Mechanical injury or infection with Helmintosporium maydis race T or O en proxidase acti in leaves of two corn ibreds which differ in their susceptibility to the fungal race T. Increases in act were found in the soluble fraction extracted from tissues with 20 mm phosphate buffer (pH 6), and in the ionicafUy bound fraction extracted fromn wail debris with 0.6 to 1 M NaCa; the covalently bound wai peroxidase fractin was unaffected. Mechanical injury and infection with either race enhanced the same distinctive cathodic isoforms present in the soluble fraction or in both the soluble and lonicaUy bound fractons.During the firt 20 to 22 hours after ioculation no differences were found in the enzym enhancement in relaton either to the corn line or fungal race. During the second day, the weaker the infection symptom, the greater was the peroxidase encement. infected with the fungus. The same isoperoxidases (IPs) which reacted to infection reacted also to mechanical injury. Such a similarity was also found between mechanically injured and tobacco mosaic virus-infected tobacco leaves (5). The similarities in the reaction to mechanical and viral or fungal injuries were observed not only in the distinctive IPs that increased in activity, but also in their distribution between cell protoplast and walls, thus supporting the assumption that the infection-induced peroxidase enhancement may result from a nonspecific response to injury. Without eliminating the possibility of peroxidase contribution to the plant protection against a parasite, we postulated that the often observed relatively small increases in the enzyme activity in compatible hosts may be a consequence of metabolic disorders rather than the cause of a lower resistance. Vance et al. (23, 24) have recently reported that attempted penetration of Helminthosporwum avenae into leaf discs of a noncompatible host, reed canarygrass, was accompanied by an increase in peroxidase activity, in particular in the walls at the penetration sites, and in wall lignification. Exposure of the inoculated discs to cycloheximide solutions at 10 or 25 itg/ml for 18 hr resulted in inhibition of peroxidase enhancement and lignin formation and in penetration of the fungus into the leaf tissue. The authors have suggested that the resistance mechanism involves an induction of cathodic IPs in the challenged tissue.This report represents a continuation of previous studies on the Funk corn N inbred and its male sterile counterpart and deals with the IP reaction to two H. maydis races, T and 0, differing from each other in the production of the host-specific T toxin. The great compatibility between the Tms inbred and the fungal race T is thought to be due to the susceptibility of the host organelle membranes to the T toxin (2, 19). Both Tms and N inbreds exhibit a low susceptibility to the race 0, nonproducer of the toxin. Special attention has been paid to the effect of cycloheximide on peroxidase enhancement induced by mechanical injury and infection. In view of the implication of et...