Rubisco Activation State Research Articles

Dark chilling imposes metabolic restrictions on photosynthesis in soybean

ABSTRACTDark chilling inhibited photosynthesis in two soybean [Glycine max (L.) Merr.] cultivars (Fiskeby V and Maple Arrow). The inhibition of CO2 assimilation was characterized by a simultaneous decrease in stomatal conductance (Gs) and intercellular CO2 concentration (Ci) in Maple Arrow, whereas a similar decrease in Gs in Fiskeby V occurred without any change in Ci. Dark chilling had little effect on total ribulose‐1, 5‐bisphosphate carboxylase/oxygenase (Rubisco) activity, Rubisco protein content or Rubisco activation state in the subsequent light period. Chilling reduced the abundance of the nocturnal Rubisco inhibitor, 2‐carboxyarabinitol 1‐phosphate, only in Fiskeby V. The abundance of Rubisco small subunit transcripts was enhanced in both cultivars as a result of dark chilling. Dark chilling decreased the maximal extractable activities and activation states of stromal fructose‐1,6‐bisphosphatase (FBPase) and NADP‐malate dehydrogenase but had no effect on sucrose phosphate synthase or leaf sucrose and starch contents. It is concluded that dark chilling‐induced limitations on CO2 assimilation are predominantly due to metabolic restrictions rather than to direct effects on electron transport reactions and that stromal FBPase is particularly susceptible to dark chilling.

Impaired Photosynthesis in Cucumber (Cucumis sativus L.) by High Root-zone Temperature Involves ABA-induced Stomatal Closure and Reduction in Ribulose-1,5-bisphosphate Carboxylase/oxygenase Activity

Effects of high root-zone temperature (RT) on ABA content, photosynthesis and its related parameters in cucumber (Cucumis sativus L., cv. Suyo) were investigated. Plants were grown in water culture at a RT of either 30°C or 3°C for 10 days. Other growth conditions were 30/25°C air temperatures (day/night), a 14-hr photoperiod, and a light intensity of 250 μmol·m·s-1. Plant growth was severely inhibited at 38°C RT as compared with that at 30°C RT. At 38°C RT, leaves did not wilt and the leaf color was normal green, but showed a marked increase in ABA content after day 6 with a significant decrease in stomatal leaf conductance. In both RT treatments, leaf water potential declined at day 6, which was more pronounced at 38°C RT than at 30°C RT, but the difference was relatively small. The photochemical activity of chloroplasts was not affected by high RT. However, the CO2 exchange rate (CER) of leaves decreased linearly after day 4 in the high RT treatment. Peeling the epidermis of the abaxial leaf surface to eliminate the stomatal limitation to CER resulted in a significant but not complete recovery of CER, indicating that the high RT limited CER through a decrease in not only stomatal aperture but also photosynthetic capacity of mesophyll cells. After day 6 of high RT treatment, ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) exhibited a gradual decrease in the initial level and the activation state. The results indicate that ABA-induced stomatal closure and reduction in the activation state of Rubisco are responsible for impaired photosynthesis in cucumber grown at elevated RT.

Sensitivity of photosynthesis in a C4 plant, maize, to heat stress.

Our objective was to determine the sensitivity of components of the photosynthetic apparatus of maize (Zea mays), a C4 plant, to high temperature stress. Net photosynthesis (Pn) was inhibited at leaf temperatures above 38 degrees C, and the inhibition was much more severe when the temperature was increased rapidly rather than gradually. Transpiration rate increased progressively with leaf temperature, indicating that inhibition was not associated with stomatal closure. Nonphotochemical fluorescence quenching (qN) increased at leaf temperatures above 30 degrees C, indicating increased thylakoid energization even at temperatures that did not inhibit Pn. Compared with CO(2) assimilation, the maximum quantum yield of photosystem II (F(v)/F(m)) was relatively insensitive to leaf temperatures up to 45 degrees C. The activation state of phosphoenolpyruvate carboxylase decreased marginally at leaf temperatures above 40 degrees C, and the activity of pyruvate phosphate dikinase was insensitive to temperature up to 45 degrees C. The activation state of Rubisco decreased at temperatures exceeding 32.5 degrees C, with nearly complete inactivation at 45 degrees C. Levels of 3-phosphoglyceric acid and ribulose-1,5-bisphosphate decreased and increased, respectively, as leaf temperature increased, consistent with the decrease in Rubisco activation. When leaf temperature was increased gradually, Rubisco activation acclimated in a similar manner as Pn, and acclimation was associated with the expression of a new activase polypeptide. Rates of Pn calculated solely from the kinetics of Rubisco were remarkably similar to measured rates if the calculation included adjustment for temperature effects on Rubisco activation. We conclude that inactivation of Rubisco was the primary constraint on the rate of Pn of maize leaves as leaf temperature increased above 30 degrees C.

