AMPLITUDE AND POLARITY OF THE LIGHT GRADIENT PHOTOVOLTAGE FROM CHLOROPLASTS

The primary charge separation in reaction centers embedded in the phot osynthetic membranes induces an electric polarization in chloroplast s uspensions. Photovoltages elicited by short non-saturating flashes, we re observed almost 20 years ago by Witt and Zickler(1973, FEBS Lett, 3 7, 307-310) and Fowler and Kok (1974, Progress in Photobiology, Frankf urt: Deutsche Gesellschaft). The photovoltage was interpreted as the r esult of the so-called ''light-gradient'' effect, in which the stronge r excitation of the membrane facing the light source compared to the s hadowed one creates a difference of dipole density in these two membra nes. Owing to the antiparallel orientation of reaction centers in oppo site thylakoid membranes, a small potential difference results. It was thought that the polarity of this potential difference could be deduc ed from the known position of electron carriers in the photosynthetic reaction center. However, the observed polarity was often opposite to that predicted by this model. Also, the measured photovoltage amplitud es could not be quantitatively related to experimental parameters. In the present paper, we show that the ''classical'' explanation of the l ight-gradient effect does not hold true and we give an alternative exp lanation that is based on light propagation and interference in pigmen ted multilayers. A model calculation is carried out for a pair of memb ranes simulating stroma lamellae of chloroplasts. It predicts a wavele ngth-dependent light distribution as well as the polarity of the photo voltage. For low intensities, the amplitude is found to be proportiona l to the intensity of the incoming light, to the optical density, and to the reciprocal of the dielectric constant of the sample. When the m embranes contain no chromophores or when the absorption coefficient is low, the predicted polarity is opposite to that expected from the cla ssical picture. The model is tested with a set of experimental photovo ltage data obtained at different wavelengths.