Photoacoustic characteristics of leaves of atrazine-resistant weed mutants

The photosynthetic characteristics of leaves of atrazine-resistant and-susceptible biotypes of several weed species (Solanum nigrum, Senecio vulgaris, Epilobium ciliatum and Chenopodium album) were compared using the photoacoustic method. Analysis of the dependence of the photoacoustic signal of the modulation frequency indicated that, in Solanum, Epilobium and Senecio, the relative quantum yield of O₂ evolution (estimated by the ratio of the amplitude of the O₂ signal, A_OX, to that of the photothermal signal, A_PT) was substantially reduced in the atrazine-resistant mutant, without any changes in the O₂ diffusion characteristics of the leaves. In contrast, in Chenopodium, atrazine-resistance was associated with a concomitant change in and in the leaf diffusion parameters. This latter change suggests that the leaf internal anatomy was modified in the resistant Chenopodium. Measurements of the Emerson enhancement indicated that the reduction of observed in the atrazine-resistant mutants was caused by a marked decrease in the photochemical potential of PS II (). The study of the light intensity dependence of the A_OX/A_PT ratio showed that saturation of O₂ evolution occurred at the same light level (around 2000 mol m^-2 s^-1) in both types of plants. However, the relative maximal rate of O₂ evolution was slightly lower (-10%) in the atrazine-resistant biotype as compared to the wild type. Reduced and light-saturated rate of O₂ evolution were also measured in atrazine-resistant weed biotypes using a conventional Clark-type O₂ electrode.Abbreviations A_OX modulated O₂ evolution component of the photoacoustic signal - A_PT photothermal component of the photoacoustic signal - Atrazine 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine - E Emerson enhancement - PS II and PS I photosystems II and I, respectively - Q_A primary electron acceptor of PS II - Q_B secondary electron acceptor of PS II - quantum yield of O₂ evolution     

Sulfite inhibition of photochemical activity of intact pea leaves

Sulfite treatment of pea leaf disks in light caused a significant decrease in the relative quantum yield of photosynthetic oxygen evolution and energy storage (ES) as measured by photoacoustic (PA) spectroscopy. The inhibition was concentration dependent and was less in darkness than in light, indicating light-dependent inhibitory site(s) on the photosynthetic electron transport chain. Further, in darksulfite-treated leaves, the energy storage was more affected than the relative quantum yield of oxygen evolution, suggesting that photophosphorylation and/or cyclic electron transport around PS I are sites of sulfite action in darkness. The R_fd values, the ratio of fluorescence decrease (fd) to the steady-state fluorescence (fs), decreased significantly in leaves treated with sulfite in light but were not affected in dark-treated ones, confirming the photoacoustic observations. Similarly, the ratio of variable fluorescence (F_v) to maximum fluorescence (F_m), a measure of PS II photochemical efficiency, was affected by sulfite treatment in light and not changed by treatment in darkness. An attempt was made to explain the mechanism of sulfite action on photosynthetic electron transport in light and in darkness.Abbreviations A_PT amplitude of photothermal signal - A_ox amplitude of oxygen signal - ES energy storage - fd fluorescence decrease - fs steady-state fluorescence - F_m maximum fluorescence - F_v variable fluorescence - PA photoacoustic(s)     

The efficiency of electron transfer from QA − to the donor side of Photosystem II decreases during induction of photosynthesis: Evidences from chlorophyll fluorescence and photoacoustic techniques

