Fluorescence Properties of Wild-Type Chlamydomonas reinhardi and Three Mutant Strains Having Impaired Photosynthesis

The wild-type strain of Chlamydomonas reinhardi and 3 mutant strains ac-21, ac-141, and ac-115 have been compared for their fluorescence (and luminescence) properties. The different fluorescence levels, the rapid and slow photochemical responses affecting fluorescence, and the intensity of luminescence have been studied under various conditions: air, nitrogen, 3(p-chlorophenyl)-1,1-dimethylurea. The strain ac-21 exhibits fluorescence properties only quantitatively different from those of the wild-type strain, and it is believed to be affected in some component of the electron transport chain between the 2 light reactions. Both ac-141 and ac-115 have an abnormally high initial fluorescence level; ac-115 does not show the normal photochemical response associated with System II and has a very low luminescence. Mutant strains ac-141 and ac-115 both seem to be modified in the System II photochemical center. These conclusions are compared with a previous analysis based on absorbance changes of cytochrome 559.     

Photosynthetic Properties of ac-31, a Mutant Strain of Chlamydomonas reinhardi Devoid of Chloroplast Membrane Stacking

A pale-green mutant strain of Chlamydomonas reinhardi, ac-31, is characterized by the absence of any stacking of its chloroplast membranes. The capacity for photosynthetic electron transport, phosphorylation, and CO₂ fixation in ac-31 is substantial, and it is concluded that these photosynthetic activities occur within the single membrane. The photosynthetic capacities of wild type and ac-31 as a function of increasing light intensity are compared. Saturation is attained at higher light intensities in ac-31, and the kinetics of the 2 sets of curves are distinctly different. The possibility that energy transfer is enhanced by membrane stacking is suggested by these results. The repeatedly-observed correlation between reduced stacking and disfunctional Photosystem II activities is discussed in view of the observation that ac-31 has no stacking but retains a functional Photosystem II.     

The Photosynthetic Electron Transport Chain of Chlamydomonas reinhardi. VII. Photosynthetic Phosphorylation by a Mutant Strain of Chlamydomonas reinhardi Deficient in Active P700

Electron transport activity and absorbance changes associated with P700 were investigated in a mutant strain of Chlamydomonas reinhardi with impaired photosynthesis. This mutant strain, ac-8oa, cannot reduce NADP with electrons from either water or dye and ascorbate, but it has considerable Hill activity. The mutant strain shows none of the absorbance changes characteristic of P700. Although unable to carry out cyclic photosynthetic phosphorylation, ac-8oa is able to synthesize ATP when ferricyanide is provided as an electron acceptor.

These observations lead to the conclusion that a site for the coupling of photosynthetic phosphorylation with electron transport must exist between the 2 photochemical systems.

     

Limiting Factors in Photosynthesis: V. Photochemical Energy Supply Colimits Photosynthesis at Low Values of Intercellular CO(2) Concentration

Although there is now some agreement with the view that the supply of photochemical energy may influence photosynthetic rate (P) at high CO₂ pressures, it is less clear whether this limitation extends to P at low CO₂. This was investigated by measuring P per area as a function of the intercellular CO₂ concentration (C_i) at different levels of photochemical energy supply. Changes in the latter were obtained experimentally by varying the level of irradiance to normal (Fe-sufficient) leaves of Beta vulgaris L. cv F58-554H1, and by varying photosynthetic electron transport capacity using leaves from Fe-deficient and Fe-sufficient plants. P and C_i were determined for attached sugar beet leaves using open flow gas exchange. The results suggest that P/area was colimited by the supply of photochemical energy at very low as well as high values of C_i. Using the procedure developed by Perchorowicz et al. (Plant Physiol 1982 69:1165-1168), we investigated the effect of irradiance on ribulose bisphosphate carboxylase (RuBPCase) activation. The ratio of initial extractable activity to total inducible RuBPCase activity increased from 0.25 to 0.90 as leaf irradiance increased from 100 to 1500 microeinsteins photosynthetically active radiation per square meter per second. These data suggest that colimitation by photochemical energy supply at low C_i may be mediated via effects on RuBPCase activation.     

