If you've had an answer, I can't find it--this site is new to me. I'd love to know. It has nothing to do with their being in pots, as I've consistently had the problem with my two plants in the ground. The yellowing begins as you say at the bottom and if left on the plant the leaves go brown and die. I water them & occasionally feed them with comfrey or chicken manure but nothing seems to prevent the problem.