Light and the maintenance of photosynthetic competence in leaves of Populus balsamifera L. during short-term exposures to high concentrations of sulfur dioxide

Leaves of Populus balsamifera grown under full natural sunlight were treated with 0, 1, or 2 l SO₂·1^-1 air under one of four different photon flux densities (PFD). When the SO₂ exposures took place in darkness or at 300 mol photons·m^-2·s^-1, sulfate accumulated to the levels predicted by measurements of stomatal conductance during SO₂ exposure. Under conditions of higher PFD (750 and 1550 mol·m^-2·s^-1), however, the predicted levels of accumulated sulfate were substantially higher than those obtained from anion chromatography of the leaf extracts. Light-and CO₂-saturated capacity as well as the photon yield of photosynthetic O₂ evolution were reduced with increasing concentration of SO₂. At 2 l SO₂·1^-1 air, the greatest reductions in both photosynthetic, capacity and photon yield occurred when the leaves were exposed to SO₂ in the dark, and increasingly smaller reductions in each occurred with increasing PFD during SO₂ exposure. This indicates that the inhibition of photosynthesis resulting from SO₂ exposure was reduced when the exposure occurred under conditions of higher light. The ratio F _v/F _M (variable/maximum fluorescence emission) for photosyntem II (PSII), a measure of the photochemical efficiency of PSII, remained unaffected by exposure of leaves to SO₂ in the dark and exhibited only moderate reductions with increasing PFD during the exposure, indicating that PSII was not a primary site of damage by SO₂. Pretreatment of leaves with SO₂ in the dark, however, increased the susceptibility of PSII to photoinhibition, as such pretreated leaves exhibited much greater reductions inF _V/F _M when transferred to moderate or high light in air than comparable control leaves.Abbreviations and symbols A₁₂₀₀ photosynthetic capacity (CO₂-saturated rate of O₂ evolution at 1200 mol photons·m^-2·s^-1) - F_o instantaneous fluorescence emission - F_M maximum fluorescence emission - F_V variable fluorescence emission - PFD photon flux density (400–700 nm) - PSII photosystem II     

Photoinhibition of photosynthesis under natural conditions in ivy (Hedera helix L.) growing in an understory of deciduous trees

We investigated to what extent south-exposed leaves (E-leaves) of the evergreen ivy (Hedera helix L.) growing in the shadow of two deciduous trees suffered from photoinhibition of photosynthesis when leaf-shedding started in autumn. Since air temperatures drop concomitantly with increase in light levels, changes in photosynthetic parameters (apparent quantum yield, _i and maximal photosynthetic capacity of O₂ evolution, P_max; chlorophyll-a fluorescence at room temperature) as well as pigment composition were compared with those in north-exposed leaves of the same clone (N-leaves; photosynthetic photon flux density PPFD< 100 mol · m^–2 · s^–2) and phenotypic sun leaves (S-leaves; PPFD up to 2000 mol · m^–2 · s^–1).In leaves exposed to drastic light changes during winter (E-leaves) strong photoinhibition of photosynthesis could be observed as soon as the incident PPFD increased in autumn. In contrast, in N-leaves the ratio of variable fluorescence to maximum fluorescence (F_V/F_Mm) and _i did not decline appreciably prior to severe frosts (up to -12° C) in January. At this time, _i was reduced to a similar extent in all leaves, from about 0.073 mol O₂ · mol^–1 photons before stress to about 0.020. Changes in _i were linearly correlated with changes in f_v/f_m (r = 0.955). The strong reduction in F_V/F_M on exposure to stress was caused by quenching in F_M. The initial fluorescence (F₀), however, was also quenched in all leaves. The diminished fluorescence yield was accompanied by an increase in zeaxanthin content. These effects indicate that winter stress in ivy primarily induces an increase in non-radiative energy-dissipation followed by photoinhibitory damage of PSII. Although a pronounced photooxidative bleaching of chloroplast pigments occurred in January (especially in E-leaves), photosynthetic parameters recovered completely in spring. Thus, the reduction in potential photosynthetic yield in winter may be up to three times greater in leaves subjected to increasing light levels than in leaves not exposed to a changing light environment.Abbreviations and Symbols F₀, F_M initial and maximal fluorescence yield when all PSII centres are open and closed - F_V variable fluorescence (F_M-F₀) - P_max maximal photosynthetic capacity at 1000 umol · m^–2 · s^–1 PPFD and CO₂ saturation - PPFD photosynthetic photon flux density - _i apparent quantum yield of photosynthetic O₂ evolution - E-leaves, N-leaves shade leaves exposed, not exposed to drastic light changes during winter - S-leaves sun leaves from an open ivy stand Dedicated to Professor Otto Härtel on the occasion of his 80th birthdayThis work was supported by the Austrian Fonds zur Förderung der wissenschaftlichen Forschung.     

