Ribulose-1,5-bisphosphate Carboxylation Research Articles

The Role of Electron Transport in Determining the Temperature Dependence of the Photosynthetic Rate in Spinach Leaves Grown at Contrasting Temperatures

The temperature response of the uncoupled whole-chain electron transport rate (ETR) in thylakoid membranes differs depending on the growth temperature. However, the steps that limit whole-chain ETR are still unclear and the question of whether the temperature dependence of whole-chain ETR reflects that of the photosynthetic rate remains unresolved. Here, we determined the whole-chain, PSI and PSII ETR in thylakoid membranes isolated from spinach leaves grown at 30 degrees C [high temperature (HT)] and 15 degrees C [low temperature (LT)]. We measured temperature dependencies of the light-saturated photosynthetic rate at 360 microl l(-1) CO2 (A360) in HT and LT leaves. Both of the temperature dependences of whole-chain ETR and of A360 were different depending on the growth temperature. Whole-chain ETR was less than the rates of PSI ETR and PSII ETR in the broad temperature range, indicating that the process was limited by diffusion processes between the PSI and PSII. However, at high temperatures, whole-chain ETR appeared to be limited by not only the diffusion processes but also PSII ETR. The C3 photosynthesis model was used to evaluate the limitations of A360 by whole-chain ETR (Pr) and ribulose bisphosphate carboxylation (Pc). In HT leaves, A360 was co-limited by Pc and Pr at low temperatures, whereas at high temperatures, A360 was limited by Pc. On the other hand, in LT leaves, A360 was solely limited by Pc over the entire temperature range. The optimum temperature for A360 was determined by Pc in both HT and LT leaves. Thus, this study showed that, at low temperatures, the limiting step of A360 was different depending on the growth temperature, but was limited by Pc at high temperatures regardless of the growth temperatures.

Manipulation of light and CO2 environments of the primary leaves of bean (Phaseolus vulgaris L.) affects photosynthesis in both the primary and the first trifoliate leaves: involvement of systemic regulation

Possible involvement of systemic regulation of the photosynthetic properties of young leaves by the local environments and/or photosynthate production of the mature leaves were examined using Phaseolus vulgaris plants. When primary leaves (PLs) were treated with air containing 150 microL CO2 L(-1) with the other plant parts in ambient air at a photosynthetic photon flux density (PPFD) of 300 micromol photon m(-2) s(-1), decreases in the photosynthetic rate measured at 360 microL CO2 L(-1) and a PPFD of 300 micromol photon m(-2) s(-1) (A360) were markedly retarded in both PLs and the first trifoliate leaves (TLs) as compared to plants treated with 400 microL CO2 L(-1). Conversely, when PLs were treated with 1000 microL CO2 L(-1), decreases in A360 were accelerated in both PLs and TLs. Shading of PLs accelerated the decrease in PL A360, and delayed the decrease in TLs. In the CO2 treatments, changes in A360 in TLs were mainly attributed to the changes in ribulose bisphosphate (RuBP) carboxylation rate, while the shading of PLs caused increases in both the RuBP carboxylation and regeneration rates in TLs. The ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco) activity on chlorophyll basis, an indicator of sun/shade acclimation, differed both among PLs and among TLs in accordance with the redox state of photosystem II (PSII) in PLs. Although carbohydrate contents of TLs were not affected by any manipulation of PLs, changes in the photosynthetic capacities of TLs acted to compensate for changes in PL photosynthesis. These results clearly indicate that the CO2 and shade treatments of PLs not only affect photosynthetic properties of the PLs themselves, but also systemically affected the photosynthetic properties of TLs. Possible roles of the redox state and photosynthate concentration in PLs in regulation of photosynthesis in PLs and TLs are discussed.

