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.     

转C4光合酶基因水稻的CO2交换和荧光特性

用转PEPC、PPDK、NADP_ME、PEPC PPDK酶基因水稻 (OryzasativaL .)及原种为材料 ,研究了光合作用对光照、温度、CO2 的响应和光抑制条件下的叶绿素荧光特性 ,结果如下 :1.转C4 光合酶基因水稻的饱和光合速率比原种高 ,其中转PEPC、PEPC PPDK双基因水稻的光饱和点比原种高 2 0 0 μmol·m-2 ·s-1,饱和光合速率比原种分别高5 1.6 %和 5 8.5 % ;转PEPC基因水稻的羧化效率比原种高 49.3% ,CO2 补偿点降低 2 6 .2 % ;在高温 (35℃ )下 ,转PEPC基因水稻的光合速率比原种高 17.5 %。 2 .经光抑制处理 8d后 ,转PEPC、PEPC PPDK酶基因水稻的PSⅡ光化学效率 (Fv/Fm)和光化学猝灭 (qP)下降 2 0 % - 30 % ,非光化学猝灭 (qN)增加了约 30 % ;但原种的Fv/Fm 和qP下降了 5 0 %多 ,qN变化不明显 ,表明转C4 光合基因水稻耐光抑制能力增强。这些结果为用生物技术提高水稻光合效率研究提供了新的依据和途径     

转C4光合酶基因水稻的CO2交换和荧光特性

用转PEPC、PPDK、NADP-ME、PEPC+PPDK酶基因水稻(Oryza sativa L.)及原种为材料 ,研究了光合作用对光照、温度、CO2的响应和光抑制条件下的叶绿素荧光特性,结果如下: 1.转C4光合酶基因水稻的饱和光合速率比原种高,其中转PEPC、PEPC+PPDK双基因水稻的光饱和点比原种高200 μmol*m-2*s-1,饱和光合速率比原种分别高51.6%和 58.5%;转PEPC基因水稻的羧化效率比原种高49.3%,CO2补偿点降低26.2%;在高温(35 ℃)下,转PEPC基因水稻的光合速率比原种高17.5%.2.经光抑制处理8 d后,转PEPC、PEPC +PPDK酶基因水稻的PSⅡ光化学效率(Fv/Fm)和光化学猝灭(qP)下降20%- 30%,非光化学猝灭(qN)增加了约30%;但原种的Fv/Fm和qP下降了5 0%多,qN变化不明显,表明转C4光合基因水稻耐光抑制能力增强.这些结果为用生物技术提高水稻光合效率研究提供了新的依据和途径.     

A new measurement technique reveals rapid post-illumination changes in the carbon isotope composition of leaf-respired CO2

We describe an open leaf gas exchange system coupled to a tunable diode laser (TDL) spectroscopy system enabling measurement of the leaf respiratory CO(2) flux and its associated carbon isotope composition (delta(13)C(Rl)) every 3 min. The precision of delta(13)C(Rl) measurement is comparable to that of traditional mass spectrometry techniques. delta(13)C(Rl) from castor bean (Ricinus communis L.) leaves tended to be positively related to the ratio of CO(2) produced to O(2) consumed [respiratory quotient (RQ)] after 24-48 h of prolonged darkness, in support of existing models. Further, the apparent fractionation between respiratory substrates and respired CO(2) within 1-8 h after the start of the dark period was similar to previous observations. In subsequent experiments, R. communis plants were grown under variable water availability to provide a range in delta(13)C of recently fixed carbohydrate. In leaves exposed to high light levels prior to the start of the dark period, CO(2) respired by leaves was up to 11 per thousand more enriched than phloem sap sugars within the first 10-15 min after plants had been moved from the light into the dark. The (13)C enrichment in respired CO(2) then decreased rapidly to within 3-7 per thousand of phloem sap after 30-60 min in the dark. This strong enrichment was not observed if light levels were low prior to the start of the dark period. Measurements of RQ confirmed that carbohydrates were the likely respiratory substrate for plants (RQ > 0.8) within the first 60 min after illumination. The strong (13)C enrichment that followed a high light-to-dark transition coincided with high respiration rates, suggesting that so-called light-enhanced dark respiration (LEDR) is fed by (13)C-enriched metabolites.     