Chlorophyll fluorescence in the resurrection plant Selaginella lepidophylla (Hook. & Grev.) Spring during high-light and desiccation stress,and evidence for zeaxanthin-associated photoprotection

The function of photosystem (PS)II during desiccation and exposure to high photon flux density (PFD) was investigated via analysis of chlorophyll fluorescence in the desert resurrection plant Selaginella lepidophylla (Hook. and Grev.) Spring. Exposure of hydrated, physiologically competent stems to 2000 mol · m^–2 · s^–1 PFD caused significant reductions in both intrinsic fluorescence yield (F_O) and photochemical efficiency of PSII (F_V/F_M) but recovery to pre-exposure values was rapid under low PFD. Desiccation under low PFD also affected fluorescence characteristics. Both F_V/F_M and photochemical fluorescence quenching remained high until about 40% relative water content and both then decreased rapidly as plants approached 0% relative water content. In contrast, the maximum fluorescence yield (F_M) decreased and non-photochemical fluorescence quenching increased early during desiccation. In plants dried at high PFD, the decrease in F_V/F_M was accentuated and F_O was reduced, however, fluorescence characteristics returned to near pre-exposure values after 24-h of rehydration and recovery at low PFD. Pretreatment of stems with dithiothreitol, an inhibitor of zeaxanthin synthesis, accelerated the decline in F_V/F_M and significantly increased F_O relative to controls at 925 mol · m^–2 · s^–1 PFD, and the differences persisted over a 3-h low-PFD recovery period. Pretreatment with dithiothreitol also significantly decreased non-photochemical fluorescence quenching, increased the reduction state of Q_A, the primary electron acceptor of PSII, and prevented the synthesis of zeaxanthin relative to controls when stems were exposed to PFDs in excess of 250 mol · m^–2 · s^–1. These results indicate that a zeaxanthin-associated mechanism of photoprotection exists in this desert pteridophyte that may help to prevent photoinhibitory damage in the fully hydrated state and which may play an additional role in protecting PSII as thylakoid membranes undergo water loss.Abbreviations and Symbols DTT dithiothreitol - EPS epoxidation state - F_O yield of instantaneous fluorescence at open PSII centers - F_M maximum yield of fluorescence at closed PSII centers induced by saturating light - F_M F_M determined during actinic illumination - F_V yield of variable fluorescence (F_M-F_O) - F_V/F_M photochemical efficiency of PSII - q_P photochemical fluorescence quenching - q_NP non-photochemical fluorescence quenching of Schreiber et al. (1986) - NPQ non-photochemical fluorescence quenching from the Stern-Volmer equation - PFD photon flux density - RWC relative water content This paper is based on research done while W.G.E. was on leave of absence at Duke University during the fall of 1990. We would like to thank Dan Yakir, John Skillman, Steve Grace, and Suchandra Balachandran and many others at Duke University for their help and input with this research. Dr. Barbara Demmig-Adams provided zeaxanthin for standard-curve purposes.     

Oxygen dependence of photoinhibition at low temperature in intact protoplasts of Valerianella locusta L.

Photoinhibition of photosynthesis in vivo is shown to be considerably promoted by O₂ under circumstances where energy turnover by photorespiration and photosynthetic carbon metabolism are low. Intact protoplasts of Valerianella locusta L. were photoinhibited by 30 min irradiation with 3000 mol photons · m^–2 · s^–1 at 4° C in saturating [CO₂] at different oxygen concentrations, corresponding to 2–40% O₂ in air. The photoinhibition of light-limited CO₂-dependent photosynthetic O₂ evolution increased with increasing oxygen concentration. The uncoupled photochemical activity of photosystem II, measured in the presence of the electron acceptor 1,4-benzoquinone, and maximum variable fluorescence, F_v, were strongly affected and this inhibition was closely correlated to the O₂ concentration. The effect of O₂ did not saturate at the highest concentrations applied. An increase in photoinhibitory fluorescence quenching with [O₂], although less pronounced than in protoplasts, was also observed with intact leaves irradiated at 4° C in air. Initial fluorescence, F_o, was slightly (about 10%) increased by the inhibitory treatments but not influenced by [O₂]. A long-term cold acclimation of the plants did not substantially alter the O₂-sensitivity of the protoplasts under the high-light treatment. From these experiments we conclude that oxygen is involved in the photoinactivation of photosystem II by excess light in vivo.Abbreviations and Symbols Chl chlorophyll - F_o initial fluorescence - F_M maximum fluorescence - F_v maximum variable fluorescence - PCO photorespiratory carbon oxidation - PCR photosynthetic carbon reduction - PFD photon flux density - q_N non-photochemical quenching - q_P photochemical quenching - _S quantum efficiency of electron transport of photosystem II This study was financially supported by the Deutsche Forschungs-gemeinschaft (SFB 189) and the Foundation for Fundamental Biological Research (BION), which is subsidised by the Netherlands Organization for the Advancement of Pure Research (NWO).     

