Development of photosystems 2 and 1 during leaf growth in grapevine seedlings probed by chlorophyll a fluorescence transient and 820 nm transmission in vivo

Chlorophyll (Chl) a fluorescence transient and 820-nm transmission kinetic were investigated to explore the development of photosynthetic apparatus in grapevine leaves from emergence to full expansion. In this study, all leaves at various developing stages exhibited typical Chl a fluorescence transient. In newly initiating leaves, the maximum quantum yield of primary photochemistry (ϕ_P0) was slightly lower (<10 %) than that in fully expanded leaves. Nevertheless, the fluorescence rise from O to J step was clearly speeded up in young leaves compared with that in fully expanded leaves. Additionally, a distinct K step appeared in young leaves at high irradiances. With leaf development, the efficiency that a trapped exciton can move an electron into the electron transport chain further than Q_A ⁻ (Ψ₀), the quantum yield of electron transport beyond Q_A (ϕ_E0), electron transport flux per excited cross section (ET₀/CS₀), the amount of active photosystem (PS) 2 reaction centres per excited cross section (RC/CS₀), and the performance index on cross section basis (PI_CS) increased gradually and rapidly. Young leaves had strikingly lower amplitude of transmission at 820 nm. A linear relationship between Ψ₀ and the transmission at 820 nm (I₃₀/I₀) was evident. Based on these data, we suggest that (1) the primary photochemistry of PS2 may be not the limiting step of the photosynthetic capacity during leaf growth under natural irradiance; (2) oxygen evolving complex (OEC) might be not fully connected to PS2 at the beginning of leaf growth; (3) though there are a few functional PS1 and PS2 at the early stages of leaf development, they match perfectly.     

Photosystem 2 is more tolerant to high temperature in apple (<Emphasis Type="Italic">Malus domestica</Emphasis> Borkh.) leaves than in fruit peel

Tolerance of photosystem 2 (PS2) to high temperature in apple (Malus domestica Borkh. cv. Cortland) leaves and peel was investigated by chlorophyll a fluorescence (OJIP) transient after exposure to 25 (control), 40, 42, 44, and 46 °C in the dark for 30 min. The positive L-step was more pronounced in a peel than in leaves when exposed to 44 °C. Heat-induced K-step became less pronounced in leaves than in peel when exposed to 42 °C or higher temperature. Leaves had negative L-and K-steps relative to the peel. The decrease of oxygen-evolving complex (OEC) by heat stress was higher in the peel than in the leaves. OJIP transient from the 46 °C treated peel could not reach the maximum fluorescence (F_m). The striking thermoeffect was the big decrease in the relative variable fluorescence at 30 ms (V_I), especially in the leaves. Compared with the peel, the leaves had less decreased maximum PS2 quantum efficiency (F_v/F_m), photochemical rate constant (K_P), F_m and performance index (PI) on absorption basis (PI_abs) and less increased minimum fluorescence (F₀) and non-photochemical rate constant (K_N), but more increased reduction of end acceptors at PS1 electron acceptor side per cross section (RE₀/CS₀) and per reaction center (RE₀/RC₀), quantum yield of electron transport from Q_A ⁻ to the end acceptors (ϕ _R0) and total PI (PI_abs,total) when exposed to 44 °C. In conclusion, PS2 is more thermally labile than PS1. The reduction of PS2 activity by heat stress primarily results from an inactivation of OEC. PS2 was more tolerant to high temperature in the leaves than in the peel.     

Inhibition of photosynthetic oxygen evolution and electron transfer from the quinone acceptor Q<Subscript>A</Subscript><Superscript>−</Superscript> to Q<Subscript>B</Subscript> by iron deficiency

The effect of iron deficiency on photosynthetic electron transport in Photosystem II (PS II) was studied in leaves and thylakoid membranes of lettuce (Lactuca sativa, Romaine variety) plants. PS II electron transport was characterized by oxygen evolution and chlorophyll fluorescence parameters. Iron deficiency in the culture medium was shown to affect water oxidation and the advancement of the S-states. A decrease of maximal quantum yield of PS II and an increase of fluorescence intensity at step J and I of OJIP kinetics were also observed. Thermoluminescence measurements revealed that charge recombination between the quinone acceptor of PS II, Q_B, and the S₂ state of the Mn-cluster was strongly perturbed. Also the dark decay of Chl fluorescence after a single turnover white flash was greatly retarded indicating a slower rate of Q_A ⁻ reoxidation.     

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.     

