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)     

Nitrogen supply as a factor influencing photoinhibition and photosynthetic acclimation after transfer of shade-grown Solanum dulcamara to bright light

We have compared the ability of shadegrown clones of Solamum dulcamara L. from shade and sun habitats to acclimate to bright light, as a function of nitrogen nutrition before and after transfer to bright light. Leaves of S. dulcamara grown in the shade with 0.6 mM NO ₃ ^- have similar photosynthetic properties as leaves of plants grown with 12.0 mM NO ₃ ^- . When transferred to bright light for 1–2 d the leaves of these plants show substantial photoinhibition which is characterized by about 50% decrease in apparent quantum yield and a reduction in the rate of photosynthesis in air at light saturation. Photoinhibition of leaf photosynthesis is associated with reduction in the variable component of low-temperature fluorescence emission, and with loss of in-vitro electron transport, especially of photosystem II-dependent processes.We find no evidence for ecotypic differentiation in the potential for photosynthetic acclimation among shade and sun clones of S. dulcamara, or of differentiation with respect to nitrogen requirements for acclimation. Recovery from photoinhibition and subsequent acclimation of photosynthesis to bright light only occurs in leaves of plants provided with 12.0 mM NO ₃ ^- . In these, apparent quantum yield is fully restored after 14 d, and photosynthetic acclimation is shown by an increase in light-saturated photosynthesis in air, of light-and CO₂-saturated photosynthesis, and of the initial slope of the CO₂-response curve. The latter changes are highly correlated with changes in ribulose-bisphosphate-carboxylase activity in vitro. Plants supplied with 0.6 mM NO ₃ ^- show incomplete recovery of apparent quantum yield after 14 d, but CO₂-dependent leaf photosynthetic parameters return to control levels.Symbols and abbreviations F_o initial level of fluorescence at 77 K - F_m maximum level of fluorescence at 77 K - F_v variable components of fluorescence at 77 K (F_v=F_m-F_o) - PSI, PSII photosystem I and II, respectively - RuBP ribulose-1,5-bisphosphate - RuBPCase ribulose-1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39)     

Reversible photoinhibition of unhardened and cold-acclimated spinach leaves at chilling temperatures

The photoinhibition of photosynthesis at chilling temperatures was investigated in cold-acclimated and unhardened (acclimated to +18° C) spinach (Spinacia oleracea L.) leaves. In unhardened leaves, reversible photoinhibition caused by exposure to moderate light at +4° C was based on reduced activity of photosystem (PS) II. This is shown by determination of quantum yield and capacity of electron transport in thylakoids isolated subsequent to photoinhibition and recovery treatments. The activity of PSII declined to approximately the same extent as the quantum yield of photosynthesis of photoinhibited leaves whereas PSI activity was only marginally affected. Leaves from plants acclimated to cold either in the field or in a growth chamber (+1° C), were considerably less susceptible to the light treatment. Only relatively high light levels led to photoinhibition, characterized by quenching of variable chlorophyll a fluorescence (F_V) and slight inhibition of PSII-driven electron transport. Fluorescence data obtained at 77 K indicated that the photoinhibition of cold-acclimated leaves (like that of the unhardened ones) was related to increased thermal energy dissipation. But in contrast to the unhardened leaves, 77 K fluorescence of cold-acclimated leaves did not reveal a relative increase of PSI excitation. High-light-treated, cold-acclimated leaves showed increased rates of dark respiration and a higher light compensation point. The photoinhibitory fluorescence quenching was fully reversible in low light levels both at +18° C and +4° C; the recovery was much faster than in unhardened leaves. Reversible photoinhibition is discussed as a protective mechanism against excess light based on transformation of PSII reaction centers to fluorescence quenchers.Abbreviations F_O initial fluorescence - F_M maximal fluorescence - F_V devariable fluorescence (f_m-f_o) - PFD photon flux density - PS photosystem - SD standard deviation The authors thank the Deutsche Forschungsgemeinschaft and the Academy of Finland for financial support.     

