The Collaborative International Dictionary of English v.0.48:

Photodynamics \Pho`to*dy*nam"ics\, n. [Photo- + dynamics.]
   (Plant Physiol.)
   The relation of light to the movements of plants and their
   organs; the study of the phenomena of curvatures induced by
   the stimulus of light. -- Pho`to*dy*nam"ic,
   Pho`to*dy*nam"ic*al, a.
   [Webster 1913 Suppl.]