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     

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     

Action spectra for the inhibition of growth in radish hypocotyls

In etiolated seedlings of Raphanus sativus L. the inhibition of hypocotyl elongation by continuous light showed a major bimodal peak of action in the red and far-red, and two minor peaks in the blue regions of the spectrum. It is argued that, under conditions of prolonged irradiation, phytochrome is the pigment controlling the inhibition of hypocotyl elongation by red and far-red light, but that its mode of action in far-red is different from that in red. A distinct pigment is postulated for blue light.Abbreviations B blue - FR far red - G green - R red - HIR high irradiance reaction - P_r and P_fr red and far red absorbing forms of phytochrome - R red     

Phytochrome-mediated germination control of maize caryopses

Maize caryopses sown in water germinate equally well either in darkness or under any light regime. However, when they are imbibed in mannitol solutions, continuous far-red light proves to be strongly inhibitory on the final germination as compared to darkness. Similar but less pronounced inhibition is also exhibited by continuous red or blue light. Intermittent far-red light can partially substitute for continuous far-red light in inhibiting maize caryopsis germination, and its effect is reversed to the intermittent red light level when red light is given immediately after each far-red illumination. These results are interpreted as a proof of existence and involvement of phytochrome in the germination control of maize caryopses, though its manifestation is realized only under osmotic stress.Abbreviations D darkness - FR far-red - R red - B blue - c-FR, c-R, c-B continuous FR, R, B, resp. - i-FR, i-R intermittent FR, R, resp.     

Chloroplast movement in Mougeotia induced by blue light pulses

The profile-to-face chloroplast movement in the green alga Mougeotia has been induced by strong blue and near-ultraviolet light pulses (6 J m^-2). Simultaneously, strong red or far-red light (10 W m^-2) was applied perpendicularly to the inducing beam. The response was measured photometrically. Against the far-red background the reciprocity law was found to hold for pulse durations varying two orders of magnitude. The action spectrum exhibited a maximum near 450 nm and a distinct increase in near-ultraviolet. The time-course and the spectral dependence of pulse responses of chloroplasts in Mougeotia were similar to those recorded for other plants which are sensitive only to blue. This points to an alternative sensor system active in the short-wavelength region in addition to the phytochrome system.Abbreviations FR far-red light - Pr red absorbing form of phytochrome - Pfr far-red absorbing form of phytochrome - R red light This paper is dedicated to the memory of Professor Jan Zurzycki     

Photoreception and photoresponses in the radish hypocotyl

In etiolated hypocotyls of Raphanus sativus L. the growth responses to continuous red, far-red and blue light have been distinguished on the bases of photoreceptive sites and regions of physiological response. Blue light appeared to retard a fairly mature stage of elongation, acting immediately and directly on the cells irradiated. Far-red light caused a marked inhibition of all stages of elongation after a lag period, and the stimulus could be transmitted from the hook region. The effect of red light was complex and consisted of one promotive and two inhibitory responses.Abbreviations B blue - FR far-red - R red     

An action spectrum in the blue for inhibition of germination of lettuce seed

Summary Using a 2-h irradiation period at constant quantum irradiance, a complete action spectrum for inhibition of germination of lettuce seed has been obtained. Action maxima were near 470 and 720 nm, the latter being the most active wavelength. It was also shown, under conditions where light inhibition cannot occur, that phytochrome potentiation of germination is maximal at all wavelengths below 700 nm, including the highly active blue region. Evidence was presented for promotion of germination by a 2-h irradiation in the red which cannot be explained on the basis of conversion of phytochrome to the active form.Abbreviations Bl blue - FR far-red, P_FR far-red-absorbing form of phytochrome - R red Supported in part by funds provided for biological and medical research by the State of Washington Initiative Measure No. 171 and the Graduate School Research Funds.     

