The effect of ammonium and nitrate on carbondioxide compensation point and enzymes associated with carbondioxide exchange in wheat

The carbondioxide compensation point (), dry matter production, and the activities of nitrate reductase (NR), glycolate oxidase (GO), ribulose 1,5-bisphosphate carboxylase (RuBPC) and phosphoenolpyruvate carboxylase (PEPC) were measured in wheat, grown on media, containing nitrate or ammonium. Significantly higher and lower dry matter was observed in plants supplied with ammonium-nitrogen (NH₄-N), as compared to those supplied with nitrate-nitrogen (NO₃-N). The activities of NR and PEPC were higher in plants grown on NO₃-N than to those grown on NH₄-N. There were no significant differences in the activities of GO and RuBPC irrespective of whether NO₃-N or NH₄-N was supplied. None of the enzymes was found to be associated directly with the .PEPC activity accounted the measured differences in the and biomass production between NH₄-N and NO₃-N supplied plants. The relationship between PEPC and the is discussed.     

The photosynthetic characteristics of differently shaped leaves in Populus euphratica Olivier

There is a distinct leaf shape polymorphism within a single plant of P. euphratica Olivier. The anatomical structure, carbon isotope discrimination (Δ¹³C), and stomatal and photosynthetic behaviour were investigated in broad-ovate (BOL) and lanceolate (LL) leaves, located at the top and bottom in crown, respectively, of a mature Euphrates poplar growing in its native habitat. Both types of leaves had a non-Kranz anatomy and low Δ¹³C values. However, Δ¹³C of a LL was in average 3.2‰ larger than that of a BOL. In comparison with the LL, the BOL had a smaller stomatal conductance, causing subsequent decreases in transpiration rate and ratio of CO₂ concentrations in intercellular spaces to air. Carbon assimilation rate and water use efficiency were higher in the BOLs than in the LLs. The BOL exhibited C₄-like enzymological features, the activity of glycollate oxidase, and the ratio of activities of ribulose-1,5-bisphosphate carboxylase (RuBPC) to phosphoenolpyruvate carboxylase (PEPC) was lower in BOL than in LL throughout the whole growing season. The lowered ratio of RuBPC/PEPC in BOL was mainly associated with a marked decline in the activity of RuBPC, and only a slight increase in the activity of PEPC. These differences might contribute to microclimate adaptation in both types of leaves. This revised version was published online in June 2006 with corrections to the Cover Date.     

Ribulose-1,5-bisphosphate carboxylase and phosphoenolpyruvate carboxylase activities of photoautotrophic callus of Platycerium coronarium (Koenig ex O.F. Muell.) Desv. under CO2 enrichment

The in vitro activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) were measured in cell-free extracts of Platycerium coronarium callus cultured for up to 42 days under photoautotrophic conditions with CO₂ enrichment. With an increase in CO₂ in the culture environment to 10% (v/v) at low light, the apparent photoautotrophic fixation of CO₂ by Rubisco declined, whereas the non-photoautotrophic CO₂ fixation by PEPC activity was enhanced. Hence, photosynthesis appears to play a lesser role in providing carbon skeletons and energy with prolonged culture in a CO₂-enriched environment. Instead, the anaplerotic supply of C-skeletons by PEPC may be important under such a situation. Short-term H¹⁴CO₃-fixation experiments indicated that photoautotrophic callus cultured for 3 weeks with 10% CO₂ enrichment assimilated less ¹⁴CO₂ than the control (0.03% CO₂). Analyses of ¹⁴C-metabolites indicated that about 50% of the total soluble ¹⁴CO₂ fixed was in the organic acid fraction and 35% in the amino acid fraction. Despite the changes in the in vitro Rubisco/PEPC activity-ratio, no significant change in the ¹⁴C distribution pattern was apparent in response to increasing sucrose or CO₂ concentrations. The suppression of Rubisco activity and total chlorophyll content in high sucrose or elevated CO₂ concentrations suggests an inhibition of the capacity for photoautotrophic callus growth under these conditions. This revised version was published online in June 2006 with corrections to the Cover Date.     

