Expanding knowledge of the Rubisco kinetics variability in plant species: environmental and evolutionary trends

The present study characterizes the kinetic properties of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) from 28 terrestrial plant species, representing different phylogenetic lineages, environmental adaptations and photosynthetic mechanisms. Our findings confirm that past atmospheric CO₂/O₂ ratio changes and present environmental pressures have influenced Rubisco kinetics. One evolutionary adaptation to a decreasing atmospheric CO₂/O₂ ratio has been an increase in the affinity of Rubisco for CO₂ (K_c falling), and a consequent decrease in the velocity of carboxylation (k_cat^c), which in turn has been ameliorated by an increase in the proportion of leaf protein accounted by Rubisco. The trade‐off between K_c and k_cat^c was not universal among the species studied and deviations from this relationship occur in extant forms of Rubisco. In species adapted to particular environments, including carnivorous plants, crassulacean acid metabolism species and C₃ plants from aquatic and arid habitats, Rubisco has evolved towards increased efficiency, as demonstrated by a higher k_cat^c/K_c ratio. This variability in kinetics was related to the amino acid sequence of the Rubisco large subunit. Phylogenetic analysis identified 13 residues under positive selection during evolution towards specific Rubisco kinetic parameters. This crucial information provides candidate amino acid replacements, which could be implemented to optimize crop photosynthesis under a range of environmental conditions.     

Would transformation of C3 crop plants with foreign Rubisco increase productivity? A computational analysis extrapolating from kinetic properties to canopy photosynthesis

Genetic modification of Rubisco to increase the specificity for CO₂ relative to O₂ (τ) would decrease photorespiration and in principle should increase crop productivity. When the kinetic properties of Rubisco from different photosynthetic organisms are compared, it appears that forms with high τ have low maximum catalytic rates of carboxylation per active site (k^c_c). If it is assumed that an inverse relationship between k^c_c and τ exists, as implied from measurements, and that an increased concentration of Rubisco per unit leaf area is not possible, will increasing τ result in increased leaf and canopy photosynthesis? A steady‐state biochemical model for leaf photosynthesis was coupled to a canopy biophysical microclimate model and used to explore this question. C₃ photosynthetic CO₂ uptake rate (A) is either limited by the maximum rate of Rubisco activity (V_cmax) or by the rate of regeneration of ribulose‐1,5‐bisphosphate, in turn determined by the rate of whole chain electron transport (J). Thus, if J is limiting, an increase in τ will increase net CO₂ uptake because more products of the electron transport chain will be partitioned away from photorespiration into photosynthesis. The effect of an increase in τ on Rubisco‐limited photosynthesis depends on both k^c_c and the concentration of CO₂ ([CO₂]). Assuming a strict inverse relationship between k^c_c and τ, the simulations showed that a decrease, not an increase, in τ increases Rubisco‐limited photosynthesis at the current atmospheric [CO₂], but the increase is observed only in high light. In crop canopies, significant amounts of both light‐limited and light‐saturated photosynthesis contribute to total crop carbon gain. For canopies, the present average τ found in C₃ terrestrial plants is supra‐optimal for the present atmospheric [CO₂] of 370 µmol mol^?1, but would be optimal for a CO₂ concentration of around 200 µmol mol^?1, a value close to the average of the last 400 000 years. Replacing the average Rubisco of terrestrial C₃ plants with one having a lower and optimal τ would increase canopy carbon gain by 3%. Because there are significant deviations from the strict inverse relationship between k^c_c and τ, the canopy model was also used to compare the rates of canopy photosynthesis for several Rubiscos with well‐defined kinetic constants. These simulations suggest that very substantial increases (> 25%) in crop carbon gain could result if specific Rubiscos having either a higher τ or higher k^c_c were successfully expressed in C₃ plants.     

