Inhibition of Gibberellic Acid-induced Germination by Abscisic Acid and Reversal by Cytokinins

The germinatioil of seed can be controlled by ap)plication of exogenous hormones at physiological concenitrations. It is therefore possible that the n-atural control of germination involves an interplay of hormones, both promoters and inh,,ibitors. WXe have shown that cytokinins overcome the inhibitoryaction of several natural inhibitors in the germination of the photosensitive Grand Rapids lettuce seed (4,5). There are other reports of cytokinin-inhibitor antagonism in germinationl and dormancy (6, 7, 10, 12, 14). Chrispeels and Varner have shown that gibberellic acid (GA3) induced a-amylase synthesis in the excised barley aleurone is inhibited by abscisic acid and revrersed only partially by excess GA) (2). In the intact barlev seed, like&% ise. GA3 fails to reverse abscisic acid inhibited a-amylase production and coleoptile growth (8, 9). Cytokinins, such as kinetin and ben'zyladenine. on the otlher hand, nearly completely reverse the abscisic acid inhiblition of a-amylase synthesis and coleoptile grovth in the seed (9). This communication will show that another GA. induced process, namely dark germination of Grand Rapids lettuce seed, is inhibited by abscisic acid and is reversed by the cvtokinin, kinetin but not by excess GA3. Lettuce seeds (Lactitca s.lizta var. Grand Rapids. Ferry Morse Seed Company. 1967) were germinated at 200 and 250 in total darkness in 5 cm Petri plates on 2 layers of filter paper soaked in 2 ml of test solut"ion (4 replicates of 50 seeds each). Germination was recorded after 48 hours. Emiiergence of the radicle was taken as criterion of germination. The effect of abscisic acid anid kinetin on GA: induced dark germination was examinied at 20( (fig 1) and 250 (fig 2). A concentration of 0.05 mM GA, was sufficient for coml)lete indutction of germination at 200 i(fig 1, curve BP wvhile at 250 much higher concentration of GA (5 mI-M) was