Trifolium repensL Research Articles

Grass Species as Living Mulches – Comparison of Weed Populations and Their Biodiversity in Apple Tree Rows and Tractor Alleys

The durability of four grass living mulches, population of annual and perennial weeds, and their biodiversity in tree rows and tractor alleys were evaluated in the first 4 years after cover crop sowing. The experiment was established in a young semi-dwarf apple orchard 1 year after the planting of the tree ‘Chopin’ cv. Soil coverage was satisfactory for the three cover crops: red fescue, Kentucky bluegrass, and perennial ryegrass. The sods of the grasses exhibited 100% soil surface cover in the tree rows and drive alleys, starting from 1 year after sowing to the end of the study period. Only the maintenance of the blue fescue resulted in low average soil development. Annual and perennial weed populations were lower in the tree rows than in the drive alleys. It was determined meticulous and manual soil preparation prior to all grass sowing under tree crowns, followed by precise mowing of grasses and nitrogen fertilization in subsequent years. The highest soil surface cover by the total annual taxa was observed immediately after the emergence of the living mulch. Total perennial weed populations increased in the following vegetation seasons. <i>Trifolium repens</i> L. and <i>Taraxacum offcinale</i> (L.) Web. dominated all the grass living mulches evaluated. Only the red fescue sod effectively limited the infestation of perennial weed after 4 years of living mulch maintenance in the tree row. More than half of the annual and perennial weed taxa occurred sporadically in all living mulches. In both the tree rows and tractor alleys, the soil surface cover was not more than 1%, and these species contributed to the increase in orchard biodiversity.

Weed Occurrence in a Young Apple Orchard Mulched With Two Different Organic Materials

Abstract In this study, conducted at the Research Station of the Wrocław University of Environmental and Life Sciences, Poland, we sought to determine the communities of weed species and their temporal occurrence in a young apple orchard mulched with spent mushroom compost or Miscanthus straw applied to tree rows. A herbicide fallow treatment was used as a control. During the first year of application, both organic mulches protected against the germination of weed seeds stored within the soil. In subsequent years, however, annual weeds occurred in the mulched tree rows, the most aggressive of which was Chenopodium album L. Annual increases in population densities were noted over the 4 subsequent years of research, particularly in the rows receiving spent mushroom compost. Perennial species in the family Poaceae and genus Malva were present in soil receiving both organic mulches. Trifolium repens L. was the perennial weed most often noted in the Miscanthus mulch, whereas Taraxacum officinale Web. was more characteristic of the spent mushroom compost. Commencing from the spring of the third year following apple tree planting, weed infestation associated with the spent mushroom compost was similar to that observed in the herbicide fallow, thus necessitating three annual applications of herbicide to further maintain the orchard. The insufficient weed suppression obtained with this mulch precludes its recommendation as an effective weed management system. Although Miscanthus straw provided extended tree row protection from weed infestation, herbicide intervention was also required. Notably, however, despite the fact that Miscanthus straw provided conditions more favorable to tree growth, apple tree yields and fruit quality tended to be similar under the three investigated orchard soil management systems.

Retention and Transmission of Grapevine Leafroll-Associated Virus 3 by Pseudococcus calceolariae

Grapevine leafroll-associated virus 3 (GLRaV-3), an economically significant pathogen of grapevines, is transmitted by Pseudococcus calceolariae, a mealybug commonly found in New Zealand vineyards. To help inform alternative GLRaV-3 control strategies, this study evaluated the three-way interaction between the mealybug, its plant host and the virus. The retention and transmission of GLRaV-3 by P. calceolariae after access to non-Vitis host plants (and a non-GLRaV-3 host) White clover (Trifolium repens L. cv. “Grasslands Huia white clover”), Crimson clover (T. incarnatum), and Nicotiana benthamiana (an alternative GLRaV-3 host) was investigated. For all experiments, P. calceolariae first instars with a 4 or 6 days acquisition access period on GLRaV-3-positive grapevine leaves were used. GLRaV-3 was detected in mealybugs up to 16 days on non-Vitis plant hosts but not after 20 days. GLRaV-3 was retained by second instars (n = 8/45) and exuviae (molted skin, n = 6/6) following a 4 days acquisition period on infected grapevines leaves and an 11 days feeding on non-Vitis plant hosts. Furthermore, GLRaV-3 was transmitted to grapevine (40−60%) by P. calceolariae second instars after access to white clover for up to 11 days; 90% transmission to grapevine was achieved when no alternative host feeding was provided. The 16 days retention period is the longest observed in mealybug vectoring of GLRaV-3. The results suggest that an alternative strategy of using ground-cover plants as a disrupter of virus transmission may be effective if mealybugs settle and continue to feed on them for 20 or more days.

