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, persistence, and emergence of flaxleaf fleabane (Conyza bonariensis [L.] Cronquist)

Flaxleaf fleabane ( Conyza bonariensis [L.] Cronquist) is a difficult-to-control weed in dryland minimum tillage farming systems in the northern grains region of Australia. Experiments under field and controlled environments were conducted to study the seed germination, emergence, and persistence of C. bonariensis . The base, optimum, and maximum temperatures for seedling germination were estimated at 4.2, 20, and 35°C, respectively, with light stimulating germination. The soil types and burial depths had significant effects on seed persistence and emergence. The persistence pattern showed an initial rapid drop, followed by a slow but steady decline over time. After 3 years of burial, there were ≈ 7.5%, 9.7%, and 1.3% viable seeds at 10, 5, and 0–2 cm soil depths, respectively. Conyza bonariensis predominantly emerged from the soil surface or from the burial depth of 0.5 cm. Very few seeds emerged from a depth of 1 cm and none emerged from ≥ 2 cm. The emergence was greater in lighter soils. On-farm monitoring of the field emergence over time in a light sandy loam soil showed that 99% emergence occurred in late autumn, early and late winter, and a small fraction of 1% emerged in mid-spring. The predominant emergence from the soil surface suggests that the fleabane problem is a result of weed species shift toward the minimum tillage systems that provide moist conditions for better emergence compared to conventional tillage systems. The seed longevity of at least 3 years in the soil and the prolonged emergence patterns between autumn and spring have posed great difficulties for the long-term management of C. bonariensis .     

Biology and management of Phalaris minor in wheat under a rice/wheat system

Summary Germination of Phalaris minor declined with the increase in duration of imbibition in water from 30 min to 72 h at temperatures above 22 °C . Germination was reduced down to 10 cm and 2 cm soil depth by wheat straw burning in puddled and non-puddled soil, respectively, with maximum reduction near the soil surface. The dormancy of P . minor seed was not more than 60 days under field conditions. In puddled soil, 38–60% of the viable seeds of P. minor remained concentrated in the upper 5-cm layer. Germination decreased with an increase in soil depth. In total, 15% of seeds stored in the laboratory emerged from 10-cm depth, whereas seeds did not germinate below 4.2-cm depth under field conditions. Depth of emergence of P . minor was shallower in zero tillage compared with the conventional method of wheat sowing. The seeds retrieved from rice soils kept under continuous submergence for 60 days exhibited 26% and 57% loss of germination over semi-submergence and semi-wet conditions respectively. There was 100% loss of germination in 10-month-old seeds retrieved from the soil under rice-growing conditions. Plant density of P. minor was lower in zero tillage than with the conventional method of wheat sowing. Cross-ploughing in the upper 2–5 cm of soil (shallow tillage) and drill-sowing of wheat 1 week after shallow tillage reduced germination of P. minor by 44% and 37% and increased grain yield by 21% and 47% over zero-tillage and conventional methods respectively.     

Germination characteristics and emergence time of annual Bromus species of differing weediness in Sweden

The germination ecology of four annual Bromus species, which differ in weediness on arable land in southern Sweden, was investigated. The most problematic species is Bromus sterilis , while Bromus hordeaceus frequently occurs on arable land. In contrast, Bromus arvensis is a rare weed, and Bromus tectorum is found infrequently in fields despite being a widespread ruderal species. Five experiments were conducted to identify germination characteristics that could explain differences in habitat and abundance: (i) intraspecific variation in dormancy level; (ii) germination response to different light conditions; (iii) light and temperature interactions at germination; (iv) timing of seedling emergence; and (v) seed persistence in soil. Bromus sterilis and B. tectorum behaved similarly in all tests. For both these species, there were large differences in dormancy level among populations and strong inhibition of germination by light. In addition, emergence from seeds sown on the soil surface was both delayed and reduced compared with buried seeds. In contrast, B. hordeaceus and B. arvensis showed generally weak dormancy, and germination was only slightly inhibited by light. It was concluded that germination characteristics alone do not explain the differences in weediness between these four species.     

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.     

