Leaf Orientation of Soybean Seedlings: II. Receptor Sites and Light Stimuli
- Raymon Donahue * and
- Virginia Seymour Berg*
Unifoliolate leaves of seedlings of soybean [Glycine max (L.) Merr. cv. Northrup King S1346] exhibit diaheliotropic movements when well watered and paraheliotropic movements when water stressed. The movements originate at the pulvinus at the base of each leaf lamina. The receptor site for the light stimulus was located by covering portions of the leaf surface with India ink and observing the effect on leaf movement; the leaf lamina, leaf veins, and pulvinus were tested. The pulvinus was the receptor site for light that stimulates both diaheliotropic and paraheliotropic leaf movement. The role of different wavelengths of light in diaheliotropism and paraheliotropism was determined using filters that remove broad bands of light wavelengths. Blue light was necessary for both diaheliotropism and paraheliotropism. Interference filters were used to expose soybean leaves to selected blue-light wavelengths. The amount of diaheliotropic leaf movement at each wavelength was used to obtain an action spectrum. There was one peak between 410 nd 440 nm and one between 470 and 490 nm. A nonphytochrome photoreceptor located in the pulvinus used blue light, in conjunction with other factors, to determine leaf orientation and thereby regulate light interception of leaves of soybean seedlings.
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