Responses of microbial communities and metabolic activities in the rhizosphere during phytoremediation of Cd-contaminated soil

Phytoremediation is an effective way to repair heavy metal contaminated soil and rhizosphere microorganisms play an important role in plant regulation. Nevertheless, little information is known about the variation of microbial metabolic activities and community structure in rhizosphere during phytoremediation. In this study, the rhizosphere soil microbial metabolic activities and community structure of Trifolium repensL. during Cd-contaminated soil phytoremediation, were analyzed by Biolog EcoPlate™ and high-throughput sequencing. The uptake in the roots of Trifolium repensL. grown in 5.68 and 24.23 mg/kg Cd contaminated soil was 33.51 and 84.69 mg/kg respectively, causing the acid-soluble Cd fractions decreased 7.3% and 5.4%. Phytoremediation significantly influenced microbial community and Trifolium repensL. planting significantly increased the rhizosphere microbial population, diversity, the relative abundance of plant growth promoting bacteria (Kaistobacter and Flavisolibacter), and the utilization of difficultly metabolized compounds. The correlation analysis among substrate utilization and microbial communities revealed that the relative abundance increased microorganisms possessed stronger carbon utilization capacity, which was beneficial to regulate the stability of plant-microbial system. Collectively, the results of this study provide fundamental insights into the microbial metabolic activities and community structure during heavy metal contaminated soil phytoremediation, which may aid in the bioregulation of phytoremediation.

Investigation on microbial community in remediation of lead-contaminated soil by Trifolium repensL.

Trifolium repensL. is a plant with strong adaptability and large biomass, which possess great potential for phytoremediation. However, little is known concerning its remediation effects and changes in rhizosphere microbial activity and community structure under heavy metal pressure. The aims of this study were to evaluate lead accumulation of Trifolium repensL., study microbial lead resistance, metabolism and community structure characteristics in rhizosphere soils. The accumulated Pb concentration of Trifolium repensL. was observed in 100 and 500 mg/kg Pb contained soil at 55.81 and 90.3 mg/kg, respectively, which cause the decrease of acid-soluble fractions in rhizosphere soil. In the progress of lead-contaminated soil phytoremediation by Trifolium repensL., Pb resistance and metabolic activities of microorganisms have been prompted gradually. In addition, the microbial community composition and abundance were investigated using Illumina sequencing and quantitative PCR. The result showed that after phytoremediation, beneficial microorganisms, such as Flavisolibacter, Kaistobacter, and Pseudomonas, increased, becoming the dominant genera. This study has provided insight into the distribution and activity of the microbial community.

Urbanization drives the evolution of parallel clines in plant populations.

Urban ecosystems are an increasingly dominant feature of terrestrial landscapes. While evidence that species can adapt to urban environments is accumulating, the mechanisms through which urbanization imposes natural selection on populations are poorly understood. The identification of adaptive phenotypic changes (i.e. clines) along urbanization gradients would facilitate our understanding of the selective factors driving adaptation in cities. Here, we test for phenotypic clines in urban ecosystems by sampling the frequency of a Mendelian-inherited trait-cyanogenesis-in white clover (Trifolium repens L.) populations along urbanization gradients in four cities. Cyanogenesis protects plants from herbivores, but reduces tolerance to freezing temperatures. We found that the frequency of cyanogenic plants within populations decreased towards the urban centre in three of four cities. A field experiment indicated that spatial variation in herbivory is unlikely to explain these clines. Rather, colder minimum winter ground temperatures in urban areas compared with non-urban areas, caused by reduced snow cover in cities, may select against cyanogenesis. In the city with no cline, high snow cover might protect plants from freezing damage in the city centre. Our study suggests that populations are adapting to urbanization gradients, but regional climatic patterns may ultimately determine whether adaptation occurs.

