Recovery of non-target plants affected by airborne herbicides.

This study was conducted to determine the effects of airborne herbicides on the photosynthesis of non-target plants and to interpret and evaluate the observed effects airborne herbicides have on non-target plants. The study involved the exposition of sunflower (Helianthus annuus L.) at different growth stages for 24 h in a wind tunnel to a range of various concentrations (0.012 to 4.104 micrograms/m3) of the herbicide bromoxynil, an inhibitor of photosynthesis. By means of chlorophyll fluorescence induction analysis, the quantum yield of electron transport at photosystem II was calculated as phi PSII = (Fm'-Ft)/Fm'. In order to evaluate the ecological significance of the observed effects, the ability of the plants to recover was investigated. Results show, that their is a risk for sunflower plants of being affected by airborne herbicides; bromoxynil concentrations in ambient air > 0.265 microgram/m3 impair the photosynthetic activity of exposed leaves and accordingly > 0.611 microgram/m3 the photosynthetic activity of newly developed leaves. Compared to monitoring data of bromoxynil in the atmosphere, it can be concluded, that plants remote from agricultural fields may be not affected, whereas plants growing near treated fields are potentially at risk. However, it has to be taken into account that plants are able to recover from injury up to a certain concentration and exposure time. The ability to recover has been found to be related to the growth stage and the development stage of the leaf. Although younger plants are more sensitive than older plants, they can compensate a decrease in the photosynthetic activity better than older plants. Thus, the ability to recover has to be considered, when estimating the ecotoxicological potential of airborne herbicides on non-target plants and consideration should be given to implement this information into a non-target-risk assessment.