Uptake and Action of Metribuzin in Soybeans (Glycine max) and Two Weed Species as Monitored by Chlorophyll Fluorescence

Abstract

Terminal fluorescence (FT) was monitored following root or foliar uptake of metribuzin in soybean, velvetleaf, and smooth pigweed seedlings grown in solution culture. These species are known to be tolerant, moderately susceptible, and susceptible, respectively, to metribuzin under field conditions. The indirect monitoring of herbicide metabolism by fluorescence is complicated by several factors (particularly photoinhibition) and is difficult to interpret when plants are kept in the light. Rapid declines in FTat metribuzin concentrations causing high photosystem II reaction center inhibition were interpreted, in conjunction with fresh weight and injury data, as resulting from photoinhibition in velvetleaf and smooth pigweed, and from a combination of photoinhibition and herbicide metabolism in soybeans. At lower concentrations, FTdeclines were apparently not affected by photoinhibition but were probably representative of herbicide metabolism. These results suggested a significant rate of metribuzin metabolism in soybeans, a minor degree of metabolism in smooth pigweed beginning after 12 h, and no detectable metabolism in velvetleaf. FTmonitored over a 24-h dark period following foliar absorption showed declines in soybeans indicating a modest degree of metribuzin metabolism (at intermediate herbicide doses) but no FTdeclines attributable to herbicide metabolism in velvetleaf or smooth pigweed. Results indicate that the inhibition of a high percentage of photosystem II reaction centers is required before photoinhibition and photooxidative leaf damage can result from photosystem II inhibitor herbicides such as metribuzin.