Rubisco Activation State Research Articles

Acclimation of photosynthesis to low temperature inSpinacia oleraceaL. II. Effects of nitrogen supply

The photosynthetic capacity of leaves of N-sufficent plants of Spinacia oleracea L. increases following transfer a constant temperature of 10°C for 10 d compared to plants maintained at 25°C. The effects of nitrogen nutrition on this low temperature acclimation have been investigated in respect of CO2 assimilation, the activities and activation states of key enzymes and the partitioning of recently fixed carbon. N-deficiency greatly restricted acclimation of photosynthetic CO2 assimilation to low temperature at both ambient and at saturating CO2 concentrations, indicating a restriction on accilmatory changes in both ribulose1,5-bisphosphate carboxylase-oxygenase (Rubisco) and the reactions of ribulose1,5-bisphosphate regeneration. Nitrogen limitation led to an increase in the partitioning of recently-fixed carbon into starch. Total protein increased during acclimation in both N-sufficient and N-deficient leaves and was much less affected than were the activities of enzymes. Increases in the activation state of Rubisco and the stromal fructose-1,6-bisphosphatase occurred in response to low temperature, but increases in the activities of Rubisco, sucrose-phosphate synthase or the cytosolic fructose1,6-bisphosphatase could not be sustained in N-deficient plants throughout the period of acclimation, although the activities of these enzymes declined less precipitately than in non-acclimated N-deficient plants. These data are all consistent with the view that increases in the activities of key enzymes of carbon assimilation are a pre-requisite for acclimation to low temperature and that these increases are restricted under N-limitation.

The Inhibition of Photosynthesis after Exposure of Bean Leaves to Various Low Levels of CO2

Mature first leaves of Phaseolus vulgaris L. were exposed to low partial pressures of CO2 (7, 6 and 0 Pa COJ for 24 h. After exposure of leaves to 6 Pa CO2 for 24 h, there was a reduction in the carbon exchange rate (CER) at all partial pressures of CO2 at which measurements were made. After exposure to 7 Pa CO2, the CER decreased only at high partial pressures of CO2. The rates of electron transport from water to methyl viologen, through the whole chain, decreased in parallel with the decrease in CER measured at 90 Pa CO2. One site of inhibition in leaves exposed for 24 h to 6 Pa CO2 appeared to be the intersystem electron-transport chain since there were no significant changes in the activities of PSI and PSII, as determined from the level of P-700 and measurement of fluorescence, respectively. Another inhibitory phenomenon appeared to be a negative change in the activation state of Rubisco, while the level of Rubisco was unaffected by the exposure to 6 Pa CO2. These decreases in photosynthetic activity caused by depletion of CO2 explains at least in part, the inhibition of photosynthesis that is caused by rain treatment [Ishibashi and Terashima (1995) Plant Cell Environ. 18: 431].

Control of photosynthesis in barley mutants with reduced activities of glutamine synthetase and glutamate synthase

Heterozygous mutants of barley (Hordeum vulgare L. cv. Maris Mink) with decreased activities of chloroplastic glutamine synthetase (GS) between 97 and 47% of the wild type and ferredoxin dependent glutamate synthase (Fd-GOGAT) down to 64% of the wild type have been used to study aspects of glyoxylate metabolism and the effect of glyoxylate on the activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in vivo. In the leaf, the extractable activities of serine:glyoxylate aminotransferase decreased with a decrease in GS whereas activities of glutamate and alanine:glyoxylate aminotransferase increased, pointing to a re direction of amino donors from serine to glutamate and alanine. Under ambient conditions, the leaf contents of glutamate and alanine declined continuously with a decrease in GS, in parallel with the decrease in total amino acids. Glycine, serine and asparagine contents decreased with a decrease in GS to approximately 70% of the wild type, but increased again with a further decrease in GS. At high irradiances and at low CO2 concentrations, glyoxylate contents exhibited a pronounced minimum between 60% and 80% GS. With a further decrease in GS, glyoxylate contents recovered and approached values similar to the wild type. The activation state of Rubisco showed a negative correlation with glyoxylate contents, indicating that a decrease in GS feeds back on the first step of carbon assimilation and photorespiration. The activation state of stromal fructose-1,6-bisphosphatase was unaffected by a decrease in GS or Fd-GOGAT, whereas the activation state of NADP dependent malate dehydrogenase changed in a complex manner. The CO2photocompensation point, Γ*, was appreciably increased in mutants with 47% GS. ‘Mitochondrial respiration’ in the light (Rd) was reduced with a decrease in GS. Relative rates of CO2 release into CO2-free air between the wild type and the 47%-GS mutant correlated with determinations of Γ*. These data are consistent with the view that when GS is decreased there is an increased oxidative decarboxylation of glyoxylate resulting from a decreased availability of amino donors for the transamination of glyoxylate to glycine, and that when GS activities are lower than 70% of the wild type an additional mechanism operates to reduce the photorespiratory loss of ammonia.

