Dose-response studies are an important tool in weed science. The use of such studies has become especially prevalent following the widespread development of herbicide resistant weeds. In the past, analyses of dose-response studies have utilized various types of transformations and equations which can be validated with several statistical techniques. Most dose-response analysis methods 1) do not accurately describe data at the extremes of doses and 2) do not provide a proper statistical test for the difference(s) between two or more dose-response curves. Consequently, results of dose-response studies are analyzed and reported in a great variety of ways, and comparison of results among various researchers is not possible. The objective of this paper is to review the principles involved in dose-response research and explain the log-logistic analysis of herbicide dose-response relationships. In this paper the log-logistic model is illustrated using a nonlinear computer analysis of experimental data. The log-logistic model is an appropriate method for analyzing most dose-response studies. This model has been used widely and successfully in weed science for many years in Europe. The log-logistic model possesses several clear advantages over other analysis methods and the authors suggest that it should be widely adopted as a standard herbicide dose-response analysis method.

Polyphenol oxidase (PPO) activity is highly related to the undesirable browning of wheat-based end products, especially Asian noodles. Characterization of PPO genes and the development of their functional markers are of great importance for marker-assisted selection in wheat breeding. In the present study, complete genomic DNA sequences of two PPO genes, one each located on chromosomes 2A and 2D and their allelic variants were characterized by means of in silico cloning and experimental validation. Sequences were aligned at both DNA and protein levels. Two haplotypes on chromosome 2D showed 95.2% sequence identity at the DNA level, indicating much more sequence diversity than those on chromosome 2A with 99.6% sequence identity. Both of the PPO genes on chromosomes 2A and 2D contain an open reading frame (ORF) of 1,731 bp, encoding a PPO precursor peptide of 577 amino acids with a predicted molecular mass of approximately 64 kD. Two complementary dominant STS markers, PPO16 and PPO29, were developed based on the PPO gene haplotypes located on chromosome 2D; they amplify a 713-bp fragment in cultivars with low PPO activity and a 490-bp fragment in those with high PPO activity, respectively. The two markers were mapped on chromosome 2DL using a doubled haploid population derived from the cross Zhongyou 9507/CA9632, and a set of nullisomic-tetrasomic lines and ditelosomic line 2DS of Chinese Spring. QTL analysis indicated that the PPO gene co-segregated with the two STS markers and was closely linked to SSR marker Xwmc41 on chromosome 2DL, explaining from 9.6 to 24.4% of the phenotypic variance for PPO activity across three environments. In order to simultaneously detect PPO loci on chromosomes 2A and 2D, a multiplexed marker combination PPO33/PPO16 was developed and yielded distinguishable DNA patterns in a number of cultivars. The STS marker PPO33 for the PPO gene on chromosome 2A was developed from the same gene sequences as PPO18 that we reported previously, and can amplify a 481-bp and a 290-bp fragment from cultivars with low and high PPO activity, respectively. A total of 217 Chinese wheat cultivars and advanced lines were used to validate the association between the polymorphic fragments and grain PPO activity. The results showed that the marker combination PPO33/PPO16 is efficient and reliable for evaluating PPO activity and can be used in wheat breeding programs aimed for noodle and other end product quality improvement.

The two primary sites of herbicide action in photosynthetic electron transport are the inhibition of photosystem II (PS II) electron transport and diversion of electron flow through photosystem I (PS I). PS II electron transport inhibitors bind to the D1 protein of the PS II reaction center, thus blocking electron transfer to plastoquinone. Inhibition of PS II electron transport prevents the conversion of absorbed light energy into electrochemical energy and results in the production of triplet chlorophyll and singlet oxygen which induce the peroxidation of membrane lipids. PS I electron acceptors probably accept electrons from the iron-sulfur protein, Fa/Fb. The free radical form of the herbicide leads to the production of hydroxyl radicals which cause the peroxidation of lipids. Herbicide-induced lipid peroxidation destroys membrane integrity, leading to cellular disorganization and phototoxicity.

