Do fresh cut cedar logs release an enzyme that will harm plants planted directly into it?

Debbi, see my reply to your original question below

I have seen plans for cedar planters on the internet and they have not said anything about having to seal the wood with anything.

If you are planting a plant directly into a log, if it doesn't have a hollowed out area large enough to contain a substantial amount of earth for plant roots, then as Moon grower said in a previous reply, that it won't work.

If you have adapted the log into a proper planter, as for as the log type is concerned, while I don't know about any enzyme specifically, I do know cedar wood is used for making planters. You could always paint it on the inside with wood preservative that is plant friendly. Your garden centre or local DIY store should be able to advise.

If the log is going to lie directly on the ground then it will tend to rot - it you make little "feet" for it raising it up off the ground slightly then it will preserve it for longer.