The physiological ecology of cyclic adenosine 3′:5′-monophosphate (cAMP) in vascular aquatic plants II. cAMP as a modulator of carbon assimilation

Abstract

Laboratory and in situ experiments on the submersed aquatic macrophyte species Scirpus subterminalis Torr., Potamogeton praelongus Wulf. and Najas flexilis (Willd.) Rostk. & Schmidt were conducted to test the hypothesis that exogenous or endogenous cyclic adenosine 3′:5′-monophosphate (cAMP) may serve as a positive or negative modulator of carbon assimilation in macrophytes. Leaf sections or discs were incubated in artificial media amended with quantities of cAMP found in lake water (0.2–10 nmol 1 −1). In both short-term (30 s) and long term (3 h) experiments, 14C-assimilation rates in Scirpus and Potamogeton were significantly stimulated by inclusion of as little as 0.2 nM cAMP in incubation media. Najas leaf sections were unaffected by cAMP addition. Our evidence indicated that the putative modulation process involved exogenous cAMP and not tissue cAMP alterations. Stimulation of C-assimilation by cAMP was blocked by the photosynthetic uncoupler DCMU, did not occur in dark-incubated plants or in senescent leaves, and commenced immediately upon addition of nucleotide to incubation media. A synergistic relationship between media Ca 2+ ion concentration and cAMP addition was found in Scirpus and cAMP stimulated C-assimilation over a wide range of photosynthetically-available carbon concentrations. Collectively, the data were consistent with the view that cAMP and similar cyclic nucleotides may interact directly with active bicarbonate assimilation complexes at the surface of macrophyte leaves, thereby modulating carbon assimilation rates.