Cold-hardening-induced resistance to photoinhibition of photosynthesis in winter rye is dependent upon an increased capacity for photosynthesis

Analyses of chlorophyll fluorescence and photosynthetic oxygen evolution were conducted to understand why cold-hardened winter rye (Secale cereale L.) is more resistant to photoinhibition of photosynthesis than is non-hardened winter rye. Under similar light and temperature conditions, leaves of cold-hardened rye were able to keep a larger fraction of the PS II reaction centres in an open configuration, i.e. a higher ratio of oxidized to reduced Q_A (the primary, stable quinone acceptor of PSII), than leaves of non-hardened rye. Three fold-higher photon fluence rates were required for cold-hardened leaves than for non-hardened leaves in order to establish the same proportion of oxidized to reduced Q_A. This ability of cold-hardened rye fully accounted for its higher resistance to photoinhibition; under similar redox states of q_a cold-hardened and non-hardened leaves of winter rye exhibited similar sensitivities to photoinhibition. Under given light and temperature conditions, it was the higher capacity for light-saturated photosynthesis in cold-hardened than in non-hardened leaves, which was responsible for maintaining a higher proportion of oxidized to reduced Q_A. This higher capacity for photosynthesis of cold-hardened leaves also explained the increased resistance of photosynthesis to photoinhibition upon cold-hardening.Abbreviations F_m and F'_m fluorescence when all PSII reaction centres are closed in dark- and light-acclimated leaves, respectively - F_o and F'_o fluorescence when all PSII reaction centres are open in darkness and steady-state light, respectively - F_v variable fluorescence (F'_m-F'_o) under steady-state light conditions - F_v/F_m the ratio of variable to maximum fluorescence as an expression of the maximum photochemical yield of PSII in dark-acclimated leaves - Q_A the primary, stable, quinone electron acceptor of PSII - q_N non-photochemical quenching of fluorescence due to high energy state (pH) - q_p photochemical quenching of fluorescence - RH cold-hardened rye - RNH non-hardened rye This work was supported by a Natural Sciences and Engineering Research Council of Canada (NSERCC) Operating Grant to N.P.A.H. G.Ö. was supported by an NSERCC International Exchange Award and by the Swedish Natural Science Research Council.     

Dithiothreitol,an inhibitor of violaxanthin de-epoxidation,increases the susceptibility of leaves ofNerium oleander L. to photoinhibition of photosynthesis

Leaves ofNerium oleander L. plants, which had been previously kept in a shaded glasshouse for at least two months, were fed 1 mM dithiothreitol (DTT) through their petioles, either for 12h in darkness (overnight) or for 2h in low light (28 μmol photons·m⁻²·s⁻¹), in each case followed by a 3-h exposure to high light (1260 μmol photons·m⁻²·s⁻¹). During exposure to high light, violaxanthin became converted to zeaxanthin in control leaves, to which water had been fed, whereas zeaxanthin did not accumulate in leaves treated with DTT. Total carbon gain was not reduced by DTT during the photoinhibitory treatment. Exposure to high light led to a decrease in the photochemical efficiency of photosystem II, measured as the ratio of variable over maximum fluorescence emission,F _v/F _M, at both 298 K and 77K. The decrease was much more pronounced in the presence of DTT, mainly owing to a sustained increase in the instantaneous fluorescence,F _o. By contrast, in the control leaves,F _o determined immediately after the high-light treatment showed a transient decrease below theF _o value obtained before the onset of the photoinhibitory treatment (i.e. after 12 h dark adaptation), followed by a rapid return (within seconds) to this original level ofF _o during the following recovery period in darkness. Incubation of leaves with DTT led to large, sustained decreases in the photon-use efficiency of photosynthetic O₂ evolution by bright light, whilst the capacity of photosynthetic O₂ evolution at light and CO₂ saturation was less affected. In the control leaves, only small reductions in the photon yield and in the photosynthetic capacity were observed. These findings are consistent with previous suggestions that zeaxanthin, formed in the xanthophyll cycle by de-epoxidation of violaxanthin, is involved in protecting the photosynthetic apparatus against the adverse effects of excessive light.     