Effects of blue and red light on unrolling of rice leaves

Unrolling of the second leaf of 8-day-old rice (Oryza sativa L.) seedlings was promoted by weak blue light (B), but not by red light (R). The effect of B was counteracted by irradiation with R just before or after the B. The counteracting effect of R was reversed by subsequent irradiation with far-red light but not by B, even if B was applied for 10 h. The B was effective when the region 0.5–2 cm from the tip of the leaf was irradiated. These results indicate that in rice photoreceptors for blue light located in the region 0.5–2 cm from the tip of the leaf play a key role in leaf unrolling and that a B-absorbing pigment and phytochrome participate in leaf unrolling in a closely related manner.Abbreviations B blue light - R red light - FR far-red light - W white light - D dark This work was presented at the Annual Meeting of the Japanese Society of Plant Physiologists on April 4, 1978, in Hiroshima     

The loci of perception for phytochrome control of internode growth in light-grown mustard: Promotion by low phytochrome photoequilibria in the internode is enhanced by blue light perceived by the leaves

Under continuous white light (WL), extension growth of the first internode in Sinapis alba L. was promoted by low red (R): far-red (FR) ratios reaching the stem and-or the leaves. Conversely, the growth promotion by end-of-day light treatments was only triggered by FR perceived by the leaves and cotyledons, while FR given to the growning internode alone was tatally ineffective. Continuous WL+FR given to the internode was also in-effective if the rest of the shoot remained in darkness. Both the background stem growth, and the growth promotion caused by either an end-of-day FR pulse or continuous WL+FR given to the internode, increased with increasing fluence rates of WL given to the rest of the shoot. The increase by WL of the growth-stimulatory effect of low phytochrome photoequilibria in the internode appears to be mediated by a specific blue-light-absorbing photoreceptor, as blue-deficient light from sodium-discharge lamps, or from filtered fluorescent tubes, promoted background stem growth similarly to WL but did not amplify the response to the R:FR ratio in the internode. Supplementing the blue-deficient light (94 mol·m^-2·s^-1) with low fluence rates of blue (<9 mol·m^-2·s^-1) restored the promotive effect of low R:FR reaching the internode.Abbreviations BL blue light - FR far-red light - PAR photosynthetically active radiation - Pfr/P ratio between the FR-absorbing form and total phytochrome - R red light - SOX low-pressure sodium lamp - WL white light Supported by the Consejo Nacional de Investigaciones Cientificas y Técnicas (República Argentina) and the ORS scheme (UK)     

Light quantity and quality interactions in the control of elongation growth in light-grown Chenopodium rubrum L. seedlings

The spectral control of hypocotyl elongation in light-grown Chenopodium rubrum L. seedlings has been studied. The results showed that although the seedlings responded to changes in the quantity of combined red and far-red radiation, they were also very sensitive to changes in the quantity of blue radiation reaching the plant. Altering the proportion of red: far-red radiation in broad waveband white light caused marked differences in hypocotyl extension. Comparison of the responses of green and chlorophyll-free seedlings indicated no qualitative difference in the response to any of the light sources used, although photosynthetically incompetent plants were more sensitive to all wavelengths. Blue light was found to act primarily of a photoreceptor which is different from phytochrome. It is concluded that hypocotyl extension rate in vegetation shade is photoregulated by the quantity of blue light and the proportion of red: far-red radiation. In neutral shade, such as that caused by stones or overlying soil, hypocotyl extension appears to be regulated primarily by the quantity of light in the blue waveband and secondarily by the quantity of light in the red and far-red wavebands.Abbreviations B blue - FR far-red - k ₁, k ₂ rate constants for photoconverison of Pr to Pfr and Pfr to Pr, respective - k ₁/k ₁ +k ₂= phytochrome photoequilibrium - k ₁ +k ₂= phytochrome cycling rate - Pr=R absorbing form of phytochrome - Pfr=FR absorbing form of phytochrome - P_tot Pr+Pfr - PAR photosynthetically active radiation = 400–700 nm - R red - WL white light     

Action spectra for the light inhibition of flowering and its reversal inLemna paucicostata T-101

