An Excel tool for deriving key photosynthetic parameters from combined gas exchange and chlorophyll fluorescence: theory and practice

Combined photosynthetic gas exchange and modulated fluorometres are widely used to evaluate physiological characteristics associated with phenotypic and genotypic variation, whether in response to genetic manipulation or resource limitation in natural vegetation or crops. After describing relatively simple experimental procedures, we present the theoretical background to the derivation of photosynthetic parameters, and provide a freely available Excel‐based fitting tool (EFT) that will be of use to specialists and non‐specialists alike. We use data acquired in concurrent variable fluorescence–gas exchange experiments, where A/C_i and light–response curves have been measured under ambient and low oxygen. From these data, the EFT derives light respiration, initial PSII (photosystem II) photochemical yield, initial quantum yield for CO₂ fixation, fraction of incident light harvested by PSII, initial quantum yield for electron transport, electron transport rate, rate of photorespiration, stomatal limitation, Rubisco (ribulose 1·5‐bisphosphate carboxylase/oxygenase) rate of carboxylation and oxygenation, Rubisco specificity factor, mesophyll conductance to CO₂ diffusion, light and CO₂ compensation point, Rubisco apparent Michaelis–Menten constant, and Rubisco CO₂‐saturated carboxylation rate. As an example, a complete analysis of gas exchange data on tobacco plants is provided. We also discuss potential measurement problems and pitfalls, and suggest how such empirical data could subsequently be used to parameterize predictive photosynthetic models.     

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.     

Leaf gas exchange and chlorophyll a fluorescence in soybean leaves infected by Phakopsora pachyrhizi

Asian soybean rust (ASR), caused by Phakopsora pachyrhizi, is one of the most important foliar diseases affecting soybean production worldwide. This study aimed to investigate the photosynthetic performance (leaf gas exchange, chlorophyll (Chl) a fluorescence images and photosynthetic pigment pools) of soybean plants sprayed with Acibenzolar‐S‐Methyl (ASM) and the fungicide epoxiconazole + pyraclostrobin (Epo+Pyr) and further inoculated with P. pachyrhizi. The ASR symptoms progressed much faster on the leaves of plants from the control treatment (water spray) in comparison with the ASM and Epo+Pyr treatments. In general, the values for the leaf gas exchange parameters net carbon assimilation rate (A), stomatal conductance to water vapour (g_s), internal CO₂ concentration (C_i) and transpiration rate (E) increased for the infected plants sprayed with ASM or Epo+Pyr in comparison with plants from the control treatment. The values for the initial fluorescence (F_o), maximal fluorescence (F_m), maximal photosystem II quantum efficiency (F_v/F_m), effective photosystem II quantum yield (Y(II)) and quantum yield of regulated energy dissipation (Y(NPQ)) were consistently higher for the ASM and Epo+Pyr treatments in comparison with the control treatment at advanced stages of fungal infection. By contrast, the values for quantum yield of non‐regulated energy dissipation (Y(NO) were significantly lower for the ASM and Epo+Pyr treatments. The concentrations of total Chl a+b and carotenoids significantly increased for infected plants sprayed with ASM and Epo+Pyr in comparison with plants from the control treatment. The results of this study demonstrated that the spray of soybean plants with either ASM or Epo+Pyr contributed to reduce the negative effect of ASR on the photosynthesis of soybean plants.     

Effects of photoinhibition on whole-plant carbon gain assessed with a photosynthesis model

A canopy photosynthesis model was modified to assess the effect of photoinhibition on whole‐plant carbon gain. Photoinhibitory changes in maximum quantum yield of photosystem II (F_v/F_m) could be explained solely from a parameter (Lflux) calculated from the light micro‐environment of the leaves. This relationship between F_v/F_m and the intercepted cumulative light dose, integrated and equally weighted over several hours was incorporated into the model. The effect of photoinhibition on net photosynthesis was described through relationships between photoinhibition and the shaping parameters of the photosynthetic light‐response curve (quantum use efficiency, convexity, and maximum capacity). This new aspect of the model was then validated by comparing measured field data (diurnal courses of F_v/F_m) with simulation results. Sensitivity analyses revealed that the extent of photoinhibitory reduction of whole‐plant photosynthesis was strongly dependent on the structural parameters (LAI and leaf angle). Simulations for a Mediterranean evergreen oak, Quercus coccifera, under climatic conditions which cause mild photoinhibition revealed a daily loss of 7·5–8·5% of potential carbon gain in the upper sunlit canopy layers, a 3% loss in the bottom canopy, and an overall loss of 6·1%. Thus, this canopy photoinhibition model (CANO‐PI) allows the quantitative evaluation of photoinhibition effects on primary production.     

