My labmate is doing research on the interaction between the plant circadian rhythm and the plant immune system. Their various immune hormones (along with all kinds of other things) are modulated by a rhythm that anticipates the diurnally-varying likelihood of fungal infection and can be phase-shifted not just by light but by humidity. The hormones that modulate this are systemic and get carried throughout the plant and can easily be taken up through roots.

After poking his plants in the lab he came up with the idea of getting our tomato plants we were growing together this summer to sacrifice a little bit of biomass in favor of fungus resistance by watering them with dilute aspirin (a slightly modified version of a plant immune system hormone originally extracted from willow bark) in the early mornings, and we discovered that other people had been doing this successfully for decades without any particular known mechanism. That chemical is easy enough for them to make and some plants (like willows) are absolutely full of it. Would not be surprised if they secreted it into the soil and if it bled over into adjacent plants. They also do not limit their interactions to other plants - I have seen research to the effect that most plants actively secrete sugars into the soil around their roots to attract bacteria which break down minerals and nutrients into forms they can absorb, and that they actively allow many symbiotic fungi into their roots without mounting immune responses.

Harry froze in sudden realization just as the forkful of carrots was about to enter his mouth.

That couldn't, couldn't possibly be true, surely no wizard would be stupid enough to do THAT...

And Harry knew, with a dreadful sinking feeling, that of course they would be that stupid. Salazar Slytherin had probably never considered the moral implications of snake intelligence for even one second, just like it hadn't ever occurred to Salazar that Muggleborns were intelligent enough to deserve personhood rights. Most people just didn't see moral issues at all unless someone else was pointing them out...

"Harry?" said Terry from beside him, sounding like he was afraid he would regret asking. "Why are you staring at your fork like that?"

"I'm starting to think magic should be illegal," said Harry. "By the way, have you ever heard any stories about wizards who could speak with plants?"

How much reliable is the International Journal of Parapsychology?

And yet those horrible vegetarians continue to murder & eat these sentient lifeforms!

The discussion seems to be interesting, maybe we can use the same criteria to decide whether plants are intelligent that are used to decide whether computers are intelligent?

If plants are intelligent without neurons that raises the likelihood that there something to human intelligence that also beyond neurons. As a result head cryonics is less likely to offer full recovery of human minds.