The amplitudes ratio of the fast and slow phases (A_fast/A_slow) in the kinetics of the dark relaxation of variable chlorophyll fluorescence (F_V) was studied after various periods of illumination of dark-adapted primary barley leaves. Simultaneously, photosynthetic activity was monitored using the photoacoustic technique and the photochemical and non-photochemical fluorescence quenching parameters. The ratio A_fast/A_slow changed with the preceding illumination time in a two-step manner. During the first stage of photosynthetic induction (0–20 s of illumination), characterized by a drop in O₂-dependent photoacoustic signal following an initial spike and by a relatively stable small value of photochemical F_V quenching, the ratio A_fast/A_slow remained practically unaltered. During the second stage (20–60 s of illumination), when both the rate of O₂ evolution and the photochemical F_V quenching were found to be sharply developed, a marked increase in the above ratio was also observed. A linear correlation was found between the value of the photochemical quenching and the ratio A_fast/A_slow during the second phase of photosynthetic induction. It is concluded that the slow phase appearing in the kinetics of F_V dark relaxation is not due to the existence of Photosystem II reaction centres lacking the ability to reduce P700⁺ with high rates, but is instead related to the limitation of electron release from Photosystem I during the initial stage of the induction period of photosynthesis. This limitation keeps the intersystem electron carriers in the reduced state and thus increases the probability of back electron transfer from Q_A ^– to the donor side of Photosystem II.Abbreviations A_fast/A_slow the ratio of magnitudes between the fast and slow phases of dark relaxation of variable fluorescence - F_O initial level of chlorophyll fluorescence - F_V variable chlorophyll fluorescence (F-F_O) - (F_V)_S the yield of variable chlorophyll fluorescence under saturating pulse in illuminated leaves - (F_V)_M the yield of variable chlorophyll fluorescence under saturating pulse in dark-adapted leaves - PA photoacoustic - PSI Photosystem I - PS II Photosystem II - qN non-photochemical quenching - qQ photochemical quenching     

Functioning of photosystems I and II in pea leaves exposed to heat stress in the presence or absence of light

Fluorimetric, photoacoustic, polarographic and absorbance techniques were used to measure in situ various functional aspects of the photochemical apparatus of photosynthesis in intact pea leaves (Pisum sativum L.) after short exposures to a high temperature of 40 ° C. The results indicated (i) that the in-vivo responses of the two photosystems to high-temperature pretreatments were markedly different and in some respects opposite, with photosystem (PS) II activity being inhibited (or down-regulated) and PSI function being stimulated; and (ii) that light strongly interacts with the response of the photosystems, acting as an efficient protector of the photochemical activity against its inactivation by heat. When imposed in the dark, heat provoked a drastic inhibition of photosynthetic oxygen evolution and photochemical energy storage, correlated with a marked loss of variable PSII-chlorophyll fluorescence emission. None of the above changes were observed in leaves which were illuminated during heating. This photoprotection was saturated at rather low light fluence rates (around 10 W · m^–2). Heat stress in darkness appeared to increase the capacity for cyclic electron flow around PSI, as indicated by the enhanced photochemical energy storage in far-red light and the faster decay of P ₇₀₀ ⁺ (oxidized reaction center of PSI) monitored upon sudded interruption of the far-red light. The presence of light during heat stress reduced somewhat this PSI-driven cyclic electron transport. It was also observed that heat stress in darkness resulted in the progressive closure of the PSI reaction centers in leaves under steady illumination whereas PSII traps remained largely open, possibly reflecting the adjustment of the photochemical efficiency of undamaged PSI to the reduced rate of photochemistry in PSII.Abbreviations B₁ and B₂ fraction of closed PSI and PSII reaction centers, respectively - ES photoacoustically measured energy storage - F_o, F_m and F_s initial, maximal and steady-state levels of chlorophyll fluorescence - P₇₀₀ reaction center of PSI - PS (I, II) photosystem (I, II) - V = (F_s – F_o)/(F_m – F_o) relative variable chlorophyll fluorescence We wish to thank Professor R. Lannoye (ULB, Brussels) for the use of this photoacoustic spectrometer and Mrs. M. Eyletters for her help.     