Changes in germinability, dormancy and viability of Amaranthus retroflexus as affected by depth and duration of burial

Changes in dormancy and viability of Amaranthus retroflexus seeds were examined after placement in pots that were buried in the field. Seeds were placed in woven nylon envelopes on the soil surface or buried at depths of 2.5, 5 or 10 cm. After 1, 3, 6, 9 and 12 months seeds were exhumed and their germinability was tested to assess changes in dormancy and viability. Depletion of seed stocks placed on the soil surface was partly because of in situ germination that did not exceed 21% and did not vary significantly over the 12-month study period. Less germination of buried seeds occurred in situ , and seeds that did not germinate appeared to acquire dormancy. Exhumed seeds germinated readily; germinability was linearly related to the depth of burial, with those retrieved from the surface germinating least. Cyclical changes in germinability occurred during the 12-month burial period, but this influence was identical for seeds buried at all depths. Germinability was greatest after periods with the lowest mean monthly temperatures and least during the hottest periods. The stimulation of remaining ungerminated seeds exhumed at each period, by the addition of ethephon to the germination medium, provided further evidence of a seasonal acquisition of dormancy, and it was concluded that other unknown factors besides cyclical changes in seasonal temperature were responsible. Irrespective of placement, all seeds lost viability at an exponential rate over time. However, the decline was most rapid for those placed on the surface, whereas the loss in viability became less with increased depth of burial. Possible explanations for this adaptation of enhanced survival when buried are discussed.     

Burial depth and cultivation influence emergence and persistence of Phalaris paradoxa seed in an Australian sub-tropical environment

Emergence and persistence characteristics of Phalaris paradoxa seeds in no- and minimum-till situations and at different burial depths were studied in a sub-tropical environment. Three experiments were carried out using naturally shed seeds. In the first experiment, seedlings emerged from May through to September each year, although the majority of seedlings emerged in July. In the second experiment with greater seed density, cultivation in March of each year stimulated seedling emergence, altered the periodicity of emergence and accelerated the decline of seeds in the seedbank compared with plots that received no cultivation. The majority of seedlings in the cultivated plots emerged in May whereas the majority of seedlings in the undisturbed plots emerged in July. Emergence accounted for only 4–19% of the seedbank in both experiments over 2 years. Seed persistence was short in both field experiments, with less than 1% remaining 2 years after seed shed. In the third experiment, burial depth and soil disturbance significantly influenced seedling emergence and persistence of seed. Seedlings emerged most from seed mixed in the top 10 cm when subjected to annual soil disturbance, and from seed buried at 2.5 and 5.0 cm depths in undisturbed soil. Emergence was least from seed on the soil surface, and buried at 10 and 15 cm depths in undisturbed soil. Seeds persisted longest when shed onto the soil surface and persisted least when the soil was tilled. These results suggest that strategic cultivation may be a useful management tool, as it will alter the periodicity of emergence allowing use of more effective control options and will deplete the soil seedbank more rapidly.     

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.     

Estimation of seed production by Stellaria media, Sinapis arvensis and Tripleurospermum inodorum in arable crops

Summary Seed production by Sinapis arvensis , Tripleurospermum inodorum and Stellaria media growing alone or in crops of spring wheat, spring field beans and spring linseed was studied in 11 experiments. Seed numbers for S. arvensis and T. inodorum were measured directly from samples harvested in late summer but for S. media were estimated from measurements of flower number and seeds/flower. Seed numbers were regressed against plant dry matter. In general, a linear log/log plot ( y  =  mx  +  c ) fitted the data for each species quite well ( R ² = 0.75, 0.89, 0.89 for S. arvensis , T. inodorum and S. media respectively), but there were small differences between crops and years in the individual regression lines, especially the intercept values. Estimated seed production for a 'mean' 10-g plant (dry matter) in the three crops for S. arvensis , T. inodorum and S. media was 590, 6300 and 8600, respectively. Seed production tended to be lower for the weeds in wheat than it was in field beans and linseed, or in the plants grown alone, primarily due to the differences in plant weight arising from the differing competition exerted by the crops involved .     

Chloris polydactyla (L.) Sw., a perennial Poaceae weed: Emergence, seed production, and its management in Brazil

Experiments were conducted to evaluate the plant emergence and seed production of Chloris polydactyla and the efficacy of herbicides for its control. The plants emerged mainly when the seeds were placed on the soil surface at ≤ 3 cm depth. Isolated plants produced a great amount of seeds. The pre-emergence herbicides, acetochlor, atrazine + simazine, s-metolachlor, alachlor, and trifluralin, were effective for C. polydactyla control. The postemergence herbicides, clodinafop-propargil, haloxyfop-methyl, clethodim, fluazifop- p -butil, tepraloxydim, sethoxydim, and quizalofop- p -tefuril showed satisfactory control of the plants at a 20 cm height with six leaves. During the flowering stage (85 cm plant height), only glyphosate was effective in controlling C. polydactyla .     

