Grasslands Huia Research Articles

Chemical and biological induction of resistance to the clover cyst nematode (Heterodera trifolii) in white clover (Trifolium repens)

AbstractPossible induction of resistance to the clover cyst nematode, Heterodera trifolii, in white clover, Trifolium repens, by application of two chemical inducers and of soil-borne Pseudomonas-like spp. and Bacillus cereus was investigated. Salicylic acid and benzo(1,2,3)thiadiazole-7-carbothioic acid-S-methyl ester, applied as a root drench in growth cabinet soil bioassays, affected development of H. trifolii in cvs Haifa and Grasslands Huia. Treatments reduced fecundity of the nematodes, increasing the proportions of distorted females and of females with fewer eggs compared to water-treated controls. Application of pectinolytic fluorescent pseudomonad strains P29 and P80, and B. cereus strain B1, induced a response similar to that resulting from the chemical induction. Both live and dead cells of P29, but not cell-free culture filtrate, induced these effects on H. trifolii. A non-pectinolytic, fluorescent pseudomonad, strain P37, had no effect. From the nature of the responses, it is concluded that the effects of both the chemical and bacterial agents have similarities to resistance. From the timing of applications and known properties of both agents, these effects seem similar to induced systemic resistance.

Determination of management and topographic influences on the balance between resident and ‘Grasslands Huia’ white clover (Trifolium repens) in an upland pasture using isozyme analysis

An investigation was made during 1988 to test the hypothesis that ‘Grasslands Huia’ white clover (Trifolium repens L.) could be eliminated under close sheep grazing. The effects of grazing management, topography and fertilizer on the contribution of Huia plants to the white clover population in an 85 ha experimental upland pasture ecosystem in the southern North Island, New Zealand (lat. 40° 20′ S, long. 175° 50′ E, 125–350 m altitude) were quantified 11 years after oversowing. Replicated sampling sites (108 in total) were located on nine combinations of slope and aspect within grazing management treatments comprising rotational grazing with cattle (RC), rotational grazing with sheep (RS) and continuous grazing with sheep (CS), with high and low fertilizer treatments in each case. White clover occurrence, leaf area, phosphoglucoisomerase-2 (PGI-2) allele frequencies and the proportion of Grasslands Huia plants in the white clover population were determined at each site. White clover frequency was lower on steeper slopes. Aspect, slope and grazing management affected area of individual clover leaves. The proportion of Huia plants in the white clover population averaged 54·9, 49·0 and 33·6% for RC, RS and CS, respectively (P < 0·039, 5 D.F.). Fertilizer and topography did not affect the proportion of Huia. It was concluded that although Huia did persist after 11 years of close sheep grazing, its contribution to the total white clover population was unsatisfactory in some cases, and use of better adapted cultivars is suggested.

Growth of White Clover Ecotypes, Cultivars, and Germplasms in the Southeastern USA

ABSTRACTNaturalized populations, or ecotypes, of white clover (Trifolium repens L.) are found in many southeastern U.S. pastures, often persisting despite adverse environmental and cultural conditions. Our objective was to compare the growth, morphology, and vegetative persistence of white clover ecotypes (small‐ to medium‐leaf) with selected cultivars (large‐leaf) and improved germplasm under grazing in four southeastern states. Seed or stolons of white clover ecotypes were collected primarily from grazed pastures in spring 1994 in Louisiana, Mississippi, Alabama, and Georgia, and increased by intercrossing. Plots of seven ecotype collections, ‘Osceola’, ‘Regal’, ‘Louisiana S‐1’, ‘Grasslands Huia’, ‘Grasslands Prestige’, Brown Loam Synthetic No. 2 germplasm, and Southern Regional Virus Resistant germplasm were established in October 1995 in each state. Plots were stocked continuously with cattle (Bos taurus) (5‐cm stubble) from January 1996 to January 1998. Stolon length and axillary bud viability, and number of stolon apices, rooted nodes, and seedlings were measured every 3 mo. Stolon length of the ecotypes usually exceeded that of the cultivars and germplasm at all locations, particularly in the winter and spring. By the end of the experiment, the ecotypes also exhibited greater stolon branching than the cultivars and germplasm (1580 vs. 320 branches m−2). Superior vegetative growth of the ecotypes was attributed to a greater proportion of the stolon nodes being both branched and rooted, particularly during the summer. White clover ecotypes represent a valuable source of germplasm to incorporate into breeding programs or to develop directly into more persistent cultivars.

