Fluorescence lifetime,yield, energy transfer and spectrum in photosynthesis, 1950–1960*

The fluorescence lifetime of chlorophyll agives information about the primary photo-physical events in photosynthesis. Most of the light energy absorbed by chlorophylls is utilized for photochemistry. There are two main additional pathways competing for the absorbed light energy: fluorescence and radiationless internal conversion (heat). Only a few percent of the absorbed energy proceeds along these two pathways. This historical minireview focuses on the first direct measurements of the lifetime of chlorophyll fluorescence, the time it takes to transfer energy from phycoerythrin to chlorophyll a, and the discovery of the fluorescence band at 720 nm (F720; then attributed to a dimer of chlorophyll). These works were carried out during the the late 1950s to the early 1960s in the laboratory of Professor Eugene Rabinowitch at the University of Illinois, Urbana-Champaign [Brody (1995) Photosynth Res 43: 67–74]. This Minireview is dedicated to Professor Eugene Rabinowitch (1901–1973), mentor of the author (Steve Brody) as well as of the editor, and authors classmate (Govindjee). The career and contributions of Eugene Rabinowitch are available in a dedication by Bannister (1972). This revised version was published online in June 2006 with corrections to the Cover Date.     

渗透胁迫对绿豆叶圆片叶绿素荧光的影响

绿豆叶圆片用0、-0．3、-0．6、-1．2、-1．8、-2．4MPa等6个渗透梯度处理24h后,其叶绿素荧光的反应表明：光系统Ⅱ的潜在量子产量(FD／Fm)和电子传递受体(QA)的库容(pool size)均随胁迫强度的增大而减小,但QA库容的下降明显比Fg／Fm的下降缓和。结合原初荧光Fo和最大荧光Fm的变化,分析认为本实验中渗透胁迫并未导致明显的光系统Ⅱ反应中心的失活或破坏。造成光系统Ⅱ潜在量子效率降低的原因主要是通过光系统Ⅱ反应中心到Ⅱ的电子传递受到阻抑。     

两种生境条件下6种牧草叶绿素含量及荧光参数的比较

昆仑山前山牧场海拔较高, 策勒绿洲海拔相对较低, 两者生境差异较大。以昆仑山前山牧场和策勒绿洲边缘两种不同生境条件下生长的6种牧草: 冰草(Agropyron cristatum)、无芒雀麦(Bromus inermis)、矮生高羊茅(Festuca elata)、披碱草(Elymus dahuricus )、红豆草(Onobrychis pulchella)及和田大叶(Medicago sativa var. luxurians)为试验材料, 研究了不同生境条件下牧草叶片叶绿素含量及叶绿素荧光动力学参数的变化情况。结果显示: (1)在两种生境条件下, 昆仑山前山牧场生境生长的牧草叶绿素a、叶绿素b、总叶绿素的含量明显较高, 生长在策勒绿洲生境的牧草品种叶绿素a/b值较高; (2)昆仑山前山牧场生境牧草最大荧光、光系统II (PSII)最大光化学效率、PSII潜在活性和单位面积反应中心的数量的值明显高于策勒绿洲生境品种, 而初始荧光、单位反应中心吸收的光能、单位反应中心捕获的能量、单位反应中心耗散的能量、荧光诱导曲线初始斜率值则低于策勒绿洲生境品种。因此, 两种生境下环境因子发生了改变, 对牧草产生综合的胁迫作用; 策勒绿洲生境明显对牧草生长产生了抑制, 策勒绿洲生境牧草的色素含量降低以及PSII的机构遭到损坏, 导致反应中心一部分失活或裂解, 剩余有活性的反应中心的效率增加, 昆仑山生境则相对比较适宜牧草生长; 两种生境不同牧草叶绿素含量和叶绿素荧光参数的变化幅度不同。     

Photodynamic effects of hypericin on photosynthetic electron transport and fluorescence of Anacystis nidulans (Synechococcus 6301)

