Comparative effects of light during growth on the photosynthetic properties of NADP-ME type C4 grasses from open and shaded habitats. I. Gas exchange,leaf anatomy and ultrastructure*

Abstract Maximum photosynthetic rate (P_max in Zea mays L. was reduced to a much greater extent by neutral shading during growth than in the shade-adapted C₄ grass Paspalum conjugatum Berg., although under a high light regime the P_max of Z. mays was two-fold higher than that of P. conjugatum. In both species the shade-induced reductions of P_max were not of stomatal origin since the intercellular CO₂ concentration (C_i) was not decreased by growth under low light levels. The C_i of P. conjugatum (～200 μPa Pa^?1) measured at air levels of CO₂ and high photon flux densities was 30% greater than that of Z. mays and, concomitantly, leaf water use efficiency was less. As with P_max, specific leaf weight, leaf thickness and chlorenchyma volume were reduced to a greater extent by shading in Z. mays than in P. conjugatum. In contrast to Z. mays, bundle sheath chloroplasts of P. conjugatum contained well-defined stacks of grana. Mesophyll chloroplasts of P. conjugatum developed under a high light regime also contained large amounts of starch. This was not the case with Z. mays.     

Effect of nitrate limitation on the photosynthetically active pools of aspartate and malate in maize, a NADP malic enzyme C4 plant

After two weeks of moderate N restriction, growth of 3-week-old Zea mays L. plants was less than half that of the control and aspartate and malate levels in the leaves were severely suppressed (45 and 65% decrease, respectively). Since in NADP malic enzyme type C₄ plants, such as maize, malate and aspartate are intermediates in the C₄ photosynthetic pathway, the operation of the latter was investigated. Moderate nitrogen deficiency had only a small effect on the rate of photosynthesis (20% decrease) measured under 1000 umol m^?2 s^?1 irradiance. ¹⁴CO₂ pulse-¹²CO₂ chase experiments combined with measurements of in vitro photosynthetic enzyme activities demonstrated the operation of a typical C⁴ photosynthetic pathway in N-restricted plants. The turnover rates of malate and aspartate molecules involved in the C₄ cycle were determined by the loss of label in the carbon 4 moiety of these molecules during the chase period. It is shown that N restriction did not alter the turnover of malate but greatly accelerated that of aspartate. The amounts of malate and aspartate moving through photosynthetically active pools were estimated using a kinetic model. For malate, the size of this pool appeared to be only slightly diminished whereas for aspartate the size of the corresponding pool decreased by a factor of 3. It is proposed that under moderate NO₃^? deficiency, despite deviations in malate metabolism leading to a pronounced decrease in the size of its cellular pool, a large amount of malate remained in the operation of the C₄ pathway. By contrast, the participation of aspartate in the operation of the C₄ pathway was greatly reduced.     

The influence of light quality on C4 photosynthesis under steady-state conditions in Zea mays and Miscanthus×giganteus: changes in rates of photosynthesis but not the efficiency of the CO2 concentrating mechanism

Differences in light quality penetration within a leaf and absorption by the photosystems alter rates of CO₂ assimilation in C₃ plants. It is also expected that light quality will have a profound impact on C₄ photosynthesis due to disrupted coordination of the C₄ and C₃ cycles. To test this hypothesis, we measured leaf gas exchange, ¹³CO₂ discrimination (Δ¹³C), photosynthetic metabolite pools and Rubisco activation state in Zea mays and Miscanthus × giganteus under steady‐state red, green, blue and white light. Photosynthetic rates, quantum yield of CO₂ assimilation, and maximum phosphoenolpyruvate carboxylase activity were significantly lower under blue light than white, red and green light in both species. However, similar leakiness under all light treatments suggests the C₄ and C₃ cycles were coordinated to maintain the photosynthetic efficiency. Measurements of photosynthetic metabolite pools also suggest coordination of C₄ and C₃ cycles across light treatments. The energy limitation under blue light affected both C₄ and C₃ cycles, as we observed a reduction in C₄ pumping of CO₂ into bundle‐sheath cells and a limitation in the conversion of C₃ metabolite phosphoglycerate to triose phosphate. Overall, light quality affects rates of CO₂ assimilation, but not the efficiency of CO₂ concentrating mechanism.     

