外源钙对干旱胁迫下烤烟幼苗光系统Ⅱ功能的影响

以叶绿素快相荧光动力学曲线(OJIP)为探针,研究了外源钙对干旱胁迫下烤烟幼苗光系统Ⅱ(PSⅡ)功能的影响.结果表明:干旱胁迫降低了烤烟幼苗PSⅡ原初光能转换效率(Fv/Fm)和电子传递速率(ETR),抑制了光合作用的原初过程,烤烟幼苗叶片发生了明显的光抑制.叶面喷施10.0 mmol·L-1CaCl2溶液后烤烟叶片的光合电子传递能量比例(ФEo)在干旱胁迫下的降低幅度明显小于对照(喷施清水),电子转运效率(ET0/RC)在干旱胁迫下明显高于对照.叶面喷施CaC12溶液增加了PSⅡ捕获光能用于光合电子传递的比例、剩余有活性反应中心的效率和电子传递链中的能量传递,使烤烟叶片的光系统Ⅱ在干旱胁迫下保持相对较高的活性,从而提高了烤烟幼苗的抗旱能力.     

Quantifying and monitoring functional photosystem II and the stoichiometry of the two photosystems in leaf segments: approaches and approximations

Given its unique function in light-induced water oxidation and its susceptibility to photoinactivation during photosynthesis, photosystem II (PS II) is often the focus of studies of photosynthetic structure and function, particularly in environmental stress conditions. Here we review four approaches for quantifying or monitoring PS II functionality or the stoichiometry of the two photosystems in leaf segments, scrutinizing the approximations in each approach. (1) Chlorophyll fluorescence parameters are convenient to derive, but the information-rich signal suffers from the localized nature of its detection in leaf tissue. (2) The gross O(2) yield per single-turnover flash in CO(2)-enriched air is a more direct measurement of the functional content, assuming that each functional PS II evolves one O(2) molecule after four flashes. However, the gross O(2) yield per single-turnover flash (multiplied by four) could over-estimate the content of functional PS II if mitochondrial respiration is lower in flash illumination than in darkness. (3) The cumulative delivery of electrons from PS II to P700(+) (oxidized primary donor in PS I) after a flash is added to steady background far-red light is a whole-tissue measurement, such that a single linear correlation with functional PS II applies to leaves of all plant species investigated so far. However, the magnitude obtained in a simple analysis (with the signal normalized to the maximum photo-oxidizable P700 signal), which should equal the ratio of PS II to PS I centers, was too small to match the independently-obtained photosystem stoichiometry. Further, an under-estimation of functional PS II content could occur if some electrons were intercepted before reaching PS I. (4) The electrochromic signal from leaf segments appears to reliably quantify the photosystem stoichiometry, either by progressively photoinactivating PS II or suppressing PS I via photo-oxidation of a known fraction of the P700 with steady far-red light. Together, these approaches have the potential for quantitatively probing PS II in vivo in leaf segments, with prospects for application of the latter two approaches in the field.     

