The Green Trail (1975)

Set in the USA, the plot concerns Thomas Norton, a researcher in the field of lie detection, who provides his services to courts and private industry. One day he idly attaches electrodes to a plant in his office, and is surprised to find it responds with recognisable emotional reactions to the stimuli he gives it. He pursues this research, and keeps a plant wired up in his lab. When a woman who lives in his building is mysteriously murdered in his lab, the plant is the only witness to the crime.