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.     

Survival and growth of seedlings of Rumex obtusifolius L. and Rumex crispus L. in newly sown grassland

The survival and growth of seedlings of Rumex obtusifolius L. and Rumex crispus L. were investigated from 1982 to 1984. A sward was established by the sowing of seeds of Dactylis glomerata and Trifolium repens and managed under two cutting frequencies. Five hundred seeds of each Rumex species were sown m^?2. The total number of emergent seedlings of each Rumex species over two seasons was 230–360 m^?2. About 85% of them emerged within 3 months of sowing. Both Rumex species showed the same pattern of survivorship. Survival was clearly enhanced by frequent cutting during the second season. About 20% of the emerged plants survived through to the third season. Plants that emerged during the first season only flowered during the second season. The flowering percentage of total surviving plants was significantly higher in R. obtusifolius than in R. crispus. Rumex obtusifolius was higher in dry matter of aerial parts defoliated during the second season and more individual plants survived through to the third season than for R. crispus. Moreover, dry matter production of grass and clover was depressed by 25–30% in mixtures with R. obtusifolius. compared with production in mixtures with R. crispus.     

Interference of Sinapis arvensis L. and Chenopodium album L. in spring rapeseed (Brassica napus L.)

Interference of Sinapis arvensis L. (wild mustard) and Chenopodium album L. (lamb's-quarters) in spring rapeseed (Brassica napus L.) was studied under field conditions in 1983 and 1984. Both weed species interfered with rapeseed early in the growing season, causing significant reductions in rapeseed dry weight by June of each year. Sinapis arvensis caused greater rapeseed grain yield reductions than did C. album. With weed densities of 20–80 plants m^?2, rapeseed grain yield reductions ranged from 19 to 77% with S. arvensis but only 20 to 25% with C. album. Rapeseed yield reductions caused by mixtures of both weed species ranged from being less than to being equal to the sum of reductions caused by each weed alone, depending on the weed density and year of study. Both weed species were prolific seed producers capable of returning large quantities of seed to the soil. With weed densities ranging from 10–80 plants m^?2, S. arvensis produced 5700–30 100 seeds m^?2 while C. album produced 3100–63 600 seeds m^?2.     

Threshold level and seed production of velvetleaf (Abutilon theophrasti Medicus) in maize

Four tests were carried out in 1980 and 1981 to determine: (a) the economic threshold density of Abutilon theophrasti Medicus (velvetleaf) in maize, and (b) seed production with varying densities of infestation, both in the presence and in the absence of maize. The infestation was artificially created, and the density of the weed ranged from 0 to 80 plants m^?2. The economic threshold, calculated using the Cousens (1987) model, varied between 0?3 plants m^?2 and 2?4 plants m^?2, depending on the variables considered. The presence of maize reduced the seed-rain of A. theophrasti by 50%. This seed-rain reached its maximum level at 20–30 plants m^?2 in maize, and at 30–35 plants m^?2 in weed monoculture. However, with only 4–5 plants m^?2 in competition with maize, A. theophrasti produced 8–10 thousand seeds m^?2. The usefulness of threshold density in weed management is debatable when one considers the ecological characteristics of the A. theophrasti seed, and the great capacity of seed production of this weed.     

Comparison of chlorsulfuron and metsulfuron for control of Allium vineale L.

The response of natural and planted stands of Allium vineale L. to chlorsulfuron and metsulfuron was determined in field experiments in Illinois, U.S.A., in 1982 and 1983. In natural stands very low rates of either herbicide controlled A. vineale. Chlorsulfuron, applied in the spring at 20 g ha^?1, reduced aerial bulblet production by 99% but when it was applied in the fall it reduced aerial bulblet production by only 59%. All rates of metsulfuron (5–20 g ha^?1), at both times of application, effected better than 94% reduction in plant density and yield of aerial bulblets. In a glasshouse experiment, plants derived from aerial bulblets were more susceptible to chlorsulfuron than plants derived from soft offset bulbs. Soft offset bulbs were, therefore, used to establish a uniform stand of A. vineale in the field. In this experiment, non-linear regression analysis showed that metsulfuron was two to three times more active than chlorsulfuron. GR₉₅ values calculated from the computed regression equations show that metsulfuron and chlorsulfuron, applied in April at 5 and 15 g ha^?1, respectively, reduced aerial and underground bulb production by 95%.     

Volunteer barley interference in fibre flax (Linum usitatissimum L.)

