Longevity of seeds of four annual grass and two dicotyledon weed species as related to placement in the soil and straw disposal technique

Persistence of seeds of the four winter annual grass species, Alopecurus myosuroides , Bromus sterilis , Bromus hordeaceus and Vulpia myuros , were studied under field conditions. The studies also included the two dicotyledon species, Tripleurospermum inodorum and Galium aparine . Seed samples were stored in the field at different depths in the plough layer for a year. Following germination, seedling emergence was taken as an indicator of field persistence. In a supplementary investigation, seed samples were placed on the soil surface or buried at 2 cm for a month and then tested for viability. Persistence of the two Bromus species was very short, irrespective of depth and duration of burial. Persistence of the other species was positively correlated with incorporation of seeds to a few centimetres depth. Increasing the depth of incorporation below 2 cm had a variable influence on persistence. Persistence of seeds placed directly on the soil surface was short, whether they were left there for a year or a month. Whether seeds at the soil surface were left uncovered or covered with chopped straw had limited influence on seed persistence. However, straw cover significantly increased persistence of A. myosuroides seeds and there was the same tendency with T. inodorum .     

A review of the relationship between primary and secondary dormancy,with reference to the volunteer crop weed oilseed rape (Brassica napus)

Fresh seeds of oilseed rape (Brassica napus) are reported to be nondormant and nonphotoblastic. However, a portion of the seeds can be induced into a light‐requiring state (secondary dormancy) for germination and also exhibit dormancy cycling. Thus, if seeds become buried in the soil they can form a persistent seedbank and become a serious volunteer weed in succeeding crops. The capacity of nondormant seeds of B. napus to be induced into secondary dormancy is contrary to results of studies on fresh nondormant seeds of some other species. A reanalysis of published and unpublished data shows that fresh seeds of this species have some degree of primary dormancy and that there is a significant relationship between primary dormancy and the capacity to enter secondary dormancy. However, most germination tests on B. napus have not been done in enough detail to detect primary dormancy (or not) in fresh seeds of this species. The usefulness of information on the relationship between primary dormancy and the capacity of the seeds to enter secondary dormancy is discussed in relation to management of weedy volunteers of this species.     

Study of seasonal variation in dormancy of Spergula arvensis L. seeds in a condensed annual temperature cycle

Changes in dormancy of Spergula arvensis seeds were studied during pre-incubation at constant temperatures and under a temperature regime that condensed the annual temperature cycle into 73 days. Each day in the regime represented the mean day and night temperatures and day lengths of 5 successive days of an average year in The Netherlands. Incubation occurred in water or loamy sand, in darkness. Germination of the seeds was tested in water or KNO₃ over a range of temperature. Seeds were irradiated with saturating doses of red light. In half of the treatments, pre-incubated seeds were dehydrated at the transfer to the conditions of the germination test. Breaking of dormancy occurred under conditions of 'spring'. It did not depend on exposure to low‘winter’temperatures, but was induced by rising 'spring’temperatures. Seeds developed secondary dormancy in late‘autumn'. The expression of the changes in dormancy that were induced during pre-incubation depended on the conditions of the germination test. Light, nitrate and dehydration stimulated germination. The experiments predicted that field emergence from nitrate-poor soils that have not been dehydrated will be restricted to a short period in autumn, whereas disturbance of nitrate-rich soils followed by a dry spell will stimulate germination of S. arvensis seeds from early spring to late autumn. The data presented good explanations for the cosmopolitan character and the serious weediness of this species. Its classification as a summer or winter annual is discussed.     

Seed dormancy,germination, emergence and seed longevity in Galinsoga parviflora and G. quadriradiata

Galinsoga quadriradiata (hairy galinsoga) and Galinsoga parviflora (smallflower galinsoga, gallant soldier) are very troublesome weeds in many vegetable row crops in Europe. To optimise management strategies for Galinsoga spp. control, an in‐depth study of germination biology was performed. Germination experiments were conducted to evaluate the impact of light and alternating temperatures on germination of a large set of Galinsoga populations. Seedling emergence was investigated by burying seeds at different depths in a sandy and sandy loam soil. Dormancy of fresh seeds harvested in autumn was evaluated by studying germination response in light at 25/20°C with and without nitrate addition. Seed longevity was investigated in an accelerated ageing experiment by exposing seeds to 45°C and 100% relative humidity. Galinsoga spp. seeds required light for germination; light dependency varied among populations. Seedling emergence decreased drastically with increasing burial depth. Maximum depth of emergence varied between 4 and 10 mm depending on soil type and population. In a sandy soil, emergence percentages were higher and seedlings were able to emerge from greater depths than in a sandy loam soil. Freshly produced G. parviflora seeds, harvested in autumn, showed a varying but high degree of primary dormancy and were less persistent than G. quadriradiata seeds that lack primary dormancy. Lack of primary dormancy of freshly harvested G. quadriradiata seeds and light dependency for germination may be used to optimise and develop Galinsoga management strategies.     

