Effects of light flashes on the dark closure of Albizzia julibrissin pinnules

An electronic flash unit is used to deliver, at the beginning of a 10 min dark period and within a few ms, large doses of light to Albizzia julibrissin pinnules, to ascertain their effects on the rate of pinnule closing. In a series of alternating light flashes at 710 and 550 nm, the first 710 nm light flash significantly retards closing. A following light flash at 550 nm negates the far-red induced delay. The second 710 nm light flash delays closing less effectively than the first when given within 4 s after the green flash, but is just as effective when given after 30 s. The delay brought about by the second 710 nm light flash is again abolished by a light flash at 550 nm. A light flash at 660 nm has no effect on pinnule closing by itself and is also ineffective in reversing the far-red induced delay. A series of ten 710 nm light flashes becomes most effective in delaying closure when there is a dark interval of one min between flashes. The closing delay induced by a 710 nm light flash escapes reversal by a 550 nm light flash when the dark interval between the two flashes exceeds 2–3 min. A 750 nm light flash has no retarding effect on pinnule closing, but it becomes effective when preceded by a 660 nm or 550 nm light flash. The results obtained are suggested to be due to light absorbed by phytochrome and an unknown photoreceptor with green, far-red photoreversal property.     

Are two photoreceptors involved in the flowering of a long-day plant?

The effect of daylength extension with narrow spectral bands on the flowering of a long-day plant, Brassica campestris L. cv. Ceres, was investigated to obtain clues to the identity of the photoreceptor involved. Extension of a 9 h photoperiod with 5 h of light pulses at various wavelengths resulted in maximal flowering occurring after irradiation at 710 nm, less at 730 nm, and none at 550, 660 and 750 nm. Flowering at 710 and 730 nm was negated by simultaneous exposures at 550 nm, but not at 660 nm. A short preirradiation at 660 nm enabled a following irradiation at 750 nm to induce flowering. This latter induction was prevented by 550 nm irradiation.
Short flashes of light at 710 nm induced flowering that was negated by a following flash at 550 nm but not at 660 nm. The negation by 550 nm radiation was prevented by subsequent flashes at 710 nm, indicating photoreversibility. A flash at 660 nm enabled subsequent light flashes at 750 nm to initiate flowering that was reversed by a following 550 nm flash.
From the results showing the necessity of red and far-red lights, it is proposed that flowering in this long-day plant is due to two photoreceptors - one is phytochrome and the other an unknown pigment with far-red, green photoreversible properties. By using fluence response data, it is deduced that the unidentified photoreceptor has weak absorption bands in the far-red, but has a strong absorption band in the green. Flowering is induced when effects of red light absorbed by phytochrome interact with effects of far-red light absorbed by the unidentified photoreceptor.     

AN INTERPRETATION OF DOSE-RESPONSE CURVES FOR LIGHT-INDUCED CELL ELONGATION IN FERN PROTONEMATA

Protonemata of Onoclea sensibilis and Diyopteris filix-mas elongate in response to both red and far-red light. The promotion caused by far-red is larger than that caused by red light. This phenomenon differs from a typical response to phytochrome, the photoreceptor pigment immediately suggested by the activity of red and far-red light. The phenomenon has been explained by two different hypotheses, one of which holds that phytochrome is solely responsible for the response, whereas the other postulates an interaction between phytochrome and P₅₈₀, a yellow-green light absorbing pigment, to account for the response. The hypothesis that phytochrome is the sole photoreceptor leads to some specific predictions concerning the shapes of the dose-response curves for light-induced protonema elongation. These predictions were tested with both continuous and short-term irradiation. In all instances saturating far-red light caused greater elongation than did saturating red light, and no dose of red light duplicated the activity of saturating far-red light. Other experiments tested the interactions of red and far-red light and the effects of different doses of yellow-green light on protonema elongation. The results of many of the experiments were not in agreement with the hypothesis that phytochrome is the sole photoreceptor, whereas they were in agreement with the assumption that filament elongation is controlled by both phytochrome and P₅₈₀.     

