Effects of chronic ozone and elevated atmospheric CO2 concentrations on ribulose-1,5-bisphosphate in soybean (Glycine max)

Ribulose-1,5-bisphosphate (RuBP) pool size was determined at regular intervals during the growing season to understand the effects of tropospheric ozone concentrations, elevated atmospheric carbon dioxide concentrations and their interactions on the photosynthetic limitation by RuBP regeneration. Soybean (Glycine max [L.] Merr. cv. Essex) was grown from seed to maturity in open-top field chambers in charcoal-filtered air (CF) either without (22 nmol O₃ mol^?1) or with added O₃ (83 nmol mol^?1) at ambient (AA, 369 μmol CO₂ mol^?1) or elevated CO₂ (710 μmol mol^?1). The RuBP pool size generally declined with plant age in all treatments when expressed on a unit leaf area and in all treatments but CF-AA when expressed per unit ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) binding site. Although O₃ in ambient CO₂ generally reduced the RuBP pool per unit leaf area, it did not change the RuBP pool per unit Rubisco binding site. Elevated CO₂, in CF or O₃-fumigated air, generally had no significant effect on RuBP pool size, thus mitigating the negative O₃ effect. The RuBP pools were below 2 mol mol^?1 binding site in all treatments for most of the season, indicating limiting RuBP regeneration capacity. These low RuBP pools resulted in increased RuBP regeneration via faster RuBP turnover, but only in CF air and during vegetative and flowering stages at elevated CO₂. Also, the low RuBP pool sizes did not always reflect RuBP consumption rates or the RuBP regeneration limitation relative to potential carboxylation (%RuBP). Rather, %RuBP increased linearly with decrease in the RuBP pool turnover time. These data suggest that amelioration of damage from O₃ by elevated atmospheric CO₂ to the RuBP regeneration may be in response to changes in the Rubisco carboxylation.     

Ribulose-1,5-bisphosphate regeneration limitation in rice leaf photosynthetic acclimation to elevated CO2

Our previous study has demonstrated that both RuBP carboxylation limitation and RuBP regeneration limitation exist simultaneously in rice grown under free-air CO₂ enrichment (FACE, about 200 μmol mol⁻¹ above the ambient air CO₂ concentration) conditions [G.-Y. Chen, Z.-H. Yong, Y. Liao, D.-Y. Zhang, Y. Chen, H.-B. Zhang, J. Chen, J.-G. Zhu, D.-Q. Xu, Photosynthetic acclimation in rice leaves to free-air CO₂ enrichment related to both ribulose-1,5-bisphosphate carboxylase limitation and ribulose-1,5-bisphosphate regeneration limitation. Plant Cell Physiol. 46 (2005) 1036–1045]. To explore the mechanism for forming of RuBP regeneration limitation, we conducted the gas exchange measurements and some biochemical analyses in FACE-treated and ambient rice plants. Net CO₂ assimilation rate (A_net) in FACE leaves was remarkably lower than that in ambient leaves when measured at the same CO₂ concentration, indicating that photosynthetic acclimation to elevated CO₂ occurred. In the meantime the maximum electron transport rate (ETR) (J_max), maximum carboxylation rate (V_cmax) in vivo, and RuBP contents decreased significantly in FACE leaves. The whole chain electron transport rate and photophosphorylation rate reduced significantly while ETR of photosystem II (PSII) did not significantly decrease and ETR of photosystem I (PSI) was significantly increased in the chloroplasts from FACE leaves. Further, the amount of cytochrome (Cyt) f protein, a key component localized between PSII and PSI, was remarkably declined in FACE leaves. It appears that during photosynthetic acclimation the decline in the Cyt f amount is an important cause for the decreased RuBP regeneration capacity by decreasing the whole chain electron transport in FACE leaves.     