Rapid-cycling CAM; an hypothetical variant of photosynthetic metabolism

The currently recognized forms of CAM photosynthesis do not represent all possible variants on the theme. It is predicted here that there may exist an undiscovered variant in which the CO₂-acquiring and CO₂-reducing phases of CAM alternate over time periods shorter (possibly much shorter) than the diel cycle. The process would occur entirely in the photoperiod and may be likened to a form of C₄ photosynthesis in which the same photosynthetic cell alternates between performing the functions of a C₄ mesophyll and C₄ bundle sheath cell. Rapid-cycling CAM, which would be unlikely to be detected by methods commonly used to measure photosynthesis, could provide a CO₂-concentrating mechanism in both unicellular and multicellular plants.     

Evolution of C4 photosynthetic genes and overexpression of maize C4 genes in rice

C3 plants including many agronomically important crops exhibit a lower photosynthetic efficiency due to inhibition of photosynthesis by O₂ and the associated photorespiration. C4 plants had evolved the C4 pathway to overcome low CO₂ and photorespiration. This review first focuses on the generation of a system for high level expression of the C4-specific gene for pyruvate, orthophosphate dikinase (Pdk), one of the key enzyme in C4 photosynthesis. Based on the results with transgenic rice plants, we have demonstrated that the regulatory system controlling thePdk expression in maize is not unique to C4 plants but rice (C3 plant) posses a similar system. Second, we discussed the possibility of the high level expression of maize C4-specific genes in transgenic rice plants. Introduction of the maize intact phosphoenolpyruvate carboxylase gene (Ppc) caused 30–100 fold higher PEPC activities than non-transgenic rice. These results demonstrated that intact C4-type genes are available for high level expression of C4 enzymes in rice plants. The extended abstract of a paper presented at the 13th International Symposium in Conjugation with Award of the International Prize for Biology “Frontier of Plant Biology”     

Light and temperature dependence of the rate and degree of activation of pyruvate,Pi dikinase in vivo in maize

Activation of pyruvate,Pi dikinase by light was studied in leaf discs of maize which were illuminated for 1 h at light intensities ranging from approximately 3% to 50% of full sunlight and at temperatures of 10, 22.5, and 35°C. At the highest light intensity the degree of activation was similar and relatively independent of temperature between 10 and 35°C. Under low light the degree of activation was high at 10°C but decreased rapidly with increasing temperature. There was a similar effect of light and temperature on the activation of NADP-malate dehydrogenase.At low temperature, the rate of activation of pyruvate,Pi dikinase was relatively low and independent of the light intensity used and the rate of inactivation in the dark was extremely low. At high temperature, the rate of activation was high and dependent on the light intensity used while the rate of dark inactivation was also relatively high. The degree of activation is discussed in relation to the possible influence of light and temperature on the turnover between the active and inactive forms of pyruvate,Pi dikinase during illumination.This research was supported by the Japan-U.S. Cooperative Research Program (The Japan Society for the Promotion of Science, NFS Grant INT 78-17245), NSF Grant PCM 77-09284, by the Japanese Ministry of Education and by the College of Agriculture and Life Sciences, University of Wisconsin, Madison, Wisconsin.     

