Picosecond Photochemistry of P700-Enriched and Vitamin K1-Depleted Photosystem I Particles Isolated from Spinach

Picosecond transient absorption changes, with a laser intensityas low as one photon absorbed per single reaction center, weremeasured with vitamin K₁-depleted and P700-enriched particleswhich were obtained by ether treatment of spinach PS-I particles.When P700 was in the oxidized state, a bleaching that correspondedto about one-seventh of the ground state absorption was observedjust after a laser flash (0 picosecond delay). A major partof the bleaching decayed with a lifetime of about 35 picoseconds,which corresponds to the relaxation of the excited antenna chl-ato the ground state. By contrast, when P700 was in the reducedstate, the bleaching observed at a 0 ps delay was broader, especiallyon the longer wavelength side than the ground state absorption,probably because of the generation of the excited state of P700.About one half of the bleaching decayed within 35 ps and theremaining half, which had a broad spectrum and a peak around682 nm, was conserved up to 2 ns. This long-lived bleachingprobes no picosecond decay of the radical pair P700⁺-A₀^–because electrons were not transferred from A₀¹ to A₁ in vitaminK₁-depleted particles. After addition of vitamin K₃, an analogof vitamin K₁, to the reduced particles, the bleaching around685 nm decayed successively with an apparent rate of about 150picosecond, while the bleaching around 700 nm was conservedfor up to 2 nanosecond. Thus, the bleaching remaining at 2 nsresembled the difference spectrum of P700, suggesting a subnanosecondquenching of A₀¹ by the externally added vitamin K₃. These observationssupport a recent proposal that the secondary electron acceptorA₁, in photosystem I, is vitamin K₁. ³Permanent address: Optics Laboratory, Korea Standards ResearchInstitute, Daedok Science Town, Chungnam 300-31, Korea. (Received October 24, 1988; Accepted April 14, 1989)     

Coupling Ratios H+/e=3 versus H+/e=2 in Chloroplasts and Quantum Requirements of Net Oxygen Exchange during the Reduction of Nitrite, Ferricyanide or Methylviologen

Using intact and osmotically ruptured chloroplasts, ratios ofcoupling between deposition of protons in the intrathylakoidspace and light-dependent transport of electrons from waterto an external acceptor were determined. The data indicate couplingbetween proton and electron transport at a ratio of H⁺/e=3 withmethylviologen as electron acceptor in thylakoids and with nitriteas electron acceptor in intact chloroplasts. With ferricyanideas electron acceptor in thylakoids, values close to H⁺/e=2 wereobserved. Evidence is discussed that H⁺/e=3 is a fixed valuein intact chloroplasts at levels of thylakoid energization sufficientfor supporting effective carbon assimilation. In the presence of methylviologen and ascorbate, the minimumquantum requirement of oxygen uptake by thylakoids was about2.7 quanta of 675 nm light per O₂ indicating an e/O₂ ratio of1.33. In the absence of ascorbate, and with KCN present in additionto methylviologen, e/O₂ ratios up to 4 were observed. The minimumquantum requirement of oxygen evolution by thylakoids in thepresence of ferricyanide and by intact chloroplasts in the presenceof nitrite was about 8 quanta/O₂. (Received May 1, 1995; Accepted October 2, 1995)     

Dibromothymoquinone (DBMIB) Replaces the Function of QA at 77 K in the Isolated Photosystem II Reaction Center (D1-D2-Cytochrome b559) Complex: Difference Spectrum of the P680+(DBMIB-) State

Transient absorbance changes of the primary electron donor chlorophylla (P680) and acceptor pheophytin a (H) were measured at 77 Kby nanosecond laser spectroscopy in the D1-D2-cytochrome b₅₅₉photosystem II reaction center complex containing dibromomethylisopropylbenzoquinone (DBMIB). After the laser excitation of the reactioncenter in the presence of DBMIB, only the P680⁺-(DBMIB^-) statewas detected. P680⁺ mainly decayed with a t_1/e of 11 ms. Inthe absence of DBMIB, the excitation produced the P680⁺H^- radicalpair. The radical pair produced the triplet state (P680^T) witha t_1/e of 50 ns, and P680^T then decayed with a t_1/e of 2.1 ms.It was concluded that H_- was oxidized by DBMIB in a time rangefaster than the detecting time resolution (3.5 ns) even at 77K. The rapid oxidation of H^- by DBMIB was also confirmed bythe suppression of delayed fluorescence with a decay t_1/e of50 ns. The P680⁺(DBMIB^-)/P680(DBMIB) difference spectrum exhibiteda Q_y, band with a peak at 682 nm with a shoulder at 673 nm.The spectral shape was almost temperature insensitive between77 and 265 K. The feature of this spectrum in the wavelengthrange between 330 and 720 nm was compared with that of P680_T/P680or H^-/H at 77 K. (Received May 8, 1996; Accepted June 24, 1996)     

Preparation and Characterization of P700-Enriched Photosystem-I Complexes from the Thermophilic Cyanobacterium, Synechococcus sp.

