Effects of nitrogen supply on the acclimation of photosynthesis to elevated CO2

A common observation in plants grown in elevated CO₂ concentration is that the rate of photosynthesis is lower than expected from the dependence of photosynthesis upon CO₂ concentration in single leaves of plants grown at present CO₂ concentration. Furthermore, it has been suggested that this apparent down regulation of photosynthesis may be larger in leaves of plants at low nitrogen supply than at higher nitrogen supply. However, the available data are rather limited and contradictory. In this paper, particular attention is drawn to the way in which whole plant growth response to N supply constitutes a variable sink strength for carbohydrate usage and how this may affect photosynthesis. The need for further studies of the acclimation of photosynthesis at elevated CO₂ in leaves of plants whose N supply has resulted in well-defined growth rate and sink activity is emphasised, and brief consideration is made of how this might be achieved.Abbreviations A rate of CO₂ assimilation - C_i internal CO₂ concentration - PCR photosynthetic carbon reduction - Rubisco Ribulose 1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose 1,5-bisphosphate     

Photosynthetic acclimation in trees to rising atmospheric CO2: A broader perspective

Analysis of leaf-level photosynthetic responses of 39 tree species grown in elevated concentrations of atmospheric CO₂ indicated an average photosynthetic enhancement of 44% when measured at the growth [CO₂]. When photosynthesis was measured at a common ambient [CO₂], photosynthesis of plants grown at elevated [CO₂] was reduced, on average, 21% relative to ambient-grown trees, but variability was high. The evidence linking photosynthetic acclimation in trees with changes at the biochemical level is examined, along with anatomical and morphological changes in trees that impact leaf- and canopy-level photosynthetic response to CO₂ enrichment. Nutrient limitations and variations in sink strength appear to influence photosynthetic acclimation, but the evidence in trees for one predominant factor controlling acclimation is lacking. Regardless of the mechanisms that underlie photosynthetic acclimation, it is doubtful that this response will be complete. A new focus on adjustments to rising [CO₂] at canopy, stand, and forest scales is needed to predict ecosystem response to a changing environment.Abbreviations A/C_i photosynthesis as a function of internal [CO₂] - J_max maximum rate of electron transport - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - Vc_max maximum rate of carboxylation The U.S. Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Photosynthetic acclimation of Pinus taeda (loblolly pine) to long-term growth in elevated pCO2 (FACE)

Growth in elevated pCO₂ generally leads to a stimulation of net CO₂ uptake rate. However, with long‐term growth the magnitude of this stimulation is often reduced. This phenomenon, termed acclimation, has been largely attributed to a loss of Rubisco (ribulose 1,5 bisphosphate carboxylase). The mechanism by which Rubisco content declines with long‐term growth is not certain. There is evidence for a sugar‐mediated, selective down‐regulation of Rubisco protein and also for a non‐selective loss of total leaf nitrogen, which impacts Rubisco levels indirectly. Over a season, and including needles at different developmental stages, we investigated these two potential mechanisms in well‐developed Pinus taeda grown for approximately 2·5 years in elevated (56 Pa) pCO₂ using free air CO₂ enrichment technology. Photosynthetic acclimation, as manifested by a decrease in the activity of Rubisco measured both in vivo (? 25%, via gas exchange) and in vitro (? 35%, via enzyme assays), was observed with growth in elevated pCO₂. This acclimation was observed in one‐year‐old needles but not in current‐year needles. Needles exhibiting acclimation had reduced levels of Lsu Rubisco (? 25%) and an increased foliar carbohydrate content (+ 30%) but showed no evidence of a decrease in needle nitrogen or total protein content. These data support the concept that photosynthetic acclimation in elevated pCO₂ is caused by a selective down‐regulation of Rubisco.     

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].     

