Stem and leaf tissues ofStellaria longipes Goldie (prairie ecotype) exhibit circadian rhythmicity in the activity and mRNA abundance for 1-aminocyclopropane-1-carboxylic acid oxidase (EC 1.4.3). The steady-state mRNA levels and enzymatic activity levels fluctuated with a period of approximately 24 h and reached their maxima by the middle of the light phase and minima by the middle of the dark phase. The oscillations showed damping under constant light, constant dark and constant temperature conditions, indicating that the rhythm is entrained by an external signal. The results indicate that light/dark cycles have greater entraining effects than temperature cycles. A 15-min red light pulse, but not a blue light pulse, could reset rhythm in continuous darkness, suggesting the possible role of a red-light signal transduction pathway in the circadian regulation of 1-aminocyctopropane-l-carboxylic acid oxidase. Key words: 1-aminocyclopropane-l-carboxylic acid oxidase (activity, mRNA) -Circadian rhythm -Light -Stellaria -Temperature ethylene dynamics may represent one physiological aspect of different levels of phenotypic plasticity in the ecotypes (Emery 1995). Emery et al. (1994a) observed daily fluctuations in ethylene production from the ramets of the prairie ecotype of Stellaria longipes. Rhythms in ethylene evolution have also been reported in other plant systems, including cotton (Lipe and Morgan 1973;Rikin et al. 1984), tomato (E1-Beltagy et al. 1976), bean (Morgan et al. 1990), and rice (Michiyama and Saka 1988). Ievenish and Kreicbergs (1992) reported endogenous rhythmicity of ethylene production in growing intact seedlings of barley, wheat and rye.
Earlier we studied the role of 1-aminocyclopropane-1carboxylic acid (ACC) accumulation and activity of ACC oxidase [-the enzyme mediating the oxidation of the immediate precursor of ethylene, ACC (Adams and Yang 1979)] in ethylene evolution. These preliminary experiments suggested rhythmic fluctuations in the activity of ACC oxidase both in vivo and in vitro (Emery 1995). We now report the existence of a light-and temperatureentrained endogenous rhythm in the accumulation of mRNA and in-vitro enzymatic activity for ACC oxidase.