Steady-state Photosynthesis Research Articles

Effects of cadmium on growth of Helianthus annuus seedlings: physiological aspects

Sunflower seedlings (Helianthus annuus hybrid Select) were grown in a complete nutrient solution in the absence or presence of Cd2+ (10 and 20 μM). Analyses were performed to establish whether there was a differential effect of Cd2+ on mature and young leaves. After 7 d the growth parameters as well as the leaf area had decreased in both mature and young leaves. Accumulation of Cd2+ in the roots exceeded that in the shoots. Seedlings treated with Cd2+ exhibited reduced contents of chlorophyll and CO2 assimilation rate, with a greater decrease in young leaves. The photochemical efficiency of photosystem II (PSII) was not altered by Cd2+ treatment in either mature or young leaves, although during steady‐state photosynthesis in young leaves there was a significant alteration in the following parameters: quantum yield of electron transport by PSII (ΦPSII), photochemical quenching (qP), non‐photochemical quenching (qNP), and excitation capture efficiency of PSII (Φexc).

Effects of Salinity on Chlorophyll Fluorescence and Photosynthesis of Barley (Hordeum Vulgare L.) Grown Under a Triple-Line-Source Sprinkler System in the Field

In flag leaves of four cultivars of barley (Hordeum vulgare L.) grown in the field under a triple-line-source sprinkler system, that produces a linear soil salinity gradient, a decrease in net carbon dioxide assimilation rate (PN) and stomatal conductance for water vapour (gs) was found. These changes were related to salinity tolerance at moderate salinity. With increasing salinity, PN was saturated at low irradiances and stomatal frequencies increased. A decrease in photosystem 2 (PS2) efficiency was not found in the field after dark adaptation even at high salinity. Salinity induced only small decreases in the actual PS2 efficiency at midday steady-state photosynthesis, indicating that the photosynthetic electron transport was little affected by salinity. Therefore, using PS2 efficiency estimates in attached leaves is probably not a useful tool to screen barley genotypes grown under saline conditions in the field for salinity tolerance. In contrast, excised flag leaves from high salinity plots, once in the laboratory, exhibited a decrease in the variable to maximum chlorophyll fluorescence ratio as compared to excised leaves from control plants. On the other hand, the PN rate might allow for a good discrimination between tolerant and non-tolerant cultivars.

Remote and Near-Contact Chlorophyll Fluorescence during Photosynthetic Induction in Iron-Deficient Sugar Beet Leaves

A comparison of time-resolved, laser-induced remote sensing and pulse amplitude modulation near-contact techniques has been carried out during photosynthetic induction in iron-deficient sugar beet leaves. Iron deficiency caused increases in the mean chlorophyll fluorescence lifetime. These increased chlorophyll fluorescence lifetimes occurred in dark-adapted leaves, during a sudden increase in light intensity and also at steady-state photosynthesis. Chlorophyll fluorescence lifetimes were correlated with the extent of nonphotochemical and/or photochemical quenching. During most of the photosynthetic induction period, Fe-deficient leaves showed lower actual PSII efficiencies than control leaves, due to decreases in photochemical quenching and intrinsic PSII efficiency. During photosynthetic induction Fe deficiency decreased the proportion of light absorbed by the PSII antenna that is used in photochemistry and increased the proportion dissipated thermally within the PSII antenna, the later being well correlated with nonphotochemical quenching. Laser instrumentation offers new perspectives for monitoring effects of stress conditions in plants at large spatial scales.