We investigated the photodynamic action of hypericin, a natural naphthodianthrone, on photosynthetic electron transport and fluorescence of the cyanobacterium Anacystis nidulans (Synechococcus 6301). The most drastic effect was the inactivation of photosynthetic oxygen evolution in the presence of the electron acceptor phenyl-p-benzoquinone in aerobic cells which required 1 hypericin/5 chlorophyll a for half-maximal effect. Anaerobic A. nidulans was only partially inactivated and variable chlorophyll a fluorescence remained unperturbed suggesting that photoreaction center II was not a target. Further, hypericin, stimulated photoinduced oxygen uptake in the presence of methylviologen in aerobic cells. This action was less specific than the inactivation of oxygen evolution (1 hypericin/0.5–0.7 chlorophyll a for half-maximal effect). Results point to the involvement of molecular oxygen in two ways. Type I mechanism (Henderson BW and Dougherty TJ (1992) Photochem Photobiol 55: 145–157) in which ground state oxygen reacts with excited substrate triplets appears probable for the inactivation of oxygen evolution. On the other hand, Type II mechanism in which excited oxygen singlets react with ground state substrate molecules appears probable in the stimulation of methylviologen mediated oxygen uptake.Abbreviations Chl chlorophyll - DAD diaminodurene - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - Hepes N-[2-hydroxyethyl]-N-[ethanesulfonic acid] - MV methyl viologen - PBQ phenyl-p-benzoquinone - PPFD photosynthetic photon flux density - PS I, PS II Photosystems I and II - RC I, RC II reaction centers of PS I and PS II     

Modification of excitation energy distribution to photosystem I by protein phosphorylation and cation depletion during thylakoid biogenesis in wheat

The effects of protein phosphorylation and cation depletion on the electron transport rate and fluorescence emission characteristics of photosystem I at two stages of chloroplast development in light-grown wheat leaves are examined. The light-harvesting chlorophyll a/b protein complex associated with photosystem I (LHC I) was absent from the thylakoids at the early stage of development, but that associated with photosystem II (LHC II) was present. Protein phosphorylation produced an increase in the light-limited rate of photosystem I electron transport at the early stage of development when chlorophyll b was preferentially excited, indicating that LHC I is not required for transfer of excitation energy from phosphorylated LHC II to the core complex of photosystem I. However, no enhancement of photosystem I fluorescence at 77 K was observed at this stage of development, demonstrating that a strict relationship between excitation energy density in photosystem I pigment matrices and the long-wavelength fluorescence emission from photosystem I at 77 K does not exist. Depletion of Mg²⁺ from the thylakoids produced a stimulation of photosystem I electron transport at both stages of development, but a large enhancement of the photosystem I fluorescence emission was observed only in the thylakoids containing LHC I. It is suggested that the enhancement of PS I electron transport by Mg²⁺-depletion and phosphorylation of LHC II is associated with an enhancement of fluorescence at 77 K from LHC I and not from the core complex of PS I.     

Responses of photosystem I compared with photosystem II to high-light stress in tropical shade and sun leaves

Sun and shade leaves of several plant species from a neotropical forest were exposed to excessive light to evaluate the responses of photosystem I in comparison to those of photosystem II. Potential photosystem I activity was determined by means of the maximum P700 absorbance change around 810 nm (ΔA_810max) in saturating far-red light. Leaf absorbance changes in dependence of increasing far-red light fluence rates were used to calculate a ‘saturation constant’, K_s, representing the far-red irradiance at which half of the maximal absorbance change (ΔA_810max/2) was reached in the steady state. Photosystem II efficiency was assessed by measuring the ratio of variable to maximum chlorophyll fluorescence, F_v/F_m, in dark-adapted leaf samples. Strong illumination caused a high degree of photo-inhibition of photosystem II in all leaves, particularly in shade leaves. Exposure to 1800–2000 μ mol photons m⁻² s⁻¹ for 75 min did not substantially affect the potential activity of photosystem I in all species tested, but caused a more than 40-fold increase of K_s in shade leaves, and a three-fold increase of K_s in sun leaves. The increase in K_s was reversible during recovery under low light, and the recovery process was much faster in sun than in shade leaves. The novel effect of high-light stress on the light saturation of P700 oxidation described here may represent a complex reversible mechanism within photosystem I that regulates light-energy dissipation and thus protects photosystem I from photo-oxidative damage. Moreover, we show that under high-light stress a high proportion of P700 accumulates in the oxidized state, P700⁺. Presumably, conversion of excitation energy to heat by this cation radical may efficiently contribute to photoprotection.     