Photosynthetic Responses of Leaves to Water Stress, Expressed by Photoacoustics and Related Methods : II. The Effect of Rapid Drought on the Electron Transport and the Relative Activities of the Two Photosystems

The effect of rapid dehydration of detached tobacco leaves (Nicotiana tabacum L.) on the photochemical apparatus of photosynthesis was studied in vivo by a combination of methods: photoacoustics, chlorophyll a fluorescence, and cytochrome f difference spectroscopy. It was shown that the inhibition of gross O₂ evolution was mainly caused by inactivation of PSII: (a) The saturation curve of cytochrome-f photooxidation by farred (>710 nanometers) light was resistant to the stress, leading to the conclusion that photosystem I (PSI) was largely unaffected by the stress. (b) The extent of the chlorophyll a variable fluorescence arising from photosystem II (PSII) decreased with the progression of the stress, but was largely unaffected when the leaf was preincubated with electron donors to PSII, such as hydroxylamine. It is concluded that the drought damage to PSII occurred on the photooxidative side. Despite the extensive inhibition of PSII and the relative preservation of PSI, the apparent PSII/PSI activity balance was somewhat larger in stressed leaves than in the control, as indicated by photoacoustic measurements of Emerson enhancement. These measurements were performed continuously under conditions which favor transitions to either state 1 or 2, showing that the transition to state 2 was considerably inhibited. Simultaneous measurements of chlorophyll fluorescence induction at 680 and 730 mm at room temperature were also used to probe changes in energy distribution between PSII and PSI and indicated that the transition from a dark adapted state to state 2 was also affected in water-stressed leaves. The saturation curve of the far-red light effect in Emerson enhancement was not changed by the stress, giving another independent evidence for the drought resistance of PSI activity. This apparent preservation of the imbalance in photochemical activities in favor of PSII, despite the fact that PSII is strongly inhibited, and PSI is not, supports a previous suggestion that the electron transfer between the two photosystems is not random but that a large extent of PSII and PSI units are specifically linked.     

Photoinhibition of photosynthesis in chilled potato leaves is not correlated with a loss of Photosystem-II activity

When 23 °C-grown potato leaves (Solanum tuberosum L.) were irradiated at 23 °C with a strong white light, photosynthetic electron transport and Photosystem-II (PS II) activity were inhibited in parallel. When the light treatment was given at a low temperature of 3 °C, the photoinhibition of photosynthesis was considerably enhanced, as expected. Surprisingly, no such stimulation of photoinhibition was observed with respect to the PS II function. A detailed functional analysis of the photosynthetic apparatus, using in-vivo fluorescence, absorbance, oxygen and photoacoustic measurements, and artificial electron donors/acceptors, showed a pronounced alteration of PS I activity during light stress at low temperature. More precisely, it was observed that both the pool of photooxidizeable reaction center pigment (P₇₀₀) of PS I and the efficiency of PS I to oxidize P₇₀₀ were dramatically reduced. Loss of P₇₀₀ activity was shown to be essentially dependent on atmospheric O₂ and to require a continued flow of electrons from PS II, suggesting the involvement of the superoxide anion radical which is produced by the interaction of O₂ and the photosynthetic electron-transfer chain through the Mehler reaction. Mass spectrometric measurements of O₂ exchange by potato leaves under strong illumination did not reveal, however, any stimulation of the Mehler reaction at low temperature, thus leading to the conclusion that O₂ toxicity mainly resulted from a chilling-induced inhibition of the scavenging system for O₂-radicals. Support for this interpretation was provided by the light response of potato leaves infiltrated with an inhibitor (diethyldithiocarbamate) of the chloroplastic Cu-Zn superoxide dismutase. It was indeed possible to simulate the differential inhibition of the PS II photochemical activity and the linear electron transport observed during light stress at low temperature by illuminating at 23 °C diethyldithiocarbamate-poisoned leaves. The experimental data presented here suggests that (i) the previously reported resistance of PS I to photoinhibition damage in-vivo is not an intrinsic property of PS I but results from efficient protective systems against O₂ toxicity, (ii) PS I is photoinhibited in chilled potato leaf due to the inactivation of this PS I defence system and (iii) PS I is more sensitive to superoxide anion radicals than PS II.Abbreviations PS - Photosystem - E - Emerson enhancement - _open ^p and ^P maximal and actual quantum yields of PS II photochemistry - DDC - diethyldithiocarbamate - Q_A and Q_B - primary and secondary (quinone) electron acceptors of PS II - P₆₈₀ and P₇₀₀ - reaction center pigments of PS II and PS I, respectively - SOD - superoxide dismutase     

