Pyruvate orthophosphate dikinase of c(3) seeds and leaves as compared to the enzyme from maize

Pyruvate orthophosphate dikinase (PPDK) was found in various immature seeds of C₃ plants (wheat, pea, green bean, plum, and castor bean), in some C₃ leaves (tobacco, spinach, sunflower, and wheat), and in C₄ (maize) kernels. The enzyme in the C₃ plants cross-reacts with rabbit antiserum against maize PPDK. Based on protein blot analysis, the apparent subunit size of PPDK from wheat seeds and leaves and from sunflower leaves is about 94 kdaltons, the same as that of the enzyme from maize, but is slightly less (about 90 kdaltons) for the enzyme from spinach and tobacco leaves. The amount of this enzyme per mg of soluble protein in C₃ seeds and leaves is much less than in C₄ leaves. PPDK is present in kernels of the C₄ plant, Zea mays in amounts comparable to those in C₄ leaves.

Regulatory properties of the enzyme from C₃ tissues (wheat) are similar to those of the enzyme from C₄ leaves with respect to in vivo light activation and dark inactivation (in leaves) and in vivo cold lability (seeds and leaves).

Following incorporation of ¹⁴CO₂ by illuminated wheat pericarp and adjoining tissue for a few seconds, the labeled metabolites were predominantly products resulting from carboxylation of phosphoenolpyruvate, with lesser labeling of compounds formed by carboxylation of ribulose 1,5-bisphosphate and operation of the reductive pentose phosphate cycle of photosynthesis. PPDK may be involved in mechanisms of amino acid interconversions during seed development.

     

Expression of cold-tolerant pyruvate,orthophosphate dikinase cDNA,and heterotetramer formation in transgenic maize plants

Maize is a typical C₄ plant of the NADP-malic enzyme type, and its high productivity is supported by the C₄ photosynthetic cycle, which concentrates atmospheric CO₂ in the leaves. The plant exhibits superior photosynthetic ability under high light and high temperature, but under cold conditions the photosynthetic rate is significantly reduced. Pyruvate orthophosphate dikinase (PPDK), a key enzyme of the C₄ pathway in maize, loses its activity below about 12 °C by dissociation of the tetramer and it is considered as one possible cause of the reduction in the photosynthetic rate of maize at low temperatures. To improve the cold stability of the enzyme, we introduced a cold-tolerant PPDK cDNA isolated from Flaveria brownii into maize by Agrobacterium-mediated transformation. We obtained higher levels of expression by using a double intron cassette and a chimeric cDNA made from F. bidentis and F. brownii with a maximum content of 1mg/g fresh weight. In leaves of transgenic maize, PPDK molecules produced from the transgene were detected in cold-tolerant homotetramers or in heterotetramers of intermediate cold susceptibility formed with the internal PPDK. Simultaneous introduction of an antisense gene for maize PPDK generated plants in which the ratio of heterolologous and endogenous PPDK was greatly improved. Arrhenius plot analysis of the enzyme extracted from one such plant revealed that the break point was shifted about 3 °C lower than that of the wild type.     

Cytosolic pyruvate,orthophosphate dikinase functions in nitrogen remobilization during leaf senescence and limits individual seed growth and nitrogen content

The protein content of seeds determines their nutritive value, downstream processing properties and market value. Up to 95% of seed protein is derived from amino acids that are exported to the seed after degradation of existing protein in leaves, but the pathways responsible for this nitrogen metabolism are poorly defined. The enzyme pyruvate,orthophosphate dikinase (PPDK) interconverts pyruvate and phosphoenolpyruvate, and is found in both plastids and the cytosol in plants. PPDK plays a cardinal role in C₄ photosynthesis, but its role in the leaves of C₃ species has remained unclear. We demonstrate that both the cytosolic and chloroplastic isoforms of PPDK are up‐regulated in naturally senescing leaves. Cytosolic PPDK accumulates preferentially in the veins, while chloroplastic PPDK also accumulates in mesophyll cells. Analysis of microarrays and labelling patterns after feeding ¹³C‐labelled pyruvate indicated that PPDK functions in a pathway that generates the transport amino acid glutamine, which is then loaded into the phloem. In Arabidopsis thaliana, over‐expression of PPDK during senescence can significantly accelerate nitrogen remobilization from leaves, and thereby increase rosette growth rate and the weight and nitrogen content of seeds. This indicates an important role for cytosolic PPDK in the leaves of C₃ plants, and allows us to propose a metabolic pathway that is responsible for production of transport amino acids during natural leaf senescence. Given that increased seed size and nitrogen content are desirable agronomic traits, and that efficient remobilization of nitrogen within the plant reduces the demand for fertiliser applications, PPDK and the pathway in which it operates are targets for crop improvement.     