Effect of long-term photoinhibition on growth and photosynthesis of cold-hardened spring and winter wheat

The effect of repeated exposure to high light (1200 mol · m^–2 · s^–1 photosynthetic photon flux density, PPFD) at 5° C was examined in attached leaves of cold-grown spring (cv. Katepwa) and winter (cv. Kharkov) wheat (Triticum aestivum L.) over an eight-week period. Under these conditions, Kharkov winter wheat exhibited a daily reduction of 24% in F_V/F_M (the ratio of variable to maximal fluorescence in the dark-adapted state), in contrast to 41% for cold-grown Katepwa spring wheat. Both cultivars were able to recover from this daily suppression of F_V/F_M such that the leaves exhibited an average morning F_V/F_M of 0.651 ± 0.004. Fluorescence measurements made under steady-state conditions as a function of irradiance from 60 to 2000 mol · m^–2 · s^–1 indicated that the yield of photosystem II (PSII) electron transport under light-saturating conditions was the same for photoinhibited and control cold-grown plants, regardless of cultivar. Repeated daily exposure to high light at low temperature did not increase resistance to short-term photoinhibition, although zeaxanthin levels increased by three- to fourfold. In addition, both cultivars increased the rate of dry-matter accumulation, relative to control plants maintained at 5° C and 250 mol · m^–2 · s^–1 PPFD (10% and 28% for Katepwa and Kharkov, respectively), despite exhibiting suppressed f_v/f_m and reduced photon yields for O₂ evolution following daily high-light treatments. Thus, although photosynthetic efficiency is suppressed by a longterm, photoinhibitory treatment, light-saturated rates of photosynthesis are sufficiently high during the high-light treatment to offset any reduction in photochemical efficiency of PSII. We suggest that in these cold-tolerant plants, photoinhibition of PSII may represent a longterm, stable, down-regulation of photochemistry to match the overall photosynthetic demand for ATP and reducing equivalents.Abbreviations and Symbols Chl chlorophyll - HL high light - PPFD photosynthetic photon flux density - F_O minimum fluorescence in the dark-adapted state - F_M maximum fluorescence in the dark-adapted state - F_V maximum variable fluorescence in the dark-adapted state (F_M-F_O) - F_V/F_V photosynthetic efficiency of the dark-adapted state - f_V/f_M photosynthetic efficiency of the light-adapted steady state - q_P photochemical quenching parameter - q_N non-photochemical quenching parameter - _e yield of electron transport and equals q_P · f_V/f_M - 1-q_O F_O quenching parameter - _app apparent photon yield. The assistance of Amy So is gratefully acknowledged. This research 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 the Swedish Natural Sciences Research Council.     

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     

Persistent effects of low concentrations of SO2 and NO2 on photosynthesis in Scots pine (Pinus sylvestris) needles

In an open-field experiment, 50-year-old trees of Scots pine (Pinus sylvestris L.) were fumigated with low concentrations of SO₂ and NO₂ (10–15 nl I^?1) during the growing season in four consecutive years (1988 to 1991). Results from the autumn and early winter of 1991 and 1992 are presented. The maximum photochemical efficiency of photosystem II (PSII), as indicated by the ratio of variable to maximum fluorescence (F_v/F_M) was assessed in current and one-year-old needles from the top and the bottom of the canopy. Furthermore, simultaneous measurements of photosynthetic O₂ evolution and chlorophyll fluorescence were made in current-year needles at 20°C. In general, the F_v/F_M ratio as well as the gross rate of O₂ evolution in needles of fumigated trees was not significantly different from that in needles of control trees during the fumigation period. However, both current and one-year-old needles sampled in November and December 1991 from the top of the canopy of fumigated trees had significantly lower F_v/F_M values than corresponding needles of control trees. Similar differences in F_v/F_M correlated with the treatments were observed in needles from the bottom of the canopy, indicating that the depression of F_v/F_M in needles of fumigated trees was not due to an increased susceptibility to photoinhibition. In 1992, when no fumigation occurred, differences in F_v/F_M between the treatments were not significant during autumn and early winter. The gross rate of O₂ evolution at high irradiances was significantly lower in current-year needles of fumigated trees sampled in November and December 1991 than in those of control trees. Furthermore, a nearly identical linear relationship between the quantum yield of PSII electron transport determined from chlorophyll fluorescence and the quantum yield of O₂ evolution (gross rate of O₂ evolution/PPFD) was found during autumn and early winter. This appeared to be largely a result of changes in the thermal energy dissipation within PSII. The observed differences in photosynthetic characteristics correlated with the different treatments after the fumigation period is suggested to be mainly caused by increased sensitivity of the needles of fumigated trees to low and subfreezing temperatures. However, current-year needles of fumigated trees tended to have a lower N content than those of control trees, which may partly explain the differences in gross photosynthesis between fumigated and control trees.     

