Modelling of light-induced chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence rise (O-J-I-P transient) and changes in 820 nm-transmittance signal of photosynthesis

Theoretical modelling is often overlooked in photosynthesis research even if it can significantly help with understanding of explored system. A new model of light-induced photosynthetic reactions occurring in and around thylakoid membrane is introduced here and used for theoretical modelling of not only the light-induced chlorophyll (Chl) a fluorescence rise (FLR; the O-J-I-P transient), reflecting function of photosystem II (PSII), but also of the 820 nmtransmittance signal (I₈₂₀), reflecting function of photosystem I (PSI) and plastocyanin (PC), paralleling the FLR. Correctness of the model was verified by successful simulations of the FLR and I₈₂₀ signal as measured with the control (no treatment) sample but also as measured with 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone- (inhibits electron transport in cytochrome b ₆/f) and methylviologen- (accepts electrons from iron-sulphur cluster of PSI) treated samples and with the control sample upon different intensities of excitation light. From the simulations performed for the control sample, contribution of the oxidised donor of PSI, P700, and oxidised PC to the I₈₂₀ signal minimum (reflects maximal accumulations of the two components) was estimated to be 75% and 25%, respectively. Further in silico experiments showed that PC must be reduced in the dark, cyclic electron transport around PSI must be considered in the model and activation of ferredoxin-NADP⁺-oxidoreductase (FNR) also affects the FLR. Correct simulations of the FLR and I₈₂₀ signal demonstrate robustness of the model, confirm that the electron transport reactions occurring beyond PSII affect the shape of the FLR, and show usefulness and perspective of theoretical approach in studying of the light-induced photosynthetic reactions.     

Experimental and theoretical study on high temperature induced changes in chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence oscillations in barley leaves upon 2 % CO<Subscript>2</Subscript>

Oscillations in many of photosynthetic quantities with a period of about 1 min can be routinely measured with higher plant leaves after perturbation of the steady state by sudden change in gas phase. Among all hypotheses suggested so far to explain the oscillations, an effect of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activation status to control the oscillations is highly probable, at least upon high temperature (HT) treatment when in vivo RuBPCO activity controlled by RuBPCO activase (RuBPCO-A) decreases. Therefore, we measured the oscillations in fluorescence signal coming from barley leaves (Hordeum vulgare L. cv. Akcent) after their exposure for various time intervals to different HTs in darkness. We also evaluated steady state fluorescence and CO₂ exchange parameters to have an insight to functions of electron transport chain within thylakoid membrane and Calvin cycle before initiation of the oscillations. The changes in period of the oscillations induced by moderate HT (up to 43 °C) best correlated with changes in non-photochemical fluorescence quenching (q_N) that in turn correlated with changes in gross photosynthetic rate (P _G) and rate of RuBPCO activation (k_act). Therefore, we suggest that changes in period of the oscillations caused by moderate HT are mainly controlled by RuBPCO activation status. For more severe HT (45 °C), the oscillations disappeared which was probably caused by an insufficient formation of NADPH by electron transport chain within thylakoid membrane as judged from a decrease in photochemical fluorescence quenching (q_P). Suggestions made on the basis of experimental data were verified by theoretical simulations of the oscillations based on a model of Calvin cycle and by means of a control analysis of the model.     

Effect of long-term drought stress on leaf gas exchange and fluorescence parameters in C<Subscript>3</Subscript> and C<Subscript>4</Subscript> plants

