What kind of treatment should I use on bottom wall plates to prevent decay?
I assume you're talking about the bottom or sole plate of framing which is laid horizontally, correct?
If you have wood resting on a concrete floor then there isn't much you can do. The floor will forever wick water into the wood from below and rot it out; even when it's not raining.
If you put some sort of sealant in the corner where the wood meets the floor then you'll prevent excessive water from flowing into the wood but concrete absorbs water and will just distribute it into the bottom of the wood anyways. This would also lock in moisture and prevent any chance of it drying.
If you fully seal the wood with some sort of paint then it will likely hasten the rot since the bottom will still be wet and have no source of fresh air drying.

So, what can you do?
You can buy pressure treatment solution, run thick bead of caulk along the floor 1/4&quot; away from the bottom plate, and pour that solution into the makeshift &quot;pool&quot;. You need the wood to absorb this solution so the longer it sits the better. When you're ready then pull up the caulk and finish your renovation.