Personally, I’m more a fan of the binary/discrete idea. I tend to go with the following definitions:

• Animate: capable of responding to stimuli
• Sentient: capable of recognizing experiences and debating the next best action to take
• Conscious: aware of the delineation between self and not self
• Sapient: capable of using abstract thinking and logic to solve problems without relying solely on memory or hardcoded actions (being able to apply knowledge abstractly to different but related problems)

If you could prove that plants have the ability to choose to scream rather than it being a reflexive response, then they would be sentient. Like a tree “screaming” only when other trees are around to hear.

If I cut myself my body will move away reflexively, it with scab over the wound. My immune system might “remember” some of the bacteria or viruses that get in and respond accordingly. But I don’t experience it as an action under my control. I’m not aware of all the work my body does in the background. I’m not sentient because my body can live on its own and respond to stimuli, I’m sentient because I am aware that stimuli exist and can choose how to react to some of them.

If you could prove that the tree as a whole or that part of a centralized control system in the tree could recognize the difference between itself and another plant or some mycorrhiza, and choose to respond to those encounters, then it would be conscious. But it seems more likely that the sharing of nutrients with others, the networking of the forest is not controlled by the tree but by the natural reflexive responses built into its genome.

Also, If something is conscious, then it will exhibit individuality. You should be able to identify changes in behavior due to the self referential systems required for the recognition of self. Plants and fungi grown in different circumstances should respond differently to the same circumstances.

If you taught a conscious fungus to play chess and then put it in a typical environment, you would expect to see it respond very differently than another member of its species who was not cursed with the knowledge of chess.

If a plant is conscious, you should be able to teach it to collaborate in ways that it normally would not, and again after placing it in a natural environment you should see it attempt those collaborations while it’s untrained peers would not.

Damn now I want to do some biology experiments…