Activity regulation and physiological impacts of maize C4-specific phosphoenolpyruvate carboxylase overproduced in transgenic rice plants

Phosphoenolpyruvate carboxylase (PEPC) was overproduced in the leaves of rice plants by introducing the intact maize C₄-specific PEPC gene. Maize PEPC in transgenic rice leaves underwent activity regulation through protein phosphorylation in a manner similar to endogenous rice PEPC but contrary to that occurring in maize leaves, being downregulated in the light and upregulated in the dark. Compared with untransformed rice, the level of the substrate for PEPC (phosphoenolpyruvate) was slightly lower and the product (oxaloacetate) was slightly higher in transgenic rice, suggesting that maize PEPC was functioning even though it remained dephosphorylated and less active in the light. ¹⁴CO₂ labeling experiments indicated that maize PEPC did not contribute significantly to the photosynthetic CO₂ fixation of transgenic rice plants. Rather, it slightly lowered the CO₂ assimilation rate. This effect was ascribable to the stimulation of respiration in the light, which was more marked at lower O₂ concentrations. It was concluded that overproduction of PEPC does not directly affect photosynthesis significantly but it suppresses photosynthesis indirectly by stimulating respiration in the light. We also found that while the steady-state stomatal aperture remained unaffected over a wide range of humidity, the stomatal opening under non-steady-state conditions was destabilized in transgenic rice. This revised version was published online in August 2006 with corrections to the Cover Date.     

Growth,14CO2 fixation,activites of photosystems,ribulose 1,5-bisphosphate carboxylase and nitrate reductase in trees as affected by simulated acid rain

In seedlings of the tropical tree speciesErythrina variegata Lam. andHardwickia binata Roxb. exposed to different acidic mist (H₂SO₄, pH 5, 3 and 2) for 5 d significant reduction in seedling growth, biomass accumulation and¹⁴CO₂ fixation were determined. In isolated chloroplasts a decrease in the activities of photosystem 2 and whole electron transport chain was observed only at pH 3 and 2, but no significant change in photosystem 1 activity was observed. SDS-PAGE analysis of crude leaf extracts of ribulose 1,5-bisphosphate carboxylase (RuBPC) indicated a significant loss of 55 and 15 kDa polypeptides at pH 2 inErythrina. The reduction in the RuBPC activity in seedlings grown under acidic mists correlated well with CO₂ fixation.     

Activities of Ribulose Bisphosphate Carboxylase and Phosphoenolpyruvate Carboxylase and C-Bicarbonate Fixation during in Vitro Culture of Pinus radiata Cotyledons

The activities of ribulose bisphosphate carboxylase (RuBPC) and phosphoenolpyruvate carboxylase (PEPC), as indicators of autotrophic and nonautotrophic CO₂ fixation, were measured in excised cotyledons of Pinus radiata D. Don cultured for 21 days under shoot-forming (SF) and nonshoot-forming (NSF) conditions. The activity of RuBPC was found to increase in both SF and NSF cultures during the initial 5 days of culture. However, it leveled off from day 5 to day 10 and subsequently began to decrease until the end of the culture period under the SF conditions. In contrast, in the NSF cultures, RuBPC activity increased until day 15, when it was twofold higher than the maximum activity found in the SF cultures. An increase in PEPC activity of about 2.5 times the level of activity in freshly excised cotyledons was observed during the initial 5 days of culture under the SF conditions. PEPC activity began to decline after day 5 until it reached the level of activity seen in NSF cotyledons by day 15. In contrast, the activity of PEPC did not show any significant increase during the initial 10 days of culture under the NSF conditions. The K_m (phosphoenolpyruvate) of PEPC from SF cotyledons was about 35% higher than that of NSF cotyledons. Cotyledons from two culture periods (days 5 and 15) were incubated for 15 seconds with NaH¹⁴CO₃. The label in the malate and asparatate fractions as a percentage of total ¹⁴C incorporation was 3 times higher in the SF cotyledons than in the NSF cotyledons. A higher incorporation of ¹⁴C into products of photosynthesis under the NSF conditions was also observed.     

