Physiological Responses of Soybean (Glycine max)Plants to Metribuzin

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Abstract
Soybean [Glycine max (L.) Merr. ‘Hark’] plants responded similarly to a sublethal application of metribuzin [2-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] or to low light intensities. Both treatments increased soluble amino acid, ATP, nitrate reductase, and nitrate levels (up to 67, 140, 190, and 10,000%, respectively) and decreased soluble reducing sugars (33%), malate levels (86%), and the chlorophyll a/b ratio (24%). The similar response of light-limited and of chemically photosynthesis-inhibited plants is thought to be induced by a carbohydrate stress or another parameter related to a decreased rate of photosynthetic electron flow. However, during the induction process in shaded plants, light quality might play an important role in some cases. The results obtained with metribuzin and soybean plants are compared to and discussed together with (a) the results obtained by other authors working with s-triazine herbicides and (b) results obtained with methabenzthiazuron [N-(benzothiazol-2-yl)-N,N′-dimethylurea] in earlier experiments.