The temperature acclimation of photosynthetic responses to CO2 in Zea mays and its relationship to the activities of photosynthetic enzymes and the CO2-concentrating mechanism of C4 photosynthesis

Abstract Associations between photosynthetic responses to CO₂ at rate-saturating light and photosynthetic enzyme activities were compared for leaves of maize grown under constant air temperatures of 19, 25 and 31°C. Key photosynthetic enzymes analysed were ribulose bisphosphatc (RuBP) carboxylase, phosphoenolpyruvate (PEP) carboxylase, NADP-malic enzyme and pyruvate, P_i dikinasc. Rates of CO₂-saturated photosynthesis were similar in leaves developed at 19°C and 25°C but were decreased significantly by growth at 31°C. In contrast, carboxylation efficiency differed significantly between all three temperature regimes. Carboxylation efficiency was greatest in leaves developed at 19°C and decreased with increasing temperature during growth. The changes of carboxylation efficiency were highly correlated with changes in the activity of pyruvate, P_i dikinase (r= 0.95), but not with other photosynthetic enzyme activities. The activities of these latter enzymes, including that of RuBP carboxylase, were relatively insensitive to temperature during growth. The sensitivity of quantum yield to O₂ concentration was lower in leaves grown at 19°C than in leaves grown at 31°C. These observations support the novel hypothesis that variation in the capacity for CO₂ delivery to the bundle sheath by the C₄ cycle, relative to the capacity for net assimilation by the C₂ cycle, can be a principal determinant of C₄ photosynthetic responses to CO₂.     

Distribution of N, Rubisco and photosynthesis in Pinus pinaster and acclimation to light

The light–nitrogen hypothesis suggests canopy photosynthesis is maximized when there is a positive relationship between irradiance received by foliage, its nitrogen content (per unit area N_area), and maximum rate of photosynthesis (A_max). Relationships among relative irradiance and N_area, allocation of nitrogen within the photosynthetic apparatus to Rubisco and chlorophyll, and A_max were examined in Pinus pinaster Ait. needles up to 6 years of age. Measurements were made before bud break in August 1998, and in May 1999 after the first ‘winter’ rains. In August, N_area in P. pinaster needles decreased from 5·1 to 5·7 g m^?2 in sunlit 1‐year‐old needles to 2·3 g m^?2 in shaded 6‐year‐old needles. In May, N_area was 5–40% less but spatial trends were the same. At both sampling dates, A_max was less in old shaded needles compared with young sunlit needles, and was thus consistent with the light–nitrogen hypothesis. Relationships between N_area and A_max were positive at both dates yet varied in strength and form. Allocation of nitrogen within the photosynthetic apparatus was qualitatively consistent with acclimation to light (i.e. Rubisco/Chl decreased with shading), but quantitatively suboptimal with respect to photosynthesis owing to consistent over‐investment in Rubisco. This over‐investment increased with height in the canopy and was greater in May than in August.     

Factors underlying genotypic differences in the induction of photosynthesis in soybean [Glycine max (L.) Merr.]

Crop leaves are subject to continually changing light levels in the field. Photosynthetic efficiency of a crop canopy and productivity will depend significantly on how quickly a leaf can acclimate to a change. One measure of speed of response is the rate of photosynthesis increase toward its steady state on transition from low to high light. This rate was measured for seven genotypes of soybean [Glycine max (L.) Merr.]. After 10 min of illumination, cultivar ‘UA4805’ (UA) had achieved a leaf photosynthetic rate (P_n) of 23.2 μmol · m^?2 · s^?1, close to its steady‐state rate, while the slowest cultivar ‘Tachinagaha’ (Tc) had only reached 13.0 μmol · m^?2 · s^?1 and was still many minutes from obtaining steady state. This difference was further investigated by examining induction at a range of carbon dioxide concentrations. Applying a biochemical model of limitations to photosynthesis to the responses of P_n to intercellular CO₂ concentration (C_i), it was found that the speed of apparent in vivo activation of ribulose‐1:5‐bisphosphate carboxylase/oxygenase (Rubisco) was responsible for this difference. Sequence analysis of the Rubisco activase gene revealed single nucleotide polymorphisms that could relate to this difference. The results show a potential route for selection of cultivars with increased photosynthetic efficiency in fluctuating light.     

