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.
Further research is required.
The finding that spermidine and spermine protect rice leaves against paraquat toxicity is intriguing. Whether spermidine and spermine would reduce paraquat toxicity in other plant systems is still to be determined. The possibility that rice leaves with high levels of spermidine and spermine can be used to select paraquat-tolerant rice plants should also be examined. Over the years, our research on polyamines has received generous support from the National Science Council of the Republic of China. Literature Cited Bachrach, U. 1973.