Interaction of phytohormones and far-red irradiation on the nyctinastic closing of Albizzia julibrissin pinnules

In order for far-red radiation at 760 nm to delay dark closing of Albizzia julibrissin pinnules, red light must be given simultaneously with or just prior to it. Studies have been made to determine whether a phytohormone can replace this red light requirement. Abscisic acid, gibberellin, kinetin, and indole-3-acetic acid have been found to replace the red light. Indole-3-carboxylic acid and a cytokinin antagonist are ineffective. In this hormone and far-red interaction, all hormones are effective at μ M or lower concentrations. The hormones show no interaction with red light at 660 nm. Simultaneous irradiation at 550 nm negates the effect of hormone and far-red interaction in delaying leaflet closing. These results are additional evidence that an unidentified far-red absorbing pigment could be involved with phytochrome in some far-red-mediated processes.     

Role of boron in the far-red delay of nyctinastic closure of albizzia pinnules

Boron has been found to be necessary for the delaying action of 710 nanometer irradiation on the nyctinastic closing of Albizzia julibrissin pinnules. It is effective only over a narrow micromolar range. In contrast, the delaying action of 470 nanometer irradiation on closure is inhibited by boron. The effect of boron on leaflet closing occurs rapidly. Boron is suggested to be involved in a phytochrome action that precedes and is required for a delaying effect on pinnule closing subsequently induced by an unknown far-red absorbing pigment.     

Action spectrum and characteristics of the light activated disappearance of phytochrome in oat seedlings

The action spectrum for the light-activated destruction of phytochrome in etiolated Avena seedlings has been determined. There are 2 broad maxima, one between 380 and 440 mμ, the other between 600 and 700 mμ. peaking at about 660 mμ. On an incident energy basis, the red region of the spectrum is more efficient than the blue by about one order of magnitude in activating phytochrome disappearance. Both the red absorbing as well as the far-red absorbing forms of phytochrome are destroyed after exposure of Avena seedling to either red or blue light.

From the action spectrum and photoreversibility of pigment loss, we conclude that phytochrome acts as a photoreceptor for the photoactivation of its metabolically-based destruction. We suggest that another pigment might also be associated with the disappearance of phytochrome in oat seedlings exposed to blue light.

     

Interactions of green or red light with blue light on the dark closure of Albizzia pinnules

The interactions of green or red light with blue light on the dark closing of Albizzia julibrissin Durazz. pinnules have been investigated. Irradiations at 430, 450 and 470 nm progressively delay dark closing with increasing photon fluence rates. Red or green light alone has no effect. However, when the blue fluence rate is low, both red and green light interact with it and increase the delaying effect of the blue light. When the blue fluence rate is high, green light interacts with it to negate some of the effectiveness of the blue light, while red light has no effect. This is similar to results obtained previously with far-red light. It is suggested that the same unidentified photoreceptor is operating in both the far-red and blue regions. The results also indicate the presence of a blue-only absorbing photoreceptor whose action is increased by phytochrome.     

Far-red light-induced changes in intracellular potentials of spinach mesophyll cells: interaction with red light

In green plants, the large bioelectric changes that photosynthetically active light stimulates make it difficult to observe electrical potential changes related to phytochrome photoconversion. As a first step towards distinguishing between photosynthetic and phytochrome effects, we showed that red light enhances far-red stimulated intracellular potential changes in spinach (Spinacia oleracea) leaf mesophyll cells.

For a dark-adapted leaf, the response to far-red light increased during the first 10 to 30 exposures of 2.5 minutes, after which it was constant. The intracellular potential depolarized by an average of 0.3 millivolts during each 2.5-minute far-red light period, and returned to the resting value during each subsequent dark period. Continuous supplementary red light (at 1-5% of the fluence rate of the far-red light that stimulated the depolarizations) increased the response to far-red 2- to 3-fold. Supplementary red light did not amplify the response to alternating 702 nanometers light and dark periods. The Emerson enhancement effect thus does not seem to explain amplification of the response to 730 nanometers light by supplementary red light. This does not prove that photosynthetic pigments are not involved in some other way.

