Good question, LJM1979! Coincidentally, I was thinking about this just a few days ago.
I haven't researched the number of plants in the world, though I would guess it's at least a few orders of magnitude more than the number of animals, especially if you count phytoplankton.
Hedonic Treader described well the case one might make for plant pain: "functional plant movement in response to integrity damage, and corresponding movement-controlling information processing of some sort." This seems like a far cry from consciousness, but given how little we know about these things, the probability is, as Hedonic Treader said, much bigger than 1 in a trillion.
Still, the probability is a lot lower than for insects. Whether the reduced probability of plant sentience is enough to make expected plant pain lower than expected insect/zooplankton pain is unclear. Another consideration, as Hedonic Treader pointed out, is that plants respond much slower than animals, so maybe the
effective clock speed for plants is (orders of magnitude?) slower than for animals, which might be enough to dampen out their otherwise potentially massive expected value.
I don't know if plants would have net positive lives in the wild. Like with animals, most plant offspring probably die soon after pushing out of their seeds. Only a lucky few mature to adulthood, I would guess. Plants can also have stresses due to hostile weather, lack of food, drought, being eaten alive by herbivores, etc.
So I'm personally doubtful about LJM1979's suggestion that plant sentience could reverse the balance of pleasure/pain in nature. At least it would add more uncertainty, but even a priori, it's not clear why we'd assume that plant lives are net good (especially since the lives of the organisms we do know about -- small, short-lived animals -- are probably net bad). Sure, plants look peaceful, but who knows what kind of stress, fear, or deprivation of basic needs is going on inside their "heads" (internal communication systems). (Yes, that sentence was mainly tongue-in-cheek with wording, but the point was serious.
)
If plant lives aren't worth living, a lot of the interventions that we would support to reduce short-lived r-selected wild-animal populations would also reduce plant populations. Putting up parking lots, reducing plankton populations, palliating climate change, desertification, and land salination are things that tend to reduce not only animal populations but also plant populations. In general, a first-order approximation for the amount of animal life in an ecosystem is the amount of plant life, although it's possible this isn't always true. For example,
eutrophication may increase plant biomass while decreasing animal biomass, but this suggestion remains counterintuitive and hence speculative.
As far as diets, the standard reply is that most meat production requires killing many times more plants than eating plants directly, so even if plants feel pain, it's better to be veg. This is probably true even when we consider wild animals because global warming will probably increase plant abundance in the long term.
Whether plants suffer or not also doesn't change the thrust of
my futurism concerns: Terraforming, panspermia, and sentient simulations will create at least as much plant life as animal life.
All in all, the things I'm working on aren't much affected if plants are sentient. Regardless, the issue doesn't keep me up at night, and I haven't formally included it in my calculations. The reason may be because I might decide that I just don't care about the computations that plants do as being conscious suffering. I'm
allowed to do this if I want to.
Now, if you show that the operations that plants do is really similar to operations done by animals that I care about, I may change my mind, but I think it's pretty unlikely the correspondence will be that strong.
As far as David Pearce's comments, I hold a very
different view. I don't believe there is a "hard" binding problem or that quantum mechanics is at all relevant. But I still agree with David on many things and owe a great deal to him; this is just a friendly intellectual disagreement.
by 
!
).
