EVIDENCE FOR MASS-FLOW IN FLOWERING INDIVIDUALS OF THE SUBMERSED VASCULAR PLANT MYRIOPHYLLUM-HETEROPHYLLUM

Myriophyllum heterophyllum Michx. is a rhizomatous submersed aquatic p lant that produces a short, emergent floral spike. We hypothesized tha t lacunar pressures in emergent spikes should be at or near atmospheri c pressure and that a mass flow of gases from submersed stems through the rhizome to emergent stems may occur as lacunar O-2 concentrations and pressures in submersed stems increase during photosynthesis. We ex amined the potential for a pressure gradient (Delta P) to develop alon g this pathway by measuring diurnal changes in lacunar gas composition and pressure in submersed stems of nonflowering plants and emergent s tems of flowering individuals. Methane release from emergent spikes wa s also monitored during three diurnal cycles to evaluate the hypothesi s that the Delta P is maintained by the release of lacunar gases to th e atmosphere. Lacunar O-2 concentrations and pressures in submersed st ems increased at sunrise and reached maximum levels by midday. Althoug h O-2 fluctuated similarly in emergent stems, lacunar pressures remain ed at or near atmospheric pressure, indicating that a Delta P is gener ated between submersed and emergent stems during photosynthesis. Metha ne release from emergent spikes increased as lacunar pressures increas ed, indicating that rhizome gases are transported through emergent ste ms by mass flow and the Delta P is maintained by venting lacunar gases from emergent spikes. The potential for mass flow in both flowering a nd nonflowering individuals is discussed.