The development of antenna complexes of barley (Hordeum vulgare cv. Akcent) under different light conditions as judged from the analysis of 77 K chlorophyll a fluorescence spectra

Using 77 K chlorophyll a (Chl a) fluorescence spectra in vivo, the development was studied of Photosystems II (PS II) and I (PS I) during greening of barley under intermittent light followed by continuous light at low (LI, 50 μmol m⁻² s⁻¹) and high (HI, 1000 μmol m⁻² s⁻¹) irradiances. The greening at HI intermittent light was accompanied with significantly reduced fluorescence intensity from Chl b excitation for both PS II (F685) and PS I (F743), in comparison with LI plants, indicating that assembly of light-harvesting complexes (LHC) of both photosystems was affected to a similar degree. During greening at continuous HI, a slower increase of emission from Chl b excitation in PS II as compared with PS I was observed, indicating a preferred reduction in the accumulation of LHC II. The following characteristics of 77 K Chl a fluorescence spectra documented the photoprotective function of an elevated content of carotenoids in HI leaves: (1) a pronounced suppression of Soret region of excitation spectra (410–450 nm) in comparison with the red region (670–690 nm) during the early stage of greening indicated a strongly reduced excitation energy transfer from carotenoids to the Chl a fluorescing forms within PS I and PS II; (2) changes in the shape of the excitation band of Chl b and carotenoids (460–490 nm) during greening under continuous light confirmed that the energy transfer from carotenoids to Chl a within PS II remained lower as compared with the LI plants. This revised version was published online in June 2006 with corrections to the Cover Date.     

Deriving room temperature excitation spectra for photosystem I and photosystem II fluorescence in intact leaves from the dependence of <Emphasis Type="Italic">F</Emphasis><Subscript>V</Subscript>/<Emphasis Type="Italic">F</Emphasis><Subscript>M</Subscript> on excitation wavelength

The F ₀ and F _M level fluorescence from a wild-type barley, a Chl b-less mutant barley, and a maize leaf was determined from 430 to 685 nm at 10 nm intervals using pulse amplitude-modulated (PAM) fluorimetry. Variable wavelengths of the pulsed excitation light were achieved by passing the broadband emission of a Xe flash lamp through a birefringent tunable optical filter. For the three leaf types, spectra of F _V/F _M (=(F _M − F ₀)/F _M) have been derived: within each of the three spectra of F _V/F _M, statistically meaningful variations were detected. Also, at distinct wavelength regions, the F _V/F _M differed significantly between leaf types. From spectra of F _V/F _M, excitation spectra of PS I and PS II fluorescence were calculated using a model that considers PS I fluorescence to be constant but variable PS II fluorescence. The photosystem spectra suggest that LHC II absorption results in high values of F _V/F _M between 470 and 490 nm in the two wild-type leaves but the absence of LHC II in the Chl b-less mutant barley leaf decreases the F _V/F _M at these wavelengths. All three leaves exhibited low values of F _V/F _M around 520 nm which was tentatively ascribed to light absorption by PS I-associated carotenoids. In the 550–650 nm region, the F _V/F _M in the maize leaf was lower than in the barley wild-type leaf which is explained with higher light absorption by PS I in maize, which is a NADP-ME C₄ species, than in barley, a C₃ species. Finally, low values of F _V/F _M at 685 in maize leaf and in the Chl b-less mutant barley leaf are in agreement with preferential PS I absorption at this wavelength. The potential use of spectra of the F _V/F _M ratio to derive information on spectral absorption properties of PS I and PS II is discussed.     

