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Phototropic Leaflet Reorientation in Alfalfa in Response to Vectorial Light¹

The phototropic reorientation of alfalfa (Medicago sativa L.) leaflets in response to vectorial light involves a complex series of movements in which the elevational angle of the leaflets either increases or decreases, the compass direction of the leaflet midrib changes and the leaflet rotates about its midrib axis. The exact combination of movements depends upon the leaflet position on the plant and the azimuth and elevational angle of the vectorial light source. The photoreceptor for leaflet reorientation is sensitive to blue light, insensitive to red and far red light and is located in the pulvinule. The time required for leaflet reorientation was dependent upon the vectorial photosynthetic photon flux density (PPFD). At high PPFD (850 to 1600 µmol m^-2 s^-1) reorientation proceeded rapidly with the full response requiring 45 to 60 min. As the PPFD was decreased the rate of reorientation also decreased. At the lowest PPFD tested (100 µmol m^-2 s^-I) the response only occurred after a 2 h lag period but then proceeded at a rapid rate with the leaflet attaining its final position in 30 rain. Upon completion of the reorientation response the adaxial surface of the leaflet was nearly perpendicular to the light source. The final position was independent of PPFD.

Key Words: Medicago sativa (L.) • Solar tracking • Diaheliotropism • Paraheliotropism • Blue light response

² Postgraduate research associate and associate professor, Dep. of Agronomy and Range Sci., Univ. of Calif., Davis, CA 95616.

Received for publication October 1, 1983.