From Wikipedia:

Well, I do not seem how Wikipedia's definition is different from mine's. Wikipedia just gives an anthropocentric definition of intelligence.

So to speak,

Or as I said, "...intelligence (at the level of an organism) is the ability to percieve internal and external cues, computete them and react accordingly to ensure the organism's survival..."

as well as:

Or as I said, "...and well-being.."

A plant's intelligence is therefore its ability to percieve changes in weather, temperature, humidity, soil composition, sunlight and direction of it, daylength, presence of pathogens, presence of allies etc etc. in order to ensure its survival and thriving. 

Humans ability to reason, to plan, to solve problems, to think abstractly, to comprehend ideas, to use language, and to learn (collectively, the "intelligence" ) are simply the humans' tools for survival and therefore not much different from a plant's intelligence in their very core definition!