A spatially explicit population model of the effect of spatial scale of heterogeneity in grass–clover grazing systems

SUMMARYThe benefits of using white clover (Trifolium repensL.) as a source of nitrogen (N) and nutritious feed in pasture grazed by ruminant livestock have been widely recognized. However, clover is considered inadequate and unreliable as the main source of N input, since its abundance in pasture is patchy, low (typically <0·20) and shows great year-to-year variation. This is thought to be due to the metabolic costs of N fixation, competition with grass, the preference for clover by grazing animals and patchy dung and urine deposition. One solution suggested by a number of authors is to increase the heterogeneity within the pasture by spatially separating clover from grass. This method of pasture management, in order to sustain higher clover content in both the sward and diet of grazing animals, would remove inter-specific competition and equalize grazing pressure, allowing clover to grow unimpeded in greater abundance than previously observed. An existing spatially explicit grass–clover simulation model, developed to investigate the intrinsic spatial and temporal variability within mixed grass–clover swards, was modified and then used to examine the impact of spatial separation on the content, variability and patchiness of clover in pasture. The results show that spatial separation increases both the content and spatial aggregation of clover and reduces year-to-year variation compared with a mixed pasture that fluctuates around a lower mean. The same model was also used to examine the impact of spatial separation across a range of spatial scales, from narrow strips to complete separation, as a means of managing the concerns over disruption to the N cycle within the pasture. The present study shows the importance of the initial sowing arrangement of plant species in sustaining a high content of clover within a pasture in the short term, to at least 20 years depending on the scale of separation, and demonstrates that the spatial separation of clover from grass within a grazed pasture may overcome some of the limitations associated with the use of clover in conventional grass–clover pastures. Results are discussed in terms of benefits to both herbage dry matter production and animal performance.

Benefits of mixing grasses and legumes for herbage yield and nutritive value inNorthernEurope andCanada

AbstractIncreased biodiversity may improve ecosystem services, including herbage yield. A mixture experiment was carried out at five sites inNorthernEurope and one inCanada to investigate whether mixtures of grasses and legumes would give higher herbage yield than monocultures. Resistance of the mixtures to weed invasion and nutritive value of the herbage were also investigated. The experimental layout followed a simplex design, where four species differing in specific functional traits, timothy (Phleum pratenseL.), smooth meadow grass (Poa pratensisL.), red clover (Trifolium pratenseL.) and white clover (Trifolium repensL.), were grown in monocultures and eleven different mixtures with systematically varying proportions of the four species. Positive diversity effects (DE) were observed, leading to greater herbage dry‐matter (DM) yield in mixtures than expected from species sown in monocultures. For centroid mixtures, theDEgenerated on average an additional 32, 25 and 21% of theDMyield than would be expected from the monocultures in the first, second and third year respectively. On average, the mixtures were 9, 15 and 7% more productive than the most productive monoculture (transgressive overyielding) in the first, second and third year respectively. These benefits persisted over the three harvest years of the experiment and were consistent among most sites. This positive effect was not accompanied by a reduction in herbage digestibility and crude protein concentration that is usually observed with increasedDMyield. Mixtures also reduced the invasion of weeds to <5% of herbage yield compared to monocultures (10–60% of herbage yield).

