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

After imbibition of water by seeds under favorable circumstances, the growth by which the embryo becomes a young seedling occurs by both expansion of cells originally present in the dormant embryo and rnitotic divisions resulting in an increase in cell number (15). Germination of lettuce seed has been of special interest to plant physiologists because it can be stimulated by gibberellins, kinetin, or light (8). The sequence of the onset of cell division and elongation in germinating lettuce seed has been studied by Evenari et al (4), who found that the beginnings of mitotic activity and of cell elongation seemed to coincide in time. No mitotic activity was found in seeds prevented from germinating by withholding favorable light treatments. Their data suggested that, "The start of mitoses, the beginning of cell elongation and the protrusion of the rootlet are correlated events during germination" (4). Cell division and expansion do not begin simultaneously during germination of many other kinds of seeds. Cellular expansion precedes mitosis by many hours during germination of corn (14) or barley (2), and by several days in the broad bean (16). On the other hand, mitotic activity occurs several days before germination in pine seeds (5). Similarly, the number of cells per embryonic axis of after-ripening cherry seeds increased weeks before there was a detectable increase in axis length, which suggests that mitosis also precedes cell expansion in cherry seeds (11). Because division and expansion do not begin simultaneously during germination of many species of seeds, it is possible that lettuce might behave similarly under selected conditions. We therefore attempted to separate in time the inception of cell division from that of expansion during lettuce seed germination. The results show not only that the beginning of mitotic activity can be made either to precede or to follow cell expansion, but also that mitotic activity can be completely separated from germination. These results are of general importance in understanding the mechanisms of germination for all types of seeds.