photo-epinasty

from The Century Dictionary.

• noun In botany, an epinastic movement, or state of curvature observed in certain organs when exposed to intense light, due to a more active growth of the dorsal surface. Compare epinasty.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Bot.) A disproportionately rapid growth of the upper surface of dorsiventral organs, such as leaves, through the stimulus of exposure to light.