MORPHO-FUNCIONAL GRADIENTS IN SUPERFICIAL AND DEEP TISSUES ALONG TOBACCO STEM - POLYAMINE LEVELS, BIOSYNTHESIS AND OXIDATION, AND ORGANOGENESIS IN-VITRO

In order to determine how polyamine metabolism operates and where it i s localized in mature or developing tissues, superficial explants (two cell layers: epidemis and subepidermis) and deep tissue (pith) explan ts were excised at different levels (7th to 22nd internode) along the stem of tobacco (Nicotiana tabacum L.) plants in the vegetative stage. They were grouped in 5 stocks (sits 1 - 5) and subjected to: histolog ical observations, determinations of free and conjugated polyamine tit ers and related biosynthetic (arginine and ornithine decarboxylases, A DC and ODC, respectively) and oxidative (diamine oxidase, DAO) activit ies, and in vitro culture. Cell area increased basipetally along the s tem both in superficial and pith explants; lignification occurred in t he basal pith parenchyma. ADC and ODC activities decreased basipetally , predominated in the supernatant rather than the particulate fraction and in superficial tissues rather than deep tissues. No DAO activity was detectable under physiological conditions. Free polyamines were mo re concentrated in the upper sites in superficial explants and in the lower sites in pith explants while conjugated ones, more abundant in s uperficial layers, did not show any particular trend along the stem. T he morphogenic response in pith explants decreased basipetally up to s ites 3 - 4. The mean number of vegetative shoots was higher in hormone -supplemented (anomalous shoots) than in hormone-free (normal shoots) medium and sites 2 and 5 produced more shoots per explant than the oth ers. Most superficial explants did not exhibit any response in vitro a nd some of them produced callus or were swollen; the presence of hormo nes in the culture medium induced shoot formation on a small percentag e of explants. The mean number of shoots per superficial explant, lowe r than in pith explants, was not significantly different along the ste m.