Phytochrome-mediated germination control of Datura stramonium L. seeds after seed burial

A series of experiments investigated the light sensitivity of buried weed seeds both within and outside the soil gas microenvironment. Light sensitivity of Datura stramonium L. seeds was found to be increased markedly by a period of soil burial; seeds showed a pronounced germination response even in far-red light (724 nm). Despite this elevated light sensitivity (even to less than 0.01% of incident light), soil overlying buried seeds was sufficient to neutralize the germination trigger. Furthermore, in situ irradiation of buried seeds was found to be virtually ineffective as a germination trigger, showing that the gaseous environment surrounding buried seeds may represent the main obstacle to germination. It is suggested in this study that the soil environment restricts the removal of the germination-inhibiting products of fermentation metabolism. This suggestion is supported by the finding that gas exchange within the seed:soil complex, achieved by flushing with nitrogen, partially restores seed light sensitivity. In addition, it was found that with increasing sowing depth, the germination response was attenuated and also became less dependent on seed phytochrome photoequilibrium conditions. This demonstrates that the phenomenon of germination photoinduction can be reversed by other environmental factors that reduce or eliminate phytochrome physiological activity. Finally, it was shown that seed very low fluence response (VLFR) is triggered only when gas exchange around buried seeds co-occurs with light exposure.     

Longevity,dormancy and germination of Cyanus segetum

Cyanus segetum is an iconic, colourful weed in arable fields that provides ecological and societal services. To understand better both the infestation dynamics of C. segetum as an abundant, harmful weed and maintain sustainable populations where it provides beneficial services, we compared information on seed dormancy, seed longevity and germination conditions in two populations. Persistence of seeds buried in the soil was low, with <10% viable after 3 years. Periodic dormancy cycling was observed over the 4 years in the soil, with a maximum of dormant seeds in the spring and a minimum in the autumn; however, 20% of the seeds were non‐dormant all the time. Seeds of C. segetum were positive photosensitive, but light requirement varied among populations. Base water potential for germination was ?1 MPa. Base temperature ranged from 1 to 2°C. Optimum temperature for germination was about 10 to 15°C, but the mean thermal time varied greatly between populations, from 80 to 134 day °C. Photoperiod and temperature combinations had no effect on germination percentage, but both reduced the germination rate. Burial deeper than 2 cm greatly reduced germination and seedling emergence strongly decreased at depths >0.5 cm. No seeds buried deeper than 8 cm emerged. Low seed longevity and a wide range of germination conditions could partly explain the rapid disappearance of C. segetum populations after herbicide application began in western Europe. However, yearly sowing in restoration areas does not seem to be essential.     

Environmental and physiological factors affecting the fate of seeds of yarrow (Achillea millefolium L.) in arable land in New Zealand

Yarrow (Achillea millefolium L.) has arisen as a problem weed in arable land in New Zealand. A spring-sown yarrow population of 58 plants m^?2 produced approximately 243000 seeds. These were capable of immediate germination in the autumn, but virtually no germination occurred in the winter and up to 80–100% at other times of the year. Seeds are positively photoblastic, but a considerable percentage of seed (>30%) may germinate in the dark after chilling, coat pricking, exposure to high nitrate concentrations or to alternating temperatures. Seed buried at 32 cm in undisturbed soil lost viability slowly over 4 years while seed in the top 8 cm lost viability almost completely within 2 years.     

Effects of the seed weight and burial depth on the seed behavior of common ragweed (Ambrosia artemisiifolia)

Common ragweed (Ambrosia artemisiifolia L.) is one of the annual plants that were described recently as invasive weeds in Europe. This species is described as an invasive plant that produces seeds that are highly variable. Its production of variably sized seeds is regarded as promoting its spread in different environments. Experiments were carried out to determine the influence of the seed weight and temperature on germination and the influence of the seed weight and burial depth on seedling emergence. The seeds were divided into a number of classes of weight and the seed weight effect on germination was evaluated by Petri dish assays. In another experiment, the seeds were buried at different depths in a clay soil/sand mix to estimate the burial effect on germination and seedling emergence. The germination level of A. artemisiifolia was high overall, between 76.8% and 94.2%. The seed germination was modified by temperature but it was not influenced by the seed weight. The amounts of germination and seedling emergence were greater for the seeds on the soil surface and decreased with an increasing burial depth, from 2 to 8 cm. No germination or emergence was observed for the seeds that were buried at 10 and 12 cm. The lightest seeds were more sensitive to burial. A greater level of seedling emergence for those seeds that were placed near the soil surface could explain the success of this species in open habitats, where the probability of deeper burial is low. After high seed production, the management of A. artemisiifolia in fields could be partly achieved through soil tillage, burying seeds below 10 cm, and not carrying out deep soil tillage the following year.     

