Diclofop-methyl {methyl 2-[4-(2,4-dichlorophenoxy)phenoxy] propanoate} loses part of its activity on wild oats (Avena fatua L.) when it is applied in combination with ester or amine formulations of MCPA {[4-chloro-o-tolyl)oxy] acetic acid.} Uptake of diclofop-methyl is reduced in the presence of commercial formulations of MCPA, particularly amine formulations, which are only partly compatible with the diclofop-methyl formulation. With added MCPA, regardless of formulation, hydrolysis of diclofop-methyl to the acid diclofop is slowed down, both in intact wild oat plants and in a crude leaf extract. At the same time, conversion of any acid formed to inactive conjugates is accelerated in the presence of MCPA. It is concluded that the loss of activity of diclofop-methyl by MCPA can be accounted for by the effects of MCPA on uptake and metabolic conversion of diclofop-methyl.

Canada thistle [Cirsium arvense (L.) Scop. # CIRAR] root samples were collected at monthly intervals over a 2-yr period from two locations to determine seasonal fluctuations in the presence and growth potential of root buds. Root bud growth was highest during late fall and winter months following death of the aerial shoots. Root fragments incubated at a constant 15 C for 2 weeks in continuous light consistently had more buds than nonincubated roots, which indicated active bud differentiation during the incubation period. Incubated roots produced 3 to 9 cm of new shoot length/cm root length. There were no obvious seasonal patterns in the presence of root buds or their ability to elongate at different times of the year. Carbohydrate reserves were stored preferentially in roots rather than in developing root buds or the bases of shoots. These reserves ranged from as low as 3% of root fresh weight during spring months to as high as 26% in late fall months, although the levels did not increase consistently during summer months over the locations and years of this study.

Peach trees treated with the herbicide mixture of 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) and 3-amino-1,2,4-triazole plus ammonium thiocyanate (amitrole-T) had higher leaf nitrogen and more growth than trees where the weeds were controlled by hand hoeing or a black plastic mulch. Apple trees had higher leaf nitrogen and more growth when treated with simazine and amitrole-T than with no weed control. In both peaches and apples more growth and higher leaf nitrogen resulted from the herbicide treatments than from supplemental nitrogen treatments, indicating that the herbicide influenced the nitrogen metabolism of these trees.

Analysis of soil samples taken periodically from field plots on Dundee silty clay loam treated with single or dual annual applications of 4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline (nitralin) or α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) at 0.84 or 1.68 kg/ha indicated dissipation of the herbicides to a low residual level within 3 to 4 months following each application. No accumulation was noted with either herbicide regardless of frequency or rate of application. Nitralin phytotoxicity to sorghum (Sorghum bicolor L. ‘Dekalb 57E’) decreased more rapidly in soil that had received prior treatment with nitralin than in soil with no previous treatment. A microbiological influence is suggested because this effect was nullified by autoclaving the soil prior to herbicide treatment. This effect was not apparent with trifluralin. A survey of 250 fields indicated a significant, inverse correlation between nitralin and trifluralin residue level and soil pH. Nitralin persistence was greater in acid than in neutral soil.

Growth dynamics, water relations, and photosynthesis of soybean [Glycine max (L.) Merr. ‘Ransom’], common cocklebur (Xanthium pensylvanicum Wallr.), jimsonweed (Datura stramonium L.), prickly sida (Sida spinosa L.), sicklepod (Cassia obtusifolia L.), smooth pigweed (Amaranthus hybridus L.), spurred anoda [Anoda cristata (L.) Schlect.], and velvetleaf (Abutilon theophrasti Medic.) were compared in a controlled-environment greenhouse programmed for 32C day and 23C night temperatures. Net photosynthetic rates, net assimilation rates, and water-use efficiency on a whole-plant or single-leaf basis were greatest in the C4-plant, smooth pigweed. Total dry-matter production at 29 days after planting was greatest in common cocklebur and least in jimsonweed. Interspecific differences in dry-matter production were highly positively correlated with leaf area duration and negatively correlated with net assimilation rate. Threshold leaf water-potential levels causing stomatal closure varied among species. The stomata of jimsonweed were the most sensitive to water stress and those of prickly sida were the least sensitive.

During the first season, tubers of purple nutsedge, Cyperus rotundus L. planted in Piedmont Plateau soil at Experiment, Georgia at 1-ft intervals produced 3,090,000 plants and 4,420,-000 tubers and bulbs per acre. Tubers planted at 3-ft intervals produced 2,320,000 plants and 2,760,000 tubers and bulbs. The differences were not pronounced at the end of the second season. A single cultivation at the beginning of the second year did not appreciably affect the number of tubers and bulbs formed. Continuous heavy shade during the second year reduced tuber-bulb formation. No apparent interaction between shading and cultivation occurred.