Effects of adaptation to winter stress on biomass production, growth and morphology of three contrasting white clover cultivars

Changes in biomass production and partitioning, together with morphology and growth of three white clover (Trifolium repens L.) cultivars after exposure to winter stress were recorded in a controlled environment study. The cultivars used were Grasslands Huia (H, New Zealand), AberHerald (AH, bred in UK from material of Swiss origin) and Sandra (S, Sweden), sub‐populations of these (survival populations), and a wild ecotype from the Uppsala area. The sub‐populations were collected after 2 (with the exception of S) and 4 years of winter stress, respectively, at Uppsala (59°49′ N, 17°39′ E). In the controlled environment study the photoperiod was 12 h and there were two temperature regimes, namely 24/18 and 12/9°C (day/night), respectively. The experimental plants were cloned from eight different plants of each population. There were few interactions between populations and temperature regimes. Biomass production was lowest in the Swedish populations and in the surviving populations of the other cultivars collected after 4 years of exposure to winter stress. However, biomass partitioning, with the exception of allocation to branches, remained unaffected. Morphology differed significantly between cultivars, but, with the exception of internode length, was unaffected by exposure to winter stress. Internode length was significantly reduced in surviving populations of both H and AH after exposure to four winters, when its value was similar to that of the Swedish populations. The reduced internode length was a result of a greater reduction in the stolon extension rate than in the leaf appearance rate of survival populations compared with the original populations. It is hypothesised that reduced internode length would have implications for the carbon economy of the plant.

Effect of four acid soils on root growth of white clover seedlings using a soil-on-agar procedure

White clover (Trifolium repens L.) is widely distributed in the Appalachian region, except on highly acid soils. We used a procedure where a thin layer of soil is placed on top of solidified water agar to characterize effects of acid soil on seedling root growth. Our objectives were to evaluate the soil-on-agar technique by using four soils (non-limed and limed) with diverse chemical characteristics and to relate root emergence to the chemical properties of the soils. We used three white clover cultivars, ‘Grasslands Huia’, ‘Grasslands Tahora’ and ‘Sacramento’. Daily counts of root emergence from soil into agar were made for 12 d. Liming hastened white clover root emergence in three of the four soils. Days to 40% emergence were closely related (P < 0.01) to soil pH\(_{{\text{H}}_{\text{2}} {\text{O}}} \) and to species of soil solution Al that are associated with Al toxicity in dicotyledonous plants. The r2 values for the regression of days to 40% root emergence on \({\text{pH}}_{{\text{(H}}_{\text{2}} {\text{O)}}} ,{\text{ Al}}^{{\text{3 + }}} ,{\text{ Al(OH)}}^{{\text{2 + }}} {\text{ and (Al}}^{{\text{3 + }}} {\text{ + Al(OH)}}^{{\text{2 + }}} {\text{ + Al(OH)}}_{\text{2}}^{\text{ + }} {\text{)}}\) were 0.95, 0.96, 0.94 and 0.96, respectively. Apparently, the primary factor responsible for delayed root emergence in the soil-on-agar procedure was Al toxicity. Because of the close relationship between root emergence and activity of toxic species of soil solution Al, we propose that the soil-on-agar technique should be useful for characterizing the response of many small-seeded species to Al.

Effects of an autumn defoliation on overwintering, spring growth and yield of a white clover/grass sward