Common reed (Phragmites australis) control is influenced by the timing of herbicide application

Herbicides are typically used as the primary method of weed control. Since common reed (Phragmites australis subsp. australis) infestations in terms of density are relatively large in the State of Nebraska, USA, determining the most appropriate timing of herbicide application is critical for developing a weed management programme. Therefore, several field studies were conducted in 2007 and 2008 at three locations along the Platte River, Nebraska, with the aim of assessing the effect of herbicide selection and timing of application on common reed control. Three herbicides (glyphosate, imazapyr, and imazamox) were applied either alone at two doses or as two-way mixtures on three growth stages of common reed, including vegetative, flowering, and seed filling stages. Both doses of imazapyr (280 and 560 g active ingredient [a] ha^?1) provided the highest level of control (≥ 92%) across all three timings, while imazamox provided the lowest level of control. For example, imazamox applied alone at 280 and 560 g a ha^?1 provided poor control (≤ 60%) across all three timings at the highest rating date. Imazapyr and glyphosate provided the highest levels of control (90%) by the end of the first growing season and into the next growing season (390 to 450 days after treatment, DAT), regardless of the herbicide application time. Imazamox and glyphosate provided the lowest level of control (< 30%) at the first application time at 450 DAT, but slightly improved control with later timings (74% and 85% control at the flowering and seed filling stages, respectively). Stem density decreased in all herbicide applications and timings except for imazamox at both doses during the vegetative growth stage, which was not significantly different than the untreated control.     

Seed dormancy and soil seedbank of the invasive weed Chenopodium hybridum in north‐western China

Seed dormancy and persistence in the soil seedbank play a key role in timing of germination and seedling emergence of weeds; thus, knowledge of these traits is required for effective weed management. We investigated seed dormancy and seed persistence on/in soil of Chenopodium hybridum, an annual invasive weed in north‐western China. Fresh seeds are physiologically dormant. Sulphuric acid scarification, mechanical scarification and cold stratification significantly increased germination percentages, whereas dry storage and treatments with plant growth regulators or nitrate had no effect. Dormancy was alleviated by piercing the seed coat but not the pericarp. Pre‐treatment of seeds collected in 2012 and 2013 with sulphuric acid for 30 min increased germination from 0% to 66% and 62% respectively. Effect of cold stratification on seed germination varied with soil moisture content (MC) and duration of treatment; seeds stratified in soil with 12% MC for 2 months germinated to 39%. Burial duration, burial depth and their interaction had significant effects on seed dormancy and seed viability. Dormancy in fresh seeds was released from October to February, and seeds re‐entered dormancy in April. Seed viability decreased with time for seeds on the soil surface and for those buried at a depth of 5 cm, and 39% and 10%, respectively, were viable after 22 months. Thus, C. hybridum can form at least a short‐lived persistent soil seedbank.     

Seed dynamics in populations of A vena sterilis ssp. ludoviciana

Seed populations of Avena sterilis ssp. ludoviciana (Durieu) Nyman were monitored in a naturally occurring infestation throughout its life cycle. Considering the large weed population present (298panicles m^?2), total seed production was relatively low: 3838 seeds m^?2. Only 68% of these seeds were recovered from the soil surface and a further 3% were removed with wheat grain and straw during harvest operations. The numbers of seeds from the stubble between mid-July and mid-September were relatively low (10%). Ploughing the stubble in October buried most of the recently produced seed rain and resulted in a relatively uniform vertical distribution of the seedbank. Maximum seed persistence in the soil ranged from 27 to 43 months (depending on the experimental technique used to do the study). Seed decline followed an exponential pattern on a yearly basis, with the greatest decline taking place between October and April (57–90% in year 1 and 10–40% in year 2), Between May and September the buried seed populations remained practically constant. Seedbank depletion was primarily due to seedling production (25%) and ‘lethal’ germination (24%). Although the depth of burial had very little effect on seed survival, the mode of seed disappearance was closely related to their depth in the soil. Seed depletion through ‘lethal’ germination increased with increasing depth in the soil, whereas depletion through seedling emergence decreased with increasing depth.     

