Role of purine metabolism in thidiazuron-induced somatic embryogenesis of geranium (Pelargonium×hortorum) hypocotyl cultures

Somatic embryogenesis in geranium ( Pelargonium × hortorum Bailey cv. Scarlet Orbit Improved) was achieved by culturing hypocotyl sections on Murashige and Skoog (1962) (MS) medium containing 10 μ M thidiazuron (TDZ) (induction medium) for 3 days and subsequently transferring the sections onto a basal medium lacking any plant growth regulators (expression medium). Addition of the purine analogue 2.6-diaminopurine (DAP) to the somatic embryo induction medium completely inhibited the embryogenic response as well as chlorophyll accumulation without affecting enlargement of the treated tissues. Addition of 20 μ M adenine sulphate to the expression medium, i.e during embryo growth and development phase, completely reversed the DAP-induced inhibition of the embryogenic response while addition during the induction phase caused only a 50% reversal of the inhibition. Analysis of endogenous levels of plant growth substances indicated that TDZ alone elevated the levels of auxins, cy-tokinins and abscisic acid while the presence of DAP during the induction phase caused a further increase in the levels of adenine and adenosine. These findings indicate a possible critical role for purines in embryogenesis from geranium hypocotyl tissues. However, the conversion of cytokinin bases to their corresponding nucleotide forms was not evident as the levels of isopentenyl adenine and zeatin increased during the second day of culture.