MECHANISMS AND RATES OF O-2 TRANSFER TO AND THROUGH SUBMERGED RHIZOMES AND ROOTS VIA AERENCHYMA

Transfer of oxygen (O-2) to roots in waterlogged or flooded soil is es sential for plant survival. This paper reviews mechanisms for the tran sport of O-2 in flooded environments. Plants adapted to wetland habita ts develop aerenchyma that facilitate internal gas-phase transport mec hanisms for O-2. Molecular diffusivity in the gas phase is approximate to 10 000 times greater than in water, and molecular diffusion throug h plant gas-space systems may be the only transport mechanism in many cases. Several mass-flow mechanisms have been proposed and evaluated. Bulk flow of gases produced by solubilization and export of respired C O2 to surrounding water or soil water is one mechanism; however, flood ed sediments could generate CO2 also. This non-throughflow convection mechanism is enhanced by tidal cycles in mangrove swamps. Several mech anisms based on Knudsen-scale diffusion of O-2 through < 0.1 mu m pore s (down a partial pressure gradient, but into pressurized lacunae) are effective for some emergent aquatic species. One mechanism is convect ive throughflow caused by differential pressurization of different aer ial plant parts based on temperature differences (''thermal transpirat ion, thermodiffusion, or thermo-osmosis''). Under certain experimental circumstances, humidity-induced differential pressurization can be ev en more effective in causing convective throughflow than thermo-osmosi s. These two mechanisms were discovered in the late 1800s. Recently, d ifferences in mean molar mass of gas have been shown to generate gas f low in leaves from a physical effect which can be described by Graham' s law of diffusion. Venturi-induced convective flow across broken culm s of Phragmites has also been demonstrated, and stagnation pressure of wind onto broken culms might also cause convective throughflow. Micro baroms due to atmospheric turbulence and a ''heat-tube'' type of mass transfer are additional possible mechanisms that have not been explore d. In summary, humidity-induced pressurization and thermo-osmosis clea rly enhance convective throughflow of gases in some plants, and their potential for O-2 transport in other plants should be investigated fur ther.