Starch turnover in shoot‐forming tobacco callus

Tobacco callus grown under shoot‐forming conditions or in the presence of gibberellic acid, which inhibits shoot formation, was incubated in [14C]‐sucrose at three different periods in culture and then replanted. Evolution of 14CO2 occurred during the 10 day post‐incubation period. Most of the radioactivity was incorporated into the ethanol‐soluble fraction, which lost most of its label after 24 h. Starch was the major ethanol‐insoluble component and post‐incubation synthesis occurred in this fraction for 24 h or longer. Greater net synthesis of starch occurred in shoot‐forming tissue and the loss of label from starch began later than in tissue cultured in the presence of gibbe‐rellic acid. Newly synthesized starch was not immediately utilised in the organogenic process, but its utilization could be correlated with the shoot‐forming process.