Measuring chlorophyll fluorescence is a rapid and noninvasive technique for assaying photosynthetic apparatus status in plants. Chlorophyll fluorescence measurements were performed in greenhouse to characterize how the fluorescence induction curve (Kautsky curve) and its parameters were affected by ametryn (a PSI! inhibiting herbicide) in R- and S-junglerice biotypes. The experiment was conducted in a completely randomized design with four replications. The maximum quantum efficiency of PSII photochemistry (FjFm), the relative changes at J step (Fvj) and the area between the Kautsky curve and maximum fluorescence (Fm) (Area) of S-biotype decreased dramatically at much lower doses (100 g ai ha-1 ) than in the R-biotype at 4 HAS. The R-biotype showed decrease in the fluorescence parameters only at high concentration of ametryn at the same time. There were positive significant correlation between Fvi taken at 4 HAS and fresh weight taken 28 DAS of the R- and S-biotypes as 0.71 and 0.86, respectively (p:'S 0.01). The chlorophyll fluorescence parameters are suitable and practical indicators for monitoring photosynthesis inhibition; also, they could be useful in detecting biotypes showing resistance to PSII-inhibiting herbicides.

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