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.     

Seasonal changes in seed dormancy of Solanum nigrum and Solanum physalifolium

The seed dormancy cycle in Solanum nigrum and Solanum physalifolium was studied in relation to seasonal temperature. Seed lots of both species were buried in pots outdoors in a randomised complete block design with four replicates from November 2004 to November 2006. At regular intervals, samples of the seeds were randomly exhumed and tested for germination in incubators at three temperatures and light/darkness regimes. For both species, low winter temperature weakened dormancy and high temperature strengthened it. Dormancy induction mainly occurred from August to October in both species after experiencing warm temperatures. An exception from the general pattern of seed dormancy was however observed; seed germination percentages were temporarily reduced in early spring, followed by a peak in germination, before the main period of strong dormancy in S. nigrum . The same phenomenon was observed in S. physalifolium during June in the first year. This short-lived dormancy induction might explain the late emergence of the species. Seed dormancy enables the species to maximise its chance of survival by regulating germination timing to favourable conditions. Therefore, information on the dormancy cycle can be used to predict seedling emergence and optimise weed control operations.     

SEED GERMINATION OF AGROSTIS GIGANTEA ROTH

Summary. To assess the role of seed production and seedling behaviour in the biology of the perennial grass weed Agrostis gigantea Roth, seed germination and seedling emergence of the species were investigated in the glasshouse and in the laboratory. These studies showed that the species may produce a large number of seeds: panicles collected from a cereal field in 1971 and 1972 contained about a thousand viable seeds in both years. Mature seeds of the species are mostly non-dormant and germinate readily on moist soil surfaces. Freshly-harvested seeds require light and alternating tempratures for germination, but older seeds can germinate in constant temparature in the dark. Covering seeds with 0·6 cm soil halved the percentage emergence and few seedlings emerged from seeds sown deeper than 3±8 cm, but many seeds survived under enforced dormancy. A. gigantea seeds may also persist long in frequently cultivated shallow soil. The species flowers late but its seeds become viable quickly—in 1969 a third of the number that were finally viable became so within a week of flowering.
Germination des semenes d' Agrostis gigantean Roth.     

Germination, emergence and seedling growth of Hypericum perforatum L.

Germination of new seeds (1–6 months old) of Hypericum perforation L. was restricted by high temperatures (16h/8h, 20/30°C), darkness and a chemical inhibitor in exudate from the capsule, whereas germination of old seeds (9 years) was only restricted by the inhibitor. The effect of the chemical inhibitor and high temperatures was overcome, respectively, by washing seeds in water and by reducing temperatures to constant 15°C. Calcium in solution from CaCO₃ and from three different soils did not prevent the germination of new or old seeds or of seeds collected from five different locations. There were differences in the germination characteristics and dormancy mechanisms of seeds collected from different localities, Restriction of the emergence of seedlings by a covering of > 2 mm of soil appeared to be due to lack of seedling vigour rather than to lack of light. Seedling growth was much slower than in other pasture species. Thus the requirements for germination of H. perforatum of low temperature and moisture to wash away the chemical inhibitor favour its establishment but the slow growth of its seedling restricts its emergence and renders it extremely susceptible to competition from other plants.     

