The etioplasts of dark-grown bean leaves showed ATPase (adenosine triphosphatase) activity which had a pH optimum of 8.5, was stimulated by dithiothreitol and unaffected by light-triggering. Bean chloroplasts showed a low activity of dark-induced ATPase with a pH optimum of 8.5 and a substantial amount of light-triggered activity with a pH optimum of 8.0. The light-triggered activity depended on dithiothreitol and Mg2+ and was promoted by phenazine methosulphate. Light-triggered ATPase activity was completely inhibited by 20mum-dicyclohexylcarbodi-imide. Etioplasts developed light-triggered ATPase activity in response to 30 min illumination of the etiolated leaves. During the 48 h of light-induced greening of dark-grown leaves there was a 70% increase of the chloroplast ATPase activity found after light-triggering and a 30% fall in the dark-induced activity, both expressed on a per leaf basis. As the larger part of these changes occurred during the first 30 min of illumination, it is concluded that most or all of the chloroplast ATPase was present in the etioplast, a conclusion identical with that of Lockshin et al. (1971) for maize. During 48 h of greening there was a tenfold increase in the amount of thylakoid membrane in the leaf together with an 83% fall in the ATPase activity per m2 of thylakoid membrane, measured after light-triggering.