Detection of photosynthetic energy storage in a photosystem I reaction center preparation by photoacoustic spectroscopy

Thermal emission and photochemical energy storage were examined in photosystem I reaction center/core antenna complexes (about 40 Chl a/P700) using photoacoustic spectroscopy. Satisfactory signals could only be obtained from samples bound to hydroxyapatite and all samples had a low signal-to-noise ratio compared to either PS I or PS II in thylakoid membranes. The energy storage signal was saturated at low intensity (half saturation at 1.5 W m^-2) and predicted a photochemical quantum yield of >90%. Exogenous donors and acceptors had no effect on the signal amplitudes indicating that energy storage is the result of charge separation between endogenous components. Fe(CN)₆ ^-3 oxidation of P700 and dithionite-induced reduction of acceptors F_A-F_B inhibited energy storage. These data are compatible with the hypothesis that energy storage in PS I arises from charge separation between P700 and Fe-S centers F_A-F_B that is stable on the time scale of the photoacoustic modulation. High intensity background light (160 W m^-2) caused an irreversible loss of energy storage and correlated with a decrease in oxidizable P700; both are probably the result of high light-induced photoinhibition. By analogy to the low fluorescence yield of PS I, the low signal-to-noise ratio in these preparations is attributed to the short lifetime of Chl singlet excited states in PS I-40 and its indirect effect on the yield of thermal emission.Abbreviations FFT fast Föurier transform - HA hydroxyapatite - I₅₀ half saturation intensity for energy storage - PA photoacoustic - PS photosystem - PS I-40 photosystem I reaction center/core antenna complex containing about 40 Chl a/P700 - 201-1 photoacoustic energy storage signal - S/N signal-to-noise     

Short-term responses of Photosystem I to heat stress

When 23°C-grown potato leaves (Solanum tuberosum L.) were exposed for 15 min to elevated temperatures in weak light, a dramatic and preferential inactivation of Photosystem (PS) II was observed at temperatures higher than about 38°C. In vivo photoacoustic measurements indicated that, concomitantly with the loss of PS II activity, heat stress induced a marked gas-uptake activity both in far-red light (>715 nm) exciting only PS I and in broadband light (350–600 nm) exciting PS I and PS II. In view of its suppression by nitrogen gas and oxygen and its stimulation by high carbon-dioxide concentrations, the bulk of the photoacoustically measured gas uptake by heat-stressed leaves was ascribed to rapid carbon-dioxide solubilization in response to light-modulated stroma alkalization coupled to PS I-driven electron transport. Heat-induced gas uptake was observed to be insensitive to the PS II inhibitor diuron, sensitive to the plastocyanin inhibitor HgCl₂ and saturated at a rather high photon flux density of around 1200 E m^–2 s^–1. Upon transition from far-red light to darkness, the oxidized reaction center P700⁺ of PS I was re-reduced very slowly in control leaves (with a half time t_1/2 higher than 500 ms), as measured by leaf absorbance changes at around 820 nm. Heat stress caused a spectacular acceleration of the postillumination P700⁺ reduction, with t_1/2 falling to a value lower than 50 ms (after leaf exposure to 48°C). The decreased t_1/2 was sensitive to HgCl₂ and insensitive to diuron, methyl viologen (an electron acceptor of PS I competing with the endogenous acceptor ferredoxin) and anaerobiosis. This acceleration of the P700⁺ reduction was very rapidly induced by heat treatment (within less than 5 min) and persisted even after prolonged irradiation of the leaves with far-red light. After heat stress, the plastoquinone pool exhibited reduction in darkness as indicated by the increase in the apparent Fo level of chlorophyll fluorescence which could be quenched by far-red light. Application (for 1 min) of far-red light to heat-pretreated leaves also induced a reversible quenching of the maximal fluorescence level Fm, suggesting formation of a pH gradient in far-red light. Taken together, the presented data indicate that PS I responded to the heat-induced loss of PS II photochemical activity by catalyzing an electron flow from stromal reductants. Heat-stress-induced PS I electron transport independent of PS II seems to constitute a protective mechanism since block of this electron pathway in anaerobiosis was observed to result in a dramatic photoinactivation of PS I.Abbreviations PFD photon flux density - PS Photosystem - Apt and Aox amplitude of the photothermal and photobaric components of the photoacoustic signal, respectively - P700 reaction center pigment of PS I - Fo and Fm initial and maximal levels of chlorophyll fluorescence, respectively - Fv=Fm Fo-variable chlorophyll fluorescence - Q_A primary (stable) electron acceptor of PS II - DCMU (diuron) 3-(3,4-dichlorophenyl)-1,1-dimethylurea - Cyt cytochrome     