Explaining patterns of species dominance in the shrub steppe systems of the Junggar Basin （China） and Great Basin （USA）

Natural scientists have long recognized that regions with similar climate tend to have similar vegetation.Preliminary observations suggest that shrub steppe communities of the western US and western China may be two such regions with similar annual precipitation,temperature,land use,and vegetation.These cold dry shrub steppes have traditionally been grazed.Despite these similarities,patterns of species dominance are different.Annual species that are rare in China become dominant when introduced to the United States.The objective of this study was to investigate how climate,land use and community structure may explain these patterns of species dominance.Community structure and grazing intensity were measured at 5 sites in each region.This information was combined with a broader review of the literature describing the history of grazing in both basins.Climate was analyzed based on a spatially-gridded,interpolated weather time series(monthly records) and climatological summary(1961-1990 mean conditions) data set from the Climate Research Unit.We found that differences in summer precipitation and winter minimum temperature,land use intensity,and shrub size may all contribute to the dominance of annual species in the Great Basin,particularly Bromus tectorum.In particular,previous work indicates that summer precipitation and winter temperature drive the distribution of Bromus tectorum in the Great Basin.As a result,sites with wet summers and cold springs,similar to the Chinese sites,would not be expected to be dominated by Bromus tectorum.A history of more intense grazing of the Chinese sites,as described in the literature,also is likely to decrease fire frequency,and decreases litter and shrub dominance,all of which have been demonstrated to be important in Bromus tectorum establishment and ultimate dominance.Further research is necessary to determine if other annuals that follow the same pattern of scarcity in the Junggar Basin and dominance in the Great Basin are responding to the same influences.     

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, emergence, and dormancy of Mimosa pudica

Mimosa pudica (common sensitive plant) is a problematic weed in many crops in tropical countries. Eight experiments were conducted to determine the effects of light, seed scarification, temperature, salt and osmotic stress, pH, burial depth, and rice residue on the germination, seedling emergence, and dormancy of M. pudica seeds. Scarification released the seeds from dormancy and stimulated germination, though the germination of the scarified seeds was not influenced by light. The scarification results indicate that a hard seed coat is the primary mechanism that restricts germination. The germination increased markedly with the exposure to high temperature "pretreatment" (e.g. 150°C), which was achieved by placing non-scarified seeds in an oven for 5 min followed by incubation at 35/25°C day/night temperatures for 14 days. The germination of the scarified seeds was tolerant of salt and osmotic stress, as some seeds germinated even at 250 mmol L⁻¹ NaCl (23%) and at an osmotic potential of −0.8 MPa (5%). The germination of the scarified seeds was >74% over a pH range of 5–10. The seedling emergence of the scarified seeds was 73–88% at depths of 0–2 cm and it gradually decreased with an increasing depth, with no seedling emergence at the 8 cm depth. The rice residue applied to the soil surface at rates of ≤6 t ha⁻¹ did not influence the seedling emergence and dry weight. The information gained from this study identifies some of the factors that facilitate M. pudica becoming a widespread weed in the humid tropics and might help in developing components of integrated weed management practises to control this weed.     

Annual cycles of germinability and differences between primary and secondary dormancy in buried seeds of Echinochloa crus-galli

The seasonal changes in percentage of dormant seeds of Echinochloa crus-galli in the field were recorded for 4 years. The lots of seeds were wrapped in nylon fabric, buried 20 cm under the grass sward and exhumed at monthly intervals. The proportion of seeds germinating under light conditions at a constant temperature of 25 °C fluctuated between 0% and 96%, with maxima in May–July and minima in September–November. Small between-year differences in the course of summer dormancy induction and its winter termination were probably caused by variation of weather conditions.
Attributes of dormancy innate to seeds after maturation (primary dormancy) and dormancy induced in buried seeds during the summer (secondary dormancy) were compared by investigating the rate of dormancy termination during storage of (a) dry seeds at 25 °C, (b) imbibed seeds at 5°C and (c) in seeds buried under field conditions during October–June. Percentage of germination increased faster in secondary than primary dormant seeds at both constant 25 °C and 5 °C. The seeds with primary and secondary dormancy also differed in the response to `germination pre-treatment', a 10-day exposure of imbibed seeds at 25 °C that causes germination of the non-dormant fraction of seed materials. After this treatment the time to resuming germination in primary dormant seeds was substantially increased, whereas the secondary dormant seeds were much less affected. Annual variation in the proportion of germinable seeds explains the low efficiency of autumn soil cultivation for decreasing reserves of E. crus-galli seeds in the soil.     