Variable photosynthetic acclimation in consecutive cohorts of Scots pine needles during 3 years of growth at elevated CO2 and elevated temperature

In this experiment, the photosynthetic acclimation of successive needle cohorts of Scots pine were studied during 3 years of growth at elevated CO₂ and temperature. Naturally regenerated Scots pine (Pinus sylvestris L.) trees were subjected to elevated CO₂ concentration (+CO₂, 700 p.p.m), elevated temperature (+T, ambient +2 to +6 °C) and to a combination of elevated CO₂ and temperature (+CO₂ + T) in closed‐top chambers, starting in August 1996. Trees growing in chambers with ambient CO₂ and ambient temperature served as controls (AmbC). Elevated CO₂ influenced the dark reactions more than the light reactions of photosynthesis, as in the 1996 and 1997 cohorts the carboxylation capacity of Rubisco was reduced in the first and second year of exposure, but there was no consistent change in chlorophyll fluorescence. Net photosynthesis measured at growth concentration of CO₂ was higher at +CO₂ than at AmbC on only one measuring occasion, was generally lower at +T and was not changed at +CO₂ + T. However, trees grown at +T tended to invest more nitrogen (N) in Rubisco, as Rubisco/chlorophyll and the proportion of the total needle N bound to Rubisco occasionally increased. The interaction of +CO₂ and +T on Rubisco was mostly negative; consequently, in the second and third year of the experiment the carboxylation capacity decreased at +CO₂ + T. In the 1996, 1997 and 1998 cohorts, the structural N concentration of needles was lower at +CO₂ than at AmbC. Elevated CO₂ and elevated temperature generally had a positive interaction on N concentration; consequently, N concentration in needles decreased less at +CO₂ + T than at +CO₂. At +CO₂ + T, the acclimation response of needles varied between years and was more pronounced in the 1‐year‐old needles of the 1997 cohort than in those of the 1998 cohort. Thus, acclimation was not always greater in 1‐year‐old needles than in current‐year needles. In the +CO₂ + T treatment, elevated temperature had a greater effect on acclimation of needles than elevated CO₂.     

The temperature response of CO2 assimilation, photochemical activities and Rubisco activation in Camelina sativa, a potential bioenergy crop with limited capacity for acclimation to heat stress

The temperature optimum of photosynthesis coincides with the average daytime temperature in a species’ native environment. Moderate heat stress occurs when temperatures exceed the optimum, inhibiting photosynthesis and decreasing productivity. In the present study, the temperature response of photosynthesis and the potential for heat acclimation was evaluated for Camelina sativa, a bioenergy crop. The temperature optimum of net CO₂ assimilation rate (A) under atmospheric conditions was 30–32?°C and was only slightly higher under non-photorespiratory conditions. The activation state of Rubisco was closely correlated with A at supra-optimal temperatures, exhibiting a parallel decrease with increasing leaf temperature. At both control and elevated temperatures, the modeled response of A to intercellular CO₂ concentration was consistent with Rubisco limiting A at ambient CO₂. Rubisco activation and photochemical activities were affected by moderate heat stress at lower temperatures in camelina than in the warm-adapted species cotton and tobacco. Growth under conditions that imposed a daily interval of moderate heat stress caused a 63?% reduction in camelina seed yield. Levels of cpn60 protein were elevated under the higher growth temperature, but acclimation of photosynthesis was minimal. Inactivation of Rubisco in camelina at temperatures above 35?°C was consistent with the temperature response of Rubisco activase activity and indicated that Rubisco activase was a prime target of inhibition by moderate heat stress in camelina. That photosynthesis exhibited no acclimation to moderate heat stress will likely impact the development of camelina and other cool season Brassicaceae as sources of bioenergy in a warmer world.     

Light adaptation/acclimation of photosynthesis and the regulation of ribulose-1,5-bisphosphate carboxylase activity in sun and shade plants

The consequences of light adaptation and acclimation of photosynthesis on photosynthetic nitrogen use efficiency (NUE), particularly as it relates to the efficiency of ribulose-1,5-bisphosphate carboxylase (Rubisco) use in photosynthetic CO₂ assimilation, was studied in the sun species Glycine max and the shade species Alocasia macrorrhiza. Both G. max and A. macrorrhiza were found to possess the capacity for light acclimation of CO₂ assimilation, but over distinctly different ranges of photon flux density (PFD). For each species, light acclimation of photosynthesis had little effect on the rate of photosynthesis per unit Rubisco protein or the light response of Rubisco carbamylation and CA 1P metabolism. In contrast, photosynthesis per unit Rubisco protein was significantly higher in G. max than in A. macrorrhiza, due in part to a lower total (fully carbamylated) molar activity (activity per unit enzyme) of A. macrorrhiza Rubisco than that of G. max. Comparison of the light response of Rubisco regulatory mechanisms between G. max and A. macrorrhiza indicated some degree of adaptation, such that carbamylation was higher and CA 1P levels lower at lower PFDs in the shade species than the sun species. However, this adjustment was not sufficient for Rubisco in low light grown A. macrorrhiza to be fully active at the growth PFD. Photosynthesis in A. macrorrhiza appeared to become RuBP regeneration-limited at lower PFDs than G. max, and this was probably the determinant of the light saturated rate of photosynthesis in the shade species. The low efficiency of Rubisco use in A. macrorrhiza was a major contributing factor to its five- to sixfold lower photosynthetic NUE than G. max. Shade species such as A. macrorrhiza appear to make far from maximal use of Rubisco protein N.     

