Common sense, for one, which while obviously not definitive shouldn't be immediately discounted. But here[0] is just one such scientific argument.

[0] https://www.cell.com/trends/plant-science/fulltext/S1360-138...

It is almost a situation of definitions. Nobody has a precise test of consciousness, so we can only come up with a list of things that are, aren't and might be conscious.

Plants are on the non-conscious list, so if they appear to have anything that appears consciousnesses like then we just rule that out as sufficient for a test of being conscious. If plants are conscious to you then the word just doesn't have a meaning/is far removed from the standard meaning.