Behaviour of buried and surface-sown seeds of Parthenium hysterophorus

Parthenium hysterophorus L. seeds were buried at a depth of 5 cm for periods of 2–24 months to determine their longevity. The majority (73.7%) of these seeds were still viable after 24 months of burial. The remainder could not be recovered (18.0%) or were no longer viable (8.3%). There was a log-linear decline in persistence of germinable seeds over time, which indicated a constant rate of loss and a half-life of about 6 years. Seedling emergence from surface-sown seeds was also studied. Although there was considerable rainfall (31 mm), seedlings did not emerge during the first month of this experiment. In the succeeding 3 months, there was substantial seedling emergence after rainfall, and 51.4% of seeds had germinated by the end of the fourth month. After 5 months had passed, further seedling emergence was not detected, and intact seeds could not be located. These findings suggest that seed incorporation into the soil is important to the long-term persistence of P . hysterophorus seeds. In an initial test of germination, unburied seeds from the same seed lot exhibited a degree of innate dormancy, and this may explain the delayed germination observed in the surface-sown seeds. In the seed burial and recovery experiment, innate dormancy was lost after 2 months of burial in the field, although in situ germination of buried seed remained low for at least 24 months. Therefore, it appears that more than one dormancy mechanism may contribute to the persistence of P. hysterophorus seeds.     

Germination characteristics and seedbank of the alien species Leucaena leucocephala (Fabaceae) in Brazilian forest: ecological implications

Plant species invasiveness is frequently associated with rapid proliferation and production of seeds that can persist in the soil for long periods of time. Leucaena leucocephala (Fabaceae) is an alien and invasive species, for example in Brazilian forest and savannah ecosystems. This study quantified the invasive potential of this species by analysing its seed rain (using seed collectors), seed longevity in the soil (stored in buried bags) and the germination capacity of the soil seedbank (by collecting soil samples in the study area). Our results showed that seed rain occurred throughout the year, although more intensely from July to September, with about 5500 seeds m^?2 year^?1 being released. The numbers of seeds in the buried bags diminished over time and intact seeds showed low germinability (approximately 15%), although their viability remained >80% of the recovered seeds after two years of in situ storage. The germinability of seeds collected directly from the soil was approximately 40%, indicating that more than half of the seeds of soil seedbank were dormant (physical dormancy). Leucaena leucocephala produces large numbers of seeds and is able to form a persistent short‐lived seedbank (viability 1–5 years). These factors may contribute significantly to its invasive potential, which makes it difficult to control this species once it becomes established. As control costs become higher over time, immediate public efforts are needed to counter this threat.     

Germination ecology, emergence and host detection in Cuscuta campestris

Trials were carried out to study the germination and dormancy of Cuscuta campestris Y. (dodder) seeds and factors influencing the success of early parasitisation of sugarbeet. Primary dormancy can be removed by seed scarification. Germination was negligible at 10°C and optimal at 30°C, while it was not influenced by light. Seed burial induced a cycle of induction and breaking of secondary dormancy. Seedling emergence was inversely proportional to the depth of seed burial and only seed buried within 5 cm of the soil surface emerged. Storage of C. campestris seeds in a laboratory for 12 years resulted in the loss of primary dormancy, enabling the germination of all viable seeds. Host infection (i.e. protrusion of parasite haustoria from host tissue) was heavily influenced by host growth stage. Tropism towards a host was due to the perception of light transmitted by green parts of sugarbeet plants. Insertion of a transparent glass sheet between host leaves and parasite seedlings did not modify this response. This phototropism permitted Cuscuta to identify host plants with high chlorophyll content as a function of the lower red/far red ratio of transmitted light.     