Field experiments were conducted in 1989 and 1990 to determine the relationship between yield loss in fibre flax and the density of volunteer barley. At volunteer densities of 50–100 plants m^?2, plants which emerged 7 days before those emerging synchronously with the flax produced a significantly greater reduction in stem height (1990), stem dry weight (DW) at flowering (1989 and 1990) and seed boll DW (1990). The relationships between flax stem and boll DW and volunteer density were described at two growth stages (flowering and maturity) using a linear model. The 1990 model indicated that where volunteer tiller densities ranged from 0 to 350 m^?2, interference by 90 volunteer tillers m^?2 (30 plants m^?2) reduced stem DW by 2–9% at the flowering stage. These losses increased to 9–18% for stem DW plus 7–18% for seed boll DW when plants were grown to seed maturity. Due to the high value of flax fibres and seed it is concluded that an economic threshold based on the control of volunteer barley can be reached at low volunteer densities (30 plants m^?2) especially if both fibre and oilseed were to be harvested.     

Efficacy of UV‐C radiation to reduce seedborne anthracnose (Colletotrichum acutatum) from Andean lupin (Lupinus mutabilis)

The potential of UV‐C radiation of Andean lupin (Lupinus mutabilis) seeds to eradicate seedborne infections of anthracnose caused by Colletotrichum acutatum was investigated. UV‐C doses from 0 to 691.2 kJ m^?2 (resulting from 0 to 96 h of exposure time) on disease incidence reduction and germination on artificially and naturally infected seed were evaluated. The degree of incidence reduction and seed germination was dependent on the dose of UV‐C. The UV‐C doses of 86.4 kJ m^?2 and higher reduced incidence from 6% to 7% to undetectable levels, but these UV‐C doses also reduced seed germination. UV‐C can deleteriously affect physiological processes and overall growth. To assess its impact, L. mutabilis seeds irradiated with UV‐C doses of 57.6 and 86.4 kJ m^?2 were grown. Seedlings grown from noninfected seed and UV‐C treated seed showed an increased concentration of chlorophyll and protein contents, as well as an increase in the activation of defence enzymes peroxidase and catalase, in comparison with plants grown from infected seed. UV‐C doses resulted in seed emergence and seedling dry weight rates that were similar to the noninfected control or better than the fungicide control. Moreover, 57.6 kJ m^?2 reduced transmission of the pathogen from seed to the plantlets by 80%, while 86.4 kJ m^?2 apparently eradicated the pathogen, under greenhouse conditions. The use of UV‐C, first reported here, is advantageous for controlling anthracnose in lupin.     

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.     

Enhanced weed‐crop competition effects on growth and seed production of herbicide‐resistant and herbicide‐susceptible annual sowthistle (Sonchus oleraceus)

Enhanced crop competition could aid in the management of annual sowthistle (Sonchus oleraceus L.), a dominant weed of Australian cropping systems. A two‐year pot study was conducted to evaluate the effect of wheat (Triticum aestivum L.) planting densities (0, 82, and 164 wheat plants/m²) on growth and seed production of glyphosate‐resistant (GR) and glyphosate‐susceptible (GS) biotypes of annual sowthistle. Without competition, both biotypes produced a similar number of leaves and biomass, but the GS biotype produced 80% more seeds (46,050 per plant) than the GR biotype. In competition with 164 wheat plants/m², the number of leaves in the GR and GS biotypes was reduced by 62 and 61%, respectively, in comparison with the no‐competition treatment, and similarly, weed biomass was reduced by 78 and 77%, respectively. Compared to no‐competition treatment, the seed production of GR and GS biotypes was reduced by 33 and 69%, respectively, when grown with 82 wheat plants/m², but increasing wheat density from 82 to 164 plants/m² reduced the number of seeds only in the GS biotype (81%). Both biotypes produced greater than 6,000 seeds per plant when grown in competition with 164 plants/m², suggesting that increased crop density should be integrated with other weed management strategies for efficient control of annual sowthistle.     

SPATIAL GROWTH OF SORGHUM HALEPENSE (L.) PERS.*

Summary. Small plants of Sorghum halepense (L.) Pers. were planted in the field and grown without competition from weeds and crops for 2½ years. The plant spread by means of subterranean rhizomes from which aerial shoots developed at increasing distances from the plant centre, and by tillering around these aerial shoots to form clumps. No preferential direction of expansion was detected and established patches developed an approximately circular shape. Aerial growth stopped completely in the cold season. During the warm season the mean area increment amounted to 1·3 m²/month and was similar in both years of observations. After 2½ years of growth, patches had extended up to 3·4 m from the initial sprig and had a mean area of 17 m². In the second summer of growth, shoot density reached 190/m² and mean seed production was 84 g, or 28 000 seeds per plant. About 80% of rhizome dry weight was present in the upper 20 cm of soil and rhizomes did not penetrate deeper than 40 cm. About 80% of rhizome dry weight was present within 1 m radius from the plant centre. Croissance dans l'espace de Sorghum halepense (L.) Pers.     