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     

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)     

A portable multi-flash kinetic fluorimeter for measurement of donor and acceptor reactions of Photosystem 2 in leaves of intact plants under field conditions

A newly-developed field-portable multi-flash kinetic fluorimeter for measuring the kinetics of the microsecond to millisecond reactions of the oxidizing and reducing sides of photosystem 2 in leaves of intact plants is described and demonstrated. The instrumental technique is a refinement of that employed in the double-flash kinetic fluorimeter (Joliot 1974 Biochim Biophys Acta 357: 439–448) where a low-intensity short-duration light pulse is used to measure the fluorescence yield changes following saturating single-turnover light pulses. The present instrument uses a rapid series of short-duration (2 s) pulses to resolve a complete microsecond to millisecond time-scale kinetic trace of fluorescence yield changes after each actinic flash. Differential optics, using a matrix of optical fibers, allow very high sensitivity (noise levels about 0.05% F_max) thus eliminating the need for signal averaging, and greatly reducing the intensity of light required to make a measurement. Consequently, the measuring pulses have much less actinic effect and an entire multi-point trace (seven points) excites less than 1% of the reaction centers in a leaf. In addition, bu combining the actinic and measuring pulse light in the optical fiber network, the tail of the actinic flash can be compensated for, allowing measurements of events as rapidly as 20 s after the actinic flash. This resolution makes practical the routine measurement of the microsecond turnover kinetics of the oxygen evolving complex in leaves of intact plants in the field. The instrument is demonstrated by observing flash number dependency and inhibitor sensitivity of the induction and decay kinetics of flash-induced fluorescence transients in leaves of intact plants. From these traces the period-two oscillations associated with the turnover of the two-electron gate and the period-four oscillations associated with the turnover of the oxygen evolving complex can be observed. Applications of the instrument to extending our knowledge of chloroplast function to the whole plant, the effects on plants of environmental stress, herbicides, etc, and possible applications to screening of mutants are discussed.Abbreviations DCMU 3-(3,4-Dichlorophenol)-1,1-dimethylurea - PS 2 photosystem 2 - PS 1 photosystem 1 - P₆₈₀ primary electron donor of the PS 2 reaction center - Q_A primary acceptor quinone of PS 2 - Q_B secondary acceptor quinone of PS 2 - CCCP carbonyl cyanide-m-chlorophenylhydrazone - Y_z donor to P₆₈₀ ⁺ - F₀ level of fluorescence with all PS 2 centers open - F_max maximum level of fluorescence with all PS 2 centers closed - P₆₈₀Q_A Open reaction centers with P₆₈₀ reduced and Q_A oxidized (low fluorescence) - P₆₈₀Q_A ^- Closed reaction centers, in which P₆₈₀ is reduced (high fluorescence) - P₆₈₀ ⁺Q_A ^- Closed reaction centers, in which P₆₈₀ is oxidized (low fluorescence)     

Changes in Photosystem II fluorescence in Chlamydomonas reinhardtii exposed to increasing levels of irradiance in relationship to the photosynthetic response to light

The effects of a 60 min exposure to photosynthetic photon flux densities ranging from 300 to 2200 mol m^–2s^–1 on the photosynthetic light response curve and on PS II heterogeneity as reflected in chlorophyll a fluorescence were investigated using the unicellular green alga Chlamydomonas reinhardtii. It was established that exposure to high light acts at three different regulatory or inhibitory levels; 1) regulation occurs from 300 to 780 mol m^–2s^–1 where total amount of PS II centers and the shape of the light response curve is not significantly changed, 2) a first photoinhibitory range above 780 up to 1600 mol m^–2s^–1 where a progressive inhibition of the quantum yield and the rate of bending (convexity) of the light response curve can be related to the loss of Q_B-reducing centers and 3) a second photoinhibitory range above 1600 mol m^–2s^–1 where the rate of light saturated photosynthesis also decreases and convexity reaches zero. This was related to a particularly large decrease in PS II centers and a large increase in spill-over in energy to PS I.Abbreviations Chl chlorophyll - DCMU 3,(3,4-dichlorophenyl)-1,1-dimethylurea - F_M maximal fluorescence yield - F_pl intermediate fluorescence yield plateau level - F₀ non-variable fluorescence yield - F_v total variable fluorescence yield (F_M-F₀) - initial slope to the light response curve, used as an estimate of initial quantum yield - convexity (rate of bending) of the light response curve of photosynthesis - LHC light-harvesting complex - P_max maximum rate of photosynthesis - PQ plastoquinone - Q photosynthetically active photon flux density (400–700 nm, mol m^–2s^–1) - PS photosystem - Q_A and Q_B primary and secondary quinone electron acceptor of PS II     