Responses of Rubisco and sucrose-metabolizing enzymes to different CO 2 in a C 3 tropical epiphytic orchid Oncidium Goldiana

Experiments were conducted in controlled growth chambers to examine the responses of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), sucrose phosphate synthase (SPS) and sucrose synthase (SS), to long term CO 2 enrichment in the tropical epiphytic orchid hybrid Oncidium Goldiana. Furthermore, subsequent changes in activities of these enzymes were monitored following transfer to ambient air. After 2 months of growth, relative growth rate (RGR) in plants grown at elevated CO 2 doubled compared to those grown at ambient CO 2. Concomitantly, there was a decline in Rubisco activity, Rubisco activation state and Rubisco protein content in CO 2-enriched plants. In contrast, there was a significant up-regulation of SPS and SS activities. Both leaf sucrose and starch contents were significantly higher in plants grown at elevated CO 2 than that at ambient CO 2, while leaf total nitrogen content was markedly lower in CO 2-enriched plants. Following transfer to ambient CO 2, leaf SPS and SS activities, sucrose and starch content in CO 2-enriched plants declined sharply within 10 days to the same level of control plants without CO 2 enrichment. Contrary to our expectation, after 20 days of growth at ambient air, Rubisco activity in CO 2-enriched plants remained much lower than that of control plants, although there was some increase of leaf Rubisco activity. This is primarily due to the consistently low levels of Rubisco protein content in the leaves. Leaf SPS activity was closely associated with leaf sucrose accumulation and SS activity was closely associated with starch accumulation. The activities of SPS and SS in leaf extracts were also closely associated with leaf assimilation rates. It is suggested that (1) the up-regulation of leaf SPS and SS activities might be an acclimation response to optimize the utilization and export of organic-carbon with the increased rate of inorganic-carbon fixation; (2) SPS and SS might play an important role in carbon partitioning between sucrose and starch in O. Goldiana under elevated CO 2 conditions.

The activation state of Rubisco directly limits photosynthesis at low CO(2) and low O(2) partial pressures.

Using gas exchange, enzyme assays, and theoretical modeling of photosynthetic responses to light and CO(2), we investigated whether decarbamylation of the active site of Rubisco at low CO(2) and low light leads to a condition where the activation state of Rubisco directly limits the rate of net CO(2) assimilation. Photosynthetic limitation by a reduction in the activation state of Rubisco would be indicated as a decline in the initial slope of the photosynthetic CO(2) response relative to what is predicted using theoretical models. In bean (Phaseolus vulgaris) and oat (Avena sativa), we saw no discrepancy between predicted and observed initial slope values at 200 and 400 mbar O(2), indicating no limitation by the carbamylation state of Rubisco. At 30 mbar O(2) and light saturation, we also saw no discrepancy between predicted and observed initial slope values; however, at subsaturating light intensity, our observed initial slope values were less than the modeled initial slope values that corresponded to an RuBP regeneration limitation. Moreover, significant reduction of the Rubisco activation state occurred in both species at 30 mbar O(2) and 30 mubar CO(2). When the model was reprogrammed to account for observed levels of Rubisco deactivation, the predicted and measured initial slope values at low O(2) and low PPFD were similar, indicating the reduction in carbamylation state accounted for the discrepancy. We interpret this as evidence for a direct limitation of the carbamylation state of Rubisco, probably because of a CO(2) limitation for carbamate formation. This limitation was only observed at intercellular CO(2) levels below what is encountered in vivo. At physiologically relevant CO(2) levels in situ, the leaves maintained sufficient Rubisco activity to avoid cabamylation state limitations in the steady state.

Resistance is useful: diurnal patterns of photosynthesis in C3 and crassulacean acid metabolism epiphytic bromeliads.

