Assessment of the potential of the blue light gradient in soybean pulvini as a leaf orientation signal

Blue light gradients in the pulvini of soybean (Glycine max var. Northrup King S1346) leaves with different laminar orientations were examined with a fiber optic microprobe. The gradients changed markedly as a function of both incident light angle and leaf position and were determined largely by the amount of light present in the adaxial side of the pulvinus. The steepest gradient for inclined leaves was with light incident at 90° whereas for declined leaves it occurred when the light was incident at 150°. A proposed pulvinar mechanism which allows for the detection of light direction and leaf position by using the blue light gradient as an orientation signal could account for solar tracking by soybean.