Novel Evidence for a Reversible Dissociation of Light-harvesting Complex II from Photosystem II Reaction Center Complex Induced by Saturating Light Illumination in Soybean Leaves

After saturating light illumination for 3 h the potential photochemical efficiency of photosystem Ⅱ （PSII） （FJF,, the ratio of variable to maximal fluorescence） decreased markedly and recovered basically to the level before saturating light illumination after dark recovery for 3 h in both soybean and wheat leaves, indicating that the decline in FJ/Fm is a reversible down-regulation. Also, the saturating light illumination led to significant decreases in the low temperature （77 K） chlorophyll fluorescence parameters F685 （chlorophyll a fluorescence peaked at 685 nm） and F685/F735 （F735, chlorophyll a fluorescence peaked at 735 nm） in soybean leaves but not in wheat leaves. Moreover, trypsin （a protease） treatment resulted in a remarkable decrease in the amounts of PsbS protein （a nuclear gene psbS-encoded 22 kDa protein） in the thylakoids from saturating light-illuminated （SI）, but not in those from darkadapted （DT） and dark-recovered （DRT） soybean leaves. However, the treatment did not cause such a decrease in amounts of the PsbS protein in the thylakoids from saturating light-illuminated wheat leaves. These results support the conclusion that saturating light illumination induces a reversible dissociation of some light-harvesting complex Ⅱ （LHClI） from PSII reaction center complex in soybean leaf but not in wheat leaf.     

Photoinhibition of photosynthesis: effect on chlorophyll fluorescence at 77K in intact leaves and in chloroplast membranes of Nerium oleander

The effect of exposing intact leaves and isolated chloroplast membranes of Nerium oleander L. to excessive light levels under otherwise favorable conditions was followed by measuring photosynthetic CO₂ uptake, electron transport and low-temperature (77K=-196°C) fluorescence kinetics. Photoinhibition, as manifested by a reduced rate and photon (quantum) yield of photosynthesis and a reduced electron transport rate, was accompanied by marked changes in fluorescence characteristics of the exposed upper leaf surface while there was little effect on the shaded lower surface. The most prominent effect of photoinhibitory treatment of leaves and chloroplasts was a strong quenching of the variable fluorescence emission at 692 nm (F_v,692) while the instantaneous fluorescence (F_o,692) was slightly increased. The maximum and the variable fluorescence at 734 nm were also reduced but not as much as F_M,692 and F_v,692. The results support the view that photoinhibition involves an inactivation of the primary photochemistry of photosystem II by damaging the reaction-center complex. In intact leaves photoinhibition increased with increased light level, increased exposure time, and with decreased temperature. Increased CO₂ pressure or decreased O₂ pressure provided no protection against photoinhibition. With isolated chloroplasts, inhibition of photosystem II occurred even under essentially anaerobic conditions. Measurements of fluorescence characteristics at 77K provides a simple, rapid, sensitive and reproducible method for assessing photoinhibitory injury to leaves. The method should prove especially useful in studies of the occurrence of photoinhibition in nature and of interactive effects between high light levels and major environmental stress factors.Abbreviations and symbols PFD photon flux area density - PSI, PSII photosystem I, II - F_M, F_O, F_V maximum, instantaneous, variable fluorescence emission C.I.W.-D.P.B. Publication No. 773     