Photosystem II efficiency in low chlorophyll, iron-deficient leaves

Iron deficiency (iron chlorosis) is the major nutritional stress affecting fruit tree crops in calcareous soils in the Mediterranean area. This work reviews the changes in PS II efficiency in iron-deficient leaves. The iron deficiency-induced leaf yellowing is due to decreases in the leaf concentrations of photosynthetic pigments, chlorophylls and carotenoids. However, carotenoids, and more specifically lutein and the xanthophylls of the V+A+Z (Violaxanthin+ Antheraxanthin+Zeaxanthin) cycle are less affected than chlorophylls. Therefore, iron-chlorotic leaves grown in either growth chambers or field conditions have increases in the molar ratios lutein/chlorophyll a and (V+A+Z)/chlorophyll a. These pigment changes are associated to changes in leaf absorptance and reflectance. In the chlorotic leaves the amount of light absorbed per unit chlorophyll increases. The low chlorophyll, iron-deficient leaves showed no sustained decreases in PS II efficiency, measured after dark adaptation, except when the deficiency was very severe. This occurred when plants were grown in growth chambers or in field conditions. However, iron-deficient leaves showed decreases in the actual PS II efficiency at steady-state photosynthesis, due to decreases in photochemical quenching and intrinsic PS II efficiency. Iron-chlorotic leaves were protected not only by the decrease in leaf absorptance, but also by down-regulation mechanisms enhancing non-photochemical quenching and thermal dissipation of the light absorbed by PS II within the antenna pigment bed.

Transgenic potato plants with altered expression levels of chloroplast NADP-malate dehydrogenase: interactions between photosynthetic electron transport and malate metabolism in leaves and in isolated intact chloroplasts

The contribution of the malate valve in the regulation of steady-state photosynthesis was studied in transgenic potato (Solanum tuberosum L. cv Desiree) plants with altered expression of plastidic NADP-dependent malate dehydrogenase (NADP-MDH; EC 1.1.1.82). Mutant plants were obtained after transformation with the homologous Nmdh gene in antisense orientation, or with the Nmdh gene from pea (Pisum sativum L.) in sense orientation. A total number of nine stable sense and antisense lines with 10% or 30%, and 400% of wild-type NADP-MDH capacity were selected. Intact chloroplasts were isolated from leaves of wild-type and mutant plants. In chloroplasts from sense transformants the increased enzyme amount was activated as in wild-type chloroplasts, but increased rates of oxaloacetate-dependent malate formation were only measured upon partial uncoupling. In contrast, chloroplasts from antisense transformants produced only little malate upon oxaloacetate addition. Measurements with intact leaves during steady-state photosynthesis yielded no differences in gas-exchange parameters and chlorophyll fluorescence. The leaf malate content was unchanged in NADP-MDH underexpressors, but twice as high in overexpressing plants. The altered NADP-MDH expression clearly influences the redox state of ferredoxin, especially in low light. Furthermore, the malate valve can successfully compete for electrons with cyclic electron flow, but the conditions under which this occurs are quite artificial.

Modifications in photosystem II photochemistry in senescent leaves of maize plants

Analyses of CO2 exchange and chlorophyll fluorescence were carried out to assess photosynthetic performance during senescence of maize leaves. Senescent leaves displayed a significant decrease in CO2 assimilatory capacity accompanied by a decrease in stomatal conductance and an increase in intercellular CO2 concentration. The analyses of fluorescence quenching under steady-state photosynthesis showed that senescence resulted in an increase in nonphotochemical quenching and a decrease in photochemical quenching. It also resulted in a decrease in the efficiency of excitation energy capture by open PSII reaction centres and the quantum yield of PSII electron transport, but had very little effect on the maximal efficiency of PSII photochemistry. The results determined from the fast fluorescence induction kinetics indicated an increase in the proportion of QB-non-reducing PSII reaction centres and a decrease in the rate of QA reduction in senescent leaves. Theoretical analyses of fluorescence parameters under steady-state photosynthesis suggest that the increase in the non-photochemical quenching was due to an increase in the rate constant of thermal dissipation of excitation energy by PSII and that the decrease in the quantum yield of PSII electron transport was associated with a decrease in the rate constant of PSII photochemistry. Based on these results, it is suggested that the decrease in the quantum yield of PSII electron transport in senescent leaves was downregulated by an increase in the proportion of QB-non-reducing PSII reaction centres and in the non-photochemical quenching. The photosynthetic electron transport would thus match the decreased demand for ATP and NADPH in carbon assimilation which was inhibited significantly in senescent leaves.