Increased photosynthetic activities and thermostability of photosystem II with leaf development of elm seedlings (Ulmus pumila) probed by the fast fluorescence rise OJIP

Experiments were conducted to investigate the photosynthetic activity and thermostability of photosystem II (PSII) in elm seedling (Ulmus pumila) leaves from initiation to full expansion. During leaf development, photosynthesis, measured as CO₂ fixation, increased gradually and reached a maximum value when leaves were fully developed. In parallel with the increase of carbon assimilation, chlorophyll content increased. The chlorophyll a fluorescence measurements showed that the maximum quantum yield of PSII primary photochemistry (φ_po), the efficiency with which the energy of trapped excitons is converted into the electron transport beyond Q_A (Ψ_o) and the quantum yield of electron transport beyond Q_A (φ_Eo) increased gradually. The low light experiments confirmed these results independently. When subjected to heat stress, young leaves exhibited progressively lower φ_po and maximal fluorescence (F_m) values with considerably higher minimal fluorescence (F_o) than mature leaves, demonstrating that PSII in newly initiating leaves is more sensitive to heat stress. Further analysis revealed that PSII structure in newly initiating leaves showed a robust alteration under heat stress, which was reflected by the clear K phase in the OJIP curves. Therefore, we suggest that the enhanced thermostability of PSII in the case of leaf growth might be associated with an improvement of the stability of the oxygen-evolving complex (OEC) to heat stress during leaf development.     

Characterization of photosystem II heterogeneity in response to high salt stress in wheat leaves (<Emphasis Type="Italic">Triticum aestivum</Emphasis>)

The effect of high salt stress on PS II heterogeneity was investigated in wheat (Triticum aestivum) leaves. On the basis of antenna size, PS II has been classified into three forms, i.e., α, β, and γ centers while on the basis of electron transport properties of the reducing side of the reaction centers, two distinct forms of PS II have been suggested, i.e., Q_B reducing centers and Q_B non-reducing centers. The chlorophyll a (Chl a) fluorescence transients, which can quantify PS II behavior, were recorded using PEA to derive OJIP in vivo with high time resolution and further analyzed according to JIP test. Our results showed that with an increase in the salt concentration during growth, the number of Q_B non-reducing centers increased. In antenna size heterogeneity the number of β and γ centers increased while the number of α centers decreased. A change in the energetic connectivity between the PS II units was also observed. Recovery studies showed that antenna heterogeneity was completely recovered from damage at 0.5 M NaCl concentration and partially recovered at 1 M NaCl concentration while reducing side heterogeneity showed no recovery at all after 0.5 M onwards.     

Novel effects of methyl viologen on photosystem II function in spinach leaves

Methyl viologen (MV) is a well-known electron mediator that works on the acceptor side of photosystem I. We investigated the little-known, MV-induced inhibition of linear electron flow through photosystem II (PS II) in spinach-leaf discs. Even a low [MV] decreased the (1) average, light-adapted photochemical efficiency of PS II traps, (2) oxidation state of the primary quinone acceptor Q_A in PS II during illumination, (3) photochemical efficiency of light-adapted open PS II traps, (4) fraction of absorbed light energy dissipated constitutively in a light-independent manner or as chlorophyll (Chl) a fluorescence emission, (5) Chl a fluorescence yield corresponding to dark-adapted open reaction-center traps (F _o) and closed reaction-center traps (F _m), and (6) half-time for re-oxidation of Q_A⁻ in PS II after a single-turnover flash. These effects suggest that the presence of MV accelerates various “downhill” electron-transfer steps in PS II. Therefore, when using the MV to quantify cyclic electron flow, the inhibitory effect of MV on PS II should be taken into account.     

The acceptor side of photosystem II is damaged more severely than the donor side of photosystem II in ‘Honeycrisp’ apple leaves with zonal chlorosis

To investigate the photoinhibition of photosynthesis in ‘Honeycrisp’ apple (Malus domestica Borkh. cv. Gala) leaves with zonal chlorosis, we compared pigments, CO₂ assimilation and chlorophyll (Chl) a fluorescence (OJIP) transient between chlorotic leaves and normal ones. Chl and carotenoids (Car) contents, Chl a/b ratio, and absorptance were lower in chlorotic leaves than in normal ones, whereas Car/Chl ratio was higher in the former. Although CO₂ assimilation and stomatal conductance were lower in chlorotic leaves, intercellular CO₂ concentration did not differ significantly between the two leaf types. Compared with normal leaves, chlorotic ones had increased deactivation of oxygen-evolving complexes (OEC), minimum fluorescence (F _o), dissipated energy, relative variable fluorescence at L-, W-, J- and I-steps, and decreased maximum fluorescence (F _m), maximum quantum yield for primary photochemistry (F _v /F _m or TR_o/ABS), quantum yield for electron transport (ET_o/ABS), quantum yield for the reduction of end acceptors of photosystem I (PSI) (φ_Ro and RE_o/ABS), maximum amplitude of IP phase, amount of active photosystem II (PSII) reaction centers (RCs) per cross section (CS) and total performance index (PI_tot,abs). In conclusion, photoinhibition occurs at both the donor (i.e., the OEC) and the acceptor sides of PSII in chlorotic leaves. The acceptor side is damaged more severely than the donor side, which possibly is the consequence of over-reduction of PSII due to the slowdown of Calvin cycle. In addition to decreasing light absorptance by lowering Chl level, energy dissipation is enhanced to protect chlorotic leaves from photo-oxidative damage.     