This paper originates from a presentation at the IIIrd International Congress on Crassulacean Acid Metabolism, Cape Tribulation, Queensland, Australia, August 2001 Diurnal patterns of photosynthesis in response to environmental variables were investigated in an obligate C3 and a facultative C3-crassulacean acid metabolism (CAM) bromeliad species. A midday depression of photosynthesis occurred in both C3 groups, mediated as a decrease in stomatal conductance in response to increased vapour pressure difference. The response was associated with a reduction in Rubisco activation state during the period of maximum photon flux density. In contrast, the switch to CAM resulted in a strong shift in the pattern of Rubisco carbamylation, with full enzyme activation delayed until the midday period. For the first time it is demonstrated that the pattern of Rubisco activation differs between C3 and CAM plants of the same species under identical conditions. Despite large differences in Rubisco content between C3 and CAM plants, neither the amount of Rubisco or enzyme activity is thought to be limiting for photosynthesis, and it is suggested that Rubisco may function as a nitrogen store. Extreme CO2 diffusion limitation resulted in low rates of atmospheric CO2 assimilation that were associated with high rates of photosynthetic electron transport, and it is likely that photorespiration constitutes a significant electron sink over the entire diurnal course. Leaf morphological and physiological adaptations to drought stress are necessary for the epiphytic lifestyle but limit CO2 assimilation and confound the likelihood of high productivity.

Photosynthetic pigments, photosynthesis and plastid ultrastructure in RbcS antisense DNA mutants of tobacco (Nicotiana tabacum).

RbcS antisense DNA mutants of tobacco have reduced amounts of ribulose bisphosphate carboxylase oxygenase (Rubisco). We found that carotenoid and chlorophyll contents decrease in parallel as Rubisco is decreased, however, pigment levels are not significantly altered until Rubisco levels are reduced sharply. The mutants have normal Chl a/Chl b ratios and normal plastid ultrastructures, suggesting that reductions in Rubisco do not dramatically alter the composition of the thylakoid membranes. Nevertheless, chlorophyll fluorescence measurements, in which developmentally homogenous leaves were sampled, showed that there is reduced photosynthetic capacity of PSII and an enhanced photosensitivity in the mutants, especially in transgenics with severe reductions in Rubisco content. Support for this conclusion comes from several observations: 1) light saturation occurs at a lower light intensity in the mutants, resulting in an earlier closure of PS II (lower photochemical quenching); 2) the mutants have reduced photosynthetic efficiency (lower deltaF/Fm'); and 3) the mutants have a slower recovery of Fv/Fm. We found that acclimation to increasing light intensies in the mutants appears to involve an enhanced inactivation of PSII reaction centers as well as an increased activation of photoprotective mechanisms, notably an engagement of the xanthophyll cycle at lower than normal light intensities. We conclude that the photosensitivity of the antisense mutants is due, in part, to a limitation in Rubisco activation state.

Responses of Rubisco activity in vivo to light intensity and CO2 in rice leaves

The light response of photosynthesis and Rubisco activity at 36 Pa CO2 was first examined in leaves of the wild-type and rbcS antisense rice with 40% wild type Rubisco. Whereas light-saturated photosynthesis was about 2-fold greater in the wild-type plants than in the rbcS antisense plants, the initial slope of the light response curve was not different between the plants. This was caused by higher activation state of Rubisco in the antisense plants under low light. Rubisco inhibition was not different between the plants, and it decreased with increasing light intensity. No significant inhibition was found around light-saturation point. The activation state of NADP-malate dehydrogenase (MDH) was not correlated with Rubisco activation, but correlated with the electron transport rate at PS II measured by Chl fluorescence. The CO2 response curve at 1500 µmol quanta m-2 s-1 was next examined. While the initial slope was about 2-fold greater in the wild-type plants than in the antisense plants, the difference decreased with increasing CO2 partial pressure. Rubisco activation maintained at high levels (more than 80%) in both plants irrespective of CO2 partial pressure (5 to 120 Pa). In spite of the high light conditions, a significant Rubisco inhibition was observed around 100 Pa CO2 for both plants. This inhibition was greater in the antisense plants (about 30% of maximal activity) than in the wild type plants (20%). The NADP-MDH activation was not correlated with Rubisco activation, but correlated with the quantum yield of PS II.

Effect of heat stress on the inhibition and recovery of the ribulose-1,5-bisphosphate carboxylase/oxygenase activation state.

