Very little is known about the effects of herbicides on polyamine levels in plants. Minton et al. The protective effect of polyamines against paraquat has not been examined in higher plants. In considering the possible mechanism for reduction of paraquat toxicity, inhibition of paraquat uptake by spermidine and spermine from the medium is one possibility. Since pretreatment with spermidine and spermine followed by treatment with paraquat also results in protection against paraquat toxicity, the effect of spermidine and spermine cannot be explained by direct competition between paraquat and spermidine or spermine.
Paraquat is known to increase lipid peroxidation in leaf tissue.
Thus the elevated activities of enzymes such as catalase, peroxidase, as well as the activities of enzymes in the Halliwell-Asada pathway in rice leaves, which would prevent excessive accumulation of such toxic products, might protect against paraquat toxicity. It appears that reduction of paraquat toxicity by spermidine and spermine is due to the increased activities of catalase and peroxidase. Although the involvement of polyamines in plant growth has been proposed for more than 20 years, recent experiments suggest that polyamines play no role in regulating growth inhibition by those stress conditions, under which the decline of polyamine levels usually occurs. On the contrary, evidence indicates that putrescine accumulation is a factor causing stress-induced growth inhibition of plants. We do not know the mechanism of growth inhibition caused by elevated putrescine levels.