Lemna paucicostata Hegelm. T-101, a short-day plant, flowers when plants preirradiated with red light (R) for 24 h are subjected to inductive darkness for 72 h followed by two short-day cycles (6 h R+ 18 h dark). However, flowering is inhibited by blue-or far-red-light pulses applied at the beginning of the inductive dark period. These inhibitory light effects are fully reversible by a R pulse. The action spectra for the inhibitory light effect and for its reversal show that the light pulses act exclusively through phytochrome. It is concluded that a low level of Pfr at the beginning of the inductive dark period prevents flowering.Abbreviations R red (light) - B blue (light) - FR far-red (light)     

Blue- and red-light action in photoorientation of chloroplasts in Adiantum protonemata

An action spectrum for the low-fluencerate response of chloroplast movement in protonemata of the fern Adiantum capillus-veneris L. was determined using polarized light vibrating perpendicularly to the protonema axis. The spectrum had several peaks in the blue region around 450 nm and one in the red region at 680 nm, the blue peaks being higher than the red one. The red-light action was suppressed by nonpolarized far-red light given simultaneously or alternately, whereas the bluelight action was not. Chloroplast movement was also induced by a local irradiation with a narrow beam of monochromatic light. A beam of blue light at low energy fluence rates (7.3·10^-3-1.0 W m^-2) caused movement of the chloroplasts to the beam area (positive response), while one at high fluence rates (10 W m^-2 and higher) caused movement to outside of the beam area (negative response). A red beam caused a positive response at fluence rates up to 100 W m^-2, but a negative response at very high fluence rates (230 and 470 W m^-2). When a far-red beam was combined with total background irradiation with red light at fluence rates causing a low-fluence-rate response in whole cells, chloroplasts moved out of the beam area. When blue light was used as background irradiation, however, a narrow far-red beam had no effect on chloroplast distribution. These results indicate that the light-oriented movement of Adiantum chloroplasts is caused by red and blue light, mediated by phytochrome and another, unidentified photoreceptor(s), respectively. This movement depends on a local gradient of the far-red-absorbing form of phytochrome or of a photoexcited blue-light photoreceptor, and it includes positive and negative responses for both red and blue light.Abbreviations BL blue light - FR far-red light - Pfr far-red-absorbing form of phytochrome - Pr red-absorbing form of phytochrome - R red light - UV ultraviolet     

Rapid photomodulation of stem extension in light-grownSinapis alba L.

Treatment of the whole of aSinapis alba plant with supplementary far-red light (FR), in back-ground white light (WL), induces a rapid increase in stem extension rate. This rapid increase is regulated by the light environment of the stem itself. Supplementary FR to the stem increases extension rate after a lag period of 10–15 min. A lag period of 3–4 h follows FR irradiation of the leaf, before an increase in extension rate is detectable. When the stem is given supplementary FR, the change in extension rate which is induced increases with increasing FR fluence rate, and with decreasing phytochrome photoequilibrium. There is no difference between the effects of supplementary FR _max 719 nm and supplementary FR _max 739 nm for these relationships. The increase in extension rate induced by supplementary FR is reversed by an increase in the fluence rate of red light (R). These data indicate that the response is controlled by phytochrome photoequilibrium.Abbreviations B blue light - FR far-red light - R red light - WL white light - Pfr far-red absorbing form of phytochrome - Pr red absorbing form of phytochrome - P_tot total phytochrome level (=Pr+Pfr); -Pfr/Ptot, measured - ER difference in stem extension rate, before and after treatment     

Photocontrol of germination of cucumis anguria l

Germination ofCucumis anguria was inhibited by white, blue (B), and far-red (FR) irradiation and promoted by darkness and red (R) irradiation. The effect of white light was greater when supplied after rather than before the dark period. Darkness was more effective in reversing the effect of FR than FR in reversing the effect of darkness. FR was also more effective than B. When darkness followed B pretreatments, final germination percentage was higher than with FR pretreatment. R fully reversed the inhibitory effect of FR.     

Interaction of light and temperature on seed germination of Rumex obtusifolius L.