Effects of rapidly imposed water deficit on photosynthetic parameters of three C<Subscript>4</Subscript> grasses

Water deficit, when rapidly imposed on three C₄ grasses of the different metabolic subtypes, Paspalum dilatatum Poiret (NADP-malic enzyme), Cynodon dactylon (L.) Pers (NAD-malic enzyme) and Zoysia japonica Steudel (phosphoenolpyruvate carboxykinase), caused decreases in photosynthetic rates, in the quantum yield of PS II and photochemical quenching, and in the activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC). The results provide evidence for non-stomatal limitations of photosynthesis differing in nature between the three species.     

The occurrence of both C3 and C4 photosynthetic characteristics in a single Zea mays plant

The activities of the carboxylating enzymes ribulose-1,5-biphosphate (RuBP) carboxylase and phosphoenolpyruvate (PEP) carboxylase in leaves of three-week old Zea mays plants grown under phytotron conditions were found to vary according to leaf position. In the lower leaves the activity of PEP carboxylase was lower than that of RuBP carboxylase, while the upper leaves exhibited high levels of PEP carboxylase. Carbon dioxide compensation points and net photosynthetic rates also differed in the lower and upper leaves. Differences in the fine structure of the lowermost and uppermost leaves are shown. The existence of both the C₃ and C₄ photosynthetic pathways in the same plant, in this and other species, is discussed.Abbreviations PEP phosphoenolpyruvate - RuBP ribulose-1,5-biphosphate     

CYANOBACTERIAL ACCLIMATION TO RAPIDLY FLUCTUATING LIGHT IS CONSTRAINED BY INORGANIC CARBON STATUS1

Acclimation to rapidly fluctuating light, simulating shallow aquatic habitats, is altered depending on inorganic carbon (C_i) availability. Under steady light of 50 μmol photons·m^?2·s^?1, the growth rate of Synechococcus elongatus PCC7942 was similar in cells grown in high C_i (4 mM) and low C_i (0.02 mM), with induced carbon concentrating mechanisms compensating for low C_i. Growth under fluctuating light of a 1‐s period averaging 50 μmol photons·m^?2·s^?1 caused a drop in growth rate of 28%±6% in high C_i cells and 38%±8% in low C_i cells. In high C_i cells under fluctuating light, the PSI/PSII ratio increased, the PSII absorption cross‐section decreased, and the PSII turnover rate increased in a pattern similar to high‐light acclimation. In low C_i cells under fluctuating light, the PSI/PSII ratio decreased, the PSII absorption cross‐section decreased, and the PSII turnover remained slow. Electron transport rate was similar in high and low C_i cells but in both was lower under fluctuating than under steady light. After acclimation to a 1‐s period fluctuating light, electron transport rate decreased under steady or long‐period fluctuating light. We hypothesize that high C_i cells acclimated to exploit the bright phases of the fluctuating light, whereas low C_i cells enlarged their PSII pool to integrate the fluctuating light and dampen the variation of the electron flux into a rate‐restricted C_i pool. Light response curves measured under steady light, widely used to predict photosynthetic rates, do not properly predict photosynthetic rates achieved under fluctuating light, and exploitation of fluctuating light is altered by C_i status.     

Differential inhibition of photosynthesis under drought stress in <Emphasis Type="Italic">Flaveria</Emphasis> species with different degrees of development of the C<Subscript>4</Subscript> syndrome

The effect of drought stress (DS) on photosynthesis and photosynthesis-related enzyme activities was investigated in F. pringlei (C₃), F. floridana (C₃–C₄), F. brownii (C₄-like), and F. trinervia (C₄) species. Stomatal closure was observed in all species, probably being the main cause for the decline in photosynthesis in the C₃ species under ambient conditions. In vitro ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) and stromal fructose 1,6-bisphosphatase (sFBP) activities were sufficient to interpret the net photosynthetic rates (P _N), but, from the decreases in P _N values under high CO₂ (C _a = 700 μmol mol^{− 1}) it is concluded that a decrease in the in vivo rate of the RuBPCO reaction may be an additional limiting factor under DS in the C₃ species. The observed decline in the photosynthesis capacity of the C₃–C₄ species is suggested to be associated both to in vivo decreases of RuBPCO activity and of the RuBP regeneration rate. The decline of the maximum P _N observed in the C₄-like species under DS was probably attributed to a decrease in maximum RuBPCO activity and/or to decrease of enzyme substrate (RuBP or PEP) regeneration rates. In the C₄ species, the decline of both in vivo photosynthesis and photosynthetic capacity could be due to in vivo inhibition of the phosphoenolpyruvate carboxylase (PEPC) by a twofold increase of the malate concentration observed in mesophyll cell extracts from DS plants.     