Salinity and Nitrogen Effects on Photosynthesis, Ribulose-1,5-Bisphosphate Carboxylase and Metabolite Pool Sizes in Phaseolus vulgaris L

Salinity (100 millimolar NaCl) was found to reduce photosynthetic capacity independent of stomatal closure in Phaseolus vulgaris. This reduction was shown to be a consequence of a reduction in the efficiency of ribulose-1,5-bisphosphate (RuBP) carboxylase (RuBPCase) rather than a reduction in the leaf content of photosynthetic machinery. In control plants, photosynthesis became RuBP-limited at approximately 1.75 moles RuBP per mole 2-carboxyarabinitol bisphosphate binding sites. Salinization caused the RuBP pool size to reach this limiting value for CO₂ fixation at much lower values of intercellular CO₂. Plants grown at low nitrogen and ± NaCl became RuBP limited at similar RuBP pool sizes as the high nitrogen-grown plants. At limiting RuBP pool sizes and equal values of intercellular CO₂ photosynthetic capacity of salt-stressed plants was less than control plants. This effect of salinity on RuBPCase activity could not be explained by deactivation of the enzyme or inhibitor synthesis. Thus, salinity reduced photosynthetic capacity by reducing both the RuBP pool size by an effect on RuBP regeneration capacity and RuBPCase activity by an unknown mechanism when RuBP was limiting.     

Polygonum sachalinense alters the balance between capacities of regeneration and carboxylation of ribulose‐1,5‐bisphosphate in response to growth CO2 increment but not the nitrogen allocation within the photosynthetic apparatus

The limiting step of photosynthesis changes depending on CO₂ concentration and, in theory, photosynthetic nitrogen use efficiency at a respective CO₂ concentration is maximized if nitrogen is redistributed from non‐limiting to limiting processes. It has been shown that some plants increase the capacity of ribulose‐1,5‐bisphoshate (RuBP) regeneration (evaluated as J_max) relative to the RuBP carboxylation capacity (evaluated as V_cmax) at elevated CO₂, which is in accord with the theory. However, there is no study that tests whether this change is accompanied by redistribution of nitrogen in the photosynthetic apparatus. We raised a perennial plant, Polygonum sachalinense, at two nutrient availabilities under two CO₂ concentrations. The J_max to V_cmax ratio significantly changed with CO₂ increment but the nitrogen allocation among the photosynthetic apparatus did not respond to growth CO₂. Enzymes involved in RuBP regeneration might be more activated at elevated CO₂, leading to the higher J_max to V_cmax ratio. Our result suggests that nitrogen partitioning is not responsive to elevated CO₂ even in species that alters the balance between RuBP regeneration and carboxylation. Nitrogen partitioning seems to be conservative against changes in growth CO₂ concentration.     

The relationship between carbon-dioxide-limited photosynthetic rate and ribulose-1,5-bisphosphate-carboxylase content in two nuclear-cytoplasm substitution lines of wheat,and the coordination of ribulose-bisphosphate-carboxylation and electron-transport capacities

Photosynthesis in two cultivars of Triticum aestivum was compared with photosynthesis in two lines having the same nuclear genomes but with cytoplasms derived from T. boeoticum. The in-vitro specific activity of ribulose-1,5-bisphosphate carboxylase (RuBPCase; EC 4.1.1.39) isolated from lines with T. boeoticum cytoplasm was only 71% of that of normal T. aestivum. By contrast, the RuBPCase activities calculated from the CO₂-assimilation rate at low partial pressures of CO₂, p(CO₂), were the same for all lines for a given RuBPCase content. This indicates that both types of RuBPCase have the same turnover numbers in-vivo of 27.5 mol CO₂·(mol enzyme)^–1·s^–1 (23°). The rate of CO₂ assimilation measured at normal p(CO₂), p _a =340 bar, and high irradiance could be quantitatively predicted from the amount of RuBPCase protein. The maximum rate of RuBP regeneration could also predict the rate of CO₂ assimilation at normal ambient conditions. Therefore, the maximum capacities for RuBP carboxylation and RuBP regeneration appear to be well-balanced for normal ambient conditions. As photosynthetic capacity declined with increasing leaf age, the capacities for RuBP carboxylation and RuBP regeneration declined in parallel.Abbreviations PAR photosynthetically active radiation - RuBP(Case) ribulose-1,5-bisphosphate (carboxylase)     

A Model Describing the Regulation of Ribulose-1,5-Bisphosphate Carboxylase, Electron Transport, and Triose Phosphate Use in Response to Light Intensity and CO(2) in C(3) Plants