The purpose of this study was to determine cross-resistance patterns among wild oat lines resistant to acetyl-CoA carboxylase (ACCase) inhibitors and to determine which, if any, cross-resistant type was more common than another. Discriminatory concentrations of two aryloxyphenoxy-propionates (APP) and three cyclohexanediones (CHD) were determined using a petri-dish bioassay. These concentrations were then applied to 82 resistant wild oat lines identified in previous studies. In addition, two resistant standards (UM1 and UM33) and a susceptible standard (UM5) were included in the experiments. Coleoptile lengths expressed as percentages of untreated controls were used to assess the level of resistance to each herbicide. Large variations were observed among wild oat lines and herbicides. However, cluster analysis summarized the relationship between the five herbicides (variables) and the wild oat lines into three main cross-resistance types. Type A included wild oat lines with high resistance to APP herbicides and no or low resistance to CHD herbicides. Types B and C included those with low to moderate resistant and high levels of resistance to all five herbicides, respectively. Type C was the most common cross-resistance type. Relationships among herbicides were determined using pairwise correlation and principal component analysis (PCA). All correlations were high between APP herbicides and between CHD herbicides but not between APP and CHD herbicides. The first two axes of the PCA accounted for 88.4% of the total variance, with the first axis correlated to the CHD herbicides and the second axis correlated to the APP herbicides. In the PCA, wild oat lines were segregated into the three types identified in the cluster analysis. Although CHD and APP herbicides bind at the same region on the ACCase, resistant wild oat lines respond differently to them.

Investigations of the behavior of 3-isopropyl-1H-2,1,3-benzothiadiazin-(4)3H-one 2,2-dioxide (bentazon) in water solutions have shown that it was anionic in neutral solutions by exhibiting a strong movement toward the anode in an electrical field. The herbicide was not adsorbed by any of twelve selected Illinois soils or by the cation exchange resin, carboxymethylcellulose (CMC), but was almost completely adsorbed by charcoal and by the anion exchanger, diethylaminoethylcellulose (DEAE). Bentazon moved with the water front on soil thin-layer plates as well as through soil columns.

Although many studies have been made on the behavior of herbicides in soils, there is still insufficient information to make intelligent use of many herbicides. Certain theories on herbicidal behavior in soils are well established in some regions, but the applicability of such theories to other regions under different soil and environmental conditions is not always known.

Laboratory studies were conducted to evaluate variation in germination response of eight annual bluegrass ecotypes (‘Augusta 4’, ‘Augusta 8’, ‘Augusta 14’, ‘Augusta 17’, ‘Auburn’, ‘Birmingham’, ‘Columbia’, and ‘Purchased’) to photoperiod, temperature, and fenarimol, a fungicide–herbicide used for preemergence annual bluegrass. Seed collected from greenhouse-grown plants and stored for > 2 mo were evaluated under 18 environments (three day and night temperatures by six day and night durations). There was a significant ecotype by environment interaction affecting annual bluegrass germination. High temperature markedly restricted germination, with only the Birmingham ecotype exceeding 20% germination at day and night temperatures of 39 and 29 C, respectively. Maximum germination of all ecotypes was observed at a day and night temperature of 19 and 10 C, respectively. Maximum germination for a specific photoperiod was not consistent across ecotypes; however, all ecotypes germinated to some degree in complete darkness, which indicates that maintaining a dense turf canopy to eliminate annual bluegrass germination may not be completely effective. Ecotypes did not differ with respect to root length response to fenarimol but did vary with respect to shoot length response. Purchased and Columbia shoot growth were the most tolerant to increasing fenarimol concentrations. This information will be used to develop improved management strategies for annual bluegrass.

Reports arose from major Chinese wheat production regions that flixweed was not controlled by tribenuron after the herbicide was continuously used for several years. Flixweed seeds were collected from wheat fields that had been treated with tribenuron repeatedly over 3 to 15 yr or from road sides and remote hills that had never received tribenuron in Jiangsu, Hebei, Shanxi, Sichuan, Shandong, Shaanxi, and Henan provinces, and Tianjin and Beijing metropolises in China. The response of various biotypes to tribenuron was determined by whole plant experiments in the greenhouse. The experiments demonstrated that 11 of 32 flixweed biotypes were susceptible to tribenuron. The remaining 21 biotypes expressed moderate to high levels of tribenuron resistance with resistance indices ranging from 4 to > 1,500. DNA sequence analysis of acetolactate synthase (ALS) genes of selected biotypes 6, 7, 27, and 29 revealed a point mutation at position 197 of the ALS gene, as numbered relative to the protein sequence of Arabidopsis, where proline was substituted by leucine in biotype 7 and by threonine in biotype 29. These mutations are known to confer resistance to ALS-inhibiting herbicides and are responsible for the high resistance of these biotypes to tribenuron. The results also indicate that tribenuron resistance in flixweed is widespread in China and management programs to control these resistant populations are warranted.