Up-regulation of cyclic electron flow and down-regulation of linear electron flow in antisense-<Emphasis Type="Italic">rca</Emphasis> mutant rice

To investigate how excess excitation energy is dissipated in a ribulose-1,5-bisphospate carboxylase/oxygenase activase antisense transgenic rice with net photosynthetic rate (P _N) half of that of wild type parent, we measured the response curve of P _N to intercellular CO₂ concentration (C _i), electron transport rate (ETR), quantum yield of open photosystem 2 (PS2) reaction centres under irradiation (F_v′/F_m′), efficiency of total PS2 centres (Φ_PS2), photochemical (q_P) and non-photochemical quenching (NPQ), post-irradiation transient increase in chlorophyll (Chl) fluorescence (PITICF), and P700⁺ re-reduction. Carboxylation efficiency dependence on C _i, ETR at saturation irradiance, and F_v′/F_m′, Φ_PS2, and q_P under the irradiation were significantly lower in the mutant. However, NPQ, energy-dependent quenching (q_E), PITICF, and P700⁺ re-reduction were significantly higher in the mutant. Hence the mutant down-regulates linear ETR and stimulates cyclic electron flow around PS1, which may generate the ΔpH to support NPQ and q_E for dissipation of excess excitation energy.     

How to correctly determine the different chlorophyll fluorescence parameters and the chlorophyll fluorescence decrease ratio R<Subscript>Fd</Subscript> of leaves with the PAM fluorometer

This contribution is a practical guide to the measurement of the different chlorophyll (Chl) fluorescence parameters and gives examples of their development under high-irradiance stress. From the Chl fluorescence induction kinetics upon irradiation of dark-adapted leaves, measured with the PAM fluorometer, various Chl fluorescence parameters, ratios, and quenching coefficients can be determined, which provide information on the functionality of the photosystem 2 (PS2) and the photosynthetic apparatus. These are the parameters F_v, F_m, F₀, F_m′, F_v′, NF, and ΔF, the Chl fluorescence ratios F_v/F_m, F_v/F₀, ΔF/F_m′, as well as the photochemical (q_P) and non-photochemical quenching coefficients (q_N, q_CN, and NPQ). q_N consists of three components (q_N = q_E + q_T + q_I), the contribution of which can be determined via Chl fluorescence relaxation kinetics measured in the dark period after the induction kinetics. The above Chl fluorescence parameters and ratios, many of which are measured in the dark-adapted state of leaves, primarily provide information on the functionality of PS2. In fully developed green and dark-green leaves these Chl fluorescence parameters, measured at the upper adaxial leaf side, only reflect the Chl fluorescence of a small portion of the leaf chloroplasts of the green palisade parenchyma cells at the upper outer leaf half. Thus, PAM fluorometer measurements have to be performed at both leaf sides to obtain information on all chloroplasts of the whole leaf. Combined high irradiance (HI) and heat stress, applied at the upper leaf side, strongly reduced the quantum yield of the photochemical energy conversion at the upper leaf half to nearly zero, whereas the Chl fluorescence signals measured at the lower leaf side were not or only little affected. During this HL-stress treatment, q_N, q_CN, and NPQ increased in both leaf sides, but to a much higher extent at the lower compared to the upper leaf side. q_N was the best indicator for non-photochemical quenching even during a stronger HL-stress, whereas q_CN and NPQ decreased with progressive stress even though non-photochemical quenching still continued. It is strongly recommended to determine, in addition to the classical fluorescence parameters, via the PAM fluorometer also the Chl fluorescence decrease ratio R_Fd (F_d/F_s), which, when measured at saturation irradiance is directly correlated to the net CO₂ assimilation rate (_{P N}) of leaves. This R_Fd-ratio can be determined from the Chl fluorescence induction kinetics measured with the PAM fluorometer using continuous saturating light (cSL) during 4–5 min. As the R_Fd-values are fast measurable indicators correlating with the photosynthetic activity of whole leaves, they should always be determined via the PAM fluorometer parallel to the other Chl fluorescence coefficients and ratios.     

High photosynthetic efficiency of a rice (Oryza sativa L.) xantha mutant

Comparative analysis revealed that a xantha rice mutant (cv. Huangyu B) had higher ratios of chlorophyll (Chl) a/b and carotenoids/Chl, and higher photosynthetic efficiency than its wild type parent (cv. II32 B). Unexpectedly, the mutant had higher net photosynthetic rate (P _N) than II32 B. This might have resulted from its lower non-photochemical quenching (q_N) but higher maximal photochemical efficiency (F_V/F_M), higher excitation energy capture efficiency of photosystem 2 (PS2) reaction centres (F_V′/F_M′), higher photochemical quenching (q_P), higher effective PS2 quantum yield (Φ_PS2), and higher non-cyclic electron transport rate (ETR). This is the first report of a chlorophyll mutant that has higher photosynthetic efficiency and main Chl fluorescence parameters than its wild type. This mutant could become a unique material both for the basic research on photosynthesis and for the development of high yielding rice cultivars.     