Recovery from winter depression of photosynthesis in pine and spruce

Summary Recovery from winter depression of photosynthesis was studied in Pinus sylvestris, Pinus conforta and Picea abies by means of chlorophyll fluorescence and gas exchange measurements. During the winter 1986–1987 the fluorescence yield was low and no variable fluorescence was detectable before the end of March. In the field recovery of variable fluorescence/maximum fluorescence (F_v/F_m) during spring was slow for all three species studied. The temperature dependence of recovery was confirmed from measurements of the potential rate of recovery of F_v/F_m at different temperatures in the laboratory. At 20° C, F_v/F_m increased from 0.1 to 0.8 within 3 days. Recovery of F_v/F_m was paralleled by an increase in apparent photon yield. No significant differences could be demonstrated between the studied tree species in potential rate of recovery in the laboratory or in actual recovery in the field.     

The effect of UV-B radiation on photosynthesis and respiration of phytoplankton,benthic macroalgae and seagrasses

Several species of marine benthic algae, four species of phytoplankton and two species of seagrass have been subjected to ultraviolet B irradiation for varying lengths of time and the effects on respiration, photosynthesis and fluorescence rise kinetics studied. No effect on respiration was found. Photosynthesis was inhibited to a variable degree in all groups of plants after irradiation over periods of up to 1 h and variable fluorescence was also inhibited in a similar way. The most sensitive plants were phytoplankton and deep-water benthic algae. Intertidal benthic algae were the least sensitive to UV-B irradiation and this may be related to adaptation, through the accumulation of UV-B screening compounds, to high light/high UV-B levels. Inhibition of variable fluorescence (F_v) of the fluorescence rise curve was a fast and sensitive indicator of UV-B damage. Two plants studied, a brown alga and a seagrass, showed very poor recovery of F_v over a period of 32 h.Abbreviations F_m- fluorescence yield with reaction centres closed - F_o- fluorescence yield with reaction centres open - F_v- variable fluorescence - PAR- photosynthetically active radiation - P680- primary donor of Photosystem II - O- primary quencher of Photosystem II - Q_A- primary quinone acceptor of Photosystem II - UV-B- ultraviolet B     

Photoinhibition at chilling temperature

The effects of moderate light at chilling temperature on the photosynthesis of unhardened (acclimated to +18° C) and hardened (cold-acclimated) spinach (Spinacea oleracea L.) leaves were studied by means of fluorescence-induction measurements at 20° C and 77K and by determination of quantum yield of O₂ evolution. Exposure to 550 mol photons·m^-2·s^-1 at +4° C induced a strong photoinhibition in the unhardened leaves within a few hours. Photoinhibition manifested by a decline in quantum yield was characterized by an increase in initial fluorescence (F _o) and a decrease in variable fluorescence (F _v) and in the ratio of variable to maximum fluorescence (F _V/F _M), both at 77K and 20° C. The decline in quantum yield was more closely related to the decrease in the F _V/F _M ratio measured at 20° C, as compared with F _V/F _M at 77K. Quenching of the variable fluorescence of photosystem II was accompanied by a decline in photosystem-I fluorescence at 77K, indicating increased thermal de-excitation of pigments as the main consequence of the light treatment. All these changes detected in fluorescence parameters as well as in the quantum yield of O₂ evolution were fully reversible within 1–3 h at a higher temperature in low light. The fast recovery led us to the view that this photoinhibition represents a regulatory mechanism protecting the photosynthetic apparatus from the adverse effects of excess light by increasing thermal energy dissipation. Long-term cold acclimation probably enforces other protective mechanisms, as the hardened leaves were insensitive to the same light treatment that induced strong inhibition of photosynthesis in unhardened leaves.Abbreviations F ₀ initial fluorescence - F _M maximum fluorescence - F _V variable fluorescence (F _M-F ₀ - PFD photon flux density - PS photosystem     

Photosynthetic response of Ulva rotundata to light and temperature during emersion on an intertidal sand flat

