"No, not a tomato! A lemon!"
Translation:Nej, inte en tomat! En citron!
Not sure about the exact grammatical reasons, but it's the wrong word order. Just like in English, in this case: you can't really say No, a tomato not.
Edit: hmm. Actually, you probably can say that in English, although it sounds contrived. Anyway, doesn't work in Swedish. :)