Greenhouse experiments were conducted to determine the effects of glufosinate on ammonium accumulation and photosynthetic inhibition in Palmer amaranth. Glufosinate applied at 410 g ha−1 reduced glutamine synthetase activity and enhanced ammonium content 30 min after treatment. Glufosinate application resulted in rapid inhibitions of the photosynthetic rate and stomatal conductance during the first 2 h after treatment (HAT), whereas the ammonium concentration increased over the same time period. Ammonium content 6 HAT was 22 times higher in treated plants than in untreated plants, whereas the photosynthetic rate of treated plants decreased by 63%. At 24 HAT, the ammonium content was 53 times higher in treated plants than in untreated plants; however, no further inhibition of photosynthesis occurred. Photosynthetic inhibition in Palmer amaranth coincided with the rapid accumulation of ammonium and decrease in stomatal conductance shortly after glufosinate application.

Experiments were conducted to determine adaptation characteristics to reduced irradiance of velvetleaf, common lambsquarters, eastern black nightshade, tumble pigweed, and soybean. Plants were grown to the 5- to 8-leaf stage in the greenhouse at ambient radiation (850 μE·m–2·s–1), and 26 and 13% of ambient radiation. Tumble pigweed, a C4 plant, had the highest light-saturated photosynthetic rates at all growth irradiances, while common lambsquarters had the highest rates of the four C3 species. All species adjusted to reduced irradiance by decreasing light-saturated photosynthesis, leaf respiration rates, root:shoot ratios, and leaf densities, while increasing leaf area ratios (LAR)3. As growth irradiance was reduced, support tissues (roots, stems, and petioles):leaf ratios did not change for common lambsquarters or velvetleaf, increased for soybean, and decreased for eastern black nightshade and tumble pigweed, indicating superior adaptation of the latter two species for efficient light harvesting at reduced growth irradiances. Of these five species, eastern black nightshade had the lowest respiration rate, the highest LAR, and the lowest support:leaf ratio for optimum adaptation to shaded environments.

Weedy red rice is a highly troublesome weed of rice in the United States and throughout the world. Effective management of this weed has remained challenging to U.S. farmers, partly because of the biological diversity among red rice populations, resistance to or avoidance of control measures, and genetic similarities with crop rice that allow crossing between the two plant types. The aim of this research was to identify simple sequence repeat (SSR) marker loci that will unambiguously differentiate between U.S. weedy red rice, commercial rice cultivars, and their hybrids, to characterize the genetic diversity and structure of U.S. weedy red rice accessions in relation to Oryza collections from international sources, and to relate genetic and geographic variability within U.S. weedy red rice. Thirty-one SSR markers were used to analyze 180 worldwide Oryza entries and 80 U.S. weedy red rice and U.S. rice cultivars. Twenty-six of the 31 SSR marker loci were highly informative with respect to genetic distinctions between U.S. weedy red rice and U.S. rice cultivars. U.S. red rice are accessions clustered into two main SSR-based collections, awnless strawhull (SA−) and awned blackhull (BA+), according to genetic distance analysis and principal coordinate analysis. Genetic structure analysis clearly identified SA− and BA+ red rice, rice–red rice hybrids, commercial japonica rice cultivars, indica rice, and a number of international and wild Oryza spp. standards (e.g., Oryza nivara, Oryza rufipogon, and Oryza glaberrima) as genetically distinct groups. U.S. SA− red rice exhibited greater spatial structure than did BA+ in that the genetic makeup of SA− accessions changed nearly twice as much with geographic distance as compared to BA+. However, the overall genetic variability within SA− red rice accessions was less than for BA+ accessions, suggesting that the SA− types may be genetically less compatible than BA+ types with other Oryza plants such as rice or other red rice types present in U.S. rice fields. Several of the awned red rice entries exhibited evidence of natural hybridization with different red rice types. Our results suggest that the SA− and BA+ red rice collections have different genetic backgrounds. SA− accessions generally associated most closely with indica-like red- or white-bran Oryza sativa cultivar standards, while BA+ accessions generally associated more closely with O. nivara or O. nivara–like O. sativa entries. Although the U.S. red rice accessions appear not to have descended directly from introductions of the worldwide Oryza standards analyzed, an Oryza red-pericarp entry from Niger (UA 1012; PI 490783) was genetically very similar to some U.S. BA+ accessions.