Modifications in photosystem II photochemistry in senescent leaves of maize plants

Analyses of CO 2 exchange and chlorophyll fluorescence were carried out to assess photosynthetic performance during senescence of maize leaves. Senescent leaves displayed a significant decrease in CO 2 assimilatory capacity accompanied by a decrease in stomatal conductance and an increase in intercellular CO 2 concentration. The analyses of fluorescence quenching under steady-state photosynthesis showed that senescence resulted in an increase in non-photochemical quenching and a decrease in photochemical quenching. It also resulted in a decrease in the efficiency of excitation energy capture by open PSII reaction centres and the quantum yield of PSII electron transport, but had very little effect on the maximal efficiency of PSII photochemistry. The results determined from the fast fluorescence induction kinetics indicated an increase in the proportion of Q B -non-reducing PSII reaction centres and a decrease in the rate of Q A reduction in senescent leaves. Theoretical analyses of fluorescence parameters under steady-state photosynthesis suggest that the increase in the non-photochemical quenching was due to an increase in the rate constant of thermal dissipation of excitation energy by PSII and that the decrease in the quantum yield of PSII electron transport was associated with a decrease in the rate constant of PSII photochemistry. Based on these results, it is suggested that the decrease in the quantum yield of PSII electron transport in senescent leaves was downregulated by an increase in the proportion of Q B -non-reducing PSII reaction centres and in the non-photochemical quenching. The photosynthetic electron transport would thus match the decreased demand for ATP and NADPH in carbon assimilation which was inhibited significantly in senescent leaves.

Photosynthesizing marine microorganisms can constitute a source of CO2 rather than a sink

The unexpected finding that certain major marine photosynthetic microorganisms can serve as a source of CO2 rather than a sink emerged during measurements of inorganic carbon fluxes associated with the CO2-concentrating mechanism. During steady-state photosynthesis, CO2 was evolved at sustained rates up to 5-fold that of photosynthesis; the steady-state external CO2 concentration reached was significantly higher than that at CO2-HCO3- equilibrium. The evolved CO2 originated from HCO3- taken up and intracellularly converted to CO2 in a light-dependent process. Our results bear implications for carbon cycling in the marine environment; the use of naturally-observed stable carbon isotope fractionations as paleobarometer and productivity probe; and for intracellular energy balance and pH regulation.Key words: carbonic anhydrase, CO2 evolution, cyanobacteria, photosynthesis, Synechococcus.

Photoinhibitory damage is modulated by the rate of photosynthesis and by the photosystem II light-harvesting chlorophyll antenna size.

We investigated the effect of photosynthetic electron transport and of the photosystem II (PSII) chlorophyll (Chl) antenna size on the rate of PSII photoinhibitory damage. To modulate the rate of photosynthesis and the light-harvesting capacity in the unicellular chlorophyte Dunaliella salina Teod., we varied the amount of inorganic carbon in the culture medium. Cells were grown under high irradiance either with a limiting supply of inorganic carbon, provided by an initial concentration of 25 mM NaHCO3, or with supplemental CO2 bubbled in the form of 3% CO2 in air. The NaHCO3-grown cells displayed slow rates of photosynthesis and had a small PSII light-harvesting Chl antenna size (60 Chl molecules). The half-time of PSII photodamage was 40 min. When switched to supplemental CO2 conditions, the rate of photodamage was retarded to a t1/2 = 70 min. Conversely, CO2-supplemented cells displayed faster rates of photosynthesis and a larger PSII light-harvesting Chl antenna size (500 Chl molecules). They also showed a rate of photodamage with t1/2 = 40 min. When depleted of CO2, the rate of photodamage was accelerated (t1/2 = 20 min). These results indicate that the in-vivo susceptibility to photodamage is modulated by the rate of forward electron transport through PSII. Moreover, a large Chl antenna size enhances the rate of light absorption and photodamage and, therefore, counters the mitigating effect of forward electron transport. We propose that under steady-state photosynthesis, the rate of light absorption (determined by incident light intensity and PS Chl antenna size) and the rate of forward electron transport (determined by CO2 availability) modulate the oxidation/reduction state of the primary PSII acceptor QA, which in turn defines the low/high probability for photodamage in the PSII reaction center.