THE INFLUENCE OF LIGHT AND CARBON DIOXIDE ON PHOTOSYNTHESIS

1. An optical system is described which furnishes an intensity of 282,000 meter candles at the bottom of a Warburg manometric vessel. With such a high intensity available it was possible to measure the rate of photosynthesis of single fronds of Cabomba caroliniana over a large range of intensities and CO₂ concentrations. 2. The data obtained are described with high precision by the equation KI = p/(p ² _max. – p ²)^½ where p is the rate of photosynthesis at light intensity I, K is a constant which locates the curve on the I axis, and p _max. is the asymptotic maximum rate of photosynthesis. With CO₂ concentration substituted for I, this equation describes the data of photosynthesis for Cabomba, as a function of CO₂ concentration. 3. The above equation also describes the data obtained by other investigators for photosynthesis as a function of intensity, and of CO₂ concentration where external diffusion rate is not the limiting factor. This shows that for different species of green plants there is a fundamental similarity in kinetic properties and therefore probably in chemical mechanism. 4. A derivation of the above equation can be made in terms of half-order photochemical and Blackman reactions, with intensity and CO₂ concentration entering as the first power, or if both sides of the equation are squared, the photochemical and Blackman reactions are first order and intensity and CO₂ enter as the square. The presence of fractional exponents or intensity as the square suggests a complex reaction mechanism involving more than one photochemical reaction. This is consistent with the requirement of 4 quanta for the reduction of a CO₂ molecule.     

Separate action spectra for the two photochemical systems of photosynthesis

Action spectra have been obtained which are believed to represent the 2 photochemical systems in the marine green alga, Ulva lobata. Measurement of transient rates of O₂ uptake in the presence of DCMU furnished the data for the spectrum representing system I. The action spectrum for system II refers to O₂ evolution rates after the Ulva was pretreated in an atmosphere of high O₂ concentration. Tissue responses after high O₂ exposure suggests an inhibition of the photosynthetic functioning of system I.     

Boron excess affects photosynthesis and antioxidant apparatus of greenhouse Cucurbita pepo and Cucumis sativus

This study aimed to evaluate the behavior of zucchini (Cucurbita pepo L.) and cucumber (Cucumis sativus L.) under boron (B) excess. Plants were grown under greenhouse conditions in a sandy soil–peat mixture using a nutrient solution containing 0.2 (control), 10 and 20 mg L^?1 B. Visible symptoms were quantified and leaf B accumulation, gas exchanges, chlorophyll (Chl) a fluorescence, malondialdehyde by-products and antioxidants were investigated 20 days after the beginning of the treatments. Boron toxicity induced oxidative load and leaf necrotic burns coupled with the reduction of leaf growth and biomass accumulation in both species. Boron excess resulted in a decrease of Chl a/b ratio, potential (F_v/F_m) and actual (Φ_PSII) PSII quantum efficiency, photosynthetic rate (Pn), stomatal conductance (g_s), and transpiration (E) as well. A general stimulation of the antioxidant enzymes ascorbate peroxidase, catalase and superoxide dismutase was observed, and a significant increase in the oxidized form of ascorbate and glutathione was evidenced for treated plants of both species. A difference between the two species was observed: C. pepo appeared to be more sensitive to B stress being damaged at all B concentration. C. sativus grown at 10 mg L^?1 B in nutrient solution showed some down-regulated mechanisms, i.e. increase in Chl b content and a good photochemical PSII efficiency as well as a higher amount of constitutive antioxidant molecules, that, however, are not sufficient to contrast the negative effects of B.     

Severe water deficit-induced ethylene production decreases photosynthesis and photochemical efficiency in flag leaves of wheat

Wheat (Triticum aestivum L.) cv. Jimai22 was used to evaluate the effect of ethylene evolution rate (EER) and 1-aminocyclopropane-1-carboxylic acid (ACC) and their relations with photosynthesis and photochemical efficiency in plants well-watered (WW) and under a severe water deficit (SWD). SWD caused a noticeable reduction in the grain mass. The marked increases in both EER and the ACC concentration were observed under SWD; it was reversed effectively by exogenous spermidine (Spd) or amino-ethoxyvinylglycine (AVG). Thermal images indicated that SWD increased obviously the temperature of flag leaves, mainly due to the decrease in transpiration rate under SWD. Exogenous Spd or AVG decreased to some extent the temperature of the flag leaves. The strong decline in photosynthetic rate (P _N) and stomatal conductance as well as the photodamage of PSII were also observed under SWD after 14 and 21 days after anthesis (DAA). Intercellular CO₂ concentration was reduced at 7 DAA, but slightly increased at 14 and 21 DAA under SWD, indicating that the decreased P _N at 7 DAA might result from stomatal limitations, while the decline after 14 and 21 DAA might be attributed to nonstomatal limitations. Correlation analysis suggested that EER and ACC showed negative relations to photosynthesis and photochemical efficiency. Data obtained suggested that the effects of SWD were mediated predominantly by the increase in EER and ACC concentration, which greatly decreased the leaf photosynthesis and photochemical efficiency, and, therefore, reduced the grain mass. Application of Spd or AVG reduced the EER and ACC, and thus positively influenced photosynthesis and photochemical efficiency under SWD.     