Germination,emergence, and persistence of Sonchus oleraceus,a major crop weed in subtropical Australia

Sonchus oleraceus (common sowthistle) is a dominant weed and has increased in prevalence in conservation cropping systems of the subtropical grain region of Australia. Four experiments were undertaken to define the environmental factors that favor its germination, emergence, and seed persistence. Seeds were germinated at constant temperatures between 5 and 35°C and water potentials between 0 and ?1.4 MPa. The maximum germination rate of 86–100% occurred at 0 and ?0.2 MPa, irrespective of the temperature when exposed to light (12 h photoperiod light/dark), but the germination rate was reduced by 72% without light. At water potentials of ?0.6 to ?0.8 MPa, the germination rate was reduced substantially by higher temperatures; no seed germinated at a water potential >?1.0 MPa. Emergence and seed persistence were measured over 30 months following seed burial at 0 (surface), 1, 2, 5, and 10 cm depths in large pots that were buried in a south‐eastern Queensland field. Seedlings emerged readily from the surface and 1 cm depth, with no emergence from below the 2 cm depth. The seedlings emerged during any season following rain but, predominantly, within 6 months of planting. Seed persistence was short‐term on the soil surface, with 2% of seeds remaining after 6 months, but it increased with the burial depth, with 12% remaining after 30 months at 10 cm. Thus, a minimal seed burial depth with reduced tillage and increased surface soil water with stubble retention has favored the proliferation of this weed in any season in a subtropical environment. However, diligent management without seed replenishment will greatly reduce this weed problem within a short period.     

Germination and seedling emergence of glyphosate‐resistant and susceptible biotypes of goosegrass (Eleusine indica[L.] Gaertn.)

Effects of environmental factors on the germination and seedling emergence of glyphosate‐resistant (R) and ‐susceptible (S) biotypes of Eleusine indica (L.) Gaertn. were examined under laboratory and greenhouse conditions. The R biotype exhibited a higher germination percentage compared with the S biotype at constant temperatures of 20 and 35°C under dark conditions, and alternating temperatures of 30/25°C, and 35/25°C during a 12 h photo period. For both biotypes, germination was optimal at alternating temperatures of 30/20°C and 35/20°C. However, there was no significant difference (P > 0.05) in the germination between the R and S biotypes at these temperature regimes. The germination of both biotypes was inhibited by osmotic stress imposed by a water potential of ?0.80 MPa. When the moisture stress was released and the seeds were subsequently transferred to distilled water, the germination was enhanced to approximately 90% and 16% for the R and S biotype seeds, respectively. Higher emergence rates were obtained in shallow seed depths (0 or 2 cm) compared to deep depths. Emergence percentage of the R biotype was higher than that of the S biotype at 0 cm and 2 cm depths. The maximum emergence percentage of the R biotype was higher than that of S biotype when seeds were sown on the surface of either loamy or clay loam soil taken from three different sites.     

Dormancy cycle and viability of buried seeds of Papaver rhoeas

Seeds of three Spanish Papaver rhoeas populations (two resistant to both tribenuron‐methyl and 2,4‐dichlorophenoxyacetic acid and one susceptible to both herbicides) were buried at 2, 8 and 20 cm depth in Lleida (Catalonia, Spain). At various intervals up to 77 months they were exhumed and tested for germination in an agar medium and for viability in a tetrazolium test. A similar annual dormancy cycle was found for each population in 3 years. Maximum germination occurred between September and December. Practically no seeds germinated between February and May. Burial depth influenced the germination cycle in the first 16 months after burial, and higher germination was found to occur in the seeds that were buried at 2 or 8 cm, but no consistent effect was observed thereafter. In a second study, burial depth had no effect on the loss of dormancy of P. rhoeas seeds in the first 2 months after burial. Viability of the buried seeds decreased slowly throughout the 77‐month experimental period and, at the end of this period, it was 53, 72 and 61% for seeds buried 2, 8 and 20 cm respectively. No significant differences were observed between the three populations.     

