Botanical Gazette, 118(4): 199-208 (June, 1957)
Photoreversible Control of Elongation of Pinto Beans and Other Plants under Normal Conditions of Growth
R. J. Downs, S. B. Hendricks, H. A. Borthwick


1. Internodes of young seedlings of Pinto bean (Phaseolus vulgaris L.), grown under fluorescent lights, elongated to about three or more times their normal length in response to low radiant energies of unfiltered incandescent-filament light applied at the close of daily high-energy fluorescent-light periods. The lengthening response was due to the far-red radiant energy of the supplemental light and was completely and repeatedly reversible with red. Maximum lengthening occurred when the plants received cycles of about 4 hours of light and 20 hours of dark. Plants that received no daily high-energy period of light did not elongate in response to far-red treatments unless given sugar solution through their severed hypocotyls.

2. The lengthening response was found to result from the action of a photoreversible pigment which also controls flowering, seed germination, and certain other photoregulated phenomena. Experiments with bean internode elongation show that this pigment is present and functioning in the plant at all times; no period of preliminary light or darkness is required for its generation.

3. Plants of several other bean varieties, both bush and pole, were found to respond like Pinto to far-red and red treatments as also did sunflower (Helianthus annuus L.) and two species of ornamental morning-glory (Ipomoea hederacea Jacq. [var. 1] and Ipomoea var. Scarlett O'Hara [var. 2]). Elongation of internodes resulted primarily from light action on the internode itself, but possibility of translocation of an effect from the leaves was not excluded.

4. Wide differences in internode length were induced by different temperature treatments and by treatments with different concentrations of gibberellic acid. Regardless of the degree of lengthening induced by such treatments, further elongation resulting from far red was reversed by red given after far red.

5. An elongative influence was obtained from blue fluorescent lamps by removing the red with a blue cellophane filter. Elongation was minimized by removal of the far red with a liquid Cu(NO3)2 filter.

6. Internodes of a given mean length were produced in two ways: (a) by promotion with the correct amount of far-red energy and (b) by inhibition with the correct amount of red after initial saturation with far red. The length attained, regardless of the procedure, depended on a particular fraction of the active pigment being in the far-red-absorbing form and the rest in the red-absorbing form. A method was illustrated whereby the fractions of total pigment in each form were calculated from results of a simple physiological experiment and without knowledge of the chemical composition of the pigment.