Is there any particular reason to believe that a salads might be capable of consciousness? No.
Is there any particular reason to believe that brains might be capable of consciousness? Yes - namely the fact that most brains insist on describing themselves as such. Does this imply brains are conscious if and only if they insist on describing themselves as such? No. No more than than a bird is only capable of flight when it's actually literally soaring in the air.
Is there any particular reason to believe that a salads might be capable of consciousness? No
How can you tell without "any satisfactory theory of consciousness"?
This seems significant:
http://www.theguardian.com/science/2015/aug/18/first-almost-fully-formed-human-brain-grown-in-lab-researchers-claim