The effect of seed weight on the early growth and nodulation of white clover

The results of growing, in agar culture, nodulated and un-nodulated white clover seedlings from three seed-weight categories, and the subsequent growth of selected plants in soil in pots, are reported. Seed-weight variations greatly influenced plant growth, made substantial differences in nodulation parameters and nitrogen fixation, and contributed appreciably to the observed variability. The relationship of such effects to nitrogen fixation and the relevance to breeding for superior symbiosis are discussed. The growth of plants, as determined by agar culture technique, was not predictive of their subsequent performance in the soil in pots, and restricting seed weight variations did not improve the prediction. The importance of such effects and the relevance of tube culture to breeding work are discussed.     

The effect of wood charcoal on the growth and nodulation of garden peas in pot culture

Summary In a factorial pot experiment on garden peas investigating the effect of growing medium, inoculation, nitrogen fertilisation and charcoal addition, shoot growth was significantly better in John Innes potting compost than in 50/50 peat/sand compost and when receiving nitrogen fertilisation but was depressed by the presence of wood charcoal.In the peat/sand medium, root systems were smaller but carried more and larger nodules. The number of nodules was increased by inoculation but the addition of carbon decreased root size, nodule number and nodule size.     

The influence of water supply on the growth and phosphorus uptake of italian ryegrass and white clover in pot culture

Summary White clover appears to be much more sensitive, in terms of dry-matter production, to variations of soil moisture than Italian ryegrass. The results show that a variation of 5 in the percentage of water (moist-soil basis) on either side of the field-capacity figure can cause a statistically significant effect in the dry weight of Italian ryegrass in pot culture, although a slightly larger percentage may be necessary for white clover to be affected as standard errors in pot culture with this crop tend to be higher.With an adequate supply of soil phosphorus the greater the amount of readily available water the greater the percentage of P₂O₅ in Italian ryegrass; with white clover it rises to a maximum then falls off under very wet Statistically significant differences in percentage P₂O₅ for both crops can be obtained whit those variations in the water supply which caused significant differences in dry weight.     

A comparison of the nodulation of diploid and tetraploid varieties of red clover inoculated with different Rhizobial strains

Summary The effectiveness of three different strains ofRhizobium trifolii on a diploid and a tetraploid variety of red clover (Trifolium pratense) grown in aseptic mineral-agar culture was compared. It was found that with two of the rhizobial strains tested, the tetraploid plants were slower to nodulate than the diploid plants, but that a hundred per cent nodulation was obtained in both cases. With the third strain, however, a sixty-six per cent nodulation of the diploid plants was reduced to an eight per cent nodulation of the tetraploids.In general, over a given period, fewer nodules were formed on the tetraploid plants, but these tended to be larger than on the diploids. The fresh weight of the tetraploid plants always exceeded that of the diploids, but less so in cases of poorly nodulated plants.     

The effect of short term withdrawal of NaCl stress on nodulation of white clover

The number of nodules formed by white clover (Trifolium repens L.) released from NaCl stress for 3 days (137 mol m^-3) at different periods was examined. The NaCl stress-free periods were, 0 to 3 days prior to rhizobial inoculation, 0 to 3, 3 to 6, and 6 to 9 days after rhizobial inoculation. Plants not subjected to NaCl stress at 0 to 3 days after inoculation had 28.7 nodules per plant (74% of control), while plants continuously stressed had 5.2 nodules (13% of control). A NaCl stress-free period immediately after inoculation was the best among the stressed treatments, indicating that the early stage of nodulation was more sensitive than the later stages. Microscopic observation showed that imposing NaCl stress during the first 3 days after inoculation suppressed root hair curling to 9.1% of control, while the numbers of rhizobia attached to roots counted by dilution plates were not affected. Thus, there were no significant effects of NaCl stress on rhizobia. The sensitivity of the early stage of infection to NaCl stress was attributed to the inhibition of root hair curling.     