A field study was performed on triticale, field bean, maize and amaranth, to find differences between studied species in physiological alterations resulting from progressive response as injuries and/or acclimation to long-term soil drought during various stages of plant development. The measurements of leaf water potential, electrolyte leakage, chlorophyll a fluorescence, leaf gas exchange and yield analysis were done. A special emphasis was given to the measurements of the blue, green, red and far-red fluorescence. Beside, different ratios of the four fluorescence bands (red/far-red: F ₆₉₀/F ₇₄₀, blue/red: F ₄₄₀/F ₆₉₀, blue/far-red: F ₄₄₀/F ₇₄₀ and blue/green: F ₄₄₀/F ₅₂₀) were calculated. Based on both yield analysis and measurements of physiological processes it can be suggested that field bean and maize responded with better tolerance to the water deficit in soil due to the activation of photoprotective mechanism probably connected with synthesis of the phenolic compounds, which can play a role of photoprotectors in different stages of plant development. The photosynthetic apparatus of those two species scattered the excess of excitation energy more effectively, partially through its transfer to PS I. In this way, plants avoided irreversible and/or deep injuries to PS II. The observed changes in the red fluorescence emission and in the F _v/F _m for triticale and amaranth could have occurred due to serious and irreversible photoinhibitory injuries. Probably, field bean and maize acclimatized more effectively to soil drought through the development of effective mechanisms for utilising excitation energy in the photosynthetic conversion of light accompanied by the mechanism protecting the photosynthetic apparatus against the excess of this energy.     

Selective effects of H<Subscript>2</Subscript>O<Subscript>2</Subscript> on cyanobacterial photosynthesis

The sensitivity of phytoplankton species for hydrogen peroxide (H₂O₂) was analyzed by pulse amplitude modulated (PAM) fluorometry. The inhibition of photosynthesis was more severe in five tested cyanobacterial species than in three green algal species and one diatom species. Hence the inhibitory effect of H₂O₂ is especially pronounced for cyanobacteria. A specific damage of the photosynthetic apparatus was demonstrated by changes in 77 K fluorescence emission spectra. Different handling of oxidative stress and different cell structure are responsible for the different susceptibility to H₂O₂ between cyanobacteria and other phytoplankton species. This principle may be potentially employed in the development of new agents to combat cyanobacterial bloom formation in water reservoirs.     

Rapid effect of light on the K+ channel in the plasmalemma ofNitella

Summary Chlorophyll fluorescence, plasmalemma potential and resistance were measured simultaneously and subjected to a kinetic analysis. It was found that the light-induced changes of all three signals have two time constants in common. The faster one (₄=ca. 20 sec) was assigned to the action of light-induced proton uptake across the thylakoid membrane on the plasmalemma H⁺ pump. The slower one (_5a=40 sec) is related to the light action of an unknown photosynthetic process on the potassium channel. The action on the K⁺ channel was revealed from the reversal potential of the related effect on membrane potential. The comparison of the data with findings of other authors led to the hypothesis that the unknown photosynthetic mechanism is the depletion of NADP⁺, which stimulates the uptake of Ca²⁺ from the cytosol, which is required for the NAD-kinase. The resulting change in cytosolic Ca²⁺ modulates the number of open K⁺ channels.     

Photosynthesis and fluorescence responses of C<Subscript>4</Subscript> plant <Emphasis Type="Italic">Andropogon gerardii</Emphasis> acclimated to temperature and carbon dioxide

Increase in both atmospheric CO₂ concentration [CO₂] and associated warming are likely to alter Earths’ carbon balance and photosynthetic carbon fixation of dominant plant species in a given biome. An experiment was conducted in sunlit, controlled environment chambers to determine effects of atmospheric [CO₂] and temperature on net photosynthetic rate (P _N) and fluorescence (F) in response to internal CO₂ concentration (C _i) and photosynthetically active radiation (PAR) of the C₄ species, big bluestem (Andropogon gerardii Vitman). Ten treatments were comprised of two [CO₂] of 360 (ambient, AC) and 720 (elevated, EC) μmol mol⁻¹ and five day/night temperature of 20/12, 25/17, 30/22, 35/27 and 40/32 °C. Treatments were imposed from 15 d after sowing (DAS) through 130 DAS. Both F-P _N/C _i and F-P _N/PAR response curves were measured on top most fully expanded leaves between 55 and 75 DAS. Plants grown in EC exhibited significantly higher CO₂-saturated net photosynthesis (P _sat), phosphoenolpyruvate carboxylase (PEPC) efficiency, and electron transport rate (ETR). At a given [CO₂], increase in temperature increased P _sat, PEPC efficiency, and ETR. Plants grown at EC did not differ for dark respiration rate (R _D), but had significantly higher maximum photosynthesis (P _max) than plants grown in AC. Increase in temperature increased Pmax, R _D, and ETR, irrespective of the [CO₂]. The ability of PEPC, ribulose-1,5-bisphosphate carboxylase/oxygenase, and photosystem components, derived from response curves to tolerate higher temperatures (>35 °C), particularly under EC, indicates the ability of C₄ species to sustain photosynthetic capacity in future climates.     

