Etioplasts, etiochloroplasts, and chloroplasts ofAvena sativaL. purified on a Percoll gradient were subjected to increasing electric field strengths in the orifice of a hydrodynamically focussing Coulter Counter. The change in resistance of the orifice when an organelle is present correlates well with the size of the plastid for field strengths up to about 3.5 kV cm-1. Beyond this field strength, depending on the size of the organelle, the size is underestimated. The underestimation of the size is caused by the dielectric breakdown of the envelope membranes once a critical membrane potential has been exceeded. Beyond breakdown the signal of the particle is predominately determined both by the internal conductivity and the increased membrane conductivity. Measurements of the breakdown voltage of different developmental stages of the plastids reveal that the breakdown voltage decreases from 1.2 V in etioplasts to about 0.9 V in chloroplasts after 48 h illumination. The decrease in breakdown voltage can be explained in terms of increasing incorporation of proteins into the inner envelope membrane during development.
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