披针叶茴香叶绿素荧光参数对不同光环境的响应

以天目山披针叶茴香(Illicium lanceolatum)4年生栽培幼苗为对象,经不同光环境(自然全光照,50%光照和20%光照)处理后,采用雅欣理1611植物效率仪测试,并进行光系统Ⅱ(PSⅡ)快速叶绿素荧光诱导动力学分析(JIP-test),以探讨其光适应机制,为高莽草酸含量植株高效栽培技术提供理论依据。结果显示:(1)随着遮光程度增加,披针叶茴香叶片叶绿素a(Chl a)、叶绿素b(Chl b)和总叶绿素含量(Chl(a+b))呈上升趋势,均与全光照存在显著差异;Chl a/Chl b值分别降低了22.92%和31.56%,均与全光照差异极显著。(2)随遮光程度增强,PSⅡ最大光能转换效率(Fv/Fm)降低,50%和20%光照处理的Fv/Fm值分别比全光照下降了1.34%和2.79%,且20%光照处理与全光照差异显著。(3)随遮光程度增强,50%光照和20%光照处理叶片单位面积光合机构含有的反应中心数目(RC/CSo)分别比全光照减少了2.94%和13.63%,单位反应中心以热能形式耗散的能量(DIo/RC)分别增加了2.2%和62.9%。研究表明,50%光照处理下披针叶茴香叶片用于光合电子传递的能量占吸收光能的比例变化不显著,而在20%光照处理则显著降低,即50%遮光环境有利于披针叶茴香提高光能利用效率,促进其生长和增加生物量积累。     

Assessing the relationship between respiratory acclimation to the cold and photosystem II redox poise in Arabidopsis thaliana

We examined the effect of manipulating photosystem II (PSII) redox poise on respiratory flux in leaves of Arabidopsis thaliana. Measurements were made on wild-type (WT) plants and npq4 mutant plants deficient in non-photochemical quenching (NPQ). Two experiments were carried out. In the first experiment, WT and mutant warm-grown plants were exposed to three different irradiance regimes [75, 150 and 300 micromol photosynthetically active radiation (PAR)], and leaf dark respiration was measured in conjunction with PSII redox poise. In the second experiment, WT and mutant warm-grown plants were shifted to 5 degrees C and 75, 150 or 300 micromol PAR, and dark respiration was measured alongside PSII redox poise in cold-treated and cold-developed leaves. Despite significant differences in PSII redox poise between genotypes and irradiance treatments, neither genotype nor growth irradiance had any effect upon the rate of respiration in warm-grown, cold-treated or cold-developed leaves. We conclude that changes in PSII redox poise, at least within the range experienced here, have no direct impacts on rates of leaf dark respiration, and that the respiratory cold acclimation response is unrelated to changes in chloroplast redox poise.     

Evidence for the contribution of the S-state transitions of oxygen evolution to the initial phase of fluorescence induction

Fluorescence induction of isolated spinach chloroplasts was measured by using weak continuous light. It is found that the kinetics of the initial phase of fluorescence induction as well as the initial fluorescence level F_j are influenced by the number of preilluminating flashes, and shows damped period 4 oscillation. Evidence is given to show that it is correlated with the S-state transitions of oxygen evolution. Based on the previous observations that the S states can modulate the fluorescence yield of Photosystem II, a simulating calculation suggests that, in addition to the Photosystem II centers inactive in the plastoquinone reduction, the S-state transitions can also make a contribution to the intial phase of fluorescence induction.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - F₀ non-variable fluorescence level emitted when all PS II centers are open - F_i initial fluorescence level immediately after shutter open - F_pt intermediate plateau fluorescence level - F_m maximum fluorescence level emitted when all PS II centers are closed - PS II Photosystem II - Q_A primary quinone acceptor of PS II - Q_B secondary quinone acceptor of PS II     

Energy transfer for low temperature fluorescence in PS II mutant thylakoids

The Chl-protein complexes of three maize (Zea mays L.) mutants and one barley (Hordeum vulgare L.) mutant were analyzed using low temperature Chl fluorescence emissions spectroscopy and LDS-polyacrylamide gel electrophoresis. The maize mutants hcf-3, hcf-19, and hcf-114 all exhibited a high Chl fluorescence (hcf) phenotype indicating a disruption of the energy transfer within the photosynthetic apparatus. The mutations in each of these maize mutants affects Photosystem II. The barley mutant analyzed was the well characterized Chl b-less mutant chlorina-f2, which did not exhibit the hcf phenotype. Chlorina-f2 was used because no complete Chl b-less mutant of maize is available. Analysis of hcf-3, hcf-19, and hcf-114 revealed that in the absence of CP43, LHC II can still transfer excitation energy to CP47. These results suggest that in mutant membranes LHC II can interact with CP47 as well as CP43. This functional interaction of LHC II with CP47 may only occur in the absence of CP43, however, it is possible that LHC II is positioned in the thylakoid membranes in a manner which allows association with both CP43 and CP47.Abbreviations hcf high chlorophyll fluorescence - LDS lithium dodecyl sulfate - LHC II light-harvesting complex of Photosystem II - LHC I light-harvesting complex of Photosystem I - CPIa chlorophyll-protein complex consisting of LHC I and the PS I core complex - CPI chlorophyll-protein complex consisting of the PS I core complex - CP47 47 kDa chlorophyll-protein of the Photosystem II core - CP43 43 kDa chlorophyll-protein of the Photosystem II core - CP29 29 kDa chlorophyll-protein of Photosystem II - CP26 26 kDa chlorophyll-protein of Photosystem II - CP24 24 kDa chlorophyll-protein of Photosystem II - fp free pigments     

