Chloroplasts, Kinetin and Protein Synthesis

The effect of kinetin on protein synthesis of isolated chloroplasts was investigated by following the incorporation of ¹⁴C‐leucine into isolated chloroplasts from Nicotiana tabacum. The incorporation activity varied greatly during the year, being largest in the winter and smallest in the summer. Conversely, the relative effect of kinetin on the incorporation of ¹⁴C‐leucine, whether applied as a pretreatment to the leaves or directly in the incubation medium, was largest in the summer and smallest or absent altogether in the winter. Kinetin did not prolong the net incorporation period, which lasted about 40 min, but only enhanced the initial rate of the reaction. Chloroplasts extracted from leaves that had been detached for 24 or 48 h displayed very little of their original, pre‐aged incorporation activity and treating the leaves with kinetin did not, essentially, prevent this loss. It was concluded that the major effect of kinetin upon chloroplasts may be related primarily to an effect upon hydration and permeability of the chloroplast and its membranes, and not to an effect directly upon its machinery for protein synthesis.