H. A. Borthwick

Action of Light on Lettuce-Seed Germination

1. The observations of Flint and McAlister that imbibed seed of some lettuce varieties can be promoted in germination by irradiation in the red portion of the spectrum and inhibited in the infrared were verified. 2. The action spectra for promotion and inhibition were measured in detail for wave lengths greater than 4000 A. Maximum sensitivity for promotion was found in the region 6400-6700 A (red) and for inhibition in the region 7200-7500 A (infrared). 3. Absorption of radiation in the red or in the infrared region changes the effective pigment into the infrared- or the red-absorbing form, respectively. The alternation of form can be repeated many times. 4. The action spectrum for lettuce-seed germination is the same as that effective for photoperiodic control of floral initiation. The two phenomena involve the same initial photoreaction. 5. The photoreaction involves monomolecular isomerization of the effective pigment. 6. A reaction by which the pigment changes from the infrared- to the red-absorbing form occurs in darkness. This reaction was elsewhere found to be the one by which duration of darkness is measured in photoperiodic control of floral initiation. 7. Seed of one lettuce variety, Great Lakes, that did not require radiation for germination, became sensitive to radiation when held imbibed at 35⚬ C.

Action Spectrum for the Photoperiodic Control of Floral Initiation of Short-Day Plants

1. The purposes of this investigation were to obtain quantitative data on the photoreactions that prevent flowering of short-day plants, from which an action spectrum relating wave length to photoperiodic effectiveness of light could be derived, and to draw such inferences concerning the nature of the photoreactions as the action spectrum would permit. 2. Experiments designed to give the action spectrum made use of a specially designed prism spectrograph having a dispersion of 15 A. per cm. at 5000 A. At this wave length and with an effective slit width of 100 A. the energy was about 3000 ergs per sq. cm. per second with the slit illuminated by a carbon arc operated at 12 kw. input. 3. Plants investigated were soybean, Soja max (L.) Piper var. Biloxi, and cocklebur, Xanthium saccharatum Wallr. To facilitate irradiation, the foliar surface of the plants of each species at the beginning of an experiment was reduced to a single leaflet or leaf, respectively. 4. The experimental treatment was based on the fact that flowering of short-day plants receiving short photoperiods can be prevented by interrupting each of the accompanying long dark periods at or near the middle with a brief period of irradiation. In these experiments minimal energies required to prevent floral initiation were determined for many narrow regions of the spectrum. 5. The reciprocity law was tested and found to hold for the factors of time and intensity within the ranges used for dark-period interruption. Flowering response of both soybean and cocklebur depends on the total radiant energy used to interrupt the dark period. 6. Minimal energies necessary to prevent flowering depend upon the number of hours of darkness elapsing before the plants are irradiated but are constant over a 2-hour period beginning 30 minutes before the middle of the dark period. 7. Action spectra for soybean and cocklebur are similar in several respects. The limit of effectiveness of radiation for preventing flowering of both plants is near 7200 A., at the red end of the spectrum. Maximum effectiveness for both plants occurs over a broad region extending from about 6000 to 6800 A. A minimum of effectiveness of radiation for both plants occurs in the region of 4800 A., and effectiveness increases again at shorter wave lengths in the visible portion of the spectrum. 8. Ratios of energies required to prevent flowering in the regions of maximum and of minimum effectiveness were different for the two plants, being about 1:200 for cocklebur and 1:60 for soybean. 9. The possibility that chlorophyll may be the effective pigment in the photoperiodic reaction is examined.

