Subterranean Clover Seeds Research Articles

Time of sowing and cultivar effects on hardseededness and germination of subterranean clover seeds

ABSTRACT The annual regeneration from seeds of subterranean clover plants after initial sowing depends on the hardseededness (HSmax, %), its breakdown rate (HSbreak, %/ °C) and the germinability of the seeds over time. Hardseededness is a physical mechanism of dormancy that prevents germination of seeds and is ecologically important for the persistence of subterranean clover in a range of environments. The effects of genotype (G) and sowing time (environment, E) on hardseedednes and germination were investigated for ‘Antas’, ‘Denmark’, ‘Leura’, ‘Monti’, ‘Narrikup’ and ‘Woogenellup’ harvested from six different sowing dates (June 2015–March 2016) in a field experiment in Canterbury, New Zealand. ‘Antas’ had a lower HSmax (53%) than ‘Monti’ (86%). HSbreak was affected by the G x E interaction with a doubling of HSbreak from 0.011%/°C for the June sowing date to ∼0.02%/°Cd for the February sowing date for ‘Denmark’, ‘Leura’, ‘Monti’ and ‘Narrikup’. There was less sensitivity of ‘Antas’ and ‘Wogenellup’ to sowing time. There was an increase in the percentage of abnormal seedlings as the sowing date advanced from winter to late-summer. The developed parameters enabled estimation of seedling emergence and indicate cultivars which have a competitive advantage of an early seedling regeneration in temperate regions.

Harvesting subterranean clover seed – current practices, technology and issues

Subterranean clover (Trifolium subterraneum L.) is Australia’s most widely sown annual pasture legume. Its widespread use as a pasture plant requires a well-functioning seed production industry, and Australia is the only significant producer of subterranean clover seed globally. However, the sustainability of this industry is under threat due to its reliance on ageing harvest equipment and the resultant environmental impacts. In order to evaluate seed harvesting practices, technology, and issues, we report on case studies, workshops, and a survey of seed producers across southern Australia. The Horwood Bagshaw Clover Harvester, designed in the 1950s, remains the most popular subterranean clover seed harvester. We discuss its use and modifications, and document several contemporary issues facing the seed production industry. Issues are primarily soil erosion and degradation; the expensive, slow and labour-intensive harvest process; and poor reliability and maintainability of harvesters that are now at least 30 years old. We conclude the root cause of these issues is the suction harvest technology utilised by the Horwood Bagshaw Clover Harvester. Analysis of the current harvest system is provided to support the development of new approaches to harvest subterranean clover seeds.

Effects of dark or of red, blue or white light on germination of subterranean clover seeds

Effects of dark or of red, blue or white light on germination of subterranean clover seeds

Effects of rhizobium, arbuscular mycorrhizal fungi and anion content of simulated rain on subterranean clover

An experiment was conducted to determine the extent to which rhizobia, mycorrhizal fungi, and anions in simulated rain affect plant growth response to acid deposition. Germinating subterranean clover seeds were planted in steam-pasteurized soil in pots and inoculated with Rhizobium leguminosarum, Glomus intraradices, Glomus etunicatum, R. leguminosarum + G. intraradices, R. leguminosarum + G. etunicatum, or no microbial symbionts. Beginning 3 weeks later, plants and the soil surface were exposed to simulated rain in a greenhouse on 3 days week −1 for 12 weeks. Rain solutions were deionized water amended with background ions only (pH 5.0) or also adjusted to pH 3.0 with HNO 3 only, H 2SO 4 only, or a 50/50 mixture of the two acids. Glomus intraradices colonized plant roots poorly, and G. intraradices-inoculated plants responded like nonmycorrhizal plants to rhizobia and rain treatments. Variation in plant biomass attributable to different rain formulations was strongest for G. etunicatum-inoculated plants, and the effect of rain formulation differed with respect to nodulation by rhizobia. The smallest plants at the end of the experiment were noninoculated plants exposed to rains (0.38 g mean dry weight total for 3 plants pot −1). Among nonnodulated plants infected by G. etunicatum, those exposed to HNO 3 rain were largest, followed by plants exposed to HNO 3 + H 2SO 4, pH 5.0, and H 2SO 4 rain, in that order. Among plants inoculated with both R. leguminosarum + G. etunicatum, however, the greatest biomass occurred with pH 5.0 rains, resulting in the largest plants in the study (1.00 g/3 plants). Treatment-related variation among root and shoot biomass data reflected those for whole-plant biomass. Based on quantification of biomass and N concentrations in shoot and root tissues, total N content of plants inoculated with G. etunicatum alone and exposed to the HNO 3 + H 2SO 4 rains was approximately the same as plants inoculated with R. leguminosarum + G. entunicatum and exposed to pH 5 rains. Thus, the acid-mixture rains and rhizobia under no acid deposition provided approximately equal amounts of N in biomass. The significant interactions among rain formulation and the symbiotic status of the plants suggest that conclusions concerning the impact of acid deposition on plants in the environment cannot be considered reliable because most experiments on which such assessments are based have not tested confounding influences of microorganisms and precipitation characteristics.

