COMPONENTS OF RESISTANCE IN BARLEY TO STEM RUST - PRE-PENETRATION ANDPOST-PENETRATION DEVELOPMENT IN SEEDLING AND ADULT-PLANT LEAVES

Seven barley (Hordeum vulgare) cultivars or breeding lines with differ ent levels of resistance to stem rust and the susceptible wheat cultiv ar Little Club (Triticum aestivum) were inoculated with races QCC, QFC , and TPM of Puccinia graminis f.sp. tritici and examined using light microscopy to associate the sequence of histological events with pheno typic expression of resistance. There were no significant differences in urediniospore germination, appressorium formation, or substomatal p enetration in any of the race-cultivar combinations in seedling leaves . Formation of primary haustorium mother cells (HMCs) was not affected by the presence or absence of the resistance gene Rpg1 when inoculate d with race QCC, but was reduced considerably in lines possessing this gene after inoculation with races QFC and TPM. Development of all rac es was arrested during primary HMC formation to a greater extent in al l barley lines than in Little Club wheat. The expression of resistance in barley to P. g. tritici was associated with the frequency of post- penetration abortion, the degree of reduction in colony growth, and th e incidence of colony-associated necrosis of host cells. The rankings of the host lines based on these histological events were in agreement with the rankings for receptivity and urediniospore production determ ined previously on stem tissues of adult barley plants.