A model of the regulation of the activity of ribulose-1,5-bisphosphate carboxylase, electron transport, and the rate of orthophosphate regeneration by starch and sucrose synthesis in response to changes in light intensity and partial pressures of CO₂ and O₂ is presented. The key assumption behind the model is that nonlimiting processes of photosynthesis are regulated to balance the capacity of limiting processes. Thus, at CO₂ partial pressures below ambient, when a limitation on photosynthesis by the capacity of rubisco is postulated, the activities of electron transport and phosphate regeneration are down-regulated in order that the rate of RuBP regeneration matches the rate of RuBP consumption by rubisco. Similarly, at subsaturating light intensity or elevated CO₂, when electron transport or Pi regeneration may limit photosynthesis, the activity of rubisco is downregulated to balance the limitation in the rate of RuBP regeneration. Comparisons with published data demonstrate a general consistency between modelled predictions and measured results.     

Gas Exchange Analysis of the Fast Phase of Photosynthetic Induction in Alocasia macrorrhiza

When leaves of Alocasia macrorrhiza that had been preconditioned in 10 micromoles photons per square meter per second for at least 2 hours were suddenly exposed to 500 micromoles photons per square meter per second, there was an almost instantaneous increase in assimilation rate. After this initial increase, there was a secondary increase over the next minute. This secondary increase was more pronounced in high CO₂ (1400 microbars), where assimilation rate was assumed to be limited by the rate of regeneration of ribulose 1,5-bisphosphate (RuBP). It was absent in low CO₂ (75 microbars), where RuBP carboxylase/oxygenase (Rubisco) was assumed to be limiting. It was therefore concluded that it represented an increase in the capacity to regenerate RuBP. This fast-inducing component not only gained full induction rapidly, but also lost it rapidly in low photon flux density (PFD) with a half time of 150 to 200 seconds. It was concluded that in environments with fluctuating PFD, this fast-inducing component is an important factor in determining a leaf's potential for photosynthetic carbon gain. It is especially important during brief periods (<30 seconds) of high PFD that follow moderately long periods (1 to 10 minutes) of low PFD.     

Influence of Etherel and Gibberellic Acid on Carbon Metabolism,Growth, and Essential Oil Accumulation in Spearmint (Mentha Spicata)

Controlled environment chamber and glasshouse studies were conducted on six herbaceous annual species grown at 350 (AC) and 700 (EC) mol(CO₂) mol^-1 to determine whether growth at EC resulted in acclimation of the apparent quantum yield of photosynthesis (QY) measured at limiting photosynthetic photon flux density (PPFD), or in acclimation of net photosynthetic rate (P _N) measured at saturating PPFD. It was also determined whether acclimation in P _N at limiting PPFD was correlated with acclimation of carboxylation efficiency or ribulose-1,5-bisphosphate (RuBP) regeneration rate measured at saturating PPFD. Growth at EC reduced both the QY and P _N at limiting PPFD in three of the six species. The occurrence of photosynthetic acclimation measured at a rate limiting PPFD was independent of whether photosynthetic acclimation was apparent at saturating measurement PPFD. At saturating measurement PPFD, acclimation to EC in the apparent carboxylation efficiency and RuBP regeneration capacity also occurred independently. Thus at least three components of the photosynthetic system may adjust independently when leaves are grown at EC. Estimates of photosynthetic acclimation at both high and low PPFD are necessary to accurately predict photosynthesis at the whole plant or canopy level as [CO₂] increases.     

Photosynthesis and Ribulose 1,5-Bisphosphate Concentrations in Intact Leaves of Xanthium strumarium L

The interacting effects of the rate of ribulose 1,5-bisphosphate (RuBP) regeneration and the rate of RuBP utilization as influenced by the amount and activation of RuBP carboxylase on photosynthesis and RuBP concentrations were resolved in experiments which examined the kinetics of the response of photosynthesis and RuBP concentrations after step changes from a rate-saturating to a rate-limiting light intensity in Xanthium strumarium. Because RuBP carboxylase requires several minutes to deactivate in vivo, it was possible to observe the effect of reducing the rate of RuBP regeneration on the RuBP concentration at constant enzyme activation state by sampling very soon after reducing the light intensity. Samples taken over longer time periods showed the effect of changes in enzyme activation at constant RuBP regeneration rate on RuBP concentration and photosynthetic rate. Within 15 s of lowering the light intensity from 1500 to 600 microEinsteins per square meter per second the RuBP concentration in the leaves dropped below the enzyme active site concentration, indicating that RuBP regeneration rate was limiting for photosynthesis. After longer intervals of time, the RuBP concentration in the leaf increased as the RuBP carboxylase assumed a new steady state activation level. No change in the rate of photosynthesis was observed during the interval that RuBP concentration increased. It is concluded that the rate of photosynthesis at the lower light intensity was limited by the rate of RuBP regeneration and that parallel changes in the activation of RuBP carboxylase occurred such that concentrations of RuBP at steady state were not altered by changes in light intensity.     