Silencing of tomato mitochondrial uncoupling protein disrupts redox poise and antioxidant enzymes activities balance under oxidative stress

Plant mitochondrial uncoupling proteins (pUCPs) play important roles in generation of metabolic thermogenesis, response to stress situation, and regulation of energy metabolism. Although the signaling pathways for the pUCPs-regulated plant energy metabolism and thermogenesis are well studied, the role of pUCPs in the regulation of plant stress tolerance has not been fully substantiated. Here we showed that mitochondrial uncoupling protein was required for effective antioxidant enzymes activities, chlorophyll fluorescence and redox poise in tomato under oxidative stress using virusinduced gene silencing approach. Silencing of LeUCP gene reduced maximal quantum yield of PSII (Fv/Fm) and photochemical quenching coefficient (qP), as well as mitigated activation of antioxidant enzymes and related genes expression. The content of reduced ascorbate and reduced glutathione, redox ratio of ascorbate and L-galactono-1,4-lactone dehydrogenase (GalLDH; EC 1.3.2.3) activity were all decreased in the leaves of LeUCP gene-silenced plant. However, malondialdehyde content was increased under methylviologen (MV) stress. ROS accumulation was increased significantly following MV and heat stress treatments. Meanwhile, LeUCP gene silencing aggravated accumulation of H₂O₂ and O ₂ ^·? in leaves. Taken together, these results strongly suggest that LeUCP gene plays critical role in maintaining the redox homeostasis and balance in antioxidant enzyme system under oxidative stress.     

SO(2) Effect on Photosynthetic Activities of Intact Sugar Maple Leaves as Detected by Photoacoustic Spectroscopy

Short-term (4 hours) effect of different concentrations of SO₂ fumigation on in vivo photochemical activities of sugar maple (Acer saccharum Marsh.) leaves was investigated using photoacoustic spectroscopy. The relative quantum yield of O₂ evolution (ratio of O₂ signal to the photothermal signal) and photochemical energy storage are increased by 0.05 microliter per liter of SO₂. This increase is more pronounced in 5 to 7 year old saplings than in 3 month old seedlings. Both oxygen-relative quantum yield and energy storage of seedlings are inhibited by increased concentrations of SO₂ and the inhibition is concentration dependent. The inhibition is greater in seedlings than in saplings at 2 microliters per liter of SO₂, indicating the more susceptible nature of seedlings. The present study indicates a concentration dependent differential effect of SO₂ on photochemical activities of sugar maple leaves.     

Anaerobiosis Induced State Transition: A Non Photochemical Reduction of PQ Pool Mediated by NDH in Arabidopsis thaliana

Background

Non photochemical reduction of PQ pool and mobilization of LHCII between PSII and PSI are found to be linked under abiotic stress conditions. The interaction of non photochemical reduction of PQ pool and state transitions associated physiological changes are critically important under anaerobic condition in higher plants.

Methodology/Findings

The present study focused on the effect of anaerobiosis on non-photochemical reduction of PQ pool which trigger state II transition in Arabidopsis thaliana. Upon exposure to dark-anaerobic condition the shape of the OJIP transient rise is completely altered where as in aerobic treated leaves the rise is unaltered. Rise in F _o and F _J was due to the loss of oxidized PQ pool as the PQ pool becomes more reduced. The increase in F_o′ was due to the non photochemical reduction of PQ pool which activated STN7 kinase and induced LHCII phosphorylation under anaerobic condition. Further, it was observed that the phosphorylated LHCII is migrated and associated with PSI supercomplex increasing its absorption cross-section. Furthermore, evidences from crr2-2 (NDH mutant) and pgr5 mutants (deficient in non NDH pathway of cyclic electron transport) have indicated that NDH is responsible for non photochemical reduction of the PQ pool. We propose that dark anaerobic condition accelerates production of reducing equivalents (such as NADPH by various metabolic pathways) which reduce PQ pool and is mediated by NDH leading to state II transition.

Conclusions/Significance

Anaerobic condition triggers non photochemical reduction of PQ pool mediated by NDH complex. The reduced PQ pool activates STN7 kinase leading to state II transition in A. thaliana.     