干旱和遮荫对马尾松幼苗生长和光合特性的影响

为探究马尾松对干旱和遮荫胁迫的生理响应规律和适应机制,以2年生马尾松幼苗为对象,设置对照(CK)、模拟干旱(DR)、遮荫(LL)以及干旱与遮荫的交互处理(DRLL)4种环境,研究干旱和遮荫对马尾松幼苗的生长和光合生理特性的影响。结果表明:(1)在干旱、遮荫和二者的交互处理下,马尾松幼苗的基径和株高增长量均显著减小,且二者的交互处理加重了干旱和遮荫单一处理下的减小趋势,二者交互作用的影响符合"相互作用理论"。(2)在干旱处理下,针叶长度和比叶面积减小,净光合速率、蒸腾速率、气孔导度和细胞间隙CO_2浓度显著降低,水分利用效率显著增加,光合色素含量基本不变。(3)在遮荫处理下,针叶长度和比叶面积增大,净光合速率、蒸腾速率、气孔导度显著降低,光合色素含量显著增加,水分利用效率和细胞间隙CO_2浓度基本未发生改变。(4)二者的交互处理下,针叶长度、比叶面积减小,净光合速率、蒸腾速率和气孔导度显著降低,且降低程度大于干旱单一处理,光合色素含量显著增加,但其增加程度小于遮荫单一处理。说明干旱和遮荫均能抑制马尾松植株的生长,但其光合生理特性在干旱和遮荫胁迫下分别表现出不同的响应特征。在干旱处理下,通过牺牲针叶长度和比叶面积、调节气孔导度达到保水目的来适应干旱环境;在遮荫处理下,通过增大针叶长度、比叶面积和光合色素含量来响应遮荫环境。在二者的交互处理下,干旱胁迫导致马尾松针叶长度、比叶面积和气孔导度的减小趋势不仅未因遮荫环境而得到缓解,而且还削弱了对光合色素的合成能力,因此,二者的交互作用会进一步加重对马尾松生长的抑制。     

Photosynthetic carbon metabolism during leaf ontogeny in Zea mays L.: Enzyme studies

The activities of several enzymes, including ribulose-1,5-diphosphate (RuDP) carboxylase (EC 4.1.1.39) and phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) were measured as a function of leaf age in Z. mays. Mature leaf tissue had a RuDP-carboxylase activity of 296.7 mol CO₂ g^-1 fresh weight h^-1 and a PEP-carboxylase activity of 660.6 mol CO₂ g^-1 fresh weight h^-1. In young corn leaves the activity of the two enzymes was 11 and 29%, respectively, of the mature leaves. In senescent leaf tissue, RuDP carboxylase activity declined more rapidly than that of any of the other enzymes assayed. On a relative basis the activities of NADP malic enzyme (EC 1.1.1.40), aspartate (EC 2.6.1.1) and alanine aminotransferase (EC 2.6.1.2), and NAD malate dehydrogenase (EC 1.1.1.37) exceeded those of both PEP and RuDP carboxylase in young and senescent leaf tissue. Pulse-chase labeling experiments with mature and senescent leaf tissue show that the predominant C₄ acid differs between the two leaf ages. Labeling of alanine in senescent tissue never exceeded 4% of the total ¹⁴C remaining during the chase period, while in mature leaf tissue alanine accounted for 20% of the total after 60 s in ¹²CO₂. The activity of RuDP carboxylase during leaf ontogeny in Z. mays parallels the development of the activity of this enzyme in C₃ plants.Abbreviations RuDP ribulose-1,5-diphosphate - PEP phosphoenol pyruvate - PGA 3-phosphoglycerate     

Correlations between photosynthetic enzymes,CO2-fixation and plastid structure in an albino mutant of Zea mays L.

