Evidence in the literature has generally supported either of two paraquat resistance mechanisms: an increase in activity of oxygen radical-scavenging enzymes in resistant plants which affords protection from active oxygen species formed by paraquat; and sequestration of paraquat away from its site of action in the chloroplast. Evidence for the first model relies primarily on measurement of increased enzyme activity and cross-resistance to other oxygen radical-generating stresses in resistant plants. The sequestration model is supported by data showing decreased translocation of paraquat and absence of paraquat injury in plant systems that do not have increased levels of protective enzymes. An alteration in paraquat transport at one of several plant cell membranes could confer resistance by modifying movement of paraquat into the compartment bounded by that membrane. Properties of the plasmalemma, chloroplast envelope, and tonoplast that may be important to paraquat transport are discussed and data supporting or discounting specific membrane alterations in resistant plants are presented. Finally, the possibility that both mechanisms may work in concert is addressed.

All actively metabolizing cells have an electrical potential difference, negative on the interior, across their membranes. This electrochemical potential gradient is generated primarily by proton-pumping ATPases and provides the driving force for the transport of various ionic and neutral solutes. It is a key element in the energy metabolism of cells. Such factors as alteration of transport processes, energy metabolism, cytoplasmic pH, and membrane permeability have a direct effect on the magnitude of the membrane potential. In a brief survey, diclofop-methyl, diclofop, hydroxydiclofop, CGA 82725, haloxyfop-methyl, haloxyfop, bentazon, dinoseb, 4-hydroxy CIPC, and 2-hydroxy CIPC caused rapid depolarizations of the membrane potential of oat coleoptiles. Chlorsulfuron, dimethipin, propham, CIPC, dicamba, alachlor, metolachlor, napthalic anhydride, and paraquat had no measurable effects. The depolarizing effects of diclofop-reported earlier are used to illustrate the methods and interpretation of plant cell membrane potential measurements. Diclofop and diclofop-methyl affect the membrane properties of sensitive plant cells. Diclofop irreversibly depolarized the membrane potential and increased the proton permeability of sensitive cells but not resistant cells. It also increased the ATPase activity of isolated membrane vesicles. The mechanism through which diclofop exerted its effect is not fully understood. The equipment and techniques required for the intercellular recording of membrane potentials and resistance are described as well as the limitations of the techniques. A method not used in herbicide studies but with great potential for studies of herbicide interactions with membranes is patch clamp. A brief introduction to the methods will be given.

Aryloxyphenoxypropanoate (APP) herbicides, such as diclofop, depolarize membranes in parenchyma cells of coleoptiles and root tips, and isolated tonoplast or plasma membrane vesicles from a variety of plant species. Some APP-resistant biotypes of rigid ryegrass and wild oat repolarize membranes after removal of herbicide from a bathing medium. The repolarization ability does not require presence of either APP-insensitive acetyl coenzyme A carboxylase or an increased capacity for herbicide detoxification. The kinetics of depolarization and repolarization depend upon the herbicide, the herbicide concentration, the biotype, and the pH of the bathing solution. For rigid ryegrass, depolarization in the presence of diclofop acid is more rapid than in the presence of diclofop-methyl, and 50% depolarization required about 4 μM diclofop acid. Both the nonherbicidal S(–) and the herbicidal R(+) enantiomers of diclofop acid depolarized membranes in susceptible and resistant ryegrass. Susceptible biotypes regenerated transmembrane potentials following removal of the S(–) but not the R(+) enantiomer, whereas resistant biotypes repolarized following exposure to either enantiomer or a mixture of the two. The herbicide 2,4-D affected, in a complex manner, the ability of both susceptible and resistant ryegrass biotypes to depolarize and repolarize. It is postulated that the intracellular concentration of diclofop acid in susceptible and resistant plants is not the same due to differences in the partitioning of diclofop acid between the extracellular spaces and the cytoplasm. The mechanism producing the postulated difference is unknown, but observations on the proton extrusion capacity of both ryegrass and wild oats, the responses of ryegrass to [K+] and PCMBS, and the single-gene inheritance pattern of resistance in wild oats indicate that changes in the diclofop sensitivity of a plasma membrane protein involved in the generation of proton or ion gradients may be involved.