Enhancement of susceptivity to photoinhibition and photooxidation in rice chlorophyll <Emphasis Type="Italic">b</Emphasis>-less mutants

Two rice chlorophyll (Chl) b-less mutants (VG28-1, VG30-5) and the respective wild type (WT) plant (cv. Zhonghua No. 11) were analyzed for the changes in Chl fluorescence parameters, xanthophyll cycle pool, and its de-epoxidation state under exposure to strong irradiance, SI (1 700 μmol m⁻² s⁻¹). We also examined alterations in the chloroplast ultrastructure of the mutants induced by methyl viologen (MV) photooxidation. During HI (0–3.5 h), the photoinactivation of photosystem 2 (PS2) appeared earlier and more severely in Chl b-less mutants than in the WT. The decreases in maximal photochemical efficiency of PS2 in the dark (F_v/F_m), quantum efficiency of PS2 electron transport (Φ_PS2), photochemical quenching (q_P), as well as rate of photochemistry (P_rate), and the increases in de-epoxidation state (DES) and rate of thermal dissipation of excitation energy (D_rate) were significantly greater in Chl b-mutants compared with the WT plant. A relatively larger xanthophyll pool and 78–83 % conversion of violaxanthin into antheraxanthin and zeaxanthin in the mutants after 3.5 h of HI was accompanied with a high ratio of inactive/total PS2 (0.55–0.73) and high 1–q_P (0.57–0.68) which showed that the activities of the xanthophyll cycle were probably insufficient to protect the photosynthetic apparatus against photoinhibition. No apparent difference of chloroplast ultrastructure was found between Chl b-less mutants and WT plants grown under low, LI (180 μmol m⁻² s⁻¹) and high, HI (700 μmol m⁻² s⁻¹) irradiance. However, swollen chloroplasts and slight dilation of thylakoids occurred in both mutants and the WT grown under LI followed by MV treatment. These typical symptoms of photooxidative damage were aggravated as plants were exposed to HI. Distorted and loose scattered thylakoids were observed in particular in the Chl b-less mutants. A greater extent of photoinhibition and photooxidation in these mutants indicated that the susceptibility to HI and oxidative stresses was enhanced in the photosynthetic apparatus without Chl b most likely as a consequence of a smaller antenna size.     

Study on energy transfer between carotenoid and chlorophyll a in cytochrome b 6 f complex from Bryopsis corticulans

The excitation energy transfer between carotenoid and chlorophyll (Chl) in the cytochrome b ₆ f complex from Bryopsis corticulans (B. corticulans), in which the carotenoid is 9-cis-α-carotene, was investigated by means of fluorescence excitation and sub-microsecond time-resolved absorption spectroscopies. The presence of efficient singlet excitation transfer from α-carotene to Chl a was found with an overall efficiency as high as ∼ ∼24%, meanwhile the Chl a-to-α-carotene triplet excitation transfer was also evidenced. Circular dichroism spectroscopy showed that α-carotene molecule existed in an asymmetric environment and Chl a molecule had a certain orientation in this complex.Bin-Xing Li and Ping Zuo contributed equally to this work.     

Diurnal variation of gas exchange, chlorophyll fluorescence, and xanthophyll cycle components of maize hybrids released in different years