The dormancy of wild oat seed (Avena fatua L.) from plants grown under various temperature and soil moisture conditions

Wild oat plants of types fA, fB and fC were grown at a constant 15 or 20°C during the period of seed maturation. Seed of the three types differed little in dormancy when grown at 15°C, but at 20°C a larger proportion of seeds of type fA were dormant compared with fB or fC. Overall, dormancy of seed produced at 15 and 20°C was 97 and 63% respectively. Plants of another collection of type fB were grown from seed at 15 or 20°C with or without water stress applied only from the time of panicle emergence. Water stress and high temperature reduced viable seed production. Seed dormancy was tested immediately after collection by planting the seed in soil, and by Petri dish tests. Further Petri dish tests were made after 6 months storage. Seedling emergence in the first autumn from seeds of plants matured without water stress at 15°C was 10% compared with 30% for seeds grown at 20°C. Seeds grown with water stress at 15°C gave 47%, and at 20°C 78% emergence. The majority of emergence from seeds formed at 15°C without water stress occurred in the second spring after burial. Petri dish tests support these findings and suggest that seeds produced in hot dry summers are less dormant than those produced in cool moist ones.     

Seed biology of Mikania micrantha in Viti Levu,Fiji

Mikania micrantha (mile‐a‐minute) reproduces both by seed and vegetatively. A study to determine the possible pollinators, seed production rates, temperature and salinity limitations to germination and its seedbank size and persistence was conducted in Viti Levu, Fiji. Representatives of the Diptera had the greatest percentage of all floral visits (38%), followed by Hymenoptera (34%) and Lepidoptera (27%), while the honeybee was the most recurring visitor (18% of all visits). Flower heads (capitula) within the inflorescence commonly formed four viable seeds, resulting in 60 820 filled seed being produced per m². However, the seedbanks formed were not massive (600 seed m^?2) and they were moderately persistent (T₅₀: 1–3 years). Seed germination from both high and moderate rainfall regions occurred rapidly, under a wide range of temperature regimes, with no primary dormancy being observed. This study indicates that the seed reproductive success of M. micrantha in the two rainfall regions of Fiji is due to a number of factors, including the production of large numbers of flowers, successful pollination by local insects and the subsequent production of a large number of viable seeds. These seeds have high viability, no dormancy and are capable of forming small‐to‐medium seedbanks that are moderately long‐lived. In addition, seeds can germinate under a wide range of temperature and salinity conditions. This knowledge on seed production, biology, longevity and salt tolerance is vital in the development of management plans of M. micrantha in Fiji.     

Systemic effects of thiamethoxam and chlorantraniliprole seed treatments on adult Lissorhoptrus oryzophilus (Coleoptera: Curculionidae) in rice

BACKGROUND: Feeding assays using adult rice water weevils and foliage of plants treated as seeds with chlorantraniliprole and thiamethoxam at different rates were conducted to evaluate the systemic adulticidal and feeding effects. Dose–mortality relationships were determined for thiamethoxam seed treatments by combining leaf area lost due to feeding and insecticide residues analyzed by LC/MS/MS. Changes in adulticidal activity of thiamethoxam were also investigated by contrasting adult mortalities at the 5–6‐leaf and tillering stages of rice. RESULTS: Adult weevil mortalities and leaf consumption rates on foliage were affected in thiamethoxam but not in chlorantraniliprole treatments when rice was at the 6–7‐leaf stage. The LD₅₀ for weevils feeding on thiamethoxam‐treated rice at the 2–3‐leaf stage was 447 pg insecticide weevil^?1 (95% CL: 25–830 pg weevil^?1) but was lower (142 pg weevil^?1; 95% CL: 102–180 pg weevil^?1) in experiments with 3–4‐leaf‐stage plants. Mortalities on leaves from 5–6‐leaf‐stage plants were consistently higher than on leaves from tillering plants. Thiamethoxam residues measured by ELISA increased with seed treatment rate and differed between plant stages. CONCLUSION: The LD₅₀ values developed in this study are the first values for leaf‐feeding insects on foliage of plants treated as seeds with thiamethoxam. The attrition of adulticidal activity of thiamethoxam in foliage of older plants may help to explain the reduced effectiveness of seed treatments against rice water larvae that is seen at later stages of rice growth in field studies. The differential activity of these two seed treatments on adults suggests that adult mortality contributes to the field efficacy of thiamethoxam but not to that of chlorantraniliprole. Copyright © 2012 Society of Chemical Industry     