Seasonal response of photosynthesis to elevated CO2 in loblolly pine (Pinus taeda L.) over two growing seasons

AbstractTrees growing in natural systems undergo seasonal changes in environmental factors that generate seasonal differences in net photosynthetic rates. To examine how seasonal changes in the environment affect the response of net photosynthetic rates to elevated CO2, we grew Pinus taeda L. seedlings for three growing seasons in open‐top chambers continuously maintained at either ambient or ambient + 30 Pa CO2. Seedlings were grown in the ground, under natural conditions of light, temperature nd nutrient and water availability. Photosynthetic capacity was measured bimonthly using net photosynthetic rate vs. intercellular CO2 partial pressure (A‐Ci) curves. Maximum Rubisco activity (Vcmax) and ribulose 1,5‐bisphosphate regeneration capacity mediated by electron transport (Jmax) and phosphate regeneration (PiRC) were calculated from A‐Ci curves using a biochemically based model. Rubisco activity, activation state and content, and leaf carbohydrate, chlorophyll and nitrogen concentrations were measured concurrently with photosynthesis measurements. This paper presents results from the second and third years of treatment.Mean leaf nitrogen concentrations ranged from 13.7 to 23.8 mg g−1, indicating that seedlings were not nitrogen deficient. Relative to ambient CO2 seedlings, elevated CO2 increased light‐saturated net photosynthetic rates 60–110% during the summer, but < 30% during the winter. A relatively strong correlation between leaf temperature and the relative response of net photosynthetic rates to elevated CO2 suggests a strong effect of leaf temperature. During the third growing season, elevated CO2 reduced Rubisco activity 30% relative to ambient CO2 seedlings, nearly completely balancing Rubisco and RuBP‐regeneration regulation of photosynthesis. However, reductions in Rubisco activity did not eliminate the seasonal pattern in the relative response of net photosynthetic rates to elevated CO2. These results indicate that seasonal differences in the relative response of net photosynthetic rates to elevated CO2 are likely to occur in natural systems.

Regulation of Photosynthetic Induction State in High- and Low-Light-Grown Soybean and Alocasia macrorrhiza (L.) G. Don.

Alocasia (Alocasia macrorrhiza [L.] G. Don) and soybean (Glycine max [L.]) were grown under high or low photon flux density (PFD) conditions to achieve a range of photosynthetic capacities and light-adaptation modes. The CO2 assimilation rate and in vivo linear electron transport rate (Jf) were determined over a range of PFDs and under saturating 1-s-duration lightflecks applied at a range of frequencies. At the same mean PFD, the assimilation rate and the Jf were lower under the lightfleck regimes than under constant light. The activation state of two, key enzymes of the photosynthetic carbon reduction cycle pathway, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and fructose-1,6-bisphosphatase, and the photosynthetic induction states (ISs) were also found to be lower under flashing as compared to continuous PFD. Under all conditions, the IS measured 120 s after an increase in PFD to constant and saturating values was highly correlated with the Rubisco activation state and stomatal conductances established in the light regime before the increase. Both the fructose-1,6-bisphosphatase and Rubisco activities established in a particular light regime were highly correlated with the mean Jf in that regime. The relationships between enzyme activation state and Jf and between IS and enzyme activation state were similar in soybean and Alocasia and were not affected either by growth-light regime, and hence photosynthetic capacity, or by flashing versus constant PFD. The common relationship between the linear Jf and the activation state of key enzymes suggests that electron transport may be the determinant of the signal regulating IS, at least to the extent that the IS is controlled by the activation state of key stromal enzymes.

Acclimation of photosynthesis in relation to Rubisco and non‐structural carbohydrate contents and in situ carboxylase activity in Scirpus olneyi grown at elevated CO2 in the field