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

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

Long‐term measurements of chlorophyll a fluorescence using the JIP‐test show that combined abiotic stresses influence the photosynthetic performance of the perennial ryegrass (Lolium perenne) in a managed temperate grassland

Several experiments have highlighted the complexity of stress interactions involved in plant response. The impact in field conditions of combined environmental constraints on the mechanisms involved in plant photosynthetic response, however, remains understudied. In a long‐term field study performed in a managed grassland, we investigated the photosynthetic apparatus response of the perennial ryegrass (Lolium perenne L.) to environmental constraints and its ability to recover and acclimatize. Frequent field measurements of chlorophyll a fluorescence (ChlF) were made in order to determine the photosynthetic performance response of a population of L. perenne. Strong midday declines in the maximum quantum yield of primary photochemistry (F_VF_M) were observed in summer, when a combination of heat and high light intensity increased photosynthetic inhibition. During this period, increase in photosystem I (PSI) activity efficiency was also recorded, suggesting an increase in the photochemical pathway for de‐excitation in summer. Strong climatic events (e.g. heat waves) were shown to reduce electron transport between photosystem II (PSII) and PSI. This reduction might have preserved the PSI from photo‐oxidation. Periods of low soil moisture and high levels of sun irradiance increased PSII sensitivity to heat stress, suggesting increased susceptibility to combined environmental constraints. Despite the multiple inhibitions of photosynthetic functionality in summer, the L. perenne population showed increased PSII tolerance to environmental stresses in August. This might have been a response to earlier environmental constraints. It could also be linked to the selection and/or emergence of well‐adapted individuals.     

Combined gas exchange characteristics,chlorophyll fluorescence and response curves as selection traits for temperature tolerance in maize genotypes

Maize is a low-temperature (LT)-sensitive plant and its physiological responses towards LT of temperate regions developed is an adaptive trait. To further our understanding about the response of maize to LT at the physiological and photosynthesis level, we conducted Infrared Gas Analysis (IRGA using LICOR6400-XT in 45-day-old grown two maize genotypes, one from temperate region (Gurez-Kashmir Himalayas), viz., Gurez local (Gz local), and another from tropics (Gujarat), viz., GM6. This study was carried out to evaluate the underlying physiological mechanisms in the two differentially temperature-tolerant maize genotypes. Net photosynthetic rate (A/P_N), 18.253 in Gz local and 25.587 (µmol CO₂ m^?2 s^?1) in GM6; leaf conductance (gs), 0.0102 in Gz local and 0.0566 (mmol H₂O m^?2 s^?1) in GM6; transpiration rate (E), 0.5371 in Gz local and 2.9409 (mmol H₂O m^?2 s^?1) in GM6; and water use efficiency (WUE), 33.9852 in Gz local and 8.7224 (µmol CO₂ mmol H₂O^?1) in GM6, were recorded under ambient conditions. Also, photochemical efficiency of photosystem II (PSII) (F_v/F_m), 0.675 in Gz local and 0.705 in GM6; maximum photochemical efficiency (F_v′/F_m′), 0.310234 in Gz local and 0.401391 in GM6; photochemical quenching (qP), 0.2375 in Gz local and 0.2609 in GM6; non-photochemical quenching (NPQ), 2.0036 in Gz local and 1.1686 in GM6; effective yield of PSII (ФPSII), 0.0789 in Gz local and 0.099 in GM6; and electron transport rate (ETR), 55.3152 in Gz local and 68.112 in GM6, were also evaluated in addition to various response curves, like light intensities and temperature. We observed that light response curves show the saturation light intensity requirement of 1600 µmol for both the genotypes, whereas temperature response curves showed the optimum temperature requirement for Gz local as 20 °C and for GM6 it was found to be 35 °C. The results obtained for each individual parameter and other correlational studies indicate that IRGA forms a promising route for quick and reliable screening of various stress-tolerant valuable genotypes, forming the first study of its kind.