AbstractAn established sward of binary mixtures of meadow fescue (Festuca pratensis) and white clover (Trifolium repens) (either AberHerald, Grasslands Huia or Sandra) was subjected to (A) no further defoliation, (B) a defoliation in late September or (C) a defoliation in late October after four harvests had been taken during the grazing season. About a tonne of dry matter (DM) was removed by the autumn defoliations. There were two levels of nitrogen application in spring, either 0 or 90 kg ha−1. The development of grass and clover morphology and population sizes from early autumn until the first harvest the following year was followed by regular sampling of the above‐ground material. Stolons were analysed for total non‐structural carbohydrates (TNCs), and the temperature at stolon level was continuously recorded. There were no interactions between autumn defoliation, clover cultivar or nitrogen treatments on any of the parameters studied. White clover growing‐point numbers and stolon morphological characteristics were reduced in size during the winter and did not recover during the spring. A defoliation in late September resulted in the greatest reduction, whereas there were no differences between the other two treatments. The grass tiller population increased from early autumn until the last sampling occasion in May, but both autumn defoliations resulted in a smaller increase. Defoliation in late September had the greatest impact. The TNC content of white clover stolons fell from about 350 g kg−1 to 150 g kg−1 DM from late autumn until late April. There were small differences between the treatments, but a defoliation in late September resulted in a significantly lower level in late autumn. The temperature amplitude at stolon level was consistently greater in plots defoliated in late September. Total DM harvested in spring was 4367, 2564 and 3536 kg ha−1, of which 388, 352 and 460 kg ha−1 was white clover, from treatments A, B and C respectively. It is concluded that an autumn defoliation may affect the overwintering of white clover negatively, but that the effect on the grass may be even more detrimental.

Determination of optimum nutrient element ratios in plant tissue

A new approach for determining optimum nutrient element ratios in plant tissue is presented. Essential steps in the procedure involve: a) measuring patterns of response to pairs of nutrient elements in factorial fertilizer trials, b) modeling the yield response surface using a bivariate, Mitscherlich‐related response function, c) defining balanced nutrition in terms of the parameters of the response surface, d) identifying combinations of P and S fertilizer resulting in balanced nutrition, and e) determining from plant chemical analysis the ratio of nutrients in plant tissue in nutritionally balanced combinations. The approach is illustrated by data from a phosphorus (P) by sulfur (S) factorial field fertilizer trial on a mown mixed white clover (Trifolium repens cv Grasslands Huia) and ryegrass (Lolium perenne L cv Grasslands Nui) sward. Different parameters of yield [total dry matter production, clover dry matter production, clover nitrogen (N) uptake] required different ratios of S:P in fertilizer and consequently in plant tissue for nutritional balance. Also, plant tissue S:P ratios for balanced nutrition declined as the level of nutrition increased. Economic optimum S:P fertilizer ratios were higher than those ratios required for nutrient element balance due to the lower cost and higher effectiveness per kilogram of fertilizer S compared with fertilizer P. Ratios of S and P to N in clover tissue were useful indicators of the adequacy of S and P for clover which was dependent on N2 fixation for its N supply. It is suggested that a nutrient element index system showing both relative and absolute nutrient element status might be built around N as an internal standard for legumes dependent on N2 fixation and possibly also for non‐legumes.

Resistance of seedling white clover cultivars to redlegged earth mite &lt;i&gt;Halotydeus destructor&lt;/i&gt;

Two glasshouse pot bioassays were conducted in Perth, Western Australia, in 1994 which compared the seedling resistance of subterranean clover (Trifolium subterraneum L.) with New Zealand white clover (Trifolium repens L.) cultivars ‘Grasslands Huia’, ‘Grasslands Tahora’, ‘Prop’, ‘Grasslands Pitau’ and ‘Grasslands Kopu’ to redlegged earth mite (Halotydeus destructor) (Acarina: Penthaleidae). Plant wet weights, mite development and survival and plant damage scores were monitored. Subterranean clover was more resistant to redlegged earth mite than white clover in both experiments. The white clover cultivars, ‘Grasslands Pitau’ and ‘Grasslands Kopu’ had lower cotyledon damage scores and higher yields than the other white clover cultivars.