Seasonal changes in the temperature response of photosynthesis in canopy leaves of Quercus crispula in a cool-temperate forest

Understanding seasonal changes in photosynthetic characteristics of canopy leaves is indispensable for modeling the carbon balance in forests. We studied seasonal changes in gas exchange characteristics that are related to the temperature dependence of photosynthesis in canopy leaves of Quercus crispula Blume, one of the most abundant species in cool-temperate forests in Japan. Photosynthetic rate and ribulose-1,5-bisphosphate (RuBP) carboxylation capacity (V(cmax)) at 20 degrees C increased from June to August and then decreased in September. The activation energy of V(cmax), a measure of the temperature dependence of V(cmax), was highest in summer, indicating that V(cmax) was most sensitive to leaf temperature at this time. The activation energy of V(cmax) was significantly correlated with growth temperature. Other parameters related to the temperature dependence of photosynthesis, such as intercellular CO(2) partial pressure and temperature dependence of RuBP regeneration capacity, showed no clear seasonal trend. It was suggested that leaf senescence affected the balance between carboxylation and regeneration of RuBP. The model simulation showed that photosynthetic rate and its optimal temperature were highest in summer.

Physiological basis of seasonal trend in leaf photosynthesis of five evergreen broad-leaved species in a temperate deciduous forest

The physiological basis of photosynthesis during winter was investigated in saplings of five evergreen broad-leaved species (Camellia japonica L., Cleyera japonica Thunb., Photinia glabra (Thunb.) Maxim., Castanopsis cuspidata (Thunb.) Schottky and Quercus glauca Thunb.) co-occurring under deciduous canopy trees in a temperate forest. We focused on temperature dependence of photosynthetic rate and capacity as important physiological parameters that determine light-saturated rates of net photosynthesis at low temperatures during winter. Under controlled temperature conditions, maximum rates of ribulose bisphosphate carboxylation and electron transport (Vcmax) and Jmax, respectively) increased exponentially with increasing leaf temperature. The temperature dependence of photosynthetic rate did not differ among species. In the field, photosynthetic capacity, determined as Vcmax and Jmax at a common temperature of 25 degrees C (Vcmax(25) and Jmax(25)), increased until autumn and then decreased in species-specific patterns. Values of Vcmax(25) and Jmax(25) differed among species during winter. There was a positive correlation of Vcmax(25) with area-based nitrogen concentration among leaves during winter in Camellia and Photinia. Interspecific differences in Vcmax(25) were responsible for interspecific differences in light-saturated rates of net photosynthesis during winter.

Seasonal changes in temperature dependence of photosynthetic rate in rice under a free-air CO(2) enrichment.

Influences of rising global CO(2) concentration and temperature on plant growth and ecosystem function have become major concerns, but how photosynthesis changes with CO(2) and temperature in the field is poorly understood. Therefore, studies were made of the effect of elevated CO(2) on temperature dependence of photosynthetic rates in rice (Oryza sativa) grown in a paddy field, in relation to seasons in two years. Photosynthetic rates were determined monthly for rice grown under free-air CO(2) enrichment (FACE) compared to the normal atmosphere (570 vs 370 micromol mol(-1)). Temperature dependence of the maximum rate of RuBP (ribulose-1,5-bisphosphate) carboxylation (V(cmax)) and the maximum rate of electron transport (J(max)) were analysed with the Arrhenius equation. The photosynthesis-temperature response was reconstructed to determine the optimal temperature (T(opt)) that maximizes the photosynthetic rate. There was both an increase in the absolute value of the light-saturated photosynthetic rate at growth CO(2) (P(growth)) and an increase in T(opt) for P(growth) caused by elevated CO(2) in FACE conditions. Seasonal decrease in P(growth) was associated with a decrease in nitrogen content per unit leaf area (N(area)) and thus in the maximum rate of electron transport (J(max)) and the maximum rate of RuBP carboxylation (V(cmax)). At ambient CO(2), T(opt) increased with increasing growth temperature due mainly to increasing activation energy of V(cmax). At elevated CO(2), T(opt) did not show a clear seasonal trend. Temperature dependence of photosynthesis was changed by seasonal climate and plant nitrogen status, which differed between ambient and elevated CO(2).

