There's plenty to suggest that a central nervous system (or something like it) is responsible for sentience. There's not much (if anything) to suggest that plants have sentience.

If you think otherwise, I think the burden of proof falls on you. I can't disprove that plants have a heightened level of awareness that is imperceptible to humans, but I also can't disprove that I am god and I created the universe.

Also, what are your feelings on fruitarianism? If you only eat the part of plants that is meant to fall off and be eaten to help it spread, then you are in the clear morally, no? The plant body remains wholly in tact and no worse for the wear. There's enough fruit-like plants that I'm pretty sure you can get a complete balanced diet off only fruit-like things. The only problem I can think of is mostly with how expensive fruits are compared to grains and legumes (actually, are legumes a fruit-like part of the plant?) and what not, so it's not as economically viable as veganism.