     

Phytochrome-mediated effects of near-ultraviolet radiation in the induction of flowering in etiolated Lemna paucicostata T-101, a short-day plant

Induction of flowering of etiolated Lemna paucicostata Hegelm. T-101, a short-day plant, was inhibited by far-red (FR) or blue light (BL) applied at the beginning of a 72-h inductive dark period which was followed by two short days. In either case the inhibition was reversed by a subsequent exposure of the plants to near-ultraviolet radiation (NUV), with a peak of effectiveness near 380 nm. Inhibition by BL or FR and its reversion by NUV are repeatable, i.e., NUV is acting in these photoresponses like red light although with much lower effectiveness. Thus, it is considered that NUV acts through phytochrome and no specific BL and NUV photoreceptor is involved in photocontrol of floral induction on this plant.Abbreviations BL blue light - FR far-red light - NUV near ultraviolet radiation - P red-absorbing form of phytochrome - P_fr far-red absorbing form of phytochrome - R red light     

Wavelength Dependence in the Red and Far-Red for Induction of Phytochrome Increase in Mung Bean Hooks

The wavelength dependence for a radiation induced increase of phytochrome in mung bean hooks (Vigna radiata L.), preirradiated with red light, was determined between 640 to 800 nm. Radiation between 640 to 700 nm and 780 to 800 nm had little effect on phytochrome concentration in hooks pretreated with red. Two bands of far-red light, one at 710 nm and the other at 750 to 760 nm, were found to increase phytochrome content about four times. Besides the requirement for a photochemical process, one or more dark processes appear to be necessary for the induction of phytochrome increase.     

Evidence for phytochrome involvement in light-mediated stomatal movement in Phaseolus vulgaris L.

Observations made with primary leaves of Phaseolus vulgaris L. demonstrated that phytochrome modulates light-induced stomatal movement. Removal of the far-red-absorbing form of the pigment (Pfr) with far-red (FR) radiation decreased the time required by the stomata to reach maximal opening following a dark-to-light transition; this effect of FR was fully reversible with red. Removal of Pfr with FR also decreased the time required to reach maximal closure following a light-to-dark transition, and the rate of closure was dependent on the final irradiation treatment before darkness. No evidence was found for phytochrome involvement in determining stomatal aperture under constant conditions of either darkness of light.Abbreviations and symbols Chl chlorophyll - D darkness - FR far-red - phytochrome photostationary state - Pfr, Pr FR- and R-absorbing forms of phytochrome, respectively - R red     

Phytochrome intermediates and action spectra for light perception by dry seeds

It has previously been demonstrated that far-red irradiation of dry Lactuca sativa L. seeds results in inhibition of subsequent germination. Although red has no effect on dry seeds, a red irradiation following a farred irradiation reverses the effect of far-red. This phenomenon is most noticeable in seeds with artificially raised levels of phytochrome in the far-red absorbing form. Qualitatively similar results have been found for the seeds of Plantago major L., Sinapis arvensis L., and Bromus sterilis L. Action spectra studies on Plantago seeds show that the action peaks for promotion and inhibition of germination of hydrated seeds are at 660 and 730 nanometers, respectively. The action spectrum for inhibition of subsequent germination following irradiation of dry seeds is qualitatively and quantitatively similar to that for hydrated seeds, with an action peak at 730 nanometers, indicating absorption by phytochrome in the far-red absorbing form. However, the action spectrum for the reversal of this far-red effect on dry seeds has a broad peak at 680 nanometers and subsidiary peaks at 650 and 600 nanometers. It is proposed that this effect is due to light absorption by the phytochrome intermediate complex meta-Fa, and that the action spectrum reflects the in vivo absorption properties of this intermediate.     