ABSTRACTStands of Scirpus olneyi, a native saltmarsh sedge with C3 photosynthesis, had been exposed to normal ambient and elevated atmospheric CO2 concentrations (Ca) in their native habitat since 1987. The objective of this investigation was to characterize the acclimation of photosynthesis of Scirpus olneyi stems, the photosynthesizing organs of this species, to long‐term elevated Ca treatment in relation to the concentrations of Rubisco and non‐structural carbohydrates. Measurements were made on intact stems in the Held under existing natural conditions and in the laboratory under controlled conditions on stems excised in the field early in the morning. Plants grown at elevated Ca had a significantly higher (30–59%) net CO2 assimilation rate (A) than those grown at ambient Ca when measurements were performed on excised stems at the respective growth Ca. However, when measurements were made at normal ambient Ca, A was smaller (45–53%) in plants grown at elevated Ca than in those grown at ambient Ca. The reductions in A at normal ambient Ca, carboxylation efficiency and in situ carboxylase activity were caused by a decreased Rubisco concentration (30–58%) in plants grown at elevated Ca; these plants also contained less soluble protein (39–52%). The Rubisco content was 43 to 58% of soluble protein, and this relationship was not significantly altered by the growth CO2 concentrations. The Rubisco activation state increased slightly, but the in situ carboxylase activity decreased substantially in plants grown at elevated Ca. When measurements were made on intact stems in the field, the elevated Ca treatment caused a greater stimulation of,A (100%) and a smaller reduction in carboxylation efficiency (which was not statistically significant) than when measurements were made on excised stems in the laboratory. The possible reasons for this arc discussed.Plants grown at elevated Ca contained more non‐structural carbohydrates (25–53%) than those grown at ambient Ca. Plants grown at elevated Ca appear to have sufficient sink capacity to utilize the additional carbohydrates formed during photosynthesis.Overall, our results are in agreement with the hypothesis that elevated Ca leads to an increased carbohydrate concentration and the ensuing acclimation of the photo‐synthetic apparatus in C3 plants results in a reduction in the protein complement, especially Rubisco, which reduces the photosynthetic capacity in plants grown at elevated Ca, relative to plants grown at normal ambient Ca. Nevertheless, when compared at their respective growth Ca, Scirpus olneyi plants grown at elevated Ca in their native habitat maintained a substantially higher rate of photosynthesis than those grown at normal ambient Ca even after 8 years of growth at elevated Ca.

The Effects of Chilling in the Light on Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Activation in Tomato (Lycopersicon esculentum Mill.).

Photosynthesis rate, ribulsoe-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activation state, and ribulose bisphosphate concentration were reduced after exposing tomato (Lycopersicon esculentum Mill.) plants to light at 4[deg]C for 6 h. Analysis of lysed and reconsituted chloroplasts showed that activity of the thylakoid membrane was inhibited and that Rubisco, Rubisco activase, and other soluble factors were not affected. Leaf photosynthesis rates and the ability of chilled thylakoid membranes to promote Rubisco activation recovered after 24 h at 25[deg]C. Thylakoid membranes from control tomato plants were as effective as spinach thylakoids in activating spinach Rubisco in the presence of spinach Rubisco activase. This observation is in sharp contrast to the poor ability of spinach Rubisco activase to activate tomato Rubisco (Z.-Y. Wang, G.W. Snyder, B.D. Esau, A.R. Portis, and W.L. Ogren [1992] Plant Physiol 100: 1858-1862). The ability of thylakoids from chilled tomato plants to activate Rubisco in the assay system was greatly inhibited compared to control plants. These experiments indicate that chilling tomato plants at 4[deg]C interferes with photosynthetic carbon metabolism at two sites, thioredoxin/ferredoxin reduction (G.F. Sassenrath, D.R. Ort, and A.R. Portis, Jr. [1990] Arch Biochem Biophys 282: 302-308), which limits bisphosphatase activity, and Rubisco activase, which reduces Rubisco activation state.

An evaluation of direct and indirect mechanisms for the ?sink-regulation? of photosynthesis in spinach: Changes in gas exchange, carbohydrates, metabolites, enzyme activities and steady-state transcript levels after cold-girdling source leaves