Developmental regulation of photosynthate distribution in leaves of rice

mRNA expression patterns of genes for metabolic key enzymes sucrose phosphate synthase (SPS), phosphoenolpyruvate carboxylase (PEPC), pyruvate kinase, ribulose 1,5-bisphosphate carboxylase/oxygenase, glutamine synthetase 1, and glutamine synthetase 2 were investigated in leaves of rice plants grown at two nitrogen (N) supplies (N_0.5, N_3.0). The relative gene expression patterns were similar in all leaves except for 9^th leaf, in which mRNA levels were generally depressed. Though increased N supply prolonged the expression period of each mRNA, it did not affect the relative expression intensity of any mRNA in a given leaf. SPS V_max, SPS limiting and PEPC activities, and carbon flow were examined. The ratio between PEPC activity and SPS V_max was higher in leaves developed at the vegetative growth stage (vegetative leaves: 5^th and 7^th leaves) than in leaves developed after the ear primordia formation stage (reproductive leaves: 9^th and flag leaves). PEPC activity and SPS V_max decreased with declining leaf N content. After using ¹⁴CO₂ the ¹⁴C photosynthate distribution in the amino acid fraction was higher in vegetative than in reproductive leaves when compared for the same leaf N status. Thus, at high PEPC/SPS activities ratio, more ¹⁴C photosynthate was distributed to the amino acid pool, whereas at higher SPS activity more ¹⁴C was channelled into the saccharide fraction. Thus, leaf ontogeny was an important factor controlling photosynthate distribution to the N- or C-pool, respectively, regardless of the leaf N status.     

The photosynthetic characteristics of differently shaped leaves in Populus euphratica Olivier

There is a distinct leaf shape polymorphism within a single plant of P. euphratica Olivier. The anatomical structure, carbon isotope discrimination (Δ¹³C), and stomatal and photosynthetic behaviour were investigated in broad-ovate (BOL) and lanceolate (LL) leaves, located at the top and bottom in crown, respectively, of a mature Euphrates poplar growing in its native habitat. Both types of leaves had a non-Kranz anatomy and low Δ¹³C values. However, Δ¹³C of a LL was in average 3.2‰ larger than that of a BOL. In comparison with the LL, the BOL had a smaller stomatal conductance, causing subsequent decreases in transpiration rate and ratio of CO₂ concentrations in intercellular spaces to air. Carbon assimilation rate and water use efficiency were higher in the BOLs than in the LLs. The BOL exhibited C₄-like enzymological features, the activity of glycollate oxidase, and the ratio of activities of ribulose-1,5-bisphosphate carboxylase (RuBPC) to phosphoenolpyruvate carboxylase (PEPC) was lower in BOL than in LL throughout the whole growing season. The lowered ratio of RuBPC/PEPC in BOL was mainly associated with a marked decline in the activity of RuBPC, and only a slight increase in the activity of PEPC. These differences might contribute to microclimate adaptation in both types of leaves.     

Changes in photosynthetic apparatus during dark incubation of detached leaves from control and ultraviolet-B treatedVigna seedlings

Changes in various components of photosynthetic apparatus during the 4 d dark incubation at 25°C of detached control and ultraviolet-B (UV-B) treatedVigna unguiculata L. leaves were examined. The photosynthetic apparatus was more degraded in younger control seedlings and for a longer time UV-B treated seedlings than in the older or for a shorter time UV-B treated seedlings. This was shown by determining the losses in chlorophyll (Chl) and protein contents, variable fluorescence yield, photosystem (PS) 2, PS1 and ribulose-1,5-bisphosphate carboxylase (RuBPC) activities, and photosynthetic¹⁴CO₂ fixation. In contrast, the Car/Chl ratio increased during the dark incubation due to less expressed degradation of Car.     

Photosynthetic Metabolism and Activity of Carboxylating Enzymes in Emergent,Floating, and Submersed Leaves of Hydrophytes

Radioisotope techniques were used to compare photosynthetic CO₂ fixation, activities of carboxylating enzymes, and the composition of photosynthates in 42 species of aquatic plants (emergent, floating, and submersed hydrophytes) collected from rivers Sysert' and Iset' in Sverdlovsk oblast (Russia). The submersed leaves, in comparison with the emergent and floating leaves, featured lower rates of potential photosynthesis (by 2.2 mg CO²/(dm² h) on average), low content of the fraction I protein, and low activity of Rubisco and phosphoenolpyruvate carboxylase (PEPC). The averaged activities of Rubisco and PEPC were diminished in submersed leaves by 10 and 1 mg/(dm² h), respectively. Different hydrophyte groups showed similar composition of assimilates accumulated after 5-min photosynthesis and did not differ in this respect from terrestrial plants. However, the incorporation of ¹⁴C into sucrose and starch in submersed leaves (30 and 9% of total labeling, respectively) was lower than in emergent and floating leaves (45 and 15%, respectively). At the same time, the incorporation of ¹⁴C into C₄ acids (malate and aspartate) was 1.5 times higher in submersed leaves than in other leaf types. Analysis of leaf differentiation, the Rubisco/PEPC activity ratio, the PEPC activity, and the composition of primary photosynthates in the pulse–chase experiments revealed no evidence of the C₄ effect in the submersed hydrophytes examined. The adaptation of hydatophytes to specific conditions of an aquatic environment was structurally manifested in the reduction (by a factor of 3–5) in the number of chloroplasts per 1 cm² leaf area. This small number of chloroplasts was responsible for low photosynthetic rates in submersed leaves, although metabolic activities of individual chloroplasts were similar for all three hydrophyte groups.     

