A review, particularly of the responses to herbicides of photosynthetic processes at the molecular level, with some discussion of the effects of herbicides on cell membranes.

Field experiments were conducted for 2 yr on pronamide [3,5-dichloro-N-(1,1-dimethyl-2-propynyl)benzamide] treatments in the Piedmont region of Georgia to aid the transition of overseeded cool-season turf to warm-season turf in early spring. Pronamide applied to overseeded perennial ryegrass (Lolium perenne L. ‘Game’ and ‘Manhattan’) gradually reduced the growth of perennial ryegrass and permitted bermudagrass [Cynodon dactylon (L.) Pers. ‘Tifdwarf’] to initiate spring growth with little competition. Total turfgrass cover and turf quality ratings in pronamide treated plots were lower than ratings for untreated plots during the transition period. However, the reduction in turf quality and stand was minimal when pronamide was applied March 20 at 0.8 kg/ha. The turf quality and stand was 76 and 88% of the untreated check on April 23 and May 9, respectively, but the turf fully recovered within 2 weeks. The turf quality was higher in plots treated with pronamide on March 20 than in untreated check throughout June. The optimum date of promanide treatment in the Piedmont Region for transition of cool-season grass to warm-season grass was March 20, when compared to applications made on February 28, April 9, or April 29.

Greenhouse and laboratory studies were conducted to examine the activity and foliar absorption of foramsulfuron in giant foxtail and woolly cupgrass with various adjuvants. Adjuvant selection was important for giant foxtail control. Foramsulfuron provided 90% or greater giant foxtail control with the addition of methylated seed oil (MSO) or MSO plus 28% urea ammonium nitrate (UAN). When a crop oil concentrate (COC) or a nonionic surfactant (NIS) was added to foramsulfuron, giant foxtail control was only 20%. However, when 28% UAN was added to COC or NIS, control was increased to 90 and 85%, respectively. Foramsulfuron absorption and control were closely related in giant foxtail. Foliar absorption of 14C-foramsulfuron in giant foxtail ranged between 35 and 90% 24 h after treatment (HAT) depending on adjuvant selection. The rate of absorption was greatest when MSO plus 28% UAN was added to foramsulfuron and absorption was maximized 4 HAT. Foramsulfuron absorption in woolly cupgrass reached its maximum levels 2 HAT with all adjuvant combinations. Although the rate of foramsulfuron absorption was quicker in woolly cupgrass, absorption trends by adjuvants were similar to those in giant foxtail. Maximum absorption of 14C-foramsulfuron in woolly cupgrass was 84% with the addition of MSO plus 28% UAN. However, even with high levels of absorption, woolly cupgrass control with foramsulfuron was poor and may be related to rapid metabolism to nonphytotoxic compounds.

The potential dispersal of Benghal dayflower seeds by mourning doves was studied in southern Georgia, U.S.A. The gut contents (both crop and gizzard) of mourning doves harvested in the autumn months were investigated to determine if mourning doves fed on Benghal dayflower and whether seeds can survive conditions in the bird gut. Research indicated that mourning doves fed selectively on Benghal dayflower with some harvested birds containing hundreds of Benghal dayflower seeds and capsules in their guts. Further, some seeds recovered remained highly viable. Germination rates in seeds taken from bird crops were similar to controls over the first 4 wk of germination and enhanced over control treatments during the latter 16 wk of a 20-wk germination study. Ultimately, seeds extracted from dove crops had 92% germination as compared to 80% for control seeds. Seeds extracted from dove gizzards had 45% germination, about half that of controls. Benghal dayflower seeds have a structurally reinforced seed coat that probably aids in survival of mechanical damage through bird intestinal tracts. Benghal dayflower seeds exposed to 1.0 M HCl treatment for 2 h had little loss in viability, successfully germinating after such treatment. When evaluating mechanisms for the eradication of Benghal dayflower from agricultural crops, consideration needs to be given to the large number of mourning doves and other bird species that visit cropland and potentially aid in its dispersal.

