Solanum viarum, commonly known as tropical soda apple (TSA), is native to Brazil and Argentina but has become a harmful weed in many countries with tropical climates. This study was conducted to reassess the seed biology of TSA found in South Africa. Cold stratification (14 d), acid scarification (20% H2SO4 for 5 min), and sandpaper scarification (30 s) significantly improved percentage germination when compared to the control. The highest germination (99.5%) was achieved when seeds were germinated in 50% Hoagland's nutrient solution (HS). The lowest germination (66%) was recorded in the absence of phosphorus (P) under alternating light conditions. HS without nitrogen (N) completely inhibited seed germination of TSA under constant light conditions. These findings are useful in controlling TSA by amending the levels of N and P in soils. Seed germination of TSA was significantly enhanced by different concentrations of smoke-water and butenolide solution. Smoke-water dilution of 1:500 v/v and butenolide concentration of 10−8M showed the highest seedling vigor indices (6,688 and 6,666, respectively) in comparison to the control (1,251) and gibberellic acid (GA3) concentrations (< 5,327). These findings suggest that germination of seeds or seedbanks of TSA might be successfully stimulated using smoke solutions. Subsequently, patches of seedlings emerging after treatment can be mechanically uprooted to reduce the infestation of TSA. However, justifying this with field trials is essential.

Control of seedling and rhizome johnsongrass [Sorghum halepense (L.) Pers. # SORHA] with five herbicides was evaluated after postemergence application in various diluents and spray volumes using one to three types of applicators from 1983 to 1985. The rate of each herbicide required to control seedling johnsongrass using paraffinic oil as the diluent in applications with an external- or internal-mixing air-assist sprayer at a spray volume of 9.4 L/ha was one-half the rate required when water plus 1.25% (v/v) paraffinic oil concentrate (POC) or water plus 1.25% (v/v) soybean oil concentrate (SOC) were the diluents in applications with a conventional sprayer equipped with fan jet spray nozzles at a spray volume of 187 L/ha. Rates were also reduced when once-refined soybean oil was the diluent at a spray volume of 9.4 L/ha, except for the ethyl ester of quizalofop {(±)-2-[4-[(6-chloro-2-quinoxalinyl)oxy] phenoxy] propanoic acid}. Sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} was equally effective on seedling johnsongrass growing in soybeans [Glycine max (L.) Merr. ‘DPL 105’] when applied at 224 g ai/ha with an internal-mixing air-assist sprayer or a controlled-droplet applicator (CDA) at a spray volume of 9.4 L/ha and a conventional sprayer at a spray volume of 187 L/ha, regardless of diluent. Conventional application required the use of 1.25% (v/v) SOC or POC as part of the diluent to be effective. For three of the five herbicides evaluated on rhizome johnsongrass, paraffinic oil at a spray volume of 9.4 L/ha was the only diluent that resulted in season-long control. Rhizome johnsongrass control with this diluent also resulted in significantly higher soybean yields than all other diluent-spray volume treatments.

Seeds of five yellow foxtail [Setaria lutescens (Weigel) Hubb.] biotypes, collected from Connecticut, Iowa, Massachusetts, Pennsylvania and California, were tested for dormancy and germination characteristics. Seeds of the California biotypes, while dormant at time of dispersal, germinated readily following a 4-month after-ripening period in dry storage. California biotype seeds did not require stratification. The eastern biotypes also showed high initial dormancy. Prolonged dry storage (16 months) or stratification overcame this dormancy to varying degrees. All five biotypes germinated over the range of 15 to 35 C, but the California biotype exhibited much higher germination at 35 C than any eastern biotype. The California biotype germinated more rapidly, by 1 to 3 days, at any given temperature than the eastern biotypes. These differences in dormancy and germination requirements between the five biotypes are considered as evidence to support a hypothesis of genetically controlled, physiological differences between geographically separated biotypes.