Effect of Heat Stress During Grain Filling on Phosphoenolpyruvate Carboxylase and Ribulose-1,5-bisphosphate Carboxylase/Oxygenase Activities of Various Green Organs in Winter Wheat

In two winter wheat (Triticum aestivum L.) cultivars differing in their response to high temperature, JD8 (tolerant) and J411 (sensitive) we studied the effect of heat stress on the activities of phosphoenolpyruvate carboxylase (PEPC) and ribulose-1,5-bisphosphate carboxylase (RuBPC) in green organs during grain-filling. There were significantly higher PEPC activities and lower RuBPC activities in each of the non-leaf organs (awn, glume, lemma, peduncle, and sheath) than in the flag leaf blade. Under heat stress for 12 d, the activity of RuBPC quickly declined and the activity of PEPC first increased and later declined in all organs, resulting in a great increase of the PEPC/RuBPC ratios in the organs, particularly in non-leaf organs which had a higher PEPC/RuBPC than the flag leaf blade in all times. The PEPC activity and PEPC/RuBPC ratio in every organ of JD8 were higher than those in the same organ of J411. Thus the differences in PEPC activities and PEPC/RuBPC may be associated with the differences in photosynthetic heat tolerance among the organs of the same plant or between the two cultivars.     

Sucrose enhances phosphoenolpyruvate carboxylase activity of in vitro solanum tuberosum L. under non-limiting nitrogen conditions

Summary The effect of sucrose on in vitro potato (ev. Kennebec) metabolism was evaluated. Plants were grown in three different media: Murashige and Skoog basal medium containing high nitrogen concentration with 0 or 20 g l⁻¹ sucrose; or modified medium containing reduced nitrogen amount and 20 g l⁻¹ sucrose. Plants fed with 20 g l⁻¹ sucrose and high N exhibited higher phosphoenolpyruvate carboxylase (PEPC) and pyruvate kinase activities and high PEPC protein concentration at 7, 20 and 33 d of culture compared to those grown with 20 g l⁻¹ sucrose and low N, or with 0 g l⁻¹ sucrose and high nitrogen (control). The highest accumulation of starch and sucrose was found in plants grown with sucrose and low nitrogen. This accumulation occurred concomitantly with a reduced enzyme activity resulting from a low utilization of α-ketoglutarate by nitrogen assimilation, when plants were grown with reduced nitrogen. Our investigations on tricarboxylic acid cycle activity showed that sucrose led to the reduction of organic acid amounts in both leaves and roots when high nitrogen was supplied to plants. This was probably due to the intense exit of α-ketoglutarate, which was confirmed by measurements of cytosolic isocitrate dehydrogenase activity. The low leaf glutamine/glutamate ratio observed in plants grown with 20 g l⁻¹ sucrose and high nitrogen compared to their counterparts cultivated with low nitrogen might be due to glutamine conversion into proteins when nitrogen assimilation was intense. These results demonstrate that sucrose enhanced PEPC activity by increasing protein synthesis. They also suggest that sucrose metabolism is involved in the replenishment of the tricarboxylic acid cycle by providing carbon skeletons required to sustain phosphoenolpyruvate utilization during high nitrate assimilation.     