Regulation of Rubisco activation in antisense plants of tobacco containing reduced levels of Rubisco activase

Following an increase in photon flux density (PFD), ribulose bisphosphate carboxylase/oxygenase (Rubisco) undergoes a slow activation which substantially limits the rate of photosynthesis. This activation process is mediated in part by Rubisco activase. Antisense DNA plants of tobacco were used to quantify the degree to which activase limits Rubisco activation. Reductions in leaf activase content caused proportional reductions in the rate of Rubisco activation following a PFD increase from 110 to 1200 micromol m(-2) sec(-1). This was the case for activase levels up to and slightly beyond normal wild-type activase levels. Activase therefore has a flux control coefficient of unity with respect to the Rubisco activation flux. Such a high control coefficient has rarely been measured for any metabolic system, and this is the highest control coefficient measured for an important photosynthetic flux. In contrast, the rate of Rubisco inactivation in leaves following a drop in PFD of 1200 to 110 micromol m(-2) sec(-1) was unchanged by a 60% reduction in activase levels. Despite the high degree of control that activase exerts over the rate of activation, and thus non-steady-state photosynthesis, it was shown that steady-state photosynthesis was largely unaffected by activase concentration until it was reduced below approximately 15% of the wild-type level. The significance of these results and their implications for published models of Rubisco activation are discussed.

Unifying model for the photoinactivation of Photosystem II in vivo under steady-state photosynthesis

We present a unifying mechanism for photoinhibition based on current obsevations from in vivo studies rather than from in vitro studies with isolated thylakoids or PS II membranes. In vitro studies have limited relevance for in vivo photoinhibition because very high light is used with photon exposures rarely encountered in nature, and most of the multiple, interacting, protective strategies of PS II regulation in living cells are not functional. It is now established that the photoinactivation of Photosystem II in vivo is a probability and light-dosage event which depends on the photons absorbed and not the irradiance per se. As the reciprocity law is obeyed and target theory analysis strongly suggests that only one photon is required, we propose that a single dominant molecular mechanism occurs in vivo with one photon inactivating PS II under limiting, saturating or sustained high light. Two mechanisms have been proposed for photoinhibition under high light, acceptor-side and donor-side photoinhibition [see Aro et al. (1994) Biochim Biophys Acta 1143: 113–134], and another mechanism for very low light, the low-light syndrome [Keren et al. (1995) J Biol Chem 270: 806–814]. Based on the exciton-radical pair equilibrium model of exciton dynamics, we propose a unifying mechanism for the photoinactivation of PS II in vivo under steady-state photosynthesis that depends on the generation and maintenance of increased concentrations of the primary radical pair, P680+Pheo−, and the different ways charge recombination is regulated under varying environmental conditions [Anderson et al. (1997) Physiol Plant 100: 214–223]. We suggest that the primary cause of damage to D1 protein is P680+, rather than singlet O2 formed from triplet P680, or other reactive oxygen species.