Summary We have investigated the diurnal response of photosynthesis and variable photosystem II (PSII) chlorophyll fluorescence at 77 K for thalli of the chlorophyte macroalga, Ulva rotundata, grown in outdoor culture and transplanted to an intertidal sand flat in different seasons. The physiological response in summer indicated synergistic effects of high PFD and aerial exposure, the latter probably attributable to temperature, which usually increased by 8 to 10° C during midday emersion. Except at extreme emersed temperatures in summer (38° C), the light-saturated photosynthesis rate (P_m) did not decline at midday. In contrast, light-limited quantum yield of photosynthetic O₂ exchange () and the ratio of variable to maximum fluorescence yield (F_v/F_m) reversibly declined during midday low tides in all seasons. Shade-grown thalli exhibited a fluorescence response suggestive of greater photodamage to PSII, whereas sun-grown thalli had greater photoprotective capacity. The fluorescence decline was smaller when high tide occurred at midday, and was delayed during morning cloudiness. These results suggest that the diurnal response to PFD in this shallow water species is modified by tidal and meteorological factors. U. rotundata has a great capacity for photoprotection which allows it to tolerate and even thrive in the harsh intertidal environment.Abbreviations F_o instantaneous yield of chlorophyll fluorescence - F_m maximum yield of fluorescence - F_v variable yield (F_m–F_o) of fluorescence - PFD photon flux density (400–700 nm) - P_m light-saturated rate of photosynthesis - PSH photosystem II - Q_A electron acceptor of PSII - light-limited quantum yield of photosynthesis     

Effects of photoinhibitory treatment on CO2 assimilation, the quantum yield of CO2 assimilation, D1 protein, ascorbate, glutathione and xanthophyll contents and the electron transport rate in vine leaves

The responses of photosynthesis to high light and low temperature were studied in vines cultivated in the greenhouse in low light. Exposure to high light (1000 /umol m^?2 s^?1) or low temperature (5 °C) alone had no measurable effect on the photosynthetic processes, but the combination of high light and low temperature caused rapid loss of photosynthetic capacity and a decrease in the efficiency of photosynthetic energy conversion. After a 15 h exposure to 5°C at high light, the F_v/sb/F_mratio had decreased by 80% and the apparent quantum yield by 75%. Nevertheless, when the leaves were returned to low light at 22°C, these parameters recovered rapidly. The foliar pools of ascorbate and glutathione decreased in the first hours of photoinhibitory treatment while the zeaxanthin content increased from negligible levels to about 50% of the total foliar xanthophyll pool. There was a clear correlation between the zeaxanthin content of the leaves and their F_v/F_m ratio during both photoinhibition and recovery. However, there was also a good correlation between the decrease in theF_v F_m ratio and the measured decrease in the total foliar levels of the antioxidants ascorbate and glutathione. The amount of D, protein diminished over the same period as the zeaxanthin levels were increasing. This approach, involving simultaneous measurements of several parameters considered to influence photosystemy II activity, clearly demonstrates that measured decreases in F_v/F_m may not simply be related to zeaxanthin levels or to amounts of D₁ protein alone but result from multifactoral influences.     

The ratio of variable to maximum chlorophyll fluorescence from photosystem II,measured in leaves at ambient temperature and at 77K,as an indicator of the photon yield of photosynthesis

The response of a number of species to high light levels was examined to determine whether chlorophyll fluorescence from photosystem (PS) II measured at ambient temperature could be used quantitatively to estimate the photon yield of O₂ evolution. In many species, the ratio of the yield of the variable (F_V) and the maximum chlorophyll fluorescence (F_M) determined from leaves at ambient temperature matched that from leaves frozen to 77K when reductions in F_V/F_M and the photon yield resulted from exposure of leaves to high light levels under favorable temperatures and water status. Under conditions which were less favorable for photosynthesis, F_V/F_M at ambient temperature often matched the photon yield more closely than F_V/F_M measured at 77K. Exposure of leaves to high light levels in combination with water stress or chilling stress resulted in much greater reductions in the photon yield than in F_V/F_M (at both ambient temperature and 77K) measured in darkness, which would be expected if the site of inhibition was beyond PSII. Following chilling stress, F_V/F_M determined during measurement of the photon yield in the light was depressed to a degree more similar to that of the depression of photon yield, presumably as a result of regulation of PSII in response to greatly reduced electron flow.Abbreviations and Symbols F_o yield of instantaneous fluorescence - F_M yield of maximum fluorescence - F_V yield of variable fluorescence - PFD photon flux density (400–700 nm) - PSI (II) photosystem I (II) This work was supported by the Deutsche Forschungsgemeinchaft. W.W.A. gratefully acknowledges the support of Fellowships from the North Atlantic Treaty Organization and the Alexander von Humboldt-Stiftung. We also thank Maria Lesch for plant maintenance.     