Summary An albino seedling of Zea mays L. was investigated for its potential for CO₂-assimilation. In the mesophyll the number, dimensions and fine structure of chloroplasts are drastically reduced but to a lesser extent in the bundle sheath. Chlorophyll concentration is zero and carotenoid concentration almost zero. Albinism also exerts a strong influence on the stroma of bundle sheath chloroplasts; ribulose-1.5-biphosphate carboxylase (EC 4.1.1.39) activity and glyceraldehyde-3-phosphate dehydrogenase (NADP) (EC 1.2.1.13) activity is not detectable. The C₄-enzymes phosphoenolpyruvate carboxylase (EC 4.1.1.31) and malate dehydrogenase (decarboxylating) (EC 1.1.1.40) and the non-photosynthetic linked enzymes malate dehydrogenase (NAD) (EC 1.1.1.37), aspartate-2-oxoglutarate aminotransferase (EC 1.1.1.37), aspartate-2-oxoglutarate aminotransferase (EC 2.6.1.1.) and glyceraldehyde-3-phosphate dehydrogenase (NAD) (EC 1.2.1.1.) are present in the albino seedling with activities comparable to those in etiolated maize seedlings. The potential for CO₂ fixation of the albino seedlings exceeds that of comparable dark seedlings considerably. The results are discussed with regard to enzyme localization of the C₄ pathway of photosynthesis.Abbreviations Aspartate aminotransferase L-aspartate-2-oxoglutarate aminotransferase-EC 2.6.1.1. - GAPDH (NAD) glyceraldehyde-3-phosphate dehydrogenase (NAD dep.)-EC 1.2.1.12 - GAPDH (NADP) glyceraldehyde-3-phosphate dehydrogenase (NADP dep.)-EC 1.2.1.13 - malic enzyme malate dehydrogenase (NADP dep., decarboxylating)-EC 1.1.1.40 - MDH malate dehydrogenase (NAD dep.)-1.1.1.37 - PEP carboxylase phosphoenolpyruvate carboxylase-EC 4.1.1.31 - RuDP carboxylase ribulose-1.5-biphosphate carboxylase-EC 4.1.1.39     

Response of plasmodesmata formation in leaves of C4 grasses to growth irradiance

Rapid metabolite diffusion across the mesophyll (M) and bundle sheath (BS) cell interface in C₄ leaves is a key requirement for C₄ photosynthesis and occurs via plasmodesmata (PD). Here, we investigated how growth irradiance affects PD density between M and BS cells and between M cells in two C₄ species using our PD quantification method, which combines three‐dimensional laser confocal fluorescence microscopy and scanning electron microscopy. The response of leaf anatomy and physiology of NADP‐ME species, Setaria viridis and Zea mays to growth under different irradiances, low light (100 μmol m^?2 s^?1), and high light (1,000 μmol m^?2 s^?1), was observed both at seedling and established growth stages. We found that the effect of growth irradiance on C₄ leaf PD density depended on plant age and species. The high light treatment resulted in two to four‐fold greater PD density per unit leaf area than at low light, due to greater area of PD clusters and greater PD size in high light plants. These results along with our finding that the effect of light on M‐BS PD density was not tightly linked to photosynthetic capacity suggest a complex mechanism underlying the dynamic response of C₄ leaf PD formation to growth irradiance.     

Ecology of SO2 resistance: III. Metabolic changes of C3 and C4 Atriplex species due to SO2 fumigations

Summary The photosynthetic processes of two ecologically-matched, herbaceous Atriplex species differed in their response to SO₂ fumigations. Atriplex triangularis, a C₃ species, was more sensitive than the C₄ species, A. sabulosa. This difference in sensitivity can be attributed in part to the higher conductance of the C₃ species in normal air and saturating light as well as greater stimulation of stomatal opening following exposure to SO₂. In addition, photosynthetic mechanisms of the C₃ species had higher intrinsic SO₂ sensitivity than the C₄ species. Differences between photosynthetic responses of these two species may also reflect differences in morphological configuration of mesophyll tissues and greater SO₂ sensitivity of the initial photosynthetic carboxlating enzyme of the C₃ species. It is likely that certain of the differences in photosynthetic SO₂ sensitivity of these contrasting C₃ and C₄ Atriplex species are characteristic of C₃ and C₄ plants in general.Abbreviations PEP carboxylase phosphoenolpyruvate carboxylase - RuBP carboxylase ribulose-1,5-bisphosphate carboxylase     

Photosynthetic activities of isolated bundle sheath cells in relation to differing mechanisms of C-4 pathway photosynthesis.