Effect of Husk morphology on grain development and topography in rice

Kernels grown within loosened glumes in three varieties of paddy were darker in color and had a smoother surface than those grown under normal conditions. The thickness of the pericarp plus seed coat layers was 33.6 ±2.8 µm, and the thickness of the aleurone layers was 21.7 ± 2.5 µm in grains of the first type, while in the normal grains, these dimensions were 13.0 ± 1.4 and 26.9 ± 2.9 µm respectively. The kernels which developed within loosened glumes tended to taper towards the distal end. They were lighter in weight than normal grains by 32 to 67 percent, the weight loss being less in the bolder variety. The lemma-palea interlocking depth was positively correlated with the groove depth on the kernel and with the clearance between husk and kernel. All three parameters showed a positive correlation with grain breadth. A low lemma-palea interlocking depth and a smaller clearance between husk and kernel are technologically desirable characteristics in rice. The reclasping of the two glume components after pollination was essential for the normal development of the rice grain.     

湛江5种红树林树种光合作用特性及光合固碳能力研究

应用Li-6400便携式光合作用测定系统,对湛江市特呈岛5种红树林树种的净光合速率日变化和光合作用—光响应曲线进行了测定,探讨了各树种的光合作用特性以及主要影响因子并评估其固碳能力大小。结果表明:在自然光照条件下,秋茄和红海榄叶片净光合速率(Pn)的日变化曲线呈单峰型,白骨壤、木榄和桐花树为双峰型,光合"午休"现象明显,而且峰值分别出现在10:00和14:00左右。其中,白骨壤和木榄的光合午休主要由气孔限制因素引起,桐花树主要由非气孔限制因素引起。通径分析表明,光合有效辐射(PAR)是影响白骨壤和桐花树叶片Pn的主要决策因子,而叶面大气蒸汽压亏缺(VPD)是主要限制因子;影响秋茄和红海榄叶片Pn的主要决策因子是气孔导度(Gs),主要限制因子是叶温(Tl);影响木榄叶片Pn的主要决策因子是气孔导度(Gs)。基于叶片净光合作用速率的各树种日净固碳量存在显著性差异(P0.01),秋茄的日净固碳量最大(13.83 g·m-2·d-1),其次为白骨壤和桐花树(9.48和8.24 g·m-2·d-1),木榄和红海榄的较小(6.72和6.30 g·m-2·d-1)。5种红树林树种的光补偿点(LCP)介于28.3~137.0μmol·m-2·s-1之间,显示了阳生植物的特性。光饱和点(LSP)介于169.3~1189.3μmol·m-2·s-1之间,桐花树最大,红海榄最小。5种红树林树种的表观量子效率(AQY)存在极显著差异(P0.01),白骨壤最高为0.064 mol·mol-1,木榄最低,仅为0.005 mol·mol-1。5种红树林植物叶片的光响应参数与日净固碳量的关联度大小顺序为最大净光合速率(Pmax)、LSP-LCP、AQY、LSP、LCP。     

Elevated pyruvate,orthophosphate dikinase (PPDK) activity alters carbon metabolism in C3 transgenic potatoes with a C4 maize PPDK gene

Changes in carbon metabolism and δ¹³C value of transgenic potato plants with a maize pyruvate,orthophosphate dikinase (PPDK; EC 2.7.9.1) gene are reported. PPDK catalyzes the formation of phospho enol pyruvate (PEP), the initial acceptor of CO₂ in the C₄ photosynthetic pathway. PPDK activities in the leases of transgenic potatoes were up to 5.4‐fold higher than those of control potato plants (wild‐type and treated control plants). In the transgenic potato plants, PPDK activity in leaves was negatively correlated with pyruvate content (r²= 0.81), and was positively correlated with malate content (r²= 0.88). A significant increase in the δ¹³C value was observed in the transgenic potato plants, suggesting a certain contribution of PEP carboxylase as the initial acceptor of atmospheric CO₂. These data suggest that elevated PPDK activity may alter carbon metabolism and lead to a partial operation of C₄‐type carbon metabolism. However, since parameters associated with CO₂ gas exchange were not affected, the altered carbon metabolism had only a small effect on the total photosynthetic characteristics of the transgenic plants.     