Stress-induced chlorosis and increase in D1-protein turnover precede photoinhibition in spinach suffering under magnesium/sulphur deficiency

To test wether chlorosis is induced by photoinhibitory damage to photosystem II (PSII), onset of chlorosis and loss of PSII function were compared in young spinach (Spinaciae oleracea L.) plants suffering under a combined magnesium and sulphur deficiency. Loss of chlorophyll already occurred after the first week of deficiency and preceded any permanent functional inhibition of the photosynthetic apparatus. Permanent disturbancies of photosynthetic electron transport measured in isolated thylakoids and of PSII function, determined via the ratio of variable fluorescence to maximal fluorescence, F_v/F_m, could be detected only after the second week of deficiency. After the third week, the plants had lost about 60% of their chlorophyll; even so, fluorescence data indicated that 85% of the existing PSII was still capable of initiating photosynthetic electron transport. However, quenching analysis of steady-state fluorescence showed an early increase in non-photochemical quenching and in down-regulated PSII centres with low steady-state quantum efficiency. Together with the down-regulation of PSII centres, a 1.4-fold increase in D1-protein synthesis, measured as incorporation of [¹⁴C]leucine, could be observed at the end of the first week before any loss of D1 protein, chlorophyll or photosynthetic activity could be detected. Immunological determiation by Western-blotting did not show a change in D1-protein content; thus, at this time, D1 protein was not only faster synthesised but was also faster degraded than before the imposition of mineral deficiency. The increased turnover was high enough to prevent any loss or functional inhibition of PSII. After 3 weeks, D1-protein synthesis on a chlorophyll basis was further stimulated by a factor of 2. However, this was not enough to prevent a net loss of D1 protein of about 70%, showing that the D1-protein was now degraded faster than it was synthesised. Immunological determination and electron-transport measurements showed that together with the loss of D1 protein the other polypetides of PSII were also degraded, resulting in a specific loss of PSII centres. The degradation of PSII centres prevented a large accumulation of damaged PSII centres. We assume that the decrease in PSII centres initiates the breakdown of the other thylakoid proteins.Abbreviations Fo yield of intrinsic fluorescence when all PSII centres are open in the dark - F_m yield of maximal fluorescence when all reaction centres are closed - F_m fluorescence yield when all reaction centres are closed under steady-state conditions - F_v yield of variable fluorescence, (difference between F_o and F_m) - F yield of variable fluorescence under steady-state conditions, difference between F_m and F_t, the fluorescence yield under steady-state conditions - PFD photon flux density - Q_A primary quinone acceptor of PSII - Q_B secondary quinone acceptor of PSII - q_p photochemical quenching - q_n non-photochemical quenching This work was supported by grants from the Bundesminister für Forschung und Technologie and the German Israeli Foundation. The authors thank Prof. I. Ohad (Department of Biological Chemistry, Hebrew University, Jerusalem, Israel) for fruitful discussions.     

Effects of extreme high temperature,drought and elevated CO2 on photosynthesis of the Mojave Desert evergreen shrub,Larrea tridentata

The interaction of extreme temperature events with future atmospheric CO₂ concentrations may have strong impacts on physiological performance of desert shrub seedlings, which during the critical establishment phase often endure temperature extremes in conjunction with pronounced drought. To evaluate the interaction of drought and CO₂ on photosynthesis during heat stress, one-year-old Larrea tridentata[DC] Cov. seedlings were exposed to nine days of heat with midday air temperature maxima reaching 53 °C under three atmospheric CO₂ concentrations (360, 550 and 700 mol mol^–1) and two water regimes (well-watered and droughted). Photosynthetic gas exchange, chlorophyll fluorescence and water potential responses were measured prior to, during and one week following the high temperature stress event. Heat stress markedly decreased net photosynthetic rate (A _net), stomatal conductance (g _s), and the photochemical efficiency of photosystem II (F _v/F _m) in all plants except for well-watered L. tridentata grown in 700 mol mol^–1 CO₂. A _net and g _s remained similar to pre-stress levels in these plants. In droughted L. tridentata, A _net was ca. 2× (in 550 mol mol^–1 CO₂) to 3× (in 700 mol mol^–1 CO₂) higher than in ambient-CO₂-grown plants, while g _s and F _v/F _m were similar and low in all CO₂ treatments. Following heat stress, g _s in all well-watered plants rose dramatically, exceeding pre-stress levels by up to 100%. In droughted plants, g _s and A _net rose only in plants grown at elevated CO₂ following release from heat. This recovery response was strongest at 700 mol mol^–1 CO₂, which returned to A _net and g _s values similar to pre-heat following several days of recovery. Extreme heat diminished the photosynthetic down-regulation response to growth at elevated CO₂ under well-watered conditions, similar to the action of drought. Ambient-CO₂-grown L. tridentata did not show significant recovery of photosynthetic capacity (A _\max and CE) after alleviation of temperature stress, especially when exposed to drought, while plants exposed to elevated CO₂ appeared to be unaffected. These findings suggest that elevated CO₂ could promote photosynthetic activity during critical periods of seedling establishment, and enhance the potential for L. tridentata to survive extreme high temperature events.     

