Herbicides that inhibit the electron transport on the photosystem II (PSII) generate oxidative stress. This stress normally is attenuated by carotenoids, which dissipate the electron energy as heat. Hence, we hypothesize that mixtures between PSII inhibitors and carotenoid inhibitors can result in synergism. The objective of this work was to verify the existence of synergism in mixtures of herbicides from both mechanisms of action. Two experiments were conducted in 1998-99 growing season, one with corn, in the USA, and another with soybean in Brazil. At the corn trial, the treatments consisted of isoxaflutole at 52.5g ha-1, atrazine at 840g ha-1, the mixture of both herbicides, and a weedy control. At the soybean trial, the treatments consisted of metribuzin at 280g ha-1, clomazone at 560g ha-1, the mixture of both herbicides, with additional isolated treatments of metribuzin at 560g ha-1, clomazone at 1,120g ha-1, a weed-free and a weedy control. At the corn crop, assessment included the densities of Abutilon theophrasti (ABUTH), Ipomoea hederacea (IPOHE) and Amaranthus retroflexus (AMARE) at 52 days after the treatments (DAT); and the corn grain yield. At the soybean crop, assessment included Bidens pilosa (BIDPI) density, at 30 and 55 DAT; BIDPI cover, control, dry matter and leaf area, at 55 DAT; and soybean grain yield. Apparent herbicide antagonism occurred at the ABUTH, AMARE and IPOHE control, however, herbicide synergism was observed in the corn grain yield. BIDPI results demonstrated synergism for weed cover and control. Despite the complexity to demonstrate herbicide synergism in the field, the results indicate, at least to one species, the occurrence of synergism in the mixture between PSII inhibitors and carotenoid inhibitors.
herbicide mixtures; atrazine; isoxaflutole; clomazone; metribuzin
