Mode of action of piperonyl butoxide as herbicide synergist of atrazine and terbutryn in maize

Piperonyl butoxide, a known insecticide synergist, enhances herbicidal activity of atrazine and terbutryn in maize seedling(s). When applied to maize seedling in combination with either atrazine or terbutryn, piperonyl butoxide enhances foliar uptake of both herbicides and leads to a light-dependent damage to membrane integrity, indicating free radical involvement. Rapid changes in the glutathione level were also observed in plants treated either with piperonyl butoxide alone or in combination with atrazine. Furthermore, in piperonyl butoxide-treated maize, terbutryn (but not atrazine) degradation is inhibited as compared to those treated with terbutryn alone. In vitro studies using microsomal fraction(s) isolated from etiolated maize seedlings have shown that either piperonyl butoxide or terbutryn bind to cytochrome P-450 monooxygenases forming a distinct shift in absorbance spectrum with a peak at 438 nm. Piperonyl butoxide caused about 50% inhibition in terbutryn conversion to polar products by maize microsomal fraction(s) containing NADPH and O₂, indicating that piperonyl butoxide inhibits terbutryn degradation mediated by cytochrome P-450 monooxygenases. Piperonyl butoxide had little effect on atrazine degradation in treated plants. High concentration(s) of piperonyl butoxide were needed to inhibit the in vitro activity of glutathione-S-transferase, isolated from maize leaves. We suggest that the synergistic effect of piperonyl butoxide is due to enhanced herbicide penetration, induced membrane damage, and/or inhibition of cytochrome P-450-mediated herbicide metabolism.