Changes in crude protein fractions of forage legumes during the spring growth and summer regrowth period

SUMMARYOnly a few previous studies have analysed the crude protein (CP) fractions of the Cornell Net Carbohydrate and Protein System during the growth period of forage legumes. The objective of the present study was to investigate the changes in CP fractions during the spring growth and summer–autumn regrowth period of five forage legume species (alfalfa (also known as lucerne,Medicago sativaL.), birdsfoot trefoil (Lotus corniculatusL.), kura clover (Trifolium ambiguumM.B.), red clover (Trifolium pratenseL.) and white clover (Trifolium repensL.)) grown in binary mixtures with perennial ryegrass (Lolium perenneL.) and also in pure stands (two red clover cultivars). Additionally, the specific polyphenol oxidase (PPO) activity was measured photometrically in the leaves of pure red clover swards. In both pure and mixed cropping, CP fraction A increased with advancing maturity, except for the legumes from mixed cropping in the summer–autumn growth period 2004 and 2005. The variation of CP fraction A was mostly positively related to the N yield and the amount of dinitrogen fixation. Although CP fraction A of pure red clover was negatively correlated with the specific PPO activity in the spring growth period, the specific PPO activity was less relevant for the variation of CP fraction A with respect to the whole growing season. CP fraction B generally made up the largest proportion of the CP. Pure red clover stands showed reducing amounts of CP fraction C during the growth period, whereas in legumes grown with ryegrass an increase was usually observed. Despite these differences, there was generally an increase of CP fraction C when the content of non-structural carbohydrates decreased. Red clover and birdsfoot trefoil herbage contained the highest proportions of CP fraction C in the CP, regardless of growth period and year. In conclusion, red clover and birdsfoot trefoil had a more favourable CP composition for ruminant nutrition compared to the other legume species, and in red clover this could not be clearly attributed to the specific PPO activity.

Genetic resources in Trifolium and their utilization in plant breeding

Clovers (Trifoliumspecies) are a large and widespread genus of legumes. Recent work supports the Mediterranean origin of the genus in the Early Miocene period, and centres of diversity for clovers occur in the Eastern Mediterranean, East Africa and South America. A number of clovers are of agricultural importance as forage species in grassland agriculture around the world, particularly in temperate areas. White clover (Trifolium repensL.) is the most important legume of grazed pastures, and red clover (T. pratense) is widely cut and conserved as a winter feed. We consider the current state of collected resources in clovers, which have been collated in the most important databases. These are European Internet Search Catalogue (EURISCO), germplasm resources information network and system-wide information network for genetic resources together with the collection held at AgResearch, New Zealand. In total, collections have been made of 204 species with 48 species having more than 100 accessions in these databases. As expected, the majority of accessions are of the agriculturally important species. The geographical origin of collected material is outlined and, for the major species, accessions are broken down according to their status, e.g. wild population, breeders' lines. We then describe some of the ways in which genetic resources of white and red clovers and their relatives have been used in the breeding of these species. These include introgression of stress tolerance traits, targetting improvements in resource use efficiency and increasing seed yield.

Effects of a cold treatment of the root system on white clover( Trifolium repensL.) morphogenesis and nitrogen reserve accumulation

The ability of white clover (Trifolium repens L.) to undergo cold acclimation is an important determinant of its persistence in mixed swards since growth rate at low temperatures sustains higher clover contents at the start of spring. During a re-growth period following defoliation, a gradual exposure of the root system (cv. Grasslands Huia) led to some physiological and morphological changes of cold-adaptive significance, similar to those developed by clover ecotypes originating in northern areas of Europe. Thus, cold exposure of the root system resulted in small-leaved prostrate forms of white clover after one month of re-growth. Similarly, cold exposure increased the ability of plants to store nitrogen since the application of low temperatures to the root system enhanced soluble protein accumulation in roots and in stolons. More specifically, cold exposure of the roots induced gene expression of a vegetative storage protein (17.3 kDa VSP) in both organs. These results demonstrate that the root system of clover plants should be a site of perception of the low-temperature stimulus, and gave rise to the question of the transduction of the cold signal from the roots to the aerial parts. On the basis of this study and taking into account molecular aspects concerning the clover VSP, it is suggested that this protein could participate in cold acclimation in addition to its role in nitrogen storage.