Effects of dose and application timing on the seed production of three weed species treated with MCPA or tribenuron-methyl

The seed production of Fallopia convolvulus (L.) A. Love, Galium spurium L, and Thlaspi arvense L., treated with MCPA or tribenuron-methyl, was studied in a pot experiment. The herbicides were applied in doses from 1/16 to 1/l of a full dose at five different growth stages. The largest reduction of seed production was observed on plants treated when one true leaf had developed (F. convolvulus, both herbicides) or at the beginning of the generative phase (G. spurium treated with tribenuron-methyl and T. arvense treated with either herbicide). Tribenuron-methyl (0.375 g a.i. ha^?1) or MCPA (0.1875 kg a.i. ha^?1) applied at bolting stage was enough to reduce the seed production of T. arvense by 83% and 100%, respectively. The seed production of F. convolvulus, treated with 0.75 kg MCPA at the cotyledon stage, was reduced by 70%. Plants of G. spurium with 5–9 axillary shoots, treated with 3 g tribenuron-methyl, produced 45% fewer seeds than the control. Both herbicides (at least at the highest doses) reduced seed production of all three species more than shoot dry matter.     

Morphological and physiological variability in black nightshade (Solanum spp.)

Plants from two black nightshade (Solanum spp.) seed sources were compared for differences in morphology at seedling and mature plant stages, flowering and physiological responses to pre-germination seed treatment, growth responses to temperature, and response to herbicides. Differences in the morphology of the two Solanum spp. were evident from seedling to maturity. The plants grown from the Michigan (MI) seed source had a deep purple abaxial leaf surface at the three to four-leaf stage, a yellow anther column, and toothed proximal half in the mature leaf, whereas the plants grown from the California (CA) seed source had no unusual abaxial coloration or toothed proximal margin and had a dark brown anther column. Differences in response to herbicides were seen between plants grown from the two seed sources following pre-plant-incorporated, pre-emergence, and post-emergence herbicide applications. The plants from the MI seed source were up to five times more susceptible to chloramben, applied pre-plant-incorporated at 1.68 kg ha^?1, and to ethalfluralin, applied preemergence at 1.12 kg ha^?1, than plants from the CA seed source. Plants from both seed sources have been described as Solanum nigrum L. The plants from MI have been identified elsewhere as Solanum ptycanthum Dun. and those from CA as Solanum scabrum Mill. This identification could resolve conflicting reports among researchers regarding chemical control measures.     

Interference between velvetleaf (Abutilon theophrasti Medic.) and soybean (Glycine max (L.) Merr.) I. Growth

Three field experiments were conduced to evaluate the effect of Abutilon theophrasti on dry matter, branching and flowering node, and seed production of soybean. Low populations (2.4–4.7 plants m^?2) of the weed reduced soybean dry matter, flowering node and seed production. The effect was most pronounced on soybean seed yield, and less on flowering node and dry weight production. These effects were due to the presence of A. theophrasti with soybean, and not to changes in plant population. The effect on soybean branching node production was inconsistent between years.     

Flower infection of Brassica oleracea with Xanthomonas campestris pv. campestris results in high levels of seed infection

During seed production, Brassica seed may become infected with Xanthomonas campestris pv. campestris after systemic colonization of plants upon leaf infection, or alternatively, after flower infection. Polytunnel experiments were conducted in 2007 and 2008 to study the relative importance of these colonization routes resulting in seed infection. Cauliflower plants (Brassica oleracea) were spray-inoculated at the 8-leaf stage, after formation of cauliflowers or during flowering, at which stage leaves or blossoms were inoculated. Inoculation at all stages resulted in a relatively high percentage of systemic infection; the average estimated infection incidences for stem base and peduncle infections were 16 % and 19 %, respectively. When seed samples were examined by dilution plating for deep-seated infection following hot water treatment, Xcc was detected in 61 % of the 23 seed samples harvested from plants with inoculated flowers. However, symptom development in seedlings raised from the seeds could not be confirmed in a grow-out test under favourable conditions for Xcc infection at a high RH (>95 %) and a relative high temperature (28 °C). Xcc was not detected in 59 seed samples harvested from leaf-inoculated plants with the exception of one sample from plants inoculated at peduncle formation. In a third polytunnel experiment carried out in 2009, the population dynamics of Xcc on inoculated flowers was investigated. Following spray-inoculation of flowers, 52 % of the flowers were infected with Xcc. During development of siliques, infection incidence decreased slowly and at 56 dpi, 20 % of the superficially disinfected siliques were infected with Xcc. It was estimated that 0.18 % of the seeds was infected and that 1–10 % of the infected siliques contained infected seeds. The implications of these results for control of Xcc in a seed production crop are discussed.     