Adjustment of leaf photosynthesis to shade in a natural canopy: rate parameters

The present study was performed to investigate the adjustment of the rate parameters of the light and dark reactions of photosynthesis to the natural growth light in leaves of an overstorey species, Betula pendula Roth, a subcanopy species, Tilia cordata P. Mill., and a herb, Solidago virgaurea L., growing in a natural plant community in Järvselja, Estonia. Shoots were collected from the site and individual leaves were measured in a laboratory applying a standardized routine of kinetic gas exchange, Chl fluorescence and 820 nm transmittance measurements. These measurements enabled the calculations of the quantum yield of photosynthesis and rate constants of excitation capture by photochemical and non-photochemical quenchers, rate constant for P700⁺ reduction via the cytochrome b₆f complex with and without photosynthetic control, actual maximum and potential (uncoupled) electron transport rate, stomatal and mesophyll resistances for CO₂ transport, K_m(CO₂) and V_m of ribulose-bisphosphate carboxylase-oxygenase (Rubisco) in vivo. In parallel, N, Chl and Rubisco contents were measured from the same leaves. No adjustment toward higher quantum yield in shade compared with sun leaves was observed, although relatively more N was partitioned to the light-harvesting machinery in shade leaves ( H. Eichelmann et al., 2004 ). The electron transport rate through the Cyt b₆f complex was strongly down-regulated under saturating light compared with darkness, and this was observed under atmospheric, as well as saturating CO₂ concentration. In vivo V_m measurements of Rubisco were lower than corresponding reported measurements in vitro, and the k_cat per reaction site varied widely between leaves and growth sites. The correlation between Rubisco V_m and the photosystem I density was stronger than between V_m and the density of Rubisco active sites. The results showed that the capacity of the photosynthetic machinery decreases in shade-adjusted leaves, but it still remains in excess of the actual photosynthetic rate. The photosynthetic control systems that are targeted to adjust the photosynthetic rate to meet the plant's needs and to balance the partial reactions of photosynthesis, down-regulate partial processes of photosynthesis: excess harvested light is quenched non-photochemically; excess electron transport capacity of Cyt b₆f is down-regulated by ΔpH-dependent photosynthetic control; Rubisco is synthesized in excess, and the number of activated Rubisco molecules is controlled by photosystem I-related processes. Consequently, the nitrogen contained in the components of the photosynthetic machinery is not used at full efficiency. The strong correlation between leaf nitrogen and photosynthetic performance is not due to the nitrogen requirements of the photosynthetic apparatus, but because a certain amount of energy must be captured through photosynthesis to maintain this nitrogen within a leaf.     

Elevated CO2 and water-availability effect on gas exchange and nodule development in N2-fixing alfalfa plants

N₂-fixing alfalfa plants were grown in controlled conditions at different CO₂ levels (350 μmol mol^?1 versus 700 μmol mol^?1) and water-availability conditions (WW, watered at maximum pot water capacity versus WD, watered at 50% of control treatments) in order to determine the CO₂ effect (and applied at two water regimes) on plant growth and nodule activity in alfalfa plants. The CO₂ stimulatory effect (26% enhancement) on plant growth was limited to WW plants, whereas no CO₂ effect was observed in WD plants. Exposure to elevated CO₂ decreased Rubisco carboxylation capacity of plants, caused by a specific reduction in Rubisco (EC 4.1.1.39) concentration (11% in WW and 43% in WD) probably explained by an increase in the leaf carbohydrate levels. Plants grown at 700 μmol mol^?1 CO₂ maintained control photosynthetic rates (at growth conditions) by diminishing Rubisco content and by increasing nitrogen use efficiency. Interestingly, our data also suggest that reduction in shoot N demand (reflected by the TSP and especially Rubisco depletion) affected negatively nodule activity (malate dehydrogenase, EC 1.1.1.37, and glutamate-oxaloacetate transaminase, EC 2.6.1.1, activities) particularly in water-limited conditions. Furthermore, nodule DM and TSS data revealed that those nodules were not capable to overcome C sink strength limitations.     

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

Trees 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-C_i) curves. Maximum Rubisco activity (Vc_max) and ribulose 1,5-bisphosphate regeneration capacity mediated by electron transport (J_max) and phosphate regeneration (PiRC) were calculated from A-C_i 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.     

