Aluminium modulation of the photosynthetic carbon reduction cycle in Zea mays

Two-weeks-old maize (Zea mays L. cv. XL-72.3) plants were submitted to Al concentrations of 0-81 g m-3 for 20 d, after which the A1 concentration-dependent effects on CO2 uptake by the mesophyll tissue and subsequent CO2 assimilation in the photosynthetic carbon reduction cycle of bundle sheath cells were investigated. The net photosynthetic rate (PN) and stomatal conductance (gs) increased continuously up to 27 g m-3 Al, whereas the intercellular CO2 concentration showed minimum values with the 27 g m-3 Al treatment. Moreover, the starch and saccharide concentrations, and fructose-1,6-bisphosphatase did not change significantly with increasing Al concentrations. The photosynthetic electron transport rates along with photosystems 2 and 1 started falling from 9 g m-3 Al onwards, while thylakoid acyl lipid composition did not show a clear pattern. With the Al concentration at 81 g m-3, NADP-malate dehydrogenase activity decreased to minimum values, whereas the opposite occurred with those of pyruvate dikinase, NADP-malic enzyme, and phosphoenolpyruvate carboxylase. Thus in vivo Al concentrations modulate the photosynthetic reduction cycle, possibly by interacting with the carbon flow rate exported to the cytosol. Although the inhibition of NADP-malate dehydrogenase activity might limit pyruvate dikinase, NADP-malic enzyme, and phosphoenolpyruvate carboxylase activities, in vivo the balance between phosphoenolpyruvate production and its carboxylation remains unaffected.     

Variations in the Photosynthesis Rate and Activity of Photosynthetic Enzymes in Maize Leaf Tissue of Different Ages

Highly good correlations for the extractable activities of ribulose-1,5-bisphosphatecarboxylase, pyruvate,P_i dikinase, phosphoenolpyruvate carboxylase,NADP-malate dehydrogenase and NADP-malic enzyme and the rateof photosynthesis were found in maize leaves of various ages.The activities of the first two enzymes were similar to, orslightly higher than, the photosynthesis rate, whereas the activitiesof the other enzymes were 2 to 6 times higher than the photosynthesisrate. These results suggest that pyruvate,P_i dikinase and ribulose-1,5-bisphosphatecarboxylase may be rate-limiting factors in maize. (Received May 12, 1984; Accepted July 5, 1984)     

Overproduction of C4 photosynthetic enzymes in transgenic rice plants: an approach to introduce the C4-like photosynthetic pathway into rice

Four enzymes, namely, the maize C(4)-specific phosphoenolpyruvate carboxylase (PEPC), the maize C(4)-specific pyruvate, orthophosphate dikinase (PPDK), the sorghum NADP-malate dehydrogenase (MDH), and the rice C(3)-specific NADP-malic enzyme (ME), were overproduced in the mesophyll cells of rice plants independently or in combination. Overproduction individually of PPDK, MDH or ME did not affect the rate of photosynthetic CO(2) assimilation, while in the case of PEPC it was slightly reduced. The reduction in CO(2) assimilation in PEPC overproduction lines remained unaffected by overproduction of PPDK, ME or a combination of both, however it was significantly restored by the combined overproduction of PPDK, ME, and MDH to reach levels comparable to or slightly higher than that of non-transgenic rice. The extent of the restoration of CO(2) assimilation, however, was more marked at higher CO(2) concentrations, an indication that overproduction of the four enzymes in combination did not act to concentrate CO(2) inside the chloroplast. Transgenic rice plants overproducing the four enzymes showed slight stunting. Comparison of transformants overproducing different combinations of enzymes indicated that overproduction of PEPC together with ME was responsible for stunting, and that overproduction of MDH had some mitigating effects. Possible mechanisms underlying these phenotypic effects, as well as possibilities and limitations of introducing the C(4)-like photosynthetic pathway into C(3) plants, are discussed.     