Germination of Rumex obtusifolius L. seeds (nutlets) is low in darkness at 25° C. Germination is stimulated by exposure to 10 min red light (R) and also by a 10-min elevation of temperature to 35° C. A 10-min exposure to far-red light (FR) can reverse the effect of both R (indicating phytochrome control) and 35° C treatment. Fluence-response curves for this reversal of the effect of R and 35° C treatments are quantitatively identical. Treatment for 10 min with light of wavelenght 680, 700, 710 and 730 nm, after R and 35° C treatment, demonstrates that germination induced by 35° C treatment results from increased sensitivity to a pre-existing, active, far-red-absorbing form of phytochrome (Pfr) in the seeds.Abbreviations FR far-red light - P phytochrome - Pr red-absorbing form of P - Pfr far-red-absorbing form of P - R red light     

Overexpression of oat phytochrome A gene differentially affects stem growth responses to red/far-red ratio signals characteristic of sparse or dense canopies

The effects of overexpression of oat phytochrome A on neighbour detection and on stem-growth responses to changes in red light (R), far-red light (FR) and blue light (B) simulating neighbours were investigated in transgenic tobacco seedlings grown under natural radiation. In wild-type (WT) seedlings, stem extension growth was promoted: (1) by lowering the R:FR by means of daytime supplementary FR, end-of-day FR, neighbours reflecting FR, or selective light filters placed around the base of the shoot to reduce R without affecting FR; and (2) by lowering phytochrome-absorbable radiation (R+FR) reaching the stem. Transgenic seedlings only responded to reductions in R:FR involving no significant changes in FR irra-diance, i.e. end-of-day FR and filters placed around the stem to reduce R. Neither daytime supplementary R nor selective filters placed around the stem to reduce B affected stem growth in any genotype. In growing canopies, WT seedlings responded to the reduction of R:FR caused by FR reflected in neighbour plants. Transgenic seedlings responded to plant density about a week later, when mutual plant shading reduced R and (to a lesser extent) FR below sunlight levels. Overexpression of phytochrome A impaired early neighbour detection.     

A phytochrome-mediated alteration from stem to rosette growth on chicory root explants in vitro

Chicory root explants (Cichorium intybus L. var. foliosum) of two cultivars, taken before and after hydroponic forcing, were cultured in vitro in complete darkness supplemented with red and far-red light treatments. Using 5 min red light per day, the strong stem elongation occurring in complete darkness was converted to rosette formation. This reaction was reversed to stem elongation (accompanied by leaf formation) adding 15 min far-red light after the red light. Fifteen min far-red light per day alone caused the same reaction as 5 min red/15 min far-red light. Far-red light followed by red light caused rosette formation. In stems, formed under complete darkness in vitro, the presence of phytochrome was shown. No phytochrome was detected in the root fragment itself.Abbreviations R red light - FR far-red light - GA gibberellinic acid - A absorbance - FW fresh weight     

Co-action between phytochrome B and HY4 in Arabidopsis thaliana

A combination of physiological and genetic approaches was used to investigate whether phytochromes and blue light (BL) photoreceptors act in a fully independent manner during photomorphogenesis of Arabidopsis thaliana (L.) Heynh. Wild-type seedlings and phyA, phyBand hy4 mutants were daily exposed to 3 h BL terminated with either a red light (R) or a far-red light (FR) pulse. In wild-type and phyA-mutant seedlings, BL followed by an R pulse inhibited hypocotyl growth and promoted cotyledon unfolding. The effects of BL were reduced if exposure to BL was followed by an FR pulse driving phytochrome to the R-absorbing form (Pr). In the wild type, the effects of R versus FR pulses were small in seedlings not exposed to BL. Thus, maximal responses depended on the presence of both BL and the FR-absorbing form of phytochrome (Pfr) in the subsequent dark period. Impaired responses to BL and to R versus FR pulses were observed in phyB and hy4 mutants. Simultaneous irradiation with orange light indicated that BL, perceived by specific BL photoreceptors (i.e. not by phytochromes), required phytochrome B to display a full effect. These results indicate interdependent co-action between phytochrome B and BL photoreceptors, particularly the HY4 gene product. No synergism between phytochrome A (activated by continuous or pulsed FR) and BL photoreceptors was observed.Abbreviations BL blue light - D darkness - FR far-redlight - FRc continuous FR - Pfr FR-absorbing form of phytochrome - Pfr/P proportion of phytochrome as Pfr - phyA phytochrome A - phyB phytochrome B - R red light - WT wild type We thank Professors R.E. Kendrick and M. Koornneef (Wageningen Agricultural University, The Netherlands), Professor J. Chory (Salk Institute, Calif., USA) and the Arabidopsis Biological Resource Center (Ohio State University, Ohio, USA) for their kind provision of the original seed batches. This work was financially supported by CONICET, Universidad de Buenos Aires (AG 040) and Fundación Antorchas (A-12830/1 0000/9)     

