THIDIAZURON-INDUCED MORPHOGENESIS OF REGAL GERANIUM (PELARGONIUM-DOMESTICUM) - A POTENTIAL STRESS-RESPONSE

Thidiazuron (TDZ), a phenylurea derivative with cytokinin-like activit y, induced the development of outgrowths on root tissue of vegetativel y propagated geraniums (Pelargonium x hortorum, Pelargonium domesticum and Pelargonium graveolens). Root outgrowths developed as globular or elongated structures following treatment with 10-30 mu M TDZ and many of these structures differentiated to produce shoots, Analysis of roo t and shoot samples, collected daily during the outgrowth induction ph ase and at the end of the treatment period, revealed significant chang es in accumulation of manganese, iron, copper, calcium, magnesium and potassium. The markers of a stress response, proline, abscisic acid an d 4-aminobutyrate, accumulated in the TDZ-treated roots during the fir st week of induction. Assessment of the adenylate phosphate pool sizes of the TDZ-treated plants indicated a sequential increase in the endo genous levels of ATP, ADP and AMP following each application of TDZ. T he energy charge ratio was also significantly higher in TDZ-treated pl ants indicating an increase in ATP utilizing systems. Similarily, pyri dine nucleotide pool size analyses revealed that TDZ-treated plants ha d a higher level of endogenous NADP(+) in the initial 24-h period foll owing each treatment and the level of NADPH increased following the th ird application of TDZ. The ratio of NADPH/NADP(+) was significantly h igher in TDZ-treated plants throughout the treatment period. We hypoth esize that the primary effect of TDZ was through induction of a stress response in the geranium plants. In order to overcome this stress, th e plants accumulated significantly higher levels of proline, ABA and 4 -aminobutyrate. Moreover, the plants also exhibited modified metabolic processes which in turn led to increased availability of energy and r educing power required for subsequent growth and to initiate stress ad aptation mechanisms including modified cellular processes and regenera tive outgrowth development.