Effects of nickel hyperaccumulation on physiological characteristics of Alyssum murale grown on metal contaminated waste amended soil

A pot experiment was conducted to investigate the effect of nickel concentration on physiological characteristics of Alyssum murale when grown in a soil mixed with sewage sludge (at the rate of 2.8%). Two types of sludge were used: agricultural sewage sludge (S1) and industrial sewage sludge with an increasing nickel concentration (S2, S3, and S4). Results showed that Ni in shoots was higher than Ni in roots. A. murale is able to concentrate up to 12730 mg/kg Ni in leaves. The highest dry matter yield was observed with plants grown with agricultural sewage sludge. An addition of S2 and S3 increased shoot biomass. However, application of S4 reduced 40% shoot dry weight as compared to the control Ni treatment did not affect all chlorophyll fluorescence parameters. The F(v)/F(m) ratio was stable between Ni treatments. Photosynthesis rate (A) increased with agricultural sewage sludge, but remained stable with variable Ni rates from the industrial sludge. The chlorophyll content increased with S1, S2 and S3 but it remains constant with S4 when compared to the control Therefore, high nickel concentration did not affect the function of the photosynthetic machine of A. murale.     

On the long-wavelength spectral forms of chlorophyll a in Photosystem I: Spectroscopic and immunological investigations on a greening mutant of the green alga Scenedesmus obliquus

The origin of the long-wavelength chlorophyll (Chl) absorption (_peak > 680 nm) and fluorescence emission (_peak > 685 nm) has been investigated on Scenedesmus mutants (C-2A-series, lacking the ability to synthesize chlorophyll in the dark) grown at 0.3 (LL), 10 (ML) and 240 µE s^–1 m^–2(HL). LL cells are arrested in an early greening state; consequently, Chl availability determines the phenotype. LL thylakoids are totally lacking long-wavelength Chl; nonetheless, PS I and PS II are fully functional. Gel electrophoresis and Western blots indicate that four out of seven resolved LHC polypeptides seem to require a high Chl availability for assembly of functional chlorophyll-protein complexes. The PS I core-complex of ML and HL thylakoids contains long-wavelength chlorophylls, but in the PS I core-complex of LL thylakoids these pigments are lacking. We conclude that long-wavelength pigments are only present in the PS I core in the case of high Chl availability. The following hypothesis is discussed: Chl availability determines not only the LHC polypeptide pattern, but also the number of bound Chl molecules per individual pigment-protein complex. Chl-binding at non-obligatory, peripheral sites of the pigment-protein complex results in long-wavelength Chl. In the case of low Chl availability, these sites are not occupied and, therefore, the long-wavelength Chl is absent.     

Spectrofluorimetric method for the determination of large chlorophyll a/b ratios

A practical spectrofluorimetric calibration method at room temperature is described for determining large chlorophyll a/b ratios on direct extracts from plant material of pigments in 80% aqueous acetone, dimethyl sulfoxide, and N,N-dimethylformamide. The method is based on the work of Boardman and Thorne [Boardman NK and Thorne SW (1971) Biochim Biophys Acta 253: 222–231] who used diethyl ether as solvent. We repeated the calibration in diethyl ether and find significantly different parameters for the calibration curve. The range of standards in this work included solutions with chlorophyll a/b ratios of 10–125 in dimethyl sulfoxide, and of 10–220 in the other solvents. Fluorescence emission spectra were found to be a highly sensitive method for assessing chlorophyll purity. We determined the limits of sensitivity for each solvent from the calibration data. The empirically determined slope of the calibration curve was shown to be related to intrinsic properties of the chlorophylls in solution; this allows predictions of the performance of the method in other solvents.     