Mature source leaves of spinach (Spinacia oleracea L.) plants growing hydroponically in a 9 h light (350 μmol photons·m−2 · s−1)/15 h dark cycle at 20° C in a climate chamber were fitted with a cold girdle around the petiole, 2 h into the light period. Samples were taken 1, 3 and 7 h later, and at the end of the photoperiod for the following 4 d. Control samples were taken from ungirdled leaves. In the first 7 h after fitting the cold girdle there was (compared to the control leaves) a two to five-fold accumulation of sucrose, glucose, fructose and starch, a 40–50% increase of hexose-phosphates and ribulose-1,5-bisphosphate, a decrease of glycerate-3-phosphate, a small decrease in sucrose-phosphate synthase activation, an increase of fructose-2,6-bisphosphate, increased activation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), but no significant change in photosynthetic rate or stomatal conductance. Steady-state transcript levels for rbcS (small subunit of Rubisco) and atp-D (D-subunit of the thylakoid ATP synthase) decreased 30%, cab (chlorophyll-a-binding protein) decreased by 15% and agp-S (S-isoenzyme of ADP-glucose pyrophosphorylase) and nra (nitrate reductase) rose twofold. On the following days, levels of carbohydrates continued to rise and the changes of metabolites were maintained. Transcripts for rbcS, cab and atpD declined to 20, 70 and 25% of the control values. From day 3 onward the maximum activity of Rubisco declined. This was accompanied by a further increase of Rubisco activation to over 90% and, from day 4 onwards, an inhibition of photosynthesis which was associated with high internal CO2 concentration (ci), high ribulose-1,5-bisphosphate, and low glycerate-3-phosphate. When the cold-girdle was removed on day 5 there was a gradual recovery of photosynthesis and decline of ci over the next 2 d. Hexose-phosphates levels and transcripts for rbcS, cab and atp-D completely recovered within 2 d, even though the levels of carbohydrates had not fully recovered. Activity of Rubisco only reverted partly after 2 d, and Rubisco activation state and the ribulose-1,5-bisphosphate/glycerate-3-phosphate ratio were still higher than in control leaves. Transcripts for nra and agp-S were also still higher than in control leaves. It is concluded (i) that a reversible modulation of gene expression in response to the export rate plays a central role in the mid-term feedback “sink” regulation of photosynthesis, and (ii) that feedback regulation of CO2 fixation by changes of Pi are of little importance in spinach under these conditions. Further (iii) the rapid and reciprocal changes in nra and agpS transcripts, compared to rbcS, provide evidence that gene expression could also contribute to the modulation of nitrate assimilation and carbohydrate storage in conditions of decreased sink demand.

Responses of Nicotiana tabacum to CO2 enrichment at low‐photon flux density

Effects of CO2 enrichment on photosynthesis and on dry matter allocation were examined in two tobacco (Nicotiana tabaum L.) genotypes. Samsun and W38. Plants were grown from seed in controlled environment chambers at a photosynthetic photon flux density of 450 μmol m−2 s−1. Averaged over the 9 day study, net photosynthesis rates were 14.2±0.5 and 13.0±0.4 μmol m−2 s−1 in elevated (70 Pa) and in ambient (35 Pal CO2 air. respectively, when measured at the irradiance and CO2 partial pressure employed for plant growth. However. photosynthesis rates of plants grown in elevated CO2 were 50% less than those of the ambient controls on the last day of treatment, when measured al 70 Pa CO2 air and an irradiance of 900 μmol m−2 s−1 Total plant dry weight and specific leaf weight were greater (P < 0.051 in enriched‐CO2‐grown than in ambient‐CO2‐grown plants. Leaf starch, measured during the first hour of the photoperiod. increased over 7 days of treatment in elevaled‐CO2‐grown but not in ambient‐CO2‐grown plants. Ribulose 1.5‐bisphosphate carboxylase/oxygenase (Rubisco) activities of tobacco plants grown at 35 and 70 Pa CO2 air were 58.5±4.5 and 48.5±3.7 umol m−2 s−1. respectively, between days 0 and 9 of the study. Rubisco activation state. Rubisco protein concentration. soluble protein and total chlorophyll were unaffected by CO2 enrichment. The above findings demonstrated that photosynthesis was down regulated in tobacco plants after 7 to 9 days of CO2 enrichment at low photosynthetic photon flxu density, but less than at moderate irradiances.

Control of photosynthesis in barley leaves with reduced activities of glutamine synthetase or glutamate synthase

Wild-type and mutant plants of barley (Hordeum vulgate L. cv. Marls Mink) lacking activities of chloroplastic glutamine synthetase (GS) and of ferredox- in-dependent glutamate synthase (Fd-GOGAT) were crossed to generate heterozygous plants. Crosses of the F 2 generation containing GS activities between 47% and 97% of the wild-type and Fd-GOGAT activities down to 63% of the wild-type have been selected to study the control of both enzymes on photorespiratory carbon and nitrogen metabolism. There were no major pleiotropic effects. Decreased GS had a small impact on leaf protein and the total activity of ribulose-l,5-bisphosphate car- boxylase-oxygenase (Rubisco). The activation state of Rubisco was unaffected in air, but a decrease in GS influ- enced the activation state of Rubisco in low CO2. In illu- minated leaves, the amino-acid content decreased with decreasing GS, while the content of ammonium rose, showing that even small reductions in GS limit ammoni- um re-assimilation and may bring about a loss of nitro- gen from the plants, and hence a reduction in protein and Rubisco. Leaf amino-acid contents were restored, and ammonium and nitrate contents decreased, by leaving plants in the dark for 24 h. The ratios of serine to glycine decreased with a decrease in GS when plants were kept at moderate photon flux densities in air, suggesting a possi- ble feedback on glycine decarboxylation. This effect was absent in high light and low CO 2. Under these conditions ammonium contents exhibited an optimum and amino- acid contents a minimum at a GS activity of 65% of the wild-type, suggesting an inhibition of ammonium release in mutants with less than 65% GS. The leaf contents of glutamate, glutamine, aspartate, asparagine, and alanine largely followed changes in the total amino-acid contents Abbreviations: Chl = chlorophyll; FBPase = fructose-l,6-bisphos- phatase; Fd-GOGAT = ferredoxin-dependent glutamine: 2-oxo- glutarate aminotransferase; GS = glutamine synthetase; PEP- phosphoenolpyruvate; PFD = photon flux density; Rubisco = ribulose- 1,5-bisphosphate carboxylase-oxygenase