Temperature acclimation of photosynthesis: mechanisms involved in the changes in temperature dependence of photosynthetic rate

Growth temperature alters temperature dependence of the photosynthetic rate (temperature acclimation). In many species, the optimal temperature that maximizes the photosynthetic rate increases with increasing growth temperature. In this minireview, mechanisms involved in changes in the photosynthesis-temperature curve are discussed. Based on the biochemical model of photosynthesis, change in the photosynthesis-temperature curve is attributable to four factors: intercellular CO2 concentration, activation energy of the maximum rate of RuBP (ribulose-1,5-bisphosphate) carboxylation (Vc max), activation energy of the rate of RuBP regeneration (Jmax), and the ratio of Jmax to Vc max. In the survey, every species increased the activation energy of Vc max with increasing growth temperature. Other factors changed with growth temperature, but their responses were different among species. Among these factors, activation energy of Vc max may be the most important for the shift of optimal temperature of photosynthesis at ambient CO2 concentrations. Physiological and biochemical causes for the change in these parameters are discussed.

Nitrogen Partitioning in the Photosynthetic Apparatus of Plantago asiatica Leaves Grown Under Different Temperature and Light Conditions: Similarities and Differences Between Temperature and Light Acclimation

Effects of growth temperature and irradiance on nitrogen partitioning among photosynthetic components were studied. Plantago asiatica was grown under different temperature and light conditions. Growth conditions were regulated such that the Chl a/b ratio in leaves grown at a low temperature with a low irradiance was similar to that in leaves grown at a high temperature with a high irradiance, suggesting that the balance between acquisition and utilization of light energy in the photosynthetic apparatus was similar between the two growth conditions. When plotted against the leaf nitrogen content, the RuBP (ribulose-1,5-bisphosphate) carboxylase content did not significantly differ depending on growth conditions. Both high irradiance and low temperature decreased nitrogen partitioning to Chl-protein complexes. Low temperature increased nitrogen allocation to stroma FBPase (fructose-1,6-phosphatase) irrespective of growth irradiance. Gas exchange measurement indicated that the ratio of the electron transport (J(max)) to the maximum carboxylation rate (V(cmax)) was not affected by growth irradiance but by growth temperature. It is concluded that nitrogen partitioning between acquisition and utilization of light energy responds to both growth temperature and irradiance, while nitrogen partitioning between carboxylation and regeneration of RuBP responds only to growth temperature.

ANALYSIS OF THE TEMPERATURE DEPENDENCE OF CO2 ASSIMILATION RATE (STUDY CASE: GLYCINE MAXL. MERR)

The maximum rate of carboxylation (Kcmax) and maximum rate of regeneration of Ribulose bisphosphate (RuBP) (controlled by the rate of electron transport, Jmax) arc two processes governing the photosynthetic capacity of plants. Both processes are affected by temperature. This paper examines how the response of these two photosynthetic capacities to temperature determines the temperature response curve of the CO2-assimilation rate for plants grown at different temperatures, by using the concept of the Farquhar €3 photosynthesis model. The goal is to use photosynthetic parameters from CO2 and light curves to predict the temperature dependence of the CO2-assimilation rate (A) of soybean and to estimate the preferred growth temperature. Analysis shows that the optimum temperature of the assimilation rate changes with the changing temperature dependence of carboxylation and regeneration of RuBP. Key words : temperature dependence/soybean/modeling photosynthesis/preferred growth temperature.

Seasonal change in the balance between capacities of RuBP carboxylation and RuBP regeneration affects CO2 response of photosynthesis in Polygonum cuspidatum