Short-term effects of nitrate,nitrite and ammonium assimilation on photosynthesis,carbon partitioning and protein phosphorylation in maize

Maize (Zea mays L. cv. Contessa) was grown with a nitrogen supply that was just sufficient to support maximal biomass production. The third leaves from 14-to 21-d-old plants were harvested and net photosynthesis allowed to attain steady-state rates at an irradiance of either 250 or 700 mol·m^–2·s^–1. Nitrogen in the form of either KNO₃, KNO₂ or NH₄Cl was then supplied to the leaves through the transpiration stream. In all cases the addition of the nitrogen source resulted in an approximate doubling of the total amino-acid content of the leaves within 1 h. The glutamine pool increased to ten times the level found in control leaves in the light in the absence of added nitrogen. Glutamine accounted for about 21–24% of the total amino-acid content in leaves fed with 40 mM NH₄Cl. Nitrate caused a rapid, but transient inhibition of the rate of net CO₂ assimilation, accompanied by an increase in the activity of phosphoenolpyruvate carboxylase and a decrease in the maximum extractable activity of sucrose-phosphate synthase. This demonstrates that the activities of phospho-enolpyruvate carboxylase and sucrose-phosphate synthase are modulated by NO ₃ ^– in the C₄ plant maize, in a similar manner to that observed in C₃ plants. Nitrite or ammonium feeding resulted in decreased rates of CO₂ assimilation for as long as the nitrogen source was supplied. In all cases the degree of inhibition was greatest at high irradiance and least at low irradiance, even though the total amino-acid contents of the leaves were comparable at the time when maximum inhibition of CO₂ assimilation occurred. Measurements of chlorophyll-a fluorescence showed that the quantum efficiency of PSII decreased and non-radiative dissipation of excitation energy increased as CO₂ assimilation was inhibited by nitrate or nitrite. These metabolites had no direct effect on thylakoid PSII-based electron transport. Ammonium ions weakly inhibited O₂ evolution at high concentrations. The addition of nitrogen (KNO ₃ ^– , KNO₂ or NH₄Cl) caused a significant decrease in the phosphorylation state of the light-harvesting chlorophyll-a/b-binding protein of the thylakoid membranes. We conclude that the response of photosynthetic carbon assimilation and electron transport in maize is essentially similar whether nitrogen is supplied in the form of nitrate, nitrite or ammonium, with the noteworthy exception that the nitrogen-induced inhibition of photosynthesis is transient when leaves are supplied with NO ₃ ^– but sustained when NO ₂ ^– or NH ₄ ⁺ is provided. We suggest that the observed modulation of phosphoenolpyruvate carboxylase and sucrose-phosphate synthase is mediated by the increase in the endogenous level of glutamine. Furthermore, the transient nature of the inhibition of CO₂ assimilation in the case of NO ₃ ^– , but not NO ₂ ^– or NH ₄ ⁺ , may be due to regulation of nitrate reductase.Abbreviations and Symbol Chl chlorophyll - FB-Pase fructose-1,6-bisphosphatase - Gln glutamine - Glu glutamic acid - K_D index of the rate of thermal energy dissipation within the PSII antenna - LHCII light-harvesting chlorophyll-a/b-binding protein - PEPCase phosphoenolpyruvate carboxylase - PFD photon flux density - SPS sucrose-phosphate synthase - _PSII relative quantum efficiency for electron transport by PSII We wish to thank Gabriel Cornic (Structure et métabolisme des plantes, Université de Paris-Sud, Orsay, France) for useful discussion. We are grateful to Sylvie Ferrario (Laboratoire du Métabolisme, I.N.R.A., Versailles) for optimising the conditions of assay and extraction of SPS and PEPCase from maize leaves.     

The effect of pCO2 on carbon acquisition and intracellular assimilation in four marine diatoms

The effect of pCO₂ on carbon acquisition and intracellular assimilation was investigated in the three bloom-forming diatom species, Eucampia zodiacus (Ehrenberg), Skeletonema costatum (Greville) Cleve, Thalassionema nitzschioides (Grunow) Mereschkowsky and the non-bloom-forming Thalassiosira pseudonana (Hust.) Hasle and Heimdal. In vivo activities of carbonic anhydrase (CA), photosynthetic O₂ evolution, CO₂ and HCO₃⁻ uptake rates were measured by membrane-inlet mass spectrometry (MIMS) in cells acclimated to pCO₂ levels of 370 and 800 μatm. To investigate whether the cells operate a C₄-like pathway, activities of ribulose-1,5-bisphosphate carboxylase (RubisCO) and phosphoenolpyruvate carboxylase (PEPC) were measured at the mentioned pCO₂ levels and a lower pCO₂ level of 50 μatm. In the bloom-forming species, extracellular CA activities strongly increased with decreasing CO₂ supply while constantly low activities were obtained for T. pseudonana. Half-saturation concentrations (K_1/2) for photosynthetic O₂ evolution decreased with decreasing CO₂ supply in the two bloom-forming species S. costatum and T. nitzschioides, but not in T. pseudonana and E. zodiacus. With the exception of S. costatum, maximum rates (V_max) of photosynthesis remained constant in all investigated diatom species. Independent of the pCO₂ level, PEPC activities were significantly lower than those for RubisCO, averaging generally less than 3%. All examined diatom species operate highly efficient CCMs under ambient and high pCO₂, but differ strongly in the degree of regulation of individual components of the CCM such as C_i uptake kinetics and extracellular CA activities. The present data do not suggest C₄ metabolism in the investigated species.     

