Rhythmicity of L-phenylalanine ammonia-lyase activity in Spirodela oligorrhiza. Effects of darkening, abscisic acid and 1-amino-2-phenylethylphosphonic acid

L-Phenylalanine ammonia-lyase (PAL; EC 4.3.1.5) in Spirodela oligorrhiza (Kurz) Hegelm. cultivated in continuous light showed ca 48-h rhythmicity in the activity. Abscisic acid shifted the oscillation by ca 24 h. 1-Amino-2-phenylethylphosphonic acid, a competitive inhibitor of the enzyme in vitro, markedly increased PAL activity in vivo and nullified the oscillation. Rhythmicity in PAL activity did not occur in darkness. Illumination was a prerequisite for maintaining high PAL activity as well as for the occurrence of the oscillations in the enzyme activity. 'Average'enzyme activity decreased proportionally to time of cultivation of plants in the stationary culture. Transfer of the plants to fresh medium was connected with the increase of PAL activity to a maximum during the following 1-2 days. Diurnal oscillation of PAL activity with a maximum at 1800–1900 h, dependent on the presence of glucose or sucrose in the nutrient medium, was also recorded.