Effect of grain supplementation and the provision of chemical or physical fibre on marginal milk production responses of cows grazing perennial ryegrass pastures

An experiment was undertaken to establish the marginal milk production response from cereal grain supplementation by dairy cows grazing irrigated perennial pastures in spring and whether the addition of a hay supplement would improve that response. Cows were offered perennial ryegrass (Lolium perenne L.)–white clover (Trifolium repensL.) pasture at allowances (measured to ground level) of 20 or 40 kg dry matter (DM)/cow.day. Additional cows offered the low allowance were also offered supplements of pasture hay as a cube or pellet (2.5 kg DM/day), cereal grain as a pellet (5.0 kg DM/day) or cereal grain plus pasture hay (7.5 kg DM/day) as a pellet (1.75:1 DM basis) or cube (1.97:1 DM basis). The 7 treatments were replicated 3 times, and 63 cows were used (3 cows/treatment). The in vitro DM digestibility, crude protein and neutral detergent fibre (NDF) concentration of the barley and perennial pasture hay were 863 and 563 g/kg DM, 131 and 92 g/kg DM, and 182 and 666 g/kg DM, respectively. Cows from all treatments consumed herbage with an in vitro DM digestibility of 833 g/kg DM, crude protein content of 214 g/kg DM and NDF concentration of 457 g/kg DM compared with the pregrazed herbage on offer of 771, 174 and 527 g/kg DM, respectively. The mean pregrazing herbage mass of 4.13 t DM/ha consisted of perennial ryegrass (507 g/kg DM), white clover (188 g/kg DM), weeds (194 g/kg DM) and dead material (111 g/kg DM). Without supplements, cows consumed 11.2 kg DM/day at the low pasture allowance and 15.6 kg DM/day at the high allowance. Herbage intake did not decline to a large degree when barley was included in the diet, where a substitution rate of 0.2 kg reduction in pasture DM/kg DM cereal grain supplement was measured. The NDF concentration of the diet consumed was lowest (P< 0.05) for the grain pellet treatment (370 g NDF/kg DM) and highest (P<0.05) for the fibre pellet (493 g NDF/kg DM) and the fibre cube (507 g NDF/kg DM) treatments. Fat-corrected milk yield increased (P<0.05)) from 20.1 to 24.5 kg/cow.day when cows consumed 4.5 kg DM cereal grain/day with a marginal milk response of 1.0 kg 4% fat-corrected milk/kg DM concentrate supplement. Supplementation with grain or hay had no significant effect (P>0.05) on milk fat, protein or lactose concentrations. The cows offered the high allowance spent more (P<0.05) time grazing than those offered the low allowance, but there were no significant differences in rumination time. Cows spent more time ruminating when consuming the grain + fibre cube compared with the other supplement treatments, which were lower (P<0.05) and similar. Cows offered pasture at the low allowance ruminated the least. The rumen pH was not different between treatments and averaged 5.9. The ratio of acetate plus butyrate: propionate was not different between treatments and averaged 4.3. This study has shown that feeding cereal grain to cows grazing pastures high in perennial ryegrass resulted in acceptable marginal milk responses of 1 kg milk/kg DM of cereal grain in spring because of sufficient NDF in the diet. As there was adequate NDF in the diet, further supplements with pasture hay as cubes or pellets had no effect on marginal milk responses .

Molecular Cytogenetic Organization of 5S and 18S-26S rDNA Loci in White Clover (Trifolium repensL.) and Related Species

