The Onion

There is not really strong evidence of plant sentience. Here's one paper looking at it:

A. Plants do not show proactive behavior.

B. Classical learning does not indicate consciousness, so reports of such learning in plants are irrelevant.

C. The considerable differences between the electrical signals in plants and the animal nervous system speak against a functional equivalence. Unlike in animals, the action potentials of plants have many physiological roles that involve Ca2+ signaling and osmotic control; and plants’ variable potentials have properties that preclude any conscious perception of wounding as pain.

D. In plants, no evidence exists of reciprocal (recurrent) electrical signaling for integrating information, which is a prerequisite for consciousness.

E. Most proponents of plant consciousness also say that all cells are conscious, a speculative theory plagued with counterevidence.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8052213/

Though something interesting and perhaps counter intuitive to note is that even if we realized plants were sentient, a plant-based diet actually involved killing fewer plants due to the lessened need to grow feed (of which most of the energy is lost)

Well, the first step to this question is the ever infuriating "define cruelty". It's easy enough with complex vertebrates who have evolved to socially signal pain, which is almost everything we eat. It's even easy to extend it to complex vertebrates which hide pain. But it's hard enough to rigorously say whether something like an invertebrate insect or crustacean even feels pain at all. They certainly have pain responses, but is the qualia of that response in theory internal space recognizable?

It's not an easy question to approach, but it is an important one broadly.

Let's say that plants do have some kind of sentience, which is probably very limited due to the evidence we do have. Animals still have more advanced sentience that is closer to our own so it would still be the lesser evil to eat plants. Like why would you eat other people or chimps when there are other options available?

It takes a lot of mental gymnastics to be able to say that plants suffer the same way as animals. I know you're not saying this, but you do hear stuff like this based on this premise.

They are living things. We shouldn't seek to deliberately cause pain if possible. While I like how stuff like bonsai trees look, I also feel a bit bad for them, wired and snipped in so many places and forced to be grown unnaturally small.

Or people who deliberately carve graffiti into trees with a knife.

Plants and trees have interestingly complex communication networks. We barely understand their microfauna and underground microbiomes that allow forests to grow much healthier and disease-resistant than our backyards. I have a funny feeling we know a lot less than we think we know, like when scientists discovered that babies can actually feel pain, or that dogs realize when they are treated unfairly. Stuff discovered within our lifetimes, lol.