The photosynthetic induction response in wheat leaves: net CO2 uptake,enzyme activation,and leaf metabolites

The photosynthetic induction response was studied in whole leaves of wheat (Triticum aestivum L.) following 5-min, 30-min and 10-h dark periods. After the 5-min dark treatment there was a rapid burst in the rate of photosynthesis upon illumination (half of maximum after 30s), followed by a slight decrease after 1.5 more min and then a gradual rise to the maximum rate. During this initial burst in photosynthesis, there was a rapid rise in the level of 3-phosphoglycerate (PGA) and a high PGA/triose-phosphate (triose-P) ratio was obtained. In addition, after the 5-min dark treatment, ribulose-1,5-bisphosphate carboxylase (Rubisco, EC 4.1.1.39), ribulose-5-phosphate kinase (EC 2.7.1.19) and chloroplastic fructose-1,6-bisphosphatase (EC 3.1.3.11) maintained a relatively high state of activation, and maximum activation occurred within 1 min of illumination. The results indicate there is a high capacity for CO₂ fixation in the cycle upon illumination but attaining maximum rates requires an increase in the ribulose-1,5-bisphosphate (RuBP) pool (adjustment in triose-P utilization for carbohydrate synthesis versus RuBP synthesis). With both the 30-min and 10-h dark pretreatments there was only a slight rise in photosynthesis upon illumination, followed by a lag, then a gradual increase to steady-state (half-maximum rate after 6 min). In contrast to the 5-min dark treatment, the level of PGA was low and actually decreased initially, whereas the level of RuBP increased and was high during induction, indicating that Rubisco is limiting. This regulation via the carboxylase was not reflected in the initial extractable activity, which reached a maximum by 1 min after illumination. The light activation of chloroplastic fructose-1,6-bisphosphatase in leaves darkened for 30 min and 10 h prior to illumination was relatively slow (reaching a maximum after 8 min). However, this was not considered to limit carbon flux through the carbon-fixation cycle during induction since RuBP was not limiting. When photosynthesis approached the maximum steady-state rate, a high PGA/triose-P ratio and a high PGA/RuBP ratio were obtained. This may allow a high rate of photosynthesis by producing a favorable mass-action ratio for the reductive phase (the conversion of PGA to triose phosphate) while stimulating starch and sucrose synthesis.Abbreviations Chl chlorophyll - FBP fructose-1,6-bisphosphate - FBPase fructose-1,6-bisphosphatase - Fru6P fructose-6-phosphate - Glc6P glucose-6-phosphate - PGA 3-phosphoglycerate - Pi inoganic phosphate - Rubisco RuBP carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - Ru5P ribulose-5-phosphate - triose-P triose phosphates (dihydroxyacetone phosphate+glyceraldehyde-3-phosphate)     

Effects of water stress on photosynthetic electron transport,photophosphorylation, and metabolite levels of Xanthium strumarium mesophyll cells

Several component processes of photosynthesis were measured in osmotically stressed mesophyll cells of Xanthium strumarium L. The ribulose-1,5-bisphosphate regeneration capacity was reduced by water stress. Photophoshorylation was sensitive to water stress but photosynthetic electron transport was unaffected by water potentials down to-40 bar (-4 MPa). The concentrations of several intermediates of the photosynthetic carbon-reduction cycle remained relatively constant and did not indicate that ATP supply was limiting photosynthesis in the water-stressed cells.Abbreviations Hepes 4-(2-hydroxyethyl)-1-piperazinepropanesulfonic acid - PGA 3-phosphoglyceric acid - RuBP ribulose-1,5-bisphosphate     

An improved model of C3 photosynthesis at high CO2: Reversed O2 sensitivity explained by lack of glycerate reentry into the chloroplast