Photoinhibition of photosynthesis in intact kiwifruit (Actinidia deliciosa) leaves: Effect of temperature

Photoinhibition of photosynthesis was induced in attached leaves of kiwifruit grown in natural light not exceeding a photon flux density (PFD) of 300 mol·m^-2·s^-1, by exposing them to a PFD of 1500 mol·m^-2·s^-1. The temperature was held constant, between 5 and 35° C, during the exposure to high light. The kinetics of photoinhibition were measured by chlorophyll fluorescence at 77K and the photon yield of photosynthetic O₂ evolution. Photoinhibition occurred at all temperatures but was greatest at low temperatures. Photoinhibition followed pseudo first-order kinetics, as determined by the variable fluorescence (F _v) and photon yield, with the long-term steady-state of photoinhibition strongly dependent on temperature wheareas the observed rate constant was only weakly temperature-dependent. Temperature had little effect on the decrease in the maximum fluorescence (F _m) but the increase in the instantaneous fluorescence (F _o) was significantly affected by low temperatures in particular. These changes in fluorescence indicate that kiwifruit leaves have some capacity to dissipate excessive excitation energy by increasing the rate constant for non-radiative (thermal) energy dissipation although temperature apparently had little effect on this. Direct photoinhibitory damage to the photosystem II reaction centres was evident by the increases in F _o and extreme, irreversible damage occurred at the lower temperatures. This indicates that kiwifruit leaves were most susceptible to photoinhibition at low temperatures because direct damage to the reaction centres was greatest at these temperatures. The results also imply that mechanisms to dissipate excess energy were inadequate to afford any protection from photoinhibition over a wide temperature range in these shade-grown leaves.Abbreviations and symbols fluorescence yield correction coefficient - F _o, F _m, F _v instantaneous, maximum, variable fluorescence - K _D, K _F, K _P, K _T rate constants for non-radiative energy dissipation, fluorescence, photochemistry, energy transfer to photosystem I - PFD photon flux density - PSI, II photosystem I, II - _i photon yield of photosynthesis (incident light)     

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.     

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.     

The effects of low temperature acclimation and photoinhibitory treatments on Photosystem 2 studied by thermoluminescence and fluorescence decay kinetics