The germination behavior of giant foxtail and shattercane seed was investigated to determine whether environment in which maternal plants grow and progeny seed develop affects seed dormancy. Giant foxtail and shattercane seed was collected in 1985, 1991, and 1992. Plants from this seed were grown to maturity in the field and the greenhouse in 1993 and 1994. Experiments were conducted using mature progeny seed collected from these plants to determine the influence of environment of maternal growth on seed germination. Greenhouse-raised giant foxtail seed was more dormant than field-raised seed. Progeny of the 1991 giant foxtail collection had the most dormant seed, whereas progeny of the 1992 collection had the least dormant seed. Field-raised shattercane seed was more dormant than greenhouse-raised seed, but this dormancy was easily overcome by placing them in fluctuating temperatures. Moist stratification of giant foxtail seed increased its overall germination and reduced the differences in germination between individual collections and between field- and greenhouse-raised seed lots. Moist stratification of shattercane seed resulted in increased variability in germination within and among seed lots. Subjecting shattercane seed to freezing and thawing was most detrimental and caused an 89% reduction in germination due to seed death. These data indicate that before overwintering, giant foxtail seed differed in germination percentage, but overwintering reduced these differences. Shattercane seed produced in different environments had similar germination percentage. This suggests that successful overwintering depends on its ability to survive freezing and thawing conditions.

Greenhouse and laboratory studies were conducted to determine the basis of differential response of two corn hybrids to the chloroacetanilide herbicide metolachlor. In greenhouse experiments, metolachlor at 6.7 kg ha−1 reduced the height of the susceptible ‘Northrup-King 9283’ corn by 53% relative to untreated controls and caused extensive visible injury 14 d after treatment Under the same conditions, the height of metolachlor-treated ‘Cargill 7567’ corn seedlings was reduced by only 18% without any visible herbicide injury. The 14C-metolachlor was more rapidly absorbed by the emerging shoot of the metolachlor-susceptible hybrid, Northrup-King 9283. Thus, differential metolachlor tolerance may be due in part to processes at the level of herbicide uptake. Metabolism experiments revealed that both hybrids were able to conjugate 14C-metolachlor with glutathione at similar rates. However, glutathione S-transferase activity increased earlier during seedling development and reached higher activities in the metolachlor-tolerant hybrid, Cargill 7567.

Haloxyfop-methyl toxicity to johnsongrass was reduced when sprayed in combination with 2,4-D in greenhouse studies. The antagonism was not seen when 2,4-D application followed haloxyfop-methyl treatment by 72 h or preceded it by 24 h. The antagonism of haloxyfopmethyl activity was evident regardless of chemical form of 2,4-D used (DMA, BOE ester, IPA, or acid). Absorption of 14C-haloxyfop-methyl into johnsongrass leaves was slowed by the presence of 2,4-D in the treatment solution but total absorption after 72 h was not affected. Translocation of 14C was decreased when the 14C-haloxyfop-methyl was applied in combination with 2,4-D. More haloxyfop-methyl, less haloxyfop, and more ether-soluble haloxyfop-methyl metabolites were found in treated leaves when haloxyfop-methyl was applied with 2,4-D compared to haloxyfop-methyl applied alone. The changes in haloxyfop-methyl translocation and metabolism caused by 2,4-D can quantitatively account for the antagonism observed.

The relation of translocation and degradation to selectivity of 3,5-dibromo-4-hydroxybenzonitrile (bromoxynil) in winter wheat (Triticum aestivum L., var. Nugaines), a resistant species, and coast fiddleneck (Amsinckia intermedia Fisch. & Mey.), a susceptible species, was examined. Radioautographic and extraction analysis revealed that the label from 14C-bromoxynil was more mobile in coast fiddleneck than in wheat. Higher levels of soluble radioactivity were found in extracts of coast fiddleneck than wheat. Higher levels of insoluble label were found in tissue residues of wheat than coast fiddleneck. In both species, most of the activity remained in treated leaves. A high percentage of the soluble activity was attributed to 14C-bromoxynil in both wheat and coast fiddleneck. The evolution of 14CO2 by wheat treated with 14C-bromoxynil significantly exceeded that of coast fiddleneck. The difference in susceptibility between winter wheat and coast fiddleneck appears to be explained partly by differential translocation and degradation of bromoxynil.