Photosynthetic activities of bundle sheath cell strands isolated from several C₄ pathway species were examined. These included species that decarboxylate C₄ acids via either NADP-malic enzyme (Zea mays, NADP-malic enzyme-type), NAD-malic enzyme (Atriplex spongiosa and Panicum miliaceum, NAD-malic enzyme-type) or phosphoenolpyruvate carboxykinase (Chloris gayana and Panicum maximum, phosphoenolpyruvate carboxykinase-type). Preparations from each of these species fixed ¹⁴CO₂ at rates ranging between 1.2 and 3.5 μmol min^?1 mg^?1 of chlorophyll, with more than 90% of the ¹⁴C being assimilated into Calvin cycle intermediates. With added HCO₃^? the rate of light-dependent O₂ evolution ranged between 2 and 4 μmol min^?1 mg^?1 of chlorophyll for cells from NAD-malic enzyme-type and phosphoenolpyruvate carboxykinase-type species but with Z. mays cells there was no O₂ evolution detectable. Most of the ¹⁴CO₂ fixed by Z. mays cells provided with H¹⁴CO₃^? plus ribose 5-phosphate accumulated in the C-1 of 3-phosphoglycerate. However, 3-phosphoglycerate reduction was increased several fold when malate was also provided. Cells from all species rapidly decarboxylated C₄ acids under appropriate conditions, and the CO₂ released from the C-4 carboxyl was reassimilated via the Calvin cycle. Malate decarboxylation by Z. mays cells was dependent upon light and an endogenous or exogenous source of 3-phosphoglycerate. Bundle sheath cells of NAD-malic enzyme-type species rapidly decarboxylated [¹⁴C]malate when aspartate and 2-oxoglutarate were also provided, and [¹⁴C]aspartate was decarboxylated at similar rates when 2-oxoglutarate was added. Cells from phosphoenolpyruvate carboxykinase-type species decarboxylated [¹⁴C]aspartate when 2-oxoglutarate was added and they also catalyzed a slower decarboxylation of malate. Cells from NAD-malic enzyme-type and phosphoenolpyruvate carboxykinase-type species evolved O₂ in the light when C₄ acids were added. These results are discussed in relation to proposed mechanisms for photosynthetic metabolism in the bundle sheath cells of species utilizing C₄ pathway photosynthesis.     

Modelling C3 photosynthesis: A sensitivity analysis of the photosynthetic carbon-reduction cycle

A model of the C ₃ photosynthetic system is developed which describes the sensitivity of the steadystate rate of carbon dioxide assimilation to changes in the activity of several enzymes of the system. The model requires measurements of the steady-state rate of carbon dioxide assimilation, the concentrations of several intermediates in the photosynthetic system, and the concentration of the active site of ribulose 1,5-bisphosphate carboxyalse/oxygenase (Rubisco). It is shown that in sunflowers (Helianthus annuus L.) at photon flux densities that are largely saturating for the rate of photosynthesis, the steady-stete rate of carbon dioxide assimilation is most sensitive to Rubisco activity and, to a lesser degree, to the activities of the stromal fructose, 6-bisphosphatase and the enzymes catalysing sucrose synthesis. The activities of sedoheptulose 1,7-bisphosphatase, ribulose 5-phosphate kinase, ATP synthase and the ADP-glucose pyrophosphorylase are calculated to have a negligible effect on the flux under the high-light conditions. The utility of this analysis in developing simpler models of photosynthesis is also discussed.Abbreviations c _i intercellular CO₂ concentration - C _infP ^supJ control coefficient for enzyme P with respect to flux J - DHAP dihydroxyacetonephosphate - E4P erythrose 4-phosphate - F6P fructose 6-phosphate - FBP fructose 1,6-bisphosphate - FBPase fructose 1,6-bisphosphatase - G3P glyceraldehyde 3-phosphate - G1P glucose 1-phosphate - G6P glucose 6-phosphate - Pi inorganic phosphate - PCR photosynthetic carbon reduction - PGA 3-phosphoglyceric acid - PPFD photosynthetically active photon flux density - R _n ^J response coefficient for effector n with respect to flux J - R5P ribose 5-phosphate - Rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase - Ru5P ribulose 5-phosphate - RuBP ribulose 1,5-bisphosphate - S7P sedoheptulose 7-phosphate - SBP sedoheptulose 1,7-bisphosphate - SBPase sedoheptulose 1,7-bisphosphatase - SPS sucrose-phosphate synthase - Xu5P xylulose 5-phosphate - _n ^P elasticity coefficient for effector n with respect to the catalytic velocity of enzyme P This research was funded by an Australian Research Council grant to I.E.W. and was undertaken during a visity by K.A.M. to the James Cook University of North Queensland. The expert help of Glenys Hanley and Mick Kelly is greatly appreciated.     