     

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     

Photosynthesis in dynamic light: systems biology of unconventional chlorophyll fluorescence transients in Synechocystis sp. PCC 6803

Photosynthetic organisms live in a dynamic environment where light typically fluctuates around a mean level that is slowly drifting during the solar day. We show that the far-from-equilibrium photosynthesis occurring in a rapidly fluctuating light differs vastly from the stationary-flux photosynthesis attained in a constant or slowly drifting light. Photosynthetic organisms in a static or slowly drifting light can be characterized by a steady-state quantum yield of chlorophyll fluorescence emission F′ that is changing linearly with small and slow variations of the incident irradiance I+ΔI(t): F′(I+ΔI(t))≈ F_mean^′(dF)/(dI)·ΔI(t). In Synechocystis sp. PCC 6803, the linear approximation holds for an extended interval covering largely the static irradiance range experienced by the cyanobacteria in nature. The photosynthetic dynamism and, consequently, the dynamism of the chlorophyll fluorescence emission change dramatically when exposing the organism to a fluctuating irradiance. Harmonically-modulated irradiance I+ΔI · sin(2πt/T), T ≈ 1–25 s induces perpetual, far-from-equilibrium forced oscillations that are strongly non-linear, exhibiting significant hysteresis with multiple fluorescence levels corresponding to a single instantaneous level of the incident irradiance. We propose that, in nature, the far-from-equilibrium dynamic phenomena represent a significant correction to the steady-state photosynthetic activity that is typically investigated in laboratory. Analysis of the forced oscillations by the tools of systems biology suggests that the dynamism of photosynthesis observed in fluctuating light can be explained by a delayed action of regulatory agents.     

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)     

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     

Anthocyanin accumulation in the illuminated surface of maize leaves enhances protection from photo-inhibitory risks at low temperature, without further limitation to photosynthesis

At suboptimal temperatures, anthocyanins accumulate in the illuminated leaf surface of some maize genotypes and, if the anthocyanins shade chloroplasts, they can effectively reduce the risk of photo‐inhibition but also photo‐synthesis. To investigate this phenomenon, gas exchange, fluorescence, superoxide dismutase activity and xantho‐phyll composition of anthocyanin‐containing HOPI and anthocyanin‐deficient W22 maize genotypes were measured in either white or red light, where the latter is not absorbed by anthocyanins. Despite differences in light absorption in chloroplasts, photosynthesis did not differ between HOPI and W22 under either light source, suggesting that neither CO₂ supply nor photochemistry were more limiting in red leaves than in green leaves. In fact, no major differences in transpiration were detected. The ΔF/F_m (photosystem II quantum yield) of HOPI in white light was higher than in red light and higher than ΔF/F_m of W22 with either light source. This probably compensated for the lower white light absorption of HOPI chloroplasts compared with W22 because of the presence of anthocyanins and led to similar rates of calculated electron transport for both genotypes. After exposure to high white light at 5 °C, xanthophyll de‐epoxidation and superoxide dismutase activity were lower in HOPI than in W22. Further, HOPI could be exposed to a much higher irradiance than W22 before F_v/F_m was reduced to that of W22.     

Mathematical modelling of the light response curve of photoinhibition of Photosystem II

The light response curves of the acceptor and donor side mechanisms of photoinhibition of Photosystem II were calculated, using Arabidopsis as a model organism. Acceptor-side photoinhibition was modelled as double reduction of Q_A, noting that non-photochemical quenching has the same effect on the quantum yield of Q_A double reduction in closed PSII centres as it has on the quantum yield of electron transport in open centres. The light response curve of acceptor-side photoinhibition in Arabidopsis shows very low efficiency under low intensity light and a relatively constant quantum yield above light saturation of photosynthesis. To calculate the light response curve of donor-side photoinhibition, we built a model describing the concentration of the oxidized primary donor P₆₈₀⁺ during steady-state photosynthesis. The model is based on literature values of rate constants of electron transfer reactions of PSII, and it was fitted with fluorescence parameters measured in the steady state. The modelling analysis showed that the quantum yield of donor-side photoinhibition peaks under moderate light. The deviation of the acceptor and donor side mechanisms from the direct proportionality between photoinhibition and photon flux density suggests that these mechanisms cannot solely account for photoinhibition in vivo, but contribution of a reaction whose quantum yield is independent of light intensity is needed. Furthermore, a simple kinetic calculation suggests that the acceptor-side mechanism may not explain singlet oxygen production by photoinhibited leaves. The theoretical framework described here can be used to estimate the yields of different photoinhibition mechanisms under different wavelengths or light intensities.     

Influence of the Parasite Pilostyles ingae (Rafflesiaceae) on some Physiological Parameters of the Host Plant,Mimosa naguirei (Mimosaceae)*

