Phytotoxicity estimation

Urea and uracil herbicides tend to be persistent in soils and may carry over from one season to the next (299). However, there is significant variation between compounds. Bromacil is debrominated under anaerobic conditions but does not undergo further transformation (423), linuron is degraded in a field soil and does not accumulate or cause carryover problems (424), and terbacd [5902-51-2] is slowly degraded in a Russian soil by microbial means (425). The half-hves for this breakdown range from 76 to 2,475 days and are affected by several factors including moisture and temperature. Finally, tebuthiuron apphed to rangeland has been shown to be phytotoxic after 615 days, and the estimated time for total dissipation of the herbicide is from 2.9 to 7.2 years (426). 

Although arsenic is not an essential plant nutrient, small yield increases have sometimes been observed at low soil arsenic levels, especially for tolerant crops such as potatoes, com, rye, and wheat (Woolson 1975). Arsenic phytotoxicity of soils is reduced with increasing lime, organic matter, iron, zinc, and phosphates (NRCC 1978). In most soil systems, the chemistry of As becomes the chemistry of arsenate the estimated half-time of arsenic in soils is about 6.5 years, although losses of 60% in 3 years and 67% in 7 years have been reported (Woolson 1975). Additional research is warranted on the role of arsenic in crop production, and in nutrition, with special reference to essentiality for aquatic and terrestrial wildlife. 

Knapweed litter, collected from a severely infested site, was air-dried and ground to pass a 20-mesh screen. To determine the phytotoxicity of the litter, the ground material was applied to m plots on a pasture site in 1981 and on two rangeland sites in 1982. Sand (650 g) was combined with each knapweed treatment to produce an even dispersal of litter. The litter was applied in November and grass yields, estimated on a dry matter basis, were determined in the following June. 

The same three spectrophotometric reagents were compared for their abilities to differentiate mono- and polynuclear hydroxy-aluminium complexes in solutions typical of those used in phytotoxicity studies (Parker et al., 1988b). Methods based upon each of the three reagents yielded estimates of the mononuclear aluminium fraction of adequate precision for most purposes. Studies using ferron demonstrated its utility for characterising the non-mononuclear aluminium fraction using kinetic analyses. The ferron spectrophotometric procedure was preferred for its simplicity, level of precision and moderate rate of reaction with aluminium. 

In this chapter I describe how microbial activity may be estimated and what data are presently available on a) rates of microbial transformation and utilization, b) phenolic acid effects on soil and rhizosphere microbial populations, and c) the influences of soil and rhizosphere microbial populations on phenolic acid phytotoxicity. The resulting insight is then used to suggest a possible approach by which this hypothesis may be tested experimentally. 

Figure 6. Mathematical models for the effects of herbicide phytotoxicity on crop growth and yield. A represents an estimation of final yield reduction [99].

In 1995 over 1.7 billion worth of glyphosate was sold. (The total world market for herbicides is estimated to be 14 billion.) This herbicide thus makes up more than 12% of the herbicide market. It has been more than 30 years since the phytotoxic properties of glyphosate were first described, and it is still an herbicide with great unexploited potential through the use of genetically engineered crop plants resistant to it. Whether such techniques are ethically acceptable and favorable for the chemical environment and biodiversity is another question. The debate about this will probably continue for another decade or so. 

The time periods that are required for recovery from possible phytotoxicity and physical disturbance are uncertain [89], In general, recovery of vegetation from physical disturbance in arid ecosystems can take hundreds of years [90], An estimate of the minimum time to recovery could be provided by the average age of the lost vegetation [90], It should be noted that the recovery of one ecological property can be impeded by restoration or reclamation of another. For example, the maintenance of impervious soil caps over a waste disposal site involves the removal (and therefore prevention of recovery) of deeply rooted vegetation and burrowing mammals [91],... 

Estimation of Phvtotoxicity. Several investigators have used cell cultures to study herbicide phytotoxicity (3. 5. 18-20). Callus and cell suspensions have potential in the estimation of phytotoxicity, especially in detecting the inherent phytotoxicity of a molecule that fails to penetrate into or translocate in a whole plant. Alteration of the molecule to enhance penetrability may provide a usable herbicide. Structure activity relationships have also been examined to determine the comparative potency of different plant growth regulators in cell cultures in comparison to whole plants (21). 

Because cell cultures do not normally possess a fully functional photosystem, they fail to estimate the phytotoxicity of photosynthetic inhibitors unless secondary sites of action exist. The phytotoxicity and detoxification of the photosynthetic inhibitor metribuzin [4-amino-6-(l.l-dimethylethyl)-3-(methylthio)-1.2.4-tri-azin-5(4H)-one] was studied by Oswald et al. (5) in dark-grown cell suspensions of resistant and tolerant soybean cultivars. These researchers demonstrated that phytotoxicity was not restricted to photosynthesis. Enzymatic detoxification of metribuzin was inoperative in susceptible cell cultures and was suggested to be due to the... 

In general, studies of pesticide metabolism in cell cultures have shown that metabolism is qualitatively similar to that of the whole plant, but quantitative differences do exist. Whole plants or plant parts need to be used to confirm the quantitative aspects of pesticide metabolism observed in plant cell cultures. However, cell cultures can be used to estimate the phytotoxicity and metabolic fate of chemicals that exhibit poor uptake and mobility in whole plants. Thus, they provide an inq>ortant adjunct to whole plant studies. In addition, higher yields of minor or transitory metabolites can usually be achieved in cell cultures, allowing the determination of a sequence of metabolic steps in a reaction. 

Table 5.12 Examples of QSAR models for estimating phytotoxicity correlations with various parameters... 

The sulfonylureas are compounds of intermediate lipophilicity (log estimated in the range 1.5 to 2.5), and all possess the acidic sulfonylurea function (pKa 3.3 to 5.2). They are applied pre- or postemergence and kill plants by slowing cell division due to inhibition of the enzyme acetolac-tate synthase. The compounds move in both the xylem and phloem, " and their phytotoxic action is believed to be due both to direct uptake at the sites of action (e.g., root tips) and to phloem transport to the growing points. 

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