Field research was conducted to determine the potential of hyperspectral remote sensing for discriminating plots of soybean intermixed with pitted morningglory and weed-free soybean with similar and different proportions of vegetation ground cover. Hyperspectral data were collected using a handheld spectroradiometer when pitted morningglory was in the cotyledon to two-leaf, two- to four-leaf, and four- to six-leaf growth stages. Synthesized reflectance measurements containing equal and unequal proportions of reflectance from vegetation were obtained, and seven 50-nm spectral bands (one ultraviolet, two visible, and four near-infrared) derived from each hyperspectral reflectance measurement were used as discrimination variables to differentiate weed-free soybean and soybean intermixed with pitted morningglory. Discrimination accuracy was 93 to 100% regardless of pitted morningglory growth stage and whether equal or unequal proportions of reflectance from vegetation existed in weed-free soybean and soybean intermixed with pitted morningglory. Discrimination accuracy was 88 to 98% when using the discriminant model developed for one experiment to discriminate soybean intermixed with pitted morningglory and weed-free soybean plots of the other experiment. Reflectance in the near-infrared spectrum was higher for weed-free soybean compared with soybean intermixed with pitted morningglory, and this difference affected the ability to discriminate weed-free soybean from soybean intermixed with pitted morningglory.

Seasonal variation in flowering of common dandelion (Taraxacum officinale Weber) was studied in 1969 and 1970 at Bowling Green, Kentucky. At this location the dandelion flowered throughout the year. Maximum flowering occurred during April and a secondary peak occurred in September and October. Number of flower stems per square meter was not significantly correlated with precipitation, temperature, or day length. Length of stem was positively correlated with temperature and day length. The plants exhibited diurnal variation in flowering habit. Number of times (or days) the heads opened and closed, length of time in which the heads remained open each day, and length of time heads remained closed before opening into mature heads (white balls) varied with time of year.

Field studies conducted over 2 yr in Louisiana determined critical periods of Rottboellia cochinchinensis interference in Zea mays. In a duration of interference study, R. cochinchinensis was allowed to compete with Z. mays for 0 (weed-free, season-long), 2, 4, 6, 8, 10, 12, or 14 (weedy, season-long) wk, after which plots were maintained weed-free for the rest of the growing season. Rottboellia cochinchinensis biomass at time of initial removal increased linearly as weeks of interference increased. For 2 wk of interference, R. cochinchinensis biomass was greater in 1993 than 1994, but for 4 wk or more of interference, biomass was greater the second year indicating environmental conditions were more conducive to R. cochinchinensis growth in 1994. Season-long R. cochinchinensis interference reduced Z. mays height by 18% compared with the weed-free check. For both years, R. cochinchinensis reduced yields 125 kg ha−1 for each week of interference. In weed-free maintenance studies, 0, 2, 4, 6, 8, 10, 12, or 14 (weed-free, season-long) wk of weed-free conditions were provided, after which R. cochinchinensis was allowed to repopulate. Zea mays yield was equivalent for the weed-free control and plots maintained free of R. cochinchinensis for 2 wk or more. In the interference studies, season-long R. cochinchinensis interference reduced Z. mays yield at least 33% compared with the season-long weed-free check.