Chloroacetamide and thiocarbamate herbicides have many properties in common: both herbicide classes are effective only as preemergence herbicides; they inhibit early seedling growth and cause similar injury symptoms in susceptible species; they are detoxified in plants by glutathione conjugation; they have a similar spectrum of selectivity; they can be applied safely in certain susceptible grass crops when applied with antidotes; and they can inhibit the synthesis of lipids, isoprenoids, and other metabolic processes requiring coenzyme A. It can be hypothesized that these similarities are due to the ability of the chloroacetamides and the sulfoxide of thiocarbamates to bind covalently to enzymes, coenzymes, or metabolic intermediates containing sulfhydryl (-SH) groups.

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This study evaluated the effects of inhibitors on polyphenol oxidase (PPO) activity, the effect of the PPO inhibitor tropolone on noodle darkening, and the correlation of PPO activity with darkening of alkaline noodles. The PPO inhibitors tropolone and salicylhydroxamic acid (each at 1 microM) reduced kernel PPO activity by approximately 50% in three hexaploid wheat cultivars but did not inhibit PPO activity in the two very low PPO cultivars, durum Langdon, and the synthetic hexaploid-derived ID580. Tropolone (100 microg/g flour) inhibited alkaline noodle darkening (deltaL*) by 13-25% in the low PPO wheat cultivar, ID377s, and by 39-54% in the high PPO wheat cultivar, Klasic. Alkaline noodle darkening among 502 wheat samples was correlated with kernel PPO activity (r = 0.64). Results substantiate the hypothesis that PPO plays a major role in darkening of alkaline noodles. However, results also indicate that substantial darkening would occur even at zero PPO activity, as measured in the kernel PPO assay. Therefore, darkening of alkaline noodles is probably due to the cultivar-specific level of PPO activity and the presence of at least one additional darkening mechanism. Further investigation is required to identify the phenolic discoloration agent(s) and to determine the potential roles of non-PPO discoloration mechanisms, both enzymatic and nonenzymatic, in wheat products.

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.

A glutathione S-transferase (CST) isozyme from maize (Zea mays Pioneer hybrid 3906) treated with the dichloroacetamide herbicide safener benoxacor (CCA-154281) was purified to homogeneity and partially characterized. l h e enzyme, assayed with metolachlor as a substrate, was purified approximately 200-fold by ammonium sulfate precipitation, anion-exchange chromatography on Mono Q resins, and affinity chromatography on S-hexylglutathione agarose from total CST activity present in etiolated shoots. l h e purified protein migrated during sodium dodecyl sulfate-polyacrylamide gel electrophoresis (PACE) as a single band with a molecular m a s of 27 kD. Using nondenaturing PACE, we determined that the native protein has a molecular mass of about 57 kD and that the protein exists as a dimer. Two-dimensional electrophoresis revealed only a single protein with an isoelectric point of 5.75 and molecular mass of 27 kD. These results further suggest that the protein exists as a homodimer of two identical27-kD subunits. l h e enzyme was most active with substrates possessing a chloroacetamide structure. trans-Cinnamic acid and l-chloro-2,4-dinitrobenzene were not effective substrates. Apparent K,,, values for the enzyme were 10.8 WM for the chloroacetamide metolachlor and 292 p~ for glutathione. l h e enzyme was active from pH 6 to 9, with a pH optimum between 7.5 and 8. An apparently blocked amino terminus of the intact protein prevented direct amino acid sequencing. l h e enzyme was digested with trypsin, and the amino acid sequences of several peptide fragments were obtained. l h e sequence information for the isolated CST we have designated "CST I V indicates that the enzyme i s a unique maize GST but shares some homology with maize CSTs I and 111.Members of the GST (EC 2.5.1.18) family of enzymes are best known for their role in the detoxification of various exogenous compounds. These enzymes catalyze the nucleophilic attack of the thiol group of GSH, y-glutamylcysteinylglycine, at an electrophilic site of a second substrate. This reaction most frequently results in the covalent linkage of GSH to the second substrate, yielding a GSH conjugate, which is generally less toxic than the parent compound. In plants, this conjugation reaction is responsible for the metabolism of several classes of pesticides, including chloroacetamide (Fuerst and Gronwald, 1986; Breaux, 1987; O'Connell et al