High-Level Expression of Cold-Tolerant Pyruvate, Orthophosphate Dikinase from a Genomic Clone with Site-Directed Mutations in Transgenic Maize

Pyruvate, orthophosphate dikinase (PPDK) is a key enzyme in the C₄ photosynthetic pathway of maize. To improve the cold tolerance of the enzyme in maize, we designed two genomic sequence-based constructs in which the carboxy-terminal region of the enzyme was modified to mimic the amino acid sequence of the cold-tolerant PPDK of Flaveria brownii (Asteraceae). A large amount of PPDK was found to have accumulated in the leaves of many of the maize plants transformed with one of these constructs – that which introduced 17 amino acid substitutions without any alteration of the exon-intron structure – although there was a wide range of variation in the amount of PPDK among the separate plants. In contrast, the production was much less in maize transformed with the second construct in which a cDNA fragment for the same carboxy-terminal region was inserted. The specific activity of PPDK in the plants transformed with the gene with the amino acid substitutions was inversely correlated with the amount of enzyme in the leaves. In addition, the activity of the cold-tolerant recombinant enzyme was judged to be regulated by the PPDK regulatory protein, similar to that of the native PPDK. The cold tolerance of PPDK in crude leaf extracts was greatly improved in plants that produced a large amount of the engineered PPDK. The photosynthetic rate at 8°C increased significantly (by 23%, p<0.05), but there was no obvious effect at higher temperatures. These results support the hypothesis that PPDK is one of the limiting factors in the C₄ photosynthesis of maize under cold conditions.     

Photosynthetic performance at low temperature of Bouteloua gracilis Lag., a high-altitude C4 grass from the Rocky Mountains, USA

The mechanisms controlling the photosynthetic performance of C₄ plants at low temperature were investigated using ecotypes of Bouteloua gracilis Lag. from high (3000 m) and low (1500 m) elevation sites in the Rocky Mountains of Colorado. Plants were grown in controlled‐environment cabinets at a photon flux density of 700 μ mol m^?2 s^?1 and day/night temperatures of 26/16 °C or 14/7 °C. The thermal response of the net CO₂ assimilation rate (A) was evaluated using leaf gas‐exchange analysis and activity assays of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco), phosphoenolpyruvate carboxylase (PEPCase) and pyruvate,orthophosphate dikinase (PPDK). In both ecotypes, a reduction in measurement temperature caused the CO₂‐saturated rate of photosynthesis to decline to a greater degree than the initial slope of A versus the intercellular CO₂ response, thereby reducing the photosynthetic CO₂ saturation point. As a consequence, A in normal air was CO₂‐saturated at sub‐optimal temperatures. Ecotypic variation was low when grown at 26/16 °C, with the major difference between the ecotypes being that the low‐elevation plants had higher A; however, the ecotypes responded differently when grown at cool temperature. At temperatures below the thermal optimum, A in high‐elevation plants grown at 14/7 °C was enhanced relative to plants grown at 26/16 °C, while A in low‐elevation plants grown at 14/7 °C was reduced compared to 26/16 °C‐grown plants. Photoinhibition at low growth temperature was minor in both ecotypes as indicated by small reductions in dark‐adapted F_v/F_m. In both ecotypes, the activity of Rubisco was equivalent to A below 17 °C but well in excess of A above 25 °C. Activities of PEPCase and PPDK responded to temperature in a similar proportion relative to Rubisco, and showed no evidence for dissociation that would cause them to become principal limitations at low temperature. Because of the similar temperature response of Rubisco and A, we propose that Rubisco is a major limitation on C₄ photosynthesis in B. gracilis below 17 °C. Based on these results and for theoretical reasons associated with how C₄ plants use Rubisco, we further suggest that Rubisco capacity may be a widespread limitation upon C₄ photosynthesis at low temperature.     