Down-regulation of Rubisco activity under combined increases of CO<Subscript>2</Subscript> and temperature minimized by changes in Rubisco k<Subscript>cat</Subscript> in wheat

Increases in growth temperature have been observed to affect photosynthesis differently under long-term exposure to ambient- and twice ambient-air CO₂ concentrations. This study investigates the causes of this interaction in wheat (Triticum aestivum L.) grown in the field over two consecutive years under temperature gradient chambers in ambient (370 μmol mol⁻¹) or elevated (700 μmol mol⁻¹) atmospheric CO₂ concentrations and at ambient or ambient +4°C temperatures, with either a low or a high nitrogen supply. The photosynthesis-internal CO₂ response curves and the activity, activation state, k_cat and amount of Ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) were measured, as well as the soluble protein concentration in flag leaves at ear emergence and 8–15 days after anthesis. A high nitrogen supply increased V_cmax, the Rubisco amount and activity and soluble protein contents, but did not significantly change the Rubisco k_cat. Both elevated CO₂ and above ambient temperatures had negative effects on V_cmax and Rubisco activity, but at elevated CO₂, an increase in temperature did not decrease V_cmax or Rubisco activity in relation to ambient temperature. The amounts of Rubisco and soluble protein decreased with elevated CO₂ and temperature. The negative impact of elevated CO₂ on Rubisco properties was somewhat counteracted at elevated temperatures by an increase in k_cat. This effect can diminish the detrimental effects on photosynthesis of combined increases of CO₂ and temperature.     

Acclimation response of spring wheat in a free-air CO2 enrichment (FACE) atmosphere with variable soil nitrogen regimes. 1. Leaf position and phenology determine acclimation response

We have examined the photosynthetic acclimation of wheat leaves grown at an elevated CO₂ concentration, and ample and limiting N supplies, within a field experiment using free-air CO₂ enrichment (FACE). To understand how leaf age and developmental stage affected any acclimation response, measurements were made on a vertical profile of leaves every week from tillering until maturity. The response of assimilation (A) to internal CO₂ concentration (C_i) was used to estimate the in vivo carboxylation capacity (Vc_max) and maximum rate of ribulose-1,5-bisphosphate limited photosynthesis (A _sat). The total activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), and leaf content of Rubisco and the Light Harvesting Chlorophyll a/b protein associated with Photosystem II (LHC II), were determined. Elevated CO₂ did not alter Vc_max in the flag leaf at either low or high N. In the older shaded leaves lower in the canopy, acclimatory decline in Vc_max and A _sat was observed, and was found to correlate with reduced Rubisco activity and content. The dependency of acclimation on N supply was different at each developmental stage. With adequate N supply, acclimation to elevated CO₂ was also accompanied by an increased LHC II/Rubisco ratio. At low N supply, contents of Rubisco and LHC II were reduced in all leaves, although an increased LHC II/Rubisco ratio under elevated CO₂ was still observed. These results underscore the importance of leaf position, leaf age and crop developmental stage in understanding the acclimation of photosynthesis to elevated CO₂ and nutrient stress. This revised version was published online in June 2006 with corrections to the Cover Date.     

Simultaneous determination of Rubisco carboxylase and oxygenase kinetic parameters in Triticum aestivum and Zea mays using membrane inlet mass spectrometry

The lack of complete Rubisco kinetic data for numerous species is partly because of the time consuming nature of the multiple methods needed to assay all of the Rubisco parameters. We have developed a membrane inlet mass spectrometer method that simultaneously determines the rate of Rubisco carboxylation (v_c) and oxygenation (v_o), and the CO₂ and O₂ concentrations. Using the collected data, the Michaels‐Menten equations for v_c and v_o in response to changing CO₂ and O₂ concentrations were simultaneously solved for the CO₂ (K_c) and O₂ (K_o) constants, the maximum turnover rates of the enzyme for CO₂ (kcat_CO2) and O₂ (kcat_O2) and the specificity for CO₂ relative to O₂ (S_c/o). In the C₄ species Zea mays K_c was higher but K_o was lower compared with the C₃ species Triticum aestivum. The kcat_CO2 was higher and the kcat_O2 lower in Z. mays compared with T. aestivum and S_c/o was similar in the two species. The V_omax/V_cmax was lower in Z. mays and thus did not correlate with changes in S_c/o. In conclusion, this mass spectrometer system provides a means of simultaneously determining the important Rubisco kinetic parameters, K_c, K_o, kcat_CO2,kcat_O2 and S_c/o from the same set of assays.     

