An intensive field survey of an eastern Nebraska corn and soybean field was conducted to characterize the spatial structure and temporal stability of broadleaf weed seedling populations over two growing seasons. Anisotropy, the effect of direction on the relationship between observations, is present in the semivariogram for the velvetleaf and common sunflower populations in 1992 and 1993. The directional trends in aggregation are visible in kriged maps as elliptical shapes oriented east to west across the study area. In addition, there are two distinct zones of aggregation from north to south. These two distinct areas of aggregation are reflected as a ‘plateau’ in the north-south semivariogram. The distance over which this plateau extends indicates that the shape or size of the patch is contracting in the north-south direction (perpendicular to the crop row). The slope of the semivariogram in the east-west direction (aligned with the crop row) remains consistent from 1992 to 1993 suggesting that the shape of the patch is not changing. For sunflower populations, the slope of the north-south empirical semivariogram changes at 20 m, similar to the velvetleaf population semivariograms. This change, however, is reflected as a downward trend in the empirical semivariogram. The distance over which this trend occurs increases from 1992 to 1993 suggesting that seedling patch size was smaller in 1993 compared to 1992. Weed seedling establishment resulting from seed dispersal, differential seed and seedling mortality, or emergence may have resulted in the observed patch dynamics.

Acifluorfen, alachlor, glufosinate, glyphosate, paraquat, and pendimethalin were evaluated for their effects on mycelial growth and sclerotia/microsclerotia production by Rhizoctonia solani AG-1 IA and IB in culture. All of these herbicides except glufosinate and glyphosate were evaluated for effects on severity of Rhizoctonia foliar blight of soybean in the field. In laboratory studies, all herbicides reduced colony radius of R. solani. Growth reductions for IB were greater than for IA in the presence of pendimethalin, alachlor, and acifluorfen, but glufosinate reduced growth of IA more than IB. Sclerotia production by both isolates was prevented by paraquat, greatly reduced by glufosinate, but markedly less affected by the other herbicides tested. In field studies, all tested herbicides influenced severity of Rhizoctonia foliar blight when disease pressure was low, but only paraquat reduced severity when disease pressure was high.

This paper gives an overview of computer assisted molecule design using quantitative structure activity relationships (QSAR). In deriving a QSAR for a set of compounds, one should be aware of possible properties one can use to correlate with the activity of this set of compounds. The paper describes some of the traditional and also more recently used properties that can be calculated. The traditionally used properties have been those calculated for the substituent groups that were varied on a particular scaffold, whereas the more recently used properties are those calculated on the molecule as a whole. One of the most recently used properties for comparative purposes are fields around the molecules. The technique is termed comparative molecular field analysis (CoMFA). The statistical techniques used to correlate the properties with the activity are also described and, finally, a case study is described. The case study shows the application of traditional and 3D QSAR (CoMFA) techniques on some photosystem II inhibitors.

This paper focuses primarily on the means by which biochemical information can be used to identify enzymes which, upon inhibition, produce lethal phenotypes and the enzyme inhibitor design strategies that have the highest probability of not only inhibiting the enzyme but also translating that inhibition into herbicidal efficacy. The identification of an exquisitely lethal target site is the key initial component to this approach and has often been one of the most difficult steps because the attributes of a lethal site have, at best, been ill-defined. An examination of the characteristics of known targets provides some insight as to the definition of a lethal target. Recently, antisense RNA suppression of enzyme translation has been used to determine the extent of inhibition required for toxicity and offers potential as a strategy for identifying lethal target sites. After identification of a lethal target, detailed knowledge of the enzyme's chemical and kinetic mechanism as well as the protein's structure may be used to design potent inhibitors. Various types of inhibitors may be designed for a given enzyme. The advantages and disadvantages of a given type with respect to in vivo efficacy as well as the probability of herbicide resistance development will be discussed.

A convergence of research effort in a number of scientific disciplines in the early 1980s resulted in a rapid expansion of knowledge of the structure and function of the photosynthetic reaction center in bacteria and higher plants. The structure of the reaction center from photosynthetic bacteria was determined by X-ray analysis. The herbicide binding protein (the D1 protein) was identified by photoaffinity labelling and found to be an integral part of the photosynthetic reaction center complex in higher plants. Studies using herbicide-resistant mutants enabled the location of the herbicide binding niche on D1 to be determined. Quantitative Structure Activity Relationships (QSAR) of families of inhibitors and their effect on photosynthetic electron transport helped elucidate the nature of the interaction between inhibitors and receptor. Binding appeared to be predominantly hydrophobic with hydrogen bonding also having an important role. Studies with a series of highly potent inhibitors, the 2-cyanoacrylates, identified certain steric constraints in the interaction of these molecules with the binding site. The activity of these inhibitors was particularly sensitive to minor structural change and they proved to be useful probes of receptor topography. The results of structure-activity studies of the 2-cyanoacrylates combined with a refined knowledge of the three-dimensional structure of the inhibitor binding site has enabled computer-based molecular modelling of interactions of these inhibitors with the receptor. The spatial arrangement of the inhibitor functional groups within the binding domain was shown to be a critical factor in determining binding affinity.

We have developed a mathematical model for the plant vascular system that enables the prediction of a compound's phloem systemicity as a function of its partition coefficients and acid dissociation constants. The mathematical model can account for the sensitivity of systemicity to plant parameters such as plant size and pH of the phloem sap. This paper reviews this model and demonstrates how it accounts for the phloem systemic properties of most herbicides as well as that of many endogenous substances such as plant hormones. The model also can be used to design phloem systemic pesticides as illustrated for a pronematicide that successfully controls nematodes when applied foliarly to transgenic tobacco plants capable of regenerating the parent nematicide in a root specific fashion.

Eight triazolopyrimidine sulfonanilides were tested for metabolic stability in a number of crop and weed species. These data, along with in vitro determinations of activity (I50) against acetolactate synthase, successfully described the in vivo activity of these compounds in a two-parameter model. Whole plant activity increased with increasing compound stability and decreasing I50 (r2 =.78, N = 36). The difficulty in obtaining metabolic stability data during a structure optimization program prompted a study with substituent parameters in models of in vivo activity. Models describing whole plant activity in jimsonweed were developed using a series of 5-methyl triazolopyrimidine sulfonanilides that differed only in ortho and meta substituents on the aniline ring. The I50 term and clogP were most important to jimsonweed activity. Hence, in vitro activity (I50) may be a useful component of whole plant structure activity models to aid in identification of barriers to in vivo performance.