Photosynthetic energy storage of Photosystems I and II in the spectral range of photosynthetically active radiation in intact sugar maple leaves

The relative activity of Photosystems (PS) I and II in the spectral range between 400 and 720 nm was studied by measuring photosynthetic energy storage (ES) of an intact sugar maple leaf using photoacoustic spectroscopy. ES, determined with a modulated (80 Hz) monochromatic light beam in the presence of saturating intensity of background non-modulated white light, indicated the total energy stored by both photosystems (ES_T). Using background far-red light, ES of PS I (ES_{PS I}) was quantified. ES_{PS II} was derived from ES_T-ES_{PS I}. ES_T dependence on intensity and wavelength of modulated light was studied at 470, 560, 640 and 680 nm. ES_T was maximum in red light and minimum in blue light. It decreased with an increase in modulated light intensity. The ratio ES_{PS II}/ES_{PS I}, measured at 640 nm, remained nearly constant with an increase in modulated light intensity. The relative quantum yield of ES_T spectrum showed two peaks around 610 and 660 nm, and declined sharply after 680 nm, revealing a clear red drop. ES_{PS I} spectrum presented peaks around 610 and 670 nm, and a minimum between 440 and 470 nm. ES_{PS I} was observed beyond 700 nm up to 720 nm, indicating the energy stored by cyclic electron transport. ES_{PS II} spectrum showed broad peaks, around 460, 490, 600 and 660 nm, and a shoulder between 530 and 560 nm. ES_{PS II} was always higher than ES_{PS I} between 400 and 690 nm and reached zero around 700 nm.Abbreviations ES energy storage - ES_{PS I} energy storage of PS I - ES_{PS II} energy storage of PS II - ES_T energy storage of PS I and PS II - PA photoacoustic - PS I Photosystem I - PS II Photosystem II - Q_m PA signal in the absence of any background light - Q_ma PA signal in the presence of background white light - Q_mfrl PA signal in the presence of background far-red light - S/N signal to noise     

小麦叶片氮素状况与光谱特性的相关性研究

 系统分析了不同时相下两个小麦(Triticum aestivium)品种叶片含氮量及叶片氮积累量与冠层光谱反射特征的关系。结果表明,随施氮水平的增加,小麦冠层在可见光区的反射率逐渐降低,而近红外波段的反射率逐渐升高。小麦叶片氮素状况与比值指数或归一化指数显著相关,两个品种表现极为一致,可以用一个指数方程来拟合。分阶段建模并没有提高模型的精度,因此可以建立一个适用于整个生育时期的通用氮素诊断方程。叶片含氮量同光谱指数在整个生育期内的关系要优于叶片氮积累量的,其中,与叶片含氮量关系最佳的指数为红波段(660 nm)和蓝波段(460 nm)的组合(R2>0.80);与叶片氮积累量关系最佳的光谱指数为中红外波段(1 220 nm)与红波段(660 nm)的组合(R2>0.62)。     

Production of dihydrothymidine stereoisomers in DNA by gamma-irradiation

5,6-Dihydrothymidine (dDHT) is a derivative thymidine formed during gamma-irradiation. This paper demonstrates the conditions under which dDHT is formed in solutions of DNA and that dDHT is produced in the DNA of HeLa cells during gamma-irradiation. The product of dDHT by gamma-irradiation of either thymidine or DNA has been quantitated by a sensitive and specific high-pressure liquid chromatography method. dDHT is a major product of the anoxic irradiation of thymidine (G value 0.5) but is produced in substantially smaller amounts in DNA irradiated under the same conditions (G value 0.026). The presence of oxygen reduces the yield of dDHT by at least 25-fold for both irradiation substrates. In HeLa cells, 60Co irradiation under anoxia produces (6.2 +/- 0.2) X 10(-8) mol of the R isomer of dDHT per mole of cell deoxynucleotide per gray (G value 0.11). gamma-Irradiation of thymidine produces equal quantities of the R and S stereoisomers of dDHT. Irradiation of DNA produces significantly more (69%) (R)- than (S)-dDHT. DNA isolated from cultured human cells following gamma-irradiation also contains more of the R than the S form of dDHT. The conformation of double-stranded DNA favors a stereospecific production of the R isomer. Among products of gamma-irradiation of DNA, dDHT is unique in its strict requirement for anoxia during irradiation and the preferential production of a particular stereoisomer.     

