Mitotic cycle time in perennial and annual plants with various amounts of DNA and heterochromatin

Abstract

Annual Anthemideae show either a lower or a higher DNA content than their perennial relatives (14). To find out whether the higher growth rate in annuals is due to shorter cell cycles, particularly in those species that show increased DNA amounts, the mean duration of the mitotic cell cycle in the root tip meristem of seedlings of perennial and annual species of the Anthemideae (Asteraceae) was determined by tritiated thymidine autoradiography. It was found that the duration of the total cell cycle and of each phase, but especially that of G1, was shorter in all the annuals studied. This is in accordance with their type of life habit, but in variance to the increased nuclear DNA contents in some of the annual species. Since all the annuals with high DNA content display an excessive proportion of heterochromatin, a proposal is put forward to account for decreasing mitotic cycle time in spite of increased DNA amount, namely, that heterochromatin is required to shorten the duration of all three stages of interphase, G1, S, and G2.