Phosphate Deficiency in Maize. II. Enzyme Activities

Low-phosphate (P) treatment decreased photosynthetic rates ofmaize plants (Zea mays L.) by about 50% 18 to 19 days afterplanting [Usuda and Shimogawara (1991) Plant Cell Physiol. 32:497]. Low-P treatment decreased the enzyme activities differentially(by 0-49%). The significance of the decreased activities ofpyruvate.Pj dikinase (by 29%), phosphoenolpyruvate carboxylase(by 49%), and ribulose 1,5-bisphosphate carboxylase (by 42%)in the detrimental effects of low-P treatments on the ratesof photosynthesis is discussed. (Received August 9, 1991; Accepted October 1, 1991)     

Intracellular localization of certain photosynthetic enzymes in bundle sheath cells of plants possessing the C4 pathway of photosynthesis.

Bundle sheath cells were enzymatically isolated from representatives of three groups of C₄ plants: Zea mays (NADP malic enzyme type), Panicum miliaceum (NAD malic enzyme type), and Panicum maximum (phosphoenolpyruvate (PEP) carboxykinase type). Cellular organelles from bundle sheath homogenates were partially resolved by differential centrifugation and on isopycnic sucrose density gradients in order to study compartmentation of photosynthetic enzymes. A 48-h-dark pretreatment of the leaves allowed the isolation of relatively intact chloroplasts. Enzymes that decarboxylate C₄ acids and furnish CO₂ to the Calvin cycle are localized as follows: NADP malic enzyme, chloroplastic in Z. mays; NAD malic enzyme, mitochondrial in all three species; PEP carboxykinase, chloroplastic in P. maximum. The activity of NAD malic enzyme in the three species was in the order of P. miliaceum > P. maximum > Z. mays. There were high levels of aspartate and alanine aminotransferases in bundle sheath extracts of P. miliaceum and P. maximum and substantial activity in Z. mays. In all three species, aspartate aminotransferase was mitochondrial whereas alanine aminotransferase was cytoplasmic. Based on the activity and localization of certain enzymes, the concept for aspartate and malate as transport metabolites from mesophyll to bundle sheath cells in C₄ species of the three C₄ groups is discussed.     

The impact of elevated carbon dioxide on the growth and gas exchange of three C4 species differing in CO2 leak rates

Recent work has suggested that the photosynthetic rate of certain C₄ species can be stimulated by increasing CO₂ concentration, [CO₂], even under optimal water and nutrients. To determine the basis for the observed photosynthetic stimulation, we tested the hypothesis that the CO₂ leak rate from the bundle sheath would be directly related to any observed stimulation in single leaf photosynthesis at double the current [CO₂]. Three C₄ species that differed in the reported degree of bundle sheath leakiness to CO₂, Flaveria trinervia, Panicum miliaceum, and Panicum maximum, were grown for 31–48 days after sowing at a [CO₂] of 350 μl l^?1 (ambient) or 700 μl l^?1 (elevated). Assimilation as a function of increasing [CO₂] at high photosynthetic photon flux density (PPFD, 1 600 μmol m^?2 s^?1) indicated that leaf photosynthesis was not saturated under current ambient [CO₂] for any of the three C₄ species. Assimilation as a function of increasing PPFD also indicated that the response of leaf photosynthesis to elevated [CO₂] was light dependent for all three C₄ species. The stimulation of leaf photosynthesis at elevated [CO₂] was not associated with previously published values of CO₂ leak rates from the bundle sheath, changes in the ratio of activities of PEP-carboxylase to RuBP carboxylase/oxgenase, or any improvement in daytime leaf water potential for the species tested in this experiment. In spite of the simulation of leaf photosynthesis, a significant increase in growth at elevated [CO₂] was only observed for one species, F. trinervia. Results from this study indicate that leaf photosynthetic rates of certain C₄ species can respond directly to increased [CO₂] under optimal growth conditions, but that the stimulation of whole plant growth at elevated carbon dioxide cannot be predicted solely on the response of individual leaves.