PEROXIDASE ISOENZYMES IN THE DEFENSE RESPONSE OF CAPSICUM-ANNUUM TO PHYTOPHTHORA-CAPSICI

Quantitative and qualitative changes in isoperoxidase patterns from st ems of three cultivars of pepper (Capsicum annuum L.), one susceptible , one intermediate and one resistant, were found upon inoculation with Phytophthora capsici using a decapitation method. The peroxidase acti vity was determined in the intercellular fluid as well as in the cytos olic fraction of the necrotic, healthy and intermediate zones of stems of the three cultivars, 6 days after inoculation. In the intercellula r fluid, peroxidase activity of the susceptible cv. Yolo Wonder increa sed somewhat from 4.7 (healthy zone) to 12.9 (intermediate zone) mu mo l mg(-1) protein min(-1), whereas in the intermediate cv. Americano, t he peroxidase activity decreased from 123 (healthy zone) to 78 (interm ediate zone) mu mol mg(-1) protein min(-1). The most dramatic increase (5.7 to 662 mu mol mg(-1) protein min(-1)) in intercellular peroxidas e activity was found in the resistant cv. Smith-5. This, in conjunctio n with the appearance of an additional acidic isoperoxidase (pI 4.4) s pecific for the cv. Smith-5, could be the reason for the resistance of this cultivar against the fungus attack. The release of peroxidase in to the intercellular space as a defense reaction was confirmed by hist ochemical analysis, showing that peroxidase activity occurred in the i ntercellular spaces of those stems of the resistant cultivar that had not yet been invaded by the fungus, but was detected neither in the ot her cultivars nor in the intercellular spaces of such stems of the int ermediate and susceptible cultivars that contained growing mycelium of P. capsici. The lack of staining in the intercellular spaces of the s usceptible cultivars could be attributed to their low content in perox idase.