Temperature and Soil Water Effects on Dormancy and Mortality of Subterranean Clover Seed

AbstractGermination tests were conducted to investigate possible causes for a reestablishment failure of subterranean clover (Trifolium subterraneum L.). ‘Mount Barker’ subterranean clover seeds were germinated on blotters and on the soil surface or at 1 cm depth in nonfumigated and fumigated soil at four constant temperatures for 14 d, followed by constant 15°C for 8 d. Fumigation had no effect on germination. Germination on blotters was reduced to 11.1% at 25 and 30°C, compared with 94.5% at 15°C. Emergence of seedlings was reduced to 37.9% at 30°C, compared with 92.5% at 15°C. Emergence of seedlings at 30°C from soil was greater after 14 d than germination on blotters. In a second experiment, seeds buried 1 cm were germinated at soil water levels ranging from 0.0 to 0.816 kg kg−1. Temperatures were 15 or 30°C for 7 d, then 15°C for 7 d more. Seeds failed to germinate at either temperature in dry or saturated soil. Emergence after 7 d was reduced at 30°C compared with 15°C at soil moisture levels from 0.204 to 0.544 kg kg−1. After 7 more d at 15°C, emergence was similar between temperatures at soil moisture levels from 0.0 to 0.544 kg kg−1. Under saturated soil conditions, 28.5 and 80.8% of the seed rotted at 15 and 30°C, respectively. Our results suggest that subterranean clover seeds can survive in pasture soil for 14 d in an embryo dormant condition during the summer, unless the soil is saturated, in which case soft seeds tend to decay.

Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders associated with subterranean clover produces non-infective hyphae during sporulation.

Subterranean clover (Trifolium subterraneum L.) inoculated with either Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders or Glomus sp. (WUM 10(1)) was grown in 3 kg of Lancelin sand surrounding a fine mesh bag (< 38 μm) filled with 400 g of steamed sand. The mesh size was chosen to allow hyphae to penetrate but to exclude plant roots. Every week, from 3 wk after planting the original plants, subterranean clover seeds were sown into the sand inside the mesh bags. The last sowing date was 8 wk from sowing of the original plants. The seedlings, with their roots growing inside the mesh bags, were allowed to grow for 3 wk. At that stage the seedlings and the original plants were harvested and their roots assessed for mycorrhizal colonization. The percentage of root length of the original plants colonized by S. calospora decreased from the first harvest onwards, while colonization by Glomus sp. maintained a similar level at all harvests. Seedlings growing in pots whose original plants were colonized by S. calospora had a her percentage of root length colonized at the first two harvests but levels of Colonization decreased substantially thereafter. Seedlings in the Glomus sp. treatment on the other hand, had similar levels of colonization at all but the last harvest. The length of hyphae g-1 soil in the mesh hues increased throughout the experiment for both fungi. However, the length of hyphae in the S. calospora treatment increased at a faster rate. Spore production by S. calospora commenced 7 wk after sowing the original plants. These results indicate that the hyphae produced by S. colospora after internal colonization had stopped increasing were largely non-infective, and that these hyphae are primarily involved in the production of spores.