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     

Reduced sensitivity to photoinhibition following frost-hardening of winter rye is due to increased phosphate availability

The possibility of a role for phosphate metabolism in the photosynthetic regulation that occurs during frost hardening was investigated in winter rye (Secale cereale L. cv. Musketeer). Leaves of frost-hardened and non-hardened winter rye were studied during photosynthetic induction, and at steady state after being allowed to take up 20 mM orthophosphate through the transpiration stream for 3 h. At the growth irradiance (350 mol·m^-2·s^-1) frost-hardening increased the stationary rate of CO₂-dependent O₂ evolution by 57% and 25% when measured at 5 and 20° C, respectively. Frosthardening also reduced the lag phase to stationary photosynthesis by 40% at 5° C and decreased the susceptibility of leaves to oscillations during induction and after interruption of the actinic beam during steady-state photosynthesis. These responses are all indicative of increased phosphate availability in frost-hardened leaves. As reported previously by Öquist and Huner (1993, Planta 189, 150–156), frost-hardening also decreased the reduction state of Q_A, the primary, stable quinone acceptor of PSII, and decreased the sensitivity of winter rye to photoinhibition of photosynthesis. Non-hardened rye leaves fed orthophosphate also showed an increased photosynthetic capacity (25% at 20° C and light saturation), lower reduction state of Q_A, a reduced sensitivity to photoinhibition and lower susceptibility to oscillations resulting from a brief interruption of the actinic light. Thus, the data indicate that phosphate metabolism plays a key role in photosynthetic acclimation of winter rye to low temperatures.Abbreviations F_o and F_o minimal fluorescence when all PSII reaction centres are open in dark-and light-acclimated leaves, respectively - F_m and F_m maximal fluorescence when all PSII reaction centres are closed in dark-and light-acclimated leaves, respectively - F_v variable fluoresence (F_m -F_o) in dark-acclimated leaves - F_v variable fluorescence (F_m-F_o) in light-acclimated leaves - PCR photosynthetic carbon reduction - PPFD photosynthetic photon flux density - Q_A the primary, stable quinone acceptor of PSII - q_P photochemical quenching of fluorescence - q_N non-photochemical quenching of fluorescence This work was supported by the Swedish Natural Sciences Research Council. The authors are indebted to Dr. N. Huner, Department of Plant Sciences, UWO, London, Canada, for helpful discussions during the initiation of this work and for the gift of rye seeds.     

An in vivo analysis of photosynthesis during short-term O3 exposure in three contrasting species

The depressions of photosynthetic CO₂ uptake following O₃ exposures of 200 and 400 nmol mol^-1 for between 4 and 16 h were compared between Pisum sativum, Quercus robur and Triticum aestivum, and the potential causes of change identified in vivo. Photosynthetic change was examined by analysis of CO₂, O₂, O₃ and water vapour exchanges together with chlorophyll fluorescence in controlled environments. Under identical fumigation conditions, each species showed very similar rates of O₃ consumption. The light-saturated rate of CO₂ uptake showed a statistically significant decrease in each species with increasing O₃ dose. Although stomatal conductance declined in parallel with CO₂ uptake this did not account for the observed decrease in photosynthesis. The decrease in mesophyll conductance resulted primarily from a decrease in the apparent carboxylation capacity, implying in decreased activity of ribulose 1,5-bisphosphate carboxylase/oxygenase. The maximum capacity of carboxylation was consequently reduced by over 30% and 50% after 16 h fumigation with 200 and 400 nmol mol^-1 O₃ respectively. Additionally, in Q. robur, a statistically significant inhibition of the CO₂ saturated rate of photosynthesis occurred after 16 h with 400 nmol mol^-1 O₃, suggesting that the ability to regenerate ribulose 1,5-bisphosphate was also impaired. None of the species showed any significant decrease in the efficiency of light-limited photosynthesis following fumigation at 200 nmol mol^-1 O₃, but effects were apparent at 400 nmol mol^-1 O₃. The common feature in all three species was a decline in carboxylation capacity which preceded any other change in the photosynthetic apparatus.Abbreviations A_sat net CO₂ uptake rate per unit leaf area at light saturation - A net CO₂ uptake rate per unit leaf area - A_max net CO₂ uptake rate per unit leaf area at CO₂ and light saturation - c_i mole fraction of CO₂ in the intercellular air space - g_s stomatal conductance to CO₂ - F_m maximum chlorophyll fluorescence - F_v variable chlorophyll fluorescence - _c quantum yield of CO₂ uptake for absorbed light - ₀ quantum yield of oxygen evolution for incident light - PPFD photosynthetically active radiation - Rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose 1,5-bisphosphate - Vc_max maximum rate of carboxylation     

