BIOLOGIA PLANTARUM38 (2): 269-274, 1996 Germination of dormant seeds ofStylosanthes humilis as related to heavy metal ions C.A. DELATORRE and R.S. BARROS Departamento de Biologia Vegetal, Universidade Federal de Vir 36571-000 Vir MG, Brasil Abstract Cadmium, copper and zinc ions at high concentrations partially released scarified freshly-harvested seeds of Stylosanthes humilis from physiological dormancy. This response to toxic metals increased along with seed postharvest ageing. Cobalt and silver ions, and abscisic acid impaired metal-promoted germination. Additional key words: cadmium, cobalt, copper, dormancy break, silver, Townsville stylo seed, zinc. Introduction The tropical legume Townsville stylo (Stylosanthes humilis H.B.K.) is an annual forage species whose seeds possess hard integuments; when harvested these seeds also exhibit physiological dormancy. The latter is gradually lost upon seed ageing (Burin et al. 1987, Vieira and Barros 1994). Treatment of seeds with high temperatures (Gardener 1975) or specific and unspecific regulators (Ballard and Buchwald 1971, Burin et al. 1987) breaks physiological dormancy. By supplying growth regulators in several combinations to dormant seeds, Burin et al. (1987) and Vieira and Barros (1994) concluded that at least two classes of growth substances, ethylene and cytokinin, were required for germination of Townsville stylo seeds. That ethylene is essential for seed germination is shown by the following evidences: ACC, the immediate precursor of ethylene, partially breaks seed dormancy; seeds are also responsive to ethrel (2-chloroethylphosphonic acid), a well-known ethylene- releasing compound; inhibitors of ethylene biosynthesis, such as cobalt ions, and of ethylene action, such as silver ions, block germination of non-dormant seeds and of ACC-stimulated dormant seeds (Vieira and Barros 1994). Several plant systems respond to heavy metals by producing high amounts of ethylene; this stress response seems to be due to toxic effects of heavy metals at high concentrations (Matoo et al. Received 31 May 1995, accepted 31 October 1995. Abbreviations: ABA- abscisic acid; ACC- 1-aminoeyclopropane-l-carboxylic acid. Acknowledgments: CAPES, CNPq, FAPEMIG and FINEP are gratefully acknowledged for the financial support granted for the conduction of this work. 269