Effects of improved, adapted white clover (Trifolium repens L.) germplasm on the productive properties of a hill pasture

White clover (Trifolium repens L.) herbage accumulation rate, nitrogen (N) fixation rate and efficiency, and plant growth characteristics were compared in pastures based on eight white clover lines on north‐ and south‐facing sites under sheep grazing in southern North Island hill country during years 4–7 following oversowing. The lines compared included three cultivars ('Grasslands Tahora’, ‘Grasslands Prestige’, and Prop) bred for hill country and/or sheep grazing systems with (notionally, at least) a moderate‐high degree of adaptation to the environment, and two cultivars ('Grasslands Huia’ and ‘Grasslands Kopu') known to have moderate—low adaptation. The control treatment comprised pastures based on the resident clover ecotype of the site, seed of which was sown at the same rate as the other lines. Two of the adapted lines (Tahora, Prestige) yielded significantly more white clover herbage than the resident ecotype over 3 full years of measurement. Prestige displayed consistently greater cool‐season growth in late winter‐early spring than all other lines. All three adapted cultivars fixed significantly more N than the resident ecotype over 12 months (mean for resident = estimated 64 kg N/ha per year compared to mean for three cultivars = 100 kg N/ha per year). There was a close linear relationship between white clover herbage accumulation rate and clover stolon density in most years and on both aspects. The adapted lines developed greater stolon densities through superior branching ability (higher frequency of occurrence of branches at stolon nodes). There was evidence that the adapted clover lines were able to compete more effectively with grasses for growth resources at some times of the year, e.g., for mineralised N in late winter‐early spring, resulting in lower N fixation efficiencies for Tahora and Prestige (mean 43 kg N fixed/t clover DM grown, versus 122 kg N/t clover DM for the resident ecotype). There were no statistically significant effects of clover line on total annual pasture herbage accumulation, but a difference between Prestige and all other lines in total pasture growth in winter gradually strengthened over 4 years of measurement so that, in the final year measured (year 7 after sowing), a statistically significant effect of line was recorded. In this winter, Prestige‐based pastures grew 1130 kg DM/ ha over 65 days from early June to late August, whereas resident pastures grew 570 kg DM/ha. The results are discussed within the context of pasture improvement strategies in hill country. Several important effects of adapted germplasm on the productive properties of hill pastures are identified.

Identification of soil bacteria expressing a symbiotic plasmid from Rhizobium leguminosarum bv. trofolii.

A hundred strains of non-nodulating, Gram-negative, rod-shaped bacteria were isolated from clover-ryegrass pastures on three different soil types and from a sandy loam under lupins. When crossed with Escherichia coli PN200 containing the cointegrate plasmid pPN1, 11 transconjugants gained the ability to form nodules on the roots of white clover (Trifolium repens cv. Grasslands Huia). A nodA probe indicated that they had gained nodulation genes. The identities of these 11 strains and 4 others derived from earlier work on non-nodulating root nodule bacteria, were determined by ribotyping, DNA-DNA hybridization, and partial 16S rRNA sequencing. Good agreement was obtained between the three methods, and 11 of the strains were identified as Rhizobium leguminosarum (6), Rhizobium loti (2), Rhizobium etli (1), Rhizobium tropici (1), and Sinorhizobium meliloti (1). DNA-DNA hybridization indicated that the remaining four strains were related to the Rhizobium leguminosarum reference strains. The existence of several species of non-nodulating rhizobia in pasture soil, including species for which the normal host plant was absent, is discussed in relation to the fate of symbiotic plasmids from Rhizobium seed inoculants. It is also suggested that new species should be named for the geographical region from which they are first isolated rather than the host plant.

Assimilate Partitioning in Three White Clover Cultivars in the Autumn, and the Effect of Defoliation

Partitioning of recently assimilated 14 C in three cloned white clover cultivars (Aberherald, Grasslands Huia and Sandra), grown outdoors, was determined at the end of Aug., the end of Sep. and the end of Oct. to examine the relative strength of different sinks within the plant when growing conditions increasingly favour carbohydrate accumulation in relation to growth. Also, the effect of removing two out of three expanded leaves on 14 C partitioning was studied. Export of 14 C from leaves increased from late Aug. to late Oct. More 14 C was partitioned to the roots and less to the apex at the Sep. and Oct. harvests than at the harvest in Aug., irrespective of cultivar. Severe defoliation resulted in more 14 C being partitioned to the apex and less to the main stolon irrespective of cultivar and harvest occasion. Sandra (a cultivar of northern origin) generally partitioned more 14 C to the stolon and less to the apex than did the other two cultivars.