Low temperature spectrophotometry of the phototransformation of Pfr to Pr in pelletable pea phytochrome

Manabe, K. 1987. Low temperature spectrophotometry of the phototransformation of P_fr to P_r, in pelletable pea phytochrome.
Low temperature spectrophotometry was used to study the phototransformation of P_fr to P_r in 1000–7000 g pelletable fractions extracted from dark grown pea ( Pisum sativum L. cv. Alaska) epicotyls which had been irradiated with red and then far-red light. At -170°C, far-red irradiation of the pelletable phytochrome which had been pre-irradiated with saturating fluence of red light before freezing caused formation of an intermediate (named I₆₆₀), the difference spectrum of which showed a marked ab-sorbance decrease at 740 nm and a concomitant small increase at about 660 nm. The inermediate I₆₆₀ was converted to another intermediate (I₆₆₀) when it was warmed above -80°C. The difference spectrum of this intermediate showed a positive peak at 670 nm. This intermediate was photoconverted to P_fr by red irradiation and also underwent dark reversion to P_fr at -60°C. I₆₆₀ formed P_r if the temperature was above -10°C. The basic features of the phytochrome intermediates resemble those obtained in vivo and in degraded purified phytochrome.     

Photostimulation of Hydroxypyruvate Reductase Activity in Peroxisomes of Pharbitis nil Seedlings: II. Photoreceptors in Blue Light

The action spectrum for stimulation of hydroxypyruvate reductase(HPR) activity in etiolated cotyledons of Pharbitis nil showsthe three effective regions of blue (B), red (R), and far red(FR), with high B efficiency. Light duration response curvesand kinetics of stimulation in continuous light were comparedat 455 nm, 660 nm and 710 nm. Before 10 h, the efficienciesat 455 and 660 nm were higher than that at 710 nm. After 10h, lengthening of the 660 nm irradiation was ineffective whilelengthening of the 710 and 455 nm irradiation caused linearincreases in HPR activity. These results together with the effectof pre-R-irradiation on FR or B stimulation suggest that twopigments are simultaneously involved in the B light effect:phytochrome and a specific blue-absorbing pigment. Phytochromemay be the main effective pigment up to 10 h in blue light whilethe blue-absorbing pigment may be more effective for longerexposures. (Received November 22, 1983; Accepted June 20, 1984)     

Bacteriorhodopsin photoreaction: identification of a long-lived intermediate N (P,R350) at high pH and its M-like photoproduct

An alkaline suspension of light-adapted purple membrane exposed to continuous light showed a large absorption depletion at 580 nm and a small increase around 350 nm. We attribute this absorption change to an efficient photoconversion of bR570 into a photoproduct N (P,R350), which has a major absorption maximum between 550 and 560 nm but has lower absorbance than bR570. N was barely detectable at low pH, low ionic strength, and physiological temperature. However, when the thermal relaxation of N to bR570 was inhibited by increasing pH, increasing ionic strength, and decreasing temperature, its relaxation time could be as long as 10 s at room temperature. N is also photoactive; when it is present in significant concentrations, e.g., accumulated by background light, the flash-induced absorption changes of purple membrane suspensions were affected. Double-excitation experiments showed an M-like photoproduct of N,NM, with an absorption maximum near 410 nm and a much longer lifetime than M412. It may be in equilibrium with an L-like precursor NL. We suggest that N occurs after M412 in the photoreaction cycle and that its photoproduct NM decays into bR570. Thus, at high pH and high light intensity, the overall photoreaction of bR may be approximated by the two-photon cycle bR570----M412----N----(NL----NM)----bR570, whereas at neutral pH and low light intensity it can be described by the one-photon cycle bR570----M412----N----O640----bR570.(ABSTRACT TRUNCATED AT 250 WORDS)     