Photosynthetic Utilization of Radiant Energy by Temperate Lettuce Grown Under Natural Tropical Condition with Manipulation of Root-Zone Temperature

Photosynthetic utilization of radiant energy was studied by chlorophyll (Chl) fluorescence and maximum photosynthetic O₂ evolution (P _max) in temperate lettuce (Lactuca sativa L.) grown under natural tropical fluctuating ambient temperatures but with their roots exposed to two different root-zone temperatures (RZTs): a constant 20 °C-RZT (RZT₂₀) and a fluctuating ambient RZT (RZT_a) from 23 to 40 °C. On a sunny day, irrespective of RZT, F/_Fm [ratio of the variable to maximal fluorescence under irradiation (the maximal photosystem 2 quantum yield with actinic light)] decreased and non-photochemical quenching (NPQ) increased parallel to the increase of photosynthetic photon flux density (PPFD). However, RZT_a plants showed lower F/F_m and higher NPQ than RZT₂₀ plants. The electron transport rate (ETR) was much higher in RZT₂₀ plants than in RZT_a plants especially during moderately sunny days. There were no significant diurnal changes in P _max although these values of RZT₂₀ plants were much higher than those of RZT_a plants. On cloudy days, no significant diurnal changes in F/F_m and NPQ occurred, but F/F_m was higher and NPQ was lower in RZT₂₀ plants than in RZT_a plants. Diurnal changes in ETR were also observed in all plants while P _max values throughout the whole cloudy days in both RZT₂₀ and RZT_a plants were constant. Again, RZT₂₀ plants had much higher values of P _max than RZT_a plants. During RZT transfer period, all Chl fluorescence parameters measured at midday fluctuated with PPFD. Impact of RZT on these parameters was observed 2–3 d after RZT transfer. ETR and P _max measured with saturating PPFD in the laboratory did not vary with the fluctuating PPFD in the greenhouse but the effects of RZT on these two parameters were observed 3–4 d after RZT transfer. Thus RZT affects photosynthetic utilization of photon energy in temperate lettuce grown under natural tropical condition.     

Carotenoid composition and photon-use efficiency of photosynthesis inGossypium hirsutum L. grown under conditions of slightly suboptimum leaf temperatures and high levels of irradiance

Summary Cotton (Gossypium hirsutum L. var. DP 61) was grown at different temperatures during 12-h light periods, with either 1800–2000 mol photons m^–2 s^–1 (high photon flux density, PFD) or 1000–1100 mol m^–2 s^–1 (medium PFD) incident on the plants. Night temperature was 25°C in all experiments. Growth was less when leaf temperatures were below 30°C during illumination, the effect being greater in plants grown with high PFD (Winter and Königer 1991). Leaf pigment composition and the photon-use efficiency of photosynthesis were analysed to assess whether plants grown with high PFD and suboptimal temperatures experienced a higher degree of high irradiance stress during development than those grown with medium PFD. The chlorophyll content per unit area was 3–4 times less, and the content of total carotenoids about 2 times less, with the proportion of the three xanthophylls zeaxanthin + antheraxanthin + violaxanthin being greater in leaves grown at 20–21°C than in leaves grown at 33–34°C. In leaves from plants grown at 21°C and 1800–2000 mol photons m^–2 s^–1, zeaxanthin accounted for as much as 34% of total carotenoids in the middle of the photoperiod, the highest level recorded in this study. This finding is consistent with a protective role of zeaxanthin under conditions of excess light. At the lower temperatures, the photochemical efficiency of photosystem II, measured as the ratio of variable to maximum fluorescence yield (F _V/F _M) after 12-h dark adaptation, was 0.76 in medium PFD plants and 0.75 in high PFD plants compared with 0.83 and 0.79, respectively, at the higher temperatures. The photon-use efficiency of O₂ evolution () based on absorbed light between 630 and 700nm, decreased with decrease in temperature from 0.102 to 0.07 under conditions of high PFD, but remained above 0.1 at medium PFD. Owing to compensatory reactions in these long-term growth experiments, sustained differences inF _V/F _M and were much less pronounced than the differences in chlorophyll content and dry matter, particularly in plants which had developed at high PFD and low temperature. In fact, in these plants, which exhibited pronounced photobleaching, a largely functional photosynthetic apparatus was still maintained in cells adjacent to the lower leaf surfaces. This was indicated by measurements of photon use efficiencies of photosynthetic O₂ evolution with leaves illuminated first at the upper, and then at the lower surface.Abbreviations F _O yield of dark level fluorescence - F _M maximum yield of fluorescence, induced in a pulse of saturating light - F _V yield of variable fluorescence (=F _M-F _o) - PFD photon flux density - _iw photon use efficiency of O₂ evolution based on white (400–700 nm) incident light - _ir photon use efficiency based on red (630–700 nm) incident light - _aw photon use efficiency based on white absorbed light - _ar photon use efficiency based on red absorbed light     