Velvetleaf growth and canopy architecture were compared under a range of light conditions representative of competitive and noncompetitive environments typical of irrigated Mediterranean-type agroecosystems. Velvetleaf biomass and seed production exceeded those reported in the literature. Plants grown in full light produced 1,370 g dry weight and 44,200 seeds per plant and showed low relative variability. Velvetleaf grown with corn was reduced to 21 g dry weight and 349 seeds per plant, and had high relative variability for biomass and seed numbers. Velvetleaf grown with kidney bean, intraspecific neighbors, or under shadecloth had dry weights and seed numbers that were intermediate to plants grown in full light or with corn. Relative growth rate (RGR), net assimilation rate (NAR), and leaf area ratio (LAR) were assessed utilizing Richards functions, which were fitted to the primary biomass and leaf area data by weighted regression. RGR was highest for all plants early in the season, but declined later. Dynamics of NAR and LAR appeared to be correlated with increased self-shading, shading by neighbors, leaf age, and shedding of lower canopy leaves. Dynamics of specific leaf area corresponded with light availability such that the leaves exposed to full light were thicker than those exposed to shade. The branches of plants in all treatments had random azimuths and the foliage area density was concentrated along the perimeter of the plant's canopy. Velvetleaf increased the canopy radius through extensive branching when exposed to full sunlight. Leaf area distribution and branching patterns resulted in leaf area indices of less than 1.0. Leaves maintained a perpendicular angle to the sun throughout the day, but this depended on whether leaves received a consistent directional signal from the sun and not necessarily on whether they received a high-intensity signal. When shaded, the allocation of dry matter went primarily to the stem tissue, which increased the height rather than the girth of the plants. There was a 10- to 20-d delay for allocations to seed in the case of shaded plants relative to those grown in full sunlight. In brief, velvetleaf had a wide range of growth and canopy responses to a variety of light availabilities and it should have little difficulty in becoming fully established in the irrigated agroecosystems of Mediterranean-type regions.

Weed residues can impact nitrogen (N) cycling in agro-ecosystems that primarily utilize POST weed control. Quantifying this potential N source or sink may influence weed control and fertilization practices. A laboratory experiment measured the rate and quantity of N release from common lambsquarters, common ragweed, and giant foxtail. Weeds were grown in the field at four N rates (0, 67, 134, or 202 kg N ha−1) and collected at two weed heights (10 or 20 cm) to give a range of residue chemical composition. Residue chemical composition parameters of carbon : N (C : N) ratio and total N, nitrate-N, acid detergent fiber, and neutral detergent fiber concentration were measured and correlated with N release. Nitrogen release from weed residue mixed with soil was determined over a 12-wk period. Nitrogen was released from all weed residues at 12 wk. Prior to 12 wk, N was immobilized by giant foxtail grown with no N application. Prior to 4 wk, N was immobilized by 20-cm weeds grown with no N application. Nitrogen release from weed residue was negatively correlated with C : N ratio. Weed residue with a C : N ratio of < 19 (weeds grown with N application and 10-cm weeds) released 25 to 45% total N concentration within 2 wk and may contribute N within the growing season. Weed residue with a C : N ratio > 19 (giant foxtail and 20-cm weeds grown with no N) initially immobilized N and may not contribute N within the growing season.

Johnsongrass (Sorgum halepense (L.) Pers.) is a serious problem in many parts of the United States but little is known about the carbohydrate metabolism of this weed. Rapp studied the seasonal variation of various carbohydrates in Johnsongrass grown in pots. These plants were placed in the greenhouse for several weeks. The purpose of the present investigation was to study carbohydrates in field-grown plants and to obtain information on the effects of herbicidal treatments on the carbohydrate balance.

Weeds are a fundamental component of agroecosystems and, if not appropriately managed, can cause severe crop yield losses. New perspectives on weed management are required, because current approaches, such as herbicide application or soil tillage, have significant environmental and agronomic drawbacks. We propose the concept of “neutral weed communities,” which are weed communities that coexist with crops and do not negatively affect crop yield and quality compared with weed-free conditions. Management practices that promote neutral weed communities can enable reduced use of herbicides and soil tillage while enhancing ecosystem services and biodiversity. We report scientific evidence of neutral weed communities and survey ecological explanations for why different weed communities have different effects on crop production. We also propose two weed management approaches for attaining neutral weed communities. The first approach aims to maximize weed biodiversity using traditional approaches such as cropping system diversification and integrated weed management. Higher weed biodiversity is associated with lower dominance of competitive weed species that reduce crop yield. The second approach relies on modern tools such as robots and biotechnology to manipulate the density of specific weed species. This approach can remove highly problematic species and minimize niche overlap between the weeds and crops. Given the complexity of interactions among crops, weeds, and other components of the agroecosystem, we highlight the need for multidisciplinary research to illuminate mechanisms that determine the neutrality of weed communities.