The time course of photoinactivation of photosystem II in leaves revisited

Since photosystem II (PS II) performs the demanding function of water oxidation using light energy, it is susceptible to photoinactivation during photosynthesis. The time course of photoinactivation of PS II yields useful information about the process. Depending on how PS II function is assayed, however, the time course seems to differ. Here, we revisit this problem by using two additional assays: (1) the quantum yield of oxygen evolution in limiting, continuous light and (2) the flash-induced cumulative delivery of PS II electrons to the oxidized primary donor (P700(+)) in PS I measured as a 'P700 kinetics area'. The P700 kinetics area is based on the fact that the two photosystems function in series: when P700 is completely photo-oxidized by a flash added to continuous far-red light, electrons delivered from PS II to PS I by the flash tend to re-reduce P700(+) transiently to an extent depending on the PS II functionality, while the far-red light photo-oxidizes P700 back to the steady-state concentration. The quantum yield of oxygen evolution in limiting, continuous light indeed decreased in a way that deviated from a single-negative exponential. However, measurement of the quantum yield of oxygen in limiting light may be complicated by changes in mitochondrial respiration between darkness and limiting light. Similarly, an assay based on chlorophyll fluorescence may be complicated by the varying depth in leaf tissue from which the signal is detected after progressive photoinactivation of PS II. On the other hand, the P700 kinetics area appears to be a reasonable assay, which is a measure of functional PS II in the whole leaf tissue and independent of changes in mitochondrial respiration. The P700 kinetics area decreased in a single-negative exponential fashion during progressive photoinactivation of PS II in a number of plant species, at least at functional PS II contents ≥6?% of the initial value, in agreement with the conclusion of Sarvikas et al. (Photosynth Res 103:7-17, 2010). That is, the single-negative-exponential time course does not provide evidence for photoprotection of functional PS II complexes by photoinactivated, connected neighbours.     

Hyponastic leaf growth decreases the photoprotective demand, prevents damage to photosystem II and delays leaf senescence in Salvia broussonetii plants

The influence of leaf angle on the response of plants to high light was studied in Salvia broussonetii, a species endemic of the Canary Islands that shows hyponastic leaf growth. The response of vertical, naturally oriented leaves was compared with that of horizontal, artificially held leaves for 1, 13, 24 and 29 days in terms of photoinhibition [efficiency of photosystem II (PSII)], photoprotection (by the xanthophyll cycle, alpha-tocopherol and beta-carotene) and progression of leaf senescence. Vertical leaves not only showed a decreased photoprotective demand compared with horizontal leaves but also kept the maximum efficiency of PSII (F(v)/F(m) ratio) constant throughout the experiment, thus reflecting the capacity of naturally oriented leaves to avoid photooxidative stress in the field. By contrast, horizontal leaves, which were exposed to higher light intensities, showed a higher photoprotective demand (reflected by a higher de-epoxidation of the xanthophyll cycle, carotenoid losses and increases in alpha-tocopherol), damage to PSII (as indicated by decreases in the F(v)/F(m) ratio) and accelerated leaf senescence, which was associated with cell death after 24 days of high light exposure. It is concluded that hyponastic leaf growth prevents photoinhibition and decreases the photoprotective demand of leaves by reducing the incident light, which helps maintaining leaf vigor and delaying the progression of leaf senescence in S. broussonetii plants. Hyponastic leaf growth is therefore one of the first photoprotection mechanisms activated in this species to avoid the negative impact of high-light stress in the field.     

短期UV-B辐射对青藏高原美丽风毛菊PS Ⅱ光化学效率的影响

通过短期增补UV-B辐射模拟试验,研究了青藏高原典型天气(晴天、多云、阴天)下高山植物美丽风毛菊叶片的叶绿素荧光参数变化.结果表明:随天气由晴变阴,美丽风毛菊叶片暗适应3 min的PSⅡ最大光化学量子效率(Fv/Fm)显著升高,实际PSⅡ光化学效率(ФPSⅡ)和光化学猝灭系数(qp)也升高,而非光化学猝灭系数(NPQ)则降低,可见光辐射(PAR)是影响PSⅡ光能转化效率的主要因素.增补UV-B辐射后,3种典型天气下,美丽风毛菊叶片的Fv/Fm和NPQ略有降低,ФPSⅡ和qp略微增加,但对光合气体交换过程没有产生负影响.叶片净光合速率Pn和ФPSⅡ的增高趋势与增补UV-B辐射下相对较多的UV-A成分有关,同时也得益于叶片厚度的增加.UV-B辐射对叶片光合机构具有潜在负影响.     

