This paper reports data and considerations relevant to the question of what determines the polarity of the voltages induced between electrodes in a suspension of chloroplasts when irradiated with a flash of light from a laser or flash-lamp. We found positive polarity (electrode nearest the light source positive) with excitation by ns pulses at 694, 539 and 530 nm wavelength. This and the earlier finding (Mesz6na et al. (1988) Studia Biophysica 126:77-86), confirmed in this work, of negative polarity at 420 nm confirm, in part, the action spectrum reported by Gr/iber and Trissl (1981 FEBS Let 123:95-99) using 50 ps flashes. Gr/iber and Trissl also showed that swelling the chloroplasts can reverse the polarity.
Negative polarity is expected on the basis of a simple light-gradient in the sample together with what is known about photosynthetic charge movements. The cause of positive polarities has eluded explanation. Duration of flash was suspected. We tried a random series of short flashes averaging about 10/~s apart and found that all simply duplicated the first flash. If there is any effect of light following the first flash it must occur in less than about 10 #s.
We suggest that the polarity is determined by a complicated interference pattern of the light in the chloroplast which can focus it onto different parts, front or back, depending upon the wavelength of the light and the structure of the chloroplast.