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.     

Effect of environmental conditions on Alopecurus myosuroides germination. II. Effect of moisture conditions and storage length

Summary More effective control of Alopecurus myosuroides with a minimum amount of herbicide may be achievable by using models describing the impact of cropping system effects on the weed's demography. In order to develop the submodel relating the weed seedbank to the emerged seedlings, a series of laboratory experiments was set up to analyse and quangify the effects on A. myosuroides germination of length of seed storage in dry conditions, soil water potential and dryness during imbibition. The base soil water potential for germination was estimated at −1.5 MPa. A four-parameter non-linear equation depending on accumulated day–degrees was fitted to the observed germination data, resulting in the estimation of parameters illustrating germination proportion and rate. Germination proportion and rate increased with seed storage length, and were increased by short and decreased by long dry periods occurring after imbibition. These results showed that, in contrast to crop species, A. myosuroides germination cannot be expressed simply as a function of hydrothermal time, but also requires the determination of the proportion of non-dormant seeds as a function of seed age and environmental conditions.     

Assessment of the differential response of weeds to soil solarization by two methods

The efficacy of solarization in weed control under field conditions of the United Arab Emirates was evaluated by two methods: on-farm weed assessment and a seed germination test. In the on-farm weed assessment method, the weed frequency, density, and dry weight were compared in the solarized and non-solarized plots that were cultivated with cabbage. Prior to solarization, the soil was artificially infested with the seeds of 10 weeds. Generally, the densities of seven species and dry weights of five species were significantly lower in the solarized plots as compared to the control. Launea mucronata , Capsella bursa-pastoris , and Echinochloa colona were very sensitive to solarization, as they did not appear in the solarized plots. However, Portulaca oleracea and Melilotus indica were not significantly affected by soil solarization. In the second method, the germination was assessed for the seeds of four weedy species buried at three depths for different durations of solarization. The results confirmed the great sensitivity of L. mucronata and C. bursa-pastoris seeds to solarization, as all had not germinated after 15 days of solarization, even at the 15 cm depth. The seeds of E. colona , however, were less sensitive after 15 days of solarization, especially at 7.5 cm and 15 cm, respectively. The seed germination method confirmed the resistance of the P. oleracea seeds to solarization. The results emphasized the importance of the germination test to provide accurate predictions about the spatial and temporal changes of the soil seed bank in solarized farms. This would help to determine the optimal duration of solarization in each farm, depending on the kind of weeds infesting the farm.     

Seed survival and predation of Anoda cristata in soyabean crops

Anoda cristata is a troublesome annual broad-leaved weed in summer crops in the rolling Pampa in Argentina; seeds are the only source of regeneration of this species. Seed persistence or depletion is the result of survival and loss processes, including predation. The objective of this study was to determine survival at two burial depths in undisturbed soil and predation rates of A. cristata seeds in soyabean crops in different rotations and tillage systems. Survival was discontinuous and decreased to 25% after 35 months, after which no further reduction in survival was observed to the end of the experiment at 96 months. No differences in seed survival between seeds placed on the soil surface and buried 5 cm below the soil surface were found at 80 months, but at later times survival was lower for seeds placed on the soil surface. Predation rates ranged between 0.3% day⁻¹ and 6.7% day⁻¹. Of the models tested, a polynomial regression of the rate of predation with time gave the best representation of seed predation. From January to July, predation was higher in non-tillage plots in the wheat/soyabean rotation. There was no significant difference in predation rates between tillage systems in the soyabean monoculture and no difference between planting densities. Higher crop residue levels in non-tillage plots in the wheat/soyabean rotation was the dominant factor influencing seed predation, probably because such habitat favours the presence of seed predators.     