Photoinactivation of photosystem II complexes and photoprotection by non-functional neighbours in Capsicum annuum L. leaves

Leaf segments from Capsicum annuum plants grown at 100 micromol photons m(-2) s(-1) (low light) or 500 micromol photons m(-2) s(-1) (high light) were illuminated at three irradiances and three temperatures for several hours. At various times, the remaining fraction (f) of functional photosystem II (PS II) complexes was measured by a chlorophyll fluorescence parameter (1/Fo -1/Fm, where Fo and Fm are the fluorescence yields corresponding to open and closed PS II traps, respectively), which was in turn calibrated by the oxygen yield per saturating single-turnover flash. During illumination of leaf segments in the presence of lincomycin, an inhibitor of chloroplast-encoded protein synthesis, the decline of f from 1.0 to about 0.3 was mono-exponential. Thereafter, f declined much more slowly, the remaining fraction (approximately equals 0.2) being able to survive prolonged illumination. The results can be interpreted as being in support of the hypothesis that photoinactivated PS II complexes photoprotect functional neighbours (G. Oquist et al. 1992, Planta 186: 450-460), provided it is assumed that a photoinactivated PS II is initially only a weak quencher of excitation energy, but becomes a much stronger quencher during prolonged illumination when a substantial fraction of PS II complexes has also been photoinactivated. In the absence of lincomycin, photoinactivation and repair of PS II occur in parallel, allowing f to reach a steady-state value that is determined by the treatment irradiance, temperature and growth irradiance. The results obtained in the presence and absence of lincomycin are analysed according to a simple kinetic model which formally incorporates a conversion from weak to strong quenchers, yielding the rate coefficients of photoinactivation and of repair for various conditions, as well as gaining an insight into the influence off on the rate coefficient of photoinactivation. They demonstrate that the method is a convenient alternative to the use of radiolabelled amino acids for quantifying photoinactivation and repair of PS II in leaves.     

Nitrite regulates distribution of excitation energy between the two photosystems by causing state transition.

The mechanism of distribution of absorbed excitation energy between the two photosystems in the presence of nitrite has been investigated in spinach (Spinacia oleracea L.) thylakoid membranes. Nitrite inhibited PS II activity (H(2)O --> DCPIP reaction) and enhanced PS I activity (DCPIPH(2) --> MV reaction). Nitrite decreased the F(v)/F(m) ratio measured at room temperature and increased the F(730)/F(685) ratio measured at low temperature (77 K). These results suggested that nitrite caused a decrease in the excitation energy available to PS II and transferred more energy to PS I by the mechanism of state transition. Measurement of fluorescence excitation spectra at 77 K showed that nitrite increased the absorption cross-section of PS I antenna at the expense of chlorophyll b and LHC II. Based on these observations we have suggested a role of nitrite in causing state transition.     

Thermal stability studies of photosystem II complexes reconstituted into phosphatidylcholine liposomes

Light-harvesting II complexes (LHCII) and photosystem II core complexes (PSIICC) were isolated from spinach (Spinacia oleracea L.) and reconstituted into phosphatidylcholine liposomes and, under heat stress, PSIICC-LHCII proteoliposomes were found to exhibit significantly higher oxygen evolution activity than PSIICC proteoliposomes lacking LHCII. In the presence of LHCII, the temperature of a 10-min heat stress that caused semi-inactivation of oxygen-evolving activity in these liposomes increased from 34 to ～37°C and the total inactivation temperature increased from ～50 to ～60°C. Moreover, with heat stress, decreases in the absorbance and fluorescence spectra of PSIICC-LHCII proteoliposomes were smaller than in LHCII-lacking PSIICC proteoliposomes. These results demonstrated that reconstitution of PSII into liposomes with LHCII increased the antenna size and light harvesting cross-section of PSII and thus, under heat stress, enhanced PSII photochemical activity and thermal stability.     

