Ken, I see what you mean...some of the leaf edges have turned brown. But the leaves are not uniformly turning yellow, which would be an indication of leaf drop from dormancy and/or not enough moisture.
@RCanada would be able to verify this.
If your solarium is warm enough and you see some active growth, then perhaps your adeniums need some trace elements. I suggest applying a liquid solution of a starter that has Vit. B1, boron, zinc, iron, weekly for two weeks, then a weak solution of seaweed or fish emulsion every two weeks.
The caudex of the plants look good, though, and should fatten up with the above recommended treatment.