Acetolactate Synthase- (ALS) inhibiting herbicides are important components for the control of ryegrass species infesting cereal-cropping systems worldwide. Although resistance to ALS herbicides in ryegrasses has evolved more than 25 yr ago, few studies have been dedicated to elucidate the molecular mechanisms involved. To this end, we have investigated the molecular basis of chlorsulfuron, sulfometuron-methyl, and imazapyr resistance in AUS5 and AUS23, two ryegrass populations from Australia. Comparison between whole-plant herbicide assays and DNA sequencing results showed that resistance to the nonmetabolizable herbicide sulfometuron-methyl was associated with four different proline mutations at ALS codon position 197 (P197) in AUS23. In addition to three P197 amino acid changes impacting on the efficacies of the two sulfonylurea herbicides, the tryptophan to leucine target-site mutation at ALS codon position 574 (W574L) was present in AUS5, conferring resistance to both sulfometuron-methyl and imazapyr. The samples were also characterized by non target-site-based resistance impacting on the metabolizable herbicide chlorsulfuron only. Interestingly, compound mutant heterozygotes threonine/serine at ALS position 197, and plants with double mutations at positions 197 and 574 were detected, thus reflecting the ability of this outcrossing species to accumulate mutant alleles. Whole-plant dose-response assays conducted on predetermined wild-type and mutant genotypes originating from the same populations allowed for a more precise estimation of the dominant and very high levels of resistance associated with the proline to serine target-site mutation at ALS codon position 197 (P197S) and W574L mutations. The two highly efficient polymerase chain reaction- (PCR) based derived cleaved amplified polymorphic sequence (dCAPS) markers developed here will allow for quick confirmation of 197 and 574 ALS target-site resistance in ryegrass species field samples and also contribute to identify populations characterized by other likely resistance mechanisms in this important weed species.

An assay technique utilizing Chlorella vulgaris Beijerinck gave highly significant responses to low concentrations of aqueous 3-(p-chlorophenyl)-l,l dimethylurea (monuron), 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron), and 2,4-bis(isopropylamino)-6-methylmercapto-s-triazine (prometryne) following 6 days of incubation. Water extracts of diuron from soil assayed with this organism were highly correlated with diuron added to the soil. Similar correlations for oats (Avena sativa L.) grown on the soil and for diuron in the water extracts as dichloroaniline serve to substantiate the relative effectiveness of the method. Assay of soil samples from field residue plots taken 150 days after herbicidal application showed that active residues of diuron and prometryne were detectable by the organism. Although the chemical method was faster, it has the disadvantage of measuring any dichloroaniline residues rather than only the active residues. The method developed insures a considerable saving in time over herbicide assays involving higher plants and gives reasonably good quantitative data on low concentrations of toxic monuron, diuron, and prometryne.

Two methods of obtaining air samples were used over 4-month periods in 1976 and 1977 to quantitate trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) vapors arising from a silt loam. With the first method, only the air from an enclosure covering 0.37 m2 of soil was sampled. The second method used air probes to sample the air above the field. Rainfall and resultant soil moisture were related to trifluralin vaporization from field soil. Soon after trifluralin incorporation (0.84 kg/ha) and, while the soil was relatively dry, there was very little herbicide vaporization. Trifluralin vapors accumulated rapidly during a period in which several centimeters of rain fell, but with continued precipitation the vapor density declined. The influence of rainfall and resultant soil moisture upon vaporization became less apparent as the soil concentration of the herbicides declined. Less than 0.5% of the applied trifluralin was detected as vapors during each 4-month period.

The phytotoxicity and the persistence of ethyl N,N-dipropylthiocarbamate (EPTC), S-propyl butylethylthiocarbamate (pebulate), ethyl N-ethyl-N-cyclohexylthiocarbamate (hereinafter referred to as R-2063), and S-2,3-dichloroallyl N,N-diisopropylthiolcarbamate (diallate) were studied in nutrient solution and in five soil types. Both in nutrient solution and in soil, EPTC was the most toxic to barnyardgrass [Echinochloa crusgalli (L) Beauv.], followed in order by pebulate, R-2063, and diallate. Herbicidal injury, as reflected by the morphological aberrations that occurred in treated plants, was much more severe than the reduction in fresh weight. Thus, toxicity studies based on plant height had greater validity than those based on weight. The initial toxicity of each herbicide was high in those soils low in organic matter and low in soils high in organic matter. The persistence of the four thiocarbamates in the soil was associated primarily with adsorptive forces and microbial degradation. In a soil with extremely large or small adsorptive capacity, there was rapid inactivation of the herbicides because of strong adsorption to the colloids or lack of adsorption with consequent loss by volatilization. Under the latter conditions, the microbial breakdown played only a secondary role. The herbicides persisted longer in soils with more moderate adsorptive capacities but were subjected to more striking degradation by microorganisms. The patterns and rates of inactivation of the four herbicides in a given soil were quite similar. A particular concentration of each herbicide stimulated growth in barnyardgrass, as reflected both in the fresh weights and in the heights of the plants. The nature of this stimulation was not determined.

