Energy coupling for membrane hyperpolarization in Lemna: respiration rate, ATP level and membrane potential at low oxygen concentrations.

Respiration rate, ATP content and membrane potential of Lemna have been measured as a function of the concentration of dissolved oxygen. Kinetic analysis showed that within the range from 1 μM to 20 μM O2, the respiration rate of isolated mitochondria and intact plants was a hyperbolic function of the oxygen concentration. The apparent Michaelis constant (K m ) for the oxygen of respiration of intact plants (1.15±0.08 μM) is close to that for isolated mitochondria (1.07±0.06 μM), so that diffusion of oxygen within the tissue was obviously not rate-limiting under the applied experimental conditions. The ATP level decreased in parallel with the respiration rate when the oxygen concentration was reduced. In contrast, the hyperpolarization of the membrane potential above the diffusion potential had already decreased at oxygen concentrations where the respiration rate and ATP level remained practically unchanged and was completely abolished at oxygen concentrations above the K m of respiration. This result is discussed according to the current models for electrogenic pumps. It is concluded that ATP cannot be the fuel for the electrogenic process under investigation.