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The comparative analysis of plant hormones was undertaken on a 1-naphthaleneacetic acid tolerant mutant and normal tobacco (Nicotiana tabacum cv Xanthi) plantlets. The mutant plantlet was scrubby and impaired in its root morphogenesis. Degeneration of the root meristem was studied on tissue sections; it appeared very fast (as early as the 3rd or 4th day after sowing), after which the root was further transformed into a callus. Indoleacetic acid (IAA), abscisic acid (ABA), and the isopentenyladenine (iP)- and trans-zeatin(Z)-type cytokinin levels were measured in terminal buds and root tips 13 days after sowing, by enzyme linked immunosorbent assay of high performance liquid chromatography fractions. Some differences appeared between the apical buds of the two genotypes, but the mutant tobacco differed from the wild type mainly by the presence of higher levels of IAA, ABA, and iP + isopentenyladenosine (iPA) in its small root. Thus, the IAA, ABA, and iP + iPA contents were increased by a factor of 15, 7, and 24 times, respectively, in mutant root compared to wild-type tobacco roots. Previous work has shown that the mutation impairs membrane polarization effects induced by auxin at the cell level. The present results would favor the hypothesis that the mutation has also affected the control of growth regulator accumulation in tissues.