My first thought beyond "wow, what total bull" is that there is a serious philosophical battle that needs to be waged, or vacuities like "the dignity of wheat" will persist. Basically we need to get down to things broadly termed qualia - feelings, emotions, "subject experience" like color and so on. Daniel Dennett is a good place to start."Qualia" is a product of the computation power of the brain. So wheat does not have a nervous system, ergo has no computational power, ergo has no "qualia", subjective experience, the feeling of pain and so on. Once we establish all subject experience, all qualia, as a phenomenon of computation then cockamamie like the "dignity of all living things" will go away. Which, by the way, a "living thing" is roughly when you get down to it a molecule that can copy itself, so crystals are "living" too. Shall we now have a discussion on tax payer money about how much pain a crystal feels when we cut it?
I don't think you would change their mind even if you convinced them that plants have no feelings. A certain segment of society is dissatisfied with modern civilization; they are disappointed with how well a capitalist, competitive, technology-heavy society has turned out. In their worldview, the noble savage is the ideal - communal tribes living "in harmony" with nature. The modern world is evil, dirty, selfish. So, they will support whatever sentiments they can to oppose it.
Well, that simplification is not quite acceptable to me. The general definition of "living" involves more conditions. Wikipedia has a pretty good list of the 7 usual criteria. http://en.wikipedia.org/wiki/Life
I don't think crystals qualify. Still, in the end, I think I do agree with you; calling plants and\or crystals sentient life is pretty unfounded.