Resolution of low-energy chlorophylls in Photosystem I of Synechocystis sp. PCC 6803 at 77 and 295 K through fluorescence excitation anisotropy

Fluorescence excitation spectra of highly anisotropic emission from Photosystem I (PS I) were measured at 295 and 77 K on a PS II-less mutant of the cyanobacterium Synechocystis sp. PCC 6803 (S. 6803). When PS I was excited with light at wavelengths greater than 715 nm, fluorescence observed at 745 nm was highly polarized with anisotropies of 0.32 and 0.20 at 77 and 295 K, respectively. Upon excitation at shorter wavelengths, the 745-nm fluorescence had low anisotropy. The highly anisotropic emission observed at both 77 and 295 K is interpreted as evidence for low-energy chlorophylls (Chls) in cyanobacteria at room temperature. This indicates that low-energy Chls, defined as Chls with first excited singlet-state energy levels below or near that of the reaction center, P700, are not artifacts of low-temperature measurements.If the low-energy Chls are a distinct subset of Chls and a simple two-pool model describes the excitation transfer network adequately, one can take advantage of the low-energy Chls' high anisotropy to approximate their fluorescence excitation spectra. Maxima at 703 and 708 nm were calculated from 295 and 77 K data, respectively. Upper limits for the number of low-energy Chls per P700 in PS I from S. 6803 were calculated to be 8 (295 K) and 11 (77 K).Abbreviations Chl - chlorophyll - BChl - bacteriochlorophyll - LHC - light-harvesting chlorophyll - PS - Photosystem - RC - reaction center - S. 6803 - Synechocystis sp. PCC 6803     

Eugene I. Rabinowitch: A prophet of photosynthesis and of peace in the world

Photosynthesis Research - More than 45 years have passed since Eugene I. Rabinowitch died, on May 15, 1973, at the age of 75, but many still remember him as a photosynthesis giant, the...     

Fluorescence induction of Photosystem II membranes shows the steps till reduction and protonation of the quinone pool

Chlorophyll fluorescence induction (Chl-F) was investigated in Photosystem II (PSII)-enriched membranes, which predominantly include active (Q_B reducing) PSII reaction centres (RCs) and lack Photosystem I (PSI). The Chl-F curve of these preparations show a polyphasic rise from F₀, the minimal fluorescence, to F_P, the maximal fluorescence, with several intermediate transitions. Analyses of these transitions revealed three exponential rise components with lifetimes of 18 ms, 400 ms and 800 ms. The 18 ms component was assigned to the photoaccumulation of reduced Q_A. The two slowest components, of 400 ms and 800 ms, were assigned to Q_B reduction (Q_B⁻ and Q_B⁼) and further Q_B⁼ protonation (till Q_BH₂), respectively. These assignments were based on the observation of specific quenching of the phases by DCMU or by different oxidized, reduced and protonated quinones. The work is done in low light conditions which are saturating to avoid photoinhibition or PSII inactivation effects. The results suggest that the Chl-F curve observed in PSII-enriched membranes can be attributed to the sequential steps till the photoaccumulation (reduction and protonation) of plastoquinone (PQ) by PSII. These results are in good agreement with the molecular models that show a correspondence between Chl-F and PQ reduction steps, like the models that propose and explain the O-J-I-P transients.     

Photosynthesis and non-photochemical excitation quenching components of chlorophyll excitation in maize and field bean during chilling at different photon flux density

The influence of chilling (8 °C, 5 d) at two photon flux densities [PFD, L = 200 and H = 500 μmol(photon) m⁻² s⁻¹] on the gas exchange and chlorophyll fluorescence was investigated in chilling-tolerant and chilling-sensitive maize hybrids (Zea mays L., K383×K130, K185×K217) and one cultivar of field bean (Vicia faba L. minor, cv. Nadwiślański). The net photosynthetic rate (P _N) for the both studied plant species was inhibited at 8 °C. P _N of both maize hybrids additionally decreased during chilling. Changes in the quantum efficiency of PS2 electron transport (Φ_PS2) as a response to chilling and PFD were similar to P _N. Measurements of Φ_PS2/Φ_CO2 ratio showed that in field bean seedlings strong alternative photochemical sinks of energy did not appear during chilling. However, the high increment in Φ_PS2/Φ_CO2 for maize hybrids can indicate reactions associated with chill damage generation. At 8 °C the non-photochemical quenching (NPQ) increased in all plants with chilling duration and PFD. The appearance of protective (q_I,p) and damage (q_I,d) components of q_I and a decrease in q_E (energy dependent quenching) took place. NPQ components of field bean and maize hybrids differed from each other. The amount of protective NPQ (q_E + q_I,p) components as part of total NPQ was higher in field bean than in maize hybrids at both PFD. On 5^th day of chilling, the sum of q_E and q_I,p was 26.7 % of NPQ in tolerant maize hybrids and 17.6 % of NPQ in the sensitive one (averages for both PFD). The increased PFD inhibited the ability of all plants to perform protective dissipation of absorbed energy. The understanding of the genotypic variation of NPQ components in maize may have implications for the future selection of plants with a high chilling tolerance.     