The survival of Alopecurus myosuroides Huds. seeds in soil

The survival of Alopecurus myosuroides seeds was studied in soil under arable cropping and short term grass leys in which seed return was prevented. At two winter wheat sites, where weed seeds were sown, the mean annual seed decline was 73–83% over a 2- or 3-year period. The rate of decline was similar with all the cultivation systems studied: ploughing, tine cultivation and direct drilling. Seeds buried initially by ploughing, and then not disturbed by cultivation, were slightly more persistent. At five arable sites with natural populations of A. myosuroides, seed numbers declined to an average of 3% of the original amount present after 3 years, and to 1% after 4 years. Initial populations of over 50 000 seeds m^?2 were recorded. Plant populations were not always proportional to the total seed content of the soil, especially on ploughed land. Seed decline in two grass fields was similar to that under arable cropping. A. myosuroides plants were recorded in a wheat crop following a 2-year grass ley. Weed plants did not persist in the vegetative state in grass used for conservation and grazing. At all sites, appreciable quantities of seeds were still present in the soil after 2–4 years. Although a relatively small proportion of seeds survived, the actual number of seeds surviving was substantial. For this reason, it was concluded that any eradication policy is unlikely to be effective in a cropping system dominated by winter cereals.     

Investigation of wheat as a trap crop for control of Orobanche minor

Orobanche minor is a parasitic weed that attaches to the roots of red clover (Trifolium pratense) and a number of other broad‐leaved plant species in the Pacific Northwest USA. Orobanche minor seed must be stimulated by host plant exudates for germination and attachment to occur. However, plant species called false‐hosts can stimulate parasitic seed germination without attachment. These species could be utilized as trap crops to reduce the amount of parasitic seed in infested soil. Wheat (Triticum aestivum), was found to be a false‐host of O. minor; therefore, growth chamber, glasshouse and field soil experiments were conducted to evaluate the effect of six soft white winter wheats (T. aestivum), one durum wheat (Triticum turgidum), and one triticale (Triticale hexaploide) on O. minor germination. In growth chamber experiments, wheat and triticale induced 20–70% of O. minor seeds to germinate. In glasshouse studies, O. minor attachment was minimal on red clover plants grown in pots previously planted to wheat or triticale. In pots that did not receive a false‐host treatment, red clover plants averaged 4.2 O. minor attachments per plant. Red clover plants also had fewer O. minor attachments when grown in field soil taken from the plots where wheat or triticale were grown compared with plants grown in soil where no wheat or triticale were previously grown. Our results demonstrate that wheat may have the potential to be effectively integrated into an O. minor management system.     

Competitive effects of Avena fatua L. plants derived from seeds of different weights

A study was made of the caryopsis weights of Avena fatua grown in the field and in pots and of the competitive ability of A. fatua from heavy seed (≥18 mg) and light reed (≤11 mg) grown at a depth of either 25 or 75 mm with spring barley sown at 25 mm. The weight of field- and pot-grown caryopses of A. fatua types fA, fB and fC ranged from 3 to 25, 5 to 23, and 5 to 25 mg respectively. With one exception, both the field- and pot-grown samples showed a double normal distribution pattern. The mean weights of pot-grown primary and secondary caryopses of fA, fB and fC were 15 and 9, 19 and 11, and 18 and 11 mg respectively. In a competition experiment, A. fatua grown from heavy seeds sown at 75 mm depth with an equal number of barley produced 47% more panicles, 54% more seed and 56% more dry weight per plant than plants from light seeds. When barley and both seed weights of A. fatua were sown at 25 mm depth these differences were smaller, being 21, 28 and 34% respectively. When the barley was at Zadoks stage 87, and when the barley and A. fatua were grown at equal densities at 25 mm depth, the dry weight of barley was reduced from 10·4 to 7·7, and to 5·8 g per plant by light- and heavy-seeded A. fatua, respectively. When the barley was grown at 25 mm and the A. fatua at 75 mm, the weight per barley plant was reduced to 9·5 and 7·2 g by A. fatua plants grown from light and heavy seeds respectively. Reductions in numbers of grain were caused mainly by a reduction in the number of fertile heads.