Changing activities during senescence and sites of synthesis of photosynthetic enzymes in leaves of the labiate, Perilla frutescens (L.) Britt

The changing activities of several regulatory enzymes of the photosynthetic carbon reduction cycle accompanying ageing of the third leaf pair of Perilla frutescens fall into two distinct categories: firstly, enzymes which reach maximum activity prior to the completion of leaf expansion followed by a rapid decline (phosphoribulokinase, ribulose-l,5-diphosphate carboxylase, and NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase); secondly, enzymes which maintain high activity beyond completion of leaf expansion and decline only at a late stage in senescence (phosphoglycerate kinase, NADH-linked glyceraldehyde-3-phosphate dehydrogenase, alkaline fructose-1,6-diphosphatase, and ribose-5-phosphate isomerase). The introduction of the ribosomal inhibitors D-threo chloramphenicol, lincomycin, D-2-(4-methyl-2,6-dinitroanilino)-N-methyl propionamide, and cycloheximide to illuminated, detached shoot systems of 2-d-darkened Perilla plants has suggested that synthesis of ribulose diphosphate carboxylase, NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase, and possibly phosphoribulokinase, is mediated by 70 S-based chloroplastic ribosomes. A chloroplastic site of synthesis of these three photosynthetic enzymes is consistent with their early deterioration during leaf ageing.