Reversible changes in structure and function of photosynthetic apparatus of pea (Pisum sativum) leaves under drought stress

The effects of drought on photosynthesis have been extensively studied, whereas those on thylakoid organization are limited. We observed a significant decline in gas exchange parameters of pea (Pisum sativum) leaves under progressive drought stress. Chl a fluorescence kinetics revealed the reduction of photochemical efficiency of photosystem (PS)II and PSI. The non-photochemical quenching (NPQ) and the levels of PSII subunit PSBS increased. Furthermore, the light-harvesting complexes (LHCs) and some of the PSI and PSII core proteins were disassembled in drought conditions, whereas these complexes were reassociated during recovery. By contrast, the abundance of supercomplexes of PSII-LHCII and PSII dimer were reduced, whereas LHCII monomers increased following the change in the macro-organization of thylakoids. The stacks of thylakoids were loosely arranged in drought-affected plants, which could be attributed to changes in the supercomplexes of thylakoids. Severe drought stress caused a reduction of both LHCI and LHCII and a few reaction center proteins of PSI and PSII, indicating significant disorganization of the photosynthetic machinery. After 7 days of rewatering, plants recovered well, with restored chloroplast thylakoid structure and photosynthetic efficiency. The correlation of structural changes with leaf reactive oxygen species levels indicated that these changes were associated with the production of reactive oxygen species.     

Electron transport to oxygen mitigates against the photoinactivation of Photosystem II in vivo

The role of electron transport to O₂ in mitigating against photoinactivation of Photosystem (PS) II was investigated in leaves of pea (Pisum sativum L.) grown in moderate light (250 mol m^–2 s^–1). During short-term illumination, the electron flux at PS II and non-radiative dissipation of absorbed quanta, calculated from chlorophyll fluorescence quenching, increased with increasing O₂ concentration at each light regime tested. The photoinactivation of PS II in pea leaves was monitored by the oxygen yield per repetitive flash as a function of photon exposure (mol photons m^–2). The number of functional PS II complexes decreased nonlinearly with increasing photon exposure, with greater photoinactivation of PS II at a lower O₂ concentration. The results suggest that electron transport to O₂, via the twin processes of oxygenase photorespiration and the Mehler reaction, mitigates against the photoinactivation of PS II in vivo, through both utilization of photons in electron transport and increased nonradiative dissipation of excitation. Photoprotection via electron transport to O₂ in vivo is a useful addition to the large extent of photoprotection mediated by carbon-assimilatory electron transport in 1.1% CO₂ alone.Abbreviations Fm, Fo, Fv- maximal, initial (corresponding to open PS II traps) and variable chlorophyll fluorescence yield, respectively - NPQ- non-photochemical quenching - PS- photosystem - Q_A- primary quinone acceptor - qP- photochemical quenching coefficient     