Long-term patterns of seed softening in some annual pasture legumes in a low rainfall environment

Annual rates of seed softening were determined from 4 lines of burr medic (Medicago polymorpha), 1 barrel medic (M. truncatula), and 1 subterranean clover (Trifolium subterraneum) grown at Merredin in the 1 year. Measurements were also made on one of the lines of burr medic grown in 2 other environments, Gnowangerup and Eneabba, in the same year. Burrs were placed on the soil surface at Merredin and the numbers of residual hard seeds determined each year for up to 5 years in this one environment. Patterns of softening of seeds from the same seed populations were also determined in a laboratory oven with a diurnal temperature fluctuation of 60/15�C. In the field, the softening rates of the 5 medics grown at Merredin were similar, averaging 21% of the original seeds each year for the first 4 years. Seeds of the burr medic grown in a more favourable environment at Eneabba were much slower to soften (averaging 14%); hence, hardseededness in these medics was influenced more by the growing environment than by genotype. More than half of the seeds of subterranean clover softened in the field over the first summer, with declining annual proportions thereafter. There were clear differences between the clover and medics in both pattern and rate of seed softening. The lower seed-softening rate of medics than of subterranean clover was more favourable for ley systems involving frequent cropping, especially in low rainfall areas. Treatment of seeds at 60/15�C simulated field softening for subterranean clover well but produced misleading results for the medics.

Effect of depth of burial on the longevity of hard seeds of subterranean clover and annual medics

Burrs of 3 cultivars of subterranean clover (Trifolium subterraneum) and 1 cultivar each of burr medic (Medicago polymorpha) and barrel medic (M. truncatula), which had experienced 1 summer at the soil surface, were placed on the soil surface and at depths of 2, 6 and 10 cm in the soil. The numbers of residual hard seeds were determined each year for up to 4 years. There was a marked reduction in the rate of seed softening in all 3 clover cultivars with increasing depth of burial. Whereas &lt;20% of the seeds of the hardest seeded clover cultivar, Nungarin, survived 3 years at the soil surface, there was no significant decline in seed numbers during 4 years of burial at 10 cm. Even with cv. Geraldton, in which only 5% of seeds remained after 1 year of placement at the soil surface, 75% of seeds survived 4 years of burial at 10 cm. Hard seeds of both medic varieties were considerably more resilient than clover seeds at the soil surface, particularly during the first summer following seed set. However, burial had much less effect on their longevity, with no significant effect of burial to 2 cm in either medic, or of burial to 6 cm in the case of barrel medic. These results support earlier findings which showed that tillage operations associated with crop establishment which result in the burial of substantial proportions of subterranean clover seeds can lead to useful soil seed reserves. The much lesser effect of burial on seed softening of the medics, compared with subterranean clover, suggests that tillage operations will be less advantageous to medic persistence in leys.

Sulfur supply and the formation of vesicular-arbuscular mycorrhizas by Glomus fasciculatum on subterranean clover

Sulfur supply and the formation of vesicular-arbuscular mycorrhizas by Glomus fasciculatum on subterranean clover

Effect of tillage practices on the fate of hard seeds of subterranean clover in a ley farming system

In a rotation of 1 year pasture/l year crop, a subterranean clover (Trifolium subterraneum cv. Daliak) pasture was either left untilled or subjected to minimum or conventional tillage. One set of tillage treatments was imposed in each ofthree crop years while another set of treatments was imposed in only the first crop year. Regenerating clover plants were prevented from setting seed. In the first crop, 44% of the clover seeds were buried below 2 cm of soil by minimum tillage; this proportion was 65% in the conventional tillage treatment. In the first pasture regeneration year, seedling densities were highest in the no-tillage treatment. Conversely, there were more residual seeds in the tilled treatments and, in the second and third pasture regeneration years, this led to higher seedling densities than in the no-tillage treatment. The effects of tillage were more marked in the conventional than in the minimum-tillage treatment. Clover establishment was improved by repeat tillage operations which returned some of the buried seeds closer to the soil surface. Although more seedlings overall were obtained from the no-tillage treatment, the disadvantage of fewer seedlings in the tilled treatments was offset by the spread of seedling establishment over a number of pasture years. This spread, which would be more marked with harder-seeded cultivars, could be desirable in environments in which clover seed production is unreliable.