Photosynthesis, quenching of chlorophyll fluorescence and thermal energy dissipation in iron-deficient sugar beet leaves

In sugar beet (Beta vulgaris L.) iron deficiency decreased not only the photosynthetic rate but also the actual photosystem II efficiency at steady-state photosynthesis. In moderate iron deficiency, the decrease in actual photosystem II efficiency under illumination was related to closure of photosystem II reaction centers, whereas in severe iron deficiency it was associated to decreases of intrinsic photosystem II efficiency. The O2 evolution, on an absorbed light basis, decreased more than the actual photosystem II efficiency, suggesting the presence of a significant fraction of electron transport to molecular oxygen or the existence of some form of cyclic electron flow. Iron-deficient leaves reduced the excess of light absorbed that cannot be used in photosynthesis not only by decreasing absorptance, but also by dissipating a large part of the light absorbed by the photosystem II antenna. This mechanism, that protects the photosystem II reaction centers through the enhancement of energy dissipation, was related to the de-epoxidation of violaxanthin (V) to antheraxanthin (A) and zeaxanthin (Z) in iron-deficient leaves. These data provide additional support for a role of Z+A in photoprotection under conditions of excess photosynthetic light absorption.

Simulation of C1fluxes in cyanobacteria: comparison between the carboxysome hypothesis and the equilibrium proposal

Inorganic carbon fluxes were simulated by a mathematical model using an equilibrium hypothesis for a wide range of conditions in a closed system composed of air-grown cells of Synechococcus UTEX 625 in a reaction vessel connected to a mass spectrometer. The metabolic scheme took into account the input fluxes of CO2and HCO3−transport into the cells, the output fluxes of CO2and HCO3−efflux, the diversion of Q toward the formation of the internal C2pool, and photosynthetic CO2fixation. The equations expressed the variation in concentration of each inorganic species outside and inside the cell as a function of time. The input fluxes were previously characterized by their kinetic constants (K1/2and Vm) both during initial uptake occurring upon illumination of the cells and under steady-state photosynthesis conditions. The efflux rates of the various Cispecies from the cells were investigated under a wide variety of experimental conditions. Using these efflux rates, the permeability coefficients of the cell for CO2and HCO3−were calculated previously. Using the kinetic constants for CO2and HCO3−transport, the permeability coefficients of the cell for CO2and HCO3−and the geometrical characteristics of the cells, the model simulated precisely the [HCO3−]/[CO2] ratio and the [CO2] and [O2] changes in the extracellular medium as well as the rate of filling of the internal Cipool under various conditions. Accurate fitting of experimental data with calculated values were possible only when the intracellular Cispecies were assumed to be in equilibrium throughout the entire cell volume. Results are discussed and compared with those given by previous hypotheses. Key words: Synechococcus UTEX 625, blue green algae, cyanobacteria, mathematical model, active CO2transport, active HCO3−transport, steady state, photosynthesis, Ciconcentrating mechanism.

Influx and efflux of inorganic carbon during steady-state photosynthesis of air-grownAnabaena variabilis

The inward and outward fluxes of inorganic carbon in illuminated cell suspensions of air-grown Anabaena variabilis were measured by mass spectrometry under conditions of inorganic carbon disequilibrium. The inclusion of 25 mM NaCl significantly enhanced both inward inorganic carbon influx during CO2fixation and outward CO2efflux when CO2fixation was blocked by the Calvin cycle inhibitor, iodoacetamide. At low, steady-state concentrations of inorganic carbon (< 100μM), CO2fixation was nearly entirely supported by HCO3−transport in the presence of 25 mM NaCl. At approximately 150 μM inorganic carbon, the contributions of CO2and HCO3−transport to CO2fixation were about equal. Above this, CO2transport provided most of the substrate for CO2fixation. The affinity (K0.5) of photosynthesizing cells for CO2, HCO3−and total inorganic carbon was determined and mean values of 1.7, 9.5, and 8.2 μM, respectively, were determined. Maximum rates of inward CO2and HCO3−transport and CO2fixation during steady state were 255.7, 307.3, and 329.1 μmol∙mg−1Chl∙h−1, respectively. Permeability coefficients for CO2of 9.8 × 10−8 m∙s−1and 2.8 × 10−7 m∙s−1were calculated for the plasma membrane and carboxysomal surface areas, respectively, from the dark efflux rates assuming an internal pH of 7.2. A permeability coefficient for HCO3−across the plasma membrane of 7.6 × 10−9 m∙s−1was calculated from the dark inorganic carbon efflux corrected for the corresponding dark CO2efflux. Sodium sulphide (Na2S, 200 μM) blocked CO2transport. In the presence of 25 mM NaCl, net CO2efflux was approximately seven times greater than in its absence, when CO2transport and fixation were both blocked, indicating greater CO2leakage as a result of larger internal inorganic carbon pools in the presence of NaCl. The rapidity and amount of C16O2generated from the exchange of18O from18O-enriched HCO3−with water in cell suspensions suggested that the internal inorganic carbon pool may be rapidly equilibrated. Key words: Anabaena variabilis, CO2-concentrating mechanism, CO2transport, HCO3−transport, CO2efflux, permeability coefficient.

