Cross-resistance of the paraquat-resistant (R) hairy fleabane to other compounds that accept electrons from photosystem I (PSI) or produce toxic oxygen species was determined by chlorophyll loss, electron microscopy, and chlorophyll fluorescence suppression. Although the R bioype is approximately 100 x more resistant to paraquat than the susceptible (S) biotype based upon the assays for tissue damage, little or no cross-resistance was observed to a number of other PSI electron acceptors, including the bipyridilium herbicide morfamquat. A low level of resistance (approximately 10-fold) was noted to diquat and the singlet oxygen generator rose bengal. As measured by chlorophyll fluorescence suppression, the R biotype was about 100-fold resistant to paraquat, but only 10-fold resistant to diquat, and exhibited no resistance to morfamquat. Because differences observed with this protocol are direct measures of the ability of the herbicide to reach the active site and the results correlate with the level of resistance observed by chlorophyll bleaching or electron microscopy, these data suggest that compartmentalization is the major factor in paraquat resistance in hairy fleabane.

Mitotic disrupters from higher plants have been studied for their effects on animal cells, but their effects on plant cells have been studied less. The plant-derived compounds have counterparts among herbicides with respect to cellular effects and, in some cases, mode of action. Cellular effects in common are arrested prometaphases (colchicine, dinitroanilines), multipolar divisions (Vinca alkaloids, carbamates), and production of binucleate cells (caffeine, dichlobenil). The potential of these natural compounds, either directly or as the basis of new chemistries for herbicides, has remained largely untapped.

Artemisinin (qinghaosu), a sesquiterpenoid lactone peroxide constituent of annual wormwood (Artemisia annua L. # ARTAN) that is used as an antimalarial drug, was tested for phytotoxic properties. It inhibited germination of lettuce (Lactuca sativa L.) and annual wormwood, and growth of roots and shoots of lettuce, redroot pigweed (Amaranthus retroflexus L. # AMARE), pitted morningglory (Ipomoea lacunosa L. # IPOLA), annual wormwood, and common purslane (Portulaca oleracea L. # POROL) was inhibited at 33 μM. No effects of 33 μM artemisinin were detected on growth of velvetleaf (Abutilon theophrasti Medik. # ABUTH) and grain sorghum [Sorghum bicolor (L.) Moench.]. Chlorophyll content was not affected in lettuce, and chlorosis was not observed in any species tested. The probable biosynthetic precursors of artemisinin, arteannuin B and qinghao acid, had no effect on growth or chlorophyll content of lettuce; however, they inhibited lettuce seed germination. Artemisinin and cinmethylin {exo-1-methyl-4-(1-methylethyl)-2-[(2-methylphenyl)methoxy]-7-oxabicyclo [2.2.1] heptane} were equally effective in reducing growth of lettuce; however, cinmethylin had no effect on germination. Respiration of lettuce roots or cotyledons was not inhibited by artemisinin. Artemisinin only marginally increased the mitotic index of lettuce root tips at 33 μM. At the ultrastructural level, however, chromosomes were less condensed during mitosis in artemisinin-treated than control meristematic cells. The growth-inhibiting ability of artemisinin could not be reduced by feeding the plants with hydrolyzed protein or treatment with putrescine. Artemisinin is a selective phytotoxin that reduces growth by a mechanism other than mitotic disruption or inhibition of protein synthesis.

The effects of treatment with twelve metabolites of trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl) benzenamine] on the gross morphology and ultrastructure of trifluralin-susceptible (S) and -resistant (R) biotypes of goosegrass [Eleusine indica (L.) Gaertner. # ELEIN] root meristems were examined. Unlike trifluralin, ten of the twelve metabolites including the major metabolites of trifluralin caused no root tip swelling even at water-saturated concentrations. Two of the minor metabolites, 3-methoxy-2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine (TR-36M) and N-(2,6-dinitro-4-(trifluoromethyl)phenyl)-N-propylpropanamide (TR-40), caused root swelling when applied as a saturated solution. The most effective metabolite, TR-36M, caused root swelling at 10 μM or greater concentrations. Because trifluralin caused swelling of the root tips even at a 0.01 μM concentration, trifluralin is 1000-fold more effective than the most herbicidally active metabolite. Both herbicidally active metabolites are also much less effective than trifluralin at increasing the mitotic index. Structural studies indicate that the two herbicidally active metabolites arrest mitosis at prometaphase due to the loss of spindle microtubules, similar to trifluralin. Cortical microtubules are also affected by the highest concentrations of these two metabolites. A resistant biotype, in which mitosis is unaffected by trifluralin, was unaffected by any of the trifluralin metabolites.

Many problems in weed science may be solved by utilizing immunochemical techniques, although these techniques are currently underutilized by weed scientists. In this review, we describe some of the methods of greatest use to weed scientists. Antibodies may be raised against proteins, such as the targets of herbicide action or against the herbicide itself which can be linked to a larger molecule so that antibodies are elicited. Once specific antibodies are obtained, several techniques may be performed utilizing the binding phenomena of antibody-antigen interactions. Immunofluorescence and immunogold electron microscopy are utilized to obtain tissue and subcellular distributions of the protein or herbicide of interest. Quantitation of either protein or herbicide in a given sample may be performed by ELISA or “slot” and “dot” blotting. These protocols are less costly, more sensitive, and much less labor-intensive than most analytical methods. Molecular mass or charge alterations may be determined by electrophoresis and subsequent immunoblotting. With the increased exposure to immunological techniques by weed scientists and their potential utility, we predict that many more weed science problems will be addressed using these protocols.

