Effects of divergent selection on correlations between morphological traits in white clover (Trifolium repens L.)

The effects of two generations of divergent selection for stolon diameter and stolon amount on the pattern of phenotypic correlations between important agronomic traits in white clover (Trifolium repens L.) were studied. The effects of selection were: the proportions of the variances of traits under direct selection were considerably lower within selection lines than overall and this was associated with a loss of correlation between traits. Unselected traits showed higher proportions of variances remaining within the selection lines and shifts from non-significant to significant correlations from the plants overall to selection lines. The significance of these results and their relevance to strategies of germplasm improvement is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Changes induced by defoliation in the yield and digestibility of leaves and stems of perennial ryegrass (Lolium perenne L.) during reproductive development

Ten identical perennial ryegrass plots (cv. Frances) were sequentially harvested for first cut silage at 7 day intervals, with second cuts after 6 weeks regrowth and further cuts until the growing season ended. Total herbage dry matter and digestible organic matter yields increased and digestibility decreased with delayed cutting, as was expected. The opposite and counterbalancing response occurred at the second cut. This pattern of yield change was mirrored by changes in the amount of stem tissue, whereas leaf yield did not change significantly at the first harvest but declined at the second, in response to delayed cutting. Furthermore, stem digestibility declined at the first cycle of harvests from ca. 75 to 62–64% in the most delayed cutting treatments. In contrast, leaf digestibility remained high (ca. 70%) until after seed-head emergence but then decreased rapidly to ca. 56%. This decline may have been associated with accelerated leaf senescence and redistribution of assimilates, though this needs to be examined. It was concluded that although manipulating first harvest date determined the proportioning of yield and digestibility in the first and second cuts, the observation that the combined yield and digestibility in these two harvests did not vary substantially is an important result for farming practice.     

Characterization of novel inbred lines of white clover (Trifolium repens L.). I. Dynamics of plant growth and nodule development in flowing solution culture

Growth, dry matter partitioning between shoots and roots, and extent of nodulation were characterised in four novel self-fertile highly inbred lines (referred to as A, B, C and D) of white clover (Trifolium repens L.) thought to differ fundamentally in their nitrogen relations and therefore regarded as candidate material for genetic mapping. Plants were inoculated with a mixture of three strains of Rhizobium and grown in flowing nutrient solutions without N for 18 d. Half the plants were then supplied with 20 μM NO₃ ^- during a 36 d treatment period, the remaining plants acting as ‘controls’ solely dependent upon N₂ fixation for acquisition of N. Total dry matter production and shoot:root ratios were similar in all lines supplied with NO₃ ^- and in control plants of lines B and C. Growth was severely reduced in control plants of line D and to a lesser extent in control line A. These effects were attributed to a failure to develop effective N₂ fixation capacity after apparently normal infection. Mean nodule size, nodule numbers and nodule d.w. per plant were extremely low in line D compared with the other lines, irrespective of whether NO₃ ^- was supplied. Lines A, B and C differed in the severity with which NO₃ ^- decreased mean d.w. per nodule and total nodule d.w. per plant relative to corresponding control plants, with line C being least sensitive. Nitrate also decreased the total number of nodules per plant relative to control plants after day 14 in all lines except C, and most severely in line B. This revised version was published online in August 2006 with corrections to the Cover Date.     

Influence of cutting frequency on the morphology and the C and N reserve status of two cultivars of white clover (Trifolium repens L.)

The influence of cutting frequency was studied on white clover (Trifolium repens L.) cultivars (giant-Aran and dwarf-Rivendel) during two experiments set up under field conditions. Clover morphological and biochemical characteristics were measured prior to regrowth in response to various cutting interval treatments (1, 2, 3, 6 and 8 weeks of regrowth between two successive defoliations). The morphology of the aerial parts of the plant was described through the residual leaf area (RLA), the number of total growing points (TGP), stolon length and stolon and root dry matter productions per unit of cultivated area, as well as stolon dry weight per unit length. C and N reserve status was assessed by starch and vegetative storage proteins (VSP) concentration and amount measurements. Cutting frequency largely influenced the different variables. RLA and TGP increased in response to frequent cuttings while all other morphological and biochemical parameters decreased, with the exception of the soluble protein concentrations and protein amounts. Nevertheless, a significant difference in morphology and C and N reserve accumulation was shown between the two clover cultivars. In spite of a few number of leaves and growing points, the giant cultivar Aran showed higher C and N reserves, compared with the dwarf type Rivendel. However, the genetic expression of these differences was tightly linked to the cutting frequency applied to the sward, and was at its maximum in response to the most frequent treatments. Subjected to severe cuttings, both genotypes showed an increase in RLA and TGP, the greater effect of the treatment was obtained with the dwarf-type Rivendel. Frequent defoliations strongly reduced organic reserves but the cutting-induced decreases observed in starch and VSP contents were similar for each cultivar. Overall results confirmed that the morphological development of clover, partly defined by genetic performance, should also be controlled by cutting frequency, and highlighted a tight relationship between morphogenesis, reserve accumulation and sward management. Results are also discussed in terms of the use of C and N reserve contents as a further characterisation of white clover cultivars.     

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.     

Effects of previous cropping on seed yield and yield components of oil-seed rape (Brassica napus L.)

Information about the effect of the preceding crop or crop combination on the seed yield of oil-seed rape is extremely scarce. Experiments were carried out in northwest Germany to investigate the effect of different preceding crops on the growth, seed yield and yield components of oil-seed rape. The two directly preceding crops, wheat and oil-seed rape, had only a negligible and non-significant effect on the seed yield of the following oil-seed rape crop. Oil-seed rape grown after wheat had more pods per plant, due to an increase in the number of pods on the higher category branches. In contrast, the seed yield and yield components were more affected by the cropping sequence, i.e. the crops 2 years before. Averaged over two experimental years, the greatest yields were observed in oil-seed rape following the sequence peas-wheat (694 g m⁻²), whereas the smallest seed yield occurred after 2 years of oil-seed rape cropping (371 g m⁻²). The differences in the seed yield were again associated with more pods per plant, which compensated for the lower number of plants m⁻², whereas the number of seeds per pod and the mean seed weight were almost unaffected by the previous cropping. It was not possible to relate the described differences to the crop development, since differences in the biomass caused by the previous cropping were only significant at maturity. Oil-seed rape grown after 2 years of oil-seed rape had the highest ratings of stem canker (Leptosphaeria maculans) as well as verticillium wilt (Verticillium dahliae). But the general level of the diseases was low, and therefore other causes for the effects described must be considered.     

Effects of water stress applied at different growth stages to Brassica napus L. var. oleifera on yield, yield components and seed quality

In pot experiments under controlled conditions we investigated the effects of water stress on oilseed rape. Yield and yield components were mainly affected by water shortage occurring from flowering to the end of seed set. The greatest reduction (48%) was observed when only 37% of full water requirement was supplied to the plant during this stage. The number of seeds per plant was the main yield component affected. Some compensation occurred when the water supply was restored. The 1000-seed weight was only affected by a water stress from the stage when the pods were swollen until the seeds colored stage. The results demonstrated a marked reduction in oil concentration when water deficit occurred from anthesis to maturity. There was an inverse relationship between oil and protein concentration. The most marked effect observed in this experiment was on the glucosinolate concentration where increases of up to 60% were observed. These results may explain effects on seed quality of field grown oilseed rape.