Adaptation responses in C4 photosynthesis of maize under salinity

The effect of salinity on C(4) photosynthesis was examined in leaves of maize, a NADP-malic enzyme (NADP-ME) type C(4) species. Potted plants with the fourth leaf blade fully developed were treated with 3% NaCl solution for 5d. Under salt treatment, the activities of pyruvate orthophosphate dikinase (PPDK), phosphoenolpyruvate carboxylase (PEPCase), NADP-dependent malate dehydrogenase (NADP-MDH) and NAD-dependent malate dehydrogenase (NAD-MDH), which are derived mainly from mesophyll cells, increased, whereas those of NADP-ME and ribulose-1,5-bisphosphate carboxylase, which are derived mainly from bundle sheath cells (BSCs), decreased. Immunocytochemical studies by electron microscopy revealed that PPDK protein increased, while the content of ribulose-1,5-bisphosphate carboxylase/oxygenase protein decreased under salinity. In salt-treated plants, the photosynthetic metabolites malate, pyruvate and starch decreased by 40, 89 and 81%, respectively. Gas-exchange analysis revealed that the net photosynthetic rate, the transpiration rate, stomatal conductance (g(s)) and the intercellular CO(2) concentration decreased strongly in salt-treated plants. The carbon isotope ratio (δ(13)C) in these plants was significantly lower than that in control. These findings suggest that the decrease in photosynthetic metabolites under salinity was induced by a reduction in gas-exchange. Moreover, in addition to the decrease in g(s), the decrease in enzyme activities in BSCs was responsible for the decline of C(4) photosynthesis. The increase of PPDK, PEPCase, NADP-MDH, and NAD-MDH activities and the decrease of NADP-ME activity are interpreted as adaptation responses to salinity.     

Oligomeric enzymes in the C4 pathway of photosynthesis

This review deals with the factors controlling the aggregation-state of several enzymes involved in C₄ photosynthesis, namely phosphoenolpyruvate carboxylase, NAD-and NADP-malic enzyme, NADP-malic dehydrogenase and pyruvate, phosphate dikinase and its regulatory protein. All of these enzymes are oligomeric and have been shown to undergo changes in their quaternary structure in vitro under different conditions. The activity changes linked to variations in aggregation-state are discussed in terms of their putative physiological role in the regulation of C₄ metabolism.Abbreviations P-enolpyruvate phosphoenolpyruvate - NAD-ME NAD-dependent malic enzyme - NADP-ME NADP-dependent malic enzyme - NADP-MDH NADP-dependent malic dehydrogenase - PPDK pyruvate, phosphate dikinase - PPDK-RP pyruvate, phosphate dikinase regulatory protein - V_max maximal velocity - K_m Michaelis constant - CAM Crassulacean acid metabolism     

CO(2) Assimilation and Activities of Photosynthetic Enzymes in High Chlorophyll Fluorescence Mutants of Maize Having Low Levels of Ribulose 1,5-Bisphosphate Carboxylase

Photosynthetic properties were examined in several hcf (high chlorophyll fluorescence 11, 21, 42 and 45) nuclear recessive mutants of maize which were previously found to have normal photochemistry and low CO₂ fixation. Mutants usually either died after depletion of seed reserves (about 18 days after planting), or survived with slow growth up to 7 or 8 weeks. Both the activity and quantity of ribulose 1,5-bisphosphate carboxylase (Rubisco) were low in the mutants (5-25% of the normal siblings on a leaf area basis) and the loss of Rubisco tended to parallel the reduction in photosynthetic capacity. The Rubisco content in the mutants was often marginal for photosynthetic carbon gain, with some leaves and positions along a leaf having no net photosynthesis, while other leaves had a low carbon gain. Conversely, the activities of C₄ cycle enzymes, phosphoenolpyruvate carboxylase, pyruvate, Pi dikinase, NADP-malate dehydrogenase, and NADP-malic enzyme, were the same or only slightly reduced compared to the normal siblings. The mutants had about half as much chlorophyll content per leaf area as the normal green plants. However, the Rubisco activity in the mutants was low on both a leaf area and chlorophyll basis. Low Rubisco activity and lower chlorophyll content may both contribute to the low rates of photosynthesis in the mutants on a leaf area basis.     