Increased stomatal conductance induces rapid changes to photosynthetic rate in response to naturally fluctuating light conditions in rice

A close correlation between stomatal conductance and the steady-state photosynthetic rate has been observed for diverse plant species under various environmental conditions. However, it remains unclear whether stomatal conductance is a major limiting factor for the photosynthetic rate under naturally fluctuating light conditions. We analysed a SLAC1 knockout rice line to examine the role of stomatal conductance in photosynthetic responses to fluctuating light. SLAC1 encodes a stomatal anion channel that regulates stomatal closure. Long exposures to weak light before treatments with strong light increased the photosynthetic induction time required for plants to reach a steady-state photosynthetic rate and also induced stomatal limitation of photosynthesis by restricting the diffusion of CO₂ into leaves. The slac1 mutant exhibited a significantly higher rate of stomatal opening after an increase in irradiance than wild-type plants, leading to a higher rate of photosynthetic induction. Under natural conditions, in which irradiance levels are highly variable, the stomata of the slac1 mutant remained open to ensure efficient photosynthetic reaction. These observations reveal that stomatal conductance is important for regulating photosynthesis in rice plants in the natural environment with fluctuating light.     

Relationship between photosynthetic and respiratory carbon metabolism in freshwater phytoplankton

Measurements of algal carbon metabolism in the light and the dark were conducted in (1) short-term (3-h) light and dark incubations, (2) a diel (24-h) experiment, and (3) a longer-term (4-d) carbon accumulation experiment to examine the relationship between photosynthetic rates, photosynthetic carbon metabolism in the light, and respiration and carbon metabolism in the ensuing dark period in natural assemblages of freshwater phytoplankton. High rates of photosynthesis and polysaccharide synthesis in the light were followed by high rates of respiration and polysaccharide utilization in the dark. Polysaccharide was the major respiratory substrate in the dark, and small molecular weight metabolites, lipids, and protein were less important sources of metabolic energy. The protein pool accumulated carbon during dark incubations, but more slowly than during active photosynthesis in the light. Because the intracellular macromolecular pools turn over at very different rates (polysaccharide > protein and lipid), patterns of short-term photosynthetic carbon metabolism are not necessarily indicative of the biochemical composition of the phytoplankton.     

Expression of C3 and C4 photosynthetic characteristics in the amphibious plant Eleocharis vivipara: structure and analysis of the expression of isogenes for pyruvate,orthophosphate dikinase

Eleocharis vivipara, a unique leafless amphibious sedge, adopts the C4 mode of photosynthesis under terrestrial conditions and the C3 mode under submerged aquatic conditions. To analyze the molecular basis of these responses to the contrasting environments, we isolated and characterized two full-length cDNAs for a key C4 enzyme, pyruvate, orthophosphate dikinase (PPDK; EC 2.7.9.1). The isogenes for PPDK, designated ppdk1 and ppdk2, were highly homologous to one another but not identical. The PPDK1 protein, deduced from the nucleotide sequence of the cDNA, contained an extra domain at the amino terminus which, presumably, serves as a chloroplast transit peptide, while PPDK2 lacked this extra domain. It seems likely, therefore, that the ppdk1 and ppdk2 genes encode a chloroplastic and a cytosolic PPDK, respectively. Genomic Southern blot analysis revealed the existence of a small family of genes for PPDK in the genome of E. vivipara. Northern blot analysis indicate that both chloroplastic and cytosolic genes for PPDK are expressed simultaneously in the culms, a photosynthetic organ, of E. vivipara and that the pattern of expression of these genes differs between the growth forms.     

Regulatory interaction of photosynthetic nitrate utilization and carbon dioxide fixation in the cyanobacterium Anacystis nidulans

The rate of photosynthetic nitrate utilization in Anacystis nidulans is strongly influenced by the availability of carbon dioxide. This dependence can be relieved by inhibiting the metabolism of the ammonium derived from nitrate reduction. Nitrate uptake seems to be modulated through a sensitive regulatory system integrating the photosynthetic metabolism of carbon and nitrogen, with CO₂ fixation products antagonizing the inhibitory effect of ammonium derivatives.     