P700 was enriched relative to antenna pigments by treating thethylakoid membranes from thermophilic cyanobacterium (Synechococcussp.) with diethylether. The total Chi a/P700 ratio of the membraneswas 147 but decreased to 13 after the treatment with ether whichhad been 70% saturated with water. Vitamin K₁ and carotenoidswere completely removed by this treatment. The low chlorophylla to P700 ratio was retained in photosystem I reaction-centercomplexes purified from the ether-extracted membranes with TritonX-100. The midpoint potential of P700 was considerably loweredby the ether treatment of the thylakoid membranes but was partiallyreversed by the further treatment of the extracted membraneswith Triton X-100. Photo-oxidation of P700 in the purified complexeswas extremely slow under continuous illumination, indicatingthat there is no significant leakage of electrons from the primaryelectron acceptor to other bound acceptors or O₂. The photooxidationof P700 was appreciably accerelated on addition of vitamin K₃(or K₁). (Received June 23, 1988; Accepted November 25, 1988)     

Reaction of Reconstituted Acceptor Quinone and Dynamic Equilibration of Electron Transfer in the Photosystem I Reaction Center

Electron transfer mechanism in the spinach photosystem I reactioncenter that contains artificial quinones in place of phylloquinone(2-methyl-3-phytyl-1,4-naphthoquinone, vitamin K₁) as the secondaryelectron acceptor, Q_ø (or A₁) was discussed. (1) Mostof the reconstituted quinones oxidized the primary acceptorchlorophyll a, A⁰, at a rate rapid enough to compete againstthe charge recombination between A₀^– and the oxidizeddonor chlorophyll P700⁺. (2) The pathway of electron transferfrom the semiquinone^– varied depending on the redox potentialvalue of each semiquinone^– /quinone couple. Low potentialquinones reduced the tertiary acceptor iron-sulfur center, F_x,while the high potential ones reduced P700⁺ directly with a200-µs halftime. (3) The E_m value of each semiquinone^–/quinone couple in situ in the reaction center was estimatedto be shifted by about 0.3 volt to the negative side from theirhalf wave redox potential values that were measured polarographicallyin dimethylformamide. The shift seems to represent the acceptorproperty of the protein environment at the Q_ø site. (4)The E_m of reconstituted phylloquinone was estimated to be 50–80mV more negative than that of F_x. (5) The mechanism of efficientelectron transfer in the reaction center was discussed basedon the dynamic equilibria between the electron transfer componentsand on the estimated E_m values. (Received April 9, 1994; Accepted July 7, 1994)     

Time-Resolved Spectroscopy of Chlorophyll-a like Electron Acceptor in the Reaction Center Complex of the Green Sulfur Bacterium Chlorobium tepidum

The absorption changes of chlorophyll (Chl) a-like pigments(C670) were studied by ns-ms laser spectroscopy at 77 K in theuntreated and urea-treated homodimeric reaction center (RC)complex of the green sulfur bacterium Chlorobium tepidum. Theuntreated RC complex contained 9 molecules of C670 in additionto 41 molecules of Bchl a and 0.9 molecules of menaquinone-7per one primary electron donor Bchl a dimer (P840). Upon photo-oxidationof P840, C670 showed an absorption change of a red-shift withan isosbestic wavelength at 668 nm. The absorption change ofP840 decayed with time constants (t_1/e) of 55 and 37 ms at 283and 77 K, respectively, and was assigned to represent the chargerecombination between P840⁺ and FeS^–. In the urea-treatedRC complex, a bleach peaking at 670 nm with a shoulder peakat 662 nm, which is ascribable to the reduced primary electronacceptor A₀^–, was detected after the laser excitationin addition to the shift at 668 nm indicating the formationof the P840⁺A₀^– state. The P840⁺A₀^– state decayedwith a t_1/e of 43 ns at 77 K and produced a triplet state p840^Tdue to the suppression of the forward electron transfer. Theseresults indicate the two different types of C670 species inthe RC complex; the one peaking at 670 nm functions as A⁰, whilethe other peaking at 668 nm shows the electrochromic shift,which presumably functions as the accessory pigment locatedin the close vicinity of P840. (Received May 17, 1999; Accepted July 14, 1999)     