Effects of chilling temperature on photosynthetic rates, photosynthetic enzyme activities and metabolite levels in leaves of three sugarcane species

FBPase, fructose-1,6-bisphosphatase
NADP-MDH, NADP-malate dehydrogenase
NADP-ME, NADP-malic enzyme
OAA, oxaloacetic acid
PEP, phosphoenolpyruvate
PEPcase, phosphoenolpyruvate carboxylase
PPDK, pyruvate orthophosphate dikinase
Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase

The aim of this study was to investigate the mechanism of photosynthetic changes in sugarcane leaves in response to chilling temperature by using three species ( Saccharum sinense R. cv. Yomitanzan, Saccharum sp. cv. NiF4 and Saccharum officinarum L. cv. Badira) differing in origin and cold sensitivity. Yomitanzan is native to subtropical areas, Badira is native to tropical areas and NiF4 is a hybrid species containing genes of both tropical and subtropical species. At exposure to chilling temperature (10 °C), the photosynthetic rate in the leaves at either 10 °C or 30 °C showed a greater decrease in Badira than in NiF4 and Yomitanzan. After 28 h exposure of plants to the chilling temperature, the extractable activities of pyruvate, orthophosphate dikinase (PPDK) and NADP-malate dehydrogenase (NADP-MDH) increased or were relatively stable in the leaves of NiF4 and Yomitanzan, but decreased substantially in Badira. Correspondingly, there was a substantial accumulation of aspartate, and the level of alanine increased in Badira leaves during the chilling treatment. It is suggested that NADP-MDH and PPDK are key enzymes which may determine the cold sensitivity in photosynthesis of sugarcane.     

Effects of temperature and photosynthetic inhibitors on light activation of C4-phosphoenolpyruvate carboxylase

Phosphoenolpyruvate carboxylase from leaves of the C₄ plant Setaria verticillata (L.) Beauv. is activated by light; day levels of activity are reached after 30 minutes of illumination. Photoactivation is prevented by inhibitors of photosynthetic electron flow or of photophosphorylation and by D,L-glyceraldehyde, which inhibits the reductive pentose phosphate pathway.Although the extractable activity in the dark is not affected by temperature the photoactivation is prevented when both illumination and extraction are done under low temperature (5 C). High temperature (30 C) during either illumination or extraction is needed for activation. Once the enzyme is photoactivated at 30 C, a transfer of the leaves to 5 C does not abolish the extra activity.The results suggest that both unimpaired electron flow and photophosphorylation are prerequisites for the activation of phosphoenolpyruvate carboxylase. Low temperature apparently suppresses either the transport to the cytoplasm of a photosynthetic intermediate or the activating reaction itself. The inclusion of phosphoenolpyruvate in the extraction medium increases the night activity.On the basis of the available information, it is suggested that phosphoenolpyruvate could be the activator in vivo. In that case, the activation of phosphoenolpyruvate carboxylase would depend on internal CO₂ level and prior photoactivation of both pyruvate, orthophosphate, dikinase and NADP malate dehydrogenase.Abbreviations PEPCase phosphoenolpyruvate carboxylase - PEP phosphoenolpyruvate - PAR photosynthetically active radiation - CCCP carbonyl cyanide m-chlorophenylhydrazone - DCMU 3-(3, 4-dichlorophenyl)-1, 1-dimethylurea - DSPD disalicylidenpropanediamine - MV methylviologen - ME malic enzyme - MDH malate dehydrogenase - PPDK pyruvate, Pi dikinase - CAM Crassulacean Acid Metabolism     

