Diaphototropic Responses to Polarized Light in the Solar-tracking Leaf of Lupinus palaestinus Boiss. (Fabaceae)

Abstract

Summary The diaphototropic response of the palmately compound leaf lamina of Lupinus palaestinus exhibits dichroic effects in plane polarized light. The lamina discriminates between two opposite oblique (non-polarized) beams, maintained at an equivalent angle of incidence (30°) transverse to its plane of symmetry, when their fluence rate ratio is 0.8 (50 and 40 μmol m -2 s -1 , respectively), by reorienting towards the stronger beam. When fluence rate of the two beams is equal (50 μmol m -1 s -1 ), but one is polarized parallel to their common plane (∥) while its opposite is polarized transversely to that plane (⊥), the lamina reorients towards the ∥ beam and away from the ⊥ beam. When the fluence rate of the ∥ beam is reduced, while that of the ⊥ beam is maintained constant, the angular velocity of laminar reorientation is smaller. Further reduction of the ∥ fluence rate eventually results in reversal of the direction of laminar reorientation, at a progressively increasing angular velocity. The relationship between the diaphototropic response and the ∥/⊥ fluence rate ratio is linear, with equilibrium between the two beams at a value of 0.586, suggesting that the ∥ beam may be ≈ 70 % more effective than the ⊥ beam.