Chloroplast omega-6 fatty acid desaturase introduces the second double bond in the biosynthesis of 16:3 and 18:3 fatty acids, important constituents of plant membranes. It is thought to use ferredoxin as an electron donor and to act on fatty acids esterified to galactolipids, sulfolipids and phosphatidylglycerol. a (9Z)-octadecenoyl-containing glycerolipid + 2 H(+) + O2 + 2 reduced [2Fe-2S]-[ferredoxin] = a (9Z,12Z)-octadecadienoyl-containing glycerolipid + 2 H2O + 2 oxidized [2Fe-2S]-[ferredoxin] Lipid metabolism; polyunsaturated fatty acid biosynthesis. Highest levels found in expanding leaves. The histidine box domains may contain the active site and/or be involved in metal ion binding. Belongs to the fatty acid desaturase type 1 family.