Light acclimation maintains the redox state of the PS II electron acceptor Q2 within a narrow range over a broad range of light intensities

Chrysanthemum inducum-hybrid `Coral Charm', Hibiscus rosa-sinensis L. `Cairo Red' and Spathiphyllum wallisii Regel `Petit' were grown in natural light in a greenhouse at three levels of irradiance using permanent shade screens. Light acclimation of photosynthesis was characterized using modulated chlorophyll a fluorescence of intact leaves. A close correlation was found between the degree of reduction of the primary electron acceptor Q_A of Photosystem II (PS II) approximated as the fluorescence parameter 1−q_P, and light acclimation. The action range of 1−q_P was 0–0.4 from darkness to full irradiance around noon, within the respective light treatments in the greenhouse, indicating that most PS II reaction centres were kept open. In general, the index for electron transport (ETR) measured by chlorophyll fluorescence was higher for high-light (HL) than intermediate-(IL) and low-light (LL) grown plants. However, HL Chrysanthemum showed 40% higher ETR than HL Hibiscus at light saturation, despite identical redox states of Q_A. The light acclimation of the non-radiative dissipation of excess energy in the antenna, NPQ, varied considerably between the species. However, when normalized against q_P, a strong negative correlation was found between thermal dissipation and ETR measured by chlorophyll fluorescence. To be able to accommodate a high flux of electrons through PS II, the plants with the highest light-saturated ETR had the lowest NPQ/q_P. The possibility of using chlorophyll fluorescence for quantification of the energy balance between energy input and utilization in PS II in intact leaves is discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

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.     

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.     

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     

Period four oscillations in chlorophyll a fluorescence

The discovery of period four oscillations of the fluorescence yield under flashing light demonstrated that not only the redox state of the Photosystem II (PS II) electron acceptor Q_A, but also the oxygen evolving cycle (described by the S states) modulates the fluorescence yield of chlorophyll (Chl). The positive charges accumulated on the donor side of PS II act on the fluorescence yield (measured in the Q_A ⁻ state during a strong flash) through the concentration of the quencher P₆₈₀ ⁺, the oxidized form of PS II reaction center Chl a. However, the period four oscillations of the fluorescence yield detected 1 s after a strong flash (in the P₆₈₀Q_A state) have not yet been fully explained. This revised version was published online in June 2006 with corrections to the Cover Date.     

Decline of Photosynthetic Pigments,Ribulose-1,5-Bisphosphate Carboxylase and Soluble Protein Contents,Nitrate Reductase and Photosynthetic Activities,and Changes in Thylakoid Membrane Protein Pattern in Canopy Shade Grapevine (Vitis Vinifera L. cv. Moscato Giallo) Leaves

In canopy shade leaves of grapevine (Vitis vinifera L. cv. Moscato giallo) grown in the field the contents of chlorophyll (Chl), carotenoids (Car), and soluble protein per fresh mass were lower than in sun leaves. RuBPC activity, in vivo nitrate reductase activity (indicator of nitrate utilisation), apparent electron transport rate, and photochemical fluorescence quenching were also significantly reduced in canopy shade leaves. When various photosynthetic activities were followed in isolated thylakoids, canopy shade leaves exerted a marked inhibition of whole chain and photosystem (PS) 2 activity. Smaller inhibition of PS1 activity was observed even in high-level canopy shade (HS) leaves. The artificial exogenous electron donors, DPC and NH₂OH, significantly restored the loss of PS2 activity in HS leaves. Similar results were obtained when F_v/F_m was evaluated by Chl fluorescence measurements. The marked loss of PS2 activity in canopy shade leaves was due to the loss of 47, 43, 33, 28–25, 23, 17, and 10 kDa polypeptides.     

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.     

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.     

