Experiments were conducted to determine the influence of glyphosate [N-(phosphonomethyl)glycine] on extractable nitrate reductase activity during light and dark growth of soybean (Glycine max) seedlings. Glyphosate (5×10−4M), applied via root-feeding to three-day-old etiolated seedling, significantly reduced enzyme activity in roots (48 to 96 h) and leaves (96 h) of seedlings placed in the light, but had little effect on enzyme activity in cotyledons compared to enzyme levels in tissues of untreated seedlings. During dark-growth, nitrate reductase activity increased with time in cotyledons of untreated seedlings (activity about 85-fold less than in cotyledons of light-grown plants) but much lower enzyme levels were found in cotyledons of glyphosate-treated seedlings after 72 and 96 h. In leaves of dark-grown seedlings, glyphosate reduced nitrate reductase levels by 95%. Most inhibition of extractable enzyme activity occurred in newly developing organs (leaves and roots) which correlates well with reports that glyphosate is rapidly translocated to these sites. However, the fact that glyphosate inhibits growth prior to lowering enzyme activity levels indicates a secondary effect on nitrate reducta
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