Future carbon dioxide concentration decreases canopy evapotranspiration and soil water depletion by field‐grown maize

Maize, in rotation with soybean, forms the largest continuous ecosystem in temperate North America, therefore changes to the biosphere‐atmosphere exchange of water vapor and energy of these crops are likely to have an impact on the Midwestern US climate and hydrological cycle. As a C₄ crop, maize photosynthesis is already CO₂‐saturated at current CO₂ concentrations ([CO₂]) and the primary response of maize to elevated [CO₂] is decreased stomatal conductance (g_s). If maize photosynthesis is not stimulated in elevated [CO₂], then reduced g_s is not offset by greater canopy leaf area, which could potentially result in a greater ET reduction relative to that previously reported in soybean, a C₃ species. The objective of this study is to quantify the impact of elevated [CO₂] on canopy energy and water fluxes of maize (Zea mays). Maize was grown under ambient and elevated [CO₂] (550 μmol mol^?1 during 2004 and 2006 and 585 μmol mol^?1 during 2010) using Free Air Concentration Enrichment (FACE) technology at the SoyFACE facility in Urbana, Illinois. Maize ET was determined using a residual energy balance approach based on measurements of sensible (H) and soil heat fluxes, and net radiation. Relative to control, elevated [CO₂] decreased maize ET (7–11%; P < 0.01) along with lesser soil moisture depletion, while H increased (25–30 W m^?2; P < 0.01) along with higher canopy temperature (0.5–0.6 °C). This reduction in maize ET in elevated [CO₂] is approximately half that previously reported for soybean. A partitioning analysis showed that transpiration contributed less to total ET for maize compared to soybean, indicating a smaller role of stomata in dictating the ET response to elevated [CO₂]. Nonetheless, both maize and soybean had significantly decreased ET and increased H, highlighting the critical role of elevated [CO₂] in altering future hydrology and climate of the region that is extensively cropped with these species.     

Appearance and accumulation of c(4) carbon pathway enzymes in developing wheat leaves

Soluble protein has been extracted from sections of wheat leaves, from base to tip, and the content of several key enzymes of photosynthetic carbon assimilation in each section has been determined by the protein blot method. In the first leaf, ribulose 1,5-bisphosphate carboxylase (RuBPC) (EC 4.1.1.39) in the basal 0 to 1 centimeter section is about 12% the level in the tip section, whereas phosphoenolpyruvate carboxylase (EC 4.1.1.31) is present in small amounts in the basal section and does not change much in the tip. Pyruvate orthophosphate dikinase (PPDK) (EC 2.7.9.1) first appears in the 4 to 6 centimeter section and increases gradually with development to 10-fold in the tip. Malic enzyme, NADP-dependent (EC 1.3.1.37) also appears in the 4 to 6 centimeter section but remains low to the tip.

Fixation of ¹⁴CO₂ by wheat leaf base sections resulted in 42% of total incorporation into malate and aspartate, indicating β-carboxylation, whereas in the tip section these labeled compounds were only 8% of the total. Although the amount of PPDK in wheat leaves is only 1 to 3% of that in maize leaves, this C₃ PPDK may have a limited role in photosynthesis leading to formation of C₄ compounds. The possibility of a further role, similar to that in C₄ plants, but for intracellular carbon transport in wheat leaves is discussed. The presence of malic dehydrogenase, NADP-specific (EC 1.1.1.82) in wheat leaf chloroplasts was shown, a necessary though not sufficient condition for such a proposed role. Assuming each of the four enzymes associated with C₄ carbon transport were fully active in vivo during photosynthesis, PPDK would still be rate limiting, even in the leaf tip where its activity is maximal. Possible evolutionary and breeding implications are discussed.