不同光质的LED对蝴蝶兰组织培养增殖及生根的影响

以蝴蝶兰(Phalaenopsis ssp.)栽培种绿熊(Green Bear)和大辣椒(Big Chilli)为培养材料, 研究不同光质的发光二极管(LED)组合对组织培养过程中增殖及生根的影响。结果表明, 红光更有利于蝴蝶兰单芽增殖、干重、鲜重以及株高的增加, 但不利于叶片叶绿素的积累; 蓝光有利于叶片叶绿素的积累, 并能提高根系活力; 远红光则对根长和根系活力的增加作用更显著。增殖扩繁阶段的最适LED为暖白, 2种蝴蝶兰芽增殖系数分别比白色荧光灯(对照)高出53.17%和46.37%。生根诱导阶段的最适LED组合为红:蓝:远红=3:6:1, 根长及根系活力均较对照显著增加。该研究结果为LED光源在蝴蝶兰组织培养中的应用奠定了基础。     

In vivo studies of gross photosynthesis in attached leaves by means of photothermal radiometry

In photothermal radiometry, heat radiation from an illuminated object, in synchronism with incident chopped light, is observed using an infrared detector with suitable electronics. By thus measuring the heat released during pulse-wise irradiation of leaves, conclusions can be drawn as to the gross efficiency of photosynthesis: More heat means less photochemically stored energy. Saturation of photosynthesis, by employing additional strong continuous-wave background light, affords an internal photothermal radiometry signal reference corresponding to the photochemical zero efficiency level, against which the signal in the absence of saturation can be compared. Through such means, gross energy storage efficiencies approaching 30% have been observed in Caragana arborescens Lam. at low light intensities. Several other examples are given, including measurements on dark-adapted leaves and leaves infiltrated with 3-(3,4-dichlorophenyl)-1,1-dimethylurea, to support our conclusion that photothermal radiometry provides a powerful new method for in vivo studies of photosynthesis in whole, attached leaves.     

Phytochrome is not involved in the red-light-enhancement of the stomatal blue-light-response in wheat seedlings

The stomatal response to blue light (BL) in wheat seedlings ( Triticum aestivum L. cv. Starke II, Weibull) was enhanced by background red light (R). This enhancement was only slightly affected by the addition of background far-red light (FR). Under similar light treatments, the addition of FR induced a 43% transformation from the far-red-absorbing form towards the red-absorbing form of phytochrome from etiolated oat ( Avena sativa L. cv. Sol II), immobilized on phenyl-sepharose. Furthermore, the enhancement of the stomatal BL-response by 15 min R was not reversed by a subsequent irradiation with 5 min FR. It is concluded that the red-light-enhancement of the stomatal blue-light-response in wheat seedlings does not involve a change in the photostationary state of phytochrome.     

Role of conserved transmembrane cationic amino acids in the prostaglandin transporter PGT

The prostaglandin transporter "PGT" interacts electrostatically with its anionic substrate, based on inhibition by the disulfonic stilbenes [Chan, B. S. (1998) J. Biol. Chem. 273, 6689-6697], inhibition by the thiol-reactive anion sodium (2-sulfonatoethyl)methanethiosulfonate (MTSES) [Chan, B. S. (1999) J. Biol. Chem. 274, 25564-25570], and the requirement for a negatively charged 1-position carboxyl on the substrate [Itoh, S. (1996) Mol. Pharm. 50, 736-742]. Here we found that modification of positively charged residues on wild-type PGT by arginine- and lysine-specific reagents significantly inhibited transport. We previously found that the binding site of PGT is formed, at least in part, by its membrane-spanning segments [Chan, B. S. (1999) J. Biol. Chem. 274, 25564-25570]. Three charged residues within predicted transmembrane spans (E78, R560, and K613) are conserved in PGT and in related transporters. Substitution of the anionic residue E78 (E78D and E78C) produced an essentially functional transporter, whereas substitution of the cationic residues with neutral residues (R560N and K613Q) resulted in poorly functional transporters. Immunoblotting revealed similar expression levels of wild-type and mutant transporters, and immunostaining indicated correct targeting. Conservative charge substitutions (R560K, K613R, and K613H) resulted in generally functional transporters. In contrast, R560N was nonfunctional, whereas the substrate affinity of K613G decreased greater than 50-fold. Conservative substitutions retaining the charge at position 613 (K613R and K613H) restored the substrate affinity, suggesting a direct role of K613 in substrate binding. Double-neutral mutants E78G/R560C and E78G/K613C were inactive, indicating that these residues are not simply charge-paired. Our results suggest that an arginine at position 560 is critical for maximal substrate translocation, and that a positively charged side chain at position 613 contributes to electrostatic binding of the anionic substrate.     