Aluminium Toxicity Modulates Nitrate to Ammonia Reduction

Two weeks-old maize (Zea mays cv. XL-72.3) plants were exposed to Al concentrations 0 (Al0), 9 (Al9), 27 (Al27) or 81 (Al81) g m-3 for 20 d in a growth medium with low ionic strength. Thereafter, the Al concentration-dependent interactions on root nitrate uptake, and its subsequent reduction to ammonia in the leaves were investigated. Al concentrations in the roots sharply increased with increasing Al concentrations while root elongation correspondingly decreased. Root fresh and dry masses, acidification capacity, and nitrate and nitrogen contents decreased from Al27 onwards, whereas leaf nitrogen, nitrate, nitrite, and ammonia concentrations decreased starting with Al9. Electrolytic conductance increased by 60 % in root tissues from Al0 to Al81 but it did not increase significantly in the leaves. In Al9, Al27, and Al81 plants a decrease in shoot fresh and dry masses was observed. Al concentrations between 0 and 27 g m-3 increased net photosynthetic rate, stomatal conductance, and the quantum yield of photosynthetic electron transport, whereas the intercellular CO2 concentration was minimum in Al27 plants. In the leaves, nitrate reductase (E.C. 1.6.6.1) activity increased until Al27, and nitrite reductase (E.C. 1.6.6.4) activity until Al81. Hence there may be an Al mediated extracellular and intracellular regulation of root net nitrate uptake. Nitrate accumulation in the roots affects the translocation rates and, therefore, the nitrate concentration in the leaves. The in vivo reducing power generated by the photosynthetic electron flow does not limit nitrate to ammonia reduction, and the increase of maximum nitrate and nitrite reductase activities parallels the decreasing nitrate, nitrite, and ammonia concentrations.     

THE RELATIVE SIGNIFICANCE OF CO2-FIXING ENZYMES IN THE METABOLISM OF RAT BRAIN

To evaluate the relative significance of CO₂-fixing enzymes in the metabolism of rat brain, the subcellular distribution of pyruvate carboxylase, phosphoenolpyruvate carboxykinase, NADP-isocitrate dehydrogenase and NADP-malate dehydrogenase, as well as the fixation of H¹⁴CO₃^? by the cytosol and the mitochondria was investigated. Pyruvate carboxylase and phosphoenol-pyruvate carboxykinase are mainly localized in the mitochondria whereas NADP-isocitrate dehydrogenase and NADP-malate dehydrogenase are present in both the cytosol and the mitochondria. In the presence of pyruvate rat brain mitochondria fixed H¹⁴CO₃^? at a rate of about 170 nmol/g of tissue/min whereas these organelles fixed negligible amounts of H¹⁴CO₃^? in the presence of α-ketoglutarate or phosphoenolpyruvate. Rat brain cortex slices fixed H¹⁴CO₃^? at a rate of about 7 nmol/g of tissue/min and it was increased by two-fold when pyruvate was added to the incubation medium. The carboxylation of α-ketoglutarate and pyruvate by the reversal of the cytosolic NADP-isocitrate dehydrogenase and NADP-malate dehydrogenase respectively was very low as compared to that by pyruvate carboxylase. The rate of carboxylation reaction of both NADP-isocitrate dehydrogenase and NADP-malate dehydrogenase was only about 1/10th of that of decarboxylation reaction of the same enzyme. It is suggested that under physiological conditions these two enzymes do not play a significant role in CO₂-fixation in the brain. In rat brain cytosol, citrate is largely metabolized to α-ketoglutarate by a sequential action of aconitate hydratase and NADP-isocitrate dehydrogenase. The operation of the citrate-cleavage pathway in rat brain cytosol is demonstrated. The data show that among four CO₂-fixing enzymes, pyruvate carboxylase, an anaplerotic enzyme, plays the major role in CO₂-fixation in the brain.     

In vitro enzyme activities and products of 14CO2 assimilation in flag leaf and ear parts of wheat (Triticum aestivum L.)