The genusTrifolium(Leguminosae), which includes several important forage legumes of temperate and subtropical regions, is characterized by small chromosomes of uniform size. Cytogenetic mapping of the two multigene families coding for 18S-5.8S-26S rRNA and 5S rRNA in white clover (T. repens)and seven closely related species/subspecies was carried out using fluorescencein situhybridization (FISH). The 18S-26S rDNA, generally confined to nucleolus organizer regions (NORs), was consistently located proximally on the shorter arm of one or two metacentric or submetacentric chromosome pairs. DAPI-negative, diffused and highly stretched 18S-26S FISH signals, representing previously transcriptionally active NORs, were observed up to late metaphase. The flanking condensed ends of the diffused 18S-26S rDNA signals were DAPI-positive and represented perinucleolar knobs which are presumed to be transcriptionally inactive. In interphase cells, the decondensed NOR chromatin passed through the nucleolar domain. In one species (T. isthmocarpum),in addition to two NOR chromosome pairs, 18S-26S signals were also located on a third pair, with no apparent NOR association. The 5S rDNA signals were located proximally on a long arm in six species and were syntenic to the NORs in all these species exceptT. ambiguumandT. nigrescensssp.petrisavii.In two species (T. hybridum, T. isthmocarpum),5S sequences occurred in the chromosomal satellites aligned distally to the NOR sequences. The relative distribution patterns of the two types of tandemly repeated DNA sequences were species-specific except forT. nigrescensssp.petrisaviiandT. ambiguum,both of which showed identical hybridization patterns. The two types of rDNA sequences have provided molecular markers for individual identification of a set of clover chromosomes. Present findings support the allotetraploid origin of white clover (T. repens)and suggest thatT. nigrescensssp.petrisaviimay be one of the present day diploid ancestors.

Comparison of Horizontal Spread of White Clover (Trifolium repensL.) Grown Under Two Artificial Light Sources Differing in their Content of Blue Light

The effects of blue light on the growth and development of different branches were studied to determine if variations in the blue light supply modified the horizontal spread of white clover. Cuttings of white clover (‘Huia’) were grown for 56 d in two controlled chambers under conditions similar except for level of blue light (BL). The horizontal spread of white clover was characterised by the phyllochron, internode length and branching or flowering of different axes. The responses to a reduction in BL were different according to axis degree: (1) on the main axis, lowering BL slightly decreased the phyllochron and had no effect on internode length or stolon length; (2) on older primary branches, it decreased their phyllochron, but reduced stolon length due to a reduction in internode length; (3) on secondary branches, it increased the phyllochron and decreased internode length, triggering a strong reduction in stolon length (up to 54%). Moreover, an increase in BL reversed these effects. The switch to lower BL or higher BL had no effect on internode length of primary branches, but triggered changes in internode length of the main axis. These differential effects of variations in BL on the morphogenesis of different branches are discussed in relation to the clonal integration of variations in light composition. Changes in the horizontal spread of white clover induced by BL could play a role, combined with responses to low red:far-red ratio and low photosynthetic photon flux (PPF) in the strategy of horizontal colonisation and shade-avoidance of white clover.

SHORT COMMUNICATION Growth and Development of White Clover (Trifolium repensL.) as Influenced by P and K Nutrition

The effects of varying P and K supplies on the growth and morphology of white clover were investigated in a pot experiment. Plants were treated with three rates of potassium (K) and four rates of phosphorus (P) in factorial combination, and five harvests were taken between August 1993 and February 1994. During establishment, shoot and leaf development were more adversely affected by P than by K deficiency but, by the final growth period, K deficiency was the more limiting to growth, causing dramatic reductions in leaf area index (LAI), numbers of growing points, and stolon and root masses. The adverse affects of K deficiency on shoot development were attributed to the curtailment of petiole and lamina expansion which have major requirements for K. It was noted that, under K deficiency, proportionately less assimilate was partitioned to leaf production than to stolon and root growth; this may be an adaptation enabling K-deficient plants to exploit nutrient supplies in adjacent localities. The results suggest that, provided white clover can withstand ‘moderate’ P stress during establishment, its persistence in swards will probably be limited more by K deficiency than by low or inadequate P supplies. However, because the experiment was conducted outside the normal growing season and with only a single white clover cultivar, it is inappropriate to make any generalizations regarding the effects of P and K nutrition on white clover in the field.