Variation in seed dormancy and light sensitivity in Alopecurus myosuroides and Apera spica-venti

Variation in seed dormancy and light sensitivity was studied in Alopecurus myosuroides and Apera spica-venti . Seeds were collected from different populations, at different dates and from plants emerging in autumn or spring, and used in four experiments. In the first experiment, initial dormancy was investigated in light and darkness. In Expt 2, buried seeds were exhumed on 16 occasions, from September 1997 to March 2000, and germinated in light, in darkness and after a 5-s light exposure. In Expt 3, emergence was recorded for seeds sown in pots outdoors. In Expt 4, stratified seeds of A. myosuroides only were exposed to photon irradiance ranging from 0.1 to 25 600 μmol m⁻². Variation was high among seed collections, but both species showed winter annual dormancy patterns. Apera spica-venti germinated to high percentages in autumn but negligibly in spring. Alopecurus myosuroides germinated less in spring when tested in darkness and after a short light exposure and emerged poorly in spring, which reflected photo-desensitisation during cold stratification. We conclude that the peak of emergence in A. myosuroides , and to some extent in A. spica-venti , is largely regulated by exposure to light interacting with low-level dormancy. This offers valuable information regarding optimal timing of weed control measures.     

Correlation between the soil seed bank and weed populations in maize fields

Annual weed populations establish every year from persistent seed banks in the soil. This 3 year study investigated the relationship between the number of weed seeds in the soil seed bank and the resultant populations of major broadleaf and grass weeds in 30 maize fields. After planting the crop, 1 m² areas were protected from the pre-emergence herbicide application. Soil samples were collected soon after spraying to a depth of 100 mm and the weed seeds therein were enumerated. The emerged weed seedlings in the field sampling areas were counted over the following 8 weeks. Up to 67 broadleaf species and five grass weeds were identified, although not all were found at every site and some were specific to a region or soil type. For the most abundant weeds in the field plots, on average 2.1–8.2% of the seeds of the broadleaf species and 6.2–11.9% of the seeds of the grass weeds in the soil seed bank emerged in any one year, depending on the species. Overall, the results showed a strong linear relationship between the seed numbers in the soil and the seedling numbers in the field for all the grasses and for most broadleaf weeds. For some species, like Trifolium repens , only a weak relationship was observed. In the case of Chenopodium album , which had the largest seed bank, there was evidence of asymptotic behavior, with seedling emergence leveling off at high seed numbers. An estimate of the soil seed bank combined with knowledge of the germination and behavior of specific weed species would thus have good potential for predicting future weed infestations in maize fields.     

Secondary dispersal, spatial dynamics and effects of herbicides on reproductive capacity of a recently introduced population of Bromus sterilis in an arable field

An arable field was subdivided and subjected to either deep inversion ploughing or non‐inversion cultivation after viable seeds of Bromus sterilis had been sown into oilseed rape stubble. After sowing in isolated plots distributed within the field, sequences of cropping treatments for the establishment of two successive winter wheat crops were applied. Each subfield was split into an uphill and a downhill direction for soil cultivation. The field had a 10° slope. In the season following seed introduction, 2.6% of the introduced seeds had successfully germinated and established in the non‐inversion cultivation regime, when no effective graminicide was applied. Ploughing eradicated B. sterilis. Using differential global positioning system (DGPS) mapping of the whole field population, emerged plants were observed up to 8.7 m (uphill treatment) and 21.3 m (downhill treatment) of their initial source. The median distance seeds were transported was 2.3 m uphill and 4.8 m downhill. Post‐emergence application of the herbicide propoxycarbazone slightly reduced weed density and seed weight, and almost halved weed seed production. Application of fenoxaprop‐P‐ethyl was followed by higher density of plants, tillers and seeds of B. sterilis. Seed viability was unaffected by herbicide use. Thus, in the second wheat crop following seed rain, the weed population was dispersed more widely in the field, such that 20–30% of seeds were dispersed more than 5 m distance from the first year's foci of infestation. The relevance of soil cultivation to secondary dispersal of B. sterilis is discussed.     

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.     

两种雀麦种子萌发期耐热性比较研究

设置了八个变温梯度 ,三种播前预措方式及一个对照 ,分别对两种雀麦 (沙地雀麦Bromus ircutensis和无芒雀麦 Bromusinermis)种子进行萌发测试。测定了不同处理下的发芽率、发芽指数、休眠率和死亡率 ,并依此比较了这两种牧草种子在萌发期的耐热性能。结果表明 ,从发芽率和发芽指数看 ,二者并无明显的差异 ( P<0 .0 5) ,在较低的温度下二者都有较高的发芽率和发芽指数 ,当温度超过 45℃时二者都不萌发。从休眠和死亡状况可知 ,沙地雀麦种子比无芒雀麦种子有较强的耐高温性。三种预措方式中 ,三次回干效果较好     

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.