The balance between the capacities of RuBP (ribulose-1,5-bisphosphate) carboxylation (V(cmax)) and RuBP regeneration (expressed as the maximum electron transport rate, J(max)) determines the CO(2) dependence of the photosynthetic rate. As it has been suggested that this balance changes depending on the growth temperature, the hypothesis that the seasonal change in air temperature affects the balance and modulates the CO(2) response of photosynthesis was tested. V(cmax) and J(max) were determined in summer and autumn for young and old leaves of Polygonum cuspidatum grown at two CO(2) concentrations (370 and 700 micromol mol(-1)). Elevated CO(2) concentration tended to reduce both V(cmax) and J(max) without changing the J(max):V(cmax) ratio. The seasonal environment, on the other hand, altered the ratio such that the J(max):V(cmax) ratio was higher in autumn leaves than summer leaves. This alternation made the photosynthetic rate more dependent on CO(2) concentration in autumn. Therefore, when photosynthetic rates were compared at growth CO(2) concentration, the stimulation in photosynthetic rate was higher in young-autumn than in young-summer leaves. In old-autumn leaves, the stimulation of photosynthesis brought by a change in the J(max):V(cmax) ratio was partly offset by accelerated leaf senescence under elevated CO(2). Across the two seasons and the two CO(2) concentrations, V(cmax) was strongly correlated with Rubisco and J(max) with cytochrome f content. These results suggest that seasonal change in climate affects the relative amounts of photosynthetic proteins, which in turn affect the CO(2) response of photosynthesis.

The balance between RuBP carboxylation and RuBP regeneration: a mechanism underlying the interspecific variation in acclimation of photosynthesis to seasonal change in temperature.

The ratio of the capacities of ribulose-1,5-bisphosphate (RuBP) regeneration to RuBP carboxylation (Jmax / Vcmax) (measured at a common temperature) increases in some species when they are grown at lower temperatures, but does not increase in other species. To investigate the mechanism of interspecific difference in the response of Jmax / Vcmax to growth temperature, we analysed the temperature dependence of Vcmax and Jmax in Polygonum cuspidatum and Fagus crenata with the Arrhenius function. P. cuspidatum had a higher ratio of Jmax / Vcmax in spring and autumn than in summer, while F. crenata did not show such change. The two species had a similar activation energy for Vcmax (EaV) across seasons, but P. cuspidatum had a higher activation energy for Jmax (EaJ) than F. crenata. Reconstruction of the temperature response curve of photosynthesis showed that plants with an inherently higher EaJ / EaV (P. cuspidatum) had photosynthetic rates that were limited by RuBP regeneration at low temperatures and limited by RuBP carboxylation at high temperatures, while plants with an inherently lower EaJ / EaV (F. crenata) had photosynthetic rates that were limited solely by RuBP carboxylation over the range of temperatures. These results indicate that the increase in Jmax / Vcmax at low growth temperatures relieved the limitation of RuBP regeneration on the photosynthetic rate in P. cuspidatum, but that such change in Jmax / Vcmax would not improve the photosynthetic rate in F. crenata. We suggest that whether or not the Jmax / Vcmax ratio changes with growth temperature is attributable to interspecific differences in EaJ / EaV between species.

Rubisco: structure, regulatory interactions, and possibilities for a better enzyme.

Ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco) catalyzes the first step in net photosynthetic CO2 assimilation and photorespiratory carbon oxidation. The enzyme is notoriously inefficient as a catalyst for the carboxylation of RuBP and is subject to competitive inhibition by O2, inactivation by loss of carbamylation, and dead-end inhibition by RuBP. These inadequacies make Rubisco rate limiting for photosynthesis and an obvious target for increasing agricultural productivity. Resolution of X-ray crystal structures and detailed analysis of divergent, mutant, and hybrid enzymes have increased our insight into the structure/function relationships of Rubisco. The interactions and associations relatively far from the Rubisco active site, including regulatory interactions with Rubisco activase, may present new approaches and strategies for understanding and ultimately improving this complex enzyme.