Effects of Growth Temperature on the Responses of Ribulose-1,5-Biphosphate Carboxylase, Electron Transport Components, and Sucrose Synthesis Enzymes to Leaf Nitrogen in Rice, and Their Relationships to Photosynthesis.

Effects of growth temperature on the photosynthetic gas-exchange rates and their underlying biochemical properties were examined in young, fully expanded leaves of rice (Oryza sativa L.). The plants were grown hydroponically under day/night temperature regimes of 18/15[deg]C, 23/18[deg]C, and 30/23[deg]C and all photosynthetic measurements were made at a leaf temperature of 25[deg]C and an irradiance of 1800 [mu]mol quanta m-2 s-1. Growth temperature affected the photosynthetic CO2 response curve. The relative ratio of the initial slope to the CO2-saturated photosynthesis increased with rising growth temperature. This was caused mainly by an increase in CO2-limited photosynthesis for a given leaf nitrogen content with rising growth temperature. However, there was no difference in ribulose-1,5-bisphosphate carboxylase (Rubisco) content at any given leaf nitrogen content among temperature treatments. In addition, the activation state and catalytic turnover rate of Rubisco were not affected by growth temperature. The increase in CO2-limited photosynthesis with rising growth temperature was the result of an increase in the CO2 transfer conductance between the intercellular airspaces and the carboxylation sites. The amounts of total chlorophyll and light-harvesting chlorophyll a/b protein II increased for the same leaf nitrogen content with rising growth temperature, but the amounts of cytochrome f and coupling factor 1 and the activities of cytosolic fructose-1,6-bisphosphatase and sucrose-phosphate synthase were the same between plants grown at 23/18[deg]C and those grown at 30/23[deg]C. Similarly, CO2-saturated photosynthesis was not different for the same leaf nitrogen content between these treatments. For the 18/15[deg]C-grown plants, a slight decrease in the amounts of cytochrome f and coupling factor 1 and an increase in the activities of cytosolic fructose-1,6-bisphosphatase and sucrose-phosphate synthase were found, but these were not reflected in CO2-saturated photosynthesis.

Antisense RNA inhibition of Rubisco activase expression

SummaryRibulose bisphosphate carboxylase (Rubisco) activase catalyzes the activation of Rubisco in vivo. Activase antisense DNA mutants of tobacco have been generated to explore the control that activase exerts on the photosynthetic process. These mutants have up to 90% reductions in activase protein levels as a consequence of an inhibition of activase mRNA accumulation. It is shown that photosynthesis, measured as the rate of CO2 exchange (CER), is modestly decreased in plants exposed to high irradiances. The decreases in CER in the transgenic plants are accompanied by corresponding decreases in Rubisco activation, indicating that activase has a direct effect on photosynthetic rates in the antisense plants by influencing the activation state of Rubisco. It is concluded that in high light conditions, control of photosynthesis is largely shared between Rubisco and activase. Plant growth is also impaired in mutant plants that have severe reductions in activase. The inhibition of activase in the antisense plants does not have an impact on the accumulation of Rubisco large subunit or small subunit mRNAs or proteins. This indicates that the concerted expression of the genes for activase (Rca) and Rubisco (rbcL and rbcS) in response to light, developmental factors and circadian controls is not due to feedback regulation of rbcL or rbcS by the amount of activase protein.

EFFECTS OF SUCROSE ON METABOLIZE POOL SIZES AND RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE/OXYGENASE ACTIVITY IN STRAWBERRY PLANTLETS CULTIVATED IN VITRO

To identify the physiological and biochemical events leading to the negative effects of sucrose in culture medium on the photosynthetic capacity of plantlets cultivated in vitro, time-course changes in photosynthesis, metabolize pool sizes, and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity were investigated in strawberry (Fragaria × ananassa Duch. cv. Kent) plantlets following their transfer to medium with or without sucrose. When the plantlets grown in medium without sucrose were transferred to a similar medium with 30 g sucrose/liter, their net photosynthesis decreased and their level of phosphorylated compounds increased with time. In addition, initial Kcat, total Kcat, and the activation state of Rubisco decreased in these plantlets. Conversely, when the plantlets grown in medium with 30 g sucrose/liter were transferred to a similar medium without sucrose, their net photosynthesis slowly increased with time and their level of phosphorylated compounds slowly decreased. A slow increase with time of initial Kcat, total Kcat, and the activation state of Rubisco was also observed in these plantlets. The results of the present research suggest that the reduced photosynthetic capacity of strawberry plantlets cultivated in vitro in the presence of sucrose was the consequence of reduced Rubisco efficiency due to its deactivation and the possible presence of a putative tight binding inhibitor.