plants found at this site were densely covered by flowers of the parasite on their stems indicating heavy development of cellular The holoparasite/host interaction of Pilostyles ingae (Karst.) Hook. f. (Rafflesiaceae) and Mimosa naguirei Barneby (Mimosaceae) was studied in the open campo rupestre vegetation of Serra do Cipó (State of Minas Gerais, Brazil). Infected M. naguirei threads of the parasite in the bark of the hosts. Cellular threads of the parasite are likely to be richer in lipids and hence depleted in ¹³C. This may explain the significantly more negative carbon isotope ratios (δ¹³C values) of the bark of infected host plants observed as compared to other tissues of infected and non-infected host plants. Photosynthetic parameters such as potential quantum yield of photosystem II (F_v/F_m), apparent photosynthetic electron transport rates (ETR) and effective quantum yield of photosystem II (A F/F'_m) in light dependence curves, as well as δ¹³C values of leaves as a relative measure of average intercellular CO₂ partial pressure during photosynthesis over the lifetime of the leaves, which is also related to average stomatal conductance via water use efficiency, were remarkably similar. This suggests a well balanced relation between the Mimosa host and the Pilostyles parasite, in contrast to other hemiparasitic angiosperm parasite/host interactions where the parasite (e.g. Striga) is known to have strong detrimental effects on host photosynthesis.     

NPQ(T): a chlorophyll fluorescence parameter for rapid estimation and imaging of non‐photochemical quenching of excitons in photosystem‐II‐associated antenna complexes

In photosynthesis, light energy is absorbed by light‐harvesting complexes and used to drive photochemistry. However, a fraction of absorbed light is lost to non‐photochemical quenching (NPQ) that reflects several important photosynthetic processes to dissipate excess energy. Currently, estimates of NPQ and its individual components (q_E, q_I, q_Z and q_T) are measured from pulse‐amplitude‐modulation (PAM) measurements of chlorophyll fluorescence yield and require measurements of the maximal yield of fluorescence in fully dark‐adapted material (F_m), when NPQ is assumed to be negligible. Unfortunately, this approach requires extensive dark acclimation, often precluding widespread or high‐throughput use, particularly under field conditions or in imaging applications, while introducing artefacts when F_m is measured in the presence of residual photodamaged centres. To address these limitations, we derived and characterized a new set of parameters, NPQ_(T), and its components that can be (1) measured in a few seconds, allowing for high‐throughput and field applications; (2) does not require full relaxation of quenching processes and thus can be applied to photoinhibited materials; (3) can distinguish between NPQ and chloroplast movements; and (4) can be used to image NPQ in plants with large leaf movements. We discuss the applications benefits and caveats of both approaches.     

Diurnal changes in chlorophyll fluorescence and light utilization in Colocasia esculenta leaves grown in marshy waterlogged area

Diurnal cycle of chlorophyll fluorescence parameters was done in Colocasia esculenta L. (swamp taro) grown in marshy land under sun or under shade. The sun leaves maintained higher electron transport rate (ETR) and steady state to initial fluorescence ratio (F_s/F₀) than shade leaves. In spite of lower ETR, higher photochemical quenching (PQ), and effective quantum yield of photosystem 2 (Φ_PS2) was evident in shade plants compared to plants exposed to higher irradiance. ETR increased linearly with increase in irradiance more under low irradiance (r ² = 0.84) compared to higher irradiance (r ² = 0.62). The maximum quantum yield of PS 2 (F_v/F_m) did not differ much in sun and shade leaves with the exception of midday when excess of light energy absorbed by plants under sun was thermally dissipated. Hence swamp taro plants adopted different strategies to utilize radiation under different irradiances. At higher irradiance, there was faster decline in proportion of open PS 2 centers (PQ) and excess light energy was dissipated through non-photochemical quenching (NPQ). Under shade, absorbed energy was effectively utilized resulting in higher Φ_PS2.     

Effect of Glomus mosseae on chlorophyll content, chlorophyll fluorescence parameters, and chloroplast ultrastructure of beach plum (Prunus maritima) under NaCl stress

The present study was undertaken to investigate the effect of Glomus mosseae on chlorophyll (Chl) content, Chl fluorescence parameters and chloroplast ultrastructure of beach plum seedlings under 2% NaCl stress. The results showed that compared to control, both Chl a and Chl b contents of NaCl + G. mosseae treatment were significantly lower during the salt stress, while Chl a/b ratio increased significantly. The increase of minimal fluorescence of darkadapted state (F₀), and the decrease of maximal fluorescence of dark-adapted state (F_m) and variable fluorescence (F_v) values were inhibited. The maximum quantum yield of PSII photochemistry (F_v/F_m), the maximum energy transformation potential of PSII photochemistry (F_v/F₀) and the effective quantum yield of PSII photochemistry (??_PSII) increased significantly, especially the latter two variables. The values of the photochemical quenching coefficient (q_P) and the nonphotochemical quenching (NPQ) were similar between G. mosseae inoculation and noninoculation. It could be concluded that G. mosseae inoculation could protect the photosystem II (PSII) of beach plum, enhance the efficiency of primary light energy conversion and improve the primitive response of photosynthesis under salinity stress. Meanwhile, G. mosseae inoculation was beneficial to maintain the integrity of thylakoid membrane and to protect the structure and function of chloroplast, which suggested that G. mosseae can alleviate the damage of NaCl stress to chloroplast.     