The effects of shading and defoliation on nodulation and nitrogen fixation by white clover

In a glasshouse experiment, single plants of ten-weeks old white clover (Trifolium repens L.) were subjected to two levels of shading and two levels of defoliation. Nodulation and nitrogen fixation parameters were measured at six sequential harvest over four weeks. Changes in nodule number and hence nodule dry weight per plant were due to nodule decay, sloughing off and non-production and were closely related to losses in root dry weight. Severe defoliation caused degradation of leghaemoglobin, an effect which was seen in less than three days from treatment. It led also to a temporary but marked decrease in the nitrogen fixation capacity of the nodules as measured by the acetylene reduction assay. Recovery of normal activity by the nitrogenase system took about ten days. The effects of shading and defoliation on the pattern of nodulation have been described briefly.     

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.     

The effect of mixtures of rhizobium strains on the dry matter production of white clover grown in agar

The effects of mixtures of rhizobium strains on the dry matter production of S. 100 white clover were investigated by growing plants in tubes of agar with four pure strains known to give contrasts in yield effectiveness, and comparing the results with those produced by plants grown with mixtures of these strains, and with those of uninoculated plants receiving mineral nitrogen. It was found that some components of the mixtures influenced yield much more than others and these strains therefore appeared to be strong competitors. Ability to infect and capacities to fix nitrogen were shown to be mechanisms under separate genetic control; thus competitiveness and effectiveness were not necessarily linked. Variation within groups of plants inoculated with the same rhizobium strain was generally greater than within the uninoculated control receiving mineral nitrogen. The amount of variation differed from strain to strain and was largest with the intermediately effective one. The plant genotype could modify the relative competitiveness of rhizobium strains and effective association of plant and rhizobium with a high affinity for each other under competitive situations could be built up by coincidental selection of plant and bacterium.     

Effect of lime,phosphorus and mycorrhizal fungi on growth,nodulation and nitrogen fixation by white clover (Trifolium repens) grown in UK hill soils

Summary The responses of white clover (cv NZ Grasslands Huia grown in four UK hill soil types) to additions of lime and P, to inoculation with Rhizobium and mycorrhizal fungi, and to differences in soil water status were assessed in pot and field experiments. With a deep peat soil in pots, shoot production, nodulation and N fixation by clover were increased by 160, 130 and 85% respectively following inoculation with mycorrhiza, but in the field, despite a doubling of root infection, there was no response in growth. On a brown earth soil in the field inoculation with one endophyte (Glomus mosseae L1) out of four tested depressed production of white clover shoots by 42% but enhanced that of leeks (Allium porrum) by 50%; the others were without effect. With dry peaty podzol and brown earth soils in pots, clover shoot production was highest with added P when a water holding capacity of 80% was maintained, but roots from the latter had only 2.6 compared to 68 nodules per plant from the former. Further work is required to explain poor nodulation in the brown earth soils.     

The potential for breeding white clover (Trifolium repens L.) with improved nodulation and nitrogen fixation when grown with combined nitrogen

Summary Sodium nitrate applications ranging from 0.36 to 22.84 mM N were shown to depress rates of nodule formation and reduce total nitrogen fixation (acetylene reduction) in white clover plants grown in aseptic test tube culture.Low nitrate levels gave an initial depression in symbiotic activity but the reduction was of short duration and these treatments were subsequently associated with enhanced rates of nodule formation and nitrogen fixation. As a result, phenotypic variation appeared to be strongly differentially affected by the amount of nitrate present. A subsequent experiment suggested that much of the variation was a consequence of early enhancement of plant growth rates by low levels of nitrate followed by rapid depletion thus giving a transitory inhibitory effect. This was confirmed in a third experiment in which the range of nitrate concentration was held constant. Differential effects on variability in nodule formation and nitrogen fixation were then greatly reduced but there was still a residual level of plant-to-plant variation. The results have clear implications for selecting genetic variants capable of fixing di-nitrogen in the presence of combined N. The provision of a single limiting dose of combined nitrogen to a population containing individuals with inherently different growth rates can bring about variations in the phenotypic expression of symbiotic characters. These variations are unlikely to be based on genetic factors which have a direct and stable effect on nodule development and nitrogenase activity. The implications of the results for plant breeding are discussed.     