Floral initiation in Lolium temulentum L.: the role of phytochrome in the responses to red and far-red light

Summary The possibility that phytochrome is involved in the promotion of flowering by far-red light was investigated. The addition of far-red (FR) to a day extension with red (R) light promotes inflorescence initiation in Lolium. A 2-hour interruption with darkness also promoted flowering compared with the uninterrupted red light control; apex length was further increased by a 10-minute FR irradiation given before the 2-hour dark interruption and was decreased by 10-minutes of R light given in the middle: both FR promotion and R inhibition were reversed by R and FR respectively. Apex length increased approximately linearly with increasing duration of dark interruption up to at least 2 1/2 hours. When varying ratios of R:FR light were substituted for a 2-hour dark period, apex length was increasingly depressed as the % R was increased above 25%; no difference between 25% R/75% FR and 100% FR could be detected. Apex length was inversely linearly related to the calculated [Pfr]/[P] ratios above about 40% Pfr.FR promoted flowering when given during a 5-hour interruption of a day extension with R light but, between 0.25 and 0.90 J m² s^-1, there was no effect of intensity of FR; at 0.11 J m^-2 s^-1 apex length was shorter than at 0.25 J m^-2 s^-1 but longer than in darkness. When the duration of FR (from the beginning of a dark interruption of a day extension with R) was varied, apex length increased with increasing duration of FR up to 1 1/4 to 2 hours but further increasing the duration of FR did not promote flowering more.The results implicate phytochrome in the promotion of flowering by FR light. It has been demonstrated that a low [Pfr]/[P] ratio (less than present in 25% R/75% FR) is needed over a relatively long period of time: this explains why a relatively high proportion of FR light must be added to R for several hours in order to give maximum promotion of flowering. It is concluded that, in Lolium, the increased flowering response to FR light is brought about by a reduction of [Pfr]/[P] ratio at the appropriate time, although the possibility that another effect of far-red is also involved has not been rigorously excluded.     

Action spectra for changes in the “high irradiance reaction” in hypocotyls of Sinapis alba L.

Detailed action spectra are presented for the inhibition of hypocotyl extension in dark-grown Sinapis alba L. seedlings by continuous (24 h) narrow waveband monochromatic light between 336 nm and 783 nm. The results show four distinct wavebands of major inhibitory action; these are centred in the ultra-violet (_max=367 nm), blue (_max=446 nm), red (_max=653 nm) and far-red (_max=712 nm) wavebands. Previous irradiation of the plants with red light (which also decreases Ptot) causes decreased inhibitory action by all wavelengths except those responsible for the red light inhibitory response. Pre-irradiation did not alter the wavelength of the action maxima. It is concluded that ultra-violet and blue light act mainly on a photoreceptor which is different from phytochrome.Abbreviations B blue - D dark - FR far-red - HIR high irradiance reaction - HW half power bandwith - Pr R absorbing form of phytochrome - Pfr FR absorbing form of phytochrome - Ptot total phytochrome=Pr+Pfr - R red - UV ultra violet