甘蔗抗旱性生理生化鉴定指标

利用因子分析和灰色关联度分析方法研究了甘蔗叶片相对含水量、膜脂过氧化代谢、活性氧代谢、光合参数及蔗茎产量性状等指标与抗旱性的关系.结果表明,干旱胁迫下甘蔗叶片的MDA含量和PMP明显提高,而RWC、SOD活性、Chl含量、F_v/F_m、F_v/F_o、△F_v/F_t、△F_v/F_o和蔗茎单茎重(SSW)8个抗旱性指标均显著降低.SSW与其它9个生理生化指标的相关性大小依次为PMP＞SOD活性＞MDA含量＞RWC＞F_v/F_o＞F_v/F_m＞Chl含量＞F_v/F_o＞F_v/F_t,其中,SSW与F_v/F_o和ΔF_v/F_t相关性不显著.通过因子分析将10个甘蔗抗旱性指标用4个公共因子表示,累加方差贡献率达到92.08%.因子l主要是反映光合作用特性指标对甘蔗品种抗旱性起支配作用,因子2主要是反映叶片相对含水量及活性氧代谢指标对甘蔗品种抗旱性起支配作用,因子3和因子4分别只有SSW和Chl含量有较大载荷.灰色关联度分析表明,各抗旱性生理生化指标与SSW关联密切程度依次为F_v/F_m＞PMP＞F_v/F_o＞RWC＞MDA含量＞SOD活性＞ΔF_v/F_t＞Chl含量＞F_v/F_o.     

Delayed fluorescence observed in the nanosecond time region at 77 K originates directly from the photosystem II reaction center

The excited-state dynamics of delayed fluorescence in photosystem (PS) II at 77 K were studied by time-resolved fluorescence spectroscopy and decay analysis on three samples with different antenna sizes: PS II particles and the PS II reaction center from spinach, and the PS II core complexes from Synechocystis sp. PCC 6803. Delayed fluorescence in the nanosecond time region originated from the 683-nm component in all three samples, even though a slight variation in lifetimes was detected from 15 to 25 ns. The relative amplitude of the delayed fluorescence was higher when the antenna size was smaller. Energy transfer from the 683-nm pigment responsible for delayed fluorescence to antenna pigment(s) at a lower energy level was not observed in any of the samples examined. This indicated that the excited state generated by charge recombination was not shared with antenna pigments under the low-temperature condition, and that delayed fluorescence originates directly from the PS II reaction center, either from chlorophyll a(D1) or P680. Supplemental data on delayed fluorescence from spinach PS I complexes are included.     

控释肥对菊花叶片叶绿素荧光特性及观赏品质的影响

以切花菊品种‘白马’为材料,采用盆栽试验研究了控释肥对菊花叶片叶绿素荧光参数、叶绿素和养分含量及观赏品质的影响.结果表明:未施肥处理(对照)的菊花叶片PSⅡ原初光化学效率(Fv/Fm)、PSⅡ潜在光化学活性(Fv/Fo)和PSⅡ量子效率(ΦPSⅡ)与施肥处理相比显著下降;两种普通复合肥CCFA(N∶P∶K=20∶8∶10)和CCFB(N∶P∶K=14∶14∶14)处理的Fv/Fm、Fv/Fo和ΦPSⅡ在前期(30~60 d)比两个控释肥CRFA(N∶P∶K=20∶8∶10)和CRFB(N∶P∶K=14∶14∶14)处理有所增高,但在中后期(75~120 d)比两个控释肥处理显著下降.CRFA处理的Fv/Fm、ΦPSⅡ和光化学猝灭系数(qP)比CRFB处理有所增高.两种控释肥处理的非化学猝灭系数(NPQ)与对照和两种普通复合肥处理相比显著下降.各处理叶绿素含量变化规律与Fv/Fm、Fv/Fo和ΦPSⅡ基本一致.切花采收期CRFA和CRFB处理的叶片N、P、K含量以及花梗长、花梗粗、花径、花鲜质量和干质量均高于CCFA、CCFB和对照,而且CRFA处理的花鲜质量和干质量比CRFB处理显著增高.表明控释肥可以通过提...     