Gas exchange and photosynthetic performance of the tropical tree <Emphasis Type="Italic">Acacia nigrescens</Emphasis> when grown in different CO<Subscript>2</Subscript> concentrations

The photosynthetic responses of the tropical tree species Acacia nigrescens Oliv. grown at different atmospheric CO₂ concentrations—from sub-ambient to super-ambient—have been studied. Light-saturated rates of net photosynthesis (A _sat) in A. nigrescens, measured after 120 days exposure, increased significantly from sub-ambient (196 μL L⁻¹) to current ambient (386 μL L⁻¹) CO₂ growth conditions but did not increase any further as [CO₂] became super-ambient (597 μL L⁻¹). Examination of photosynthetic CO₂ response curves, leaf nitrogen content, and leaf thickness showed that this acclimation was most likely caused by reduction in Rubisco activity and a shift towards ribulose-1,5-bisphosphate regeneration-limited photosynthesis, but not a consequence of changes in mesophyll conductance. Also, measurements of the maximum efficiency of PSII and the carotenoid to chlorophyll ratio of leaves indicated that it was unlikely that the pattern of A _sat seen was a consequence of growth [CO₂] induced stress. Many of the photosynthetic responses examined were not linear with respect to the concentration of CO₂ but could be explained by current models of photosynthesis.     

Acclimation of shade-tolerant and light-resistant <Emphasis Type="Italic">Tradescantia</Emphasis> species to growth light: chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence,electron transport,and xanthophyll content

In this study, we have compared the photosynthetic characteristics of two contrasting species of Tradescantia plants, T. fluminensis (shade-tolerant species), and T. sillamontana (light-resistant species), grown under the low light (LL, 50–125 µmol photons m^?2 s^?1) or high light (HL, 875–1000 µmol photons m^?2 s^?1) conditions during their entire growth period. For monitoring the functional state of photosynthetic apparatus (PSA), we measured chlorophyll (Chl) a emission fluorescence spectra and kinetics of light-induced changes in the heights of fluorescence peaks at 685 and 740 nm (F ₆₈₅ and F ₇₄₀). We also compared the light-induced oxidation of P₇₀₀ and assayed the composition of carotenoids in Tradescantia leaves grown under the LL and HL conditions. The analyses of slow induction of Chl a fluorescence (SIF) uncovered different traits in the LL- and HL-grown plants of ecologically contrasting Tradescantia species, which may have potential ecophysiological significance with respect to their tolerance to HL stress. The fluorometry and EPR studies of induction events in chloroplasts in situ demonstrated that acclimation of both Tradescantia species to HL conditions promoted faster responses of their PSA as compared to LL-grown plants. Acclimation of both species to HL also caused marked changes in the leaf anatomy and carotenoid composition (an increase in Violaxanthin?+?Antheraxantin?+?Zeaxanthin and Lutein pools), suggesting enhanced photoprotective capacity of the carotenoids in the plants grown in nature under high irradiance. Collectively, the results of the present work suggest that the mechanisms of long-term PSA photoprotection in Tradescantia are based predominantly on the light-induced remodeling of pigment-protein complexes in chloroplasts.     

