No abstract

Lettuce seed germination can be stimulated, probably by distinct mechanisms, by any of the three widely studied morphogenic agents: gibberellin, kinetin, or red light (11). It has been shown that cellular expansion is both necessary and sufficient and that cell division is neither necessary nor sufficient for the initiation of lettuce seed germination (8 The criterion of germination was visual detection of radicle protrusion. Radicles (1-2 mm) from nongerminated seeds were excised and prepared as described previously (8). The apical 0.5 mm was examined for mitotic activity. An entire dish of seeds was sampled at one time for germination percentages and mitotic activity in nongerminated seeds. Once sampled, the dish was removed from the experiment.Respiratory rates were determined by placing 50 seeds in 1.2 ml of solution in the main compartment of Warburg flasks. We placed 0.2 ml of 20 % KOH and a fluted piece of filter paper in the center well to measure oxygen consumption by the direct method (32).The term "germination" is used here in the physio-

Haber, Alan H., and Helen J. Luippold. (Oak Ridge National Laboratory,2 Oak Ridge, Tenn.) Effects of gibberellin on gamma‐irradiated wheat. American Jour. Bot. 47(2): 140—144. Illus. 1960.–After receiving large doses of gamma rays, wheat grains can germinate and grow into small seedlings, either in water or in a solution of gibberellic acid, without undergoing any mitosis. Gibberellic acid stimulates growth of plants from grain given twice the dose that abolishes all detectable mitotic activity. Therefore, it must stimulate growth of these irradiated plants by cell expansion alone. Growth stimulation in shoots from such irradiated grain was reflected in increased dry matter content and other responses characteristic of gibberellin treatment. Comparison of the shoot height‐dose curves for plants grown in water and those in gibberellin suggest that cell division per se does not contribute greatly to the gibberellin stimulation of growth in unirradiated young wheat seedlings. The effect of gibberellic acid on plants from grain irradiated sufficiently to prevent subsequent mitotic activity was on the total extent of growth, not only on the rate. The general ineffectiveness of gibberellin on mature tissues can not be attributed to the cells having reached the maximum size that they were potentially capable of attaining, but must be caused by a physiological incapacity of the cells to respond to gibberellin treatment. These results are discussed in connection with recent reports that stimulation of growth by gibberellin can be attributed solely to an increase in cell number.

No abstract

No abstract