Plays an essential role in chain termination during de novo fatty acid synthesis. Possesses high thioesterase activity for palmitoyl-ACP versus other acyl-ACPs. Substrate preference is 16:0 > 18:1 > 18:0 > 16:1. Plays an essential role in the supply of saturated fatty acids necessary for plant growth and seed development. Contributes to 16:0 production particularly in flowers. May be involved in the synthesis of long chain fatty acid. H2O + hexadecanoyl-[ACP] = H(+) + hexadecanoate + holo-[ACP] The catalytic efficiency for 16:0 is at least 2-fold higher than for other substrates. Optimum pH is 9.7. Highly expressed in flowers. Expressed in roots, leaves, stems, siliques and seeds. Retarded growth, deformed seeds with low rate of germination and reduced levels of palmitate and stearate. Plants silencing FATB, show reduced level of palmitate in flowers and seeds. Belongs to the acyl-ACP thioesterase family.