Response of flax, canola, field pea, sunflower, field corn, lentils, and common buckwheat to soil residues of CGA-131036 and chlorsulfuron applied at 22 g ai ha–1 was determined on two soil types at Morden, Manitoba. on a fine sandy loam with a pH of 7.4 and 4.5% organic matter, the length of time required before crops showed no phytotoxicity from CGA-131036 residues was: sunflower 4 yr; canola and common buckwheat 3 yr; flax 2 yr; field pea and field corn 1 yr. on a clay loam with a pH of 6.5 and 5.3% organic matter, the corresponding duration was: lentil, canola, and sunflower 3 yr; flax and field pea 1 yr. Chlorsulfuron residues persisted somewhat longer than CGA-131036 residues on the sandy loam but not on the clay loam.

In field experiments conducted from 1985 to 1987 in Mississippi and Arkansas, jimsonweed was successfully controlled with the fungus Alternaria crassa (Sacc.) Rands. Conidial applications of the fungus provided an average of 96% control in Mississippi, and 87% control in Arkansas. Mycelial formulations were less effective than conidial applications overall, but during periods of favorable environmental conditions, weed control provided by the mycelial formulations was similar to that provided by conidial preparations. These results indicate that Alternaria crassa has potential as a mycoherbicide for controlling jimsonweed.

Many bleaching herbicides with different core structures inhibit phytoene desaturase (PD), a membrane-bound enzyme in the carotenogenic pathway catalyzing the hydrogen abstraction step at the first C40 precursor of β-carotene. Prospects are good that new PD-active herbicides will be discovered by screening for bleaching activity. Accordingly, interest in PD enzymology and molecular genetics has increased. Although active carotenogenic cell-free systems are available, no isolation of PD has been achieved since the enzyme cannot be detected in its isolated form due to complete loss of activity. A portion of the Rhodobacter PD gene was incorporated into an appropriate plasmid which could be expressed in E. coli. This system was used to produce an antibody specific against PD from higher plants as well as Rhodobacter. All PDs assayed had an apparent molecular weight of 52 to 55 kDa. A Rhodobacter gene probe hybridized with a 3.1 kbBamH I fragment from Aphanocapsa which allowed us to sequence the PD gene from this cyanobacterium. Its DNA sequence matched with the apparent molecular weight of the PD band in the western blot, and a fusion-gene product was found to be immunoreactive with the Rhodobacter PD antibody, Anacystis mutants were produced exhibiting cross-resistance against norflurazon and fluorochloridone. Apparently, this resistance is due to an altered PD with concurrent decrease of inhibitor binding affinity. Cloning of the resistant gene into the wild type is in progress.

In two field experiments over a 4-yr period, 2,4-D amine applied up to four times per year from late April to early June at 2.5, 10, and 25 ppmw produced a wide range of symptoms on ‘Concord’ grapes. When 2,4-D symptoms in July were severe, yields were reduced by as much as 85%. However, when 2,4-D symptoms in July were slight (less than 2 on a 0 to 6 visual scale), grape yields were not reduced. A second-order polynomial described well (R2 = 0.92) the relationship between symptoms and grape yield. Symptoms of 2,4-D on grape leaves did not persist beyond the year of treatment, although grape yields did not recover fully until the second year after the last 2,4-D treatment In the first year of 2,4-D treatments, low grape yields were correlated with reduced weight and number of berries per cluster. In the second and third year of treatment, low grape yields were correlated with fewer clusters per shoot in addition to lower cluster weight and fewer berries per cluster. When 2,4-D was applied one year only, soluble solids in the grape juice were reduced as much as 9%, although reductions were not related to severity of foliar symptoms. When 2,4-D was applied for three consecutive years, soluble solids in the juice were consistently higher in grapes with moderate to severe 2,4-D symptoms. There were no consistent effects of 2,4-D on juice color or titratable acidity. The pH of fresh juice was increased slightly by 2,4-D with the maximum being 10% after 3 yr of consecutive treatments. In the first subsequent non-treatment year after 3 yr of 2,4-D treatments, juice quality was similar to the quality of nontreated grapes. At a detection limit of 0.05 ppmw, 2,4-D was not detected in the harvested grapes. Based on the results of these experiments, grapes with slight 2,4-D symptoms would not be expected to have reduced yields or reduced juice quality.

The concentration of three sulfonylurea herbicides in soil was determined by a modified petri dish bioassay procedure. The method involved planting pregerminated seed of selected species in petri dishes containing 65 to 100 g of treated soil and measuring the radicle lengths after 24 h. Chlorimuron was detected in two soils at 0.002 μg g–1 using either corn, sorghum, or sicklepod as the assay species. Chlorsulfuron and CGA-131036 were detected in soils at 0.001 μg g–1 using corn, and chlorsulfuron was detected at the same level using sicklepod. In contrast to other methods, the bioassay procedure described could be completed in 48 h, including pregermination of the seed, growth of the plants on treated soil, and plant measurements.