Photosynthetic Control Factors in a Single Leaf os Sweet Potato, Ipomoea batatas Lam. 3. Estimation of in vivo rubisco activity from the CO2 exchange rate of a peeled leaf.

Rubisco (ribulose 1, 5-bisphosphate carboxylase/oxygenase) activity is the prime determinant for CO2 exchang rate (CER) at the beginning of the photosynthetic reaction, and hence the rubisco activity in a leaf can be deduced from the initial rise of CER. The activity of rubisco is also regulated by two main internal factors, the activation state of rubisco and the availability of RuBP (ribulose 1, 5-bisphosphate). A peeled leaf (Ipomoea batatas, cv. Koganesengan) without stomatal resistance was preilluminated in PAR of 900 μ mol m-2 s-1 at CO2 concentration ([CO2]) of 20, 60 or 350) μmol mollt;-1>. Directly after the dark interruption following preillumination, the response of CER of a peeled leaf (CERP1) to reillumination (PAR of 900 μmol m-2 s-1) was measured at [CO2] of 350 μmol mol-1. The dark time affected the response of CERP1. The light-activated state of rubisco, which was deduced from the response of the initial rise of CERP1 (IR-CERP1), could remain constant for at least 5 min in the dark. The leaves preillluminated at different [CO2] levels had no or little difference in IR-CERP1 which was determined after the dark interruption of 2.5 min. This is an evidence that rubisco in the intact mesophyll tissues was substantially light-activated even at such a low [CO2] level as 20 or 60 μmolmol-1] and the rubisco activity was primarily determined by the activation state of rubisco but was independent of the RuBP pool level. IR-CERP1 can be proposed as an indicator for the rubisco activation state in vivo or the rubisco activity unrestricted by RuBP availability.

Changes in Activities of Enzymes of Carbon Metabolism in Leaves during Exposure of Plants to Low Temperature

The aim of this study was to determine the response of photosynthetic carbon metabolism in spinach and bean to low temperature. (a) Exposure of warm-grown spinach and bean plants to 10 degrees C for 10 days resulted in increases in the total activities of a number of enzymes, including ribulose 1,5-bisphosphate carboxylase (Rubisco), stromal fructose 1,6 bisphosphatase (Fru 1,6-P(2)ase), sedoheptulose 1,7-bisphosphatase (Sed 1,7-P(2)ase), and the cytosolic Fru 1,6-P(2)ase. In spinach, but not bean, there was an increase in the total activity of sucrose-phosphate synthase. (b) The CO(2)-saturated rates of photosynthesis for the cold-acclimated spinach plants were 68% greater at 10 degrees C than those for warm-acclimated plants, whereas in bean, rates of photosynthesis at 10 degrees C were very low after exposure to low temperature. (c) When spinach leaf discs were transferred from 27 to 10 degrees C, the stromal Fru 1,6-P(2)ase and NADP-malate dehydrogenase were almost fully activated within 8 minutes, and Rubisco reached 90% of full activation within 15 minutes of transfer. An initial restriction of Calvin cycle fluxes was evident as an increase in the amounts of ribulose 1,5-bisphosphate, glycerate-3-phosphate, Fru 1,6-P(2), and Sed 1,7-P(2). In bean, activation of stromal Fru 1,6-P(2)ase was weak, whereas the activation state of Rubisco decreased during the first few minutes after transfer to low temperature. However, NADP-malate dehydrogenase became almost fully activated, showing that no loss of the capacity for reductive activation occurred. (d) Temperature compensation in spinach evidently involves increases in the capacities of a range of enzymes, achieved in the short term by an increase in activation state, whereas long-term acclimation is achieved by an increase in the maximum activities of enzymes. The inability of bean to activate fully certain Calvin cycle enzymes and sucrose-phosphate synthase, or to increase nonphotochemical quenching of chlorophyll fluorescence at 10 degrees C, may be factors contributing to its poor performance at low temperature.