Influence of hairy vetch seed germination and maturation on weediness in subsequent crops

Hairy vetch is a widely adopted cover crop in the United States. However, hairy vetch can become weedy in subsequent crops as seeds germinate after the cover crop growing season, which is largely attributed to seed dormancy. We conducted two field experiments to determine seed germination, viability and seed production phenology of two common hairy vetch cultivars in Blacksburg and Blackstone, Virginia, US. ‘Groff’ and ‘Purple Bounty’ seed were sown in October 2015 and May 2016 and germination was tracked until June 2017. Subsequently, ungerminated seeds were tested for viability. Both cultivars had <2% germination after the initial germination period, and <1% of seed recovered was still viable at the end of the experiments. We also conducted experiments to determine when these cultivars produce viable seed. Hairy vetch seeds were counted and tested for germination in the spring. Both cultivars produced seed beginning in late‐May, but most seed were not viable until mid‐June in Virginia. Our results indicate that seed dormancy is not the primary cause of hairy vetch weediness in subsequent crops as nearly all germination, 99% of the total germinated seed, occurred during the cover crop growing season. Also, if complete termination occurs before mid‐June in Virginia, it is unlikely viable seed will be added to the seed bank. To better utilise this cover crop species, cultivar selection and proper termination are important to prevent weediness.     

Evidence of differences in seed dormancy among populations of Bromus sterilis

Seeds of 40 populations of Bromus sterilis L. were collected in the southern and midland counties of England over a 2‐year period and grown outdoors in pots in two subsequent years. In the first year, seeds were tested in a 12‐h dark/12‐h light regime at 15 °C and in the second year the seeds were tested both in the dark/light regime and in the dark at 15 °C. There was a wide range in the degree of enforced dormancy given by the dark/light regime. Germination of freshly collected seeds in the dark/light regime after 21 days ranged from 44% to 97% in the populations tested in the first year and from 19% to 97% in populations tested in the second year. Induced dormancy was caused by light in two populations. Seeds had little innate dormancy apart from in two populations which gave 64% and 68% germination, respectively, in the dark after 21 days. A field trial in which seeds of a selected range of six populations were sown on the soil surface after harvest (August 8), showed that populations predicted to be inhibited by light in laboratory tests were also inhibited by light in the field and, depending upon the population, there was between 4% and 54% of the seeds remaining ungerminated by October 23 in the year of planting. By June of the following year, 36% of the seeds sown on the soil surface in one population still remained viable and ungerminated. The agricultural significance of the results is discussed.     

Diaspore characteristics and ecological adaptation of Bromus tectorum L. from different distribution regions

The invasion of nonnative plants is considered one of the main threats to the structure and function of North American ecosystems.Moreover,they can alter ecosystem processes and reduce biodiversity.In arid and semi-arid region of North America,the species of European annual grass Bromus tectorum L.is an outstanding example of these problems,which not only increase the fire density and change the fire regime,but replace native communities.Therefore,there are amount of researches on B.tectorum,including resource acquisition,water use efficiency and growth.Whereas the relevant research on the morphology of diaspore is scare.Diaspores have a fundamental role in seed germination and seedling establishment.Besides,as an important link between different generations,diaspores have a vital significance on individual reproduction and population extension.Hence,diaspores under selection for studying have an important implication.This study compares differences in seed morphology for Bromus tectorum collected from the United States,Kazakhstan,and Xinjiang of China.The following indices of B.tectorum diaspores were analyzed:size,thickness of covering layers,and micromorphological characteristics of the base,middle and transition area of diaspores as well as of the awn.Micromorphology of the lemma and the cross-section of the diaspore were observed by scanning electron microscopy.Results showed that thickness of the lemma and the palea of diaspores from B.tectorum-infested grasslands in the United States were reduced(P<0.05),likely because of environmental influences.This reduction facilitated the germination of diaspores and lowered the resistance of B.tectorum to adverse environmental conditions.The length of the awn also increased significantly(P<0.05),which helped in dispersal and anchoring of diaspores.Therefore,B.tectorum adapted ecologically to its new environment in the United States by strengthening its establishment ability.However,the defense capability of B.tectorum decreased.These results fit the evolution of increased competitive ability hypothesis(EICA)of invasive species.Analysis of various cells on the lemma revealed that prickle densities and collapsed,long epidermal cells were easily influenced by environmental factors such as temperature and moisture because of the physiologic function of these structures on silicon accumulation.However,the form and the position of silica cells,which were not greatly influenced by environmental factors,might be genetically controlled.Studying these structures at the microscopic level helps define the relationship between the diaspore and its environment.This study has a reference value for future studies on B.tectorum.     