Performance of active Photosystem II centers in photoinhibited pea leaves

Effects of photoinhibition on photosynthesis in pea (Pisum sativum L.) leaves were investigated by studying the relationship between the severity of a photoinhibitory treatment (measured as F_v/F_m) and several photoacoustic and chlorophyll a fluorescence parameters. Because of the observed linear relationship between the decline of F_v/F_m and the potential oxygen evolution rate determined by the photoacoustic method, the parameter F_v/F_m was used as an indicator for the severity of photoinhibition. Our analysis revealed that part of the Photosystem II (PS II) reaction centers is inactive in oxygen evolution and is also less sensitive to photoinhibition. Correcting the parameter q_P (fraction of open PS II reaction centers) for inactive PS II centers unveiled a strong increase of q_P in severely inhibited pea leaves, indicating that the inactivated active centers do no longer contribute to q_P and that photoinhibition has an all or none effect on PS II centers. Analysis of q_E (energy quenching) demonstrated its initial increase possibly associated with dephosphorylation of LHC II. Analysis of q_I (photoinhibition dependent quenching) showed that the half-time of recovery of q_I increases steeply below an F_v/F_m of 0.65. This increase of the relaxation half-time corresponds with a decrease of the electron transport rate J and tentatively indicates that the supply of ATP, needed for the recovery, starts to decrease. The data indicate the necessity of correcting for inactive centers in order to make valuable conclusions about effects of photoinhibition on photosynthetic parameters.     

Evaluation of the role of State transitions in determining the efficiency of light utilisation for CO2 assimilation in leaves

Wheat leaves were exposed to light treatments that excite preferentially Photosystem I (PS I) or Photosystem II (PS II) and induce State 1 or State 2, respectively. Simultaneous measurements of CO₂ assimilation, chlorophyll fluorescence and absorbance at 820 nm were used to estimate the quantum efficiencies of CO₂ assimilation and PS II and PS I photochemistry during State transitions. State transitions were found to be associated with changes in the efficiency with which an absorbed photon is transferred to an open PS II reaction centre, but did not correlate with changes in the quantum efficiencies of PS II photochemistry or CO₂ assimilation. Studies of the phosphorylation status of the light harvesting chlorophyll protein complex associated with PS II (LHC II) in wheat leaves and using chlorina mutants of barley which are deficient in this complex demonstrate that the changes in the effective antennae size of Photosystem II occurring during State transitions require LHC II and correlate with the phosphorylation status of LHC II. However, such correlations were not found in maize leaves. It is concluded that State transitions in C₃ leaves are associated with phosphorylation-induced modifications of the PS II antennae, but these changes do not serve to optimise the use of light absorbed by the leaf for CO₂ assimilation.Abbreviations Fm, Fo, Fv maximal, minimal and variable fluorescence yields - Fm, Fv maximal and variable fluorescence yields in a light adapted state - LHC II light harvesting chlorophyll a/b protein complex associated with PS II - q_P photochemical quenching - A₈₂₀ light-induced absorbance change at 820 nm - _{PS I}, _{PS II} relative quantum efficiencies of PS I and PS II photochemistry - _{CO 2} quantum yield of CO₂ assimilation     

Role of the xanthophyll cycle in photoprotection elucidated by measurements of light-induced absorbance changes,fluorescence and photosynthesis in leaves of Hedera canariensis

