Heterogeneity of the early G2 phase-arrested cells; cytophotometric, interferometric, and EM morphometric studies

Cells of antheridial filaments in Chara vulgaris L. blocked at the early G2 phase by a 5 day dark treatment were studied cytologically in order to determine causes of their heterogeneous response to light-induced mitotic activation. Cell size distribution patterns in dark inhibited populations were shown to be nearly consistent with those found within the control populations, comprising all the stages of interphase; cells which resume divisions in response to photoinduction attain, however, sizes characteristic of mitosis occurring in natural light conditions. Cytophotometric determinations of nuclei in cells given dark treatment indicated a complete homogeneity of populations in respect of replicated DNA contents. As evidence by nucleus-cytoplasm surface relations, interferometric measurements of their dry mass contents, and EM stereometric studies, the heterogeneity of early G2 phase arrested populations results from an imbalance between nuclear DNA-division cycle, which is continued up to the early stage of the G2 period, and the growth cycle, which in turn, due to its pronounced inhibition gives rise to the deficit of cytoplasm. Since no shortages in total volumes of structural cytoplasm components were found the number and volume of mitochondria were shown even largely increased, it is assumed that asynchrony during light-induced mitotic reactivation originates from quantitative differences of constituents within the area of ground cytoplasm.