Changes in Stolon Carbohydrates During the Winter in Four Varieties of White Clover (Trifolium repensL.) with Contrasting Hardiness

Miniswards of white clover (Trifolium repensL.) varieties Menna, Huia, AberCrest and AberHerald, with contrasting winter hardiness, were established from cloned material in flat boxes. The miniswards were placed in an unheated glasshouse with natural lighting for the winter of 1995/96. During this time there were several periods of cold weather. The plants experienced temperatures at least as severe as those experienced by unprotected plants during preceding mild winters. Growth measurements carried out monthly suggested the cloned genotypes were representative of the varieties from which they were derived. Growth continued throughout the experiment, particularly when expressed as appearance of new internodes. During the coldest weather these internodes were short, and the amount of tissue produced was small. Starch concentration showed a complex pattern of changes during the winter. Older internodes were the major site of starch storage in the autumn; starch was lost from these internodes during the winter and early spring. Starch synthesis also occurred, even during the coldest periods. Starch accumulated in all new stolon tissue. There were significant differences between varieties. AberCrest and AberHerald retained higher concentrations of starch in older stolon tissue, synthesized higher concentrations of starch in young tissue and retained these for longer. There were few differences in soluble sugar concentrations between varieties, but there were significant changes during the winter. In mid-winter, sucrose and pinitol were at their highest concentrations, whereas reducing sugars were at their lowest concentrations. This study further supports the important role of starch reserves in supporting winter survival.

Contribution from Stolons and Roots to Estimates of the Total Amount of N2Fixed by White Clover (Trifolium repensL.)

Abstract Growth and N-accumulation rates in leaves, stolons and roots of individual white clover plants were studied in three experiments using two methods. In a growth chamber experiment, the relative differences between tissues were found to be almost constant for a wide range of clover plant sizes. The stolon dry matter (DM) production was 56% and the root DM production 40% of the DM production in leaves. The N yield of stolons was 30% while N yield in roots was 34% of N yield in leaves. The effect of N application on these relations was investigated in a glasshouse experiment. Application of N reduced the root:shoot N ratio from 0.50 to 0.28, whereas the stolon+root:leaf N ratio (i.e. for abovevs.below cutting-height tissues) was only reduced from 0.97 to 0.80. In a field trial with two contrasting N regimes, growth and N accumulation were measured on individual clover plants. Dinitrogen fixation was estimated by15N isotope dilution based on analysis of leaves-only or by including stolons. Using leaves-only did not affect the calculation of percentage of clover N derived from N2fixation (% Ndfa) since the15N enrichment was found to be uniform in all parts of the clover. A correction factor of 1.7 to account for N in below cutting-height tissue is suggested when N2fixation in white clover is estimated by harvesting the leaves only.

Effect of Severity of Defoliation on the Viability of Reproductive and Vegetative Axillary Buds of Trifolium repensL.

This glasshouse experiment was performed to assess the effects of a range of constant defoliation regimes applied to cuttings of a single large-leaved genotype of Trifolium repensL. on the viability of its axillary buds. Plants were established to comprise a single main stolon (axillary branches were removed) and defoliation treatments were applied by removing the older (basal) leaves until leaf complements of 1·0, 1·5, 2·0, 2·5, 3·0 or all leaves (control) remained. Basal leaves were subsequently removed as necessary to maintain the target leaf complements. Only severe defoliation (leaf complements of 1·0 and 1·5) induced a loss of viability in axillary buds. Loss of viability was greatest in reproductive buds present within the apical bud when the treatments were first imposed. Although the most severe treatment (leaf complement 1·0) resulted in death of half the plants, in plants surviving that treatment, death of vegetative axillary buds was restricted to 21% of the vegetative buds at the three youngest node positions within the apical bud at the time of treatment application. No other treatment induced any loss of viability of vegetative buds. There was no loss of viability of axillary buds at nodes formed after the treatments were imposed. The frequency of initiation of inflorescences at nodes formed after treatments were imposed decreased as defoliation severity increased. Severe defoliation resulted in marked changes in plant morphology indicative of a sharp decrease in availability of intraplant resources. It was concluded that under severe defoliation: (1) the potential for vegetative growth (as represented by viable vegetative axillary buds) was maintained at the expense of reproductive growth; and (2) that the loss of viability of axillary buds was associated with the sudden changes in physiological processes induced by defoliation as there was no loss of viability in buds formed after plants had adjusted their phenotype to one of smaller size.