The role of the xanthophyll cycle in regulating the energy flow to the PS II reaction centers and therefore in photoprotection was studied by measurements of light-induced absorbance changes, Chl fluorescence, and photosynthetic O₂ evolution in sun and shade leaves of Hedera canariensis. The light-induced absorbance change at 510 nm (A₅₁₀) was used for continuous monitoring of zeaxanthin formation by de-epoxidation of violaxanthin. Non-radiative energy dissipation (NRD) was estimated from non-photochemical fluorescence quenching (NPQ).High capacity for zeaxanthin formation in sun leaves was accompanied by large NRD in the pigment bed at high PFDs as indicated by a very strong NPQ both when all PS II centers are closed (F'_m) and when all centers are open (F'_o). Such F_o quenching, although present, was less pronounced in shade leaves which have a much smaller xanthophyll cycle pool.Dithiothreitol (DTT) provided through the cut petiole completely blocked zeaxanthin formation. DTT had no detectable effect on photosynthetic O₂ evolution or the photochemical yield of PS II in the short term but fully inhibited the quenching of F_o and 75% of the quenching of F_m, indicating that NRD in the antenna was largely blocked. This inhibition of quenching was accompanied by an increased closure of the PS II reaction centers.In the presence of DTT a photoinhibitory treatment at a PFD of 200 mol m^-2 s^-1, followed by a 45 min recovery period at a low PFD, caused a 35% decrease in the photon yield of O₂ evolution, compared to a decrease of less than 5% in the absence of DTT. The F_v/F_m ratio, measured in darkness showed a much greater decrease in the presence than in the absence of DTT. In the presence of DTT F_o rose by 15–20% whereas no change was detected in control leaves.The results support the conclusion that the xanthophyll cycle has a central role in regulating the energy flow to the PS II reaction centers and also provide direct evidence that zeaxanthin protects against photoinhibitory injury to the photosynthetic system.Abbreviations F, F_m, F_o, F_v Fluorescence yield at actual degree of PS II center closure, when all centers are closed, when all centers are open, variable fluorescence - NPQ non-photochemical fluorescence quenching - NRD non-radiative energy dissipation - PFD photon flux density - Q_A primary acceptor PS II     

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.     

Photochemical efficiency of Photosystem II and xanthophyll cycle components in Zea mays leaves exposed to water stress and high light

The effects of two light treatments (photosynthetically active photon flux density of either 650 or 1950 µmol m^–2 s^–1) on the photochemical efficiency of Photosystem II (PS II) (measured as variable to maximum fluorescence ratio) and on the xanthophyll cycle components was studied in wilted Zea mays leaves. For comparison, these parameters were followed under the same light conditions in well-hydrated leaves maintained either in normal or CO₂-free air. The net CO₂ assimilation of dehydrated leaves declined rapidly as their relative water content (RWC) decreased from 100 to 60% while the PS II efficiency measured after a prolonged dark period of 16 h declined only when RWC leaves was lower than 60%. Furthermore, drought caused an increase in the pool size of the xanthophyll cycle pigments and the presence of a sustained elevated level of zeaxanthin and antheraxanthin at the end of the long dark period. The leaf water deficit enhanced the sensitivity of PS II efficiency to light exposure. During illumination, strong inhibition of PS II efficiency and large violaxanthin deepoxidation was observed in wilted leaves even under moderate photon flux density compared to control leaves in the same conditions. After 2 h of darkness following the light treatment, the PS II efficiency that is dependent on the previous PPFD, decreased with leaf water deficit. Moreover, zeaxanthin epoxidation led to an accumulation of antheraxanthin in dehydrated leaves. All these drought effects on PS II efficiency and xanthophyll cycle components were also obtained in well-hydrated leaves by short-term CO₂ deprivation during illumination. We conclude that the increased susceptibility of PS II efficiency to light in wilted maize leaves is mainly explained by the decrease of CO₂ availability and the resulting low net CO₂ assimilation.     

Effects of calcium deficiency on Coffea arabica. Nutrient changes and correlation of calcium levels with some photosynthetic parameters