Analysis of chlorophyll a fluoresence changes in weak light in heat treated Amaranthus chloroplasts

After preheating of Amaranthus chloroplasts at elevated temperatures (up to 45°C), the chlorophyll a fluorescence level under low excitation light rises as compared to control (unheated) as observed earlier in other chloroplasts (Schreiber U and Armond PA (1978) Biochim Biophys Acta 502: 138–151). This elevation of heat induced fluorescence yield is quenched by addition of 0.1 mM potassium ferricyanide, suggesting that with mild heat stress the primary electron acceptor of photosystem II is more easily reduced than the unheated samples. Furthermore, the level of fluorescence attained after illumination of dithionite-treated samples is independent of preheating (up to 45°C). Thus, these experiments indicate that the heat induced rise of fluorescence level at low light can not be due to changes in the elevation in the true constant F₀ level, that must by definition, be independent of the concentration of Q_A. It is supposed that the increase in the fluorescence level by weak modulated light is either partly associated with dark reduction of Q_A due to exposure of chloroplasts to elevated temperature or due to temperature induced fluorescence rise in the so called inactive photosystem II centre where Q_A are not connected to plastoquinone pool. In the presence of dichlorophenyldimethylurea the fluorescence level triggered by weak modulated light increases at alkaline pH, both in control and heat stressed chloroplasts. This result suggests that the alkaline pH accelerates electron donation from secondary electron donor of photosystem II to Q_A both in control and heat stressed samples. Thus the increase in fluorescence level probed by weak modulated light due to preheating is not solely linked to increase in true F₀ level, but largely associated with the shift in the redox state of Q_A, the primary stable electron acceptor of photosystem II.Abbreviations ADRY Acceleration of Deactivation of Reaction of Enzyme Y - CCCP Carbonyl cyanide 4-(trifluoromethoxy)-phenylhydrazone - Chl Chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - FeCN potassium ferricyanide - HEPES 4-(2-hydroxy ethyl)-1-piperazine ethane sulfonic acid - LHCP Light harvesting chlorophyll protein - MES (4-morpholine ethane sulfonic acid) - PS photosystem - Q_A and Q_B first and second consecutive electron acceptors of photosystem II - TES (2-[tris(hydroxymethyl)-methylamino]-1-ethanesulfonic acid) sulfonic acid - TRICINE N-[tris(hydroxymethyl)methyl] glycine     

Effects of different excitation and detection spectral regions on room temperature chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence parameters

The effects of different spectral region of excitation and detection of chlorophyll (Chl) a fluorescence at room temperature on the estimation of excitation energy utilization within photosystem (PS) 2 were studied in wild-type barley (Hordeum vulgare L. cv. Bonus) and its Chl b-less mutant chlorina f2 grown under low and high irradiances [100 and 1 000 μmol(photon) m⁻² s⁻¹]. Three measuring spectral regimes were applied using a PAM 101 fluorometer: (1) excitation in the red region (maximum at the wavelength of 649 nm) and detection in the far-red region beyond 710 nm, (2) excitation in the blue region (maximum at the wavelength of 461 nm) and detection beyond 710 nm, and (3) excitation in the blue region and detection in the red region (660– 710 nm). Non-photochemical quenching of maximal (NPQ) and minimal fluorescence (SV₀), determined by detecting Chl a fluorescence beyond 710 nm, were significantly higher for blue excitation as compared to red excitation. We suggest that this results from higher non-radiative dissipation of absorbed excitation energy within light-harvesting complexes of PS2 (LHC2) due to preferential excitation of LHC2 by blue radiation and from the lower contribution of PS1 emission to the detected fluorescence in the case of blue excitation. Detection of Chl a fluorescence originating preferentially from PS2 (i.e. in the range of 660–710 nm) led to pronounced increase of NPQ, SV₀, and the PS2 photochemical efficiencies (F_V/F_M and F_V′/F_M′), indicating considerable underestimation of these parameters using the standard set-up of PAM 101. Hence PS1 contribution to the minimal fluorescence level in the irradiance-adapted state may reach up to about 80 %.     

Co<Subscript>2</Subscript> assimilation and chlorophyll fluorescence in green <Emphasis Type="Italic">versus</Emphasis> red <Emphasis Type="Italic">Berberis thunbergii</Emphasis> leaves measured with different quality irradiation

Photosynthesis, photorespiration, and chlorophyll (Chl) fluorescence in green and red Berberis thunbergii leaves were studied with two different measuring radiations, red (RR) and “white” (WR). The photosynthetic and photorespiration rates responded differently to the different radiation qualities, which indicate that the carboxylase and oxygenase activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) were affected. Differences in photosynthetic rate between the two color leaves were less under RR than under WR. However, this reduced difference in photosynthetic rate was not correlated with the stomatal response to the measuring radiation qualities. Compared with the WR, the RR reduced the differences in dark-adapted minimum and maximum fluorescence, steady-state fluorescence, light-adapted maximum fluorescence, and actual photochemical efficiency (Φ_PS2) of photosystem 2 (PS2), but enlarged the difference in non-photochemical quenching between the two color leaves. Differences in both maximum quantum yield of PS2 and ratio of Φ_PS2 to quantum yield of CO₂ fixation between the two color leaves were similar under the two measuring radiations. To exclude disturbance of radiation attenuation caused by anthocyanins, it is better to use RR to compare the photosynthesis and Chl fluorescence in green versus red leaves.