FAR RED REVERSAL OF INTERNODE‐STIMULATING EFFECT OF RED LIGHT ON PEAS

THOMSON, BETTY F. (Connecticut College, New London.) Far red reversal of internodestimulating effect of red light on peas. Amer. Jour. Bot. 46(10): 740-742. Illus. 1959.-Irradiation with red light accelerates the elongation of extremely young pea internodes, reduces the elongation of partly elongated internodes, and has a little of both effects on internodes of intermediate ages. All of these effects are reversed by far red irradiation. The possible nature of the accelerating effect is discussed. IT IS well known that internodes exposed to red light during the course of their rapid elongation attain a shorter length than similar dark-grown internodes. This effect is reversed if the red light treatment is followed promptly by exposure to far red (Hendricks et al., 1956). It has also been shown (Thomson, 1951) that white light given very early in the development of an internode, well before the period of rapid elongation, does not inhibit but rather accelerates the course of elongation. Exposures at intermediate stages bring about a lesser degree of both effects. The experiments reported here were undertaken to determine whether the early stimulating effect of light is brought about by the red region of the spectrum and, if so, whether it is reversed by subsequent far red irradiation. METHOD.-Plants were grown in an air-conditioned laboratory maintained at 28?C. where the only illumination consisted of dim, morphogenetically inactive light from green fluorescent tubes (Sylvania) wrapped in green and amber cellophane (DuPont). Plants were exposed to this only briefly while they were being manipulated. Irradiations were carried out in a separate dark room. The red light source consisted of red fluorescent tubes with a maximum emission in the vicinity of 6500 A. giving an energy level at the surface of the substrate of approximately 500 ergs/cm.2/sec. The far red irradiation consisted of light from white incandescent bulbs filtered through water and a combination of red and blue cellophane; energy received from this at the surface of the substrate approximated 20,000 ergs/cm.2/sec.2 Pea seeds of a dwarf variety, 'Laurel,' were soaked for 3 hr. in distilled water, then planted in moist, washed vermiculite. When the seedlings were 3 days old, well-grown plants with epicotyls 15-20 mm. tall were selected and transferred to pint-sized plastic containers filled with fresh vermiculite. 'Received for publication May 10, 1959. This work was done at Yale University during the tenure of a National Science Foundation Faculty Fellowship. I should like to thank the members of the Yale Botany Department for their cordial hospitality and particularly Dr. A. W. Galston for use of the facilities of his laboratory and for his interest in the progress of this work. 2 Energy levels were determined by Walter F. Bertsch, whom I would like to thank for this assistance. Each experimental unit consisted of the 25 plants growing in 1 such box. At the time of irradiation, damaged or retarded individuals were discarded from all boxes. Exposures were made when the plants were either 4 or 6 days old. Experimental groups were exposed to red light for 90 min. and the red-far red groups were exposed immediately afterwards to far red for 30 min. All plants, including a dark control series for each experiment, were kept together on the same shelf except during the irradiation period. One group of dark-grown plants was harvested at the time of irradiation, and harvests of all groups were made at 1-day intervals from 6-10 days for plants exposed at 4 days and from 7-11 days for those exposed 6 days. At the time of harvest, all internodes 1?2 mm. or more long were measured macroscopically to the nearest mm. Each experiment was run twice, and since the results of separate runs were very much alike, data from the 2 are grouped together for presentation here. RESULTS.-Results of the experiments are shown graphically in fig. 1 and 2. Standard errors of the means do not exceed 2.0 mm. and are too small to show on the scale plotted. Differences between the means for red-grown and dark control plants are significant; those for red-far reds and dark controls are not. The results show clearly that the response of an etiolated internode to red light depends on its age at the time when the plant is exposed. A mature, ungerminated pea seed contains detectable primordia of 6 leaves in addition to the cotyledons, and hence the rudiments of 6 internodes, each younger than the ones below it. Three days after germination, when plants are large enough to select and expose uniformly, the first 2 internodes have developed to the point where light has only the familiar inhibition effect. Light at 4 days has a similar effect, and at 6 days both these internodes have finished elongating, even in darkness. All the younger internodes show different responses to red irradiation at different times. The third internode is inhibited by light at 6 days, when it has attained about 30% of its final dark length. Exposure 2 days earlier, when this internode is only about 1.5 mm. long, causes a reduction in the