Carbon Dioxide-Induced Oscillations in Fluorescence and Photosynthesis: Role of Thylakoid Membrane Energization in Regulation of Photosystem II Activity

The response of CO₂ fixation to a sudden increase in ambient CO₂ concentration has been investigated in intact leaf tissue from spinach (Spinacia oleracea) using a dual channel infrared gas analyzer. Simultaneous with these measurements, changes in fluorescence emission associated with a weak, modulated measuring beam were recorded. Application of brief (2-3 seconds) dark intervals enabled estimation of the dark fluorescence level (F_o) under both steady state and transient conditions. The degree of suppression of F_o level fluorescence in the light was strongly correlated with nonphotochemical quenching under all conditions. During CO₂-induced oscillations in photosynthesis under 2% O₂ the changes in nonphotochemical quenching anticipate changes in the rate of uptake of CO₂. At such low levels of O₂ and constant illumination, changes in the relative quantum efficiency of open photosystem II units were estimated as the ratio of the rate of CO₂ uptake and the photochemical quenching coefficient. Under the same conditions the relative quantum efficiency of photosystem II was found to vary inversely with the degree of nonphotochemical quenching. The relationship between changes in the rate of CO₂ uptake: photochemical quenching coefficient and nonphotochemical quenching was altered somewhat when the same experiment was conducted under 20% O₂. The results suggest that electron transport coupled to reduction of O₂ occurs to varying degrees with time during oscillations, especially when ambient O₂ concentrations are high.     

Energy transfer and the distribution of excitation energy in the photosynthetic apparatus of spinach chloroplasts

Equations are derived from our model of the photochemical apparatus of photosynthesis to show that the yield of energy transfer from Photosystem II to Photosystem I, ?_T(II→Iz), can be obtained from measurements on an individual sample of chloroplasts frozen to ?196 °C by comparing the sum of two specifically defined fluorescence excitation spectra with the absorption spectrum of the sample. Then, given that value of ?_T(II→I), the fraction of the quanta absorbed by the photochemical apparatus which is distributed initially to Photosystem I, α, can be determined as a function of the wavelength of excitation from the same fluorescence excitation spectra. The results obtained in this study of individual samples of chloroplasts frozen to ?196 °C in the absence of divalent cations, namely, that ?_T(II→I) varies from a minimum value of 0.10 when the Photosystem II reaction centers are all open to a maximum value of 0.25 when the centers are all closed and that α has a value of about 0.30 which is almost independent of wavelength for wavelengths shorter than 675 nm (α increases rapidly toward unity at wavelengths longer than 675 nm), agrees quite well with results obtained previously from comparative measurements of chloroplasts frozen to ?196 °C in the presence and absence of divalent cations.     

Fluorescence Quenching and Gas Exchange in a Water Stressed C(3) Plant, Digitalis lanata

A leaf cuvette has been adapted for use with a pulse-modulation fluorometer and an open gas exchange system. Leaf water potential (ψ) was decreased by withholding watering from Digitalis lanata EHRH. plants. At different stages of water deficiency the photochemical (q_Q) and nonphotochemical (q_E) fluorescence quenching was determined during the transition between darkness and light-induced steady state photosynthesis of the attached leaves. In addition, the steady state CO₂ and H₂O gas exchange was recorded. Following a decrease of leaf water potential with increasing water deficiency, the transition of photochemical quenching was almost unaffected, whereas nonphotochemical quenching increased. This is indicative of an enhanced thylakoid membrane energization during the transition and is interpreted as a partial inhibition of either the ATP generating or the ATP consuming reaction sequences. Complete reversion of the stress induced changes was achieved within 6 hours after rewatering. In contrast to the variations during transition, the final steady state values of q_Q and q_E remained unchanged over the entire stress range from −0.7 to −2.5 megapascals. From these results we conclude that, once established, electron transport via photosystem II and the transmembrane proton gradient remain unaffected by water stress. These data are indicative of a protective mechanism against photoinhibition during stress, when net CO₂ uptake is limited.     

Ecophysiological responses of Caragana korshinskii Kom. under extreme drought stress: Leaf abscission and stem survives

Caragana korshinskii Kom. is a perennial xerophytic shrub, well known for its ability to resist drought. In order to study ecophysiological responses of C. korshinskii under extreme drought stress and subsequent rehydration, diurnal patterns of gas exchange and chlorophyll (Chl) fluorescence parameters of photosystem II as well as Chl content were analyzed. Plant responses to extreme drought included (1) leaf abscission and using stem for photosynthesis, (2) improved instantaneous water-use efficiency, (3) decreased photosynthetic rate and partly closed stomata owing to leaf abscission and low water status, (4) decreased maximum photochemical efficiency of photosystem II (PSII) (variable to maximum fluorescence ratio, F_v/F_m), quantum efficiency of noncyclic electron transport of PSII, and Chl a and Chl b. Four days after rehydration, new leaves budded from stems. In the rewatered plants, the chloroplast function was restored, the gas exchange and Chl fluorescence returned to a similar level as control plant. The above result indicated that maintaining an active stem system after leaf abscission during extreme drought stress may be the foundation which engenders these mechanisms rapid regrowth for C. korshinskii in arid environment.     