Donation of Electrons from Cytosolic Components to the Intersystem Chain in the Cyanobacterium Synechococcus sp. PCC 7002 as Determined by the Reduction of P700+

Cells, of Synechococcus sp. PCC 7002 showed a low oxidationlevel of P700 under a far-red light at 6 W m^–2 which inducednearly complete oxidation of P700 in spinach leaves, and a strongerfar-red light was required to observe the oxidation of P700.DCMU did not affect the level of P700⁺² but 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinoneinduced the oxidation of P700 under far-red light, indicatingthat the low oxidation level of P700 was due to the donationof electrons to P700⁺² from the cytosolic respiratory donorsthrough the intersystem chain at the plastoquinone pool. Theelectron transfer from the cytosolic donors to the intersystemchain was inhibited by HgCl₂ but not by antimycin A. The reductionof P700⁺ in Synechococcus cells, after illumination by strongfar-red light was mostly accounted for by the electron flowto the inter system chain from the respiratory donors (t     

Extraction of electron acceptor A1 from pea photosystem I

Photosystem I particles were washed twice in either 0, 50 or 100% water-saturated diethyl ether. It was found that the characteristic electron spin resonance signal associated with electron acceptor A₁ was progressively lost with increasing percentage saturation of the ether. Light-induced electron flow to the terminal iron-sulphur acceptors was inhibited although these acceptors were still present and could be chemically reduced. The kinetics of optical measurements of P700⁺ re-reduction at 820 nm following ether washing were consistent with removal of electron acceptor A₁.     

Reconstitution of the Photosystem I Secondary Quinone Acceptor (A1) in the P700-Fx Core Isolated from Synechococcus PCC 6301

By treating a F_A/F_B-depleted P700-F_x core from SynechococcusPCC 6301 with diethylether, most of the phylloquinone was removedwithout loss of P700. The 1 ms decay of P700⁺ in the originalcore was replaced by the 25 ns decay, which was interpretedas the backreaction occurring in a P700⁺     

The Steady State Chlorophyll a Fluorescence Exhibits in Vivo an Optimum as a Function of Light Intensity which Reflects the Physiological State of the Plant

Modulated (690 and 730 nm), as well as direct chlorophyll (Chl)a fluorescence and changes in the concentration of the oxidizedP700 were measured under steady state conditions in leaves ofhigher plants adapted to different light intensities. All theleaf samples exhibit an optimum curve of steady state fluorescenceyield (F_s) versus the light intensity but its position withrespect to light intensity varies considerably from one speciesto another or from one sample to other even in the same plantor within the same leaf sample. However, the optimum level ofF_s was always at a moderate light intensity. By using the modulatedfluorescence technique, the system with all closed (F^l_m) oropen reaction center (F^l_o) were measured in steady state conditions.Each experimentally measured fluorescence yield was separatedinto a fluorescence emission of open (F_open = F^l_o,(1—V_s))and closed (F_closed = (F^l_m . V_s)) reaction center (RC) of photosystemII where V_s=(F_s – F^l_o)/(F^l_m – F^l_o) is the functionof fraction of closed reaction centers. With increasing lightintensity, the fraction of open RC decreased while the fractionof closed RC increased. Maximum quantum efficiency (P_o) andactual quantum efficiency (P) decreased by increasing lightintensity. An optimum level of F_s was observed, when the fractionof closed reaction centers V_s of each sample was about 0.2 showinga common quenching mechanism which determines the fluorescenceproperties under steady state condition. This explains the apparentphenomenological contradiction that the fluorescence yield understeady state conditions can increase or decrease upon an increaseof actinic light. (Received December 31, 1994; Accepted May 1, 1995)     

Light absorption by phytoplankton: development of a matching parameter for algal photosynthesis under different spectral regimes