The temporal and species dynamics of photosynthetic acclimation in flag leaves of rice (Oryza sativa) and wheat (Triticum aestivum) under elevated carbon dioxide

In this study, we tested for the temporal occurrence of photosynthetic acclimation to elevated [CO₂] in the flag leaf of two important cereal crops, rice and wheat. In order to characterize the temporal onset of acclimation and the basis for any observed decline in photosynthetic rate, we characterized net photosynthesis, g_s, g_m, C_i/C_a, C_i/C_c, V_cmax, J_max, cell wall thickness, content of Rubisco, cytochrome (Cyt) f, N, chlorophyll and carbohydrate, mRNA expression for rbcL and petA, activity for Rubisco, sucrose phosphate synthase (SPS) and sucrose synthase (SS) at full flag expansion, mid‐anthesis and the late grain‐filling stage. No acclimation was observed for either crop at full flag leaf expansion. However, at the mid‐anthesis stage, photosynthetic acclimation in rice was associated with RuBP carboxylation and regeneration limitations, while wheat only had the carboxylation limitation. By grain maturation, the decline of Rubisco content and activity had contributed to RuBP carboxylation limitation of photosynthesis in both crops at elevated [CO₂]; however, the sharp decrease of Rubisco enzyme activity played a more important role in wheat. Although an increase in non‐structural carbohydrates did occur during these later stages, it was not consistently associated with changes in SPS and SS or photosynthetic acclimation. Rather, over time elevated [CO₂] appeared to enhance the rate of N degradation and senescence so that by late‐grain fill, photosynthetic acclimation to elevated [CO₂] in the flag leaf of either species was complete. These data suggest that the basis for photosynthetic acclimation with elevated [CO₂] may be more closely associated with enhanced rates of senescence, and, as a consequence, may be temporally dynamic, with significant species variation.     

Alterations in Rubisco activity and in stomatal behavior induce a daily rhythm in photosynthesis of aerial leaves in the amphibious-plant Nuphar lutea

Nuphar lutea is an amphibious plant with submerged and aerial foliage, which raises the question how do both leaf types perform photosynthetically in two different environments. We found that the aerial leaves function like terrestrial sun-leaves in that their photosynthetic capability was high and saturated under high irradiance (ca. 1,500 μmol photons m⁻² s⁻¹). We show that stomatal opening and Rubisco activity in these leaves co-limited photosynthesis at saturating irradiance fluctuating in a daily rhythm. In the morning, sunlight stimulated stomatal opening, Rubisco synthesis, and the neutralization of a night-accumulated Rubisco inhibitor. Consequently, the light-saturated quantum efficiency and rate of photosynthesis increased 10-fold by midday. During the afternoon, gradual closure of the stomata and a decrease in Rubisco content reduced the light-saturated photosynthetic rate. However, at limited irradiance, stomatal behavior and Rubisco content had only a marginal effect on the photosynthetic rate, which did not change during the day. In contrast to the aerial leaves, the photosynthesis rate of the submerged leaves, adapted to a shaded environment, was saturated under lower irradiance. The light-saturated quantum efficiency of these leaves was much lower and did not change during the day. Due to their low photosynthetic affinity for CO₂ (35 μM) and inability to utilize other inorganic carbon species, their photosynthetic rate at air-equilibrated water was CO₂-limited. These results reveal differences in the photosynthetic performance of the two types of Nuphar leaves and unravel how photosynthetic daily rhythm in the aerial leaves is controlled.     

Regulation of Ribulose-1,5-Bisphosphate Carboxylase Activity in Alocasia macrorrhiza in Response to Step Changes in Irradiance