Experiments were conducted to determine the relative contributions of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) activation state vis-a-vis Rubisco activase and metabolite levels to the inhibition of cotton (Gossypium hirsutum L.) photosynthesis by heat stress. Exposure of leaf tissue in the light to temperatures of 40 or 45 degrees C decreased the activation state of Rubisco to levels that were 65 or 10%, respectively, of the 28 degrees C control. Ribulose-1,5-bisphosphate (RuBP) levels increased in heat-stressed leaves, whereas the 3-phosphoglyceric acid pool was depleted. Heat stress did not affect Rubisco per se, as full activity could be restored by incubation with CO2 and Mg2+. Inhibition and recovery of Rubisco activation state and carbon dioxide exchange rate (CER) were closely related under moderate heat stress (up to 42.5 degrees C). Moderate heat stress had negligible effect on Fv/Fm, the maximal quantum yield of photosystem II. In contrast, severe heat stress (45 degrees C) caused significant and irreversible damage to Rubisco activation, CER, and Fv/Fm. The rate of Rubisco activation after alleviating moderate heat stress was comparable to that of controls, indicating rapid reversibility of the process. However, moderate heat stress decreased both the rate and final extent of CER activation during dark-to-light transition. Treatment of cotton leaves with methyl viologen or an oxygen-enriched atmosphere reduced the effect of heat stress on Rubisco inactivation. Both treatments also reduced tissue RuBP levels, indicating that the amount of RuBP present during heat stress may influence the degree of Rubisco inactivation. Under both photorespiratory and non-photorespiratory conditions, the inhibition of the CER during heat stress could be completely reversed by increasing the internal partial pressure of CO2 (Ci). However, the inhibition of the CER by nigericin, a K+ ionophore, was not reversible when the Ci was increased at ambient or high temperature. Our results indicate that inhibition of photosynthesis by moderate heat stress is not caused by inhibition of the capacity for RuBP regeneration. We conclude that heat stress inhibits Rubisco activation via a rapid and direct effect on Rubisco activase, possibly by perturbing Rubisco activase subunit interactions with each other or with Rubisco.

Rubisco activation state decreases with increasing nitrogen content in apple leaves.

Based on the curvilinear relationship between leaf nitrogen content and the initial slope of the response of CO(2) assimilation (A:) to intercellular CO(2) concentrations (C:(i)) in apple, it is hypothesized that Rubisco activation state decreases with increasing leaf N content and this decreased activation state accounts for the curvilinear relationship between leaf N and CO(2) assimilation. A range of leaf N content (1.0-5.0 g m(-2)) was achieved by fertilizing bench-grafted Fuji/M.26 apple (Malus domestica Borkh.) trees for 45 d with different N concentrations, using a modified Hoagland's solution. Analysis of A:/C:(i) curves under saturating light indicated that CO(2) assimilation at ambient CO(2) fell within the Rubisco limitation region of the A:/C:(i) curves, regardless of leaf N status. Initial Rubisco activity showed a curvilinear response to leaf N. In contrast, total Rubisco activity increased linearly with increasing leaf N throughout the leaf N range. As a result, Rubisco activation state decreased with increasing leaf N. Both light-saturated CO(2) assimilation at ambient CO(2) and the initial slope of the A:/C:(i) curves were linearly related to initial Rubisco activity, but curvilinearly related to total Rubisco activity. The curvatures in the relationships of both light-saturated CO(2) assimilation at ambient CO(2) and the initial slope of the A:/C:(i) curves with total Rubisco activity were more pronounced than in their relationships with leaf N. This was because the ratio of total Rubisco activity to leaf N increased with increasing leaf N. As leaf N increased, photosynthetic N use efficiency declined with decreasing Rubisco activation state.

Rubisco activation state decreases with increasing nitrogen content in apple leaves