Few studies have examined the combined effect of herbicide-induced stress and arthropod herbivory to reduce weed fitness. The purpose of this study was to quantify the effect of arthropod herbivory on the herbicide dose–response of a perennial weed. Fluroxypyr dose–response bioassays using volunteer potato were conducted in the presence and absence of Colorado potato beetle (CPB) herbivory. Logistic model parameter estimates for leaf area, shoot biomass, tuber number, and tuber biomass were often lower with herbivory, compared with no herbivory. Greater variance of parameter estimates within herbivory plots was attributed largely to differential feeding because CPB density was not manipulated in the field. Results from short-season field studies (1,000 growing degree days [GDD] after postemergence [POST] herbicide application) indicated that herbivory had the most effect on potato during a period that coincided with high CPB density and optimal temperatures for CPB development. Season-long bioassays (> 3,100 GDD after POST) revealed that addition of herbivory reduced herbicide use 65 to > 85%, compared with the dose needed to achieve the same reduction in tuber production in the absence of herbivory. Integrated weed management systems targeting volunteer potato are more effective when fluroxypyr applications are made before periods of high herbivory. Moreover, this article describes an experimental approach contributing to optimization of combined effects of arthropod herbivory and reduced herbicide doses.

A 7-yr study was conducted to investigate successional dynamics of herbaceous vegetation after control of western juniper (Juniperus occidentalis Hook.) trees with picloram (4-amino-3,5,6-trichloropicolinic acid) pellets. Downy brome (Bromus tectorum L. # BROTE) rapidly dominated areas under dead tree canopies with an accompanying yield increase from almost none to about 1400 kg/ha within 4 yr after treatment. In the interstitial zone between tree canopies, a dramatic shift in herbaceous species occurred with tree control. Annual broadleaf species decreased, downy brome increased, and medusahead [Taeniatherum asperum (Sim.) Nevski # ELYCM] gradually became dominant. Total yield of herbaceous vegetation in the interstitial zone increased dramatically after picloram treatment, especially where few shrubs occurred. Available soil moisture, litter accumulation and decay after picloram treatment, and N in the surface layer of the soil affected responses of herbaceous vegetation, especially under dead tree canopies. Responses of annual weedy species to western juniper control emphasize the need for comprehensive manipulation of all components of the vegetation when range improvement is attempted.

A Powell amaranth population suspected to be resistant (R) to linuron was discovered in a carrot field in Keswick, Ontario, Canada, in 1999. Dose–response analysis with different herbicides and DNA sequencing of the psbA gene encoding the D1 protein of photosystem II were done to confirm the resistance and identify its basis. A calculated resistance factor indicated a 12-fold increased resistance when linuron was applied to an R population compared with a susceptible (S) population. Moreover, the R population showed 6.4- and 3.1-fold greater resistance to two other phenylurea herbicides (diuron and monolinuron), 1.8- and 1.4-fold greater resistance to two triazine herbicides (metribuzin and prometryn), and 2.6-fold greater resistance to the triazinone metribuzin. R population was also cross-resistant to bentazon and bromoxynil when compared with S population, with a calculated resistance factor of 1.4 and 2.2, respectively. The partial nucleotide sequence of the psbA gene of R populations differed at two locations when compared with S populations. The first mutation coded for a Val219Ile substitution in the deduced amino acid sequence of the D1 protein, and the second mutation was silent and encoded for a proline at position 279 in both R and S populations. The Val219Ile substitution in the psbA gene is most likely the cause of this Powell amaranth population resistance to linuron and other PSII inhibitors. This is the first recorded instance of a Val219Ile substitution in an Amaranthus species.