The effect of gum arabic on infection and nodulation of cluster clover by Rhizobium

Summary Gum arabic is an adhesive used in pelleting legume seeds with Rhizobium. The present study shows that the gum generally enhances the number of infected root hairs, promotes early nodulation and significantly increases the number of leaves during initial nodulation of cluster clover (Trifolium glomeratum) inoculated with Rhizobium trifolii under bacteriologically controlled conditions.     

Morphological compatibility of white clover and perennial ryegrass cultivars grown under two nitrate levels in flowing solution culture

The effects of nitrate (NO3-) supply on shoot morphology, vertical distribution of shoot and root biomass and total nitrogen (N) acquisition by two perennial ryegrass (Lolium perenne L.) cultivars (AberElan and Preference) and two white clover (Trifolium repens L.) cultivars (Grasslands Huia and AberHerald) were studied in flowing nutrient culture. Cultivars were grown from seed as monocultures and the clovers inoculated with Rhizobium. The 6-week measurement period began on day 34 (grasses) and day 56 (clovers) when the NO3- supply was adjusted to either 2 mmol m-3 (low nitrogen, LN) or 50 mmol m-3 (high nitrogen, HN). These treatments were subsequently maintained automatically. Plants were harvested at intervals to measure their morphology and N content. Cultivars of both species differed significantly in several aspects of their response to NO3- supply. In the grasses, the LN treatment increased the root : shoot ratio of AberElan but did not affect the distribution of root length in the root profile. In contrast, this treatment changed the root distribution of Preference compared with HN, resulting in a larger proportion of root length being distributed further down the root profile. The morphology of white clover Grasslands Huia was for the most part unaffected by the level of NO3- supply. In contrast, AberHerald exhibited different growth strategies, with LN plants increasing their stolon weight per unit length at the expense of leaf production, leaf area and stolon length, whereas HN plants showed reduced stolon thickness, greater leaf area production and stolon length per plant. Cultivars with different morphological/physiological strategies in response to NO3- supply may be of value in the construction of 'compatible mixtures' aimed at reducing oscillations in sward clover content by extending the range of conditions that allow balanced coexistence of species to occur.     

The effect of grasses on the quality of transmitted radiation and its influence on the growth of white clover Trifolium repens

Summary Plants of white clover Trifolium repens were grown under the canopies of three grass species, Lolium perenne, Agrostis tenuis and Holcus lanatus, and under simulated canopies of black polythene and controls were exposed to unfiltered natural radiation. The canopies were adjusted so that they transmitted equal intensities of Photosynthetically Active Radiation (P.A.R.). The ratio of red to far red radiation () was unchanged under the black polythene canopies but was reduced under canopies of Lolium and Agrostis and even more so under Holcus. The effect of canopy filtered radiation on the growth of clover was greatly to reduced internode length, mean number of nodes, the number of branched nodes and the number of rooted nodes and greatly to increase petiole length. The effect of canopies of Holcus was greater than that of the other grass species both in its effect on and on the responses of the clover plant to its shade.     

The effect of stem nematode on establishment and early yields of white clover

The effects of stem nematode (Ditylenchus dipsaci) infestation on the establishment of white clover sown in mixed swards with perennial ryegrass, were investigated in two field trials. Clover cultivars known to have different degrees of resistance were sown on land in which stem nematode was controlled either by aldicarb (Experiment 1) or crop rotation (Experiment 2). The establishment of white clover was greatly improved and subsequent clover yields were inversely related to stem nematode infestation. At the first harvest after sowing, clover yields were 3.6 and 3.3 times greater from aldicarb and rotation treatment plots than from check plots; over the first nine months, total clover yields were increased by 3.5 and 8.9 times by aldicarb and rotation treatments. In both trials, plots of resistant cultivars had less stem nematode infestation and yielded better than the very susceptible cultivar, S184. Stem nematode infestations eventually developed on all plots, after the establishment phase. This is the first report showing that resistant cultivars improve establishment of clover in mixed swards on stem nematode infested soils. In both experiments, plots became dominated by clover and even cv. S184 eventually produced good clover swards. Aldicarb and rotation treatments also controlled clover cyst and root lesion nematodes, and numbers of these were inversely related to first harvest yields. Other soil borne pests and diseases, although not prominent, have not been ruled out as possible confounding factors. There was no correlation between grass yield and aldicarb treatment.