     

C4 photosynthesis in C3 rice: a theoretical analysis of biochemical and anatomical factors

Engineering C₄ photosynthesis into rice has been considered a promising strategy to increase photosynthesis and yield. A question that remains to be answered is whether expressing a C₄ metabolic cycle into a C₃ leaf structure and without removing the C₃ background metabolism improves photosynthetic efficiency. To explore this question, we developed a 3D reaction diffusion model of bundle‐sheath and connected mesophyll cells in a C₃ rice leaf. Our results show that integrating a C₄ metabolic pathway into rice leaves with a C₃ metabolism and mesophyll structure may lead to an improved photosynthesis under current ambient CO₂ concentration. We analysed a number of physiological factors that influence the CO₂ uptake rate, which include the chloroplast surface area exposed to intercellular air space, bundle‐sheath cell wall thickness, bundle‐sheath chloroplast envelope permeability, Rubisco concentration and the energy partitioning between C₃ and C₄ cycles. Among these, partitioning of energy between C₃ and C₄ photosynthesis and the partitioning of Rubisco between mesophyll and bundle‐sheath cells are decisive factors controlling photosynthetic efficiency in an engineered C₃–C₄ leaf. The implications of the results for the sequence of C₄ evolution are also discussed.     

Appearance and accumulation of c(4) carbon pathway enzymes in developing maize leaves and differentiating maize a188 callus

Regenerating maize A188 tissue cultures were examined for the presence of enzymes involved in C₄ photosynthesis, for cell morphology, and for ¹⁴C labeling kinetics to study the implementation of this pathway during plant development. For comparison, sections of maize seedling leaves were examined. Protein blot analysis using antibodies to leaf enzymes showed a different profile of these enzymes during the early stages of shoot regeneration from callus from the closely-coordinated profile observed in seedling leaves. Pyruvate orthophosphate dikinase (PPDK) (EC 2.7.9.1) and phosphoenolpyruvate carboxylase (PEPC) (EC 4.1.1.31) were found in nonchlorophyllous callus while ribulose 1,5-bisphosphate carboxylase (RuBPC, EC 4.1.1.39) and malic enzyme, NADP-specific (ME-NADP) (EC 1.3.1.37) were not detectable until later.

Enzyme activity assays showed the presence of ME-NADP as well as PEPC and PPDK in nonchlorophyllous callus. However, the activities of ME-NADP and PEPC had properties similar to those of the enzymes from C₃ leaves and from etiolated C₄ leaf tissues, but differing from the corresponding enzymes in the mature leaf.

Immunoprecipitation of in vitro translation products of poly(A)RNA extracted from embryoid-forming callus showed both the 110 kilodalton precursor to chloroplast PPDK and the 94 kilodalton polypeptide. Therefore, the chloroplast tye of PPDK mRNA is present prior to the appearance of leaf morphology.

Analysis of the labeled products of ¹⁴CO₂ fixation by nonchlorophyllous calli indicated β-carboxylation to give acids of the tricarboxylic acid cycle, but no incorporation into phosphoglycerate. With greening of the callus, some incorporation into phosphoglycerate and sugar phosphates occurred, and this increased in shoots as they developed, although with older shoots the increase in β-carboxylation products was even greater. Analysis of enzyme levels in young leaf sections by protein blot and of ¹⁴C-labeling patterns in the present study are in general agreement with enzyme activity determinations of previous studies, providing additional information about PPDK levels, and supporting the model proposed for developing young leaves.

These results suggest that maize leaves begin to express C₄ enzymes during ontogeny through several stages from greening and cell differentiation as seen in the callus and then shoot formation, and finally acquire capacity for full C₄ photosynthesis during leaf development concomitant with the development of Kranz anatomy and accumulation of large amounts of enzymes involved in carbon metabolism.