Photoinhibition of photosynthesis in intact kiwifruit (Actinidia deliciosa) leaves: Effect of light during growth on photoinhibition and recovery

Photoinhibition of photosynthesis was induced in intact kiwifruit (Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson) leaves grown at two photon flux densities (PFDs) of 700 and 1300 mol·m^-2·s^-1 in a controlled environment, by exposing the leaves to PFD between 1000 and 2000 mol·m^-2·s^-1 at temperatures between 10 and 25°C; recovery from photoinhibition was followed at the same range of temperatures and at a PFD between 0 and 500 mol·m^-2·s^-1. In either case the time-courses of photoinhibition and recovery were followed by measuring chlorophyll fluorescence at 692 nm and 77K and by measuring the photon yield of photosynthetic O₂ evolution. The initial rate of photoinhibition was lower in the high-light-grown plants but the long-term extent of photoinhibition was not different from that in low-light-grown plants. The rate constants for recovery after photoinhibition for the plants grown at 700 and 1300 mol·m^-2·s^-1 or for those grown in shade were similar, indicating that differences between sun and shade leaves in their susceptibility to photoinhibition could not be accounted for by differences in capacity for recovery during photoinhibition. Recovery following photoinhibition was increasingly suppressed by an increasing PFD above 20 mol·m^-2·s^-1, indicating that recovery in photoinhibitory conditions would, in any case, be very slow. Differences in photosynthetic capacity and in the capacity for dissipation of non-radiative energy seemed more likely to contribute to differences in susceptibility to photoinhibition between sun and shade leaves of kiwifruit.Abbreviations and symbols F _o , F _m , F _v instantaneous, maximum, variable fluorescence - F _v /F _m fluorescence ratio - F _i =F _v at t=0 - F F _v at t= - K _D rate constant for photochemistry - k(F _p ) first-order rate constant for photoinhibition - k(F _r ) first-order rate constant for recovery - PFD photon flux density - PSII photosystem II - _i photon yield of O₂ evolution (incident light)     

Effect of drought stress on chlorophyll a fluorescence and electrical admittance of shoots in Norway spruce seedlings

Effects of mild and severe soil drought on the water status of needles, chlorophyll a fluorescence, shoot electrical admittance, and concentrations of photosynthetic pigments in needles of seedlings of Picea abies (L.) Karst. were examined under controlled greenhouse conditions. Drought stress reduced shoot admittance linearly with a decrease in shoot water potential (_w) and increase in water deficit (WD) and led to a decrease in concentrations of chlorophyll a, b and carotenoids. Severe water stress (shoot _w=–2.4 MPa) had a negative effect on chlorophyll a fluorescence parameters including PSII activity (F_v/F_m), and the vitality index (R_fd). Variations in these parameters suggest an inhibition of the photosynthetic electron transport in spruce needles. Water stress led to a decrease in the mobility of electrolytes in tissues, which was reflected by decreased shoot electrical admittance. After re-watering for 21 days the WD in needles decreased and the shoot water potential increased. In the re-watered plants, the chloroplast function was restored and chlorophyll a fluorescence returned to a similar level as in the control plants. This improved hydraulic adjustment in the seedlings triggered a positive effect on ion flow in the tissues and increased shoot electrical admittance. We conclude that the shoot electrical admittance and photosynthetic electron transport in leaves are closely linked to changes in water status and their decrease is among the initial responses of seedlings to water stress.