Collections of plants of medusahead, Elymus caput-medusae L., were made from natural infestations in Nez Perce County and Washington County in northern and southern Idaho, respectively. The ash and silica content (dry weight basis) decreased as the plant matured. The total ash of the entire plant contained from 72–89 per cent silica. Similar percentages of silica in ash were obtained in the culms, heads, and seeds of the plant. X-ray diffraction patterns and polarizing microscope examinations showed the mineral form of silica to be opal. Heavy deposition of silica was found in the barbs of awns, in the epidermis of the leaves, culms, glumes and seeds, and in strands beneath the epidermis.

An understanding of the physiological action of various herbicides is based in part upon a knowledge of the exact path of movement of the substance in the plant, the rapidity of transfer from one tissue to another, and the relative proportion of the total conductive tissue that is effective in transfer under any given set of conditions. Although the intercellular distribution of radioactive isotopes in both plant and animal tissues has been studied by the use of histoautoradiography, only one brief paper describes the use of this method with plant growth regulators.

The relative tolerance of cucumber and squash to amiben was established for germinating seeds, and for young plants with both foliage and root applications. In all cases, the dosage required for 50% inhibition of growth was 6 to 8 times greater for squash than for cucumber. Root applications of carbon-14 labeled amiben on reciprocal grafts of cucumber and squash plants indicated that the shoots were spared in these experiments by differential blocking of amiben transport from the roots. Shoots of either species appeared tolerant on squash roots and susceptible on cucumber roots. Radioactivity in the shoots of both species was much greater when grafted on cucumber than on squash. Foliage applications of carbon-14 labeled amiben showed more entry into cucumber than squash, but little movement to untreated plant parts in either species.

Reliable mechanical weed control requires knowledge of the achievable levels of weed control and crop damage when using certain implements in specific conditions. Quantitative methods that use weed, crop, soil, and cultivator characteristics to predict weed control and crop damage need to be developed. To that end, the relative susceptibility of weeds and crop plants to mechanical weeding and the selective ability of cultivators need to be quantified separately. The method presented in this study uses measured plant anchorage forces to quantify crop and weed sensitivity to being uprooted by a weed harrow and predicts the relationship between weed and crop uprooting by mechanical weeding. Uprooting and anchorage force of young perennial ryegrass and garden cress plants were measured in laboratory harrowing experiments on sandy soil. A nonlinear equation was introduced to describe the relationship between weed uprooting and crop uprooting. The parameters representing the selective potential of the actual crop–weed condition (K pot) and the implement selective ability (K cult) did not depend on crop uprooting. The relationship between potential weed and crop uprooting that could theoretically be obtained by a perfectly selective implement (i.e., pulling each plant with equal force) was calculated from plant anchorage force distributions measured before harrowing. The observed uprooting percentages achieved by harrowing were lower than the potential uprooting percentages. With K cult accounting for imperfect weeder selective ability, prediction accuracy was satisfactory. Field validation is required to confirm whether this method improves comparison and prediction of weeding performance of different weeding implements in different crop–weed situations.

Effects of ethylene, gibberellins, kinetin, nitrate, light, and temperature, each alone or combined with other factors, were studied on the germination of dormant seed of field pennycress (Thlaspi arvense L. # THLAR). Treated seed were incubated at a range of constant temperatures, at temperatures alternating daily between 12 and 22 C (12 h each), or at 12 C followed by transfer to 22 C. No germination ocurrred in water under any conditions. When applied in the dark KNO3 had no effect on germination under any of the temperature regimes. A combination of KNO3, light, and a shift in temperature from 12 to 22 C induced 100% germination. Four days of incubation at 12 C induced maximal seed sensitivity towards KNO3 + light, and the sensitivity remained unchanged if the incubation at 12 C was extended to 15 days. Upon transfer to 22 C, the effect of low-temperature incubation was lost within 3 days. Of the two seed lots studied, one had a relatively lower chilling requirement, but both had similar requirements for light and KNO3. Gibberellic acids4+7 (GA4+7) induced 100% germination in the dark or light at all temperature regimes. Gibberellic acid3 (GA3) was promotive under isothermal conditions, but its effectiveness was enhanced by a shift in temperature from 12 to 22 C. Kinetin or ethephon, each applied alone, had little effect on germination. Ethephon and GA3 together induced 100% germination.