A spectral matching parameter (absorption efficiency, A_e) wasdeveloped to quantify the relationship between the light absorptionspectra of phytoplankton communities and the spectral irradianceof their ambient light field. A_e was defined as the ratio betweenthe amount of radiation absorbed by the phytoplankton in situand the amount absorbed in a spectrally flat light regime. Thisapproach was applied to our measurements of spectral absorptionfor the phytoplankton communities in six lakes in High ArcticCanada that spanned a range of bio-optical conditions. A_e valueswere calculated for the light spectrum down through the watercolumn and for 11 types of artificial light source. Spectralmatching varied among lakes and with depth. There was a significantlinear relationship between the relative change in A_e with depthand the diffuse attenuation coefficient K_d (r² = 0.52, P = 0.012for K_d for the 400–700 nm waveband; r² = 0.78, P = 0.0003for K_d at 440 nm). The tabulated values for the matching parameterA_e allow the comparison of photosynthesis versus irradiance(P versus E) curves among studies using different light sources.A_e estimates also facilitate the evaluation of chromatic adaptationin natural waters, and the calculation of spectrally adjusted,in situ primary production down through a water column fromP versus E relationships under a single spectral regime.     

Heat-stabilities of Electron Transport Related to Photosystem I in a Thermophilic Blue-green Alga, Synechococcus sp.

Heat-stabilities of photosystem I reactions in a thermophilicblue-green alga, Synechococcus sp. were studied. All the reactionsexamined were highly resistant to heat as compared with thosein ordinary higher plants and algae. Cyt c-553 photooxidation in vivo was abolished by treatmentat 75?C for 5 min. By contrast, P700 photooxidation was extremelyresistant to heat and could not be completely inactivated bytreatment of the cells or isolated thylakoids at about 100?Cfor 5 min. Photooxidation of added Cyt c-553 by isolated thylakoidmembranes was more heat-stable than was this activity in cells.This suggests that heat-treatment caused a perturbation in thestructural integrity of the membranes which is required forefficient electron transfer from Cyt c-553 to P700 in situ. At higher temperatures, the inactivation of Cyt c-553 photooxidationin the membranes parallels the decrease in the rate of P700photooxidation. Spectrophotometric studies with short flashesindicated that inactivation of the electron transport from Cytc-553 to methyl viologen is due to the denaturation of a secondaryelectron acceptor of photosystem I, A₂, and possibly anotheracceptor, P430. ¹ Present address: The Solar Energy Research Group, the Instituteof Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako-shi,Saitama 351, Japan. (Received September 28, 1981; Accepted December 21, 1981)     

Orientation of Photosystem-I pigments: low temperature linear dichroism spectroscopy of a highly-enriched P700 particle isolated from spinach

The linear dichroism of Photosystem I particles containing 10 chlorophylls per P700 has been investigated at 10 K. The particles were oriented by uniaxial squeezing of polyacrylamide gels. The oxidation state of P700 was altered either by incubation of the gels with redox mediators or by low temperature illumination. The Q_Y transitions of the primary electron donor P700, of the remaining unoxidized chlorophyll in P700⁺ and of a chlorophyll molecule absorbing at 686 nm, which presumably corresponds to the primary electron acceptor A₀, are all preferentially oriented perpendicular to the gel squeezing direction. The Q_Y transition of the chlorophyll forms absorbing at 670 and 675 nm appear tilted at 40 ± 5° from this orientation axis. This orientation of the various chlorophylls is compared to that previously reported for more native Photosystem I particles.Abbreviations PSI Photosystem I - P700 primary electron donor of PSI - A₀ primary electron acceptor of PSI     

Effects of Elevated Carbon Dioxide Concentration in the Dark on the Growth of Soybean Seedlings

Previous work has shown that elevated carbon dioxide (CO₂) concentrationsin the dark reversibly reduce the rate of CO₂ efflux from soybeans.Experiments were performed exposing soybean plants continuallyto concentrations of 350 or 700 cm³ m^-3 for 24 h d^-1, or to350 during the day and 700 cm³ m^-3 at night, in order to determinethe importance of the reduced rate of dark CO₂ efflux for plantgrowth. High CO₂ applied only at night conserved carbon andincreased dry mass during initial growth compared with the constant350 cm³ m^-3 treatment. Long-term net assimilation rate was increasedby high CO₂ in the dark, without any increase in daytime leafphotosynthesis. However, leaf area ratio was reduced by thedark CO₂ treatment to values equal to those of plants continuallyexposed to the higher concentration. From days 14-21, leaf areawas less for the elevated night-time CO₂ treatment than foreither the constant 350 or 700 cm³ m^-3 treatments. For the days7-21-period, relative growth rate was significantly reducedby the high night CO₂ treatment compared with the 350 cm³ m^-3continuous treatment. The results indicate that some functionallysignificant component of respiration was reduced by the elevatedCO₂ concentration in the dark.Copyright 1995, 1999 AcademicPress Glycine max L. (Merr.), carbon dioxide, plant growth, respiration     