Anatomical features of pepper plants (Capsicum annuum L.) grown under red light-emitting diodes supplemented with blue or far-red light

Pepper plants (Capsicum annuum L. cv., Hungarian Wax) were grown under metal halide (MH) lamps or light-emitting diode (LED) arrays with different spectra to determine the effects of light quality on plant anatomy of leaves and stems. One LED (660) array supplied 90% red light at 660 nm (25nm band-width at half-peak height) and 1% far-red light between 700-800nm. A second LED (660/735) array supplied 83% red light at 660nm and 17% far-red light at 735nm (25nm band-width at half-peak height). A third LED (660/blue) array supplied 98% red light at 660nm, 1% blue light between 350-550nm, and 1% far-red light between 700-800nm. Control plants were grown under broad spectrum metal halide lamps. Plants were gron at a mean photon flux (300-800nm) of 330 micromol m-2 s-1 under a 12 h day-night photoperiod. Significant anatomical changes in stem and leaf morphologies were observed in plants grown under the LED arrays compared to plants grown under the broad-spectrum MH lamp. Cross-sectional areas of pepper stems, thickness of secondary xylem, numbers of intraxylary phloem bundles in the periphery of stem pith tissues, leaf thickness, numbers of choloplasts per palisade mesophyll cell, and thickness of palisade and spongy mesophyll tissues were greatest in peppers grown under MH lamps, intermediate in plants grown under the 660/blue LED array, and lowest in peppers grown under the 660 or 660/735 LED arrays. Most anatomical features of pepper stems and leaves were similar among plants grown under 660 or 660/735 LED arrays. The effects of spectral quality on anatomical changes in stem and leaf tissues of peppers generally correlate to the amount of blue light present in the primary light source.     

Two electrogenic mechanisms contributing to the 560 nm absorption changes in intact Bryopsis chloroplasts.

Light -induced absorbance changes at 560 nm in dark-adapted intact chloroplasts of the green alga, Bryopsis maxima were studied in the time range of 200 ms. The initial rise of the 560 nm signals consists of two major components which are both electrochromic absorbance changes of the carotenoids, siponein and/or siphonaxanthin, but different in mechanisms of the field formation. The first component (component S) is related to electron transport since it was sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) and showed at light-intensity dependence similar to that of electron transport in chloroplasts. In the presence of DCMU, component S could be restored on addition of proton-transporting electron donors such as reduced 2.6-dichlorophenol indophenol and phenazine methosulfate, but not on addition of N,N,N',N'-tetramethyl-p-phenylenediamine which does not carry protons with electrons (Trebst, A. (1974) Annu. Rev. Plant Physiol. 25, 423--458). We propose that component S is due to the electric field set up by the proton translocation across the thylakoid membrane. The second component (component R) was resistant to DCMU and DBMIB. The light-intensity dependency of component R was similar to that of cytochrome f photooxidation which showed saturation at a relatively low light intensity. The magnitude of component R was markedly reduced by phenylmercuric acetate, suggesting the participation of ferredoxin and ferredoxin-NADP oxidoreductase in the mechanism of the field formation responsible for this component. In the presence of DCMU and phenylmercuric acetate, time courses of the 560 nm changes paralleled those of cytochrome f changes. These results indicate that component R is due to the electric field formed between oxidized cytochrome f and other intersystem electron carriers located in the inner part of the thylakoid membrane and reduced electron acceptors of Photosystem I situated on the membrane surface. The complex natures of the 560 nm changes, as well as the contributions of Photosystems I and II to the absorbance changes, are explained in terms of the two electrogenic mechanisms.     

Light-induced dephosphorylation of a 65-kDa protein in the cyanobacterium Synechocystis sp. PCC6803

We found that a 65-kDa protein (p65) of Synechocystis sp. PCC 6803 is dephosphorylated in a light-dependent manner. In darkness, p65 was specifically phosphorylated and then completely dephosphorylated within 2 min upon exposure to high-intensity light. The phosphorylation of p65 recurred after 8 hours incubated in the dark following light exposure. Green (540-560 nm) and red (660 nm) light dephosphorylated p65 efficiently, with the efficiency being greater with green light. These results suggest that p65 is a novel substrate involved in the quantity and quality of light-dependent dephosphorylation in cyanobacteria.     

