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 is localized in mature or developing tissues, superficial explants (two cell layers: epidemis and subepidermis) and deep tissue (pith) explants 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: histological observations, determinations of free and conjugated polyamine titers and related biosynthetic (arginine and ornithine decarboxylases, ADC and ODC, respectively) and oxidative (diamine oxidase, DAO) activities, and in vitro culture. Cell area increased basipetally along the stem both in superficial and pith explants; lignification occurred in the 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 more concentrated in the upper sites in superficial explants and in the lower sites in pith explants while conjugated ones, more abundant in superficial layers, did not show any particular trend along the stem. The morphogenic response in pith explants decreased basipetally up to sites 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 others. Most superficial explants did not exhibit any response in vitro and some of them produced callus or were swollen; the presence of hormones in the culture medium induced shoot formation on a small percentage of explants. The mean number of shoots per superficial explant, lower than in pith explants, was not significantly different along the stem.