     

Reduction in chlorophyll content without a corresponding reduction in photosynthesis and carbon assimilation enzymes in yellow-green oil yellow mutants of maize

The photosynthetic properties of a yellow lethal mutant, Oy/oy, and two yellow-green mutants of maize which are allelic (a homozygous recessive oy/oy and a heterozygous dominant Oy/+) were examined. Although Oy/oy had little or no chlorophyll or capacity for CO₂ fixation compared to normal siblings, it had 28% as much ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) activity, and from 40% to near normal activities of C₄ cycle enzymes.Both yellow-green mutants had only half as much chlorophyll per leaf area as normal green seedlings in greenhouse-grown plants in winter and spring. However, the absorbance of light by the mutants was relatively high, as their transmittance was only 5 to 8% greater than normal leaves. In winter-grown greenhouse plants, the activities of Rubisco and several C₄ cycle enzymes in the mutants were unaffected and similar to those of normal seedlings on a leaf area basis. After allowing for small differences in leaf absorbance, the light response curves for photosynthesis in the mutants were similar on a leaf area basis but much higher on a chlorophyll basis than those of the normal seedlings. In spring-grown greenhouse plants the enzyme activities and photosynthesis rates were about 30% lower per leaf area in the yellow-green mutant leaves compared to the wild type. The maximum carboxylation efficiency (measured under low CO₂ and 1000 mol quanta m^-2 s^-1) in the mutants and normal leaves was similar on a Rubisco protein basis. The results indicate that maize can undergo a 50% reduction in chlorophyll content without a corresponding reduction in enzymes of carbon assimilation, and still maintain a high capacity for photosynthesis.Abbreviations Chl chlorophyll - PEP phosphoenolypruvate - Rubisco ribulose-1,5-bisphosphate carboxylase oxygenase This research was supported by CSIRO and by USDA Competitive Grant 86-CRCR-1-2036.     

Decrease in leaf sucrose synthesis leads to increased leaf starch turnover and decreased RuBP regeneration-limited photosynthesis but not Rubisco-limited photosynthesis in Arabidopsis null mutants of SPSA1

We investigated the individual effect of null mutations of each of the four sucrose‐phosphate synthase (SPS) genes in Arabidopsis (SPSA1, SPSA2, SPSB and SPSC) on photosynthesis and carbon partitioning. Null mutants spsa1 and spsc led to decreases in maximum SPS activity in leaves by 80 and 13%, respectively, whereas null mutants spsa2 and spsb had no significant effect. Consistently, isoform‐specific antibodies detected only the SPSA1 and SPSC proteins in leaf extracts. Leaf photosynthesis at ambient [CO₂] was not different among the genotypes but was 20% lower in spsa1 mutants when measured under saturating [CO₂] levels. Carbon partitioning at ambient [CO₂] was altered only in the spsa1 null mutant. Cold treatment of plants (4 °C for 96 h) increased leaf soluble sugars and starch and increased the leaf content of SPSA1 and SPSC proteins twofold to threefold, and of the four null mutants, only spsa1 reduced leaf non‐structural carbohydrate accumulation in response to cold treatment. It is concluded that SPSA1 plays a major role in photosynthetic sucrose synthesis in Arabidopsis leaves, and decreases in leaf SPS activity lead to increased starch synthesis and starch turnover and decreased Ribulose 1,5‐bisphosphate regeneration‐limited photosynthesis but not ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco)‐limited photosynthesis, indicating a limitation of triose‐phosphate utilization (TPU).     

