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Introduction Chlorophyll fluorescence induction is a rapid and noninvasive technique for measuring photosynthetic electron transport in plants. Recognition of fluorescence curves such as Kautsky curves is a useful tool for quantifying herbicide effects. In general, since one of the goals of measuring chlorophyll fluorescence is the early detection of the effects of herbicide before its apparent effects on weed. PSII inhibiting herbicides resistance results from a mutation in the chloroplast genome, reducing the rate of electron transport between QA and QB in PS II. Materials and Methods The seeds pre-germinated in Petri dishes at 28±2 °C in 16/8 hr (light/dark) photoperiod for 72 hr. The pre-germinated seeds were sown in pots (with 10 cm diameter) containing loam: sand 2:1 mixture (v/v). Pots were transferred to a greenhouse and grown at 30 °C and 20 °C day and night temperatures, respectively, with artificial light to provide a 16-h photoperiod. Pots were irrigated regularly to avoid any moisture stress. Ten days after planting (DAP), they were thinned to two seedlings per pot. Twenty days after weed emergence, seedlings of the pots were subjected to the post emergence application of ametryn. Different ranges of ametryn rates were used for the resistant (0, 10, 30, 100, 300, 1000, 3000, 10000 and 30000 g ai ha-1 and susceptible (0, 1, 10, 30, 100, 300, 1000 g ai ha-1) populations because of the difference in dose-response among populations. Nonionic surfactant 0.25% (v/v) was applied with the herbicide at the time of spraying. The sprayer was calibrated to deliver 220 L ha-1 at pressure of 2 atm. The aboveground biomass was harvested 28 days after treatment (DAT) and weighed. In order to analyze the data and draw the shapes, Gompertz function and SigmaPlot11 and R soft wares were used. PEA Plus software, also, was used to output the chlorophyll fluorescence data recorded during the experiment. Results and Discussion The results showed that Fvj was more sensitivity to the application of ametryn in the tested populations. So that four hours after the application of ametryn, Fvj, in some populations, especially susceptible population (24.47 g ai ha-1), decreased significantly. Regarding ED50 values based on fresh weight of populations, the highest amount of ED50 belonged to R4 (10289 g ai ha-1), R1 (6844.6 g ai ha-1), R3 (4770 g ai ha-1) and R2 (3041.0 g ai ha-1), respectively. Estimated ED50 values for Fvj susceptible and resistant populations were, in some cases, far lower than estimated ED50 values for fresh weight of populations at 28 days after treatment with herbicides. In general, the linear relationship between the above parameter and the fresh weight of the populations indicates a strong correlation between these parameters. There were differences among the resistant and susceptible populations in the application of herbicide in terms of the slope of the curve of fvj. So that the curve slope in R1 and R2 biotypes was faster than other populations. In susceptible population response, more correlation was observed between the above parameters. The purpose of this parameter was to show that chlorophyll fluorescence parameters, such as fresh weight assay or weed dry weight, can predict the effect of herbicides. Thus, the researchers are able to evaluate the herbicide's impact on the plants by spending less time and cost. Conclusion Since the main effect of photosystem II inhibiting herbicides is in the J stage (Fvj). Therefore, it is possible to use the Fvj parameter, which most likely shows the changes in electron transfer in the electron transfer chain in photosystem II. The most resistant populations (R1 and R4) based on the measurement of fresh weight, in the analysis based on the Fvj parameter, like other resistant populations, showed the significant difference with the susceptible population. Also, the sensitivity of the susceptible population response to herbicide application based on the Fvj parameter was much higher than that of fresh weight. So that it enables the researcher to quickly distinguish among the susceptible and resistant populations to the mentioned herbicides. [ABSTRACT FROM AUTHOR]