Influence of irradiance on photosynthesis and PSII photochemical efficiency in maize during short-term exposure at high CO<Subscript>2</Subscript> concentration

This work aimed to evaluate if gas exchange and PSII photochemical activity in maize are affected by different irradiance levels during short-term exposure to elevated CO₂. For this purpose gas exchange and chlorophyll a fluorescence were measured on maize plants grown at ambient CO₂ concentration (control CO₂) and exposed for 4 h to short-term treatments at 800 μmol(CO₂) mol⁻¹ (high CO₂) at a photosynthetic photon flux density (PPFD) of either 1,000 μmol m⁻² s⁻¹ (control light) or 1,900 μmol m⁻² s⁻¹ (high light). At control light, high-CO₂ leaves showed a significant decrease of net photosynthetic rate (P _N) and a rise in the ratio of intercellular to ambient CO₂ concentration (C _i/C _a) and water-use efficiency (WUE) compared to control CO₂ leaves. No difference between CO₂ concentrations for PSII effective photochemistry (Φ_PSII), photochemical quenching (q_p) and nonphotochemical quenching (NPQ) was detected. Under high light, high-CO₂ leaves did not differ in P _N, C _i/C _a, Φ_PSII and NPQ, but showed an increase of WUE. These results suggest that at control light photosynthetic apparatus is negatively affected by high CO₂ concentration in terms of carbon gain by limitations in photosynthetic dark reaction rather than in photochemistry. At high light, the elevated CO₂ concentration did not promote an increase of photosynthesis and photochemistry but only an improvement of water balance due to increased WUE.     

An estimation of CO<Subscript>2</Subscript> fixation capacity in mangrove forest using two methods of CO<Subscript>2</Subscript> gas exchange and growth curve analysis

In many coastal areas of South-East Asia, attempts have been made to revive coastal ecosystem by initiating projects that encourage planting of mangrove trees. Compared to the terrestrial trees, mangrove trees possess a higher carbon fixation capacity. It becomes a very significant option for clean development mechanism (CDM) program. However, a reliable method to estimate CO₂ fixation capacity of mangrove trees has not been established. Acknowledging the above fact, we decided to set up an estimation method for the CDM program, using gas exchange analysis to estimate mangrove productivity, we put into consideration the net CO₂ fixation of reforested Kandelia candel (5-, 10-, and 15-year-old stand). This was estimated by gas exchange analysis and growth curve analysis. In growth curve analysis, we drew a growth curve of a single stand using data of above- and below-ground biomass. In the gas exchange analysis, we calculated CO₂ fixation capacity by (1) measuring respiration rate of each organ of stand and calculating respiratory CO₂ emission from above- to below-ground biomass. (2) Measuring the single-leaf photosynthetic rate in response to light intensity and calculating the photosynthetic CO₂ absorption. (3) We also developed a model for the diurnal changes in temperature, and monthly averages based on one-day estimation of CO₂ absorption and emission, which we corrected by this model in order to estimate the net CO₂ fixation capacity in response to temperature. Comparing the biomass accumulation of the two methods constructed for the same forest, the above-ground biomass accumulation of 10-year-old forest (34.3 ton ha⁻¹ yr⁻¹) estimated by gas exchange analysis was closely compared to those of growth curve analysis (26.6 ton ha⁻¹ yr⁻¹), suggesting that the gas exchange analysis was capable of estimating mangrove productivity. The validity of the estimated CO₂ fixation capacity by the gas exchange analysis and the growth curve analysis was also discussed.     

On the quantitative relation between dark kinetics of NPQ-induced changes in variable fluorescence and the activation state of the CF<Subscript>0</Subscript>·CF<Subscript>1</Subscript>·ATPase in leaves

The variable fluorescence at the maximum F_m of the fluorescence induction (Kautsky) curve is known to be substantially suppressed shortly after light adaption due to nonphotochemical q_E quenching. The kinetic pattern of the dark decay at F_m consists of three components with rates ~20, ~1, and ~0.1 s^–1, respectively. Light adaptation has no or little effect on these rate constants. It causes a decrease in the ratio between the amplitudes of the slow and fast one with negligible change in the small amplitude of the ultra-slow component. Results add to evidence for the hypothesis that the dark-reversible decrease in variable fluorescence accompanying light adaptation during the P–S phase of the fluorescence induction curve is due to an alteration in nonphotochemical q_E quenching caused by changes in the trans-thylakoid proton motive force in response to changes in the proton conductance g_H+ of the CF₀-channel of the CF₀·CF₁·ATPase.     