Diurnal variation of gas exchange, chlorophyll (Chl) fluorescence, and xanthophyll cycle components of three maize (Zea mays L.) hybrids released in different years, i.e. Baimaya (1950s), Zhongdan2 (1970s), and Nongda108 (1990s), were compared. On cloudless days, the newer hybrids always had higher net photosynthetic rate (P _N), especially at noon, than the older ones. At noon, all the hybrids decreased their maximal yield of photosystem 2 (PS2) photochemistry (F_v/F_m) and actual quantum yield of PS2 (Φ_PS2), the newer ones always showing higher values. Generally, the newer hybrids displayed higher photochemical quenching of Chl (q_P) and lower non-photochemical quenching (NPQ). The interhybrid differences in P _N may be owing to their differential photochemical efficiency. A midday depression in P _N occurred in all hybrids, which might be caused by serious photoinhibition or by decreased stomatal conductance. However, midday depression in P _N was more obvious in the older hybrids, especially when leaves were senescent. The higher de-epoxidation state of the xanthophylls was noted in older hybrids, which was confirmed by their larger NPQ. The newer maize hybrids did not need a strong de-epoxidation state since they had a better photosynthetic quantum conversion rate and a lower NPQ.     

Analysis of Qualitative Contribution of Assimilatory and Non-Assimilatory De-Excitation Processes to Adaptation of Photosynthetic Apparatus of Barley Plants to High Irradiance

The adaptation of barley (Hordeum vulgare L. cv. Akcent) plants to low (LI, 50 µmol m^–2 s^–1) and high (HI, 1000 µmol m^–2 s^–1) growth irradiances was studied using the simultaneous measurements of the photosynthetic oxygen evolution and chlorophyll a (Chl a) fluorescence at room temperature. If measured under ambient CO₂ concentration, neither increase of the oxygen evolution rate (P) nor enhancement of non-radiative dissipation of the absorbed excitation energy within photosystem 2 (PS2) (determined as non-photochemical quenching of Chl a fluorescence, NPQ) were observed for HI plants compared with LI plants. Nevertheless, the HI plants exhibited a significantly higher proportion of Q_A in oxidised state (estimated from photochemical quenching of Chl a fluorescence, q_P), by 49–102 % at irradiances above 200 µmol m^–2 s^–1 and an about 1.5 fold increase of irradiance-saturated PS2 electron transport rate (ETR) as compared to LI plants. At high CO₂ concentration the degree of P stimulation was approximately three times higher for HI than for LI plants, and the irradiance-saturated P values at irradiances of 2 440 and 2 900 µmol m^–2 s^–1 were by 130 and 150 % higher for HI plants than for LI plants. We suggest that non-assimilatory electron transport dominates in the adaptation of the photosynthetic apparatus of barley grown at high irradiances under ambient CO₂ rather than an increased NPQ or an enhancement of irradiance-saturated photosynthesis.     

Chilling stress and chilling tolerance of sweet potato as sensed by chlorophyll fluorescence

We studied changes in the chlorophyll (Chl) fluorescence components in chilling-stressed sweet potato (Ipomoea batatas L. Lam) cv. Tainung 57 (TN57, chilling-tolerant) and cv. Tainung 66 (TN66, chilling-susceptible). Plants under 12-h photoperiod and 400 μmol m⁻² s⁻¹ irradiance at 24/20 °C (day/night) were treated by a 5-d chilling period at 7/7 °C. Compared to TN66, TN57 exhibited a significantly greater basic Chl fluorescence (F₀), maximum fluorescence (F_m), maximum fluorescence yield during actinic irradiation (F_m′ ), and the quantum efficiency of electron transport through photosystem 2, PS2 (Φ_PS2). Chilling stress resulted in decrease in the potential efficiency of PS2 (F_v/F_m), Φ_PS2, non-photochemical fluorescence quenching (NPQ), non-photochemical quenching (q_N), and the occurrence of chilling injury in TN66. Chilling increased the likelihood of photoinhibition, characterized by a decline in the Chl fluorescence of both cultivars, and photoinhibition during low temperature stress generally occurred more rapidly in TN66.     

