It is relatively simple to determine the growth rate of a microorganism culture, minute-by-minute, by monitoring the CO2 content of an air stream that has passed through the culture at a constant rate. Any change in the growth rate caused by the addition of a toxicant is readily detectable. This is the basis of a procedure for detecting sulfonylurea herbicides on vegetation. A freshwater alga (Chlorella pyrenoidosa sorokiniana) and baker's yeast (Saccharomyces cerevisiae) were the test organisms evaluated in this study. The alga was the more sensitive to sulfonylureas, and the effect of 0.01 microgram of a sulfonylurea that had been simply leached from an apple leaf was evident in 15 min. Triazine herbicides inhibit the Chlorella even more rapidly than the sulfonylureas.
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