Activities of key enzymes of Calvin cycle and C₄ metabolism, rate of ¹⁴CO₂ fixation in light and dark and the initial products of photosynthetic ¹⁴CO₂ fixation were determined in flag leaf and different ear parts of wheat viz. pericarp, awn and glumes. Compared to the activities of RuBP carboxylase and other Calvin cycle enzymes viz. NADP-glyceraldehyde-3-phosphate dehydrogenase, NAD-glyceraldehyde-3-phosphate dehydrogenase and ribulose-5-phosphate kinase, the levels of PEP carboxylase and other enzymes of C₄ metabolism viz. NADP-malate dehydrogenase, NAD-malate dehydrogenase, NADP-malic enzyme, NAD-malic enzyme, glutamate oxaloacetate transaminase genase, NADP-malic enzyme, NAD-malic enzyme, glutamate oxaloacetate transaminase and glutamate pyruvate transaminase, were generally greater in ear parts than in the flag leaf. In contrast to CO₂ fixation in light, the various ear parts incorporated CO₂ in darkness at much higher rates than flag leaf. In short term assimilation of ¹⁴CO₂ by illuminated ear parts, most of the ¹⁴C was in malate with less in 3-phosphoglyceric acid, whereas flag leaves incorporated most into 3-phosphoglyceric acid. It seems likely that ear parts have the capability of assimilating CO₂ by the C₄ pathway of photosynthesis and utilise PEP carboxylase for recapturing the respired CO₂.     

C4 Pathway Enzymes in Soybean Leaves

High-yielding soybean ( Glycine max (L.) Merr.) variety, “Heinong 40”, and one control variety, “Heinong 37”, were used as experimental materials. The activities of C4 pathway enzymes, i.e. PEPCase (phosphoenolpyruvate carboxylase), NADP-ME (NADP-malic enzyme), NADP-MDH (NADP-malate dehydrogenase), PPDK (pyruvate phosphate dikinase) and RuBPCase (ribulose-1.5-bisphosphate carbozylase) were assayed at seedling, blooming, pod-bearing and pod-filling stage. The results indicated that C4 pathway enzymes were expressed differently at different developing stages in both varieties of soybean, but the ratio of PEPCase and RuBPCase showed that the expression of C4 pathway enzymes of “Heinong 40” was higher than those of “Heinong 37” at each stage. The results showed that C4 pathway enzymes were closely related to the crop yielding potential. Therefore, it is possible to select potentially high-yielding soybean variety by the expression of C4 pathway enzymes.     

The activities and intracellular distribution of nicotinamide-adenine dinucleotide phosphate-malate dehydrogenase, phosphoenolpyruvate carboxykinase and pyruvate carboxylase in rat, guinea-pig and rabbit tissues.

1. Measurements are presented of the activity and intracellular distribution of phosphoenolypruvate carboxykinase, pyruvate carboxylase and NADP-malate dehydrogenase in rat, guinea-pig and rabbit liver and kidney cortex, together with previously obtained measurements of these enzymes in adipose tissue. 2. In all three tissues pyruvate carboxylase activity was greatest in the rat and lowest in the rabbit. 3. Guinea pig and rabbit were very similar to each other with respect to the extramitochondrial-mitochondrial distribution of phosphoenolpyruvate carboxykinase in all three tissues. 4. NADP-malate dehydrogenase was present in all three tissues in the rat, present in kidney cortex and adipose tissue in the guinea pig and absent from all tissues examines in the rabbit.     