CARBON ISOTOPE EFFECTS (13C/12C) IN BIOLOGICAL SYSTEMS

Carbon isotope (13C/12C) fractionation (CIF) occurring in cells of organisms of different types is considered. Three metabolic points are shown to have an exceptional significance for the observed carbon isotope distribution in a living matter: 1) ribulose bisphosphate carboxylation in CO2 photoassimilation; 2) glycine decarboxylation in photorespiration; and 3) pyruvate decarboxylation in respiration metabolism. Carbon isotope effects (CIE) in ribulose bisphosphate (RuBP) carboxylation is characteristic of photosynthesizing organisms. It is responsible for the observed 12C enrichment of photosynthesizing organism biomass relative to environmental CO2. According to proposed mechanism of CIF, CO2 assimilation is a discrete process, which can be described as a flow of CO2 batches into a cell. Discreteness of the flow arises due to ability of Rubisco (the key enzyme of photosynthesis) periodically operate both as carboxylase and oxygenase depending on CO2/O2 ratio in a cell. It provides periodic filling/depletion of cytoplasmic pool of CO2 in the cell and determines the appearance of isotope Releigh effect following the pool depletion. The degree of pool depletion determines the extent of 12C enrichment of biomass. A full consumption of CO2 batches entering the cell means the complete depletion of CO2 cytoplasmic pool and is considered to be the main reason for the observed 13C enrichment of C4 plants compared with C3 and CAM plants. CIE in glycine decarboxylation is the key element of CIF in photorespiration. The effect of the reaction is also amplified by the Releigh effect of photorespiratory pool depletion. The removal of CO2 enriched in 12C results in 13C enrichment of photorespiratory substrates and biomass itself. Thus the sign of CIE of photorespiration is opposite that of CO2 photoassimilation. Some factors inducing intense photorespiration can even cause 13C enrichment of biomass in respect to ambient CO2 when CIE of photorespiration prevails that of CO2 photoassimilation. Due to the coupling of photorespiration and CO2 photoassimilation via double function of Rubisco both CIE are in reciprocal relations to each other and generate two isotopically different carbon flows in photosynthesizing cells. CIE effect in pyruvate decarboxylation is responsible for the appearance of carbon isotope heterogeneity in respiration metabolism (isotopic discrepancies of fractions, metabolites and uneven intramolecular 13C distribution of biomolecules). The reaction is a crosspoint of the central metabolic pathways. It is typical of the overwhelming majority of the organisms regardless of their position on the evolutionary stairs. The mechanism of CIE is the combination of the effect in the enzymatic reaction with Releigh effect of pyruvate pool depletion. The reaction provides C2 and C3 structural units for biosyntheses of most of the cell components whose carbon isotope ratio changes in parallel with the pool depletion determining the differences in carbon isotope composition of metabolites. At a certain energy state of the organism and the corresponding energy states of the cells, the synthesis of each component in cell cycles occurs synphasically, i.e., within the same interval of pyruvate pool depletion. In such a way a certain isotopic pattern of metabolites and 13C distribution in biomass are provided. Some applications of the technique to study plants, highly organized animals and humans are shown.

Photosynthetic characteristics of Flindersia brayleyana and Castanospermum australe from tropical lowland and upland sites.

Photosynthetic responses to temperature, light and carbon dioxide partial pressure were studied in two-year-old Flindersia brayleyana F. Muell. and Castanospermum australe Cunn. & C. Fraser ex Hook. growing on coastal lowland and upland rainforest sites in tropical Queensland, Australia. Climatic conditions ranged from moist and cool (17-19 degrees C) to dry and warm (22-24 degrees C). The optimum temperature for photosynthesis was 23.7-25.6 degrees C for C. australe and 21.2-24.6 degrees C for F. brayleyana. Mean maximum rate of electron transport for each species did not differ between sites but was higher (60-62 &mgr;mol m(-2) s(-1)) in F. brayleyana than in C. australe (42-44 &mgr;mol m(-2) s(-1)). Ribulose-bisphosphate carboxylation rate did not differ significantly between sites or species. Maximum rates of photosynthesis at 1000 &mgr;Pa Pa(-1) CO(2) did not differ significantly between sites for each species, but did differ significantly between species. At 350 &mgr;Pa Pa(-1) CO(2), photosynthetic light use efficiencies of F. brayleyana and C. australe were 0.05 and 0.015, respectively, at the upland site, and the corresponding values at the lowland site were 0.025 and 0.05. In C. australe, these differences were reflected in significantly greater maximum rates of photosynthesis at 350 &mgr;Pa Pa(-1) CO(2) at the lowland site than at the upland site (5.2 versus 3.3 &mgr;mol m(-2) s(-1)).

