An investigation into the role of photosynthesis in regulating ATP levels and rates of h efflux in isolated meosphyll cells

Aerated and stirred 10-ml suspensions of mechanically isolated Asparagus sprengeri Regel mesophyll cells were used for simultaneous measurements of net H⁺ efflux and steady-state ATP levels.

Initial rates of medium acidification indicated values for H⁺ efflux in the light and dark of 0.66 and 0.77 nanomoles H⁺/10⁶ cells per minute, respectively. When the medium pH was maintained at 6.5, with a pH-stat apparatus, rates of H⁺ efflux remained constant. Darkness or DCMU, however, stimulated H⁺ efflux by 100% or more. Darkness increased ATP levels by 33% and a switch from dark to light reduced ATP levels by 31%. In the absence of aeration, illumination prevented the accumulation of respiratory CO₂ and the buffering capacity of the medium was about 50% less than that found in the nonilluminated nonaerated medium. As a result, rates of pH decline were similar even though the dark rate of H⁺ efflux was approximately 50% greater.

Proposals that photosynthesis stimulates H⁺ efflux are based on changes in the rate of pH decline. The present data indicate that photosynthesis inhibits H⁺ efflux and that changes in rates of pH decline should not be equated with changes in the rate of H⁺ efflux.

     

Stimulation of h efflux and inhibition of photosynthesis by esters of carboxylic acids

Suspensions of mechanically isolated Asparagus sprengeri Regel mesophyll cells were used to investigate the influence of various carboxyester compounds on rates of net H⁺ efflux in the dark or light and photosynthetic O₂ production. Addition of 0.15 to 1.5 millimolar malathion, α-naphthyl acetate, phenyl acetate, or p-nitrophenyl acetate stimulated H⁺ efflux and inhibited photosynthesis within 1 minute. In contrast, the more polar esters methyl acetoacetate or ethyl p-aminobenzoate had little or no effect on either of these two processes. A 0.15 millimolar concentration of α-naphthylacetate stimulated the normal rate of H⁺ efflux, 0.77 nanomoles H⁺ per 10⁶ cells per minute by 750% and inhibited photosynthesis by 100%. The four active carboxyester compounds also stimulated H⁺ efflux after the normal rate of H⁺ efflux was eliminated with 0.01 milligrams per milliliter oligomycin or 100% N₂. Oligomycin reduced the ATP level by 70%. Incubation of cells with malathion, α-naphthyl acetate, or p-nitrophenyl acetate resulted in the generation of the respective hydrolysis products ethanol, α-naphthol, and p-nitrophenol. It is proposed that inhibition of photosynthesis and stimulation of H⁺ efflux result when nonpolar carboxyester compounds enter the cell and generate acidic carboxyl groups when hydrolyzed by esterase enzymes.     

A plasmamembrane redox system and proton transport in isolated mesophyll cells

Potassium ferricyanide (K₃Fe[CN]₆) was added to aerated and stirred nonbuffered suspensions of mechanically isolated photosynthetically competent Asparagus sprengeri Regel mesophyll cells. Rates of Fe(CN)₆³⁻ reduction and H⁺ efflux were measured with or without illumination. On the addition of 1 millimolar Fe(CN)₆³⁻ to nonilluminated cell suspensions acidification of the medium indicated an H⁺ efflux of 1.54 nanomoles H⁺/10⁶ cells per minute. Simultaneous Fe(CN)₆³⁻ reduction occurred at a rate of 1.55 nanomoles Fe(CN)₆³⁻/10⁶ cells per minute. Illumination stimulated these rates 14 to 17 times and corresponding values were 26.1 nanomoles H⁺/10⁶ cells per minute and 22.9 nanomoles Fe(CN)₆³⁻/10⁶ cells per minute. These two processes appeared to be tightly coupled and were rapidly inhibited when illuminated suspensions were transferred to darkness or treated with 1 micromolar 3-(3,4-dichlorophenyl)-1,1 dimethylurea. Addition of 0.1 millimolar diethylstilbestrol eliminated ATP dependent H⁺ efflux in illuminated or nonilluminated cells but had no influence on Fe(CN)₆³⁻ dependent H⁺ efflux. Recent reports indicate that a transmembrane redox system spans the plasma membrane of root cells and is coupled to the efflux of H⁺. The present report extends these observations to photosynthetically competent mesophyll cells. The results indicate a transport process independent of ATP driven H⁺ efflux which operates with a H⁺/e⁻ stoichiometry of one.     

Regulation of glycolysis in heterotrophic cell suspension cultures of Chenopodium rubrum in response to proton fluxes at the plasmalemma

