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Effects of Ethephon and its Decomposition Products on Germination of Rice and Watergrass¹

This study aimed to determine whether ethephon, 2-chloroethyl phosphonic acid, differentially influences the germination and seedling development of rice (Oryza sativa L.) and one of its major weeds, watergrass (Echinochloa crus-galli (L.) Beauv. var. oryzicola). Seeds were germinated at pH 7.0 at 30°C in flasks under N₂. Ethephon at 10 000 mg L^–1 inhibited germination indices by 91 and 29% in watergrass and rice, respectively. Ethephon at 2000 mg L^–1 significantly reduced coleoptile growth in both species. Ethylene at 1 and 100 µL^–1 stimulated watergrass germination but did not affect rice germination and enhanced coleoptile growth of both species. Potassium or sodium salts of phosphate and chloride, when administered in a manner simulating the anions' release from ethephon, inhibited germination of watergrass and coleoptile growth of both species. Watergrass germination was inhibited completely with 100 mM mannitol or PEG 8000 (–0.42 MPa) and by 68% with 100 mM potassium phosphate. Rice germination was also inhibited by these osmotica but to a lesser extent than watergrass. Thus, at comparable experimental concentrations, ethephon, phosphate, and chloride inhibited while ethylene promoted or did not influence seed germination or coleoptile growth of both species. We conclude that in both species, the inhibition of germination and coleoptile growth by ethephon is attributed to the effects of ethephon decomposition products other than ethylene. This effect should be considered in studies using high ethephon concentrations or when using tissues highly sensitive to osmotic stress.

Key Words: Echinochloa crus-galli (L.) Beauv. • Oryza sativa L. • Ethylene • Phosphate • Osmotica

² Graduate student, School of Medicine, Johns Hopkins Univ., 720 Rutland Ave., Baltimore, MD 21205; research assistant and assistant professor of biology, Colorado College, Colorado Springs, CO 80903; assistant professor of biology, Rollins College, Winter Park, EL 32789.

Received for publication September 9, 1985.