Current models of C₃ photosynthesis incorporate a phosphate limitation to carboxylation which arises when the capacity for starch and sucrose synthesis fails to match the capacity for the production of triose phosphates in the Calvin cycle. As a result, the release of inorganic phosphate in the chloroplast stroma fails to keep pace with its rate of sequestration into triose phosphate, and phosphate becomes limiting to photosynthesis. Such a model predicts that when phosphate is limiting, assimilation becomes insensitive to both CO₂ and O₂, and is thus incapable of explaining the experimental observation that assimilation, under phosphate-limited conditions, frequently exhibits reversed sensitivity to both CO₂ and O₂, i.e., increasing O₂ stimulates assimilation and increasing CO₂ inhibits assimilation. We propose a model which explains reversed sensitivity to CO₂ and O₂ by invoking the net release of phosphate in the photorespiratory oxidation cycle. In order for this to occur, some fraction of the glycollate carbon which leaves the stroma and which is recycled to the chloroplast by the photorespiratory pathway as glycerate must remain in the cytosol, perhaps in the form of amino acids. In that case, phosphate normally used in the stromal glycerate kinase reaction to generate PGA from glycerate is made available for photophosphorylation, stimulating RuBP regeneration and assimilation. The model is parameterized for data obtained on soybean and cotton, and model behavior in response to CO₂, O₂, and light is demonstrated.Abbreviations PFD photon flux density - PGA 3-phosphoglycerate - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - TPU triose phosphate utilization     

Differential inhibition of photosynthesis under drought stress in <Emphasis Type="Italic">Flaveria</Emphasis> species with different degrees of development of the C<Subscript>4</Subscript> syndrome

The effect of drought stress (DS) on photosynthesis and photosynthesis-related enzyme activities was investigated in F. pringlei (C₃), F. floridana (C₃–C₄), F. brownii (C₄-like), and F. trinervia (C₄) species. Stomatal closure was observed in all species, probably being the main cause for the decline in photosynthesis in the C₃ species under ambient conditions. In vitro ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) and stromal fructose 1,6-bisphosphatase (sFBP) activities were sufficient to interpret the net photosynthetic rates (P _N), but, from the decreases in P _N values under high CO₂ (C _a = 700 μmol mol^{− 1}) it is concluded that a decrease in the in vivo rate of the RuBPCO reaction may be an additional limiting factor under DS in the C₃ species. The observed decline in the photosynthesis capacity of the C₃–C₄ species is suggested to be associated both to in vivo decreases of RuBPCO activity and of the RuBP regeneration rate. The decline of the maximum P _N observed in the C₄-like species under DS was probably attributed to a decrease in maximum RuBPCO activity and/or to decrease of enzyme substrate (RuBP or PEP) regeneration rates. In the C₄ species, the decline of both in vivo photosynthesis and photosynthetic capacity could be due to in vivo inhibition of the phosphoenolpyruvate carboxylase (PEPC) by a twofold increase of the malate concentration observed in mesophyll cell extracts from DS plants.     

Electron transport is the functional limitation of photosynthesis in field-grown Pima cotton plants at high temperature

Restrictions to photosynthesis can limit plant growth at high temperature in a variety of ways. In addition to increasing photorespiration, moderately high temperatures (35–42 °C) can cause direct injury to the photosynthetic apparatus. Both carbon metabolism and thylakoid reactions have been suggested as the primary site of injury at these temperatures. In the present study this issue was addressed by first characterizing leaf temperature dynamics in Pima cotton (Gossypium barbadense) grown under irrigation in the US desert south‐west. It was found that cotton leaves repeatedly reached temperatures above 40 °C and could fluctuate as much as 8 or 10 °C in a matter of seconds. Laboratory studies revealed a maximum photosynthetic rate at 30–33 °C that declined by 22% at 45 °C. The majority of the inhibition persisted upon return to 30 °C. The mechanism of this limitation was assessed by measuring the response of photosynthesis to CO₂ in the laboratory. The first time a cotton leaf (grown at 30 °C) was exposed to 45 °C, photosynthetic electron transport was stimulated (at high CO₂) because of an increased flux through the photorespiratory pathway. However, upon cooling back to 30 °C, photosynthetic electron transport was inhibited and fell substantially below the level measured before the heat treatment. In the field, the response of assimilation (A) to various internal levels of CO₂ (C_i) revealed that photosynthesis was limited by ribulose‐1,5‐bisphosphate (RuBP) regeneration at normal levels of CO₂ (presumably because of limitations in thylakoid reactions needed to support RuBP regeneration). There was no evidence of a ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) limitation at air levels of CO₂ and at no point on any of 30 A–C_i curves measured on leaves at temperatures from 28 to 39 °C was RuBP regeneration capacity measured to be in substantial excess of the capacity of Rubisco to use RuBP. It is therefore concluded that photosynthesis in field‐grown Pima cotton leaves is functionally limited by photosynthetic electron transport and RuBP regeneration capacity, not Rubisco activity.     

