Separation of Mechanisms Initiating Cell Division and Cell Expansion in Lettuce Seed Germination

Abstract

(1) At 26[degree]C lettuce seed germinates rapidly, and the beginnings of cell division and cell expansion roughly coincide in time. (2) When gamma-irradiated seeds are sown on a solution of kinetin at 26[degree]C, germination, although greatly delayed compared to that of unirradiated controls, precedes cell division by several days. (3) Results similar to those with irradiated seeds were found with unirradiated seeds germinating at 10[degree]C. The distribution of mitoses among root tips of newly-germinated seeds at 10[degree]C suggested that mitoses occurred randomly among them. (4) When seeds were germinated at 26[degree]C in solutions of mannitol, cell divisions preceded germination by many days. High concentrations of mannitol prevented germination of seeds that nevertheless underwent mitotic activity. (5) At 30[degree]C, only a very small percentage of seeds germinate. Most seeds, however, undergo cell division. The distribution of mitoses among radicle meristems of nongerminating seeds showed that mitoses were not occurring randomly among the unexpanded radicles. Rather, there was a tendency for an all-or-none characteristic in the occurrence of mitoses within individual radicles. As time progressed, the mitotic activity in these nongerminating seeds diminished. These results indicate that (a) The initiation of cell expansion and that of cell division are controlled by separate mechanisms during germination. (b) Rootlet protrusion during germination results from cellular expansion, whereas cell division plays little or no role. (c) The mechanisms by which kinetin and other agents stimulate lettuce seed germination are related to the initiation of cellular expansion and not to cell division.