Seed survival and patterns of seedling emergence studies of Rottboellia cochinchinensis (Lour.) W.D. Clayton in cultivated soils

The longevity of buried Rottboellia cochinchinensis (Lour.) W.D. Clayton seed represents a major survival mechanism for the weed, enabling the persistence of a continuing source of weed seeds in crop land. The pattern of seed persistence and depletion of R. cochinchinensis in cultivated maize soils was investigated by means of (1) studies on the effect of depth and duration of burial on the viability of the weed seeds, (2) quantitative estimation of the seed population and viability in cultivated fields, and (3) the periodicity of emergence and effects of cultivation on seed germination both in the field and in the greenhouse. The results indicated that the mode of persistence was innate (8.5%) and enforced (35%) dormancy after 1 year of burial, and that the persistency component of the seed population on cultivated soils ranged from 40.60%. The weed was able to remain viable at depths of 45 cm, indicating an excellent mechanism of escaping the effects of most soil-applied herbicides, and it was shown that tillage increases the depletion rate of the weed seed reserve by 32% per year.     

Effect of burial depth and soil water regime on the fate of Lithospermum arvense seeds in relation to burial time

Lithospermum arvense is an increasing annual weed in winter crops of the semiarid region of southern Argentina under low impact tillage systems, an agricultural practice that has become popular in recent years. Seed distribution in the soil profile under conventional tillage will change when reduced tillage is implemented, thus affecting the germination microenvironment. The effect of seed burial depth and soil water regime on field germination, enforced dormancy, innate dormancy and seed decay was studied in relation to burial time in a field experiment. In addition, the effect of burial depth on seed germination and seedling emergence was examined under laboratory controlled conditions. Field germination of buried seed ranged from 55% to 65% for shallow (2 cm) and from 5% to 30% for greater depths (20 cm). Enforced dormancy levels were significantly higher among deeper seeds. The amount of innate dormant seeds was reduced to <10% after a year of burial. Lithospermum arvense seedbanks can be classified as short-term persistent. Germination in the laboratory was unaffected by burial depth, while seedling emergence reduction was adequately described by a sigmoidal model. Results indicate that agricultural practices that accumulate L. arvense seeds near the soil surface enhance seedling recruitment.     

Factors affecting seed germination and emergence of Aegilops tauschii

Information on seed germination and emergence ecology of Aegilops tauschii is scant, despite it being a widespread invasive weed in China. We conducted this study to determine the effects of various factors on seed germination and seedling emergence in three A. tauschii populations. Seeds germinated across a wide range of temperatures (5–35°C), with germination of over 90% at 15–20°C. Germination was completely inhibited when dry seeds were exposed to a temperature of 160°C for 5 min; a similar response was observed for pre-soaked seeds at 100°C. Light was neither required for nor inhibited germination. Germination was not significantly affected by pH. Aegilops tauschii was relatively tolerant to low osmotic potential and high salt stress: over 80% of seeds germinated at −0.3 MPa, and all three populations germinated in the presence of 400 mM salt (NaCl) although salt tolerance varied among the populations. Seeds buried at depths of 1–3 cm emerged well, but emergence was completely inhibited at depths greater than 8 cm. The addition of maize straw caused a linear reduction in seedling emergence, although the rate of reduction varied among the populations. The results of this study have contributed to understanding the requirements of A. tauschii germination and emergence and optimising an integrated management system for this weed in Huang–Huai–Hai Plain of China. In addition, our study provides data for development of models to predict the geographical distribution of this weed.     

The biology of African feather grass (Pennisetum macrourum Trin.) in Tasmania, I. Seedling establishment

Seedling establishment of Pennisetum macrourum was studied in a series of glasshouse and field experiments. Maximum germination of 88% occurred at a constant temperature of 30C. In the glasshouse, highest seedling establishment was obtained from seeds buried 1 cm deep; it was less than 25% from seeds sown on the soil surface. Dormancy was induced in seeds buried 8 cm deep but only 6.4% remained viable after 6 months. Few viable seeds of P. macrourum were found in the soil and only one seedling was observed to establish in the field over a 20 month period. Up to 98% of seedlings transplanted into the field established successfully. The results suggest that reproduction by seed would not be a major problem in the control of P. macrourum.     

