Date of Award

6-1980

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

Supervisor

Prof. D. Davidson

Abstract

The mature root meristem of the broad bean, Vicia faba, consists of a steady-state, asynchronous population of cells in which rates of cell growth and of division are constant, on average, from one time to the next. Measurements of individual cells for any growth parameter, however, show considerable variation about the mean of that parameter. This study was done in order to 1) determine the pattern of cell growth and division in the root meristem during seed germination as it develops into a mature root; 2) determine how this pattern changes in different environments, and 3) determine if events that occur during germination contribute to variation in cell growth and division seen in the mature root.

Cell growth and division were manipulated by 1) changing the amount of water available for growth of germinating and mature roots and 2) treating roots with 5-amino uracil (5-AU), a drug which temporarily arrests cells at the S to G₂ transition point of the cell cycle (Socher and Davidson, 1971). Cell area, nuclear volume and nuclear protein content were used to monitor cell growth: mitotic index (M.I.) and cell cycle kinetics determined 1) from colchicine accumulation of cells in mitosis and 2) from changes in the frequency of labelled mitoses after pulse labelling cells synthesizing DNA with ³H-thymidine, were used to monitor cell division.

The results show that cells in the mature root meristem reach different steady-states that are influenced by the ambient water level present during germination. The characterisitics of each steady-state are unique in the M.I., the size ratio, nucleus: cytoplasm and the relative proportion of fast, slow and non-cycling cells.

Mature roots subjected to a change in the ambient water level appear only to modify the initial steady state established during germination; ie., roots grown initially in different amounts of water and then transferred to the same ambient water level do not attain a steady-state condition with identical mean values for different cell parameters. Treatment of germinating and mature roots with 5-AU allows cells and nuclei to grow to sizes greater than those seen in untreated roots but does not appear to interfere with normal relationships between most growth paramters. Neither the relationship between cell and nuclear size nor nuclear protein content is altered in 5-AU treated roots to produce values not found in untreated roots.

From these results it is proposed that a specific programming of cell growth and division is established during germination and is largely determined by environmental conditions present in the early stages of seeding growth. Modifications of this programmed control of cell growth can be induced by changing the external growth conditions. In addition, there are internal fluctuations, within a meristem, in the environment of identical cells; these fluctuations are thought to contribute to the variation in cell parameters that are seen in mature roots.

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