Interaction between Green and Far-Red Light on the Low Fluence Rate Chloroplast Orientation in Mougeotia

Continuous irradiation of Mougeotia with linearly polarized green light (550 nanometers, 0.2 watt per square meter) induces a change in the orientation of its chloroplast from profile to face position, if the electrical vector of the green light is vibrating normal to the cell axis. This change is complete within 25 minutes of the onset of irradiation. In contrast, if the electrical vector of the green light is parallel to the cell axis, no chloroplast reorientation is induced, even with a fluence rate as high as 3 watts per square meter. Furthermore, unpolarized far-red light (727 nanometers, 2 watts per square meter) given alone has no effect on chloroplast reorientation. Simultaneous and continuous irradiation with polarized green light, regardless of its plane of polarization, together with unpolarized far-red light, however, does lead to chloroplast reorientation. These data indicate that, in addition to the red-absorbing form of phytochrome, there exists in Mougeotia another sensory pigment absorbing green light.     

Action Spectrum for Resetting the Circadian Phototaxis Rhythm in the CW15 Strain of Chlamydomonas: I. Cells in Darkness

We have developed protocols for phase shifting the circadian rhythm of Chlamydomonas reinhardtii by light pulses. This paper describes the photobiology of phase-resetting the Chlamydomonas clock by brief (3 seconds to 15 minutes) light pulses administered during a 24 hour dark period. Its action spectrum exhibited two prominent peaks, at 520 and 660 nanometers. The fluence at 520 nanometers required to elicit a 4 hour phase shift was 0.2 millimole photon per square meter, but the pigment that is participating in resetting the clock under these conditions is unknown. The fluence needed at 660 nanomoles to induce a 4 hour phase shift was 0.1 millimole photon per square meter, which is comparable with that needed to induce the typical low fluence rate response of phytochrome in higher plants. However, the phase shift by red light (660 nanometers) was not diminished by subsequent administration of far-red light (730 nanometers), even if the red light pulse was as short as 0.1 second. This constitutes the first report of a regulatory action by red light in Chlamydomonas.     

Blue-light promotion of flowering is absent in hy4 mutants of Arabidopsis

Loss of a blue-light photoreceptor in the hy4 mutants of Arabidopsis thaliana (L.) Heynh substantially delayed flowering (>100 d to flower vs. 40–50 d), especially with blue light exposure from lamps lacking much red (R) and/or far-red (FR) light. Red night breaks were promotory but flowering was still later for the hy4-101 mutant. However, with exposure to light from FR-rich lamps, flowering of all mutants was early and no different from the wild type. Thus, flowering of Arabidopsis involves a blue-light photoreceptor and other, often more effective photoreceptors. The latter may involve phytochrome photoresponses to R and FR, but with little or no phytochrome response to blue wavelengths.Abbreviations HIR high irradiance response - FR far-red - R red - WT wild type     

An unusual effect of the far-red absorbing form of phytochrome: Photoinhibition of seed germination inBromus sterilis L.

Seeds ofBromus sterilis L. germinated between 80–100% in darkness at 15° C but were inhibited by exposure to white or red light for 8 h per day. Exposure to far-red light resulted in germination similar to, or less than, that of seeds maintained in darkness. Germination is not permanently inhibited by light as seeds attain maximal germination when transferred back to darkness. Germination can be markedly delayed by exposure to a single pulse of red light following 4 h inhibition in darkness. The effect of the red light can be reversed by a single pulse of far-red light indicating that the photoreversible pigment phytochrome is involved in the response. The response ofB. sterilis seeds to light appears to be unique; the far-red-absorbing form of phytochrome (Pfr) actually inhibiting germination.Abbreviations Pr red absorbing form of phytochrome - Pfr far-red absorbing form of phytochrome     