Short-term feedback inhibition of photosynthesis in wheat leaves supplied with sucrose and glycerol at two temperatures

The inhibition of photosynthesis by reduced sink demand or low rates of end product synthesis was investigated by supplying detached wheat (Triticum aestivum L. cv. Tauro) leaves with 50 mM sucrose, 50 mM glycerol or water through the transpiration stream for 2 h, either at 23 or 12 °C. Lowering the temperature and sucrose and glycerol feeding decreased photosynthetic oxygen evolution at high irradiance and saturating CO2. The decrease in temperature reduced the pools of sucrose and starch, and the ratio glucose 6-phosphate (G6P)/fructose 6-phosphate (F6P), while it increased the concentrations of G6P and F6P (hexose phosphates). Sucrose feeding, in contrast to glycerol feeding, increased sucrose, glucose and fructose contents and the G6P/F6P ratio. Sucrose and glycerol incubations at 23 °C, as well as decreasing the temperature in leaves incubated in water, increased the concentration of triose-phosphates (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, TP) and decreased the glycerate 3-phosphate (PGA) content, thus increasing the TP/PGA ratio; they also tended to increase the ribulose 1,5-bisphosphate (RuBP) content and the RuBP/PGA ratio. Sucrose and glycerol feeding at 12 °C and the decrease in temperature of leaves incubated in these solutions decreased TP and RuBP contents and the TP/PGA and RuBP/PGA ratios. The results suggest that the phosphate limitation caused by accumulation of end products, restriction of their synthesis and sequestration of cytosolic phosphate can inhibit photosynthesis through decreased carboxylation of RuBP or, with increased phosphate limitation, through lowered supply of ATP.

The regulation of component processes of photosynthesis in transgenic tobacco with decreased phosphoribulokinase activity

Tobacco plants (Nicotiana tabacum L.) transformed with an inverted cDNA encoding ribulose 5-phosphate kinase (phosphoribulokinase,PRK; EC 2.7.1.19) were employed to study the in vivo relationship between photosynthetic electron transport and the partitioning of electron transport products to major carbon metabolism sinks under conditions of elevated ATP concentrations and limited ribulose 1,5-bisphosphate (RuBP) regeneration. Simultaneous measurements of room temperature chlorophyll fluorescence and CO2 gas exchange were conducted on intact leaves. Under ambient CO2 concentrations and light intensities above those at which the plants were grown, transformants with only 5% of PRK activity showed 'down-regulation" of PS II activity and electron transport in response to a decrease in net carbon assimilation when compared to wild-type. This was manifested as a decline in the efficiency of PS II electron transport (ΦPS II), an increase in dissipation of excess absorbed light in the antennae of PS II and a decline in: total linear electron transport (J1), electron transport dedicated to carbon assimilation (JA) and electron transport allocated to photorespiration (JL). The transformants showed no alteration in the Rubisco specificity factor measured in vitro and calculated in vivo but had a relatively smaller ratio of RuBP oxygenation to carboxylation rates (vo/vc), due to a higher CO2 concentration at the carboxylation site (Cc). The relationship between ΦPS II and ΦCO 2was similar in transformants and wild-type under photorespiratory conditions demonstrating no change in the intrinsic relationship between PS II function and carbon assimilation, however, a novel result of this study is that this similar relationship occurred at different values of quantum flux, J1, JA, JL and vo/vc in the transformant. For both wild-type and transformants, an assessment was made of the possible presence of a third major sink for electron transport products, beside RuBP oxygenation and carboxylation, the data provided no evidence for such a sink.

Partitioning of the Leaf CO2 Exchange into Components Using CO2 Exchange and Fluorescence Measurements.