Differences between rice and wheat in ribulose-1,5-bisphosphate regeneration capacity per unit of leaf-N content

The photosynthetic rates under saturating CO₂ conditions per unit of leaf‐N content were higher in wheat than in rice. This suggested that ribulose‐1,5‐bisphosphate (RuBP) regeneration capacity is greater in wheat. Therefore, the biochemical factor(s) for this difference were examined between rice and wheat. Soluble protein‐N, insoluble‐N, and trichloroacetic acid (TCA) soluble‐N contents were found not to differ between the two species. The activities of several Calvin cycle enzymes such as RuBP carboxylase, NADP‐glyceraldehyde‐3‐phosphate dehydrogenase, phosphoglycerate kinase and chloroplastic fructose‐1,6‐bisphosphatase (cpFBPase) activities per unit of leaf‐N content were all higher in wheat than in rice. Among them, cpFBPase activity was most highly correlated with CO₂‐saturated photosynthesis. The V_max activity of sucrose‐phosphate synthase (SPS) for UDP‐glucose was almost the same between the two species and its K_m value was a little lower in rice. Chlorophyll content and its a/b ratio did not differ. Cytochrome (Cyt) f content was greater in wheat, whereas coupling factor 1 content was greater in rice. Cyt f content was highly correlated with CO₂‐saturated photosynthesis, irrespective of the two species. The results thus suggested that higher RuBP regeneration capacity in wheat leaves is most closely related to a greater Cyt f content and that another candidate is cpFBPase.     

Rubisco activity of scenedesmus ecornis: What is wrong with initial activity determination?

A procedure was devised to measure the initial and total Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activities for the green microalga, Scenedesmus ecornis. Total Rubisco activities corresponded well with photosynthetic carbon assimilation rates. Initial activities ranged from 10 to 40% of the total activities and did not correlate with photosynthetic rates. Investigations into potential causes of the reduced initial activities yielded modest increases in percentage of the total activity. Values of K_m for ribulose-1,5-bisphosphate (RuBP) were similar for both initial and CO₂-Mg²⁺ activated enzyme. Total activities increased with increasing concentrations of RuBP to 400 μm, the assay concentration. However, concentrations above the K_m, 25 μm RuBP, were inhibitory for the initial Rubisco form. Inhibition increased with increasing RuBP concentration. The addition of Mg²⁺ in the extraction solution did not prevent RuBP inhibition. The results suggest that the low initial Rubisco activities are principally due to decarbamylation of the active sites of the enzyme during extraction.     

Limiting Factors in Photosynthesis: VI. Regeneration of Ribulose 1,5-Bisphosphate Limits Photosynthesis at Low Photochemical Capacity

Earlier work (SE Taylor, N Terry [1984] Plant Physiol 75: 82-86) has shown that the rate of photosynthesis may be colimited by photosynthetic electron transport capacity, even at low intercellular CO₂ concentrations. Here we monitored leaf metabolites diurnally and the activities of key Calvin cycle enzymes in the leaves of three treatment groups of sugar beet (Beta vulgaris L.) plants representing three different in vivo photochemical capacities, i.e. Fe-sufficient (control) plants, moderately Fe-deficient, and severely Fe-deficient plants. The results show that the decrease in photosynthesis with Fe deficiency mediated reduction in photochemical capacity was through a reduction in ribulose 1,5-bisphosphate (RuBP) regeneration and not through a decrease in ribulose 1,5-bisphosphate carboxylase/oxygenase activity. Based on measurements of ATP and NADPH and triose phosphate/3-phosphoglycerate ratios in leaves, there was little evidence that photosynthesis and RuBP regeneration in Fe-deficient leaves were limited directly by the supply of ATP and NADPH. It appeared more likely that photochemical capacity influenced RuBP regeneration through modulation of enzymes in the photosynthetic carbon reduction cycle between fructose-6-phosphate and RuBP; in particular, the initial activity of ribulose-5-phosphate kinase was strongly diminished by Fe deficiency. Starch and sucrose levels changed independently of one another to some extent during the diurnal period (both increasing in the day and decreasing at night) but the average rates of starch or sucrose accumulation over the light period were each proportional to photochemical capacity and photosynthetic rate.     