Based on the curvilinear relationship between leaf nitrogen content and the initial slope of the response of CO(2) assimilation (A:) to intercellular CO(2) concentrations (C:(i)) in apple, it is hypothesized that Rubisco activation state decreases with increasing leaf N content and this decreased activation state accounts for the curvilinear relationship between leaf N and CO(2) assimilation. A range of leaf N content (1.0-5.0 g m(-2)) was achieved by fertilizing bench-grafted Fuji/M.26 apple (Malus domestica Borkh.) trees for 45 d with different N concentrations, using a modified Hoagland's solution. Analysis of A:/C:(i) curves under saturating light indicated that CO(2) assimilation at ambient CO(2) fell within the Rubisco limitation region of the A:/C:(i) curves, regardless of leaf N status. Initial Rubisco activity showed a curvilinear response to leaf N. In contrast, total Rubisco activity increased linearly with increasing leaf N throughout the leaf N range. As a result, Rubisco activation state decreased with increasing leaf N. Both light-saturated CO(2) assimilation at ambient CO(2) and the initial slope of the A:/C:(i) curves were linearly related to initial Rubisco activity, but curvilinearly related to total Rubisco activity. The curvatures in the relationships of both light-saturated CO(2) assimilation at ambient CO(2) and the initial slope of the A:/C:(i) curves with total Rubisco activity were more pronounced than in their relationships with leaf N. This was because the ratio of total Rubisco activity to leaf N increased with increasing leaf N. As leaf N increased, photosynthetic N use efficiency declined with decreasing Rubisco activation state.

Growth and physiological acclimation to temperature and inorganic carbon availability by two submerged aquatic macrophyte species, Callitriche cophocarpa and Elodea canadensis

Abstract1. Interactive effects of temperature and inorganic carbon availability on photosynthetic acclimation and growth of two submerged macrophyte species, Elodea canadensis and Callitriche cophocarpa, were examined to test the hypotheses that: (1) effects of temperature on growth rate and photosynthetic acclimation are suppressed under low inorganic carbon availability; (2) the plants compensate for the reduction in activity of individual enzymes at lower temperatures by increasing the activity per unit plant mass, here exemplified by Rubisco. The experiments were performed in the laboratory where plants were grown in a factorial combination of three temperatures (7–25 °C) and three inorganic carbon regimes.2. The relative growth rate of both species was strongly affected by growth conditions and increased by up to 4·5 times with increased temperature and inorganic carbon availability. The sensitivity to inorganic carbon was greatest at high temperature and the sensitivity to temperature greatest at high carbon concentrations.3. Photosynthetic acclimation occurred in response to growth conditions for both species. The affinity for inorganic carbon and the photosynthetic capacity, both measured at 15 °C, increased with reduced inorganic carbon availability during growth and were greater at warmer than at cooler growth temperature. The acclimative change in photosynthesis was related to the extent of temperature and inorganic carbon stress. Using data for Elodea, a negative relationship between degree of temperature stress and photosynthetic performance was found. In relation to inorganic carbon, a linear increase in CO2 affinity and photosynthetic capacity was found with increased inorganic carbon stress during growth.4. The total Rubisco activity declined with increased inorganic carbon availability during growth and with enhanced growth temperature. In addition, the activation state of Rubisco was higher at cooler than at warmer temperatures for Callitriche. This suggests that low‐temperature grown plants compensate for the temperature‐dependent reduction in activity of the individual Rubisco molecules by enhancing resource allocations towards Rubisco.

Modelling the role of Rubisco activase in limiting non-steady-state photosynthesis.

The roles of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and Rubisco activase in limiting the approach of photosynthesis to steady-state following a step increase from a low to a saturating value of photon flux density (PFD) are reviewed. This information, along with the effect of Rubisco on steady-state photosynthetic rate and the effect of Rubisco activase on maximum Rubisco activation state, is then used to construct a model to predict the optimum allocation of protein between Rubisco and Rubisco activase for plants exposed to different light environments. The model predicts that the distribution of protein that produces the maximum steady-state rate of photosynthesis does not produce the maximum activation rate for Rubisco or the maximum steady-state activation state. The latter conclusion may explain why Rubisco is rarely found to be fully activated in leaves, even at saturating PFD values. The former suggests that plants exposed to fluctuating PFD should allocate more protein to Rubisco activase than plants exposed to constant PFD. This aspect of the model is explored in more detail for lightflecks of differing duration.