Rapid strides are being made in understanding the fundamental regulation of plant growth, development, and responses to the environment due to recent advances in molecular biology. Current questions in weed science such as herbicide mechanisms of action, biodegradation, and mechanisms of weed resistance are equally approachable using such methodology. Efforts to introduce herbicide resistance into agronomically important crops are possible because of successful isolation and transfer of genes. Investigations of weed survival and competitive strategies based on developmental processes, such as seed dormancy, are currently underway using techniques designed to monitor and characterize differential gene expression. Molecular methodology also plays a key role in taxonomic studies of weed populations using restriction fragment length polymorphism (RFLP) mapping. The future potential for these and other techniques such as nucleic acid hybridization, polymerase chain reaction (PCR), gene transfer, and the use of transgenic plants is described.

A linear programming model was used to determine which crop rotations and weed management systems result in the highest net farm income for each of three farm sizes (120, 240, and 480 hectares) under alternative tillage systems. Test plot data for the years 1981 through 1988 from the Purdue University Agronomy Farm, which has highly productive, well-drained soils, were analyzed. Net incomes for no-till tillage systems on all farms in the model were consistently and significantly lower than incomes for moldboard and chisel plow tillage systems due to slightly lower yields and substantially higher herbicide costs. Generally, net farm incomes were slightly higher with a moldboard plow versus chisel plow tillage system. Also, as farm size increased, per hectare net incomes increased. About 80% of the time under moldboard or chisel plow tillage systems, the model chose as optimal the lowest of three herbicide application rates. A corn/soybean rotation was chosen as optimal on 56% of the farm area analyzed, versus 25% for continuous corn and 13% for a corn/soybean/wheat rotation.

Genetics can be a powerful adjunct to just about any kind of physiological study, including weed physiology or weed/herbicide interactions. Making, mapping, and reverting mutations is simple and straightforward. Making mutants can be as simple as isolating variant individuals from the “wild”, as uncomplicated as doing seed mutagenesis in your laboratory, or as sneaky as recovering mutants as sectors in whole plants. The overall principles for successful development of a protocol for seed mutagenesis of weeds are described and potential problem areas noted. These generalities are illustrated with a specific case history, that of chlorsulfuron. Although chlorsulfuron is accurately described as an inhibitor of the synthesis of branched chain amino acids, careful physiological examination suggests that it kills plant cells, not by starvation for amino acids, but by active toxicity of a metabolite, α-amino butyric acid, produced from a precursor available for diversion in cells with inhibited acetolactate synthase (EC 4.1.3.18, ALS). The story of dominant resistance due to an altered ALS enzyme is well known; analysis using additional mutants fleshes out the story of how chlorsulfuron works. Such analysis has the potential to help unravel other problems in weed physiology.

Tissue and cell culture offer weed scientists many opportunities to research herbicide effects on plants. This review will discuss examples in which plant cells grown in vitro have been used to study herbicide action. Plant cell and tissue culture have many advantages over the use of whole plants; however, several disadvantages that exist are discussed. Cell cultures can be established for most plant species and provide a relatively homogeneous system for studying herbicide action. Responses of plant cells to herbicides are usually correlated with responses at the whole plant level, and cells have the advantage of posing fewer physical barriers to herbicide uptake and translocation. Cell culture techniques discussed include: screening candidate herbicide compounds; investigating herbicide efficacy, mechanism of action, metabolism, and uptake; and ascertaining mechanisms of herbicide resistance, selecting for resistance, and regenerating crops.

In field studies, ‘Regal’ sweet potato greatly reduced yellow nutsedge growth when the two species were grown together using standard cultural practices. At the end of the growing season, yellow nutsedge shoot dry weight per m2 in plots where the two species were planted together was less than 10% of shoot weight in plots where nutsedge was grown alone. Presence of yellow nutsedge did not markedly affect sweet potato growth. When grown together in a greenhouse experiment designed to minimize the competitive effects of sweet potato on yellow nutsedge, yellow nutsedge growth was reduced more than 50% by sweet potato 8 and 12 weeks after planting. The most polar fraction of serially extracted sweet potato periderm tissue was highly inhibitory to yellow nutsedge root growth. These results indicate that sweet potato interference with yellow nutsedge under field conditions is partially due to allelopathy.

The utility, advantages, and disadvantages of several spectrofluorometric and in vivo spectrophotometric methods in plant science are reviewed, with emphasis on their use in weed science. Examples of the use of in vivo, dual-wavelength spectrophotometry to probe the effects of herbicides on cytochrome f oxidation/reduction, the P515 chromatic shift, phytochrome synthesis, and other processes are discussed. Use of in vivo spectrophotometry to examine effects of herbicides on phytylation of chlorophyllide, protochlorophyllide photoconversion, porphyrin accumulation, and other processes that result in spectral changes is discussed and examples are provided. Spectrofluorometric methods for pigment identification and quantification and for enzyme assays are illustrated with examples. Spectrofluorometric methods used in conjunction with high-pressure liquid chromatography (HPLC) and thin-layer chromatography (TLC) are discussed.