Benzyladenine effects on the development of the photosynthetic apparatus in Zea mays: Studies on photosynthetic activity, enzymes and (etio)chloroplast ultrastructure

Primary leaf segments from 8-day-old dark-grown, and from 4- and 8-day-old light-grown seedlings of Zea mays L. cv. Fronica, were treated with 10^-bM benzyladenine (BA) in the dark for 14 h. The segments were then studied after an exposure to light for 14 h. Photosynthetic activity (O₂ evolution and CO₂ fixation) and chlorophyll accumulation were stimulated by BA in dark-grown leaf segments with etioplastids in the earliest stage of development. In these segments BA stimulated the activities of ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39), phosphoenolpyruvate carboxylase (EC 4.1.1.31), NADP⁺-malic enzyme (EC 1.1.1.40) and pyruvate, orthophosphate dikinase (EC 2.7.9.1). In segments taken from 4- and 8-day light-grown seedlings, BA did not enhance the photosynthetic activity nor the chlorophyll accumulation. The activity of the enzymes mentioned above, was significantly enhanced by the BA-treatment. BA mainly affected grana stacking in mesophyll cell chloroplasts in primary leaf segments taken from 3- to 5-day light-grown seedlings. Stroma thylakoid development was stimulated only in leaf segments from 3-day-old plants. At the same time BA accelerated grana loss in chloroplasts of bundle sheath cells, a typical phenomenon of development in such chloroplasts. Stroma thylakoid length in these chloroplasts increased by a BA treatment in segments from 3- and 4-day light-grown plants. A significantly higher number of chloroplasts was only observed with segments taken from 8-day light-grown seedlings and treated with BA. The etiochloroplast number in segments taken from 8-day etiolated plants was significantly higher in BA-treated segments after 26 h illumination. In etiochloroplasts from both mesophyll and bundle sheath cells, BA enhanced grana stacking after illumination for 4 h or more, whereas stroma membrane length was significantly higher only after 26 h light. It is concluded that the effects of BA depend on the developmental stage. BA accelerates the development of mesophyll and bundle sheath cell (etio)chloroplasts, but does not affect the ultrastructure of mature chloroplasts.     

Regulation of photosynthetic carbon metabolism during phosphate limitation of photosynthesis in isolated spinach chloroplasts

Intact chloroplasts isolated from spinach were illuminated in the absence of inorganic phosphate (Pi) or with optimum concentrations of Pi added to the reaction medium. In the absence of Pi photosynthesis declined after the first 1–2 min and was less than 10% of the maximum rate after 5 min. Export from the chloroplast was inhibited, with up to 60% of the ¹⁴C fixed being retained in the chloroplast, compared to less than 20% in the presence of Pi. Despite the decreased export, chloroplasts depleted of Pi had lower levels of triose phosphate while the percentage of total phosphate in 3-phosphoglycerate was increased. Chloroplast ATP declined during Pi depletion and reached dark levels after 3–4 min in the light without added Pi. At this point, stromal Pi concentration was 0.2 mM, which would be limiting to ATP synthesis. Addition of Pi resulted in a rapid burst of oxygen evolution which was not initially accompanied by net CO₂ fixation. There was a large decrease in 3-phosphoglycerate and hexose plus pentose monophosphates in the chloroplast stroma and a lesser decrease in fructose-1,6-bisphosphate. Stromal levels of triose phosphate, ribulose-1,5-bisphosphate and ATP increased after resupply of Pi. There was an increased export of ¹⁴-labelled compounds into the medium, mostly as triose phosphate. Light activation of both fructose-1,6-bisphosphatase and ribulose-1,5-bisphosphate carboxylase was decreased in the absence of Pi but increased following Pi addition.It is concluded that limitation of Pi supply to isolated chloroplasts reduced stromal Pi to the point where it limits ATP synthesis. The resulting decrease in ATP inhibits reduction of 3-phosphoglycerate to triose phosphate via mass action effects on 3-phosphoglycerate kinase. The lack of Pi in the medium also inhibits export of triose phosphate from the chloroplast via the phosphate transporter. Other sites of inhibition of photosynthesis during Pi limitation may be located in the regeneratige phase of the reductive pentose phosphate pathway.Abbreviations FBP Fructose-1,6-bisphosphate - FBPase Fructose-1,6-bisphosphatase - MP Hexose plus pentose monophosphates - PGA 3-phosphoglycerate - Pi inorganic orthophosphate - RuBP ribulose-1,5-bisphosphate - RuBPCase ribulose-1,5-bisphosphate carboxylase - TP Triose Phosphate     