Immunogold localization of photosynthetic enzymes in leaves of various C4 plants, with particular reference to pyruvate orthophosphate dikinase

Cellular localization of photosynthetic enzymes was investigated by immunogold electron microscopy for leaves of nine C4 grasses (three NADP-malic enzyme (NADP-ME)subtype species, three NAD-malic enzyme (NAD-ME) subtype species, and three phosphoenolpyruvate carboxykinase (PCK) subtype species), two C4 sedges (NADP-ME subtype species) and two C4 dicots (an NADP-ME and an NADP/NAD-ME subtype species). In leaves of all species, immunogold labelling was present for phosphoenolpyruvate carboxylase in the cytosol of the mesophyll cells (MC) and for ribulose-1,5-bisphosphate carboxylase/oxygenase in the chloroplasts of the bundle sheath cells (BSC). However, considerable specific variation was found in the intercellular patterns of labelling for pyruvate orthophosphate dikinase (PPDK). In the NADP-ME grasses, two NAD-ME grasses, and the dicots, significant labelling for PPDK was present in the both the BSC and the MC chloroplasts. In the other NAD-ME grass, the PCK grasses, and the sedges, labelling for PPDK was present almost exclusively in the chloroplasts of the MC. These patterns were observed in the leaves of both young seedlings and mature plants. These results indicate that the accumulation of PPDK in leaves of C4 plants is not necessarily restricted to the MC, although the chloroplasts of the MC accumulate more than those of the BSC.Key words: C4 plants, immunolocalization, phosphoenolpyruvate carboxylase, pyruvate orthophosphate dikinase, ribulose-1,5-bisphosphate carboxylase/oxygenase.      

Influence of Oxygen and Temperature on the Dark Inactivation of Pyruvate, Orthophosphate Dikinase and NADP-Malate Dehydrogenase in Maize

The influence of oxygen and temperature on the inactivation of pyruvate, Pi dikinase and NADP-malate dehydrogenase was studied in Zea mays. O₂ was required for inactivation of both pyruvate, Pi dikinase and NADP-malate dehydrogenase in the dark in vivo. The rate of inactivation under 2% O₂ was only slightly lower than that at 21% O₂. The in vitro inactivation of pyruvate, Pi dikinase, while dependent on adenine nucleotides (ADP + ATP), did not require O₂.

The postillumination inactivation of pyruvate, Pi dikinase in leaves was strongly dependent on temperature. As temperature was decreased in the dark, there was a lag period of increasing length (e.g. at 17°C there was a lag of about 25 minutes) before inactivation proceeded. Following the lag period, the rate of inactivation decreased with decreasing temperature. The half-time for dark inactivation was about 7 minutes at 32°C and 45 minutes at 17°C. The inactivation of pyruvate, Pi dikinase in vitro following extraction from illuminated leaves was also strongly dependent on temperature, but occurred without a lag period. In contrast, NADP-malate dehydrogenase was rapidly inactivated in leaves (half-time of approximately 3 minutes) during the postillumination period without a lag, and there was little effect of temperature between 10 and 32°C. The results are discussed in relation to known differences in the mechanism of activation/inactivation of the two enzymes.

     

A pathway of the autotrophic CO2 fixation in Chloroflexus aurantiacus

Autotrophically grown cells of Chloroflexus aurantiacus B-3 were shown to possess activity of ATP-dependent malate lyase (acetylating CoA). ATP: malate lyase is supposed to be the specific enzyme of the cycle of the autotrophic CO₂ fixation, in which pyruvate synthase, pyruvate phosphate dikinase, phosphoenolpyruvate (PEP) carboxylase and malate dehydrogenase are involved as well. The main product of the CO₂ fixation cycle is glyoxylate, which could further be converted into 3-phosphoglyceric acid (3-PGA) in the reactions of either glycerate or serine pathway. The enzymes of both pathways were detected in C. auratiacus B-3. The results of the in vivo studies of glyxoylate and glycine metabolism, as well as the inhibitor analysis using fluoroacetate (FAc), isonicotinic acid hydrazide (INH), and 4-aminopterin (4-AP) confirm the operation of the proposed pathway in Chloroflexus.Abbreviations 3-PGA 3-phosphoglyceric acid - 4-AP 4-aminopterin - FAc fluoroacetate - INH isonicotinic acid hydrazide - MV methyl viologen - PEP phosphoenolpyruvate - THF tetrahydrofolate - TPP thiamine pyrophosphate     