An Analysis of the Mechanism of the Low-wave Phenomenon of Chlorophyll Fluorescence

The low-wave phenomenon, i.e., the transient drop of yield of modulated chlorophyll fluorescence shortly after application of a pulse of saturating light, was investigated in intact leaves of tobacco and Camellia by measuring fluorescence, CO(2) assimilation and absorption at 830 nm simultaneously. Limitations on linear electron flow, due to low electron acceptor levels that were induced by low CO(2), induced the low waves of chlorophyll fluorescence. Low-wave amplitudes obtained under different CO(2) concentrations and photon-flux densities yielded single-peak curves when plotted as functions of fluorescence parameters such as PhiPS II (quantum yield of Photosystem II) and qN (coefficient of non-photochemical quenching), suggesting that low-wave formation depends on the redox state of the electron transport chain. Low waves paralleled redox changes of P700, the reaction center of Photosystem I (PS I), and an additional electron flow through PS I was detected during the application of saturating pulses that induced low-waves. It is suggested that low waves of chlorophyll fluorescence are induced by increased non-photochemical quenching, as a result of the formation of a trans-thylakoid proton gradient due to cyclic electron flow around PS I.     

Tentoxin effects on variable fluorescence and P515 electrochromic absorbance changes in tentoxin-sensitive and -resistant plant species

The effects of the cyclic tetrapeptide phytotoxin, tentoxin, on variable chlorophyll fluorescence quenching and the P515 electrochromic absorbance change were examined in tentoxin-sensitive and -resistant species and an interspecies hybrid. Immediately after infiltration of leaf discs, up to 100 μM tentoxin had no effect on the O, I, D, or P portions of the fluoresence transients of either tentoxin-sensitive or -resistant plants. However, the quenching of fluorescence following maximal fluorescence was reduced by approximately fourfold in tentoxin-treated tissues of tentoxin-sensitive plants, but was unaffected in resistant plants. Tentoxin significantly increased the magnitude of the P515 electrochromic absorbance change in all tentoxin sensitive plants by an average of approximately twofold. However, it was unaffected by tentoxin in resistant species. These data suggest that there is a close correlation between interaction of tentoxin with CF₁ ATPase in vivo and the ability to cause chlorosis in developing chloroplasts.     

Studies on the Localization and Spectral Characteristics of the Fluorescence Emission of Differently Pigmented Wheat Leaves

Intensity, spectral characteristics and localization of the UV-laser (337 nm) induced blue-green and red fluorescence emission of green, etiolated and white primary leaves of wheat seedlings were studied in a combined fluorospectral and fluoromicroscopic investigation. The blue-green fluorescence of the green leaf was characterized by a maximum near 450 nm (blue region) and a shoulder near 530 nm (green region), whereas the red chlorophyll fluorescence exhibited maxima in the near-red (F690) and far-red (F735). The etiolated leaf with some carotenoids and traces of chlorophyll a, in turn, showed a higher intensity of the blue-green fluorescence with a shoulder in the green region and a strong red fluorescence peak near 684 to 690 nm, the far-red chlorophyll fluorescence maximum (F735) was, however, absent. The norfluorazone-treated white leaf, free of chlorophylls and carotenoids, only exhibited blue-green fluorescence of a very high intensity. In green and etiolated leaves the blue-green fluorescence primarily derived from the cell walls of the epidermis and the red fluorescence from the chlorophyll a of the mesophyll cells. In white leaves the blue-green fluorescence emanated from all cell walls of epidermis, mesophyll and leaf vein bundles. The shape and intensity of the blue-green and red fluorescence emission is determined by the reabsorption properties of chlorophylls and carotenoids in the mesophyll, thus giving rise to quite different values of the various fluorescence ratios F450/F690, F450/F530, F450/F735 and F690/F735 in green and etiolated leaves.