Four years of field experiments were used to assess the competitive ability of red rice, a weedy variety of rice, and ‘Mars’, a cultivar of rice. Red rice was the dominant competitor each of the 4 yr, and an average of one red rice plant reduced Mars grain yield per plant equal to the effect of four Mars plants. Intervarietal competition was more important than intravarietal competition for Mars, whereas the reverse was true for red rice. The product of the coefficients for intravarietal competition did not significantly exceed the product of the coefficients for intervarietal competition, indicating that the two varieties were competing for the same resources. Using the reciprocal yield model coefficients from 1989, grain yield losses of Mars, due to red rice densities of 4, 16, 25, and 300 plants m−2, were predicted to approximate 13, 37, 48, and 92%, respectively, at a fixed cultivar density of 100 plants m−2.

Bentazon reduced paraquat phytotoxicity to Texas panicum, sicklepod, Florida beggarweed, and peanut. Bentazon applied alone had minimal effect on these species. Smallflower morningglory was more sensitive to bentazon than to paraquat; the interaction of paraquat and bentazon varied from antagonistic to synergistic depending upon the specific combination of rates. Antagonism with tank-mixed combinations was also obtained by applying bentazon prior to paraquat, but not with the reverse. Studies with 14C-labeled herbicides revealed that each herbicide inhibited foliar penetration of the other. In field studies, adding bentazon to paraquat reduced crop injury. However, except for smallflower morningglory, weed control was also reduced.

Toxicity and translocation of nonradiolabeled and translocation of 14C-labeled sethoxydim {2-[1-(ethoxyimino)-butyl]-5-[2-(ethylthio)-propyl]-3-hydroxy-2-cyclohexen-1-one} in common bermudagrass [Cynodon dactylon (L.) Pers. ♯3 CYNDA] were evaluated for foliar treated plants grown 2 or 4 days under controlled environmental conditions. Nonradiolabel treated plants were further grown in the greenhouse for the remainder of 3 weeks. Shoot injury of intact plants was greatest (90%) and regrowth of plants clipped after 2 days treatment each was least (< 5%) at 35 C and 100% RH. At 35 C and 40% RH and at 18 C (40 and 100% RH), shoot injuries were 60 to 68%, regrowth in lengths were 82 to 87%, and in fresh weights were 55 to 74%, respectively. After 2 days treatment, absorption of 14C-activity was greatest (70%) at 35 C and 100% RH, moderate (56%) at 35 C and 40% RH, and least (33 to 43%) at 18 C and 40 and 100% RH, respectively. At 35 C, translocation was approximately 4 fold greater at 100% and 2 fold greater at 40% RH than at 18 C at either RH. Autoradiography indicated the greatest distribution of the radiolabel at 35 C.

A sorghum-sudangrass hybrid [Sorghum bicolor (L.) Moench. x Sorghum sudanense (P.) Stapf. ‘DEKALB SX17+’] (SSH) and foxtail millet [Setaria italica (L.) Beauv.] (FM) were evaluated for their effects on weed populations when grown for forage and as land conditioners prior to late-summer, no-till planting of alfalfa (Medicago sativa L. ‘Arc’). Both grasses established well in no-till situations, except for FM when excessive mulch impeded seed placement or seedling summer-annual weeds were not destroyed at planting time. SSH was the better competitor; after both were planted into a dense stand of seedling summer-annual weeds, SSH outyielded the weeds while FM did not. Both grasses suppressed weeds and enhanced subsequent alfalfa establishment. Obtaining adequate alfalfa stands was not a problem in no-till seedings: seedling populations often exceeded 300/m2 3 to 6 weeks after planting. Lack of adequate weed control was associated with stand losses in which disease was also a factor. Biennial and perennial broadleaf species were the predominant weeds. Prior cropping of SSH or FM is a valuable option for growers wishing to establish alfalfa without tillage, particularly when summer forage is useful.

The effect of surface corn (Zea mays L.) plant residue on weed control with atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] or alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] was studied in the field and greenhouse. Plant residue did not significantly affect weed control when herbicides were applied at recommended rates but had an increased influence on control as herbicide rates were reduced. In the greenhouse, simulated rain improved the control of velvetleaf (Abutilon theophrasti Medic.) and foxtail millet [Setaria italica (L.) Beauv.].

R-25788 (N,N-diallyl-2,2-dichloroacetamide) had no effect on 14CO2 evolution from carbonyl-14C-EPTC (S-ethyl dipropylthiocarbamate) treated corn (Zea mays L. ‘inbred Oh551’). Treatment with 1.0 ppmw R-25788 increased production of water-soluble and unextractable products from EPTC and increased the rate of disappearance of organic-soluble radioactivity. R-25788 increased levels of reduced and oxidized glutathione in corn roots. Pretreatment of corn in nutrient solution with R-25788 had no effect on glutathione synthetase activity in root tissue, but R-25788 at 5.0 and 500.0 nM showed an in vitro stimulation of the enzyme.