The regulation of ribulose-1,5-bisphosphate (RuBP) carboxylase (Rubisco) activity and pool sizes of RuBP and P-glycerate were examined in the tropical understory species Alocasia macrorrhiza following step changes in photon flux density (PFD). Previous gas exchange analysis of this species following a step increase in PFD from 10 to 500 micromoles quanta per square meter per second suggested that the increase in photosynthetic rate was limited by the rate of increase of Rubisco activity for the first 5 to 10 minutes. We demonstrate here that the increase in photosynthetic rate was correlated with an increase in both the activation state of Rubisco and the total k_cat (fully activated specific activity) of the enzyme. Evidence presented here suggests that a change in the pool size of the naturally occurring tight binding inhibitor of Rubisco activity, 2-carboxyarabinitol 1-phosphate, was responsible for the PFD-dependent change in the total k_cat of the enzyme. RuBP pool size transiently increased after the increase in PFD, indicating that photosynthesis was limited by the capacity for carboxylation. After 5 to 10 minutes, RuBP pool size was again similar to the pool size at low PFD, presumably because of the increased activity of Rubisco. Following a step decrease in PFD from 500 to 10 micromoles quanta per square meter per second, Rubisco activity declined but at a much slower rate than it had increased in response to a step increase in PFD. This slower rate of activity decline than increase was apparently due to the slower rate of 2-carboxyarabinitol 1-phosphate synthesis than degradation and, to a lesser degree, to slower deactivation than activation. RuBP pool size initially declined following the decrease in PFD, indicating that RuBP regeneration was limiting photosynthesis. As Rubisco activity decreased, RuBP slowly increased to its original level at high PFD. The slow rate of activity loss by Rubisco in this species suggests a biochemical basis for the increased efficiency for CO₂ assimilation of successive lightfleck use by species such as A. macrorrhiza.     

Ear of durum wheat under water stress: water relations and photosynthetic metabolism

The photosynthetic characteristics of the ear and flag leaf of well-watered (WW) and water-stressed (WS) durum wheat (Triticum turgidum L. var. durum) were studied in plants grown under greenhouse and Mediterranean field conditions. Gas exchange measurements simultaneously with modulated chlorophyll fluorescence were used to study the response of the ear and flag leaf to CO₂ and O₂ during photosynthesis. C₄ metabolism was identified by assessing the sensitivity of photosynthetic rate and electron transport to oxygen. The presence of CAM metabolism was assessed by measuring daily patterns of stomatal conductance and net CO₂ assimilation. In addition, the histological distribution of Rubisco protein in the ear parts was studied by immunocytochemical localisation. Relative water content (RWC) and osmotic adjustment (osmotic potential at full turgor) were also measured in these organs. Oxygen sensitivity of the assimilation rate and electron transport, the lack of Rubisco compartmentalisation in the mesophyll tissues and the gas-exchange pattern at night indicated that neither C₄ nor CAM metabolism occurs in the ear of WW or WS plants. Nevertheless, photosynthetic activity of the flag leaf was more affected by WS conditions than that of the ear, under both growing conditions. The lower sensitivity under water stress of the ear than of the flag leaf was linked to higher RWC and osmotic adjustment in the ear bracts and awns. We demonstrate that the better performance of the ear under water stress (compared to the flag leaf) is not related to C₄ or CAM photosynthesis. Rather, drought tolerance of the ear is explained by its higher RWC in drought. Osmotic adjustment and xeromorphic traits of ear parts may be responsible.     

Mechanisms underlying leaf photosynthetic acclimation to warming and elevated CO2 as inferred from least‐cost optimality theory

The mechanisms responsible for photosynthetic acclimation are not well understood, effectively limiting predictability under future conditions. Least‐cost optimality theory can be used to predict the acclimation of photosynthetic capacity based on the assumption that plants maximize carbon uptake while minimizing the associated costs. Here, we use this theory as a null model in combination with multiple datasets of C₃ plant photosynthetic traits to elucidate the mechanisms underlying photosynthetic acclimation to elevated temperature and carbon dioxide (CO₂). The model‐data comparison showed that leaves decrease the ratio of the maximum rate of electron transport to the maximum rate of Rubisco carboxylation (J_max/V_cmax) under higher temperatures. The comparison also indicated that resources used for Rubisco and electron transport are reduced under both elevated temperature and CO₂. Finally, our analysis suggested that plants underinvest in electron transport relative to carboxylation under elevated CO₂, limiting potential leaf‐level photosynthesis under future CO₂ concentrations. Altogether, our results show that acclimation to temperature and CO₂ is primarily related to resource conservation at the leaf level. Under future, warmer, high CO₂ conditions, plants are therefore likely to use less nutrients for leaf‐level photosynthesis, which may impact whole‐plant to ecosystem functioning.     