Photorespiration was calculated from chlorophyll fluorescence and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) kinetics and compared with CO2 evolution rate in the light, measured by three gas-exchange methods in mature sunflower (Helianthus annuus L.) leaves. The gas-exchange methods were (a) postillumination CO2 burst at unchanged CO2 concentration, (b) postillumination CO2 burst with simultaneous transfer into CO2-free air, and (c) extrapolation of the CO2 uptake to zero CO2 concentration at Rubisco active sites. The steady-state CO2 compensation point was proportional to O2 concentration, revealing the Rubisco specificity coefficient (Ksp) of 86. Electron transport rate (ETR) was calculated from fluorescence, and photorespiration rate was calculated from ETR using CO2 and O2 concentrations, Ksp, and diffusion resistances. The values of the best-fit mesophyll diffusion resistance for CO2 ranged between 0.3 and 0.8 s cm-1. Comparison of the gas-exchange and fluorescence data showed that only ribulose-1,5-bisphosphate (RuBP) carboxylation and photorespiratory CO2 evolution were present at limiting CO2 concentrations. Carboxylation of a substrate other than RuBP, in addition to RuBP carboxylation, was detected at high CO2 concentrations. A simultaneous decarboxylation process not related to RuBP oxygenation was also detected at high CO2 concentrations in the light. We propose that these processes reflect carboxylation of phosphoenolpyruvate, formed from phosphoglyceric acid and the subsequent decarboxylation of malate.

Evidence for the functioning of photosynthetic CO2-concentrating mechanisms in lichens containing green algal and cyanobacterial photobionts

The photosynthetic properties of a range of lichens containing both green algal (11 species) and cyanobacterial (6 species) photobionts were examined with the aim of determining if there was clear evidence for the operation of a CO2-concentrating mechanism (CCM) within the photobionts. Using a CO2-gas-exchange system, which allowed resolution of fast transients, evidence was obtained for the existence of an inorganic carbon pool which accumulated in the light and was released in the dark. The pool was large (500–1000 nmol · mg Chl) in cyanobacterial lichens and about tenfold smaller in green algal lichens. In Hypogymnia physodes (L.) Nyl., which contains the green alga Trebouxia jamesii, a small inorganic carbon pool was rapidly formed in the light. Carbon dioxide was released from this pool into the gas phase upon darkening within about 20 s when photosynthesis was inhibited by the carbon-reduction-cycle inhibitor glycolaldehyde. In the absence of this inhibitor, release appeared to be obscured by carboxylation of ribulose bisphosphate. The kinetics of CO2 uptake and release were monophasic. The operation of an active CCM could be distinguished from passive accumulation and release accompanying the reversible light-dependent alkalization of the stroma by the presence of saturation characteristics with respect to external CO2. In Peltigera canina (L.) Willd., which contains the cyanobacterium Nostoc sp., a larger CO2 pool was taken up over a longer period in the light and the release of this pool in the dark was slow, lasting 3–5 min. This pool also accumulated in the presence of glycolaldehyde, and under these conditions the CO2 release was biphasic. In both species, photosynthesis at low CO2 was inhibited by the carbonic-anhydrase inhibitor ethoxyzolamide (EZ). Inhibition could be reversed fully or to a considerable extent by high CO2. In Peltigera, EZ decreased both the accumulation of the CO2 pool by the CCM and the rate of photosynthesis. Free-living cultures of Nostoc sp. showed a similar effect of EZ on photosynthesis, although it was more dramatic than that seen with the lichen thalli. In contrast, in Hypogymnia, EZ actually increased the size of the CO2 pool, although it inhibited photosynthesis. This effect was also seen when glycolaldehyde was present together with EZ. Surprisingly, EZ did not alter the kinetics of either CO2 uptake or release. Taken together, the evidence indicates the operation in cyanobacterial lichens of a CCM which is capable of considerable elevation of internal CO2 and is similar to that reported for free-living cyanobacteria. The CCM of green algal lichens accumulates much less CO2 and is probably less effective than that which operates in cyanobacterial lichens.