Interactions between seedlings of perennial ryegrass and white clover cultivars in establishing swards

AbstractInteractions between perennial ryegrass (grass) and white clover (clover) cultivars were investigated at the seedling stage in two experiments: (a) a field experiment in which two clovers, AberHerald and Grasslands Huia, were grown in binary mixture with two grasses, Preference and Ba 10761; (b) a glasshouse experiment in which the same clover/grass combinations were grown in low‐N soil either with (+ N) or without (‐N) added N. In the field experiment both clovers produced larger and more complex seedlings with Preference, and this was particularly evident in Huia. In the glasshouse experiment grass dry‐matter yield was greater in the +N treatment, and this effect increased with time. Clover seedling density and development were suppressed in the +N treatment, and the development of AberHerald was affected more than Huia. Morphological measurements of the clovers showed interactions between clover, grass and N level. In the ‐N treatment Huia plants were larger and more complex than those of AberHerald, but in +N conditions there was little difference between them. Grass cultivar had an effect on clover via N‐level interactions: in +N plants there was no grass effect, but ‐N plants were significantly larger with Preference. Comparison of the root and shoot morphology of the two grasses revealed no obvious differences that would account for these effects.

The Influence of Elevated CO2Concentration on Growth of Seven Grasses and One Clover Species in a Cool Maritime Climate

Abstract The effect of elevated CO2 concentration on the growth of eight common species and cultivars in Norwegian meadows—Festuca pratensis “Salten” and “Fure”, Festuca rubra “Koket” and “Leik”, Festuca arundinaceae “Vantage”, Festuca duruiscula “Barfina”, Poa pratensis “Lavang”, Agrostis capillaris “Aros”, Dactylis- glomerata “Apelsvoll”, and Trifolium repens “Grasslands Huia”—was studied during 11 weeks (April 26-early July). The study was performed in ten 9 m2 large field chamber units in a cool (11.3°C) maritime climate under long days (15.1–18.1 h), on the south-west coast of Norway (59°N, 6°E). The different species responded differently to elevated CO2 with respect to tillering, which was enhanced in A. capillaris (81%), D. glomerata (23%) and F. pratensis (36%), but was not significantly affected in the other species. The sward length was significantly decreased by high CO2 concentration, by 20% in P. pratensis, 36% in A. capillaris, 29% in D. glomerata, 26% in F. duruiscula, 36% in F. pratensis ...

Solution concentrations required for near maximum yield in ryegrass and white clover when grown in a low ionic strength solution: Preliminary results

Abstract In a series of preliminary experiments, the effect of varying solution concentrations of several nutrients on yield in ryegrass (Lolium perenne L. cv Grasslands Nui) or white clover (Trifolium repens cv Grasslands Huia) were investigated using a still low ionic strength (2.7 x 10‐3M) nutrient solution culture technique. The concentration of the nutrients in the basal solution was (μM): 500 calcium (Ca); 100 magnesium (Mg); 300 potassium (K); 600 nitrogen (N) [150 ammonium (NH4), 450 nitrate (NO3)]; 2.5 phosphorus (P); 600 sulfur (S); 3 boron (B); 2.5 iron (Fe); 0.5 zinc (Zn); 0.5 manganese (Mn), and 0.1 copper (Cu) at pH 4.7. The solution concentrations required for 95% maximum yield in ryegrass (μM) were: < 240 for total N, 2 for P, < 240 for S, < 40 for Mg, < 200 for Ca, and < 100 for K. The < symbol indicates that yield did not decrease nor increase, suggesting that the lowest solution concentration used (shown after < symbol) was adequate for 95% maximum yield. In white clover, solution conce...

