Uptake and translocation of [14C]-labelled benzylaminopurine in apple shoots grown in vitro in relation to shoot development

Shoots of apple (rootstock A2, Malus sylvestris (L.) Mill var. domestica Borkh.) were grown in vitro on agar media containing 4.4 μ M benzylaminopurine (BAP) during the whole of a 36-day culture period or during the first 6 days of the period. Development of lateral shoots and an appreciable increase in fresh weight were obtained only when BAP was present during the whole period. Treatment for 6 days only was not sufficient for shoot proliferation although the transferred shoot continued its growth on fresh medium without BAP. Uptake of the cytokinin was studied by adding 8–¹⁴C]-benzylaminopurine to the medium and then following the distribution of the isotope in the shoots during the 36-day culture period. There was a steady, almost linear, uptake of ¹⁴C during the whole period. Most of the activity accumulated in the lower part of the shoot. It is concluded that BAP, due to slow translocation, is active mainly in the basal parts of the shoot where it stimulates formation and growth of buds. Its action is dependent on continuous supply of new BAP from the medium, indicating metabolism of BAP in the tissue.