The response to light color does not stand on its own. That is merely one parallel from many. It is true plants do not have a nervous system like animals, but they do have similar responses to stimuli. Parallels can be drawn to sight, sound/touch and smell/taste.
Sentience is another topic that is defined subjectively. From context it is clear you make a central nervous system a foundational requirement. I could also apply this to technology, so I would need clarification from you to understand what it means to you. I do not hold to a personal definition for sentience because I have found neither a universal nor scientific understanding of the idea.
As for the last paragraph: yup.
Again, all of these reactions to stimuli can be explained as direct, chemical reactions, not signals that get sent to a central unit, are processed, being “felt”, and then being reacted to. There is no one thing or being in plants like the central nervous system of animals that is capable of feeling something.
Regarding the topic of sentience, I propose looking at it like this:
There’s a range of definitions that is somewhere around it being the capacity to perceive, to be aware, to be/exist from ones own perspective. However you define it, a central nervous system or other type of similar central unit would have to be a requirement, because that is what would actually be sentient. You are your brain, your hand is just part of your body, if it was chopped off, it by itself is not sentient.
And whatever vague definition of it you go with, there’s two options: Either sentience is real, or it isn’t. If it isn’t real, literally nothing matters, gg. If it is real, non-human animals with central nervous systems, and therefore sentience and the capacity to suffer, deserve ethical consideration, and we should do what is reasonably possible to reduce their suffering and death.
Since we don’t know the answer to the existence of sentience, we should err on the side of caution. If we’re wrong, and we’re all as sentient as a rock, the inconvenience we’d have suffered in our efforts to protect fellow sentient-but-actually-not beings can’t be felt by us, no harm done. If we’re right, the suffering we’ll have prevented, in both scale and intensity, is indescribable.
However you define it, a central nervous system or other type of similar central unit would have to be a requirement, because that is what would actually be sentient
Without CNS there would be something else sophisticated enough to show sentience that would have been sentient. So to me it looks like this is not really a requirement, albeit it’s simpler to say that it is.
As a side note, I think that given how human-centric humans are (which is to be expected, really) even if we were living with another sentient species on the same planet we would argue they are not sentient for whatever reason we could come up with, and change sentience definition accordingly
