Cell Population and Quantitative Changes of DNA in the Shoot Apex of Sinapis alba During Floral Induction

Vegetative plants of Sinapis alba L, a long-day species, were induced to flower by exposure to a single 22-hr day At various times after the start of the long-day treatment, the DNA content per nucleus and the respective distribution of the 2C, intermediate, and 4C cell populations in the shoot apex were determined by Feulgen two-wavelength cytophotometry As early as 22 hr after the initiation of the long-day treatment, the amount of DNA was lower in induced plants than in controls, and the major proportion of nuclei shifted from the 4C to the 2C condition and reached a minimum DNA value at 34 hr Thereafter, the amount of DNA increased at 46 hr and then declined again at 70 hr From 30 to 70 hr, the proportion of 2C nuclei declined slightly, while the number of nuclei with intermediate DNA values increased, and the 4C nuclei generally remained low with a slight increase at 46 hr The results correlate with previously published kinetic data concerning mitotic index and DNA multiplication (Bernier, Kinet, and Bronchart 1967) and may have important implications in floral morphogenesis.