Reporter‐based forward genetic screen to identify bundle sheath anatomy mutants in A. thaliana

The evolution of C₄ photosynthesis proceeded stepwise with each small step increasing the fitness of the plant. An important pre‐condition for the introduction of a functional C₄ cycle is the photosynthetic activation of the C₃ bundle sheath by increasing its volume and organelle number. Therefore, to engineer C₄ photosynthesis into existing C₃ crops, information about genes that control the bundle sheath cell size and organelle content is needed. However, very little information is known about the genes that could be manipulated to create a more C₄–like bundle sheath. To this end, an ethylmethanesulfonate (EMS)‐based forward genetic screen was established in the Brassicaceae C₃ species Arabidopsis thaliana. To ensure a high‐throughput primary screen, the bundle sheath cells of A. thaliana were labeled using a luciferase (LUC68) or by a chloroplast‐targeted green fluorescent protein (sGFP) reporter using a bundle sheath specific promoter. The signal strengths of the reporter genes were used as a proxy to search for mutants with altered bundle sheath anatomy. Here, we show that our genetic screen predominantly identified mutants that were primarily affected in the architecture of the vascular bundle, and led to an increase in bundle sheath volume. By using a mapping‐by‐sequencing approach the genomic segments that contained mutated candidate genes were identified.     

Photosynthesis, immediate export and carbon partitioning in source leaves of C3, C3-C4 intermediate, and C4Panicum and Flaveria species at ambient and elevated CO2 levels

Immediate export in leaves of C₃‐C₄ intermediates were compared with their C₃ and C₄ relatives within the Panicum and Flaveria genera. At 35 Pa CO₂, photosynthesis and export were highest in C₄ species in each genera. Within the Panicum, photosynthesis and export in ‘type I’ C₃‐C₄ intermediates were greater than those in C₃ species. However, ‘type I’ C₃‐C₄ intermediates exported a similar proportion of newly fixed ¹⁴C as did C₄ species. Within the Flaveria, ‘type II’ C₃‐C₄ intermediate species had the lowest export rather than the C₃ species. At ambient CO₂, immediate export was strongly correlated with photosynthesis. However, at 90 Pa CO₂, when photosynthesis and immediate export increased in all C₃ and C₃‐C₄ intermediate species, proportionally less C was exported in all photosynthetic types than that at ambient CO₂. All species accumulated starch and sugars at both CO₂ levels. There was no correlation between immediate export and the pattern of ¹⁴C‐labelling into sugars and starch among the photosynthetic types within each genus. However, during CO₂ enrichment, C₄Panicum species accumulated sugars above the level of sugars and starch normally made at ambient CO₂, whereas the C₄Flaveria species accumulated only additional starch.     

The efficiency of C4 photosynthesis under low light conditions in Zea mays,Miscanthus x giganteus and Flaveria bidentis

The efficiency of C₄ photosynthesis in Zea mays, Miscanthus x giganteus and Flaveria bidentis in response to light was determined using measurements of gas exchange, ¹³CO₂ photosynthetic discrimination, metabolite pools and spectroscopic assays, with models of C₄ photosynthesis and leaf ¹³CO₂ discrimination. Spectroscopic and metabolite assays suggested constant energy partitioning between the C₄ and C₃ cycles across photosynthetically active radiation (PAR). Leakiness (φ), modelled using C₄ light‐limited photosynthesis equations (φ_mod), matched values from the isotope method without simplifications (φ_is) and increased slightly from high to low PAR in all species. However, simplifications of bundle‐sheath [CO₂] and respiratory fractionation lead to large overestimations of φ at low PAR with the isotope method. These species used different strategies to maintain similar φ. For example, Z. mays had large rates of the C₄ cycle and low bundle‐sheath cells CO₂ conductance (g_bs). While F. bidentis had larger g_bs but lower respiration rates and M. giganteus had less C₄ cycle capacity but low g_bs, which resulted in similar φ. This demonstrates that low g_bs is important for efficient C₄ photosynthesis but it is not the only factor determining φ. Additionally, these C₄ species are able to optimize photosynthesis and minimize φ over a range of PARs, including low light.     