Relationship between the suppression of photosynthesis and starch accumulation in the pod-removed bean

Effects of pod removal on the photosynthetic rates and their under lying biochemical properties were examined in leaves of bean (Phaseolus vulgaris L.). Pod removal led to decreases in the photosynthetic gas- exchange rate, stomatal conductance and intercellular CO2 pressure. H owever, these decreases in photosynthesis were not asso-ciated with changes in the amounts of ribulose-1,5-bisphosphate carb oxylase (Rubisco) and other several photo-synthetic components, but were associated with an increase in starch content. In addition, such a decrease in the photosynthetic rate was not observed when measured with O2 evolution under conditions of saturating CO2. Similarly, although a decrease in CO2-limited photosynthesis for a given Rubisco content was found in depodded plants, neither the speci fic activity nor activation state of Rubisco decreased . The decrease in the photosynthetic gas-exchange rate by pod removal was rec overed by shading the plants. CO2-limited photosynthesis per unit of Rubisco protein was also completely recovered at the same l evel in the plants with pods. At the same time, starch accumu-lation was also decreased by shading. From these results, we concluded that the suppression of photosynthesis observed in depodded beans was caused by the hindrance of CO2 diffusion due to starch accumulated in the chloroplast.

Acclimation of the summer annual species, lolium temulentum, to CO(2) enrichment

Lolium temulentum L. Ba 3081 was grown hydroponically in air (350 &mgr;mol mol(-1) CO(2)) and elevated CO(2) (700 &mgr;mol mol(-1) CO(2)) at two irradiances (150 and 500 &mgr;mol m(-2) s(-1)) for 35 days at which point the plants were harvested. Elevated CO(2) did not modify relative growth rate or biomass at either irradiance. Foliar carbon-to-nitrogen ratios were decreased at elevated CO(2) and plants had a greater number of shorter tillers, particularly at the lower growth irradiance. Both light-limited and light-saturated rates of photosynthesis were stimulated. The amount of ribulose-1, 5-bisphosphate carboxylase-oxygenase (Rubisco) protein was increased at elevated CO(2), but maximum extractable Rubisco activities were not significantly increased. A pronounced decrease in the Rubisco activation state was found with CO(2) enrichment, particularly at the higher growth irradiance. Elevated-CO(2)-induced changes in leaf carbohydrate composition were small in comparison to those caused by changes in irradiance. No CO(2)-dependent effects on fructan biosynthesis were observed. Leaf respiration rates were increased by 68% in plants grown with CO(2) enrichment and low light. We conclude that high CO(2) will only result in increased biomass if total light input favourably increases the photosynthesis-to-respiration ratio. At low irradiances, biomass is more limited by increased rates of respiration than by CO(2)-induced enhancement of photosynthesis.

Do the gas exchange characteristics of alien acacias enable them to successfully invade the fynbos?

The invasive success of alien Acacia species in the fynbos could be partially attributable to physiological differences to indigenous fynbos. In this investigation we tested the hypothesis that the greater N availability due to the N 2 -fixing ability of acacias would result in greater Rubisco activity and photosynthetic capacity, relative to fynbos species. The gas exchange characteristics, leaf N concentrations and photosynthetic N use efficiencies of Acacia saligna and Acacia longifolia were compared with those of Protea repens , Chrysanthemoides monilifera , Dodenea viscosa and Leucadendron salignum . Compared to the fynbos species, the acacias had larger leaves and higher leaf water contents, but intermediate specific leaf areas. The acacias had more than 3-fold higher leaf N concentrations, but 50% lower photosynthetic nitrogen use efficiency than the fynbos species. The acacias showed no clear photosynthetic advantage over the fynbos species, although, the former tended to have higher photosynthetic capacities and water use efficiencies. The lack of correlation between leaf N concentrations and photosynthetic capacity or Rubisco activity may have been due to differences in the utilisation of N for nonphotosynthetic processes and in the activation state of Rubisco. It seems unlikely that photosynthetic characteristics play an important role in the invasive success of alien acacias in the Fynbos biome.

Inhibition and acclimation of photosynthesis to heat stress is closely correlated with activation of ribulose-1,5-bisphosphate Carboxylase/Oxygenase

Increasing the leaf temperature of intact cotton (Gossypium hirsutum L.) and wheat (Triticum aestivum L.) plants caused a progressive decline in the light-saturated CO2-exchange rate (CER). CER was more sensitive to increased leaf temperature in wheat than in cotton, and both species demonstrated photosynthetic acclimation when leaf temperature was increased gradually. Inhibition of CER was not a consequence of stomatal closure, as indicated by a positive relationship between leaf temperature and transpiration. The activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which is regulated by Rubisco activase, was closely correlated with temperature-induced changes in CER. Nonphotochemical chlorophyll fluorescence quenching increased with leaf temperature in a manner consistent with inhibited CER and Rubisco activation. Both nonphotochemical fluorescence quenching and Rubisco activation were more sensitive to heat stress than the maximum quantum yield of photochemistry of photosystem II. Heat stress led to decreased 3-phosphoglyceric acid content and increased ribulose-1, 5-bisphosphate content, which is indicative of inhibited metabolite flow through Rubisco. We conclude that heat stress inhibited CER primarily by decreasing the activation state of Rubisco via inhibition of Rubisco activase. Although Rubisco activation was more closely correlated with CER than the maximum quantum yield of photochemistry of photosystem II, both processes could be acclimated to heat stress by gradually increasing the leaf temperature.

