An Analysis of the Relationship between Respiration and the Transmembrane Potential in Corn Roots 12

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RESUMO

The effects of cyanide, anoxia, and temperatures varying from 2 to 42 C on the cell membrane electropotential difference (PD) of washed and freshly excised corn roots have been determined. Respiration rates of freshly excised root segments in response to cyanide and to varying temperatures were also measured. The cell membrane PD of roots which had been washed for 12 to 15 hours was almost insensitive to cyanide and anoxia but sensitive to low temperature. In contrast, the cell membrane PD of freshly excised roots was reversibly depolarized by all three treatments, cyanide depolarized from −117 to −86 millivolts and the sequential imposition of anoxia further lowered the PD to −69 millivolts. Anoxia applied first depolarized maximally and the PD was not further lowered by sequential cyanide treatment. Arrhenius plot analysis of the temperature response of respiration showed an apparent transition at 13 C with an activation energy of 20.0 kilocalories per mole below and 8.8 kilocalories per mole above the transition temperatures. The energy of activation for repolarization of PD is much higher; 53.4 kilocalories per mole below 7 to 8 C and 25.4 kilocalories per mole above this apparent transition. The energy requirement for polarization of the cell membrane PD was calculated based on the temperature responses of the cell membrane PD and respiration. It was estimated that 3.5% of the energy output from respiration at 22 C is required for cell polarization. It is unlikely that ion transport is limited by energy availability below the 8 C transition in this chill sensitive species.

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