Calcium deficiency was induced in hydroponically grown 1.5-years-old coffee plants with 12–14 pairs of leaves. Calcium was given in the form of Ca(NO₃)₂: 5, 2.5, 0.1, 0.01 and 0 mM. After 71 days of Ca-treatment root and shoot as well as total biomass were decreased by severe Ca-deficiency. However, a stronger decrease was observed for shoot growth as revealed by the increase in the root/shoot ratio. New leaves were affected showing decreases in the total leaf area and in Leaf Area Duration (LAD). After 91 days of deficiency, leaf protein concentration decreased (by about 45%) in the top leaves while nitrate reductase activity (NRA) and NO₃ content showed no significant changes. Total nitrogen and mineral concentrations (P, K, Ca, Mg and Na) were also determined in leaves and roots. With the decrease in calcium concentration in Ca-deficiency conditions, we observed concomitant increases in the concentrations of K⁺, Mg²⁺ and Na⁺ in leaves (maximal changes of 32% for K⁺, 96% for Mg²⁺ and 438% for Na⁺) and in roots (108% for K⁺, 86% for Mg²⁺ and 38% for Na⁺). Accordingly, the ratio between elements changed, including the ratio N/P, showing a non-equilibrium in the balance of nutrients. Significant correlations were obtained between Ca²⁺ concentration and some photosynthetic parameters. Ca-deficiency conditions would increase the loss of energy as expressed by the rise in a_E and decrease the photochemical efficiency, which confirms the importance of this element in the stabilization of chlorophyll and in the maintenance of good photochemical efficiency at PS II level.Abbreviations Chl Chlorophyll - Fv/Fm ratio of variable to maximal fluorescence - LAD leaf area duration - LHC II light harvesting complex of PS II - NRA nitrate reductase activity - PC photosynthetic capacity - PS II photosystem II - P₆₈₀ reaction center of PS II - q_N non-photochemical quenching - q_E high-energy dependent quenching - q_p photochemical quenching - SLA specific leaf area     

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     

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.     

Light-harvesting chlorophyll a-b complex requirement for regulation of Photosystem II photochemistry by non-photochemical quenching

Recently, it has been suggested (Horton et al. 1992) that aggregation of the light-harvesting a-b complex (LHC II) in vitro reflects the processes which occur in vivo during fluorescence induction and related to the major non-photochemical quenching (qE). Therefore the requirement of this chlorophyll a-b containing protein complex to produce qN was investigated by comparison of two barley mutants either lacking (chlorina f2) or depressed (chlorina¹⁰⁴) in LHC II to the wild-type and pea leaves submitted to intermittent light (IL) and during their greening in continuous light. It was observed that qN was photoinduced in the absence of LHC II, i.e. in IL grown pea leaves and the barley mutants. Nevertheless, in these leaves qN had no (IL, peas) or little (barley mutants) inhibitory effect on the photochemical efficiency of Q_A reduction measured by flash dosage response curves of the chlorophyll fluorescence yield increase induced by a single turn-over flash During greening in continuous light of IL pea leaves, an inhibitory effect on Q_A photoreduction associated to qN developed as Photosystem II antenna size increased with LHC II synthesis. Utilizing data from the literature on connectivity between PS II units versus antenna size, the following hypothesis is put forward to explain the results summarized above. qN can occur in the core antenna or Reaction Center of a fraction of PS II units and these units will not exhibit variable fluorescence. Other PS II units are quenched indirectly through PS II-PS II exciton transfer which develops as the proportion of connected PS II units increases through LHC II synthesis.     

Photosynthetic regulation of C4 desert plant Haloxylon ammodendron under drought stress

About 20-year-old desert plants of C₄ species, Haloxylon ammodendron, growing at the southern edge of the Badain Jaran Desert in China, were selected to study the photosynthetic characteristics and changes in chlorophyll fluorescence when plants were subject to a normal arid environment (AE), moist atmospheric conditions during post-rain (PR), and the artificial supplement of soil water (SW). Results showed that under high radiation, in the AE, the species down-regulated its net assimilation rate (A) and maximum photochemical efficiency of PS II (Fv/Fm), indicating photoinhibition. However, under the PR and SW environments, A was up-regulated, with a unimodal diurnal course of A and a small diurnal change in Fv/Fm, suggesting no photoinhibition. When the air humidity or SW content was increased, the light compensation points were reduced; light saturation points were enhanced; while light saturated rate of CO₂ assimilation (A _max) and apparent quantum yield of CO₂ assimilation (Φ_C) increased. Φ_C was higher while the A _max was reduced under PR relative to the SW treatment. It was concluded that under high-radiation conditions drought stress causes photoinhibition of H. ammodendron. Increasing air humidity or soil moisture content can reduce photoinhibition and increase the efficiency of solar energy use.