The effects of low temperature acclimation and photoinhibitory treatment on Photosystem 2 (PS 2) have been studied by thermoluminescence and chlorophyll fluorescence decay kinetics after a single turnover saturating flash. A comparison of unhardened and hardened leaves showed that, in the hardened case, a decrease in overall and B-band thermoluminescence emissions occurred, indicating the presence of fewer active PS 2 reaction centers. A modification in the form of the B-band emission was also observed and is attributed to a decrease in the apparent activation energy of recombination in the hardened leaves. The acclimated leaves also produced slower Q_A ^– reoxidation kinetics as judged from the chlorophyll fluorescence decay kinetics. This change was mainly seen in an increased lifetime of the slow reoxidation component with only a small increase in its amplitude. Similar changes in both thermoluminescence and fluorescence decay kinetics were observed when unhardened leaves were given a high light photoinhibitory treatment at 4°C, whereas the hardened leaves were affected to a much lesser extent by a similar treatment. These results suggest that the acclimated plants undergo photoinhibition at 4°C even at low light intensities and that a subsequent high light treatment produces only a small additive photoinhibitory effect. Furthermore, it can be seen that photoinhibition eventually gives rise to PS 2 reaction centers which are no longer functional and which do not produce thermoluminescence or variable chlorophyll fluorescence.Abbreviations D1 The 32 kDa protein of Photosystem 2 reaction center - F_m maximum chlorophyll fluorescence yield - F₀ minimal chlorophyll fluorescence yield obtained when all PS 2 centers are open - F_i intermediate fluorescence level corresponding to PS 2 centers which are loosely or not connected to plastoquinone (non-B centers) - F_v maximum variable chlorophyll fluorescence yield (F_v=F_m–F₀) - PS 2 Photosystem 2 - Q_A and Q_B respectively, primary and secondary quinonic acceptors of PS 2 - S1, S2 and S3 respectively, the one, two and three positively charged states of the oxygen evolving system - Z secondary donor of PS 2     

Predicting CO2 gain and photosynthetic light acclimation from fluorescence yield and quenching in cyano-lichens

Modulated chlorophylla fluorescence is useful for eco-physiological studies of lichens as it is sensitive, non-invasive and specific to the photobiont. We assessed the validity of using fluorescence yield to predict CO₂ gain in cyano-lichens, by simultaneous measurements of CO₂ gas exchange and chlorophylla fluorescence in five species withNostoc-photobionts. For comparison, O₂ evolution and fluorescence were measured in isolated cells ofNostoc, derived fromPeltigera canina (Nostoc PC). At irradiances up to the growth light level, predictions from fluorescence yield underestimated true photosynthesis, to various extents depending on species. This reflected the combined effect of a state transition in darkness, which was not fully relaxed until the growth light level was reached, and a phycobilin contribution to the minimum fluorescence yield (F_o). Above the growth light level, the model progressively overestimated assimilation, reflecting increased electron flow to oxygen under excess irradiance. In cyanobacteria, this flow maintains photosystem II centres open even up to photoinhibitory light levels without contributing to CO₂ fixation. Despite this we show that gross CO₂ gain may be predicted from fluorescence yield also in cyanolichens when the analysis is made near the acclimated growth light level. This level can be obtained even when measurements are performed in the field, since it coincides with a minimum in non-photochemical fluorescence quenching (NPQ). However, the absolute relation between fluorescence yield and gross CO₂ gain varies between species. It may therefore be necessary to standardise the fluorescence prediction for each species with CO₂ gas exchange.Abbreviations CCM CO₂-Concentrating mechanism - Chl chlorophyll - C_i inorganic carbon - 0 convexity (curvature of the light response curve) - ETR electron transport rate - F_o minimum fluorescence yield - F_m maximal fluorescence yield - F_s fluorescence yield at steady-state photosynthesis - F_v variable fluorescence yield - F_v/F_m dark ratio of variable to maximal fluorescence yield after dark adaptation - F_vF_mmax ratio of variable to maximal fluorescence yield in the absence of quenching - _CO2 maximum quantum yield of CO₂ assimilation - _PS quantum yield of photosystem II photochemistry - GP gross photosynthesis - I irradiance (mol quanta·m^–2·s^–1) - NPQ non photochemical fluorescence quenching - qp photochemical fluorescence quenching     

Seasonal changes in photosystem II organisation and pigment composition in Pinus sylvestris

