to reflect a fitness trade‐off between chemical defence in herbivore‐rich areas (warmer climates) and energetic costs of producing cyanogenic components in areas of low herbivore pressure (cooler climates). Recent observational studies suggest that cyanogenic components may also be beneficial in water‐stressed environments. We investigated fitness trade‐offs associated with temperature‐induced water stress in the cyanogenesis system using manipulative experiments in growth chambers and population surveys across a longitudinal precipitation gradient in the central United States. We find that plants producing cyanogenic glucosides have higher relative fitness in treatments simulating a moderate, persistent drought stress. In water‐neutral treatments, there are energetic costs to producing cyanogenic components, but only in treatments with nutrient stress. These fitness trade‐offs are consistent with cyanogenesis frequencies in natural populations, where we find clinal variation in the proportion of plants producing cyanogenic glucosides along the precipitation gradient. These results suggest that multiple selective pressures interact to maintain this adaptive polymorphism and that modelling adaptation will require knowledge of environment‐specific fitness effects.     

Symbiotic growth of indigenons white clover (Trifolium repens) with local Rhizobium leguminosarum biovar trifolii

To study a possible adaptation of the symbiosis between white clover (Trifolium repens L.) and Rhizobium leguminosarum biovar trifolii with regard to light and temperature at northern latitudes, local seed populations of white clover and isolates of R. leguminosarum biovar trifolii from 3 different latitudes in Norway, 58°48'N, 67°20'N and 69°22'N, were used. The commercial cultivar Undrom was used as a reference plant. The experiments were done at 18 and 9°C under controlled conditions in a phytotron during the natural growing season at 69° 39'N. Growth of the plants was evaluated by number and size of leaves, dry matter production and total N-content. At 18°C the white clover plants were harvested twice while at 9°C there was only one growth period. The results from first harvest at 18°C and total growth at 9°C, showed that white clover populations from northern Norway had a lower growth potential than the population from the south and cv. Undrom. This difference was not apparent in the second growth period at 18°C. Growth of the plants from seeds to first harvest was enhanced by mineral nitrogen compared to plants dependent on Rhizobium only. However, after a second growth period dry weight and total nitrogen content of the plants with nitrogen fixation were comparable to the plants receiving mineral nitrogen. Statistical analysis showed that the most important factor for the variation in dry matter production was the plant population. Within the populations at 9°C and at first harvest at 18°C, there were no significant differences in dry matter production with different Rhizobium inoculum. In the second growth period at 18°C, different inoculum gave significantly different amount of dry matter within a population. The results showed a significant interaction between plant population and Rhizobium inoculum, and the results indicated that plants from the north gave higher yield when nodulated by Rhizobium from the north than from the south.     

Multivariate analysis of intraspecific responses to UV-B radiation in white clover (Trifolium repens L.)

White clover (Trifolium repens L.) is experiencing increased levels of ultraviolet‐B (UV‐B) radiation in temperate pastures due to the depletion of the stratospheric ozone layer. Based on 17 morphological, morphogenetic and physiological attributes, this study analysed the consequences of enhanced UV‐B on 26 white clover populations using principal components analysis (PCA). After 18 d of exposure to 13·3 kJ m ^{? 2} d ^{? 1} UV‐B in controlled environments, UV‐B significantly decreased above‐ground and below‐ground plant growth attributes, epidermal cell surface area and maximum quantum efficiency of photosystem II photochemistry (F_v/F_m). Aspects of cell division and cell expansion both were negatively affected by UV‐B. Stomatal density, specific leaf mass, root‐to‐shoot ratio and levels of UV‐B‐absorbing compounds increased in response to UV‐B. In the multivariate analysis, the main dimension of UV‐B sensitivity was characterized by changes in plant growth attributes. Alterations in partitioning within and between plant organs constituted a secondary tier of UV‐B responsiveness. Plant characteristics related to UV‐B tolerance included lower growth rate, smaller epidermal cell surface area and higher UV‐B‐induced levels of UV‐B‐absorbing compounds. The results suggest overall UV‐B tolerance for slower‐growing populations from less productive habitats with higher natural UV‐B irradiance.     

Ancestors of white clover ( Trifolium repens L.), as revealed by isozyme polymorphisms

Isozymes were used to study the putative ancestors of white clover ( Trifolium repens L.). Ten enzymes were examined, and 18 loci were resolved via starch-gel electrophoresis for accessions representing T. repens, Trifolium isthmocarpum, Trifolium nigrescens, Trifolium occidentale and Trifolium uniflorum, in addition to two more distantly related species, Trifolium alpinum and Trifolium purseglovei. Nei's genetic identities indicate that T. uniflorum and T. nigrescens are the closest relatives of T. repens. The isozyme data thus support a hypothesis that the two genomes of the tetraploid T. repens could have been derived from hybridization between T. nigrescens and T. uniflorum. This conclusion is further supported by shared alleles between T. repens, T. nigrescens and T. uniflorum. However, the origin of T. repens is somewhat obscured by the presence of shared alleles between T. repens and both T. occidentale and T. isthmocarpum, suggesting that introgression of genes from the latter two species into T. repens may also have taken place. High values of genetic identity are shared between T. occidentale with T. nigrescens and T. uniflorum, also indicative of introgression. Alternatively the presence of shared alleles among the five species may reflect their recent common ancestry.