Thidiazuron‐induced morphogenesis of Regal geranium (Pelargonium domesticum): A potential stress response

Abstract

Thidiazuron (TDZ), a phenylurea derivative with cytokinin‐like activity, induced the development of outgrowths on root tissue of vegetatively propagated geraniums (Pelargonium × hortorum, Pelargonium domesticum and Pelargonium graveolens). Root outgrowths developed as globular or elongated structures following treatment with 10–30 μM TDZ and many of these structures differentiated to produce shoots. Analysis of root and shoot samples, collected daily during the outgrowth induction phase and at the end of the treatment period, revealed significant changes in accumulation of manganese, iron, copper, calcium, magnesium and potassium. The markers of a stress response, proline, abscisic acid and 4‐aminobutyrate, accumulated in the TDZ‐treated roots during the first week of induction. Assessment of the adenylate phosphate pool sizes of the TDZ‐treated plants indicated a sequential increase in the endogenous levels of ATP, ADP and AMP following each application of TDZ. The energy charge ratio was also significantly higher in TDZ‐treated plants indicating an increase in ATP utilizing systems. Similarily, pyridine nucleotide pool size analyses revealed that TDZ‐treated plants had a higher level of endogenous NADP+ in the initial 24‐h period following each treatment and the level of NADPH increased following the third application of TDZ. The ratio of NADPH/NADP+ was significantly higher in TDZ‐treated plants throughout the treatment period. We hypothesize that the primary effect of TDZ was through induction of a stress response in the geranium plants. In order to overcome this stress, the 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 reducing power required for subsequent growth and to initiate stress adaptation mechanisms including modified cellular processes and regenerative outgrowth development.