Effect of panicle removal on photosynthetic acclimation under elevated CO<Subscript>2</Subscript> in rice

To examine the role of sink size on photosynthetic acclimation under elevated atmospheric CO₂ concentrations ([CO₂]), we tested the effects of panicle-removal (PR) treatment on photosynthesis in rice (Oryza sativa L.). Rice was grown at two [CO₂] levels (ambient and ambient + 200 μmol mol⁻¹) throughout the growing season, and at full-heading stage, at half the plants, a sink-limitation treatment was imposed by the removal of the panicles. The PR treatment alleviated the reduction of green leaf area, the contents of chlorophyll (Chl) and Rubisco after the full-heading stage, suggesting delay of senescence. Nonetheless, elevated [CO₂] decreased photosynthesis (measured at current [CO₂]) of plants exposed to the PR treatment. No significant [CO₂] × PR interaction on photosynthesis was observed. The decrease of photosynthesis by elevated [CO₂] of plants was associated with decreased leaf Rubisco content and N content. Leaf glucose content was increased by the PR treatment and also by elevated [CO₂]. In conclusion, a sink-limitation in rice improved N status in the leaves, but this did not prevent the photosynthetic down-regulation under elevated [CO₂].     

Effects of elevated CO2 and temperature on photosynthesis and Rubisco in rice and soybean

Rice (Oryza sativa L. cv. IR-72) and soybean (Glycine max L. Merr. cv. Bragg), which have been reported to differ in acclimation to elevated CO₂, were grown for a season in sunlight at ambient and twice-ambient [CO₂], and under daytime temperature regimes ranging from 28 to 40°C. The objectives of the study were to test whether CO₂ enrichment could compensate for adverse effects of high growth temperatures on photosynthesis, and whether these two C₃ species differed in this regard. Leaf photosynthetic assimilation rates (A) of both species, when measured at the growth [CO₂], were increased by CO₂ enrichment, but decreased by supraoptimal temperatures. However, CO₂ enrichment more than compensated for the temperature-induced decline in A. For soybean, this CO₂ enhancement of A increased in a linear manner by 32–95% with increasing growth temperatures from 28 to 40°C, whereas with rice the degree of enhancement was relatively constant at about 60%, from 32 to 38°C. Both elevated CO₂ and temperature exerted coarse control on the Rubisco protein content, but the two species differed in the degree of responsiveness. CO₂ enrichment and high growth temperatures reduced the Rubisco content of rice by 22 and 23%, respectively, but only by 8 and 17% for soybean. The maximum degree of Rubisco down-regulation appeared to be limited, as in rice the substantial individual effects of these two variables, when combined, were less than additive. Fine control of Rubisco activation was also influenced by both elevated [CO₂] and temperature. In rice, total activity and activation were reduced, but in soybean only activation was lowered. The apparent catalytic turnover rate (K_cat) of rice Rubisco was unaffected by these variables, but in soybean elevated [CO₂] and temperature increased the apparent K_cat by 8 and 22%, respectively. Post-sunset declines in Rubisco activities were accelerated by elevated [CO₂] in rice, but by high temperature in soybean, suggesting that [CO₂] and growth temperature influenced the metabolism of 2-carboxyarabinitol-1-phosphate, and that the effects might be species-specific. The greater capacity of soybean for CO₂ enhancement of A at supraoptimal temperatures was probably not due to changes in stomatal conductance, but may be partially attributed to less down-regulation of Rubisco by elevated [CO₂] in soybean than in rice. However, unidentified species differences in the temperature optimum for photosynthesis also appeared to be important. The responses of photosynthesis and Rubisco in rice and soybean suggest that among C₃ plants species-specific differences will be encountered as a result of future increases in global [CO₂] and air temperatures.     