Effect of burial on the softening of hard seeds of subterranean clover

Burrs of eight varieties of subterranean clover (Trifolium subterraneum L.), which had experienced one summer at the soil surface, were placed on the soil surface and at depths of 2, 6 and 10 cm in the soil. The numbers of residual hard seeds were determined after 1, 2 and 3 years. The effects of laboratory treatment at a diurnally fluctuating temperature of 60/15�C on the softening of buried seeds and of seeds stored in the laboratory for 1 and 3 years were determined. Rate of seed softening in all varieties decreased with increasing depth of burial, apparently because the soil insulated the seeds from high soil surface temperatures. Few seeds of the varieties Northam and Geraldton softened during 3 years of burial at 6 or 10 cm; while, at the other extreme, few seeds of Yarloop survived 3 years at any depth. Some evidence was found for microbial decomposition of hard seeds in the field. Seeds softened more readily at 60/15�C (in the laboratory) as the preceding periods of either laboratory storage or field burial increased. Such storage or burial experiences have a preconditioning effect on hard seeds, making them more amenable to softening once they are subjected to wide diurnal temperature fluctuations. The results indicate that soil tillage associated with cropping should build up a useful soil seed reserve of the harder seeded varieties.

Growth of Subterranean Clover in a Range Soil as Affected by Microclimate and Phosphorus Availability. I. Field Studies1

AbstractRelationships between microenvironment and phosphorus availability are important to stand establishment and winter growth of introduced annual legumes in California's foothill rangelands.Ridge, furrow, north‐facing, and south‐facing microsites were established to create variations in light and temperature microenvironments in previously tilled soils of the Sobrante and Las Posas series. An additional microsite treatment, consisting of a simulated range drill seeding, with and without a litter cover, was made on adjacent untilled soil. Phosphorus rates, applied as single superphosphate, were 0, 40, 80, and 160 kg P/ha.Pellet‐inoculated seeds of subterranean clover (Trifolium subterranean L.) were sown in rows 15 cm apart perpendicular to the microsite ridges, slopes, and furrows.Soil (1 cm) and air (3 cm above soil) temperatures were monitored hourly with thermistors.Plant sampling was done at four times from November to March to determine morphological development, plant weight, and percent P per plant. At maturity, remaining plants were sampled to estimate seed production.Large diurnal temperature differences were associated with microsite treatments and were more pronounced for soil than for air.Percent P in shoot tissue generally reached highest values in January, indicating that plant growth was restricted more during winter than was P uptake. With warmer temperatures and higher light intensities in February, growth dilution resulted in a decrease in P percentage. Small modifications of microrelief significantly altered the microenvironment, especially soil temperature. Morphological development and plant weight responses were greatest on south‐facing slopes and at the highest P level. Seed production was not influenced by P fertilization.

Time distribution of seedlings emergence from single seed crops of several annual pasture legumes

Seedling emergence from single seed crops of Dwalganup and Geraldton subterranean clover, Yamina cupped clover, Troodos and Kondinin rose clover, and Cyprus barrel medic, was studied in a low rainfall wheatbelt environment in Western Australia. The proportion of the original seed set recovered as seedlings was highest from the subterranean clovers in the first year but about the same for all genotypes in the second year. Few subterranean clover seeds were recovered in the third regeneration year either as seedlings or residual hard seeds, whereas significant proportions of the original seed of the other genotypes were obtained. More than 10 per cent of the original seed of Cyprus barrel medic and Yamina cupped clover remained ungerminated at the end of the third season. Between about 35 and 75 per cent of the original seed set was not accounted for in the genotypes studied. The proportions of the total seedling numbers that emerged following summer and early autumn rains were generally highest from the subterranean clovers and least from the cupped clover and medic. It is suggested that these different patterns of emergence within years were the result of differences in embyro dormancy rather than different patterns of hard seed breakdown.

Embryo dormancy in subterranean clover seeds. I. Environmental control

Embryo dormancy in seeds of subterranean clover (Trifolium subterraneum) is at a maximum at field maturity in the spring. High summer temperatures are the causative agent in the subsequent breakdown of embryo dormancy, the germination inhibitors in the seeds apparently being susceptible to heat. There is no evidence for germination inhibitors in the pod and burr material, nor is there any evidence to support the hypothesis of an environmentally imposed dormancy, for germination is relatively fast at 15 ? 25�C, a temperature regime equivalent to the daily temperature change of summer moist soil. There are differences between cultivars in the initial level of embryo dormancy and in the subsequent rate of breakdown.

