Comparison of Lipid Biosynthesis of Normal and Dark Kept Spinach Leaves in Photosynthetically Active Light

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Abstract
Upon illumination, dark starved spinach leaves show a 50% reduction of 14C-incorporation capacity into lipids. If 32P-incorporation is measured, the reduction is 85% compared to controls. The relative labelling of the fatty acid residues in total lipids of normal leaves was 80 - 90% compared to 30 - 50% in the darkened material. The kinetics of 14C-incorporation into lipids show that in the controls the majority of 14C was located in the monogalactosyl diglyceride fraction while in the dark kept material phosphatidyl glycerol and phosphatidyl choline were more highly labelled. Corresponding with the changed incorporation capacity a decrease in fatty acid concen­ tration in the glycolipid fraction and a simultaneous increase in the phosphatide fraction in dark pretreated leaves is observed. An illumination period of 30 - 60 minutes at 2×104 lx brings these inverse values back to normal. The most rapid labelling occurred in a glycolipid fraction lacking galactose and having a slightly more hydrophilic character than monogalactosyl diglyceride. The molar ratio of sugar, glycerol and fatty acids in this fraction is 1 : 1 : 1 - 2. The label in the fatty acid residues of this lipid fraction was approximately 10% in dark kept but 90% in control leaves. From 14C-incorporation behavior it is suggested that this lipid is a precursor of monogalactosyl diglyceride. The changes of the labelling pattern of lipids in leaves kept in prolonged darkness are similar to those obtained with 3-(3,4-dichlorophenyl) -1,1-dimethylurea treated Chlorella. A close connection of these effects to the decrease of photosynthetic capacity is therefore suspected. Also the observation that the concentration of α-linolenic acid, bound in the glycolipids, is reduced in dark kept material and increases upon subsequent light exposure, fits into this context. A pos­ sible explanation of these labelling shifts seems to be a pH shift in the various cell-compartments induced by light/dark treatment.

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