Inhibition of photosystem II of spinach by the respiration inhibitors piericidin A and thenoyltrifluoroacetone

The effects of several respiration inhibitors on photosystem II (PS II) were investigated. Among the agents tested, piericidin A and thenoyltrifluoroacetone (TTFA) inhibited the photosynthetic electron transport of spinach as measured from chlorophyll (Chl) fluorescence parameters (Fm'-F)/Fm' and Fv/Fm. Using specific donors and acceptors of electrons, we identified the sites of inhibition in and around the PS II complex; the site of inhibition by TTFA was between QA, primary quinone acceptor in PS II, and QB, secondary quinone acceptor, in the acceptor side of P680, the reaction center Chl of PS II, while inhibition by piericidin A of the acceptor side was downstream of Q(B), out of the PS II complex. Both agents also inhibited the donor side of P680, probably between tyrosine-161 of the reaction center protein of PS II and P680.     

Photoinactivation of photosystem II by cumulative exposure to short light pulses during the induction period of photosynthesis

Photoinactivation of Photosystem (PS) II in vivo was investigated by cumulative exposure of pea, rice and spinach leaves to light pulses of variable duration from 2 to 100 s, separated by dark intervals of 30 min. During each light pulse, photosynthetic induction occurred to an extent depending on the time of illumination, but steady-state photosynthesis had not been achieved. During photosynthetic induction, it is clearly demonstrated that reciprocity of irradiance and duration of illumination did not hold: hence the same cumulative photon exposure (mol m^–2) does not necessarily give the same extent of photoinactivation of PS II. This contrasts with the situation of steady-state photosynthesis where the photoinactivation of PS II exhibited reciprocity of irradiance and duration of illumination (Park et al. (1995) Planta 196: 401–411). We suggest that, for reciprocity to hold between irradiance and duration of illumination, there must be a balance between photochemical (qP) and non-photochemical (NPQ) quenching at all irradiances. The index of susceptibility to light stress, which represents an intrinsic ability of PS II to balance photochemical and non-photochemical quenching, is defined by the quotient (1-qP)/NPQ. Although constant in steady-state photosynthesis under a wide range of irradiance (Park et al. (1995). Plant Cell Physiol 36: 1163–1169), this index of susceptibility for spinach leaves declined extremely rapidly during photosynthetic induction at a given irradiance, and, at a given cumulative photon exposure, was dependent on irradiance. During photosynthetic induction, only limited photoprotective strategies are developed: while the transthylakoid pH gradient conferred some degree of photoprotection, neither D1 protein turnover nor the xanthophyll cycle was operative. Thus, PS II is more easily photoinactivated during photosynthetic induction, a phenomenon that may have relevance for understorey leaves experiencing infrequent, short sunflecks.Abbreviations D1 protein psbA gene product - DTT dithiothreitol - F_v, F_m, F_o variable, maximum, and initial (corresponding to open traps) chlorophyll fluorescence yield, respectively - NPQ non-photochemical quenching - PS Photosystem - Q_A primary quinone acceptor of PS II - qP photochemical quenching coefficient     

