Rapid Electric Responses of Oats to Phytochrome Show Membrane Processes Unrelated to Pelletability

Abstract

The electric potential difference changes observed on etiolated oat coleoptiles in response to phytochrome transformation have been further studied using contacts on the coleoptile surface. Results are given, at 0.4 second resolution, for the first 1.5 minutes after saturating flashes of light each lasting 1 second.

Responses to initial red (662 nanometers), to far red (about 700 nanometers and above) 10 minutes later, and to second red 10 minutes later still, all have time courses that are approximately Gaussian sigmoid in shape. The response to far red is of opposite sign to the response to red. Approximate magnitudes of the three changes, 1 minute after the light flash, are +6 millivolts for red, −10 millivolts for far red, +3.5 millivolts for the second red. It is argued that the observations reflect a hyperpolarization of the plasmalemma of coleoptile cells following red light and depolarization following far red. The response to red is not produced by a change in membrane permeability to K⁺. The mechanism could include a change to Na⁺ or Cl⁻ permeability or a modulation of an electrogenic pump: enhanced H⁺ extrusion, Ca²⁺ extrusion, or Cl⁻ uptake. The response to far red could be produced by the reverse of one of those changes.

The Gaussian curve is fitted to the data to determine the time at which the responses begin. Each response begins 4.5 seconds after the start of the flash of light. These delays are not related to the time course of phytochrome pelletability or redistribution in the cell. The delays may be due to some interaction of the transformed phytochrome with the plasma-lemma. Alternatively, the transformed phytochrome may interact quickly with some other structure which initiates a signal that takes 4.5 seconds to reach the plasmalemma.