Contributions of the free oxidized and QB-bound plastoquinone molecules to the thermal phase of chlorophyll-a fluorescence

Variable chlorophyll a (Chl a) fluorescence is composed of a photochemical and a thermal phases of similar amplitudes. The photochemical phase can be induced by a saturating single turnover flash (STF) and reflects the reduction of the Photosystem II (PS II) Q_A primary electron acceptor. The thermal phase requires multiple turnover flash (MTF) and is somehow related to the reduction of the plastoquinone (PQ) molecules. This article aimed to determine the relative contributions of the Q_B-bound and the free oxidized PQ molecules to the thermal phase of Chl a fluorescence. We thus measured the interactive effects of exogenous PQ (PQex), of an inhibitor (DCMU) acting at the Q_B site of PS II and of an artificial quencher, 2-methyl-1,4-naphtoquinone, on Chl a fluorescence levels induced by STF (F_F) and MTF (F_M) in spinach thylakoids. We observed that: (1) the incorporation of PQex in thylakoids stimulated photosynthetic electron transport but barely affected F_F and F_M in the absence of DCMU; (2) DCMU significantly increased the amplitude of F_F but slightly quenched F_M; (3) 2-methyl-1,4-naphtoquinone quenched F_M to a larger-extent than F_F; (4) DCMU increased the quenching effects of PQex on F_F and F_M and also, of methyl-1,4-naphtoquinone on F_F. These results indicate that: (1) the Q_B-bound and the free PQ molecules contribute to about 56% and 25%, respectively, to the thermal phase Chl a fluorescence in dark-adapted thylakoids; and (2) the thermal phase of Chl a fluorescence is more susceptible than the photochemical phase to the non-photochemical quenching effect of oxidized quinones. This revised version was published online in June 2006 with corrections to the Cover Date.     

Chlorophyll fluorescence at 680 and 730 nm and leaf photosynthesis

Chlorophyll fluorescence constitutes a simple, rapid, and non-invasive means to assess light utilization in Photosystem II (PS II). This study examines aspects relating to the accuracy and applicability of fluorescence for measurement of PS II photochemical quantum yield in intact leaves. A known source of error is fluorescence emission at 730 nm that arises from Photosystem I (PS I). We measured this PS I offset using a dual channel detection system that allows measurement of fluorescence yield in the red (660 nm < F < 710 nm) or far red (F > 710 nm) region of the fluorescence emission spectrum. The magnitude of the PS I offset was equivalent to 30% and 48% of the dark level fluorescence F₀ in the far red region for Helianthus annuus and Sorghum bicolor, respectively. The PS I offset was therefore subtracted from fluorescence yields measured in the far red spectral window prior to calculation of PS II quantum yield. Resulting values of PS II quantum yield were consistently higher than corresponding values based on emission in the red region. The basis for this discrepancy lies in the finite optical thickness of the leaf that leads to selective reabsorption by chlorophyll of red fluorescence emission originating in deeper cell layers. Consequently, red fluorescence measurements preferentially sense emission from chloroplasts in the uppermost layer of the leaf where levels of photoprotective nonphotochemical quenching are higher due to increased photon density. It is suggested that far red fluorescence, corrected for the PS I offset, provides the most reliable quantitative basis for calculation of PS II quantum yield because of reduced sensitivity of these measurements to gradients in leaf transmittance and quenching capacity. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photosystem II chlorophyll a fluorescence lifetimes and intensity are independent of the antenna size differences between barley wild-type and chlorina mutants: Photochemical quenching and xanthophyll cycle-dependent nonphotochemical quenching of fluorescence

