Critical evaluation of a screening test for detection of herbicides by inhibiting photosynthesis of isolated chloroplasts

The possibility to determine herbicides in the aqueous environment by observation of inhibiting effects on the photosynthesis of isolated chloroplasts have been critically evaluated. The photosynthetical activities of freshly isolated chloroplasts from spinach (Spinacia oleracea L.) and peas (Pisum sativum L.) have been compared under well-defined conditions. The standard for the rate of photosynthesis was the oxygen evolution, which has been detected by a Clark-type electrode. The ratio between the oxygen production rate before and after a sample addition describes the chloroplast activity. In the presence of herbicides this ratio decreases. Increasing herbicide concentrations have been determined by a normalization procedure in semilogarithmic scales using a sigmoidal calibration plot. Photosynthesis inhibiting substances, both natural and anthropogenic, can be detected collectively. That is an advantage over enzyme immunoassays which can only detect single herbicides. Clearly disadvantageous are the insufficient detection limits (e.g. 8.9 μg/l for atrazine, respectively 2.5 μg/l for terbuthylazine) and therefore the necessary preconcentration.