Bromocresol purple and ethyl red as indicators of events linked to changes in the energization of chloroplast membranes.

1. Two dyes, bromocresol purple and ethyl red, were tested for their ability to register solution and membrane changes resulting from the illumination of suspensions of isolated chloroplasts. 2. Bromocresol purple can measure the external (suspension) pH both rapidly and accurately if the following precautions are observed: (a) since bromocresol purple stimulates electron flow to oxygen, either low concentrations ( 2 2 PM) of bromocresol purple or conditions for noncyclic electron transport must be used; (b) the absorbance shifts must be converted by formula to yield changes in proton concentration. 3. The disappearance of protons from the medium when the light is turned on is bi-exponential with half-times of about 0.7 s and 8 s. The release of protons into the medium when the light is turned off is exponential with a decay half-time of about 30 s. 4. A brief initial lag ( <0.2 s) in the uptake of protons follows the onset of illumination, and there is a lag of up to 1 s in the release of protons at the end of illumination. These data suggest the presence of an intermediate pool between electron flow and proton movement. 5. One of the major components of the ethyl red response in buffered suspensions is a band shift, which is thought to represent a change in the position of ethyl red within the membrane resulting from illumination. These responses seem related to the state of coupling of the photosynthetic membrane and could serve as a sensitive indicator thereof. 6. The time relationship between ethyl red and bromocresol purple responses shows that changes in the membrane (ethyl red response) continue after the establishment of the steady state pH shift in the light. 7. Bromocresol purple and ethyl red offer promise as investigative tools for events linked to energy transduction in thylakoids.