Photosynthetic light-response curves

The shapes of photosynthetic light-response curves for leaves of Eucalyptus maculata (Hook) and E. pauciflora (Sieber ex Sprengel) were examined. Three different methods were used to measure photosynthesis: CO₂ and H₂O-vapour exchange, O₂ evolution at a 5-kPa CO₂ partial pressure, and chlorophyll fluorescence. The three methods were compared and gave good agreement when measured under equivalent conditions. However, O₂ evolution was inhibited by high CO₂ partial pressures. A non-rectangular hyperbolic curve has been used widely to describe photosynthetic light-response curves. It has three variables which define the maximum quantum yield (photosynthetic rate divided by absorbed irradiance at very low irradiances), the maximum capacity and the curvature (Θ). We found that Θ was affected by the CO₂ partial pressure, declining to a minimum of about 0.6 as CO₂ partial pressure increased to 100 Pa. Further increases in the CO₂ partial pressure began to inhibit the rate of O₂ evolution at 2000 μmol quanta · m^?2·^?1 and Θ increased back to 0.95 by 5 kPa CO₂ partial pressure. At low irradiances, photosynthesis is limited by the rate of electron transport while at high irradiances, photosynthesis is frequently limited by the activity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco). The dependence of Θ on CO₂ partial pressure arises because the transition between limitations changes as a function of the CO₂ partial pressure. The light-response curve is truncated by the transition to a Rubisco limitation and the lower the irradiance at the transition, the higher the value of Θ. There is a gradient in light absorption through the leaf which influences the photosynthetic capacity of different layers within the leaf. The gradient in photosynthetic capacity can be demonstrated by the fact that the shape of the light-response curve changes when the leaf is illuminated unilaterally onto either the adaxial or abaxial surface. We compared two Eucalyptus species which had either isolateral or dorsiventral leaf anatomy. Leaves were able to reverse completely the gradients in photosynthetic capacity following inversion of the leaves for a week, irrespective of their anatomy.     

Gas exchange and photosynthetic performance of the tropical tree <Emphasis Type="Italic">Acacia nigrescens</Emphasis> when grown in different CO<Subscript>2</Subscript> concentrations

The photosynthetic responses of the tropical tree species Acacia nigrescens Oliv. grown at different atmospheric CO₂ concentrations—from sub-ambient to super-ambient—have been studied. Light-saturated rates of net photosynthesis (A _sat) in A. nigrescens, measured after 120 days exposure, increased significantly from sub-ambient (196 μL L⁻¹) to current ambient (386 μL L⁻¹) CO₂ growth conditions but did not increase any further as [CO₂] became super-ambient (597 μL L⁻¹). Examination of photosynthetic CO₂ response curves, leaf nitrogen content, and leaf thickness showed that this acclimation was most likely caused by reduction in Rubisco activity and a shift towards ribulose-1,5-bisphosphate regeneration-limited photosynthesis, but not a consequence of changes in mesophyll conductance. Also, measurements of the maximum efficiency of PSII and the carotenoid to chlorophyll ratio of leaves indicated that it was unlikely that the pattern of A _sat seen was a consequence of growth [CO₂] induced stress. Many of the photosynthetic responses examined were not linear with respect to the concentration of CO₂ but could be explained by current models of photosynthesis.     

Low-temperature photosynthetic performance of a C4 grass and a co-occurring C3 grass native to high latitudes

The photosynthetic performance of C₄ plants is generally inferior to that of C₃ species at low temperatures, but the reasons for this are unclear. The present study investigated the hypothesis that the capacity of Rubisco, which largely reflects Rubisco content, limits C₄ photosynthesis at suboptimal temperatures. Photosynthetic gas exchange, chlorophyll a fluorescence, and the in vitro activity of Rubisco between 5 and 35 °C were measured to examine the nature of the low‐temperature photosynthetic performance of the co‐occurring high latitude grasses, Muhlenbergia glomerata (C₄) and Calamogrostis canadensis (C₃). Plants were grown under cool (14/10 °C) and warm (26/22 °C) temperature regimes to examine whether acclimation to cool temperature alters patterns of photosynthetic limitation. Low‐temperature acclimation reduced photosynthetic rates in both species. The catalytic site concentration of Rubisco was approximately 5.0 and 20 µmol m^?2 in M. glomerata and C. canadensis, respectively, regardless of growth temperature. In both species, in vivo electron transport rates below the thermal optimum exceeded what was necessary to support photosynthesis. In warm‐grown C. canadensis, the photosynthesis rate below 15 °C was unaffected by a 90% reduction in O₂ content, indicating photosynthetic capacity was limited by the capacity of P_i‐regeneration. By contrast, the rate of photosynthesis in C. canadensis plants grown at the cooler temperatures was stimulated 20–30% by O₂ reduction, indicating the P_i‐regeneration limitation was removed during low‐temperature acclimation. In M. glomerata, in vitro Rubisco activity and gross CO₂ assimilation rate were equivalent below 25 °C, indicating that the capacity of the enzyme is a major rate limiting step during C₄ photosynthesis at cool temperatures.     