Carbon metabolism in in vitro cultures of date palm: the role of carboxylases (PEPC and RubisCO)

While describing major trends of carbon metabolism during the initiation and expression of somatic embryogenesis in date palm (Phoenix dactylifera L., cv. Deglet Nour), we have investigated the role of two carboxylases, namely PEPC (Phosphoenolpyruvate carboxylase, EC 4.1.1.31) and RubisCO (Ribulose 1,5-bisphosphate carboxylase/oxygenase, EC 4.1.1.39), in embryogenic and non-embryogenic cultures. The detection of PEPC activity on polyacrylamide native gels after electrophoresis revealed the presence of 3 active isoforms in crude extracts from the embryogenic (E) callus strain, whereas only a single band was present in the non-embryogenic (NE) one. The level of PEPC specific capacity was of the same order (3.9 ± 1.2 μmol CO₂ h⁻¹ mg⁻¹ TSP) in both types of cultures. Further changes in carboxylase (PEPC and RubisCO) activities during the growth and development of somatic embryo–derived plantlets were also analysed. The PEPC/RubisCO ratio was found to progressively decrease (from 17.7 to 0.2) throughout the in vitro development of plantlets, due to a substantial depletion of PEPC activity, which decreased from 5.3 to 1.2 μmol CO₂ h⁻¹ mg⁻¹ TSP. Concomitantly, RubisCO assumed greater importance (from 0.3 to 5.3 μmol CO₂ h⁻¹ mg⁻¹ _TSP ) and became the main route for inorganic carbon fixation. Western blot analysis using polyclonal antibodies raised against PEPC and RubisCO purified from tobacco leaves confirmed this trend in terms of relative enzyme abundance. This revised version was published online in June 2006 with corrections to the Cover Date.     

Hydrogen-stimulated CO2 fixation and coordinate induction of hydrogenase and ribulosebiphosphate carboxylase in a H2-uptake positive strain of Rhizobium japonicum

H₂-uptake positive strains (122 DES and SR) and H₂-uptake negative strains SR2 and SR3 of Rhizobium japonicum were examined for ribulosebisphosphate (RuBP) carboxylase and H₂-uptake activities during growth conditions which induced formation of the hydrogenase system. The rate of ¹⁴CO₂ uptake by hydrogenase-derepressed cells was about 6-times greater in the presence than in the absence of H₂. RuBP carboxylase activity was observed in free-living R. japonicum strains 122 DES or SR only when the cells were derepressed for their hydrogenase system. Hydrogenase and RuBP carboxylase activities were coordinately induced by H₂ and both were repressed by added succinate. Hydrogenase-negative mutant strains SR2 and SR3 derived from R. japonicum SR showed no detecyable RuBP carboxylase activities under hydrogenase derepression conditions. No detectable RuBP carboxylase was observed in bacteroids formed by H₂-uptake positive strains R. japonicum 122 DES or SR. Propionyl CoA carboxylase activity was consistently observed in extracts of cells from free-living cultures of R. japonicum but activity was not appreciably influenced by the addition of H₂. Neither phosphoenolpyruvate carboxylase nor phosphoenolpyruvate carboxykinase activity was detected in extracts of R. japonicum.Abbreviations RuBP Ribulose 1,5-bisphosphate - (Na₂EDTA) (Ethylenedinitrilo)-tetraacetic acid, disodium salt - (propionyl CoA) Propionyl coenzyme A - (PEP) Phosphoenolpyruvate - (GSH) Reduced glutathione - (Tricine) N-tris(hydroxymethyl)-methylglycine     

Developmental changes of plant affecting primary photosynthate distribution in rice leaves

