phototonus

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The state of being sensitive to or irritated by light.

from The Century Dictionary.

• noun In botany, a term proposed by Sachs for the peculiar condition in which the protoplasm is capable of exhibiting irritability induced in certain plant-organs by exposure to light of a certain intensity.

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

• noun (Bot.) A motile condition in plants resulting from exposure to light.
• noun (Physiol.) An irritable condition of protoplasm, resulting in movement, due to a certain intensity of light.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun sensitivity to, or irritation caused by light