Effects of manganese toxicity on photosynthesis of white birch (Betula platyphylla var. japonica) seedlings

The effects of manganese (Mn) toxicity on photosynthesis in white birch (Betula platyphylla var. japonica) leaves were examined by the measurement of gas exchange and chlorophyll fluorescence in hydroponically cultured plants. The net photosynthetic rate at saturating light and ambient CO2 (Ca) of 35 Pa decreased with increasing leaf Mn concentrations. The carboxylation efficiency, derived from the difference in CO2 assimilation rate at intercellular CO2 pressures attained at Ca of 13 Pa and O Pa, decreased with greater leaf Mn accumulation. Net photosynthetic rate at saturating light and saturating CO2 (5%) also declined with leaf Mn accumulation while the maximum quantum yield of O2 evolution at saturating CO2 was not affected. The maximum efficiency of PSII photochemistry (Fv/Fm) was little affected by Mn accumulation in white birch leaves over a wide range of leaf Mn concentrations (2–17 mg g−1 dry weight). When measured in the steady state of photosynthesis under ambient air at 430 μmol quanta m−2 s−1, the levels of photochemical quenching (qP) and the excitation capture efficiency of open PSII (F′v/F′m) declined with Mn accumulation in leaves. The present results suggest that excess Mn in leaves affects the activities of the CO2 reduction cycle rather than the potential efficiency of photochemistry, leading to increases in QA reduction state and thermal energy dissipation, and a decrease in quantum yield of PSII in the steady state.

Analysis of limitations to CO2 assimilation on exposure of leaves of two Brassica napus cultivars to UV‐B

ABSTRACTApex and Bristol cultivars of oilseed rape (Brassica napus) were irradiated with 0.63 W m−2 of UV‐B over 5 d. Analyses of the response of net leaf carbon assimilation to intercellular CO2 concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5‐bis‐phosphate on leaf photosynthesis. Simultaneous measurements of chlorophyll fluorescence were used to estimate the maximum quantum efficiency of photosystem II (PSII) photochemistry, the quantum efficiency of linear electron transport at steady‐state photosynthesis, and the light and CO2‐saturated rate of linear electron transport. Ribulose 1,5‐bisphosphate carboxylase/oxygenase (Rubisco) content and activities were assayed in vitro. In both cultivars the UV‐B treatment resulted in decreases in the light‐saturated rate of CO2 assimilation, which were accompanied by decreases in carboxylation velocity and Rubisco content and activity. No major effects of UV‐B were observed on end‐product inhibition and stomatal limitation of photosynthesis or the rate of photorespiration relative to CO2 assimilation. In the Bristol cultivar, photoinhibition of PSII and loss of linear electron transport activity were observed when CO2 assimilation was severely inhibited. However, the Apex cultivar exhibited no major inhibition of PSII photochemistry or linear electron transport as the rate of CO2 assimilation decreased. It is concluded that loss of Rubisco is a primary factor in UV‐B inhibition of CO2 assimilation.