Embryo dormancy in subterranean clover seeds. II. Its value relative to impermeability in field germination regulation

Less than 20% of the viable seeds of three subterranean clover cultivars survived exposure to typical summer field environments in the absence of protection from embryo dormancy and impermeability. The survival rate of seeds was no higher when protected by embryo dormancy only. By contrast, when the seeds were protected by both embryo dormancy and impermeability the survival rate of viable seeds was 70% or higher. It is concluded that fully mature dry seeds of subterranean clover receive little protection from embryo dormancy during the summer, although the mechanism may well be important in preventing germination during the late seed maturation period. It is suggested that in breeding and selection programmes impermeability should receive priority as a germination-regulating mechanism.

The action of temperature fluctuations on hard seeds of subterranean clover

Burrs of three cultivars of subterranean clover (Trifolium subterraneum) were placed in soil in a cold frame at Canberra so that they were subjected to daily temperature fluctuations of the order of 20-54�C. After three months and eight months the proportion of permeable seeds was significantly increased. Such seeds were conductive to water at one specific region of the testa-the strophiole. In a further experiment, under laboratory conditions, hard seeds were subjected to temperature fluctuations of 23-60�C with cycle lengths varying from 15 minutes to 1 hour. There was no reduction in the percentage of hard seeds except in two trials when that portion of each cycle at 60�C was greater than 45 minutes.

Reversibility of Strophiolar Permeability to Water in Seeds of Subterranean Clover (Trifolium Subterraneum L.)

Subterranean clover seeds which were impermeable to water were made permeable by percussion, but the impermeability could be quantitatively restored by covering the strophiolar region with Araldite or Vaseline, indicating that the percussed strophiole was the only site of water conduction.

Dormancy Regulation in Subterranean Clover Seeds by Ethylene

In the past there have been scattered reports of breaking of dormancy in seeds by ethylene (4, 11) or by ethylene chlorohydrin (5) which apparently acts on plants through a release of ethylene (12). The advent of gas chromatographic techniques has made it possible to show that many seeds produce ethylene naturally during the germination process (8,9, 10); these reports raise the interesting possibility that the production of ethylene by the imbibed seed may contribute to the breaking of dormancy. This possibility has been examined for peanuts by Toole et al. (10) and Ketring and Morgan (6), who concluded that the ethylene production may actually contribute to the breaking of dormancy. From studies with lettuce, Abeles and Lonski (1) reached the opposite conclusion since ethylene was produced mainly by those seeds which were going to germinate, and there were only small increases in percentage germination in the presence of added ethylene. We have been studying the role of ethylene in germination of seeds of several species, and our evidence indicates that in some seeds there is a substantial response to ethylene in breaking of dormancy, and further that the ethylene produced by the seed can have a regulatory role in determining whether or not germination will be achieved.

Further studies on the role of chalcone and flavanone in biosynthesis of flavonoids

Competitive feeding experiments using 4,2′,4′-trihydroxychalcone[-carbonyl- 14C] and (−)-7,4′-dihydroxyflavanone-[T] have been carried out with seeds of subterranean clover ( Trifolium subterraneum L.) and garbonzo bean ( Cicer arietinum L.) and with cell-free extracts of garbanzo seedlings. T/ 14C ratios of the radioactive products 3,7,4′-trihydroxyflavanone (garbanzol), 7,4′-dihydroxyflavonol, 7,4′-dihydroxyflavone and 7-hydroxy-4′-methoxyisoflavone (formononetin) were determined at various times after feeding. Results obtained support the previous conclusion that chalcone is a more immediate precursor than flavanone for the biosynthesis of these flavonoids.

Studies of Dormancy in the Seeds of Subterranean Clover (Trifolium Subterraneum L.)

When imbibed dormant subterranean clover seeds were exposed to low concentrations of oxygen for up to 6 days, and then transferred to either air or 100% oxygen atmospheres, germination was markedly increased above that of seeds held only in air. Stimulation of germination was similar whether the atmosphere of the second phase was air or 100% oxygen; it was maximal when that of the first phase contained no oxygen, and became insignificant above concentrations in the region of 5% oxygen. The additional germination was roughly proportional to the duration of exposure to low oxygen concentrations, and the effects of two separated exposures to low oxygen were additive. These effects could be produced only in those dormant samples whose seeds or embryos could also be made germinable by exposure to 2�5% carbon dioxide. At higher temperatures, anaerobic conditions were less effective in breaking dormancy, paralleling the reduced efficacy of carbon dioxide at these temperatures.