Trends in the global marketing of white clover cultivars

White clover (Trifolium repens L.) is the dominant legume of temperate pastures, having been improved by breeding since the 1930s. The 1994 OECD Register lists 93 cultivars, with a further 25-30 cultivars also known to commerce. Therefore, in excess of 100 cultivars are available to fulfil a world annual market of 8500-10,500 MT. Globally, New Zealand is the major white clover production region, providing 50- 55% of the seed. Other key production regions are Denmark, USA and South America. Consumption of white clover has been relatively static for some time, ranging from 8000-10,000 MT per annum. Between 55-60% of the seed is used in the northern hemisphere (approx. 40% by Western Europe - UK, France, Germany). USA, South America and New Zealand all consume similar tonnages annually (approx. 1000-2000 MT each) with smaller tonnages used in Australia. One cultivar, Grasslands Huia, has dominated the world white clover seed market for many years and it still remains the world's major cultivar by volume. However, in the last 10 years its position has come under increasing pressure from New Zealand and international proprietary cultivars. Eventually proprietary cultivars with improved agronomic performance will supersede the consumption of commodity/public white clovers in those countries where national listing is required to obtain release and recommended listing is need to ensure successful marketing. Despite this, Grasslands Huia will continue for many years to play a major role in supplying markets where national listing is not a prerequisite to release and price is a major factor in purchase decisions. Keywords: commodity cultivar, global market, Grasslands Huia, leaf size, market share, production trends, proprietary cultivar, seed industry, Trifolium repens L., white clover

Inheritance of multifoliolate leaves in white clover

During 1988/89 selected white clover (Trifolium repens L.) genotypes were studied to determine their inheritance of multifoliolate (mf) leaves in relation to the multifoliolate percentage of the parent material. Genotypes differing in multifoliolate frequency were crossed in an incomplete diallel with plants from three Grasslands white clover cultivars, Huia, Kopu and Tahora. Each cultivar was used in three separate pair crosses, with genotypes expressing multifoliolate leaf percentages of 25%, 50% and 75% respectively. These mf plants were also pair crossed and plants from within each cultivar were pair crossed 3 times. All plants in the total of 21 pair crosses were bee pollinated. Harvested seed from each genotype was sown and raised under glasshouse conditions, and after ninety days each young plant was evaluated for mf leaf production. Leaf count results showed that 31% of cultivar x mf genotypes expressed mf leaves, while in the mf x mf programme 90% of genotypes displayed mf leaves. A distinct increase in the percentage of multifoliolate leaves occurred in crosses between mf plants and the trifoliolate cultivars when the mf genotype was the maternal parent. It is apparent that the multifoliolate character is heritable and that the percentage of mf leaves can be increased through breeding and selection. Keywords: expression levels, inheritance, multifoliolate, white clover

Pasture species effects on carcass and meat quality

Increasing concerns about food quality and animal fat in the human diet have promoted research into the effects of different forage species on carcass quality and fat content. This trial investigated the effect on carcass composition and the intensity of sheep meat flavour and odour of five different pure pasture species: Grasslands Lancelot plantain (Plantago lanceolata), Grasslands Puna chicory (Cichorium intybus), Grasslands Huia white clover (Trifolium repens), Grasslands Nui high-endophyte perennial ryegrass (Lolium perenne) and Grasslands Goldie lotus (Lotus corniculatus). Although pasture species did affect meat quality and fat content, there appeared to be an inverse relationship with carcass weight: larger lambs had more fat, which was related to difference in body size and relative maturity rather that pasture species. Pasture species also influenced muscle ultimate pH, sheep meat odour, foreign odours, and flavours, but did not affect sheep meat flavour. Keywords: Cichorium intybus, meat flavour, Lolium perenne, Lotus corniculatus, meat odour, Plantago lanceolata, Trifolium repens

Seedling White Clover Response to Defoliation

Defoliation during the early stages of seedling development can influence growth of white clover (Trifolium repens L.). Our objective was to determine the response of cultivars with diverse leaf sizes to defoliation across a range of seedling growth stages. ‘Regal’ (large‐leaf), ‘Louisiana S‐1’ (medium‐large‐leaf), ‘Grasslands Huia’ (medium‐smallleaf), and ‘Aberystwyth S184’ (small‐leaf) were grown in the greenhouse and were initially clipped to remove all leaf (except cotyledons) and petiole tissue (1.0‐cm stubble) when seedlings attained a unifoliolate leaf or one, two, four, and eight trifoliolate leaves. Seedlings were subsequently clipped in the same manner every 7 or 28 d for the next 28 d. Plant response to defoliation was assessed 7, 14, 21, and 28 d after the last clipping. Seedling shoot dry weight of all cultivars increased linearly in response to initial leaf stage when subsequent clipping occurred every 7 d after the initial defoliation. When subsequent clipping was delayed 28 d, defoliation after the four‐trifoliolate stage did not influence shoot weight of any cultivar except Regal. Despite fewer numbers of leaves after regrowth, seedling leaf area of Regal was always greater than that of the other cultivars. Number of stolon branches of Regal and Louisiana S‐1 increased in response to leaf stage at initial defoliation (0.15 branches stage−1), while that of Grasslands Huia and Aberystwyth S184 was relatively unchanged after the four‐ to six‐trifoliolate stage. Permitting white clover seedlings to attain advanced leaf stages before initial defoliation and increasing the time interval before subsequent defoliation will enhance seedling growth and potential survival.