Trapped intermediate state of plant pyruvate phosphate dikinase indicates substeps in catalytic swiveling domain mechanism

Pyruvate phosphate dikinase (PPDK) is an essential enzyme of both the C₄ photosynthetic pathway and cellular energy metabolism of some bacteria and unicellular protists. In C₄ plants, it catalyzes the ATP‐ and P_i‐dependent formation of phosphoenolpyruvate (PEP) while in bacteria and protozoa the ATP‐forming direction is used. PPDK is composed out of three distinct domains and exhibits one of the largest single domain movements known today during its catalytic cycle. However, little information about potential intermediate steps of this movement was available. A recent study resolved a discrete intermediate step of PPDK's swiveling movement, shedding light on the details of this intriguing mechanism. Here we present an additional structural intermediate that possibly represents another crucial step in the catalytic cycle of PPDK, providing means to get a more detailed understanding of PPDK's mode of function.     

Rising CO2 levels and their potential significance for carbon flow in photosynthetic cells

Abstract. In the first part of this review, I discuss how we can predict the direct short-term effect of enhanced CO₂ on photosynthetic rate in C₃ terrestrial plants. To do this, I consider: (1) to what extent enhanced CO₂ will stimulate or relieve demand on partial processes like carboxylation, light harvesting and electron transport, the Calvin cycle, and end-product synthesis; and (2) the extent to which these various processes actually control the rate of photosynthesis. I conclude that control is usually shared between Rubisco (which responds sensitively to CO₂) and other components (which respond less sensitively), and that photosynthesis will be stimulated by 25–75% when the CO₂ concentration is doubled from 35 to 70 Pa. This is in good agreement with the published responses. In the next part of the review, I discuss the evidence that most plants undergo a gradual inhibition of photosynthesis during acclimation to enhanced CO₂. I argue that this is related to an inadequate demand for carbohydrate in the remainder of the plant. Differences in the long-term response to CO₂ may be explained by differences in the sink-source status of plants, depending upon the species, the developmental stage, and the developmental conditions. In the third part of the review, I consider the biochemical mechanisms which are involved in ‘sink’ regulation of photosynthesis. Accumulating carbohydrate could lead to a direct inhibition of photosynthesis, involving mechanical damage by large starch grains or Pi-limitation due to inhibition of sucrose synthesis. I argue that Pi is important in the short-term regulation of partitioning to sucrose and starch, but that its contribution to ‘sink’ regulation has not yet been conclusively demonstrated. Indirect or ‘adaptive’ regulation of photosynthesis is probably more important, involving decreases in amounts of key photosynthetic enzymes, including Rubisco. This decreases the rate of photosynthesis, and potentially would allow resources (e.g. amino acids) to be remobilized from the leaves and reinvested in sink growth to readjust the sink-source balance. In the final part of the review, I argue that similar changes of Rubisco and, possibly, other proteins are probably also involved during acclimation to high CO₂.