Duree de vie et rendement de fluorescence de la chlorophylle in vivo. Leur relation dans differents modeles d'unites photosynthetiques

Lifetime and yield of chlorophyll fluorescence in vivo: Their relationship in different models of photosynthetic unitsWe have used phase fluorimetry to measure the relation between fluorescence lifetime (τ) and yield (Ф) of chlorophyll during the induction phase of photosynthesis in isolated chloroplasts and in vivo. This relationship is usually non-linear, curving slightly toward negative values of τ, and does not extrapolate to zero. In the discussion we examine the conditions which might give rise to curvature and a non-zero intercept. A model of connected photosynthetic units, characterized by an intersystem frequency of energy exchange, T, can account for the concavity of the experimental curves when 0.6 ? T ? 1 ns^?1.Two hypotheses are suggested to account for the non-zero intercept: the occurrence of sensitized fluorescence emission, or the existence in the initial fluorescence of a constant fraction independent of System II.     

Low temperature stress modifies the photochemical efficiency of a tropical tree species <Emphasis Type="Italic">Hevea brasiliensis</Emphasis>: effects of varying concentration of CO<Subscript>2</Subscript> and photon flux density

Two clones of Hevea brasiliensis (RRII 105 and PB 235) were grown for one year in two distinct agroclimatic locations (warmer and colder, W and C) in peninsular India. We simultaneously measured gas exchange and chlorophyll (Chl) fluorescence on fully mature intact leaves at different photosynthetic photon flux densities (PPFDs) and ambient CO₂ concentrations (C _a) and at constant ambient O₂ concentration (21 %). Net photosynthetic rate (P _N), apparent quantum yield for CO₂ assimilation (Φ_c), in vivo carboxylation efficiency (CE), and photosystem 2 quantum yield (Φ_PS2) were low in plants grown in C climate and these reductions were more predominant in RRII 105 than in PB 235 which was also reflected in their growth. We estimated in these clones the partitioning of photosynthetic electrons between CO₂ reduction (J_A) and processes other than CO₂ reduction (J^*) at low and high PPFDs and C _a. At high C _a (700 µmol mol⁻¹) most of the photosynthetic electrons were used for CO₂ assimilation and negligible amount went for other processes when PPFD was low (200–300 µmol m⁻² s⁻¹) both in the C and W climates. But at high PPFD (900-1 100 µmol m⁻² s⁻¹), J^* was appreciably high even at a high C _a. Hence at normal ambient C _a and high irradiance, electrons can be generated in the photosynthetic apparatus far in excess of what can be safely utilised for photosynthetic CO₂ reduction. However, at high C _a there was increased diversion of electrons to photosynthetic CO₂ reduction which resulted in improved photosynthetic parameters even in plants grown in C climate.     

Influences of nitrogen load on the growth and photosynthetic responses of <Emphasis Type="Italic">Quercus serrata</Emphasis> seedlings to O<Subscript>3</Subscript>

The objectives of this study were to clarify the influences of nitrogen (N) load on the growth and photosynthetic responses of Quercus serrata seedlings to O₃ and to obtain basic data for evaluating the critical levels of O₃ for protecting Q. serrata forests in Japan. The effects of O₃ and/or N load on growth and photosynthetic activity of Q. serrata seedlings were investigated during the two growing seasons. Two-year-old seedlings were assigned to 12 experimental treatments, which were comprised of the combination of four gas treatments (charcoal-filtered air and three levels of O₃ at 1.0, 1.5 and 2.0 times ambient concentration) and three N treatments (0, 20 and 50 kg ha⁻¹ year⁻¹). During the second growing season, no significant interactive effects of O₃ and N load on the growth and net photosynthetic rate of the seedlings were detected. Threrfore, we concluded that N supply to the soil at ≤50 kg ha⁻¹ year⁻¹ does not significantly influence the growth and photosynthetic responses of Q. serrata seedlings to O₃. Based on the O₃ exposure-response relationships for the whole-plant growth of the seedlings, the critical level of O₃ for Q. serrata was estimated to be approximately 36 nmol mol⁻¹ as the average 15-h O₃ concentration during the one growing season.     