White Clover Growth and Morphology under Contrasting Cutting Regimes

Climatic conditions contribute to reduced white clover (Trifolium repens L.) productivity and plant survival in the southeastern USA, particularly during the summer. Our objective was to determine the influence of cutting height and frequency on yield and plant morphological traits associated with survival of white clover under humid‐subtropic conditions. Regal (large‐leaf), Louisiana S‐1 (medium‐largeleaf),and Grasslands Huia (medium‐small‐leaf) white clover were sown on a Catalpa silty clay (fine, montmorillonitic, thermic Fluvaquentic Hapludoll) and were cut every 7, 28, and 49 d at a 2.5‐ and 10‐cm stubble height from March to November. Forage yield was measured at each harvest. Stolon growth and branching (growing point density; GPD) and post‐harvest leaf area index (LAD were measured in May, August, and November. Dry matter (DM) yield measured by season (spring, summer, and autumn) increased as leaf size increased and cutting height decreased. The effect of cutting height on stolon DM was opposite that of its effect on DM yield. After the spring, stolon DM of all cultivars was reduced by harvesting at 2.5‐ vs. 10.0‐cm stubble, regardless of cutting interval. Within a cutting height, increasing the harvest interval from 7 or 28 to 49 d generally resulted in greater stolon DM accumulation by autumn. During much of the growing season, there was a positive association between stolon GPD and post‐harvest LAI. Our results suggest that close defoliation of white clover (2.5‐cm stubble) during the summer and autumn detrimental to plant survival, particularly when combined with a short (7‐d) defoliation interval.

Responses of white clover to gaseous pollutants and acid mist: implications for setting critical levels and loads

summaryWhite clover plants (Trifolium repens L. cv. ‘Grasslands Huia’) were exposed to combinations of gaseous pollutants and acid mists for 15 wk. The treatments were (a) charcoal‐filtered air (control), (b) 40 ppb SO2+ 40 ppb NO2, (e) 40 ppb Oa with additional peaks of 2 × 3 h at 80 ppb and 1 × 1 h at 110 ppb O3 and (d) SO2+NO2+O3 (a combination of treatments b and c), these being combined with acid mist treatments (6 mm wk1) with solutions at pHs of 2.5, 3.5, 4.5 and 5.6. All gaseous pollution treatments resulted in substantial reductions in plant dry weights with additive effects of SO2+ NO2 and O3 in the SO2+ NO2+O2 treatment. There was a lower dry weight, averaged over all gas pollution treatments, for plants exposed to pH 2.5 compared with pH 4.5 or pH 5.6 mists. An interactive effect of gas x mist treatments revealed that effects of acid mists on plant dry weights were overridden by the more toxic O3 and SO2+ NO2+ O3 gas exposures. Net photosynthesis was reduced in plants exposed to pH 2.5 and pH 3.5 mists compared with plants exposed to pH 5.6 mist. Rates of stomatal conductance were reduced by exposure to the gaseous pollution treatments but not to acid mists. Water loss was greater after 55 h for leaves detached from plants exposed to all gaseous pollution treatments compared with ones from control plants. There was a greater loss of water from detached leaves exposed to pH 2.5 compared with pH 5.6 mist. The quantities (loads) of S + N in gaseous form resulting in decreased growth of T. repens were calculated to be much lower than those applied in mists with no adverse effects. It is concluded that the concepts of critical levels and critical loads of pollutants need considerable refinement for realistic assessments of the threat of pollutants to vegetation.