Converts stearoyl-ACP to oleoyl-ACP by introduction of a cis double bond between carbons 9 and 10 of the acyl chain. Required for the repression of the salicylic acid (SA) signaling pathway. 2 H(+) + O2 + octadecanoyl-[ACP] + 2 reduced [2Fe-2S]-[ferredoxin] = (9Z)-octadecenoyl-[ACP] + 2 H2O + 2 oxidized [2Fe-2S]-[ferredoxin] Binds 2 Fe(2+) ions per subunit. Lipid metabolism; fatty acid metabolism. Homodimer. Belongs to the fatty acid desaturase type 2 family.