Ten weed species have developed resistance to paraquat. Evidence supporting two potential mechanisms of resistance has been reported in several species. Resistance may be due to the rapid sequestration of paraquat thus reducing levels of paraquat at the site of action in the chloroplast; alternatively, resistance may be due to the rapid enzymatic detoxification of superoxide and other toxic forms of oxygen.

Approximately one-quarter of all herbicides that have been marketed affect mitosis as a primary mechanism of action. All of these herbicides appear to interact directly or indirectly with the microtubule. Dinitroaniline and phosphoric amide herbicides inhibit microtubule polymerization from free tubulin subunits. Because of the loss of spindle and kinetochore microtubules, chromosomes cannot move to the poles during mitosis, resulting in cells exhibiting an arrested prometaphase configuration. Nuclear membranes re-form around the chromosomal masses to form lobed nuclei. Cortical microtubules, which influence cell shape, are also absent, and, as a result, the cell expands isodiametrically. In root tips and other structures that are normally elongated, these herbicides induce a characteristic club-shaped swelling. Pronamide and MON 7200 induce similar effects, except that tufts of microtubules remain at the kinetochore region of the chromosomes. The carbamate herbicides barban, propham, and chlorpropham alter the organization of the spindle microtubules so that multiple spindles are formed. Chromosomes move to many poles and multiple nuclei result. Abnormal branched cell walls partly separate the nuclei. Terbutol induces “star anaphase” chromosome configurations in which the chromosomes are drawn into an area at the poles in a star-like aggregation. DCPA's most dramatic effect is on phragmoplast microtubule arrays. Multiple, branched, and curved phragmoplasts are found after herbicide treatment. These disrupters should prove to be useful tools in investigations of the proteins and structures required for a successful cell division.

The cellulose-biosynthesis inhibitor (CBI) herbicides all selectively inhibit the synthesis of cellulose despite significant chemical differences. With the exception of quinclorac, they are most effective in inhibiting cellulose synthesis in dicot plants. Dichlobenil and isoxaben are the oldest and best studied of these herbicides, whereas flupoxam is a more recent introduction and acts in many ways similarly to isoxaben. Quinclorac is unusual in that it seems to act as a cellulose inhibitor in grasses but as an auxinic herbicide in dicots. These herbicides inhibit cell plate formation at one of two relatively late stages without affecting microtubule function. The effects of dichlobenil are different from other CBI herbicides; dichlobenil inhibits cellulose synthesis but promotes callose synthesis in its place. Suspension cells of both Lycopersicon esculentum and Nicotiana tabacum can become habituated to normally inhibitory concentrations of dichlobenil or isoxaben by replacing the normal cellulose network in their walls with pectin and extensin. Natural resistance to CBI herbicides is rare and has only been found in red algae species. Arabidopsis lines produced by mutagenesis all share changes in active site rather than alterations in uptake, translocation, or metabolism of these herbicides. The lack of cross-resistance to different CBI herbicides of these mutants indicates that no fewer than three different sites in the cellulose biosynthesis pathway are affected by the different herbicides in this class.

Two kinds of dinitroaniline-resistant biotypes of goosegrass have been found in counties in South Carolina where trifluralin is used for weed control. One biotype, termed the resistant (R) biotype, resists trifluralin about 1000- to 10 000-fold. The other biotype is intermediate (I) in resistance between the R and susceptible (S) biotypes, resisting trifluralin about 50-fold. Growth studies and root tip squashes were used to determine the level of resistance to other dinitroaniline herbicides. The R biotype is cross resistant to all dinitroaniline herbicides, and the I biotype is highly resistant to some and is resistant to others only at a low level. The cross resistance of the I biotype to other dinitroaniline herbicides does not correlate with structural or solubility characteristics of the herbicides. Electron microscopic investigations indicate that cell plate formation (controlled by phragmoplast microtubule arrays) is affected at lower herbicide concentrations than spindle microtubule formations. The level of herbicide resistance, the pattern of cross resistance, and the ultrastructural effects induced by the herbicide indicate that the I biotype has a unique mechanism of resistance from the R biotype.

A Research Committee was established by the Weed Science Society of America to outline the direction of weed science research during the next decade. Weeds adversely affect humans in both agricultural and nonagricultural environments. It is the opinion of the research committee that weed science will be advantageously positioned for the future if research focuses on research decision processes, weed biology and ecology, weed control and management practices, herbicide resistance, issues related to transgenic plants, environmental issues, and potential benefits of weeds. These future weed science research directions endorse those of the commodity and grower input group Coalition for Research on Plant Systems (CROPS)'99, a U.S. Department of Agriculture (USDA)-supported initiative. The future of weed science is dependent on a joint effort from industry, government regulators, and the public sector consisting of grower groups, as well as USDA, Agriculture and Agri-Food Canada (AAFC), and university researchers. It is our opinion that efforts spent on these research areas will benefit not only growers, commodity groups, homeowners, and industry, but society at large, through the maintenance and improvement of the food and fiber production system, and the environment in North America.