Induction of Dormancy in Nondormant Seeds
Anwar A. Khan Department of Horticultural Sciences, New York State Agricultural Experiment Station, Cornell University, Geneva, NY 14456
Additional index words. gibberellin biosynthesis inhibitors, dormancy induction, dormancy release, germination inhibition, growth retardants, abscisic acid, tetcyclacisAbstract. A gibberellic acid (GA) biosynthesis inhibitor, tetcyclacis, induced dormancy in nondormant seeds of lettuce (Lactuca sativa L.), tomato (Lycopersicon esculentum Mill.), pepper (Capsicum annuum L.), carrot [Daucus carota var. sativus (Hoffn.)], onion (Allium cepa L.), celery (Apium graveolens L.), and impatiens (Impatiens novette ), as most of the seeds failed to germinate after washingunder conditions that permitted germination before dormancy induction. In lettuce seeds, tetcyclacis and paclobutrazol were more effective in inhibiting germination in light than in darkness. A 16to 24-h soak treatment with tetcyclacis was sufficient to induce dormancy in nearly all seeds. Tetcyclacis failed to induce dormancy if applied after 6 h presoak in water. Dormancy induced by tetcyclaciswas released by GA4+7 (a mixture of gibberellin A4 and A7), light, and moist-chilling treatments. When GA4+7 was applied with tetcyclacis, dormancy induction was prevented under both favorable, e.g., 25C, and unfavorable, e.g., 5C, or low water potential (ψ), germination conditions. Unlike tetcyclacis, abscisic acid (ABA) failed to induce dormancy in lettuce seeds. Thermodormancy induction inlettuce seeds at 35C was prevented by fluridone. However, neither ABA nor tetcyclacis countered its effect. Dormancy was also induced in lettuce seeds by ancymidol, flurprimidol, or paclobutrazol. Dormancy induced by tetcyclacis in pepper, tomato, carrot, and onion seeds was released by GA4+7, but not by irradiation or moist-chilling. Chemical names used: 5-(4-chlorophenyl)-3, 4, 5, 9,10-pentaazatetracyclo [5.4.102,6.08,11]-dodeca-3, 9-diene (tetcyclacis); 1-(4-chlorophenyl)4, 4-dimethyl-2-(1H-1, 2, 4-triazole-1-yl )-3-pentanol (paclobutrazol); α-cyclopropyl-α-(4-methoxyphenyl)- 5-pyrimidine methanol (ancymidol); α-(1-methyl)-α-[4-(trifluoromethoxy) phenyl]-5-pyrimidine-methanol (flurprimidol); 1-methyl3-phenyl-5-[3-(trifluoromethyl)phenyl]-4 (1H)-pyridinone (fluridone).
Growth-retardingcompounds such as (2-chloroethyl) trimethylammonium chloride (CCC), 1,1-dimethylpiperidinium chloride (DPC), 2'-isopropyl-4'-(trimethylammonium chloride)5'-methylphenyl piperidine carboxylate (AMO-1618), and tributyl2,4-dichloro-benzylphosphonium chloride (Phosphon D) have been used to reduce stem growth and lodging (Cathey, 1964; Jung and Rademacher, 1983). The mode of action of these growthregulators appears to be related to the inhibition of GA synthesis (before ent- kaurene), as simultaneous application of GA will cancel the growth-retarding effect (Rademacher, 1991). Highly active growth retardants such as ancymidol, tetcyclacis, paclobutrazol, and uniconazol have recently been developed (Izumi et al., 1984; Jung et al. 1980; Lever et al., 1982; Shive and Sisler, 1976) with highspecificity for oxidative steps leading from ent-kaurene to ent-kaurenoic acid in the GA biosynthesis pathway (Coolbaugh et al., 1978; Jung and Rademacher, 1983). Evidence has been presented for the primary role of GA in dormancy control and seed germination (Khan, 1971). Inhibitors of GA biosynthesis, particularly those inhibiting ent-kaurene oxidation, readily inhibit seed germination and the effectis reversed by GA. Gardner (1983) found that ancymidol inhibited the red-light-induced germination of ‘Grand Rapids’ lettuce seed, which was reversed by GA3, and he suggested that the photoreceptor, phytochrome, might exert its effect on germination via control of GA level. Similarly, Karssen et al. (1989) showed that the lightpromoted germination of Arabidopsis thaliana seeds was inhibited by...