Photosystem II efficiency in low chlorophyll,iron-deficient leaves

Iron deficiency (iron chlorosis) is the major nutritional stress affecting fruit tree crops in calcareous soils in the Mediterranean area. This work reviews the changes in PS II efficiency in iron-deficient leaves. The iron deficiency-induced leaf yellowing is due to decreases in the leaf concentrations of photosynthetic pigments, chlorophylls and carotenoids. However, carotenoids, and more specifically lutein and the xanthophylls of the V+A+Z (Violaxanthin+ Antheraxanthin+Zeaxanthin) cycle are less affected than chlorophylls. Therefore, iron-chlorotic leaves grown in either growth chambers or field conditions have increases in the molar ratios lutein/chlorophyll a and (V+A+Z)/chlorophyll a. These pigment changes are associated to changes in leaf absorptance and reflectance. In the chlorotic leaves the amount of light absorbed per unit chlorophyll increases. The low chlorophyll, iron-deficient leaves showed no sustained decreases in PS II efficiency, measured after dark adaptation, except when the deficiency was very severe. This occurred when plants were grown in growth chambers or in field conditions. However, iron-deficient leaves showed decreases in the actual PS II efficiency at steady-state photosynthesis, due to decreases in photochemical quenching and intrinsic PS II efficiency. Iron-chlorotic leaves were protected not only by the decrease in leaf absorptance, but also by down-regulation mechanisms enhancing non-photochemical quenching and thermal dissipation of the light absorbed by PS II within the antenna pigment bed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Formation of tyrosine radicals in photosystem II under far-red illumination

Photosystem II (PS II) contains two redox-active tyrosine residues on the donor side at symmetrical positions to the primary donor, P₆₈₀. Tyr_Z, part of the water-oxidizing complex, is a preferential fast electron donor while Tyr_D is a slow auxiliary donor to P₆₈₀ ⁺. We used PS II membranes from spinach which were depleted of the water oxidation complex (Mn-depleted PS II) to study electron donation from both tyrosines by time-resolved EPR spectroscopy under visible and far-red continuous light and laser flash illumination. Our results show that under both illumination regimes, oxidation of Tyr_D occurs via equilibrium with Tyr_Z ^? at pH 4.7 and 6.3. At pH 8.5 direct Tyr_D oxidation by P₆₈₀ ⁺ occurs in the majority of the PS II centers. Under continuous far-red light illumination these reactions were less effective but still possible. Different photochemical steps were considered to explain the far-red light-induced electron donation from tyrosines and localization of the primary electron hole (P₆₈₀ ⁺) on the Chl_D1 in Mn-depleted PS II after the far-red light-induced charge separation at room temperature is suggested.     

Chemical modification of photosystem II core complex pigments with sodium borohydride

The reaction of the irreversible chemical reduction of the 13¹-keto C=O group of pheophytin a (Pheo a) with sodium borohydride in reaction centers (RCs) of functionally active spinach photosystem II (PS II) core complexes was studied. Stable, chromatographically purified PS II core complex preparations with altered chromophore composition are obtained in which ～25% of Pheo a molecules are modified to 13¹-deoxo-13¹-hydroxy-Pheo a. Some of the chlorophyll a molecules in the complexes were also irreversibly reduced with borohydride to 13¹-deoxo-13¹-hydroxy-chlorophyll a. Based on the results of comparative study of spectral, biochemical, and photochemical properties of NaBH₄-treated and control preparations, it was concluded that: (i) the borohydride treatment did not result in significant dissociation of the PS II core complex protein ensemble; (ii) the modified complexes retained the ability to photoaccumulate the radical anion of the pheophytin electron acceptor in the presence of exogenous electron donor; (iii) only the photochemically inactive pheo-phytin Pheo_D2 is subjected to the borohydride treatment; (iv) the Q_x optical transition of the Pheo_D2 molecule in the RC of PS II core complexes is located at 543 nm; (v) in the Q_y spectral region, Pheo_D2 probably absorbs at ～680 nm.     

Electrometrical study of electron transfer from the terminal FA/FB iron-sulfur clusters to external acceptors in photosystem I

An electrometrical technique was used to investigate electron transfer between the terminal iron-sulfur centers F(A)/F(B) and external electron acceptors in photosystem I (PS I) complexes from the cyanobacterium Synechococcus sp. PCC 6301 and from spinach. The increase of the relative contribution of the slow components of the membrane potential decay kinetics in the presence of both native (ferredoxin, flavodoxin) and artificial (methyl viologen) electron acceptors indicate the effective interaction between the terminal 14Fe-4S] cluster and acceptors. The finding that FA fails to donate electrons to flavodoxin in F(B)-less (HgCl2-treated) PS I complexes suggests that F(B) is the direct electron donor to flavodoxin. The lack of additional electrogenicity under conditions of effective electron transfer from the F(B) redox center to soluble acceptors indicates that this reaction is electrically silent.     