Low Root Temperature Retardation of the Mineral Nitrogen Induced Decline in N 2Fixation by a Northern Ecotype of White Clover

Nodulated white clover plants ( Trifolium repensL.) of a Norwegian ecotype from Pasvik (70°N) were grown in flowing solution culture. Root temperature was 17°C until 51d after sowing, when it was lowered decrementally over 5d to 7°C in four of the eight plant culture units. After a further 24h, mineral N was supplied automatically at 20μ MNH 4NO 3in three culture units at each root temperature (7 and 17°C) over 17d. The remaining two units provided control plants solely dependent on N 2fixation at 7 and 17°C. The supply of NH 4NO 3greatly reduced the nodule biomass per plant at 17°C over 17d compared with control plants, but had little effect at 7°C. The nodule decline at 17°C accompanied an acute and progressive decrease in specific rate of N 2fixation, from 9mmolN d -1g -1nodule d.wt on day 0 to zero by day 10. Whilst initial rates of N 2fixation were lower at 7°C, the mineral N-induced decrease in fixation rates was also less severe than at 17°C and specific fixation rates recovered after reaching a minimum on day 11. N 2fixation accounted for 36% of the total uptake of N by +min.N plants during the treatment period at 7°C as opposed to only 13% at 17°C. The total N 2fixed at 7°C was 86% of that fixed at 17°C, although the specific growth rate (d.wt) at 7°C was only 55% of that at 17°C. Addition of NH 4NO 3at 7°C had little effect on the gross amount of N 2fixed subsequently. In contrast, total N 2fixation by +min.N plants at 17°C was only 24% of that fixed by the corresponding controls. The possible mechanisms by which mineral N affects N 2fixation are discussed.

Seasonal variation in the morphology of perennial ryegrass (Lolium perenne) and cocksfoot (Dactylis glomerata) plants and populations in pastures under intensive sheep grazing

SUMMARYThe morphology of independent plants of ‘Grasslands Ruanui’ perennial ryegrass (Lolium perenneL.) and three cultivars of cocksfoot (Dactylis glomerataL.) (‘Grasslands Apanui’, ‘Grasslands Kara’ and ‘Grasslands Wana’) in mixed pastures under intensive sheep grazing was studied at Palmerston North, New Zealand during 1991/92.Both perennial ryegrass and cocksfoot exhibited a similar pattern of clonal growth and population structure. Distribution of plants among various orders of branching showed a relatively stable pattern through most of the year except in spring, when stem decay and plant fragmentation exceeded apical growth and regeneration, causing an increase in the proportion of small plants and a corresponding decrease of larger plants. This pattern was similar to that previously reported for white clover (Trifolium repensL.). Nevertheless, seasonal variation in plant structure (number of leaves, tillers and stems per plant) was small, but variation in organ size (DW or length) was greater. Stolon formation through elongation of internodes occurred throughout the year, but was associated primarily with flowering.Grazing management caused no differences in plant structure between species or among the cocksfoot cultivars, but did affect the size of organs, and hence plants. Cocksfoot plants were 50–60% heavier than perennial ryegrass under rotational grazing. Under set stocking, only perennial ryegrass and Wana cocksfoot exhibited sufficient phenotypic plasticity to survive, both Kara and Apanui cocksfoot failed to persist. The only consistent difference between the species was greater flowering in perennial ryegrass than in cocksfoot, in both the proportion of plants flowering, and the number of flowering tillers per plant. Both species produced stolons throughout the year, although perennial ryegrass and Wana cocksfoot had a higher proportion of plants with stolons than Apanui and Kara cocksfoot. Length and DW of stolons per plant were similar in both species.As there was little variation in plant structure and plant density, length of stolons per unit area tended to parallel seasonal changes in pasture tiller density. The role of grazing management in the survival of tillers and plants, and subsequent performance of grass species in pastures is discussed.