The Collaborative International Dictionary of English v.0.48:

Phototropism \Pho*tot"ro*pism\, n. [Photo- + Gr. ? to turn.]
   1. (Plant Physiol.) The tendency of growing plant organs to
      move or curve under the influence of light. In ordinary
      use the term is practically synonymous with
      heliotropism.
      [Webster 1913 Suppl.]

   2. (Microbiology) exhibiting movement in a direction toward
      (positive phototropism) or away from (negative
      phototropism) a source of light.
      [PJC]

WordNet (r) 3.0 (2006):

phototropism
    n 1: an orienting response to light