Hormonal sentience, first described by Robert A. Freitas Jr., describes the information processing rate in plants, which are mostly based on hormones instead of neurons like in all major animals (except sponges).
Acacia trees produce tannin to defend themselves when they are grazed upon by animals.
There exists some sort of communication between a fungus garden and workers of the leaf-cutting ant Atta sexdens rubropilosa.
The Venus flytrap, during a 1- to 20-second sensitivity interval, counts two stimuli before snapping shut on its insect prey, a processing peak of 1 bit/s.
Plants generally take hours to respond to stimuli though, so vegetative SQs (Sentience Quotient) tend to cluster around -2.