The sucrose-ammonium inhibition of flowering Lemnapaucicostata 6746 in continuous blue light or in short days was partially overcome by epinephrine. This reversal was prevented by propranolol, an antagonist of epinephrine in animals. In ammonium-free medium, propranolol inhibited flowering, and this inhibition was completely overcome by epinephrine.Increased levels of Ca2", Pi and nitrate partially reversed the inhibition by propranolol. Concentrations of cAMP, adenine, and adenosine that partially overcame the sucrose-ammonium inhibition did not affect flowering in cultures treated with propranolol. The possibility is discussed that the effects on flowering of sucrose-ammonium, propranolol, and epinephrine were due to altered intracellular levels of cAMP or of a cAMP-like compound.Inhibition of flowering by ammonium ion and by sucrose has been observed in several species of Lemnaceae (5, 6, 16). In the SDP4 Lemna paucicostata5 strain 6746, the sucrose inhibition (11) was later shown to be due to an interaction between sucrose and ammonium ion (4).Oota (6,8) reported that inhibition of flowering by either sucrose or ammonium ion in the LDP L. gibba G3 was reversed by cAMP. The inhibition by sucrose was partially reversed by catecholamines such as epinephrine which, in animals, increase intracellular levels of cAMP. Further, the reversal was prevented by propranolol, an antagonist of epinephrine. On the basis of these and other results, Oota (7) proposed that the inhibition of flowering in L. gibba G3 was due to reduced levels of intracellular cAMP. Intracellular levels of cAMP have been shown to be lowered by sugars in certain bacteria (9) and by ammonia in slime mold (13).The purpose of the following study was to determine whether flowering responses of L. paucicostata 6746 to epinephrine and propranolol were consistent with a possible involvement of cAMP in the sucrose-ammonium inhibition. MATERIALS AND METHODSVegetative stock cultures of Lemna paucicostata 6746 were grown axenically on approximately 50 ml half-strength Hutner 'Dedicated to the memory of Dr.