Photosystem II (PS II) chlorophyll (Chl) a fluorescence lifetimes were measured in thylakoids and leaves of barley wild-type and chlorina f104 and f2 mutants to determine the effects of the PS II Chl a+b antenna size on the deexcitation of absorbed light energy. These barley chlorina mutants have drastically reduced levels of PS II light-harvesting Chls and pigment-proteins when compared to wild-type plants. However, the mutant and wild-type PS II Chl a fluorescence lifetimes and intensity parameters were remarkably similar and thus independent of the PS II light-harvesting antenna size for both maximal (at minimum Chl fluorescence level, F_o) and minimal rates of PS II photochemistry (at maximum Chl fluorescence level, F_m). Further, the fluorescence lifetimes and intensity parameters, as affected by the trans-thylakoid membrane pH gradient (pH) and the carotenoid pigments of the xanthophyll cycle, were also similar and independent of the antenna size differences. In the presence of a pH, the xanthophyll cycle-dependent processes increased the fractional intensity of a Chl a fluorescence lifetime distribution centered around 0.4–0.5 ns, at the expense of a 1.6 ns lifetime distribution (see Gilmore et al. (1995) Proc Natl Acad Sci USA 92: 2273–2277). When the zeaxanthin and antheraxanthin concentrations were measured relative to the number of PS II reaction center units, the ratios of fluorescence quenching to [xanthophyll] were similar between the wild-type and chlorina f104. However, the chlorina f104, compared to the wild-type, required around 2.5 times higher concentrations of these xanthophylls relative to Chl a+b to obtain the same levels of xanthophyll cycle-dependent fluorescence quenching. We thus suggest that, at a constant pH, the fraction of the short lifetime distribution is determined by the concentration and thus binding frequency of the xanthophylls in the PS II inner antenna. The pH also affected both the widths and centers of the lifetime distributions independent of the xanthophyll cycle. We suggest that the combined effects of the xanthophyll cycle and pH cause major conformational changes in the pigment-protein complexes of the PS II inner or core antennae that switch a normal PS II unit to an increased rate constant of heat dissipation. We discuss a model of the PS II photochemical apparatus where PS II photochemistry and xanthophyll cycle-dependent energy dissipation are independent of the Peripheral antenna size.Abbreviations Ax antheraxanthin - BSA bovine serum albumin - c_x lifetime center of fluorescence decay component x - CP chlorophyll binding protein of PS II inner antenna - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DTT dithiothreitol - f_x fractional intensity of fluorescence lifetime component x - F_m, F_m maximal PS II Chl a fluorescence intensity with all Q_A reduced in the absence, presence of thylakoid membrane energization - F_o minimal PS II Chl a fluorescence intensity with all Q_A oxidized - F_v=F_m–F_o variable level of PS II Chl a fluorescence - HPLC high performance liquid chromatography - k_A rate constant of all combined energy dissipation pathways in PS II except photochemistry and fluorescence - k_F rate constant of PS II Chl a fluorescence - LHCIIb main light harvesting pigment-protein complex (of PS II) - N_pig mols Chl a+b per PS II - NPQ=(F_m/F_m–1) nonphotochemical quenching of PS II Chl a fluorescence - PAM pulse-amplitude modulation fluorometer - PFD photon-flux density, mols photons m^–2 s^–1 - PS II Photosystem II - P680 special-pair Chls of PS II reaction center - Q_A primary quinone electron acceptor of PS II - V_x violaxanthin - w_x width at half maximum of Lorentzian fluorescence lifetime distribution x - Zx zeaxanthin - pH trans-thylakoid proton gradient - % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqef0uAJj3BZ9Mz0bYu% H52CGmvzYLMzaerbd9wDYLwzYbItLDharqqr1ngBPrgifHhDYfgasa% acOqpw0xe9v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8Wq% Ffea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8frFve9Fve9Ff0dme% GabaqaaiGacaGaamqadaabaeaafiaakeaacqGH8aapcqaHepaDcqGH% +aGpdaWgaaWcbaGaamOraiaad2gaaeqaaaaa!4989!\[< \tau > _{Fm}\],% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqef0uAJj3BZ9Mz0bYu% H52CGmvzYLMzaerbd9wDYLwzYbItLDharqqr1ngBPrgifHhDYfgasa% acOqpw0xe9v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8Wq% Ffea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8frFve9Fve9Ff0dme% GabaqaaiGacaGaamqadaabaeaafiaakeaacqGH8aapcqaHepaDcqGH% +aGpdaWgaaWcbaGaamOraiaad+gaaeqaaOGaeyypa0Zaaabqaeaaca% WGMbWaaSbaaSqaaiaadIhaaeqaaOGaam4yamaaBaaaleaacaWG4baa% beaaaeqabeqdcqGHris5aaaa!50D3!\[< \tau > _{Fo} = \sum {f_x c_x }\] average lifetime of Chl a fluorescence calculated from a multi-exponential model under F_m, F_o conditions     

Effects of water stress and high temperature on gas exchange and chlorophyll fluorescence in Triticum aestivum L.

