ABSTRACT
In addressing the pesticide effects on allelopathy of plants, this chapter adopts the broader definition of allelopathy and considers each agricultural chemical group separately. Effects of herbicides on secondary metabolism of crops could be very important in host-plant resistance relationships and in other allelopathic interactions. In some cases, herbicides may alter the concentrations of phenolic compounds in the plant by affecting the activity of peroxidases. Herbicides induce a multitude of effects in the secondary metabolism of both crops and weeds. Besides controlling pathogens of plants, fungicides have been demonstrated to affect plant metabolism. The few reports that exist on the effects of insecticides on allelopathic or secondary compounds in plants all describe effects on phenolic constituents. Some evidence exists that the treatment of plants with fungicides induces phytoalexin production to ward off microbial attack. If natural chemical defenses are to be economically exploited, such defenses would prove more economical if employed only when needed.
