Plain old water when microwaved (and cooled) and used to water plants has been damaged to the point that the plants don't recognise it as water anymore, and the plants will wither away. If you don't believe that try it yourself on two "identical" plants. Feed one tap water, and the other microwaved tap water. You can find experiments like these documented on the internet. I didn't believe them, so I tried it myself. It's true.