Seasonal changes in the germination of buried seeds of Monochoria vaginalis

This study investigates the seasonal variation of germination ability of buried seeds of Monochoria vaginalis (Burm.f.) Presl var. plantaginea Solms. The field-collected seeds were buried in a flooded or an upland field and then exhumed monthly. The exhumed seeds were germinated under four temperature regimes. The seeds exhumed from the flooded soil were dormant at the beginning of burial and proceeded into a conditional dormancy/non-dormancy/conditional dormancy cycle throughout the remaining period of the experiment. The seeds exhumed monthly from the non-flooded soil exhibited an annual dormant cycle, which is dormancy/conditional dormancy/non-dormancy/conditional dormancy/dormancy. At day and night temperatures of 25/20 °C, the exhumed seeds from both the flooded and the upland soil resembled each other in terms of seasonal variation of the germination percentage. In September and October, more seeds exhumed from upland soil failed to germinate under higher temperature than from flooded soil. Strictly avoiding exposure to light during seed exhuming and seed testing prevented the seeds from germinating. A short exposure of the exhumed seeds to light during preparation promoted dark germination when the seeds were at the non-dormant stage. The potential implications of our results for weed management strategies in rice production are discussed.     

Biology of Commelina benghalensis L. in south-eastern Queensland. 2. Seed dormancy, germination and emergence

Germination of freshly harvested seeds of Commelina benghalensis L. varied from 0–3% for small aerial seeds, 20–35% for large aerial seeds and from 33% for small underground seeds to 90% for large underground seeds. Innate dormancy of all seed types was completely overcome by clipping the seed coat. Exposure to 90°C dry heat for 2 h was also effective in increasing germination of the three strongly dormant seed types. Optimum temperature for germination varied with the different seed types. Periods of likely major weed infestation from the four seed types were predicted using soil temperature data. Exposure to light increased germination but was not essential and underground seeds responded more to light than aerial seeds. Optimum depth of emergence for the four seed types was from 0 to 50 mm and there was a positive correlation between maximum depth of emergence and seed weight.     

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.     

Environmental control of dormancy and germination in the seeds of Cenchrus longispinus (Hack.) Fern.

Seed dormancy and germination in sand burr (Cenchrus longispinus (Hack,) Fern,) were investigated in laboratory and field studies. The burrs contain two types of seeds which differed in their innate dormancy. Primary seeds formed in the upper spikelet usually germinated within a year. Secondary seeds from lower spikelets germinated slowly and remained dormant for longer periods. Dormancy was enforced at low and high temperatures, and secondary seeds apparently developed an induced dormancy when continuously exposed to high temperatures. More than 94 % of the seedlings established during spring. Light suppressed germination, and secondary seeds also developed an induced dormancy when stored in the light. Burrs sown on the soil surface had an extended period of germination lasting for more than 3 years. However, over 96 % of the seeds sown below the surface of bare soil germinated within 2 years. Deep burial did not enforce dormancy, but germination was suppressed by the presence of live vegetation. It is concluded that treatments which disturb the soil and bury the burrs will stimulate the germination of dormant seeds.     

Light, temperature and burial depth effects on Rumex obtusifolius seed germination and emergence

Trials were carried out to investigate the effects of light and temperature on germination of Rumex obtusifolius L. After several months of storage, seeds gradually lost dormancy and became photosensitive. Thermal optima for germination were between 20 °C and 25 °C in light or in darkness. At lower temperatures there was a greater demand for light, so that the greatest differences in germination percentage (between low and high temperatures) were found within the 10–15 °C temperature range. The calculated thermal minima ( x -intercept method) in light and darkness were 8.3 °C and 6.1 °C respectively. Daily temperature fluctuation increased germination even after seed irradiation with far-red light, suggesting a lower demand for the far-red-absorbing form of phytochrome. Seed burial inhibited germination in proportion to depth; however, germination inhibition was independent of seed phytochrome photo-equilibrium, which had been diversified by seed pretreatment with light. Seedlings did not emerge when seeds were buried >8 cm deep. Recovery of ungerminated seeds showed that excessive burial did not impede seedling emergence but rather prevented seed germination. However, this induction of dormancy was lost once germination processes were activated (24–48 h at 20 °C) that made germination irreversible. Temperature was also involved in inhibition, and low temperature (<15 °C) induced the least inhibition. This is discussed in terms of processes of respiration and fermentation in buried seeds.