Acclimation of peanut (Arachis hypogaea L.) leaf photosynthesis to elevated growth CO2 and temperature

Peanut (Arachis hypogaea L. cv. Florunner) was grown from seed sowing to plant maturity under two daytime CO₂ concentrations ([CO₂]) of 360 μmol mol⁻¹ (ambient) and 720 μmol mol⁻¹ (elevated) and at two temperatures of 1.5 and 6.0 °C above ambient temperature. The objectives were to characterize peanut leaf photosynthesis responses to long-term elevated growth [CO₂] and temperature, and to assess whether elevated [CO₂] regulated peanut leaf photosynthetic capacity, in terms of activity and protein content of ribulose bisphosphate carboxylase-oxygenase (Rubisco), Rubisco photosynthetic efficiency, and carbohydrate metabolism. At both growth temperatures, leaves of plants grown under elevated [CO₂] had higher midday photosynthetic CO₂ exchange rate (CER), lower transpiration and stomatal conductance and higher water-use efficiency, compared to those of plants grown at ambient [CO₂]. Both activity and protein content of Rubisco, expressed on a leaf area basis, were reduced at elevated growth [CO₂]. Declines in Rubisco under elevated growth [CO₂] were 27–30% for initial activity, 5–12% for total activity, and 9–20% for protein content. Although Rubisco protein content and activity were down-regulated by elevated [CO₂], Rubisco photosynthetic efficiency, the ratio of midday light-saturated CER to Rubisco initial or total activity, of the elevated-[CO₂] plants was 1.3- to 1.9-fold greater than that of the ambient-[CO₂] plants at both growth temperatures. Leaf soluble sugars and starch of plants grown at elevated [CO₂] were 1.3- and 2-fold higher, respectively, than those of plants grown at ambient [CO₂]. Under elevated [CO₂], leaf soluble sugars and starch, however, were not affected by high growth temperature. In contrast, high temperature reduced leaf soluble sugars and starch of the ambient-[CO₂] plants. Activity of sucrose-P synthase, but not adenosine 5′-diphosphoglucose pyrophosphorylase, was up-regulated under elevated growth [CO₂]. Thus, in the absence of other environmental stresses, peanut leaf photosynthesis would perform well under rising atmospheric [CO₂] and temperature as predicted for this century.     

Does Leaf Position within a Canopy Affect Acclimation of Photosynthesis to Elevated CO2? : Analysis of a Wheat Crop under Free-Air CO2 Enrichment

Previous studies of photosynthetic acclimation to elevated CO₂ have focused on the most recently expanded, sunlit leaves in the canopy. We examined acclimation in a vertical profile of leaves through a canopy of wheat (Triticum aestivum L.). The crop was grown at an elevated CO₂ partial pressure of 55 Pa within a replicated field experiment using free-air CO₂ enrichment. Gas exchange was used to estimate in vivo carboxylation capacity and the maximum rate of ribulose-1,5-bisphosphate-limited photosynthesis. Net photosynthetic CO₂ uptake was measured for leaves in situ within the canopy. Leaf contents of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), light-harvesting-complex (LHC) proteins, and total N were determined. Elevated CO₂ did not affect carboxylation capacity in the most recently expanded leaves but led to a decrease in lower, shaded leaves during grain development. Despite this acclimation, in situ photosynthetic CO₂ uptake remained higher under elevated CO₂. Acclimation at elevated CO₂ was accompanied by decreases in both Rubisco and total leaf N contents and an increase in LHC content. Elevated CO₂ led to a larger increase in LHC/Rubisco in lower canopy leaves than in the uppermost leaf. Acclimation of leaf photosynthesis to elevated CO₂ therefore depended on both vertical position within the canopy and the developmental stage.