Effects of seven herbicides and two herbicide formulations on the phytotoxicity of glyphosate {[N-(phosphonomethyl)glycine]} were studied in the greenhouse. All herbicide formulations tested reduced glyphosate toxicity; the degree of reduction among herbicides varied. Type of formulation did not alter the reducing effects of propazine [2-chloro-4,6-bis(isopropylamino)-s-triazine] or propachlor (2-chloro-N-isopropylacetanilide). Liquid formulations of cyanazine {2-[[4-chloro-6-(ethylamino)-s-triazin-2-yl] amino]-2-methylpropionitrile} and linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea] caused less reduction than the wettable powder formulations, but the liquid formulation of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] caused greater reduction than the wettable powder. Inert ingredients of the herbicide formulations were primarily responsible for reduced glyphosate toxicity.

In the present field study, the capability of Canada thistle to develop shoots from intact roots and root fragments at different soil depths was studied. The experiments were performed on four sites with high-density Canada thistle, with three or four replications per treatment. At each site, the soil in the plots was removed layer by layer (to 30 or 40 cm, depending on the site), within a 1 by 1-m quadrat, and spread out on a plastic sheet. All roots and other plant parts were removed, and the soil was either replaced without any root material (two sites), or the roots of the thistles were cut into 10-cm-long fragments and replaced into the source holes (two sites). The measured variables were shoot number and biomass. The number of shoots of Canada thistle decreased with increasing depth (P < 0.001) and increased with time. Additionally, the two factors interacted (P < 0.001) such that shoot development was slower from greater depths. Roots from ≤ 20 cm depth produced higher biomasses than did roots from below 20 cm depth. Replacement of root fragments did not affect the amount of biomass produced. It was concluded that the intact root system contributed considerably more to the total biomass produced by Canada thistle than did the root fragments in the upper soil layers.

Understanding weed–crop interactions is critical in predicting crop yield loss, but it is also important to understand how these interactions affect weed productivity. Therefore, research was conducted to characterize the weed relative leaf area and weed relative volume of several giant foxtail cohorts in soybean, and to assess weed density and cohort emergence time, weed relative leaf area, and weed relative volume as predictors of giant foxtail shoot biomass and fecundity. Giant foxtail cohorts emerged at VE (emergence), VC (cotyledon), V1 (first node), and V3 (third node) soybean growth stages and were thinned to densities of 0, 4, 16, 36, and 64 plants m−2. Based on weed density and cohort emergence time, the maximum shoot biomass per square meter or the maximum fecundity per square meter differed between years. In contrast, shoot biomass or fecundity per plant, as weed density approached zero, and the rate at which shoot biomass or fecundity decreased exponentially, as time increased, were similar between years. Based on the weed relative leaf area, the cohort effect on giant foxtail shoot biomass differed between years, whereas the cohort effect on giant foxtail fecundity was similar between years. Maximum giant foxtail shoot biomass per square meter or fecundity per square meter differed between years when estimated from weed relative leaf area. Based on the weed relative volume, the cohort effect on giant foxtail shoot biomass per square meter or fecundity per square meter was similar between years, as was the maximum giant foxtail shoot biomass per square meter or fecundity per square meter. The temporal stability of weed relative volume, used to describe giant foxtail shoot biomass or fecundity, may aid in improving bioeconomic weed management models.

Experiments were conducted from 1990 to 1994 to investigate the density-dependent effects of rhizome and seedling johnsongrass on the magnitude of full-season interference in field corn. Corn grain yield was decreased as rhizome johnsongrass density increased. A maximum density of 3 rhizome johnsongrass plants 9.8 m−1 of row was considered critical to avoid yield losses above 5% of full-season weed-free corn. Interference of johnsongrass plants originating from seed and growing at densities up to 128 plants 9.8 m−1 of row did not affect grain yield. Seedling johnsongrass plants growing at a constant density of 9.8 plants m−1 of row did not affect yield from corn planted at five densities.

