INFLUENCE OF STEM LACUNAR STRUCTURE ON GAS-TRANSPORT - RELATION TO THE OXYGEN-TRANSPORT POTENTIAL OF SUBMERSED VASCULAR PLANTS

The influence of stem lacunar structure on the potential of diffusion and mass flow to meet estimated root O2 demands was evaluated and comp ared in four submersed aquatic plant species. Internodal lacunae forme d large continuous gas canals which were constricted at the nodes by t hin, perforated diaphragms. Gas transport studies showed that nodes ha d little effect on diffusion, but significantly reduced mass flow. Mea sured diffusive resistances approximated those predicted by Fick's fir st law, ranged from 203 to 5107 x 10(8) s m-4 , and increased as lacun ar area decreased in Potamogeton praelongus, two Myriophyllum species and Elodea canadensis. Our analysis suggested that diffusion could sat isfy estimated root 02 demands given the development of relatively ste ep 02 gradients (0.15-0.35 mol O2 Mol-1 per 0.5 m stem) between shoots and roots. Plants with high resistances (e.g. > 750 x 10(8) s m-4) an d long lacunar pathlengths may be unable, even during active photosynt hesis, to support the 02 demands of a large root system by diffusion a lone. Measured nodal resistances to mass flow approximated those predi cted by Hagen-Poiseuille law and ranged from 46 to 2029 x 10(8) Pas m- 3. Our analysis suggested that these resistances were quite low and th at relatively small pressure differentials (<150 Pa per 0.5 m stem) co uld drive mass flow at rates which would support root 02 demands. Poss ible mechanisms whereby plant architecture may serve to maintain these pressure differentials are proposed.