Chloroplasts in higher magnetic fields align with their equatorial plane perpendicular to the field. Because of the nonrandom orientation of the chromophores in the membrane the fluorescence radiation will be partially polarized. The chloroplast concentration, magnetic field, and temperature dependence of the fluorescence polarization has been investigated. The results are compared with a simplified model calculation. It is shown that the concentration dependence can be related to the linear dichroism of the fluorescence radiation and self-adsorption. Taking these effects into account results in the calculation of a higher fluorescence polarization (FP) ratio and higher inclination of chlorophyll dipoles to the membrane plane. Analyzing the magnetic field dependence of the FP ratio, we conclude that in a magnetic field not only will be chloroplasts be aligned, but the thylakoid stacks as well. A decrease in the FP ratio was observed around 20 degrees C. It is suggested that this decrease reflects a phase transition in the photosynthetic membrane.
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