Changes in Levels of Intermediates of the C4 Cycle and Reductive Pentose Phosphate Pathway under Various Light Intensities in Maize Leaves

The rate of CO(2) assimilation and levels of metabolites of the C(4) cycle and reductive pentose phosphate pathway in attached leaves of maize (Zea mays L.) were measured over a range of light intensity from 0 to 1,900 microEinsteins per square meter per second under a saturated CO(2) concentration of 350 microliters per liter and a limiting CO(2) concentration of 133 microliters per liter. The level of ribulose 1,5-bisphosphate (RuBP) stayed almost constant (around 60 nanomoles per milligram chlorophyll [Chl]) from low to high light intensities under 350 microliters per liter. Levels of 3-phosphoglycerate (PGA) increased from 100 to 650 nanomoles per milligram Chl under 350 microliters per liter CO(2) with increasing light intensity. The calculated RuBP concentration of 6 millimolar (corresponded to 60 nanomoles per milligram Chl) was about two times above the estimated RuBP binding-site concentration on ribulose bisphosphate carboxylase-oxygenase (Rubisco) of approximately 2.6 millimolar in maize bundle sheath chloroplasts in the light. The ratio of RuBP/PGA increased with decreasing light intensity under 350 microliters per liter CO(2). These results suggest that RuBP carboxylation is under control of light intensity possibly due to a limited supply of CO(2) to Rubisco through the C(4) cycle whose activity is highly dependent on light intensity. Pyruvate level increased with increasing light intensity as long as photosynthesis rate increased. A positive relationship between levels of PGA and those of pyruvate during steady-state photosynthesis under various conditions suggests that an elevated concentration of PGA increases the carbon input into the C(4) cycle through the conversion of PGA to PEP and consequently the level of total intermediates of the C(4) cycle can be raised to mediate higher photosynthesis rate.

Photoreductive path of carbon fixation in green plant photosynthesis. Reaction pathway of six-carbon ribulose 1,5-bisphosphate carboxylation adduct intermediate

In this paper we examine the six-carbon intermediate pathway of ribulose 1,5-bisphosphate (RuBP) carboxylation reaction in photosynthesis. Based on the observed reactions of purified RuBP carboxylase, mechanisms are described for carbon dioxide assimilation leading to the hydrolytic splitting of the six-carbon intermediate to two enzyme-bound glycerate-3-P (3-PGA) molecules. It is concluded that, under photosynthetic conditions, the reduction of enzyme-bound NADP + by the chlorophyll is responsible for the rapid carboxylase turnover rate given by the lifetime, τ L = 0.4 s , which is nearly two orders of magnitude shorter than the corresponding value, τ D = 11 ± 3 s , for the dark decay of enzyme-bound RuBP. The nocturnal inhibition and photoactivation of RuBP carboxylation are described in terms of the reversible ligh-dark cycles of the NADP + NADPH redox couple and endogenous changes that accompany the 2-carboxy-D-arabinitoll-phosphate binding to the enzyme active site.

Fraction I protein.

When examining the soluble protein from spinach leaves in the ultracentrifuge, Wildman and Bonner in 1947 coined the term Fraction I protein to designate a high molecular weight protein which occurred in very high yield. Subsequent studies in a number of laboratories have shown that this protein contributes up to 50% of the soluble protein of leaves and is probably the most abundant protein in nature (Kawashima and Wildman 1970a). It occurs with remarkably little variation in structure in all plants containing chlorophyll a, ranging from Euglena and the bluegreen algae to all higher plants examined. Later Weissbach et al. (1956) succeeded in purifying the enzyme responsible for the first step in the Calvin photosynthetic cycle, the carboxylation of ribulosebisphosphate with CO2 to form 3-phosphoglycerid acid, and it became apparent that the two proteins had very similar physical properties. Subsequently they have been shown to be identical. Hence Fraction I protein is responsible for catalyzing the initial step of photosynthesis, the process on which all life as we know it depends. More recently (Bowes et al. 1971), this enzyme has been found to be responsible for a second important activity in the plant, that of ribulosebisphosphate oxygenase, an enzyme activity which converts RuBP in the presence of oxygen to 3-phospho-D-glycerate and phosphoglycolate and is the first step of photorespiration.KeywordsLarge SubunitSmall SubunitSpinach ChloroplastChloroplast ProteinBisphosphate CarboxylaseThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.