Following ablation of the main stem (decapitation), the mean nuclear protein content of the Pisum cotyledonary bud apical cells increased 35% as early as the 6th hour, just when a massive entry into the S phase was registered, and reached a maximum, with a 157% increase, at the 39th hour, when the mitotic activity was also at its highest. The ratio nuclear protein/nuclear DNA content was then multiplied by 1.8 in comparison with the ratio observed in the G0–1 noncycling nuclei of the inhibited cotyledonary bud. In the bud treated with cycloheximide, the mean nuclear protein content slowly increased following decapitation, but remained greatly inferior to that of the controls. A slight entry into the S phase was noticed at the 24th hour and the mean nuclear protein content, then maximal, increased 32% in comparison with the inhibited buds. In the control buds, the mean protoplasmic proteins content increased 116% at the 39th hour (maximal value), whereas it increased only 25% in the treated buds. Following treatment with cycloheximide, the entry into the S phase was postponed and of low amplitude. However, the recycling nuclei were able to divide. An increase in the mean nuclear and cytoplasmic protein content was a prerequisite if the entry into the S phase of G0–1, nuclei was not to be postponed.
Mitotic activity and changes in DNA content during Zea mays L. endosperm development