Mode-of-action of photosynthesis inhibitor herbicides

Committee to Review the Health Effects in Vietnam Veterans of Exposure to Herbicides. 2004. Veterans and Agent Orange. National Academies Press. Washington, D.C. . Visited January 2013.

X might be plastoquinone; the binding with the herbicides probably occurs via hydrogen bonds (figs.

Because the seeds of many weed species are quite small and germinate within 0.5 to 1.0 inch of the soil surface, it is important that soil- applied herbicides be positioned in the top 1 to 2 inches of soil to be effective. Herbicide positioning can be accomplished by mechanical incorporation or rainfall. Once a herbicide comes in contact with the plant, absorption through the roots or shoots is very important. A herbicide that is absorbed through the roots will be taken up as long as the herbicide-treated soil remains in contact with the absorbing region near the root tips. As the roots grow to greater soil depths, herbicide uptake declines. Therefore, weeds not killed before the root tips grow out of the herbicide-treated soil are likely to survive.


The health and environmental effects of many herbicides is ..

Herbicide effects on the growth and photosynthetic efficiency of Cassiopea maremetens

The effect concentration EC50 of isoproturon and metribuzin decreased by 1-2 orders of magnitude when exposure time increased from 1-2 hours to 24 hours (Table 1 & 2). This result implies that the effects of pesticides on periphyton communities largely depend on the duration of the exposure to the herbicides. This result is important to take into account in the risk evaluation of pulses of pesticides. This was also the case for hexazinone, although the effect concentration could not be estimated for the short-term exposure experiment because of inconsistency in the dose-response curves and lack of points in the calculation of NEC and EC50. The inconsistency was caused by stimulation at the three lowest concentrations (0.4, 2, and 10 µgl-1, Fig. 1) of hexazinone compared to controls and inhibition at the highest concentration. After 24 hours exposure the stimulation of the photosynthesis disappeared (Fig. 1), the dose-response curve was consistent, and NEC and EC50 for hexazinone was 2.29 and 32.88 µgl-1 (Table 1). Such stimulation in a short-term experiment is commonly found as a short-term response to toxic stress. However, the stimulation is obviously a response to the toxicant, and it is difficult to evaluate its effect.


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Herbicide toxicity refers to the potential to cause injury, illness, or other undesirable effects in mammals, birds, fish, honey bees, or other organisms. Toxicity studies measure short term (acute), mid-term (subchronic), and long term (chronic) effects. Since most herbicides affect physiological processes exclusive to plants (like photosynthesis or plant-specific hormones or enzymes), the majority of forestry herbicides have very low toxicity to humans and animals.

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In the recovery experiment following metribuzin exposure, EC50 did not reveal any increasing or decreasing trend in toxicity as function of the duration of the exposure, but varied in a range between 9.57 and 34.00 µgl-1 and NEC and LOEC were not differing much throughout the 48 h exposure experiment (Figs. 2 and Table 2). The inhibition of the photosynthesis due to metribuzin exposure was therefore not depending on the duration of the exposure to this toxicant. This was also found in the study by Bonilla et al (1998) in experiments with natural communities of microalgae and the herbicide simazine, while the inhibition due to exposure to the herbicide paraquat was dependent on the exposure time, which increased from 18 to 76 % between 30 min and 24 h of exposure.

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Metribuzin affected the periphyton sampled in September 1999 and in May 2000 very differently. The community sampled in May was less sensitive to metribuzin than the community sampled in September, and the photosynthesis in May was even stimulated at the lowest concentrations (Table 1 & 2, Figure 1 & 2). These results confirm the hypothesis that natural communities are highly variable due to influence by physical, chemical, and biological parameters, and toxic substances have different impact on and result in different effect concentrations of the algal communities depending on season, species composition, nutrient status etc. Unfortunately the group composition was not analysed in September. Communities typically consist of many different species that differ largely in sensitivity to toxicants. The difference in sensitivity found may be explained by the dominance of metribuzin tolerant/sensitive species in the communities sampled in May. In the case that tolerant species dominated the community the risk of the toxicant will be underestimated. Because of the variations in sensitivity of phytoplankton communities at different locations and at different times of the year, it has been advocated that 14C-assimilation tests with phytoplankton communities are unsuitable in hazard evaluations of toxicants (Kusk & Nyholm 1991). However, single-species tests are not a good alternative since variability in sensitivity for toxicants may differ up to three orders of magnitude between different species and no general sensitive algae species have been identified (Blanck et al. 1984, Wängberg and Blanck 1988, Källqvist and Romstad 1994).