Will photosynthesis of maize (Zea mays) in the US Corn Belt increase in future [CO2] rich atmospheres? An analysis of diurnal courses of CO2 uptake under free‐air concentration enrichment (FACE)

The C₄ grass Zea mays (maize or corn) is the third most important food crop globally in terms of production and demand is predicted to increase 45% from 1997 to 2020. However, the effects of rising [CO₂] upon C₄ plants, and Z. mays specifically, are not sufficiently understood to allow accurate predictions of future crop production. A rainfed, field experiment utilizing free‐air concentration enrichment (FACE) technology in the primary area of global corn production (US Corn Belt) was undertaken to determine the effects of elevated [CO₂] on corn. FACE technology allows experimental treatments to be imposed upon a complete soil–plant–atmosphere continuum with none of the effects of experimental enclosures on plant microclimate. Crop performance was compared at ambient [CO₂] (354 μ mol mol^?1) and the elevated [CO₂] (549 μmol mol^?1) predicted for 2050. Previous laboratory studies suggest that under favorable growing conditions C₄ photosynthesis is not typically enhanced by elevated [CO₂]. However, stomatal conductance and transpiration are decreased, which can indirectly increase photosynthesis in dry climates. Given the deep soils and relatively high rainfall of the US Corn Belt, it was predicted that photosynthesis would not be enhanced by elevated [CO₂]. The diurnal course of gas exchange of upper canopy leaves was measured in situ across the growing season of 2002. Contrary to the prediction, growth at elevated [CO₂] significantly increased leaf photosynthetic CO₂ uptake rate (A) by up to 41%, and 10% on average. Greater A was associated with greater intercellular [CO₂], lower stomatal conductance and lower transpiration. Summer rainfall during 2002 was very close to the 50‐year average for this site, indicating that the year was not atypical or a drought year. The results call for a reassessment of the established view that C₄ photosynthesis is insensitive to elevated [CO₂] under favorable growing conditions and that the production potential of corn in the US Corn Belt will not be affected by the global rise in [CO₂].     

Influence of elevated CO2 and nitrogen nutrition on photosynthesis and nitrate photo-assimilation in maize (Zea mays L.)

Measurements of CO₂ and O₂ gas exchange and chlorophyll a fluorescence were used to test the hypothesis that elevated atmospheric CO₂ inhibits nitrate (NO₃ ^– ) photo‐assimilation in the C₄ plant, maize (Zea mays L.). The assimilatory quotient (AQ), the ratio of net CO₂ assimilation to net O₂ evolution, decreases as NO₃ ^– photo‐assimilation increases so that the difference in AQ between the ammonium‐ and nitrate‐fed plants (ΔAQ) provided an in planta estimate of NO₃ ^– photo‐assimilation. In fully expanded maize leaves, NO₃ ^– photo‐assimilation was detectable only under high light and was not affected by CO₂ treatments. Furthermore, CO₂ assimilation and O₂ evolution were higher under NO₃ ^– than ammonia (NH₄⁺) regardless of CO₂ levels. In conclusion, NO₃ ^– photo‐assimilation in maize primarily occurred at high light when reducing equivalents were presumably not limiting. Nitrate photo‐assimilation enhanced C₄ photosynthesis, and in contrast to C₃ plants, elevated CO₂ did not inhibit foliar NO₃^– photo‐assimilation.     

Leaf development,gas exchange characteristics,and photorespiratory activity in maize seedlings

Five decades ago, a novel mode of CO₂ assimilation that was later described as C₄-photosynthesis was discovered on mature leaves of maize (Zea mays L.) plants. Here we show that 3- to 5-day-old developing maize leaves recapitulate the evolutionary advance from the ancient, inefficient C₃ mode of photosynthesis to the C₄ pathway, a mechanism for overcoming the wasteful process of photorespiration. Chlorophyll fluorescence measurements documented that photorespiration was high in 3-day-old juvenile primary leaves with non-specialized C₃-like leaf anatomy and low in 5-day-old organs with the typical “Kranz-anatomy” of C₄ leaves. Photosynthetic gas (CO₂)-exchange measurements on 5-day-old leaves revealed the characteristic features of C₄ photosynthesis, with a CO₂ compensation point close to zero and little inhibition of photosynthesis by the normal oxygen concentration in the air. This indicates a very low photorespiratory activity in contrast to control experiments conducted with mature C₃ sunflower (Helianthus annuus L.) leaves, which display a high rate of photorespiration.