Biochemistry of C3-photosynthesis in high CO2

The short-term responses of C₃ photosynthesis to high CO₂ are described first. Regulation of photosynthesis in the short term is determined by interaction among the capacities of light harvesting, electron transport, ribulose-1, 5-bisphosphate carboxylase (Rubisco) and orthophosphate (Pi) regeneration during starch and sucrose synthesis. Photosynthesis under high CO₂ conditions is limited by either electron transport or Pi regeneration capacities, and Rubisco is deactivated to maintain a balance between each step in the photosynthetic pathway. Subsequently, the long-term effects on, photosynthesis are discussed. Long-term CO₂ enhancement leads to carbohydrate accumulation. Accumulation of carbohydrates is not associated with a Pi-regeneration limitation on photosynthesis, and this limitation is apparently removed during long-term exposure to high CO₂. Enhanced CO₂ does not affect Rubisco content and electron transport capacity for a given leaf-nitrogen content. In addition, the deactivated Rubisco immediately after exposure to high CO₂ does not recover during the subsequent prolonged exposure. Such evidence may indicate that plants do not necessarily have an ideal acclimation response to high CO₂ at the biochemical level.     

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.     

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.     

Long- and short-term effects of decreased sink demand on carbohydrate levels and photosynthesis in wheat leaves

Wheat (Triticum aestivum L.) ears were removed to investigate long-term regulation of photosynthesis by sink demand at ambient CO₂ and 22 °C. The CO₂ level was also increased to 660 μmol mol^?1 and temperature was lowered to 5 °C to examine short-term responses of photosynthesis to low sink demand. Sink removal inhibited photosynthesis and increased leaf levels of glucose, fructose and ribulose-1, 5-bisphosphate (RuBP), and the glucose-6-phosphate (G6P)/fructose-6-phosphate (F6P) and RuBP/3-phosphoglycerate (PGA) ratios under growth conditions, but had no effect on the activity and activation state of ribulose-1, 5-bisphosphate carboxylase oxygenase (Rubisco) either under growth or short-term conditions, suggesting an inhibition of photosynthesis by decreased in vivo catalysis of Rubisco. Photosynthesis increased similarly in eared and earless shoots after a rise in CO₂ concentration, and the ratio of triose-phosphates (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, TP) to PGA was similar or higher for removed than intact ears, suggesting that feedback inhibition of photosynthesis was not caused by a limitation of ATP synthesis in chloroplasts. Under short-term conditions (660 μmol mol^?1 CO₂, 5 °C), TP and RuBP levels and the TP/PGA and TP/RuBP ratios were increased by sink removal, indicating an additional limitation of photosynthesis by the rate of RuBP regeneration.     

Can phenotypic plasticity in Rubisco performance contribute to photosynthetic acclimation?

Photosynthetic acclimation varies among species, which likely reveals variations at the biochemical level in the pathways that constitute carbon assimilation and energy transfer. Local adaptation and phenotypic plasticity affect the environmental response of photosynthesis. Phenotypic plasticity allows for a wide array of responses from a single individual, encouraging fitness in a broad variety of environments. Rubisco catalyses the first enzymatic step of photosynthesis, and is thus central to life on Earth. The enzyme is well conserved, but there is habitat-dependent variation in kinetic parameters, indicating that local adaptation may occur. Here, we review evidence suggesting that land plants can adjust Rubisco’s intrinsic biochemical characteristics during acclimation. We show that this plasticity can theoretically improve CO₂ assimilation; the effect is non-trivial, but small relative to other acclimation responses. We conclude by discussing possible mechanisms that could account for biochemical plasticity in land plant Rubisco.     