The aryloxyphenoxypropionate and cyclohexanedione herbicides, which inhibit acetyl-coenzyme A carboxylase (EC 6.4.1.2), have also been hypothesized to act at specific sites on the plasmalemma. An impermeant sulfhydryl binding agent was reported to block the diclofop acid-induced depolarization of the membrane potential (Em) in rigid ryegrass. A correlation between the antagonistic interaction with auxin herbicides both in the field and in the Em response, and the repolarization of Em in herbicide-resistant rigid ryegrass following removal of diclofop acid also provide support for this hypothesis. However, similar membrane responses in resistant grasses and broadleaf species suggest that the membrane response may not be important in the phytotoxic activity of the postemergence graminicides under field conditions. In addition, an antagonistic interaction was not observed in roots of susceptible grasses exposed to combinations of diclofop-methyl and 2,4-D. Furthermore, the repolarization of the Em in diclofop-resistant rigid ryegrass was correlated to differential acidification of the external solution and an increase in the protonated form of diclofop acid, rather than a site-specific interaction at the plasmalemma. Although the membrane response is probably not involved in herbicide phytotoxicity in agricultural systems, a higher extracellular pH in the resistant biotypes of rigid ryegrass may inhibit the movement of these weak-acid herbicides across the plasmalemma, and possibly contribute to increased herbicide tolerance.

Bioassays using cell cultures and callus tissues of leafy spurge were devised to evaluate the potential of rhizobacteria as biocontrol agents. Rhizobacteria isolated from roots of leafy spurge seedlings were screened in suspension-cultured leafy spurge cells. Cell viability was assessed using the Evan's blue bioassay 48 h after bacterial inoculation. Among the 30 isolates tested, LS102 and LS105 consistently caused intensive cell death determined by measuring the A630 of the inoculated cell cultures. Cell death was 2.5 to 3 times higher in cultures inoculated with LS105 and LS102, respectively, than in the control. Population levels of the two isolates within cell cultures and callus tissues of leafy spurge increased during the first 48 h. Leafy spurge callus tissues were inoculated with rhizobacteria either directly or by using the Host Pathogen Interaction System (HPIS). The latter exposes calli to bacteria without any physical contact. LS102 caused cellular leakage and eventually death of the callus tissue. Callus growth was reduced by about 30 to 70% when exposed to LS102 and LS105, respectively. Results suggest that these two isolates may affect leafy spurge at the cellular level by different mechanisms. A screening method based on cell cultures and callus tissues offers a good and rapid technique for detecting deleterious rhizobacteria with potential as biocontrol agents for leafy spurge.

This 4-yr experiment was conducted to develop a profitable conservation tillage system for dryland and furrow-irrigated cotton in a winter wheat-fallow-cotton cropping system at Etter, TX. In the 2-yr cropping sequence, winter wheat was planted immediately after cotton harvest in mid-November and furrow irrigated. Three residual herbicide treatment combinations with atrazine or propazine with fluometuron, or propazine alone were sprayed on stubble after wheat harvest in late June and compared to repeated applications of glyphosate and conventional disk tillage. Glyphosate was used to control weeds that escaped residual herbicides. The following March, fluometuron, prometryn, and 2,4-D were sprayed on no-tillage treatments and trifluralin was incorporated with conventional planting. Hoeing cost ha-1 to control Palmer amaranth and kochia averaged about $13 where trifluralin was used, $38 when initial treatment included fluometuron, $63 with propazine, and $93 when glyphosate alone was used to control weeds in fallow. Lint yield on dryland was about 390 kg ha-1 with disking and when glyphosate was followed by disking in spring, and 540 kg ha-1 with no-tillage. With irrigation, lint yield was 660 kg ha-1 with disking and 759 kg ha-1 with the best no-tillage treatment. Long-term profits ha-1 on dryland ranged from $340 for disking to $524 for propazine no-tillage. With irrigation, profit ha-1 ranged from $707 for disking alone to $751 when glyphosate was used after wheat harvest and followed by disking and incorporation of trifluralin the next spring before planting cotton.