Changes in the Pattern of Cell Division in the Shoot and Root Meristems ofLolium perenneduring the Transition from Vegetative to Floral Growth

Abstract

For Lolium perenne cv. Cropper, a system which resulted in 100% flowering comprised 90 short days (SD) at 4 °C (vernalization) and 30 SD at 18 °C followed by 8 long days (LD). The mitotic index and G1 and G2 percentages were measured in the shoot and root apices of plants following 2, 5 or 8 LD and in SD controls sampled at the beginning and end of induction. Identical measurements were made in plants given 48 SD at 18 °C followed by 2, 5 or 8 LD; plants remained vegetative in response to this treatment lacking vernalization. Significant increases in both mitotic index and meristem size occurred in the shoot apex in LD following the vernalizing, but not the non-vernalizing, treatment. A cluster of mitoses in the apical dome of the shoot apex was unique to the vernalized plants given 5 or 8 LD. However, an increase in root meristem size occurred regardless of vernalization, but a significant increase in the mitotic index was limited to vernalized plants given 5 or 8 LD. Whilst the vcrnalization LD treatment resulted in an increase in the G2 percentage in the shoot apex following 2, 5 or 8 LD, no such alteration was observed in the root meristem. Thus, the changes to the cell cycle which correlated with flowering were increased mitotic indices and G2 percentages in the shoot apex at each sampling time and increased mitotic indices in the root apex following 5 and 8 LD.