Effects of long‐term elevated [CO2] from natural CO2 springs on Nardus stricta: photosynthesis, biochemistry, growth and phenology

Plants of Nardus stricta growing near a cold, naturally emitting CO2 spring in Iceland were used to investigate the long‐term (> 100 years) effects of elevated [CO2] on photosynthesis, biochemistry, growth and phenology in a northern grassland ecosystem. Comparisons were made between plants growing in an atmosphere naturally enriched with CO2 (≈ 790 μmol mol–1) near the CO2 spring and plants of the same species growing in adjacent areas exposed to ambient CO2 concentrations (≈360μmol mol–1). Nardus stricta growing near the spring exhibited earlier senescence and reductions in photosynthetic capacity (≈25%), Rubisco content (≈26%), Rubisco activity (≈40%), Rubisco activation state (≈23%), chlorophyll content (≈33%) and leaf area index (≈22%) compared with plants growing away from the spring. The potential positive effects of elevated [CO2] on grassland ecosystems in Iceland are likely to be reduced by strong down‐regulation in the photosynthetic apparatus of the abundant N. stricta species.

The Effect of Elevated Partial Pressures of CO2 on the Relationship between Photosynthetic Capacity and N Content in Rice Leaves.

The effects of growth CO2 levels on the photosynthetic rates; the amounts of ribulose-1,5-bisphosphate carboxylase (Rubisco), chlorophyll (Chl), and cytochrome f; sucrose phosphate synthase activity; and total N content were examined in young, fully expanded leaves of rice (Oryza sativa L.). The plants were grown hydroponically under two CO2 partial pressures of 36 and 100 Pa at three N concentrations. The light-saturated photosynthesis at 36 Pa CO2 was lower in the plants grown in 100 Pa CO2 than those grown in 36 Pa CO2. Similarly, the amounts of Rubisco, Chl, and total N were decreased in the leaves of the plants grown in 100 Pa CO2. However, regression analysis showed no differences between the two CO2 treatments in the relationship between photosynthesis and total N or in the relationship between Rubisco and Chl and total N. Although a relative decrease in Rubisco to cytochrome f or sucrose phosphate synthase was found in the plants grown in 100 Pa CO2, this was the result of a decrease in total N content by CO2 enrichment. The activation state of Rubisco was also unaffected by growth CO2 levels. Thus, decreases in the photosynthetic capacity of the plants grown in 100 Pa CO2 could be simply accounted for by a decrease in the absolute amount of leaf N.

The Relationship between Rubisco Activity and Photosynthesis in Apple Leaves with Different Nitrogen Content

Based on the curvilinear relationship between carboxylation efficiency and leaf N in apple leaves, we hypothesized that deactivation of Rubisco accounts for the lack of response of photosynthesis to increasing leaf N under high N supply. A wide range of leaf N content (from 1.0 to 5.0 g·m–2) was achieved by fertigating bench-grafted Fuji/M26 apple trees for 6 weeks with different N concentrations using a modified Hoagland solution. Analysis of photosynthesis in response to intercellular CO2 under both 21% and 2% O2 indicated that photosynthesis at ambient CO2 was mainly determined by the activity of Rubisco. Measurements of Rubisco activity revealed that initial Rubisco activity increased with leaf N up to 3.0 g·m–2, then leveled off with further rise in leaf N, whereas total Rubisco activity increased linearly with increasing leaf N throughout the leaf N range. As a result, Rubisco activation state decreased with increasing leaf N. Photosynthesis at ambient CO2 and carboxylation efficiency were both linearly correlated with initial Rubisco activity, but showed curvilinear relationships with total Rubisco activity and leaf N. As leaf N increased, photosynthetic nitrogen use efficiency declined with decreasing Rubisco activation state.