Investigations into the »inverse contrast« of chloroplast thylakoids

Abstract

We detected an »inverse contrast« (i. e. dense lumen and lightly staining membrane) of thylakoids in some species or organs where it has not previously been described. We probed the chemical character of the dense lumenal substance by in situ extraction on ultrathin sections, and found that it was less hydrophilic in Perilla frutescens, than in Helianthus annuus, where it could be neither lipid nor protein. Its disappearance from the lumen upon prolonged illumination was accompanied by a drop in the plastidial polyphenol oxidase activity, so it may be a phenolic compound. Growing Helianthus seedlings at 30 °C (rather than at 25 °C) resulted in the density appearing in the stroma (rather than in the lumen), but still preventing the strong staining of the thylakoid membrane. Upon the removal of the dense substance from the stroma by plastid isolation the staining pattern reverted to its usual appearance in the plastids. This excludes the explanation that light membrane staining is produced by some compositional deficiency. This conclusion was supported also by gel electrophoretograms of polypeptides from such membranes. We think that the dense substance on either side of the membrane protects lipid molecules from a conformational change during solvent dehydration, which normally leads to the strong staining of the membrane