Converts palmitoyl-ACP to (4Z)-hexadec-4-enoyl-ACP by introduction of a cis double bond between carbons 4 and 5 of the acyl chain. 2 H(+) + hexadecanoyl-[ACP] + O2 + 2 reduced [2Fe-2S]-[ferredoxin] = (4Z)-hexadecenoyl-[ACP] + 2 H2O + 2 oxidized [2Fe-2S]-[ferredoxin] Binds 2 Fe(2+) ions per subunit. Homodimer. In green tissue, found in chloroplasts. In non-photosynthetic tissue, found in plastids. Found only in tissues which synthesize petroselinic acid, such as developing seeds. Belongs to the fatty acid desaturase type 2 family.