OSMOTIC EFFECT ON THE PHOTOSHRINKAGE OF CHLOROPLASTS

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Shrinkage of spinach chloroplasts by illumination and that by exposure to a high tonicity raised by addition of sucrose were investigated by means of the rapid measurement of chloroplast volume with a COULTER counter. The osmotic shock in the dark induced two steps of volume change; almost instantaneous shrinkage for the osmotic shock and follow-up gradual swelling. The final volume attained after equilibration was smaller than the original volume below 0.6M, and greater above this concentration. When chloroplasts under osmotic shock were illuminated, the photoshrinkage competed with the swelling induced by the osmotic shock, and these reverse effects were balanced at a certain volume. Photoshrinkage activity measured after equilibration decreased with increasing concentrations, and the activity curve plotted against sucrose concentration showed a stationary level of 50% of the original activity between 0.2 and 0.6M, indicating the resistance of a structure in chloroplasts to the denaturation by the osmotic effect of sucrose. The osmotic effect in the dark as well as in the light was completely reversible below 0.2 M and was partially reversible or irreversible above this concentration. Glucose showed qualitatively the same osmotic effect as sucrose.
日本植物生理学会の論文