Theoretical fluorescence induction curves derived from coupled differential equations describing the primary photochemistry of photosystem II by an exciton-radical pair equilibrium

Fluorescence induction curves were calculated from a molecular model for the primary photophysical and photochemical processes of photosystem II that includes reversible exciton trapping by open (PHQ_A) and closed (PHQ^-_A) reaction centers (RCs), charge stabilization as well as quenching by oxidized (P⁺HQ^(-)_A) RCs. For the limiting case of perfectly connected photosynthetic units (“lake model”) and thermal equilibrium between the primary radical pair (P⁺H^-) and the excited singlet state, the primary reactions can be mathematically formulated by a set of coupled ordinary differential equations (ODE). These were numerically solved for weak flashes in a recursive way to simulate experiments with continuous illumination. Using recently published values for the molecular rate constants, this procedure yielded the time dependence of closed RCs as well as of the fluorescence yield (= fluorescence induction curves). The theoretical curves displayed the same sigmoidal shapes as experimental fluorescence induction curves. From the time development of closed RCs and the fluorescence yield, it was possible to check currently assumed proportionalities between the fraction of closed RCs and either (a) the variable fluorescence, (b) the complementary area above the fluorescence induction curve, or (c) the complementary area normalized to the variable fluorescence. By changing selected molecular rate constants, it is shown that, in contrast to current beliefs, none of these correlations obeys simple laws. The time dependence of these quantities is strongly nonexponential. In the presence of substances that quench the excited state, the model predicts straight lines in Stern-Volmer plots. We further conclude that it is impossible to estimate the degree of physical interunit energy transfer from the sigmoidicity of the fluorescence induction curve or from the curvature of the variable fluorescence plotted versus the fraction of closed RCs.     

Control of the maximal chlorophyll fluorescence yield by the Q<Subscript>B</Subscript> binding site

Differences in maximal yields of chlorophyll variable fluorescence (F_m) induced by single turnover (ST) and multiple turnover (MT) excitation are as great as 40%. Using mutants of Chlamydomonas reinhardtii we investigated potential mechanisms controlling F_m above and beyond the Q_A redox level. F_m was low when the Q_B binding site was occupied by PQ and high when the Q_B binding site was empty or occupied by a PSII herbicide. Furthermore, in mutants with impaired rates of plastoquinol reoxidation, F_m was reached rapidly during MT excitation. In PSII particles with no mobile PQ pool, F_m was virtually identical to that obtained in the presence of PSII herbicides. We have developed a model to account for the variations in maximal fluorescence yields based on the occupancy of the Q_B binding site. The model predicts that the variations in maximal fluorescence yields are caused by the capacity of secondary electron acceptors to reoxidize Q_A^–.     

Kinetics of in vivo bacteriochlorophyll fluorescence yield and the state of photosynthetic apparatus of purple bacteria

The light-induced electron transport in purple bacterium Rhodobacter sphaeroides was studied in vivo by means of kinetic difference absorption spectroscopy and kinetics of bacteriochlorophyll fluorescence yield. Measurements of redox state of the oxidised primary donor and cytochrome c and the membrane potential revealed a complex pattern of changes of the electron flow. Effects of the membrane potential on the fluorescence yield were also analysed, and a model for the fluorescence induction curve is presented. The data indicate substantial positive effect of the membrane potential on the fluorescence emission in vivo. Moreover, light-induced changes in light scattering were observed, which suggests occurrence of structural changes on the level of the photosynthetic membrane.     