Variation in the annual dormancy cycle in buried seeds of the weedy winter annual Viola arvensis

Seeds of Viola arvensis collected in different years and in different months within those years were buried in soil under natural seasonal temperature cycles, and changes in their germination requirements monitored. Seeds were dormant at maturity in May or June, but nondormant by autumn. During winter, some seeds entered dormancy, while others entered conditional dormancy, i.e. retained the ability to germinate at 15/6 and 20/10^oC but not at other thermoperiods. Dormant and conditionally dormant seeds became nondormant the following summer. Seeds collected in 1981 exhibited an annual dormancy:nondormancy cycle, while those collected in 1982 exhibited an annual conditional dormancy:nondormancy cycle. The type of dormancy cycle found in these seed lots during their first year of burial persisted in subsequent years. Thirty–five and 36% of seeds collected in May 1983 and 1986, respectively, were conditionally dormant the following May, while only 5 and 9% of those collected in the same field in June 1983 and 1986, respectively, were conditionally dormant. Dormant seeds collected in 1981,1982 and 1984 and buried at 5^oC during summer germinated to 0, 33 and 0% respectively, at 15/6^oC in autumn. After the 1982 seeds became nondormant during summer, only 25% entered conditional dormancy when buried at 5^oC, but after the 1981 and 1984 seeds became nondormant, 100% entered conditional dormancy at 5^oC. Thus, the persistent seed bank of V. arvensis at a population site may consist of seeds with an annual dormancy:mondormancy cycle and others with an annual conditional dormancy:nondormancy cycle. This is the first report of the two types of annual seed dormancy cycles in the same species.     

Effects of tillage, application time and rate on metribuzin dissipation

Metribuzin efficacy and dissipation were determined in two silty clay loam soils following preplant (PP), pre-emergence (PRE) and split (PP+PRE) application to tilled and no-till soybeans in rotation with corn at Clay Center and Lincoln, Nebraska. A similar experiment was conducted in tilled and no-till soybeans in rotation with wheat at Lincoln. Corn and wheat residue in no-till plots reduced the amount of metribuzin that reached the soil by approximately 54 and 89%, respectively. No differences in weed control or soybean yield were observed between tillage treatments or time of metribuzin application in the corn-soybean rotation. However, both weed control and yield were reduced in the wheat-soybean rotation. Most of the metribuzin remained at the 0–5 cm depth, and dissipation was exponential. The mean metribuzin half-life at the 0–5 cm depth across locations, tillage treatments, application time and rates was 11 days. The metribuzin half-life was 4–19 days following PP application and 3–17 days following PRE application. The metribuzin concentration did not exceed 65 μg kg_?1 at the 5–10 cm or 10–20 cm depths in any treatment, indicating that little metribuzin had leached from the surface soil after PP or PRE application. The finding of a higher metribuzin concentration at 5-20 cm depth in tilled plots than in no-till could be attributed to higher initial soil concentrations in the absence of crop residue.     

Effects of drought on weed emergence and growth vary with the seed burial depth and presence of a cover crop

In conservation agriculture, weed seed germination could decrease with the presence of a cover crop, surface weed seed location and temporal drought in summer just after seed shedding. This study simultaneously examined the effects of a cover crop, burial depth (seed location) and hydric stress on weed emergence and early growth. It was hypothesized that drought would reduce weed emergence and the initial growth of weed seeds and that this effect would be greater when the seeds were on the soil surface and in the presence of a cover crop. Four annual weed species were chosen that are frequently found (Anisantha sterilis, Vulpia myuros, Sonchus asper, Veronica persica) and not frequently found (Alopecurus myosuroides, Poa annua, Cyanus segetum, Capsella bursa‐pastoris) in fields that implement conservation agriculture. The unburied seeds had 26% lower emergence, on average, than the buried seeds (significant for six of the eight species), hydric stress reduced emergence by 20% (for seven of the eight species) and the presence of a cover crop reduced the level of emergence by 17% (for all species). The unburied seeds with hydric stress were emerging under the “most stressful” set of factors, with a 45% decrease in emergence, compared with the seeds emerging under the “least stressful” set of factors (buried seeds without hydric stress). All the weed growth measurements (height, dry matter content and number of leaves) decreased with the presence of a cover crop. The species that are found frequently in the fields that implement conservation agriculture, compared with the species that are not frequently found in conservation agriculture fields, had higher rates of germination and a higher tolerance of hydric stress when their seeds were unburied.