The Effect of Sucrose Concentration on Cell Proliferation and Quiescence in the Apical Meristem of Excised Roots of Pisum sativum L.

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Abstract
The effects of sucrose concentration on the rate of root elongation and on cell proliferation and quiescence in the apical meristem of excised roots of Pisum sativum have been investigated and the results compared with the corresponding values obtained for roots of intact plants. The rate of root elongation was lower in the excised roots than in the intact controls and this was at least partly a consequence of differences in various aspects of cell proliferation between the two classes of root. The durations of the mitotic cycle (C) and presynthetic interphase (G1), as measured from the labelled mitoses curve, for example, were shorter in the roots of intact plants than in the excised ones. However, the most obvious difference in terms of cell proliferation between the two classes of root was in the relative sizes of the various proliferating and quiescent cell fractions. Root excision, and their subsequent culture in sucrose supplemented media, clearly resulted in decreases in both the growth fraction (GF) and the proportion of proliferating cells (p11) with a long cycle time and increases in either, or both, the fast-dividing meristematic population (p1) and the quiescent fraction (p111). A consequence of the greater GF and shorter C in the apical meristem of the roots of the intact plants compared with the excised ones, was that cell doubling time (Td) was of shorter duration in the former roots relative to the corresponding values in the latter. In the excised roots, C increased with increase in concentration of sucrose in the culture medium, while both GF and p1 were maximal, and Td minimal, in roots grown in media supplemented with 4–4⋅6 per cent sucrose. The rate of excised root growth by elongation, however, was fastest in a medium containing 3⋅3 per cent sucrose and, since this aspect of root growth is governed by both the amount and rate of cell elongation as well as cell proliferation, the concentration of sucrose supporting optimal cell elongation must be lower than that for cell proliferation. It was concluded from the results obtained on the sizes of the various proliferating and quiescent cell fractions in all of the roots investigated and from data in the literature that: (a) variation in cycle time in different proliferating cells of the root apical meristem may be a consequence of local variations in the amount of available carbohydrate; and (b) the quiescent population of cells in the excised root apical meristem consists of a fraction which is non-prolifcrative because of a lack of carbohydrate and a fraction which is insensitive to change in carbohydrate availability.