Developmental changes of plant in the regulation of photosynthate distribution of leaves were studied in hydroponically cultivated rice by the ¹⁴CO₂ tracer technique and analysis of the activity of the regulatory enzymes, sucrose phosphate synthase (SPS), phosphoenolpyruvate carboxylase (PEPC), and pyruvate kinase (PK). The distribution of primary photosynthates into sugars, amino acids, organic acids, sugar phosphates, proteins, and polysaccharides was determined by column chromatography. The relative primary photosynthate distribution to the sugar phosphate fraction was significantly larger in the 5^th leaf than in the 6^th one. Correspondingly, the V_max of PEPC was significantly higher in the 5^th than in the 6^th leaf, while no significant differences between leaves were detected in the other enzymes. As a consequence, the ratio of the V_max of SPS and PEPC was lower in the 5^th than in the 6^th leaf. As the 5^th leaf develops before panicle initiation in rice, it predominantly supports vegetative growth, while the 6^th leaf develops after panicle initiation and thus contributes mainly to reproductive growth. We conclude that the physiological properties of each leaf are regulated developmentally. When the 6^th leaf became fully expanded (corresponding to the panicle initiation stage of plant), the distribution pattern of ¹⁴C was transiently changed in the 5^th leaf, indicating that individual organs that are mainly involved in vegetative development are affected to some extent by the whole-plant-level physiological transformation that occurs at the transition from the vegetative to the reproductive stage.     

Photosynthetic metabolism in the cyanophytaOscillatoria rubescens D.C.

Short term¹⁴C labelling experiments and enzymatic activities related to primary pathways of photosynthesis have been studied in the cyanophytaOscillatoria rubescens D.C. from axenic cyclostat cultures. Responses of samples from cultures with different amounts of nitrogen are presented and compared. Variations in photosynthetic pigments are used to quantify the degree of nitrogen starvation at different levels.PEPcarboxylase activity remains low and is not affected by nitrogen starvation. RuBPcarboxylase activity is lowered to nearly two thirds of its normal metabolic rate by starvation but PEPcarboxykinase and aspartate aminotransferase activities are significantly higher in this case. Malate dehydrogenase is slightly altered and malic enzyme is never active. Starved algae replaced in fresh complete media fix rapidly¹⁴C in nitrogen compounds such as amino acids. Results are discussed in regard to both physiological and ecological characteristics ofO. rubescens. PEPcarboxykinase can play a role in making efficient use of HCO ₃ ^- .Abbreviations AAT aspartate aminotransferase - APC allophycocyanin - ASM 1 algal synthetic medium, 1st modification - DW dry weight - GAPDH glyceraldehyde 3-phosphate dehydrogenase - MDH malate dehydrogenase - PC phycocyanin - PE phycoerythrin - PEP phophoenolpyruvate - PEPC phosphoenolpyruvate carboxylase - PEPCK phophoenolpyruvate carboxykinase - PGA 3-phosphoglyceric acid - RuBPC ribulose bisphosphate carboxylase     

In vitro enzyme activities and products of 14CO2 assimilation in flag leaf and ear parts of wheat (Triticum aestivum L.)

Activities of key enzymes of Calvin cycle and C₄ metabolism, rate of ¹⁴CO₂ fixation in light and dark and the initial products of photosynthetic ¹⁴CO₂ fixation were determined in flag leaf and different ear parts of wheat viz. pericarp, awn and glumes. Compared to the activities of RuBP carboxylase and other Calvin cycle enzymes viz. NADP-glyceraldehyde-3-phosphate dehydrogenase, NAD-glyceraldehyde-3-phosphate dehydrogenase and ribulose-5-phosphate kinase, the levels of PEP carboxylase and other enzymes of C₄ metabolism viz. NADP-malate dehydrogenase, NAD-malate dehydrogenase, NADP-malic enzyme, NAD-malic enzyme, glutamate oxaloacetate transaminase genase, NADP-malic enzyme, NAD-malic enzyme, glutamate oxaloacetate transaminase and glutamate pyruvate transaminase, were generally greater in ear parts than in the flag leaf. In contrast to CO₂ fixation in light, the various ear parts incorporated CO₂ in darkness at much higher rates than flag leaf. In short term assimilation of ¹⁴CO₂ by illuminated ear parts, most of the ¹⁴C was in malate with less in 3-phosphoglyceric acid, whereas flag leaves incorporated most into 3-phosphoglyceric acid. It seems likely that ear parts have the capability of assimilating CO₂ by the C₄ pathway of photosynthesis and utilise PEP carboxylase for recapturing the respired CO₂.