How does Lobaria pulmonaria regulate photosystem II during progressive desiccation and osmotic water stress?

The effects of decreasing water potential (Ψ) on O₂ evolution and fluorescence yield at room temperature and at 77 K were investigated using the lichen Lobaria pulmonaria. Changes in Ψ were created either by atmospheric desiccation or by osmotic dehydration, with either sucrose, sorbitol or NaCl as osmoticum. Independent of the method used to establish Ψ, similar inactivation patterns were obtained and were reversible after reincubation in pure water for 10 min. Our data indicate that exposure to increasing water stress acts at two levels. In the first phase, at ‘mild’ stress, i.e. at Ψ greater than ?13, ?16 and ?20 MPa for drying, NaCl and sucrose treatments, respectively, a progressive decline in O₂ production and the fluorescence yield (ΔF/F_m′ and F_v/F_m) was correlated with increases in non-photochemical quenching (q_N). At the same time the photochemical quenching (q_p) changed only sligthly, indicating the absence of overreduction. The F_o level remained relatively constant in this first stage of water loss. A ΔpH mediated down regulation and a donor side limitation of photosystem (PS) II are discussed. When the water stress was severe, a further decrease in the fluorescence yield was observed and correlated with a considerable decrease in F_o (second phase). Kinetic analysis of the 77 K emission showed that osmotic stress and atmospheric desiccation possibly lead to an increased spillover from PS II to PS I. In addition, a strong negative effect of NaF on the recovery from dehydration was found. This may indicate a state transition mediated by the displacement/recoupling of light harvesting complex (LHC) II from/to PS II. The photoprotective role of spatial rearrangements of antenna complexes during desiccation is discussed.     

Two redox-active beta-carotene molecules in photosystem II

Photosystem II (PS II) contains secondary electron-transfer paths involving cytochrome b(559) (Cyt b(559)), chlorophyll (Chl), and beta-carotene (Car) that are active under conditions when oxygen evolution is blocked such as in inhibited samples or at low temperature. Intermediates of the secondary electron-transfer pathways of PS II core complexes from Synechocystis PCC 6803 and Synechococcus sp. and spinach PS II membranes have been investigated using low temperature near-IR spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. We present evidence that two spectroscopically distinct redox-active carotenoids are formed upon low-temperature illumination. The Car(+) near-IR absorption peak varies in wavelength and width as a function of illumination temperature. Also, the rate of decay during dark incubation of the Car(+) peak varies as a function of wavelength. Factor analysis indicates that there are two spectral forms of Car(+) (Car(A)(+) has an absorbance maximum of 982 nm, and Car(B)(+) has an absorbance maximum of 1027 nm) that decay at different rates. In Synechocystis PS II, we observe a shift of the Car(+) peak to shorter wavelength when oxidized tyrosine D (Y(D)*) is present in the sample that is explained by an electrostatic interaction between Y(D)* and a nearby beta-carotene that disfavors oxidation of Car(B). The sequence of electron-transfer reactions in the secondary electron-transfer pathways of PS II is discussed in terms of a hole-hopping mechanism to attain the equilibrated state of the charge separation at low temperatures.     

Phosphorylation of chloroplast thylakoid membrane proteins does not increase the absorption cross-section of photosystem 1

Direct measurements on the effective absorption cross-section of photosystem (PS) 1 were obtained with control (light — ATP) and phosphorylated (light + ATP) chloroplast thylakoids from spinach. The rate of light absorption by PS1 was invariant in control and phosphorylated thylakoids, suggesting a constant functional antenna size for this photosystem. We conclude that phosphorylated chlorophyll a-b light-harvesting complex from PS2 is not functionally connected with the antenna pigment of PS1.