HCO3−efflux and the regulation of the intracellular Cipool size inSynechococcusUTEX 625

Cyanobacterial cells of Synechococcus UTEX 625 were suspended in increasing extracellular inorganic carbon (Ci, up to 1 mM) under steady state photosynthesis conditions in a cuvette connected to a mass spectrometer. Photosynthetic CO2fixation, net CO2, and net HCO3−transport increased with increased extracellular Ciup to 100 μM [C1], with HCO3−transport contributing the most to the maintenance of the internal Cipool. When the extracellular [Ci] was increased above 100 μM, CO2, fixation and net CO2transport increased toward their maximum rates but net HCO3−transport progressively declined. At an extracellular [Ci] of 1 mM, the net HCO3−transport rate was only 15% of its value at 100 μM extracellular [Ci]. Under steady state photosynthesis conditions, the gross HCO3−transport rate remained at its maximum value at extracellular [Ci] above 100 μM either when CO2transport was precluded (i.e., in the presence of 200 μM Na2S) or when HCO3−transport was measured with H14CO3−and silicone oil filtration. As HCO3−efflux, calculated using the permeability coefficient determined at low [Ci], could not account fully for the decrease of the net HCO3−transport rate, Ciefflux and CO2, efflux were measured at high extracellular [Ci] during the initial period of darkness. With increasing intracellular [Ci] Ciefflux over intracellular [Ci] increased linearly but CO2, efflux over intracellular [Ci] was constant, hence the HCO3−efflux obtained by the difference between the Ciand the CO2efflux increased hyperbolically. Using HCO3−and CO2, efflux, we calculated that the permeability coefficient of the cell envelope for HCO3−but not for CO2increased with increasing intracellular [Ci]. When the HCO3−efflux was measured under steady state Ciflux conditions in the light, the permeability coefficient of the cell envelope for HCO3−also increased with increasing intracellular [Ci]. Reasons for the change in whole cell permeability to HCO3−at high extracellular [Ci] are discussed. Key words: Synechococcus UTEX 625, blue green algae, cyanobacteria, active HCO3−transport, HCO3−leakage, Ciconcentrating mechanism.

Regulation of Steady-State Photosynthesis in Isolated Intact Chloroplasts under Constant Light: Responses of Carbon Fluxes, Metabolite Pools and Enzyme-Activation States to Changes of Electron Pressure

2Lehrstuhl fur Pflanzenphysiologie, Humboldt-Universitdt, D-10099 Berlin, Germany The reactions of isolated intact spinach chloroplasts at saturating light and CO2 to changes in steady-state electron flow were followed at the various stages of photosynthesis. Alterations in the rate of electron flow were induced by the addition of oxaloacetate (OAA), nitrite or methyl viologen (MV). Two types of effect can be distinguished: (1) When a small fraction of the electrons produced are accepted by OAA or nitrite (up to 20% of the electrons produced in the light), the activation state of the NADP + -dependent malate dehydrogenase (NADP-MDH) was strongly decreased, whereas qP and the rate of O2-production were increased. qN, the stromal metabolite pools and the [ 14 C]CO2-fixation rate were only marginally influenced. (2) Higher amounts of nitrite or MV decreased O2 production and strongly inhibited [ 14 C]CO2 fixation. This treatment further increased the ATP/ADP ratio, but had little effect on the NADPH + H + /NADP + ratio. The stromal concentrations of 3PGA, DHAP and FBP, and the rates of 3PGA and DHAP export were drastically changed. In particular, the DHAP/3PGA ratio increased, and the rate of 3PGA export was decreased by minor changes in the rate of electron flow. Addition of high amounts of nitrite or MV, but not of OAA decreased the activation states of NADP-MDH and fructose 1,6-bisphosphatase (FBPase), while the activation states of NADP + -dependent glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and phosphoribulokinase (PRK) remained unchanged under all conditions.