Kinetic Mechanisms of Biological Regulation in Photosynthetic Organisms

Principles of regulation on different levels of photosynthetic apparatus are discussed. Mathematical models of isolated photosynthetic reaction centers and general system of energy transduction in chloroplast are developed. A general approach to model these complex metabolic systems is suggested. Regulatory mechanisms in plant cell are correlated with the different patterns of fluorescence induction curve at different internal physiological states of the cells and external (environmental) conditions. Light regulation inside photosynthetic reaction centers, diffusion processes in thylakoid membrane, generation of transmembrane electrochemical potential, coupling with processes of CO₂ fixation in Calvin Cycle are considered as stages of control of energy transformation in chloroplasts in their connection with kinetic patterns of fluorescence induction curves and other spectrophotometric data.     

Interactive effects of light,leaf temperature,CO2 and O2 on photosynthesis in soybean

A biochemical model of C ₃photosynthesis has been developed by G.D. Farquhar et al. (1980, Planta 149, 78–90) based on Michaelis-Menten kinetics of ribulose-1,5-bisphosphate (RuBP) carboxylase-oxygenase, with a potential RuBP limitation imposed via the Calvin cycle and rates of electron transport. The model presented here is slightly modified so that parameters may be estimated from whole-leaf gas-exchange measurements. Carbon-dioxide response curves of net photosynthesis obtained using soybean plants (Glycine max (L.) Merr.) at four partial pressures of oxygen and five leaf temperatures are presented, and a method for estimating the kinetic parameters of RuBP carboxylase-oxygenase, as manifested in vivo, is discussed. The kinetic parameters so obtained compare well with kinetic parameters obtained in vitro, and the model fits to the measured data give r ²values ranging from 0.87 to 0.98. In addition, equations developed by J.D. Tenhunen et al. (1976, Oecologia 26, 89–100, 101–109) to describe the light and temperature responses of measured CO₂-saturated photosynthetic rates are applied to data collected on soybean. Combining these equations with those describing the kinetics of RuBP carboxylase-oxygenase allows one to model successfully the interactive effects of incident irradiance, leaf temperature, CO₂ and O₂ on whole-leaf photosynthesis. This analytical model may become a useful tool for plant ecologists interested in comparing photosynthetic responses of different C₃ plants or of a single species grown in contrasting environments.Abbreviations PCO photorespiratory carbon oxidation - PCR photosynthetic carbon reduction - PPFD photosynthetic photon-flux density - RuBP ribulose bisphosphate     

Different apparent CO2 compensation points in nitrate- and ammonium-grown Phaseolus vulgaris and the relationship to non-photorespiratory CO2 evolution

The classical theory of the relationship between gas fluxes and photosynthetic electron fluxes was extended by two additional terms: J_L describing flux to electron sinks other than the Calvin cycle, and R_L accounting for light-induced changes in non-photorespiratory CO₂ evolution. R_L comprises two main components, R_r resulting from light-induced decrease in tricarboxylic acid activity, and R_S related to extra CO₂ evolution resulting from citrate-to-2-oxoglutarate conversion for N-assimilation in NO₃^– grown leaves. This extended theory was applied to two experiments. First, A–C_i curves (dependence of CO₂ flux on stomatal CO₂ concentration) revealed a higher apparent CO₂ compensation point (Γ*_app) in NO₃^–-grown plants than in NH₄⁺-grown plants. Secondly, photosynthetic electron fluxes at different light intensities were determined by means of the Genty parameter of chlorophyll fluorescence and compared with those calculated from measured CO₂ uptake. Curve-fitting based on the extended theory provided a coincidence of these two measurements and resulted in higher R_S in NO₃^–-grown than in NH₄⁺-grown plants. This difference in R_S (about 15% of the CO₂ flux bound by carboxylation) is the same as that obtained from the analysis of Γ*_app. Further, the analysis suggests that J_L related to the extra electron flux used for N-assimilation in NO₃^–-grown plants is diverted to other sinks in NH₄⁺-grown plants. SHAM decreased photosynthetic electron flow and O₂ evolution in NH₄⁺-grown plants, antimycin A in NO₃^–-grown plants. The effect of oligomycin was small. The results are discussed in terms of different mechanisms of chloroplast/mitochondrion interaction in NO₃^–- and NH₄⁺-grown plants, their effects on non-photorespiratory CO₂ evolution and on Γ*_app.