Regulation of Photosynthetic Rate of Two Sunflower Hybrids under Water Stress

The effect of short-term water stress on photosynthesis of two sunflower hybrids (Helianthus annuus L. cv Sungro-380 and cv SH-3622), differing in productivity under field conditions, was measured. The rate of CO(2) assimilation of young, mature leaves of SH-3622 under well-watered conditions was approximately 30% greater than that of Sungro-380 in bright light and elevated CO(2); the carboxylation efficiency was also larger. Growth at large photon flux increased assimilation rates of both hybrids. The changes in leaf composition, including cell numbers and sizes, chlorophyll content, and amounts of total soluble and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) protein, and in Rubisco activity and amount of ribulose-1,5-bisphosphate (RuBP) were determined to assess the factors regulating the differences in assimilation of the hybrids at high and low water potentials. The amounts of chlorophyll, soluble protein, Rubisco protein and the initial activity of Rubisco and its activation state did not differ significantly between hybrids. However, unstressed leaves of SH-3622 had more, smaller cells per unit area and 60% more RuBP per unit leaf area than that of Sungro-380. Water stress developing over 4 days decreased the assimilation of both hybrids similarly. Changes in the amounts of chlorophyll, soluble and Rubisco protein, and Rubisco activity and activation state were small and were not sufficient to explain the decrease in photosynthesis; neither was decreased stomatal conductance (or stomatal "patchiness"). Reduction of photosynthesis per unit leaf area from 25 to 5 micromoles CO(2) per square meter per second in both hybrids was caused by a decrease in the amount of RuBP from approximately 130 to 40 micromoles per square meter in SH-3622 and from 80 to 40 micromoles per square meter in Sungro. Differences between hybrids and their response to water stress is discussed in relation to control of RuBP regeneration.

A dynamic model of photosynthesis in varying light taking account of stomatal conductance, C3‐cycle intermediates, photorespiration and Rubisco activation

Abstract. A dynamic model of whole leaf C3 photosynthesis is constructed using a modified version of the Farquliar‐von Caemmerer approach. The model is designed to provide a physiological basis to understand observations of assimilation in environments with varying photon flux densities, including induction phenomena. The model couples the effect of light activation and dark deactivation of enzymes, stomatal conductance responses, and variations in the pools of carbon cycle intermediates. The dynamic components are viewed on three time scales, the slowest of which (min to h) involves changes in stomatal conductance and the activation stale of Rubisco. On a time scale of seconds to a few minutes, adjustments in pools of biochemical components of the photosynthetic pathway occurs. The most rapid time scale corresponds to the equilibration time of intercelluar CO2 concentration through gaseous diffusion and is here assumed to occur instantaneously. The model form includes a single pool for reduced intermediates including RuBP, a single pool for components of the glycolate pathway, and a third component corresponding to the activation state of Rubisco. This is coupled to a previously described model for the dynamics of stomatal conductance, giving a final model form consisting of six non‐linear ordinary differential equations, of which three control conductance dynamics and three control assimilation. The coupling between these occurs through the variable pi, the intercellular partial pressure of CO2. Only three of the parameters for the assimilation portion of the model require dynamic data to estimate. The remaining parameters are estimated from steady‐state data. The model is calibrated using previously collected data on the tropical understory plant Alocasia macrorrhiza and is shown to have qualitatively similar behaviour to that of experimental measurements using simple changes in PFD, as well as a complex sequence of such changes.

Growth at elevated CO2: photosynthetic responses mediated through Rubisco

Abstract. The global uptake of CO2 in photosynthesis is about 120 gigatons (Gt) of carbon per year. Virtually all passes through one enzyme, ribulose bisphosphate carboxylase/oxygenase (rubisco), which initiates both the photosynthetic carbon reduction, and photorespiratory carbon oxidation, cycles. Both CO2 and O2 are substrates; CO2 also activates the enzyme. In C3 plants, rubisco has a low catalytic activity, operates below its Km (CO2), and is inhibited by O2. Consequently, increases in the CO2/O2 ratio stimulate C3 photosynthesis and inhibit photorespiration. CO2 enrichment usually enhances the productivity of C3 plants, but the effect is marginal in C4 species. It also causes acclimation in various ways: anatomically, morphologically, physiologically or biochemically. So, CO2 exerts secondary effects in growth regulation, probably at the molecular level, that are not predictable from its primary biochemical role in carboxylation. After an initial increase with CO2 enrichment, net photosynthesis often declines. This is a common acclimation phenomenon, less so in field studies, that is ultimately mediated by a decline in rubisco activity, though the RuBP/Pi‐regeneration capacities of the plant may play a role. The decline is due to decreased rubisco protein, activation state, and/or specific activity, and it maintains the rubisco fixation and RuBP/Pi regeneration capacities in balance. Carbohydrate accumulation is sometimes associated with reduced net photosynthesis, possibly causing feedback inhibition of the RuBP/Piregeneration capacities, or chloroplast disruption. As exemplified by field‐grown soybeans and salt marsh species, a reduction in net photosynthesis and rubisco activity is not inevitable under CO2 enrichment. Strong sinks or rapid translocation may avoid such acclimation responses. Over geological time, aquatic autotrophs and terrestrial C4 and CAM plants have genetically adapted to a decline in the external CO2/O2 ratio, by the development of mechanisms to concentrate CO2 internally; thus circumventing O2 inhibition of rubisco. Here rubisco affinity for CO2 is less, but its catalytic activity is greater, a situation compatible with a high‐CO2 internal environment. In aquatic autotrophs, the CO2 concentrating mechanisms acclimate to the external CO2, being suppressed at high‐CO2. It is unclear, whether a doubling in atmospheric CO2 will be sufficient to cause a de‐adaptive trend in the rubisco kinetics of future C3 plants, producing higher catalytic activities.

