Plant's biological rhythm (5)
I believe the answer is:
pulse
I believe this is a double definition.
'plant's' is the first definition.
(as in a plant which produces pulses)
'biological rhythm' is the second definition.
(Other definitions for pulse that I've seen before include "beating" , "Heart rate" , "Beat or throb of heart" , "Edible seed from various pod plants" , "Excitement" .)