4-Coumarate: CoA ligase (CL) isolated from oat primary leaves shows two activity maxim a in 5 and 7 day old seedlings, the first of which corresponds with the highest flavone accumulation rate. In this plant stage most of L-phenylalanine ammonia-lyase (PAL) is located in the epidermal tissues, whereas CL dominates in mesophyll protoplasts and in the vascular elements. In the older leaves, in which PAL is only detectable in the epiderm is and in vascular elements, most of CL is located in the latter. The CL from different tissues varies significantly in substrate specificity. Investigations of intact chloroplasts from isolated protoplasts revealed that approximately half of the PAL activity can be extracted from this organelle, whereas there is no direct evidence for the existence of CL in plastids.