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     

Photosynthesis and ribulose-1,5-bisphosphate carboxylase/oxygenase in rice leaves from emergence through senescence. Quantitative analysis by carboxylation/oxygenation and regeneration of ribulose 1,5-bisphosphate

Changes in gas-exchange rates during the life span of the leaves of rice (Oryza sativa L.) were analyzed quantitatively by measuring changes in the carboxylation/oxygenation and regeneration of ribulose 1,5-bisphosphate (RuBP) at photon fluence rates of 2000 (saturating) and 500 (subsaturating) μmol quanta·m^-2·s^-1 under ambient air conditions. The RuBP levels were always higher than the active-site concentrations of RuBP carboxylase (EC 4.1.1.39), irrespective of the irradiance supplied. Analysis of the CO₂-assimilation rate as a function of intercellular CO₂ concentration indicated that RuBP regeneration does not limit CO₂ assimilation. The estimated RuBP-carboxylase/oxygenase activity in vivo was linearly correlated to the rate of CO₂ assimilation at each level of irradiance. This enzyme activity was just enough to account for the rate of CO₂ assimilation at the saturating irradiance and was 35% more than the rate of CO₂ assimilation at the subsaturating irradiance. Analysis of the assimilation rate at subsaturating irradiance as a function of intercellular CO₂ concentration indicated that a limitation caused by enzyme activation comes into play. The results indicate that the rate of CO₂ assimilation in rice leaves under ambient air conditions is limited during their entire life span by the RuBP-carboxylation/oxygenation capacity.     

Activity and quantity of ribulose bisphosphate carboxylase-and phosphoenolpyruvate carboxylase-protein in two Crassulacean acid metabolism plants in relation to leaf age,nitrogen nutrition,and point in time during a day/night cycle

Activity of ribulose 1,5-bisphosphate (RuBP) carboxylase in leaf extracts of the constitutive Crassulacean acid metabolism (CAM) plant Kalanchoe pinnata (Lam.) Pers. decreased with increasing leaf age, whereas the activity of phosphoenolpyruvate (PEP) carboxylase increased. Changes in enzyme activities were associated with changes in the amount of enzyme proteins as determined by immunochemical analysis, sucrose density gradient centrifugation, and SDS gel electrophoresis of leaf extracts. Young developing leaves of plants which received high amounts of NO ₃ ^- during growth contained about 30% of the total soluble protein in the form of RuBP carboxylase; this value declined to about 17% in mature leaves. The level of PEP carboxylase in young leaves of plants at high NO ₃ ^- was an estimated 1% of the total soluble protein and increased to approximately 10% in mature leaves, which showed maximum capacity for dark CO₂ fixation. The growth of plants at low levels of NO ₃ ^- decreased the content of soluble protein per unit leaf area as well as the extractable activity and the percentage contribution of both RUBP carboxylase and PEP carboxylase to total soluble leaf protein. There was no definite change in the ratio of RuBP carboxylase to PEP carboxylase activity with a varying supply of NO ₃ ^- during growth. It has been suggested (e.g., Planta 144, 143–151, 1978) that a rhythmic pattern of synthesis and degradation of PEP carboxylase protein is involved in the regulation of -carboxylation during a day/night cycle in CAM. No such changes in the quantity of PEP carboxylase protein were observed in the leaves of Kalanchoe pinnata (Lam.) Pers. or in the leaves of the inducible CAM plant Mesembryanthemum crystallinum L.Abbreviations CAM Crassulacean acid metabolism - RuBP ribulose 1,5-bisphosphate - PEP phosphoenolpyruvate - G-6-P glucose-6-phosphate