Effects of Photoperiod on Growth of Trees

1. The effects of photoperiod on the growth of several tree species-American elm (Ulmus americana), flowering dogwood (Cornus florida), horse chestnut (Aesculus hippocastanum), red maple (Acer rubrum), sweet gum (Liquidambar styraciflua), tulip poplar (Liriodendron tulipifera), paulownia (Paulownia tomentosa), Asian white birch (Betula mandshurica), catalpa (Catalpa bignonioides and C. speciosa), and pine (Pinus taeda, P. virginiana, and P. sylvestris)-were investigated. 2. In general, short days induced dormancy, and long days prolonged growth. The various species differed in their response to short (8-hour) days, ranging from tulip poplar, which stopped further growth after about 10 8-hour days, to elm, which required 20 weeks of 8-hour days before the plants stopped elongating new structures. However, most of the species tested seemed to required about 4 weeks of 8-hour days before they stopped growing. At higher temperatures it took a greater number of 8-hour days to induce dormancy, and at temperatures lower than 70⚬ F. growth was inhibited even on 16-hour photoperiods. 3. Catalpa, elm, birch, red maple, and dogwood apparently can be kept growing continuously by daylengths of 16 hours, whereas paulownia, sweet gum, and horse chestnut cannot. The growth of pine seedlings is intermittent on 16-hour days, but on 14 hours they can be kept growing continuously for nearly a year. There seems to be a balance between bud formation and elongation of newly formed structures that is further illustrated by sweet gum, in which the inherent tendency to form a terminal bud is only slightly exceeded by the action of continuous light in maintaining continuous elongation. 4. The effects of different qualities of supplemental light on the growth of catalpa seedlings indicate that the same photochemical reaction controlling seed germination and flowering and growth of herbaceous plant parts also controls the onset of dormancy and the elongation of new structures of woody plants. 5. It is generally conceded that many tree species require a cold period to break the dormant condition. However, in some species, such as catalpa, full development of this cold requirement occurs gradually. Before this buildup of cold requirement reaches a level adequate to maintain dormancy, long photoperiods alone may induce the resumption of growth. In other species, such as dogwood, continuous light replaces the need for a cold period, whereas in birch the terminal bud apparently requires a cold period, but the axillary buds can be induced to resume growth by long photoperiods. 6. The dormant condition of some species appears to be centered in the leaves, since defoliation at warm temperatures will cause resumption of growth of plants like sweet gum and red maple. This is not universally the case, however, because defoliation of dormant plants of catalpa, horse chestnut, and dogwood seems to have no effect.

Action Spectrum for Photoperiodic Control of Floral Initiation of a Long- Day Plant, Wintex Barley (Hordeum vulgare)

1. Wintex barley grown with an 11.5-hour photoperiod and a 12.5-hour dark period remained vegetative. If the dark period was interrupted with a brief period of irradiation of sufficient intensity, spikelet formation was stimulated. 2. By use of this technique of interrupting the dark period, quantitative data on the photoreaction that promoted flowering were obtained, and action spectra relating wave-length to photoperiodic effectiveness of light were derived. 3. The most effective time to apply the dark-period interruptions was the 2-hour period beginning 6.5 hours after the start of the dark period. Within this time and with the intensities used, the reciprocity law held. Energy required to promote flowering, if applied continuously throughout the 12.5-hour dark period, was about tenfold greater than if applied within the 2 hours near the middle of the dark period. 4. The action spectrum for the production of spikes in barley was very similar to the action spectra for the prevention of floral initiation in soybeans and in cocklebur. The long-wave-length cut-off beyond 7200 A, the positions of maximum effectiveness between 6000 and 6600 A, and the region from 5000 to 5600 A, in which effectiveness changed rapidly with small changes in wave-lengths, coincided very closely. Minimum effectiveness occurred near 4800 A for all three plants. 5. The elongation of barley stems was closely correlated with spike development. 6. The action spectra indicated that essentially the same pigment was involved in transferring energy to the photoperiodic reaction both in long-day and in short-day plants. 7. A working hypothesis based on the assumption that flowering both in long-day and in short-day plants is controlled by the same substance and that effectiveness is due to optimum concentration is discussed.

Action Spectrum for the Photoperiodic Control of Floral Initiation of the Long-Day Plant Hyoscyamus niger

1. Vegetative plants of Hyoscyamus niger can be induced to flower by application of relatively small amounts of radiant energy near the middle of dark periods that would prevent flowering if uninterrupted. 2. Interruptions of 12-hour dark periods with energies corresponding to about 100 foot-candle-minutes of light were effective in causing initiation of flower primordia. Minimum energy to cause a given effect was relatively constant if the interruption was made during the 2-hour period following the middle of the dark period. 3. The action curve for floral initiation of H. niger was readily established in the region from 7300 A to 5600 A. Limitation of energy from 5600 A to 4000 A made it impossible to establish a continuous response curve for this region, but sufficient points were obtained to establish the limits of effectiveness. This curve is essentially the same as those for control of flowering in soybean, cocklebur, and barley, and for regulation of leaf size in etiolated peas. 4. Similarity of the action curve for control of initiation of flower primordia in Hyoscyamus to the absorption of phycocyanin was noted, and the possibility that the effective pigment for the photoperiodic reaction might be some type of straight-chain tetrapyrrole was pointed out.

Photocontrol of Plant Development by the Simultaneous Excitations of Two Interconvertible Pigments. III. Control of Seed Germination and Axis Elongation

Action spectra between 4100 and 8500 A for seed germination and radicle elongation were measured for Nemophila insignis and Lactuca sativa var. Great Lakes and between 6500 and 8500 A for seed germination of Lamium amplexicaule. The responses observed are features of the continued excitation of the photomorphogenic pigment in those regions of the spectra where both pigment forms have appreciable absorptivities.

