الفهرس 
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Abstract
Seven herbicides belong to four chemical groups (triazine-, triazinone-, dinitroaniline- and urea-herbicides) were evaluated on the green alga, Scenedesmus vacuolatus (formerly named Chlorella vusca var. vaculata). This algal species that requires little culturing and handling was used as
a model plant cell to perform the current studies. Algal count and chlorophyll content were used as biomarkers for studying herbicides phytotoxicity and resistance phenomenon.
The objectives of the current study are: 1- comparing the phytotoxicity of herbicides tested; 2- predicting the development of resistance phenomenon toward the triazine herbicides; 3- evaluating the cross- and multiple-resistance between herbicides of different chemical groups; and 4- identifying mechanisms involved in the resistance phenomenon. To reach these objectives, current studies were divided into seven parts as follows:
1- Phytotoxicity of herbicides tested:
Seven herbicides belong to four chemical groups were evaluated for their inhibitory effects on the reproduction and growth of a laboratory biotype of the green alga, Scenedesmus vacuolatus (named S0). Three parameters, algal count/1ml algal suspension, chlorophyll content/1ml algal suspension and chlorophyll content/106 cells were selected as biomarkers for comparing the phytotoxicity of herbicides tested.
1.1- Inhibitory effects on algal reproduction:
Herbicides tested were evaluated on Scenedesmus vacuolatus laboratory biotype and the inhibition of cell division was used as a biomarker of phytotoxicity. The 96 hr probit data reconfirm that atrazine is the greatest inhibitory of cell division (EC50-algal count = 0.00037 ppm) > terbutryn (0.00084) > ametryn (0.00096) > prometryn (0.00163) = metribuzin (0.00212) > pendimethalin (0.00296) > isoproturon (0.00531).
1.2- Inhibitory effects on algal growth:
When the growth expressed as chlorophyll concentration measured in constant volume of algal suspension (µg chl/1ml), the 96 hr EC50s reveal that atrazine was the greatest inhibitor and isoproturon was the least effective. The inhibitory order was as follows: atrazine > ametryn = terbutryn > prometryn > metribuzin > pendimethalin = isoproturon. When the growth was expressed as µg chl/106 cells, the 96 hr data confirmed that atrazine and terbutryn were the greatest inhibitory of chlorophyll formation. Metribuzin and prometryn were in the 2nd order. Ametryn was in the 3rd order, however, it has no significant difference with prometryn. Isoproturon and pendimethalin were inferior.