Toward predicting photosynthetic efficiency and biomass gain in crop genotypes over a field season

Photosynthesis acclimates quickly to the fluctuating environment in order to optimize the absorption of sunlight energy, specifically the photosynthetic photon fluence rate (PPFR), to fuel plant growth. The conversion efficiency of intercepted PPFR to photochemical energy (ɛ_e) and to biomass (ɛ_c) are critical parameters to describe plant productivity over time. However, they mask the link of instantaneous photochemical energy uptake under specific conditions, that is, the operating efficiency of photosystem II (F_q′/F_m′), and biomass accumulation. Therefore, the identification of energy- and thus resource-efficient genotypes under changing environmental conditions is impeded. We long-term monitored F_q′/F_m′ at the canopy level for 21 soybean (Glycine max (L.) Merr.) and maize (Zea mays) genotypes under greenhouse and field conditions using automated chlorophyll fluorescence and spectral scans. F_q′/F_m′ derived under incident sunlight during the entire growing season was modeled based on genotypic interactions with different environmental variables. This allowed us to cumulate the photochemical energy uptake and thus estimate ɛ_e noninvasively. ɛ_e ranged from 48% to 62%, depending on the genotype, and up to 9% of photochemical energy was transduced into biomass in the most efficient C₄ maize genotype. Most strikingly, ɛ_e correlated with shoot biomass in seven independent experiments under varying conditions with up to r = 0.68. Thus, we estimated biomass production by integrating photosynthetic response to environmental stresses over the growing season and identified energy-efficient genotypes. This has great potential to improve crop growth models and to estimate the productivity of breeding lines or whole ecosystems at any time point using autonomous measuring systems.

Cumulative photochemical energy uptake throughout a fluctuating growing season reveals genotypic differences in photosynthetic performance, respiratory losses, and biomass production efficiency.     

Development of the Primary Photochemical Apparatus of Photosynthesis during Greening of Etiolated Bean Leaves

Seven-day-old dark-grown bean leaves were greened under continuous light. The amount of chlorophyll, the ratio of chlorophyll a to chlorophyll b, the O₂ evolving capacity and the primary photochemical activities of Photosystem I and Photosystem II were measured on the leaves after various times of greening. The primary photochemical activities were measured as the photo-oxidation of P₇₀₀, the photoreduction of C-550, and the photo-oxidation of cytochrome b₅₅₉ in intact leaves frozen to −196 C. The results indicate that the reaction centers of Photosystem I and Photosystem II begin to appear within the first few minutes and that Photosystem II reaction centers accumulate more rapidly than Photosystem I reaction centers during the first few hours of greening. The very early appearances of the primary photochemical activity of Photosystem II was also confirmed by light-induced fluorescence yield measurements at −196 C.     

THE PHOTOCHEMICAL NATURE OF THE PHOTOSENSORY PROCESS

On page 243, Vol. ii, No. 3, January 20, 1920, line 12 (counting each equation as a line) for a photochemical reaction read a reversible photochemical reaction.     

Effects of water vapor pressure deficit on photochemical and fluorescence yields in tobacco leaf tissue

The relationship between photochemical quantum yield (_s) and fluorescence yield have been investigated in leaf tissue from Nicotiana tabacum using CO₂ exchange and a modulated fluorescence measuring system. The quantum yield of CO₂ fixation at 1.6% (v/v) O₂ and limiting irradiance was reduced 20% by increasing the mean H₂O vapor pressure deficit (VPD) from 9.2 to 18.6 mbars. As [CO₂] and irradiance were varied, the intrinsic quantum yield of open photosystem II units (_s/q_Q where q_Q is the photochemical fluorescence quenching coefficient) declined linearly with the degree of nonphotochemical fluorescence quenching. The slope and y-intercept values for this function were significantly reduced when the mean VPD was 18.4 millibars relative to 8.9 millibars. Susceptibility of the leaf tissue to photoinhibition was unaffected by VPD. Elevated O₂ concentrations (20.5% v/v) reduced the intrinsic quantum yield of net CO₂ uptake due to the occurrence of O₂-reducing processes. However, the relative effect of high VPD compared to low VPD on intrinsic quantum yield was not dependent on the O₂ level. This suggests that the Mehler reaction does not mediate the response of quantum yield to elevated VPD. The results are discussed with regard to the possible role of transpiration stress in regulating dissipation of excitation by electron transport pathways other than noncyclic electron flow supporting reduction of CO₂ and/or O₂.