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.     

The effects of photoinhibition on the photosynthetic light-response curve of green plant cells (Chlamydomonas reinhardtii)

The photosynthetic response to light can be accurately defined in terms of (1) the initial slope (quantum yield); (2) the asymptote (light-saturated rate); (3) the convexity (rate of bending); and (4) the intercept (dark respiration). The effects of photoinhibition [which damages the reaction centre of photosystem II (PSII)] on these four parameters were measured in optically thin cultures of green plant cells (Chlamydomonas reinhardtii). The convexity of the light-response curve decreased steadily from a value of 0.98 (indicating a sharply bending response) to zero (indicating Michaelis-Menten kinetics) in response to increasing photoinhibition. Photoinhibition was quantified from the quantum yield of inhibited cells relative to that of control cells. The quantum yield was estimated by applying linear regression to low-light data or by fitting a non-rectangular hyperbola. Assuming the initial slope is linear allowed comparison with earlier work. However, as the convexity was lowered this assumption resulted in a significant underestimate of the true quantum yield. Thus, the apparent level of photoinhibition required for a zero convexity and the initial decrease in light-saturated photosynthesis depended upon how the quantum yield was estimated. If the initial slope of the light response was assumed to be linear the critical level of inhibition was 60%. If the linear assumption was not made, the critical level was 40%. At the level of inhibition where the convexity reached zero, the light-saturated rate of photosynthesis also began to decrease, indicating that this level of inhibition caused photosynthesis to be limited at all light intensities by the rate of PSII electron transport. At this level of inhibition the F_m-F_i signal (where F_m is maximal chlorophyll fluorescence and F_i is intermediate chlorophyll fluorescence of dark adapted cells; Briantais et al. 1988) from the fluorescence induction curve was zero and the F_i-F_o signal (where F_o is initial chlorophyll fluorescence of dark adapted cells) was 30% of the control, indicating dramatic reduction or complete elimination of one type of PSII. These data do not contradict published mathematical models showing that the ratio of the maximum speed of electron transport in PSII relative to the maximum speed of plastoquinone electron transport can determine the convexity of the photosynthetic response to light.Abbreviations and Symbols Chl chlorophyll content - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F_o, F_i, F_m initial, intermediate, and maximal Chl fluorescence of dark adapted cells - P rate of net photosynthesis per unit chlorophyll (mol-(mg Chl)^–1 · s^–1) - PSII photosystem II - PQ plastoquinone - initial slope to the light-response curve - convexity (rate of bending) of the light-response curve of photosynthesis - Q photosynthetically active photon flux density (400–700 nm, mol · m^–2 · ^–1) The present investigation was supported by the Swedish Council for Forestry and Agricultural Research, the Swedish Environmental Protection Board, and the Swedish Natural Science Research Council. We thank Dr. Deborah D. Kaska (Department of Biological Sciences, University of California, Santa Barbara, Calif., USA) for giving us Chlamydomonas algae. We thank Professor G. Öquist (Department of Plant Physiology, University of Umea, Umea, Sweden) for his encouragement, valuable comments and discussion.     

Strategies providing success in a variable habitat: II. Ecophysiology of photosynthesis of Cladophora glomerata

Cladophora glomerata (L.) Kütz. is the dominant filamentous algae of the river Ilm, Thuringia, Germany. For most of the year it can be found at open as well as at shaded sites. Photosynthetic acclimation of C. glomerata to different light intensities was detected by chlorophyll fluorescence measurements and pigment analysis. Cladophora glomerata from highlight sites showed decreased values of efficiency of open photosystem II (F_v/F_m) as compared with C. glomerata from low‐light sites. Winter populations revealed higher F_v/F_m values than summer populations. A light‐induced decrease in efficiency of the closed photosystem II was observed at increasing irradiance intensities. The decrease was higher in C. glomerata from shaded sites compared with plants from open sites. Differences in the photosynthetic electron transport rate of different populations of C. glomerata were shown by photosynthesis–irradiance curves. Summer populations from high‐light sites yielded higher maximum electron transport rates than plants from low‐light sites, whereas winter populations exhibited significantly decreased values compared with the summer populations. Results of the analysis of photosynthetic pigments corresponded with data from chlorophyll fluorescence measurements. In addition to these long‐term acclimation effects, C. glomerata expressed its ability to cope with rapid changes in the light environment by the de‐epoxidation of violaxanthin during exposure to high light intensities.