Changes in local chlorophyll (Chl) content in photoautotrophic hairy roots of pak-bung (Ipomoea aquatica) were evaluated at incident light intensities of I=11 and 22 W/m(2) by non-destructive measurement of the pigment based on color image analysis. Upon addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), 1-1'-dimethyl-4,4'-bipyridylium dichloride (paraquat) and 2,4-dichlorophenoxyacetic acid (2,4-D) to the medium at the median effective concentrations (0.40, 0.37 and 0.45 mumol/dm(3) for DCMU, paraquat and 2,4-D, respectively), the roots showed different Chl pigmentation responses when the Chl content was measured at longitudinal lengths of l(0)=2.5 mm (Chl accumulation position) and l(0)=35 mm (Chl saturation position) under light irradiation. Chl accumulation index (beta) and Chl degradation index (gamma) were determined from the changes in Chl content at l(0)=2.5 and 35 mm, respectively, during the cultures for 96 h: beta=0% (DCMU), 93.6% (paraquat) and 93.8% (2,4-D), and gamma=98.4% (DCMU), 282% (paraquat) and 86.5% (2,4-D) at I=22 W/m(2). Moreover, the bioassay system with the hairy roots was applied to the evaluation of a model sample of field water. The values of beta and gamma for the field water were determined, respectively, to be 105% and 217% at I=22 W/m(2), from which the field water tested was judged to be a "paraquat-like" toxicant against the roots.