Lipid biosynthesis in relation to chloroplast development in barley.

During greening of detached leaves from dark-grown barley seedlings, the linolenic acid content of the lipids increases in the early stages of the formation of the chloroplast lamellar system. Primarily the fraction containing monogalactosyl diglyceride is enriched with linolenic acid. Incorporation of (14)C-labeled acetate into the leaf lipids of detached whole leaves is low, but increases 10- to 20-fold during greening. Increasing percentages of label appear in linolenic acid during the first 15 hr of greening, whereafter they remain constant. A constant, relatively high amount of acetate is incorporated into lipids when slices of leaves at various stages of greening are incubated by submersion in acetate solution, a treatment that blocks further chlorophyll synthesis during incubation. At the initial greening stages 75% of the label is channeled into steroids and other unsaponifiable lipids, but in advanced stages of chloroplast development 75% of the incorporated acetate is built into phospho-, sulfo- and galacto-lipids, and only 25% is channeled into unsaponifiable lipids. Experimental variation of the physiological conditions of the tissue during incubation resulted in differences in the amount of label found in the various phospho- and galacto-lipids. The amounts of labeling of the individual fatty acids in the lipid classes studied differ markedly and could be changed by varying the conditions of incubation. Labeling of linolenic acid was found to be highest in the monogalactosyl diglyceride fraction at all stages of greening.