Triplet States in a Photosystem I Reaction Center Complex. Inhibition of Radical Pair Recombination by Bipyridinium Dyes and Naphthoquinones

Flash-induced absorption changes between 400 and 570 nm werestudied in a P700-chlorophyll a-protein complex from the thermophiliccyanobacterium Synechococcus sp. that lacked the bound secondaryelectron acceptors A₂ and P430. A positive peak at 520 nm, whichincreased linearly with the flash intensity and independentlyof the redox state of P700, is ascribed to a carotenoid triplet.Bleaching at 430 nm, which decayed with a half time of about10 µs, was abolished when P700 was oxidized with ferricyanideand saturated at a high flash intensity, indicative of its dependenceon the primary photochemistry of photosystem I. Several bipyridinium dyes and naphthoquinones suppressed the10 µs decay of the 430 nm signal in a way indicating thatthe 10 µs component represents the P700 triplet generatedby the back reaction between the reduced primary electron acceptorand oxidized P700 and that the added oxidants oxidize the reducedprimary acceptor so rapidly that back electron transfer to oxidizedP700 is prevented. Our results also show that the primary electronacceptor is located in a lypophilic environment in the chlorophyll-bindingsubunits of the photosystem I complexes. In a reaction centercomplex containing the secondary electron acceptors, the exogenousoxidants accept electrons only via P430. (Received February 23, 1984; Accepted May 1, 1984)     

Pool Size of Electrons That Can Be Donated to P700+ As Determined in Intact Leaves: Donation to P700+ from Stromal Components Via the Intersystem Chain

A method for estimation of the functional pool size of electronsthat can be donated to P700⁺ after the illumination of intactleaves with actinic light is described. The complementary areasbetween the stationary level of P700⁺ attained by irradiationwith far-red light and the oxidation curves of P700 by far-redlight after a 50-ms multiple-turnover light (MT-area), and afterthe illumination of actinic light (AL-area) were determined.Since the MT-area represents the pool size of electrons in theintersystem chain, the ratio of the AL-area to the MT-area allowsus to estimate the pooi size of electrons stored during actinicillumination that can be donated to P700⁺ The ratio of the AL-areato the MT-area was determined for intact leaves of several C₃plants, and it was found to increase to a value of two or threewith increases in the intensity of actinic light, and it wasfurther increased by anaerobiosis. The pool size of electronsthat can be donated to P700⁺ from the stroma was estimated tobe in the range of 12 and 28 electrons per P700 under aerobicconditions. These results indicate that stromal components donateelectrons to P700⁺ through the intersystem chain in chloroplastsof C₃ plants. (Received July 3, 1992; Accepted September 29, 1992)     

Inhibition of photosystem I and photosystem II in chloroplasts by UV radiation

The effects of UV radiation on the low temperature fluorescenceand primary photochemistry of PSII and PSI of spinach chloroplastswere studied. Fluorescence induction curves at –196°Cwere measured at 695 nm for PSII fluorescence and at 730 nmfor PSI fluorescence to determine both the initial F_o and finalF_M levels. The primary photochemistry of PSII was measured asthe rate of photoreduction of C-550 at – 196°C, thatof PSI as the rate of photooxidation of P₇₀₀ at –196°C.The results were analyzed in terms of a model for the photosyntheticapparatus which accounts for the yields of fluorescence andprimary photochemistry. According to this analysis UV radiationincreases nonradiative decay processes at the reaction centerchlorophyll of PSII. However, the effect of UV radiation isnot uniform throughout the sample during irradiation so thataccount must be taken of the fraction of PSII reaction centerswhich have been irradiated at any given time. UV radiation alsoinactivates P700 and causes a slight increase in nonradiativedecay in the antenna chlorophyll of PSI. All fluorescence ofvariable yield, F_V = F_M–F_o, at 730 nm is due to energytransfer from PSII to PSI so that the sensitivity of Fv to UVradiation is the same at 730 and 695 nm. ¹Present address: Department of Biology, Faculty of Science,Toho University, Narashino, Chiba 275, Japan. ²Present address: Central Research Laboratories, Fuji PhotoFilm Co., Ltd., 105 Mizonuma, Asaka-Shi, Saitama 351, Japan. (Received September 10, 1975; )     