Conifers of the boreal zone encounter considerable combined stress of low temperature and high light during winter, when photosynthetic consumption of excitation energy is blocked. In the evergreen Pinus sylvestris L. these stresses coincided with major seasonal changes in photosystem II (PSII) organisation and pigment composition. The earliest changes occurred in September, before any freezing stress, with initial losses of chlorophyll, the D1-protein of the PSII reaction centre and of PSII light-harvesting-complex (LHC II) proteins. In October there was a transient increase in F₀, resulting from detachment of the light-harvesting antennae as reaction centres lost D1. The D1-protein content eventually decreased to 90%, reaching a minimum by December, but PSII photochemical efficiency [variable fluorescence (F_v)/maximum fluorescence (F_m)] did not reach the winter minimum until mid-February. The carotenoid composition varied seasonally with a twofold increase in lutein and the carotenoids of the xanthophyll cycle during winter, while the epoxidation state of the xanthophylls decreased from 0.9 to 0.1 from October to January. The loss of chlorophyll was complete by October and during winter much of the remaining chlorophyll was reorganised in aggregates of specific polypeptide composition, which apparently efficiently quench excitation energy through non-radiative dissipation. The timing of the autumn and winter changes indicated that xanthophyll de-epoxidation correlates with winter quenching of chlorophyll fluorescence while the drop in photochemical efficiency relates more to loss of D1-protein. In April and May recovery of the photochemistry of PSII, protein synthesis, pigment rearrangements and zeaxanthin epoxidation occurred concomitantly. Indoor recovery of photosynthesis in winter-stressed branches under favourable conditions was completed within 3 d, with rapid increases in F₀, the epoxidation state of the xanthophylls and in light-harvesting polypeptides, followed by recovery of D1-protein content and F_v/F_m, all without net increase in chlorophyll. The fall and winter reorganisation allow Pinus sylvestris to maintain a large stock of chlorophyll in a quenched, photoprotected state, allowing rapid recovery of photosynthesis in spring.Abbreviations Elips early light-induced proteins - EPS epoxidation state - F₀ instantaneous fluorescence - F_m maximum fluorescence - F_v variable fluorescence - LHC II light-harvesting complex of PSII - LiDS lithium dodecyl sulfate This research was supported by the Swedish Natural Science Research Council. We wish to thank Dr. Adrian Clarke¹ (Department of Plant Physiology, University of Umeå, Sweden) for advice on electrophoresis, valuable discussion and providing antibodies. Dr. Stefan Jansson¹ and Dr. Torill Hundal (Department for Biochemistry, University of Stockholm, Sweden) provided antibodies. Jan Karlsson¹ helped with the HPLC, Dr. Marianna Krol gave advice on green gels and Dr. Vaughan Hurry (Cooperative Research Centre for Plant Sciences, Australian National University, Canberra, Australia) provided valuable discussion.     

PHOTOINHIBITION IN RED ALGAL SPECIES WITH DIFFERENT CAROTENOID PROFILES1

Members of the Rhodophyta present different carotenoid profiles. In a majority of the species, lutein constitutes >50% of the total carotenoid content, while in other species, it is replaced by zeaxanthin or antheraxanthin. Given that carotenoids have specific roles in photoprotection, different carotenoid profiles of red algae species could be related to their capacity to cope with photoinhibitory stress. Therefore, in the present work, the sensitivity to light stress of red algal species with different carotenoid profiles was investigated. Photoinhibition of photosynthesis induced by high‐light stress and the subsequent recovery in dim‐light conditions was measured using maximal PSII quantum efficiency (F_v/F_m). The degree of decrease and recovery of F_v/F_m and their respective kinetics were related to the carotenoid profile of the species. Although no relationship between sensitivity to high‐light stress and the carotenoid profile was observed, there were clear carotenoid profile‐related differences in the decrease and recovery kinetics. In species with zeaxanthin or antheraxanthin as the major carotenoid, F_v/F_m reduction and recovery was principally associated with slowly activated and relaxed processes. In contrast, in species with lutein as the major carotenoid, rapidly activated processes appear to play a major role in the down‐regulation of photosynthesis during light‐stress conditions. In these species, the repair of D1 is also important during light‐stress conditions. This finding could imply differential expression of mechanisms involved in photoprotection in red algae that seems to be related to the carotenoid profile of the species.