Action Spectrum for Interaction between Visible and Far-Red Light on Face Chloroplast Orientation in Mougeotia

The orientation of chloroplasts from profile to face position in Mougeotia can be controlled in two ways: by a typical phytochrome-mediated system or by continuous, simultaneous irradiation with far-red and visible light. In experiments with dichromatic irradiation of Mougeotia, the light conditions applied prevented the formation of a far-red-absorbing form of phytochrome gradient in the cell. An unpolarized background of far-red light and linearly polarized monochromatic light of different wavelengths and vibrating parallel to the cell axis, if given by themselves, were completely ineffective in producing any changes in chloroplast orientation. Given together, however, changes in chloroplast orientation were induced. The action spectrum for this interaction between constant far-red and variable visible light was maximal at 620 nanometers. The chloroplast response in these dichromatic light conditions required a prolonged duration of exposure to simultaneous continuous irradiation of high fluence energy. The vibrating plane of linearly polarized 620 nanometer light had no significant influence on interaction with far-red light in chloroplast movement. The results obtained are different from the typical low energy phytochrome-mediated chloroplast orientation. This new type of chloroplast photoresponse might be mediated by an unknown sensory pigment.     

The Effect of Colored Light on Pigment Synthesis in Cultured Fern Leaf Primordia

The effect of white, blue, yellow, red and far-red light on the quantitative synthesis of the primary and auxilliary photosynthetic pigments in cultured leaf primordia of Osmunda cinnamomea L. is reported. The P₆₆₀ form of the now classical photoreceptor pigment system, phytochrome, has been demonstrated to be active in chlorophyll synthesis in cultured cinnamon fern leaf primordia as shown by red/far-red reversibility of chlorophyll synthesis. Also, it is apparent from the data presented that a blue absorbing pigment (P₄₂₀) is responsible for the extensive accumulation of chlorophylls and carotenoids in these cultured leaves.     

Is There a Third Photoreceptor Involved in the Control of Chloroplast Movements in Mougeotia?

The photometric method was used to test a possibility proposed recently that a new photoreceptor with maximum activity at 620 nm is involved in mediating chloroplast rotation in Mougeotia (Z Lechowski, J Bialczyk [1988] Plant Physiol 88: 189-193). The hypothesis was tested under conditions of continuous dichromatic unilateral or mutually perpendicular irradiation with red light of wavelengths 620 or 660 (680) nanometers and far-red. When the red light was polarized parallel to the long cell axis, chloroplast response could be monitored by changing the direction of far-red irradiation. The level of the response obtained with red and far-red applied from the same direction depended on far-red intensity: at higher fluence rates the maximum response was shifted to longer wavelengths of red light. A high fluence rate of far-red inhibited the response. The absorption coefficients of Mougeotia chloroplasts were measured for the studied wave-lengths using the microphotometric method. Possible impact of absorption by the chloroplast on photoreception has been discussed. Current and previous results can be interpreted in terms of phytochrome action and do not support the involvement of the hypothetical 620 nanometer photoreceptor.     

Evidence for Involvement of Phytochrome in Tumor Development on Plants

The regulation of nonpathogenic tumorous growths on tomato plants by red and far-red radiation was studied using leaf discs floated on water and irradiated from beneath. It was found that red light (600-700 nanometers) was required for the induction of tumors on tomato (Lycopersicon hirsutum Humb. & Bonpl. Plant Introduction LA 1625), while both blue (400-500 nanometers) and green (500-600 nanometers) light had little effect on tumor development. Detailed studies with red light demonstrated that tumor development increased with increasing photon flux and duration, though duration was the more significant factor. It was observed that tumor development could be prevented by the addition of far-red irradiance to red irradiance or by providing far-red irradiance immediately following red irradiance. The effectiveness of red and farred irradiance in the regulation of tumor development indicates phytochrome involvement in this response. These findings should provide additional insight into the multiplicity of physiological factors regulating the development of nonpathogenic tumorous growths in plants.