Spectral response of corn (Zea mays) in root geotropism

The primary roots of corn (Zea mays, Wisconsin hybrid 64A ?22R) show positive geotropism following exposure to light. Thisconfirms the works of other investigators. The curvature responsebegins at about 1 hr following irradiation and reached a plateauat 5 hr. A study of wavelengths 350–760 nm, using energiesof 2.24 ? 10¹⁴ photon cm^–2 and exposure times 60 sec,shows that the most effective light is at 660 nm with lessereffectiveness at 460 and 560 nm. The responses at 660 and 460nm are reversible by a far-red (730 nm) exposure, indicativeof the possible participation of phytochrome. Analyses of freshtips of corn roots with a dual-wavelength difference photometershow the phytochrome content in the root to be about 0.16 (OD) per gram fresh wieght. The requirement of light for thegeotropic growth response of corn roots might be an adaptivephenomenon. The occurrence of photomorphogenic activity in thegreen light should be of concern to those who use green as the"safe" light in "dark" experiments. ¹Work supported by U. S. NASA and U. S. ERDA. (Received September 26, 1977; )     

The relative importance of ultraviolet and longer wavelengths in the response properties of fly visual systems

A generalized analysis of the generator potential responses of R1-6 cells of Calliphora provides remarkable information on the visual properties for the Diptera. This shows that, although these cells have two peak response sensivities for monochromatic stimuli at 350 and 480 nm under single color stimulus conditions, and when the background illumination is either zero or in the region of 450–560 nm, the sensitivity to ultraviolet light is practically eliminated for background illumination in either the ultraviolet or the region around 600 nm or when any simultaneous dynamic stimulus in the region of 480–550 nm is also applied. These results seem somewhat perplexing to an understanding of the behavioral vision properties. It also is not consistant with the concept that the ultraviolet response is initiated by a sensitizing pigment within these cells that transfers energy to the rhodopsin-metarhodopsin process. However, it strengthens other evidence that the limited condition of ultraviolet responses comes from interaction from R7,8 cells but does not play an important behavioral role in the visual system fed from cells R1-6. As discussed in this paper, any high level pattern recognition controlling behavioral response to ultraviolet stimuli comes from the R7,8 cell system.     

红光与远红光比值对切花菊‘神马’发育和外观品质的影响

为了探讨不同R/FR值对温室切花菊发育进程和品质的影响,以切花菊品种‘神马’(Chrysanthemum morifoliumcv.‘Jingba’)为试材,设计不同红光(R:660nm!10nm)与远红光(FR:730nm!10nm)比值(R/FR)为0.5、2.5、4.5、6.5的LED灯照射处理试验,以自然光为对照(CK),观测不同处理的菊花发育阶段和品质指标。结果表明:与CK相比,R/FR=2.5处理显著加快菊花的发育进程(P<0.05),R/FR=2.5处理下短日照处理到现蕾、现蕾到破蕾及破蕾到收获3个发育阶段分别比CK提前4d、8d和5d,R/FR=0.5处理发育速度最慢,3个阶段分别比CK晚4d、2d和2d;不同R/FR值处理下菊花株高、单株叶龄、单株叶面积、茎粗、花径和花梗长度均随温光效应的增加呈增加的趋势,并在R/FR=2.5时取得最大值;收获时切花菊达到A级和B级产品等级的比例分别以R/FR=2.5和4.5处理最高。本研究发现,R/FR为2.5能够显著促进菊花发育进程和提高菊花的外观品质和A级切花的比例。     

Pathways of the rise and decay of the M photointermediate(s) of bacteriorhodopsin