The Photosynthetic Apparatus of Phytoplankton from a Perennially Ice-Covered Antarctic Lake: Acclimation to an Extreme Shade Environment

Phytoplankton in perennially ice-covered Lake Bonney (Antarctica)are exposed to a limited range of light variation both in termsof intensity (<1–3% of incident) and spectral distribution(blue-green) during the austral spring and summer. This relativeconstancy is due to continuous sunlight, optical filtering throughthe 4.2 m ice cap and an absence of vertical mixing. The effectsof this unique light environment on the structure and functionof the photosynthetic apparatus were studied using measurementsof P₇₀₀ reaction center content and spectral variation in photosystemII (PSII) fluorescence kinetics. Light-induced absorbance changeat both 700 nm and 810 nm was used to measure P₇₀₀ concentration.The average ratio of total Chl/P₇₀₀ was 743 (mol mol^–1),with a range of 480 to 1,039. These ratios were low in comparisonto previous studies of phytoplankton growing in low-light culturesor algae growing beneath Arctic sea ice. A sample from the deep(17 m) layer dominated by Chlamydomonas subcaudata was grownin enriched culture media. PSII fluorescence kinetics were measuredon thylakoid preparations in the presence of DCMU under blue-green(481 nm) and red (660 nm) light. C. subcaudata utilized blue-greenlight for photosynthesis more efficiently than the photobiologicallywell characterized C. reinhardtii (strain CC-124). These results,together with pigment analyses, suggest that carotenoids inLake Bonney phytoplankton are more important in light harvestingas opposed to photoprotection. (Received March 23, 1994; Accepted December 5, 1994)     

Phototransformation of the Red-Light-Absorbing Form to the Far-Red-Light-Absorbing Form of Phytochrome in Pea Epicotyl Tissue Measured by a Multichannel Transient Spectrum Analyser

Phototransformation of the red-light-absorbing form (P_R) tothe far-red-light-absorbing form (P_FR) of phytochrome in 7-day-oldetiolated pea epicotyl hook segments was examined at 0.5C aftera red laser flash excitation using a multichannel transientspectra analyser with electrically gated photomultiplier. Effectsof a red laser pulse on the induction of phototransformationfrom P_R to P_FR were saturated at Ca. 15 mJ for flash wavelengthsof both 640 and 655 nm. The amount of P_FR induced by a saturatinglaser pulse was ca. 50% of that obtained at the photostationaryequilibrium. A difference spectrum measured 15 µs afterthe flash showed an absorbance increase at 697 nm and a decreaseat 663 nm. A difference spectrum determined 200 ms after theflash showed no such major absorbance increase. Kinetic analysisof the rapid absorbance decrease at 700 and 710 nm gave onesimple first-order reaction component having a rate constantof 2,500 s^–1. Kinetics of P_FR appearance measured by absorbanceincrease at 750 nm was resolved into three first-order reactionshaving rate constants of 5, 1.8 and 0.4 s^–1. The secondflash light of 710 nm given 2 µs and 2 ms after the firstred flash irradiation on P_R resulted in the formation of P_Rrather than P_FR. (Received February 8, 1985; Accepted April 11, 1985)     

Relationship between Germination and PFR Level in Spores of the Fern Lygodium japonicum

The relationship between germination and P_FR level in sporesof the fern Lygodium japonicum was investigated. Percent P_FRestimated from direct spectrophotometric measurement of sporesincreased with the logarithm of total fluence of 660 nm-light.The transformation from P_R to P_FR was saturated by giving ca.200 Jm^–2 of 660 nm-light and half-saturated by ca. 55J^–2 of 660 nm-light. Clear positive correlation was observedbetween % P_FR levels and germination rates in spores irradiatedwith 660 nm and/or 730 nm-light, or with 686 or 700 nm-light.The P_FR percentage in spores was raised to 16–34% by blue(415 nm) light irradiation. This P_FR level was enough to causesome germination when produced by monochromatic light of redto far-red region, but blue light did not cause any germination. After 660 nm-light irradiation, the P_FR level decreased graduallyin darkness (25±1°C) and P_FR completely disappearedin 8 h, but 730 nm-light given even 16 h after 660 nm-lightirradiation inhibited germination. ⁴Present address: Tropical Botanic Garden and Research Institute,Navaranga Road, Trivandrum 695 011, India. (Received March 15, 1983; Accepted June 4, 1983)