Plastid differentiation, acyl lipid, and fatty acid composition have been followed in successive 2-cm sections from the base (youngest tissue) to the tip (oldest tissue) of green Zea mays (maize) leaves grown under a normal diurnal light regime. Although the youngest cells (0-4 cm from the leaf base) had only proplastids with one or two grana, they contained chlorophylls a and b, monogalactosyldiglyceride, digalactosyldiglyceride, sulfolipid, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol. In the more mature sections, the plastids increased in size 5-fold, and differentiation into mesophyll and bundle-shealth chloroplasts had occurred. Concomitantly, the levels of all the lipids increased with the exception of phosphatidylcholine and phosphatidylethanolamine which decreased. With increasing cell maturity, the percentage of linolenic acid increased in all the individual acyl lipids, but palmitic acid remained constant in phosphatidylcholine, phosphatidylethanolamine, and sulfolipid. The A"-hexadecenoic acid was only detectable in the phosphatidylglycerol of the most mature maize tissue.The differentiation of chloroplast membranes has been investigated most frequently using etiolated tissue which is subsequently exposed to light. Such systems have been used with higher plants to study the changes in lipid composition and lipid biosynthesis which take place in the plant during plastid development (1,2,(17)(18)(19) The coleoptile and first leaf were removed, and the remaining one or two leaves were cut transversely into five sections (Fig. 1). The lower four sections were each 2 cm wide, and the fifth section (between 5-10 cm long) was the remainder of the leaf. Chlorophyll in the leaf sections was determined in 80% acetone according to Arnon (3). Fresh weights and dry weights of leaf material were determined using conventional techniques.For electron microscopy, a thin (1 mm) leaf slice from the middle of each of the sections A to E was cut up in a drop of 2.5% glutaraldehyde in potassium-phosphate buffer, pH 7.0, transferred to a vial, and left for 1 hr at room temperature. The glutaraldehyde was pipetted off, and the leaf sections were postfixed for 1 hr in 1 % osmium tetroxide in the same buffer. Dehydration and embedding in Spurr's resin was as described elsewhere (13).Total lipids were rapidly extracted from the leaf sections by homogenization in a Waring Blendor at 4 C with precooled chloroform-methanol, 2:1 (v/v) which contained 0.005% (w/v) 2,6 di-tertiary butyl p-cresol as antioxidant. Two successive extractions with fresh solvent were necessary to remove all lipid material. Lipid extracts were taken almost to dryness in a rotary evaporator below 40 C. Extracts were redissolved in chloroform-methanol, 2:1 (v/v), filtered through glass-fiber filter paper, and water-soluble contaminants were removed by partition with 0.1 M KCl. The chloroform layer was recovered, taken to dryness, and the lipid extract was weighed.Total lipids, dissolved in chloroform, were applied to silici...