Wheat plants grown in controlled growth chambers were exposed to drought stress (DS) and high temperature (HT) singly and in combination (DS+HT). The effects of these two stresses on net photosynthetic rate (P _N), stomatal conductance (g _s), intercellular CO₂ concentration (C _i), quantum efficiency of photosystem 2 (Φ_PS2), variable to maximum chlorophyll (Chl) fluorescence (F_v/F_m), photochemical (q_p) and non-photochemical (NPQ) Chl fluorescence, and yield were investigated. Grain yield was decreased by 21 % due to DS, while it was increased by 26 % due to HT. P _N, g _s, C _i, and Chl fluorescence were dramatically reduced to DS, HT, and their interaction, except NPQ which showed an increase due to HT.     

Changes of Donor and Acceptor Side in Photosystem 2 Complex Induced by Iron Deficiency in Attached Soybean and Maize Leaves

Photosynthesis in iron-deficient soybean and maize leaves decreased drastically. The quantum yield of photosystem 2 (PS2) electron transport (Φ_PS2), the efficiency of excitation energy capture by open PS2 reaction centres (F_v′/F_m′), and photochemical quenching coefficient (q_P) under high irradiance were lowered significantly by iron deficiency, but non-photochemical quenching (NPQ) increased markedly. The analysis of the polyphasic rise of fluorescence transient showed that iron depletion induced a pronounced K step both in soybean and maize leaves. The maximal quantum yield of PS2 photochemistry (Φ_po) decreased only slightly, however, the efficiency with which a trapped exciton can move an electron into the electron transport chain further than Q_A (Ψ₀) and the quantum yield of electron transport beyond Q_A (Ψ_Eo) in iron deficient leaves decreased more significantly compared with that in control. Thus not only the donor side but also the acceptor of PS2 was probably damaged in iron deficient soybean and maize leaves. This revised version was published online in August 2006 with corrections to the Cover Date.     

Changes in leaf photosynthesis of transgenic rice with silenced <Emphasis Type="Italic">OsBP-73</Emphasis> gene

In comparison with its wild type (WT), the transgenic (TG) rice with silenced OsBP-73 gene had significantly lower plant height, grain number per panicle, and leaf net photosynthetic rate (P _N). Also, the TG rice showed significantly lower chlorophyll (Chl), ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO), RuBPCO activase, and RuBP contents, photosystem 2 (PS2) photochemical efficiency (F_v/F_m and ΔF/F_m′), apparent quantum yield of carbon assimilation (Φ_c), carboxylation efficiency (CE), photosynthetic electron transport and photophosphorylation rates as well as sucrose phosphate synthase activity, but higher intercellular CO₂ concentration, sucrose, fructose, and glycerate 3-phosphate contents, and non-photochemical quenching of Chl fluorescence (NPQ). Thus the decreased P _N in the TG rice leaves is related to both RuBP carboxylation and RuBP regeneration limitations, and the latter is a predominant limitation to photosynthesis.     

Photosynthetic UV-B Response of Beech (Fagus sylvatica L.) Saplings

Cloned saplings of beech (7-y-old) were exposed to enhanced UV-B irradiation (+25 %) continuously over three growing seasons (1999–2001). Analysis of CO₂ assimilation, variable chlorophyll (Chl) a fluorescence, and pigment composition was performed in late summer of the third growing season to evaluate the influence of long-term elevated UV-B irradiation. This influence was responsible for the stimulation of the net assimilation rate (P _N) over a range of irradiances. The increase in P _N was partially connected to increase of the area leaf mass, and thus to the increased leaf thickness. Even a higher degree of UV-B induced stimulation was observed at the level of photosystem 2 (PS2) photochemistry as judged from the irradiance response of electron transport rate and photochemical quenching of Chl a. The remarkably low irradiance-induced non-photochemical quenching of maximum Chl a fluorescence (NPQ) in the UV-B plants over the entire range of applied irradiances was attributed both to the reduced demand on non-radiative dissipation processes and to the considerably reduced contribution of the quenching localised in the inactivated PS2 reaction centres. Neither the content of Chls and total carotenoids expressed per leaf area nor the contents of lutein, neoxanthin, and the pool of xanthophyll cycle pigments (VAZ) were affected under the elevated UV-B. However, the contributions of antheraxanthin (A) and zeaxanthin (Z) to the entire VAZ pool in the dark-adapted UV-B treated plants were 1.61 and 2.14 times higher than in control leaves. Surprisingly, the retained A+Z in UV-B treated plants was not accompanied with long-term down-regulation of the PS2 photochemical efficiency, but it facilitated the non-radiative dissipation of excitation energy within light-harvesting complexes (LHC) of PS2. Thus, in the beech leaves the accumulation of A+Z, induced by other factors than excess irradiance itself, supports the resistance of PS2 against combined effects of high irradiance and elevated UV-B.     

