The effects of AOPP, an inhibitor of phenylalanine ammonia lyase (PAL) and of OH-PAS, an inhibitor of cinnamyl-alcohol dehydrogenase (CAD) on the barley powdery mildew resistances conditioned by the dominant Mla1 (hypersensitive cell death based) and the recessive mlo5 (conditioning penetration resistance without cell death) major gene alleles were determined in leaf and coleoptile tissues. The inhibitors were applied either by uptake through cut ends of detached leaves or directly to monolayers of epidermal cells dissected from coleoptiles. In leaves, both inhibitors decreased localized autofluorescence in epidermal cell walls directly beneath primary germ tubes and appressoria. With the Mla1 resistance allele both inhibitors strongly suppressed race-specific hypersensitive epidermal cell death (as indicated by whole cell autofluorescence in leaves or by cell collapse in the coleoptile epidermis). AOPP was effective at 1 mM in tissues of both leaves and coleoptiles. OH-PAS was effective at 1 mM in leaves and at 1-11 μM coleoptile tissue. Concomitant with the reduction in hypersensitive cell death in the coleoptile epidermis was an increase in average haustorium size and percentage of germlings producing hyphae. OH-PAS was effective when applied to the coleoptile epidermis as late as 16 h after inoculation, approximately 2 h before the onset of hypersensitive cell death. Results strongly suggest that the products of CAD activity play a direct role in stimulating events (like peroxidase activity and free radical formation) leading to programmed hypersensitive epidermal cell death conditioned by Mla1 . In contrast, the recessive mlo5 resistance, which is expressed by failure of the fungus to fully penetrate barley epidermal cells and form haustoria, was unaffected in either leaves or monolayers of colcoptile epidermis by OH-PAS or AOPP. With mlo5, localized autofluorescence was reduced at attack sites by inhibitor treatment, but treatment had no effect on rates of penetration based on observations of haustorium formation. These results indicate that mlo5 resistance docs not require PAL or CAD activity for its effectiveness. Therefore barley resistances due to the major Mendelian resistance alleles Mlal (HR resistance) and mlo5 (non-HR) differ in their respective mechanisms and in the physiological bases that underlie these mechanisms.