Action of Gibberellic Acid on Lettuce Seed Germination

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Abstract
The effect of gibberellic acid (GA3) on the germination of Grand Rapids lettuce seeds has been compared with the action of red light, in the hope of elucidating the mode of action of these factors. The work was carried out by shaking the seeds directly with a GA3 solution or water throughout the period of imbibition and germination. This method was found to yield more uniform and more repeatable results than the ordinary one using moist filter paper. During the course of imbibition, the seeds gradually become sensitive to red light, and reach their maximum sensitivity 1 1/4 hours after the beginning of imbibition, when water uptake is still not complete; thereafter the sensitivity to red light gradually decreases. In seeds which have been induced to germinate by red light, the inhibitory effect of far-red light gradually decreases with increasing time interval between red and far-red treatments. Far-red light no longer inhibits the germination process after the 1st sign of visible germination occurs (8-9 hours after red light treatment at 25[degree] C). GA3 at 60 ppm induces maximal germination; germination is almost linearly proportional to concentration below this level. However, if the solution is injected directly into the seed, concentrations as low as 1 ppm are fully effective. The action of GA3 is not to control the initial rate of growth of the hypocotyl and the root, since this rate is increased only at about 16 hours after the first sign of visible germination. The time of greatest sensitivity for the induction of germination by gibberellin occurs after about 1 1/4 hours of presoaking in water; this timing is identical with the most sensitive period of the seeds to red light. Far-red light does not inhibit the germination of seeds which have been treated with GA3 so as to produce 90% germination. If the time of GA3 treatment is adjusted to give only 50% germination, and the far-red exposure increased, then far-red does partially inhibit, but this inhibition is exerted to the same extent whether the exposure is made before or after the GA3 is applied. Thus, heavy doses of far-red desensitize the seeds to gibberellin. Repeated attempts to detect a gibberellin as the end-product of the action of light in the seeds failed completely, whether the assay was for GA3 or GA5. Small amounts of gibberellin were detected in the seeds, but the quantity was not increased by a light dosage sufficient to cause maximum germination. It is deduced that GA3 acts by initiating one of the chemical reactions which result from the light reaction, so that its end product is the same as that produced by light.