Organogenesis from Solanum melongena L-(eggplant) cotyledon explants is associated with hormone-modulated enhancement of polyamine biosynthesis and conjugation

Eggplant (Solanum melongena L. cv. Violetta lunga 2) cotyledon explants gro wn on hormone-free medium (controls) or on medium containing either naphtha leneacetic acid alone (root forming) or in combination with zeatin riboside (shoot forming) showed minor differences in free polyamine titres during c ulture. In contrast, conjugated polyamines (particularly those in the trich loroacetic acid-soluble fraction) accumulated only in hormone-treated expla nts, but not in controls. The extent and the temporal changes in soluble-co njugate levels differed between root-forming and shoot-forming explants; in the former, accumulation began earlier (within 1 day of culture) and reach ed the highest levels. In both organogenic programmes, maximum conjugate ac cumulation occurred just before and during organ emergence. Adventitious ro ots and shoots were formed along the cut surfaces. The regions closest to t hese ("borders") displayed a significantly higher ratio of conjugated to fr ee spermidine and/or putrescine than the nonorganogenic regions("centres") of the explant. Ornithine decarboxylase activity was higher than arginine d ecarboxylase activity both in control and hormone-treated explants. However , both activities increased markedly on day 2 of culture in the presence of hormones. Thereafter ornithine decarboxylase activity remained high in sho ot-forming explants, but not in root-forming ones. Putrescine oxidising act ivity was also enhanced by exogenously supplied hormones starting from day 4 of culture. This activity remained high up to day 12 in the presence of a uxin plus cytokinin, whereas it peaked on day 6 in auxin-treated explants. Spermidine oxidising activity was the only enzyme activity which was consis tently higher in controls than in hormone-treated tissue. Differences betwe en, the two organogenic programmes with respect to temporal changes in poly amine content, and putrescine biosynthetic and oxidative activities are dis cussed in relation to the timing of organ formation. The latter was monitor ed both histologically and macroscopically.