The photocycle of bacteriorhodopsin (BR) was studied at alkaline pH with a gated multichannel analyzer, in order to understand the origins of kinetic complexities in the rise and decay of the M intermediate. The results indicate that the biphasic rise and decay kinetics are unrelated to a photoreaction of the N intermediate of the BR photocycle, proposed earlier by others [Kouyama et al. (1988) Biochemistry 27, 5855-5863]. Rather, under conditions where N did not accumulate in appreciable amounts (high pH, low salt concentration), they were accounted for by conventional kinetic schemes. These contained reversible interconversions, either M in equilibrium with N in one of two parallel photocycles or L in equilibrium with as well as M in equilibrium with N in a single photocycle. Monomeric BR also showed these kinetic complications. Conditions were then created where N accumulated in a photo steady state (high pH, high salt concentration, background illumination). The apparent increase in the proportion of the slow M decay component by the background illumination could be quantitatively accounted for with the single photocycle model, by the mixing of the relaxation of the background light induced photo steady state with the inherent kinetics of the photocycle. Postulating a new M intermediate which is produced by the photoreaction of N was neither necessary nor warranted by the data. The difference spectra suggested instead that absorption of light by N generates only one intermediate, observable between 100 ns and 1 ms, which absorbs near 610 nm. Thus, the photoreaction of N resembles in some respects that of BR containing 13-cis-retinal.     

Photoinhibition of white clover seed germination at low water potential

Photosensitivity of germination of white clover ( Trifolium repens L. cv. Podkowa) seeds was studied under water deficit (low water potential) conditions at 25°C. The seeds showed negative photoblastism, which was most pronounced at -0.03 MPa polyethylene glycol solution. Inhibition was observed at two different wavelength bands with maxima at 660 nm (R) and around 730 nm (FR). Red light acted identically to white light (maximum inhibition ca 50%). The effect of far-red illumination was less inhibitory (20–30%). The photoresponse required long illuminations (3 h exposures); saturation level was at 0.1 W m⁻², independently of the light quality. White clover seed germination showed no reversibility of the effects of R and FR light. Prolonged illumination with R and FR increased the inhibition, and intermittent illumination had a higher effect than a continuous one. It was concluded that the photoinhibition of germination of seeds of Trifolium repens involves a reaction dependent on the rate of phytochrome interconversion, a property that is characteristic for the high irradiance reaction.     

Phytochrome-mediated phototropism in maize seedling shoots

Unilateral irradiation with red light (R) or blue light (BL) elicits positive curvature of the mesocotyl of maize (Zea mays L.) seedlings raised under R for 2 d from sowing and kept in the dark for 1 d prior to curvature induction. The fluenceresponse curve for R-induced mesocotyl curvature, obtained by measuring curvature 100 min after phototropic induction, shows peaks in two fluence ranges, designated first positive range (from the threshold to the trough), and second positive range (above the trough). The fluence-response curve for BL is similar to that for R but shifted two orders of magnitude to higher fluences. Blue light elicits the classical first positive curvature of the coleoptile, whereas this response is not found with R. Positive mesocotyl curvature induced by either R or BL is eliminated by R given from above just before the unilateral irradiation, whereas BL-induced coleoptile curvature is not eliminated. The above results collectively offer evidence that phototropic curvature of the mesocotyl is induced by R-sensitive photosystem(s). Mesocotyl curvature in the second positive range is reduced by vertical far-red light (FR) applied after phototropic induction with R, but is not affected by FR applied before R. Unilateral irradiation with FR following vertical irradiation with a high R fluence leads to negative curvature of the mesocotyl. It is concluded that mesocotyl curvature in the second positive range results from a gradient in the amount of the FR-absorbing form of phytochrome (Pfr) established across the plant axis. Mesocotyl curvature in the first positive range is inhibited by vertical FR given either before or after phototropic induction with R. Since the FR used here is likely to produce more Pfr than the very low fluences of R eliciting the mesocotyl curvature in the first positive range, it is assumed that FR reduces the response in this case by adding Pfr at both sides of the plant axis. By rotating seedlings on a clinostat with its axis horizontal, the kinetics of mesocotyl curvature can be studied in the absence of a counteracting gravitropic response. On the clinostat, the R-induced mesocotyl curvature develops after a lag, through two successive phases having different curvature rates, the late phase is slower than the early phase. Negative curvature of the coleoptile can be induced by either R or BL; the BL-induced negative curvature is found at fluences higher than those giving positive curvature. The clinostat experiments show that the negative coleoptile curvature induced by either R or BL is a gravitropic compensation for positive mesocotyl curvature.Abbreviations BL blue light - FR far-red light - Pfr phytochrome in the far-red-absorbing form - Pr phytochrome in the red-absorbing form - R red light C.I.W.-D.P.B. Publication No. 824     

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.