Photosynthetic Carbon Fixation Characteristics of Fruiting Structures of Brassica campestris L

Activities of key enzymes of the Calvin cycle and C₄ metabolism, rates of CO₂ fixation, and the initial products of photosynthetic ¹⁴CO₂ fixation were determined in the podwall, seed coat (fruiting structures), and the subtending leaf (leaf below a receme) of Brassica campestris L. cv `Toria.' Compared to activities of ribulose-1,5-bisphosphate carboxylase and other Calvin cycle enzymes, e.g. NADP-glyceraldehyde-3-phosphate-dehydrogenase and ribulose-5-phosphate kinase, the activities of phosphoenol pyruvate carboxylase and other enzymes of C₄ metabolism, viz. NADP-malate dehydrogenase, NADP-malic enzyme, glutamate pyruvate transaminase, and glutamate oxaloacetate transaminase, were generally much higher in seed than in podwall and leaf. Podwall and leaf were comparable to each other. Pulse-chase experiments showed that in seed the major product of ¹⁴CO₂ assimilation was malate (in short time), whereas in podwall and leaf, the label initially appeared in 3-PGA. With time, the label moved to sucrose. In contrast to legumes, Brassica pods were able to fix net CO₂ during light. However, respiratory losses were very high during the dark period.     

Regulation of enzyme activity in plants by reversible phosphorylation

This paper reviews the seven specific plant enzymes which have been shown or suggested, to date, to undergo reversible covalent modification by regulatory phosphorylation, including mitochondrial pyruvate dehydrogenase (EC 1.2.4.1), chloroplastic pyruvate, orthophosphate dikinase (EC 2.7.9.1) and ribulose bisphosphate carboxylase/oxygenase (EC 4.1.1.39), cytoplasmic phosphoenolpyruvate carboxylase (EC 4.1.1.31) and 6-phosphofructo-2-kinase (EC 2.7.1.105), microsomal hydroxymethylglutaryl - CoA reductase (EC 1.1.1.34), and quinate: NAD+ oxidoreductase (EC 1.1.1.24).     

Influence of light intensity during growth on photosynthesis and activity of several key photosynthetic enzymes in a C4 plant (Zea mays)

Maize ( Zea mays L. Hybrid Sweet Corn, Royal Crest), a C₄ plant, was grown under different light regimes, after which the rate of photosynthesis and activities of several photosynthetic enzymes (per unit leaf chlorophyll) were measured at different light intensities. Plants were grown outdoors under direct sunlight or 23% of direct sunlight, and in growth chambers at photosynthetic photon flux densities of about 20% and 8% of direct sunlight. The plants grown under direct sunlight had a higher light compensation point than plants grown under lower light. At a light intensity about 25% of direct sunlight, plants from all growth regimes had a similar rate of photosynthesis. Under saturating levels of light the plants grown under direct sunlight had a substantially higher rate of photosynthesis than plants grown under the lower light regimes. The higher photosynthetic capacity in the plants grown under direct sunlight was accompanied by an increased activity of several photosynthetic enzymes and in the amount of the soluble protein in the leaf. Among five photosynthetic enzymes examined, RuBP carboxylase (EC 4.1.1.39) and pyruvate, Pi dikinase (EC 2.7.9.1) were generally just sufficient to account for rates of photosynthesis under saturating light; thus, these may be rate limiting enzymes in C₄ photosynthesis. Pyruvate, Pi dikinase and NADP-malate dehydrogenase (EC 1.1.1.82) were the only enzymes examined which were light activated and increased in activity with increasing light intensity. In the low light grown plants the activity of pyruvate, Pi dikinase closely paralleled the photosynthetic rate measured under different light levels. With the plants grown under direct sunlight, as light intensity was increased the activation of pyruvate, Pi dikinase and NADP⁺-malate dehydrogenase proceeded more rapidly than photosynthesis.