Studies were conducted to determine lethal temperatures for Cyperus esculentus and Cyperus rotundus tubers using diurnal oscillations in soil temperature with maxima of 40, 45, 50, and 55 C and a minimum of 26 C. Growth of Cyperus spp. plants was faster at 40 C than at a constant temperature of 26 C. The 45 C treatment delayed Cyperus spp. emergence but was not lethal to tubers. Tuber mortality was 100% for both Cyperus spp. with the 50 and 55 C temperature regimes. Soil solarization with thermal-infrared-retentive (TIR) films resulted in higher soil temperatures than with a 30-μm low-density polyethylene (LDPE) clear film. With TIR films, greater proportions of emerged C. rotundus plants were killed by foliar scorching, and 6 wk of soil solarization was more effective at reducing C. rotundus density than with the LDPE film. Four weeks after film removal, the lowest level of control was obtained with the LDPE film. For C. rotundus tubers planted 5 and 10 cm deep, 62% control was obtained with the LDPE film, and it decreased to 32% with a 15-cm planting depth. The best residual control was 95 and 92% with the 75-and 100-μm TIR films, respectively. With the TIR films, there was no significant change in C. rotundus control with planting depth.

Excised leaves of Canada thistle (Cirsium arvense (L.) Scop.) ecotypes and bean (Phaseolus vulgaris L., var. Red Kidney) metabolized 3-amino-l,2,4-triazole-14C (amitrole) to the same radiolabeled products, but they were produced at different rates as shown by thin-layer and paper chromatography. Bean leaves metabolized amitrole to a metabolic product (Unknown II) probably β-(3-amino-1,2,4-triazolyl-1-) α-alanine approximately three times taster than Canada thistle leaves, while Canada thistle converted Unknown II to another product (Unknown I) about 10 times faster than did bean. Thus Unknown II accumulated in bean, and Unknown I in Canada thistle. This reaction sequence in Canada thistle was evidently an irreversible consecutive first order reaction. A difference in the rate of metabolism was observed between susceptible and resistant ecotypes of Canada thistle.

Treatments with the herbicide 3-(4-bromo-3-chlorophenyl)-1-methoxy-1-methylurea (chlorbromuron) were combined with seed treatments with the insecticide, 2,2-dimethyl-2, 3-dihydrobenzofuranyl-7-N-methylcarbamate (carbofuran), to measure phytotoxic interaction effects on barley (Hordeum vulgare L. ‘Larker’) and corn (Zea mays L. ‘Michigan 400’). The combination synergistically reduced radicle length in barley seedlings and reduced the leaf area and dry weight of 7-day-old corn seedlings grown in sand culture. The chlorbromuron-carbofuran combination reduced net photosynthesis in barley and corn and increased respiration in barley. The 14C from 14C-chlorbromuron preferentially accumulated in barley and corn shoots. The carbofuran seed treatment resulted in a reduced level in barley shoots and corn roots but increased the accumulation of 14C in barley roots and corn shoots. The basis for this interaction appeared related to the increased accumulation of chlorbromuron in corn and barley shoots and reduced chlorbromuron metabolism.

Twelve different johnsongrass (Sorghum halepense L.) clones were selected from four different regions of the United States and were grown in the greenhouse. The johnsongrass clones grew differently depending upon the environment from which they were originally obtained, suggesting that geographical ecotypes of johnsongrass exist. The parameter of growth most closely correlated to the latitude from which the plants were obtained was the time required for floral initiation. Plants from a more northern latitude consistently flowered earlier than plants obtained from a more southern latitude. Differences in height, stem number, and the weight of rhizomes, shoots, and roots also supported the hypotheses that geographical ecotypes of johnsongrass exist. Susceptibility to dalapon (2,2-dichloropropionic acid) appeared not to be correlated to the latitude from which the plants were obtained.

Repeated, foliar applications of monosodium methanearsonate (MSMA) controlled established, space-planted purple nutsedge (Cyperus rotundus L.) while three other herbicides were less effective. Four to eight applications of 5.6 to 16.8 kg/ha of MSMA in 1 year destroyed most plants. There was little difference in final control from applications made at 2, 3, or 4-week intervals or from 187 to 1496 L/ha of spray solution. Clones of purple nutsedge responded similarly to herbicide treatments.