Diurnal cycle of chlorophyll fluorescence in <Emphasis Type="Italic">Phalaenopsis</Emphasis>

Chlorophyll (Chl) fluorescence of warm day/cool night temperature exposed Phalaenopsis plants was measured hourly during 48 h to study the simultaneous temperature and irradiance response of the photosynthetic physiology. The daily pattern of fluorescence kinetics showed abrupt changes of photochemical quenching (q_P), non-photochemical quenching (NPQ) and quantum yield of photosystem II electron transport (Φ_PSII) upon transition from day to night and vice versa. During the day, the course of Φ_PSII and NPQ was related to the air temperature pattern, while maximum quantum efficiency of PSII photochemistry (F_v/F_m) revealed a rather light dependent response. Information on these daily dynamics in fluorescence kinetics is important with respect to meaningful data collection and interpretation.     

Photosynthetic activity of poikilochlorophyllous desiccation tolerant plant <Emphasis Type="Italic">Reaumuria soongorica</Emphasis> during dehydration and re-hydration

Diurnal patterns of gas exchange and chlorophyll (Chl) fluorescence parameters of photosystem 2 (PS2) as well as Chl content were analyzed in Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub during dehydration and rehydration. The net photosynthetic rate (P _N), maximum photochemical efficiency of PS2 (variable to maximum fluorescence ratio, F_v/F_m), quantum efficiency of non-cyclic electron transport of PS2, and Chl content decreased, but non-photochemical quenching of fluorescence and carotenoid content increased in stems with the increasing of drought stress. 6 d after re-hydration, new leaves budded from stems. In the re-watered plants, the chloroplast function was restored and Chl a fluorescence returned to a similar level as in the control plants. This improved hydraulic adjustment in plant triggered a positive effect on ion flow in the tissues and increased shoot electrical admittance. Thus R. soongorica plants are able to sustain drought stress through leaf abscission and keep part of Chl content in stems.     

Gradual Disassembly of Photosystem 2 In Vivo Induced by Excess Irradiance. A Hypothesis Based on Changes in 77 K Fluorescence Spectra of Chlorophyll a in Barley Leaves

Effects of short-term exposure to different irradiances on the function of photosystem 2 (PS2) were studied for barley grown at low (LI; 50 μmol m⁻² s⁻¹) and high (HI; 1 100 μmol m⁻² s⁻¹) irradiances. HI barley revealed higher ability to down-regulate the light-harvesting within PS2 after exposure to high irradiance as compared to LI plants. This ability was estimated from the light-induced decreases of F685/F742 and E476/E436 in emission and excitation spectra of 77 K chlorophyll (Chl) a fluorescence in vivo which was 65 and 10 % for HI plants as compared to 30 and 2 % for LI plants, respectively. For LI plants this protective down-regulation of the light-harvesting of PS2 was saturated at 430 μmol m⁻² s⁻¹, and progressive PS2 photodamage was induced at higher irradiances. After exposure of LI segments to 2 200 μmol m⁻² s⁻¹ a pronounced maximum at 700 nm appeared in emission spectrum of 77 K Chl a fluorescence. Based on complementary analysis of 77 K excitation spectra measured at the emission wavelength 685 nm we suggest that this emission maximum may be attributed to the formation of aggregates of light-harvesting complexes of PS2 (LHC2) with part of PS2 core during progressive PS2 photodamage. Our results can be explained assuming different contributions of LHC2 and PS2 core to the total nonradiative dissipation of absorbed excitation energy for the LI and HI barley. This revised version was published online in August 2006 with corrections to the Cover Date.