Photosynthesis in the basal growing zone of barley leaves

Cell proliferation, elongation, determination and differentiation mainly take place in the basal 5 mm of a barley leaf, the so-called basiplast. A considerable portion of cDNAs randomly selected from a basiplast cDNA library represented photosynthetic genes such as CP29, RUBISCO-SSU and type I-LHCP II. Therefore, we became interested in the role of the basiplast in establishing photosynthesis. (1) Northern blot analysis revealed expression of photosynthetic genes in the basiplast, although at a low level. Analysis of basiplasts at different developmental stages of the leaves revealed maximal expression of photosynthetic genes during early leaf development. The activity of these genes shows that plastid differentiation involves the development of the photosynthetic apparatus even at this early state of leaf cell expansion. (2) This conclusion was supported by the fact that chlorophylls and carotenoids are synthesized in the basiplast. The qualitative pattern of pigment composition was largely similar to that of fully differentiated green leaves. (3) The transition from proplastids to chloroplasts progressed in the basal 5 mm of the leaf, so that the number of grana lamellae per thylakoid stack increased with distance from the meristem from zero to about five. (4) Photosynthetic function was studied by chlorophyll a-fluorescence measurements. In dark-adapted 8-day-old primary leaves, the fluorescence ratio (FP-Fo)/FP was little decreased in basiplasts as compared to leaf blades. During steady state photosynthesis, the ratio (FM'-Fo)/FM' was high in leaf blade (0.5), but low in the sheath (0.25) and in the basiplast (0.18), indicating the existence of functional, albeit low light-adapted chloroplasts in the basiplast. (5) Further on, chlorophyll a fluorescence analysis in relation to seedling age revealed efficient photosynthetic performance in the basiplast of 3- to 6-day-old seedlings which later-on differentiates into leaf blade as compared to the basiplast of 7- to 12-day-old seedlings which develops into leaf sheath and finally ceases to grow. The leaf age dependent changes in basiplast photosynthesis were reflected by changes in pigment contents and LHCP II expression both of which also revealed a maximum in the basiplast of 4-day-old seedlings.

A mechanistic model for submerged aquatic macrophyte photosynthesis: Hydrilla in ambient and elevated CO 2

There are significant knowledge gaps about the responses of submerged aquatic macrophytes to CO 2 enrichment and global warming. A mechanistic steady-state photosynthesis model for submerged aquatic macrophytes was developed to provide an analysis tool to investigate the responses of plant photosynthesis to CO 2, temperature and light. The model was based upon a general simplified scheme for inorganic carbon assimilation of submerged aquatic macrophytes which integrated the knowledge about aquatic plant photosynthesis from previous research, mainly on Hydrilla. The model includes: (1) diffusion and/or active transfer of inorganic carbon (CO 2 and/or HCO − 3) in the bathing medium into the leaf mesophyll and cytosol; (2) diffusion and/or ‘pumping’ of CO 2 through the PEPcase-related C 4 pathway into the chloroplast; (3) inter-conversions between CO 2 and HCO − 3 inside cells; (4) photosynthetic carbon reduction cycle (PCR) in the chloroplast. In the model, the PCR processes in the chloroplast were described using the widely accepted C 3 photosynthesis model. The activity of the C 4 cycle was related to environmental CO 2 ‘stress’. In this way, the model can simulate the shift between C 3-like and C 4-like photosynthesis under different environmental conditions. The model was validated using gas exchange data from Hydrilla plants grown in ambient and elevated CO 2. The model predicted quite well photosynthetic responses to incident PAR, temperature and ambient CO 2 for both ambient and elevated atmospheric CO 2 treatments. Model predictions agreed well with measured Hydrilla gas exchange data. The simulated and measured CO 2 compensation points of Hydrilla leaf photosynthesis were about 100 ppm. The light compensation point of photosynthesis was about 25 μ mol m −2 s −1 (PAR), and photosynthesis rate was saturated at about 100 μ mol m −2 s −1 (PAR). Higher pH slightly increased photosynthesis rates at ambient CO 2 (∼350 ppm). There was no significant acclimation of Hydrilla photosynthesis to elevated CO 2 within the experimental period. Simulated CO 2 compensation point decreased with increasing activity of C 4-cycle processes.