Here's an excerpt from an interesting article I read a while back. The article happens to be referring to veganism, but the remarks about plants and sentience apply here and are interesting.
"...One of the questions most frequently asked of any vegan is: “what about plants?”
Indeed, I do not know any vegan who has not gotten that question at least once and most of us have heard it many times.
Of course, no one who asks this question really thinks that we cannot distinguish between, say, a chicken and a head of lettuce. That is, if, at your next dinner party, you chop a head of lettuce in front of your guests, you will get a different reaction than if you were to carve a live chicken. If, while walking in your garden, I step on a flower intentionally, you may quite correctly be annoyed with me, but if I intentionally kicked your dog, you would be upset with me in a different way. No one really thinks of these as equivalent acts. Everyone recognizes that there is an important difference between the plant and the dog that make kicking the dog a morally more serious act than stepping on a flower.
The difference between the animal and the plant involves sentience. That is, nonhumans—or at least the ones we routinely exploit—are clearly conscious of sense perceptions. Sentient beings have minds; they have preferences, desires, or wants. This is not to say that animal minds are like human minds. For example, the minds of humans, who use symbolic language to navigate their world, may be very different from the minds of bats, who use echolocation to navigate theirs. It is difficult to know. But it is irrelevant; the human and the bat are both sentient. They are both the sorts of beings who have interests; they both have preferences, desires, or wants. The human and the bat may think differently about those interests, but there can be no serious doubt that both have interests, including an interest in avoiding pain and suffering and an interest in continued existence.
Plants are qualitatively different from humans and sentient nonhumans in that plants are certainly alive but they are not sentient. Plants do not have interests. There is nothing that a plant desires, or wants, or prefers because there is no mind there to engage in these cognitive activities. When we say that a plant “needs” or “wants” water, we are no more making a statement about the mental status of the plant than we are when we say that a car engine “needs” or “wants” oil. It may be in my interest to put oil in my car. But it is not in my car’s interest; my car has no interests.
A plant may respond to sunlight and other stimuli but that does not mean the plant is sentient. If I run an electrical current through a wire attached to a bell, the bell rings. But that does not mean that the bell is sentient. Plants do not have nervous systems, benzodiazepine receptors, or any of the characteristics that we identify with sentience. And this all makes scientific sense. Why would plants evolve the ability to be sentient when they cannot do anything in response to an act that damages them? If you touch a flame to a plant, the plant cannot run away; it stays right where it is and burns. If you touch a flame to a dog, the dog does exactly what you would do—cries in pain and tries to get away from the flame. Sentience is a characteristic that has evolved in certain beings to enable them to survive by escaping from a noxious stimulus. Sentience would serve no purpose for a plant; plants cannot “escape.”
I am not suggesting that we cannot have moral obligations that concern plants, but I am saying that we cannot have moral obligations that we owe to plants. That is, we may have a moral obligation not to cut down a tree, but that is not an obligation that we owe to the tree. The tree is not the sort of entity to which we can have moral obligations. We can have an obligation that we owe to all of the sentient creatures who live in the tree or who depend on it for their survival. We can have moral obligations to other humans and nonhuman animals who inhabit the planet not to destroy trees wantonly. But we cannot have any moral obligations to the tree; we can only have moral obligations to sentient beings and the tree is not sentient and has no interests. There is nothing that the tree prefers, wants, or desires. The tree is not the sort of entity that cares about what we do to it. The tree is an “it.” The squirrel and the birds who live in the tree certainly have an interest in our not chopping down the tree, but the tree does not. It may be wrong morally to chop down a tree wantonly but that is a qualitatively different act from shooting a deer.
Talking about the “rights” of trees, as some do, is to invite equating trees and nonhuman animals and that can only work to the detriment of the animals. Indeed, it is common to hear environmentalists talk about our responsibly managing our natural resources and including nonhuman animals as a “resource” to be managed. That is a problem for those of us who do not see nonhumans as “resources” for our use. Trees and other plants are resources that we can use. We have an obligation to use those resources wisely, but that is an obligation that we owe only to other persons, be they human or nonhuman...."