DEVELOPMENT OF CARBOXYLATION CAPACITY IN TISSUE-CULTURED STRAWBERRY FOLLOWING TRANSPLANTING.

Strawberry plantlets (Fragaria X ananassa Duch. cv. Kent) were submitted to a factorial arrangement of 2 photosynthetic photon fluxes (PPF) (80 and 150 μmol·m-2·s-1, PAR) and 2 CO2 concentrations (330 and 3000 ppm) during the in vitro rooting stage. Leaves were tagged and placed in a growth chamber tor acclimatization. Photosynthetic capability of leaves from different origins was determined by measuring the initial and total activity of Ribulose-1, 5-bisphosphate carboxylase/oxygenase (rubisco), but the contribution of Phosphoenolpyruvate carboxylase (PEPCase) to fixation was also examined High CO2 concentration and PPF significantly increased fresh weight and surface area in vitro and after 4 weeks ex vitro. Improved growth was not the result of increased autotrophy in vitro since initial rubisco activity was 10 times lower than that of de novo formed leaves and declined under high CO2 and PPF. Carbon dioxide concentration and PPF had no effect on total activity of rubisco. Low activation state and total activity of rubisco in in vitro leaves is the cause of poor photosynthetic activity in vitro Persistent in vitro leaves after 4 weeks of acclimatization did not have higher total activity of rubisco, but the activation state was 4 times larger than the corresponding activity in vitro which might thus provide for non-negligible contribution to photosynthetic carbon assimilation. The possible inhibition of photosynthesis by the presence of sugar in the medium is discussed.

Regulation of Ribulose-1,5-Bisphosphate Carboxylase Activity in Response to Light Intensity and CO2 in the C3 Annuals Chenopodium album L. and Phaseolus vulgaris L.

The light and CO(2) response of (a) photosynthesis, (b) the activation state and total catalytic efficiency (k(cat)) of ribulose-1,5-bisphosphate carboxylase (rubisco), and (c) the pool sizes of ribulose 1,5-bisphosphate, (RuBP), ATP, and ADP were studied in the C(3) annuals Chenopodium album and Phaseolus vulgaris at 25 degrees C. The initial slope of the photosynthetic CO(2) response curve was dependent on light intensity at reduced light levels only (less than 450 micromoles per square meter per second in C. album and below 200 micromoles per square meter per second in P. vulgaris). Modeled simulations indicated that the initial slope of the CO(2) response of photosynthesis exhibited light dependency when the rate of RuBP regeneration limited photosynthesis, but not when rubisco capacity limited photosynthesis. Measured observations closely matched modeled simulations. The activation state of rubisco was measured at three light intensities in C. album (1750, 550, and 150 micromoles per square meter per second) and at intercellular CO(2) partial pressures (C(1)) between the CO(2) compensation point and 500 microbars. Above a C(1) of 120 microbars, the activation state of rubisco was light dependent. At light intensities of 550 and 1750 micromoles per square meter per second, it was also dependent on C(1), decreasing as the C(1) was elevated above 120 microbars at 550 micromoles per square meter per second and above 300 microbars at 1750 micromoles per square meter per second. The pool size of RuBP was independent of C(1) only under conditions when the activation state of rubisco was dependent on C(1). Otherwise, RuBP pool sizes increased as C(1) was reduced. ATP pools in C. album tended to increase as C(1) was reduced. In P. vulgaris, decreasing C(1) at a subsaturating light intensity of 190 micromoles per square meter per second increased the activation state of rubisco but had little effect on the k(cat). These results support modelled simulations of the rubisco response to light and CO(2), where rubisco is assumed to be down-regulated when photosynthesis is limited by the rate of RuBP regeneration.