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.     

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.     

Regulation of the expression of ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) in a cyanobacterium, Synechococcus PCC7942

When cyanobacterium cells are grown under extremely low CO₂ concentration, the number of carboxysomes, structures containing ribulose-bisphosphate carboxylase (Rubisco; EC 4.1.1.39), is known to increase. This suggests that Rubisco helps to regulate photosynthesis in cyanobacteria. However, no studies have been done on the changes of Rubisco content and activity in response to the extracellular CO₂ concentration, and no information is available on its effect on photosynthesis. To elucidate the relationship between the expression responses of Rubisco and extracellular CO₂, wild-type cells (Synechococcus PCC7942) and carboxysome-lacking cells were grown under various CO₂ concentrations, and Rubisco activity was determined. In both strains, Rubisco activity increased when the cells were grown under a CO₂ concentration around, or less than, K _1/2(CO₂) of photosynthesis. In carboxysome-lacking cells, Rubisco activity increased five to six times at most, and a simultaneous increase in the rate of photosynthesis was observed. These results suggest that stimulation of expression of Rubisco occurs to compensate for the decrease in the rate of photosynthesis under CO₂-limited conditions. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Photosynthetic capacity of loblolly pine (Pinus taeda L.) trees during the first year of carbon dioxide enrichment in a forest ecosystem

Our objective was to assess the photosynthetic responses of loblolly pine trees (Pinus taeda L.) during the first full growth season (1997) at the Brookhaven National Lab/Duke University Free Air CO₂ Enrichment (FACE) experiment. Gas exchange, fluorescence characteristics, and leaf biochemistry of ambient CO₂ (control) needles and ambient + 20 Pa CO₂ (elevated) needles were examined five times during the year. The enhancement of photosynthesis by elevated CO₂ in mature loblolly pine trees varied across the season and was influenced by abiotic and biotic factors. Photosynthetic enhancement by elevated CO₂ was strongly correlated with leaf temperature. The magnitude of photosynthetic enhancement was zero in March but was as great as 52% later in the season. In March, reduced sink demand and lower temperatures resulted in lower net photosynthesis, lower carboxylation rates and higher excess energy dissipation from the elevated CO₂ needles than from control needles. The greatest photosynthetic enhancement by CO₂ enrichment was observed in July during a period of high temperature and low precipitation, and in September during recovery from this period of low precipitation. In July, loblolly pine trees in the control rings exhibited lower net photosynthetic rates, lower maximum rates of photosynthesis at saturating CO₂ and light, lower values of carboxylation and electron transport rates (modelled from A–C_i curves), lower total Rubisco activity, and lower photochemical quenching of fluorescence in comparison to other measurement periods. During this period of low precipitation trees in the elevated CO₂ rings exhibited reduced net photosynthesis and photochemical quenching of fluorescence, but there was little effect on light- and CO₂-saturated rates of photosynthesis, modelled rates of carboxylation or electron transport, or Rubisco activity. These first-year data will be used to compare with similar measurements from subsequent years of the FACE experiment in order to determine whether photosynthetic acclimation to CO₂ occurs in these canopy loblolly pine trees growing in a forest ecosystem.     

Differential CO2 effect on primary carbon metabolism of flag leaves in durum wheat (Triticum durum Desf.)

C sink/source balance and N assimilation have been identified as target processes conditioning crop responsiveness to elevated CO₂. However, little is known about phenology‐driven modifications of C and N primary metabolism at elevated CO₂ in cereals such as wheat. Here, we examined the differential effect of elevated CO₂ at two development stages (onset of flowering, onset of grain filling) in durum wheat (Triticum durum, var. Sula) using physiological measurements (photosynthesis, isotopes), metabolomics, proteomics and ¹⁵N labelling. Our results show that growth at elevated CO₂ was accompanied by photosynthetic acclimation through a lower internal (mesophyll) conductance but no significant effect on Rubisco content, maximal carboxylation or electron transfer. Growth at elevated CO₂ altered photosynthate export and tended to accelerate leaf N remobilization, which was visible for several proteins and amino acids, as well as lysine degradation metabolism. However, grain biomass produced at elevated CO₂ was larger and less N rich, suggesting that nitrogen use efficiency rather than photosynthesis is an important target for improvement, even in good CO₂‐responsive cultivars.