Phytochrome Action in Tomato-Seed Germination

Dark-germinating seeds of Ace and Porte tomato varieties can be inhibited by far-red radiation. Germination of inhibited seed is promoted by red radiation and in turn is reversed by far-red radiation, indicative of control by phytochrome. In the Ace variety initial inhibition requires only a single short far-red exposure. Prolonged far-red irradiation extending over many hours is required for inhibition of Porte seeds. The prolonged exposure can be replaced by cyclical far-red irradiation for 3.3% of the total time. This effect is considered as arising in part from conversion of Pfr to Pr as phytochrome appears in the seed by rehydration. Effects are also evident of exhaustion of endogenous factors other than Pfr needed for germination when Pfr is maintained at a low level.

Failure of Photoreversible Control of Flowering in Pharbitis nil

1. Flowering of seedlings of Japanese morning-glory, Pharbitis nil Chois., grown in nutrient solution or in soil is inhibited by far-red and repromoted by red irradiations at the beginning of 16-hour dark periods and inhibited by both red and far-red irradiations at the middle of such dark periods. Older plants are not responsive to red or far-red irradiations at the beginning of the dark period, but, at the middle, red inhibits flowering and far red repromotes it. 2. Seedlings irradiated at the beginning of the dark period flower if at least one cotyledon is either not irradiated with far red or is irradiated with red after far red. Flowering of seedlings following irradiation at the middle of the dark period occurs only if one cotyledon is not irradiated. 3. Action spectra for control of flowering by irradiation of cotyledons were measured at the start and near the middle of inductive dark periods adequate to promote flowering. The energies required at the beginning of the dark period for half-saturation of the far-red and red responses are similar in value to those required for reversible control of flowering of other photoperiodic plants near the middle of inductive dark periods. 4. Approximately the same energies are required for control of flowering by red radiation at the beginning and near the middle of the inductive dark periods. The flowering response potentiated by red radiation, however, is not reversible by far-red radiation near the middle of the dark period and is opposite to the response upon the onset of darkness. About 4 hours in darkness are required for the plant to change from the one to the other type of response.

The role of light in suppressing hypocotyl elongation in lettuce and Petunia

SummaryHypocotyl elongation in two varieties of Petunia and in Grand Rapids lettuce is shown to be affected by a high-energy reaction and by phytochrome action. These two photoreactions interact in such a way that, on the one hand, shortening of the hypocotyls due to the high-energy reaction can be entirely masked by brief terminal far-red light treatment, while on the other hand, there is no evidence of phytochrome action unless brief exposures to red light are preceded by relatively long exposure of high-intensity.The action spectra for the high-energy reaction show peak effectiveness at wavelengths of 430–450 mμ, with a minor peak at 660 mμ in Comanche Petunia, at 700 mμ in Pink Cascade Petunia, and at 720 mμ in Grand Rapids lettuce.Prior treatment with DCMU did not reduce the effect of high-intensity light on hypocotyl lengths in lettuce.The nature of the high-energy reaction, and the relation between it and phytochrome action are discussed. Besides these two photoreactions there appears to be a direct effect of light on elongation, blue light preventing, and far-red light accelerating, elongation during actual exposure.

ROLE OF PHYTOCHROME IN CONTROL OF FLOWERING OF CHRYSANTHEMUM

1. Flowering of chrysanthemum, Chrysanthemum morifolium Ramat., is prevented by dark-period interruptions of 4 hours, which convert phytochrome to the flower-inhibiting far-red-absorbing form (Pfr) and maintain it in this form throughout the light period. 2. Pfr is also maintained at an effective level by intermittent lighting provided the dark periods between successive periods of light are so short that dark reversion of Pfr to an ineffective level does not occur. If the light is from incandescent-filament lamps, the dark periods between successive light breaks must not greatly exceed 30 minutes. Cycles of light and dark from 30 minutes to 1 minute are equally effective. 3. Effective cycle length depends on the ratio of red to far-red energies in the light. As the relative amount of red increases, the amount of Pfr formed and the time required for its reversion in darkness to a level ineffective for flower inhibition also increase. For these reasons light from ruby-red, incandescent-filament, and fluorescent lamps is about equally effective in 15-minute cycles, but the effectiveness of ruby-red light fails in 30-minute cycles and of incandescent-filament in 60-minute cycles, whereas that of fluorescent light remains high in longer cycles. 4. In chrysanthemum the time required for the flower-inhibiting reactions catalyzed by Pfr is greater than the dark reversion time of Pfr to an inactive form. This fact accounts for the need of prolonged periods each night of continuous or intermittent light, a condition also encountered in control of flowering in certain long-day plants, regulation of growth in woody species, and positioning of the chloroplast in the alga Mesotaenium.