The present experiments were carried out to investigate the effect of increased fluxes of H⁺ across the plasmalemma on glycolysis in heterotrophic cell suspension cultures of Chenopodium rubrum L. (1) Increased H⁺ influx was produced by adding glucose, 6-deoxyglucose, 2-deoxyglucose, or sodium fluoride. The net influx decreased to zero after 3 min. This recovery was accompanied by an increase in the rate of O₂ uptake, but not of dark CO₂ fixation. When glucose or fluoride were added, the increase of O₂ uptake occurred without a decrease in the ATP/ADP ratio, and was large enough to provide the ATP that would be needed for compensatory H⁺ extrusion via the plasmalemma H⁺-ATPase. When 2-deoxyglucose was added, the rise of respiration was restricted by sequestration of phosphate and depletion of phosphorylated metabolites, the ATP/ADP ratio declined, and a slow net H⁺ influx started again after 4 min. (2) Alkalinisation of the medium to induce an H⁺ efflux resulted in rapid activation of dark CO₂ fixation, but not of O²-uptake. (3) A stimulation of respiration or dark CO₂ fixation was always accompanied by a decrease of phosphoenolpyruvate. This shows that the primary sites for regulation of glycolysis are pyruvate kinase and phosphoenolpyruvate carboxylase, respectively. (4) There was no consistent relation between glycolytic flux and triose-phosphates or hexose-phosphates. This shows that the reactions involved in carbohydrate mobilisation and the conversion of hexose-phosphates to triose-phosphates only have a secondary role in stimulation of glycolysis. (5) Phosphofructokinase will be stimulated as a consequence of the decrease in phosphoenolpyruvate. (6) The increase in glycolytic flux occurred independently of (in the case of 2-deoxyglucose and fluoride), or before (in the case of glucose), any increase of fructose-2,6-bisphosphate. When fructose-2,6-bisphosphate did increase (after supplying glucose), this was accompanied by an increase of triose-phosphate and fructose-1,6-bisphosphate, which otherwise remained very low. It is argued that fructose-2,6-bisphosphate increases as a consequence of the decrease of glycerate-3-phosphate, a known inhibitor of the synthesis of this regulator metabolite. However, activation of pyrophosphate fructose-6-phosphate phosphotransferase by fructose-2,6-bisphosphate does not play an obligatory role in the stimulation of glycolysis.     

Effect of temperature on ion content, ion fluxes and energy metabolism in Chara corallina

Abstract Effects of temperature on the ionic relations and energy metabolism of Chara corallina were investigated. Measurements were made of the ionic content, tracer ion fluxes, and photosynthetic and dark CO₂ fixation in isolated cells, and of O₂ exchange in photosynthesis and respiration in isolated shoot apices. The total intracellular concentration of K⁺, Na⁺ and Cl^? was the same in cells held for 5 days in non-growing medium at 15°C (the growth temperature) as in those held at 25°C or 5°C. The tracer influx in the light of all ions tested (Rb⁺, Na⁺, CH₃NH₃⁺, Cl^? and H₂PO₄^?) was lower at 5°C than at 15°C in experiments in which cells were subjected to 5°C for less than 24 h in toto. The influx at 25°C was greater than that at 15°C for H₂PO^?₄, there was no difference between the two temperatures for Na⁺, while the influx at 25°C was less than that at 15°C for Cl^?, Rb⁺ and CH₃NH₃⁺ For Cl^? and H₂PO^?₄ similar results were found in later experiments with cells grown at 20—23°C. Photosynthetic CO₂ fixation and O₂ evolution, and respiratory O₂ uptake, are greater at 25°C, and lower at 5°C, than they are at the growth temperature of 15°C. In longer-term pretreatments at the different temperatures, tracer Cl^? influx at 15°C and particularly at 25°C were lower than in short-term experiments, while the influx at 5°C was higher. It was concluded from these experiments, and from previous data on H⁺ free energy differences across the plasmalemma, that (1) the maintenance of internal ion concentrations involves a close balancing of influx and efflux of K⁺, Na⁺ and Cl^? at all experimental temperatures; (2) the regulation of the tracer fluxes of the ions is kinetic rather than thermodynamic and (3) that the tracer fluxes at low temperatures are not restricted by the rate at which respiration or photosynthesis can supply energy to them.     

Studies on the Biochemistry and Fine Structure of Silica Shell Formation in Diatoms. Photosynthesis and Respiration in Silicon-Starvation Synchrony of Navicula pelliculosa

Rates of photosynthesis, measured by oxygen electrode or by ¹⁴CO₂ fixation, dark respiration and ³²P-phosphate incorporation are reported for the silicon-starvation synchrony of the fresh water diatom Navicula pelliculosa. During late exponential growth the rates were consistent with increase in carbon mass. During silicon starvation, rates of carbon dioxide fixation, oxygen evolution and ³²P incorporation fell, and the saturating light intensity decreased from 27,000 lux to 5000 lux. Reintroduction of silicon led to immediate transients in all parameters studied, followed by a prolonged increase in rate of dark respiration and a gradual increase in apparent photosynthesis. During release of daughter cells, the rates of dark respiration decreased as photosynthesis and ³²P incorporation increased. These results are discussed in relation to effects of silicon on the energy metabolism of the diatom.     

Nitrate and Ammonium Induced Photosynthetic Suppression in N-Limited Selenastrum minutum

Nitrate-limited chemostat cultures of Selenastrum minutum Naeg. Collins (Chlorophyta) were used to determine the effects of nitrogen addition on photosynthesis, dark respiration, and dark carbon fixation. Addition of NO₃⁻ or NH₄⁺ induced a transient suppression of photosynthetic carbon fixation (70 and 40% respectively). Intracellular ribulose bisphosphate levels decreased during suppression and recovered in parallel with photosynthesis. Photosynthetic oxygen evolution was decreased by N-pulsing under saturating light (650 microeinsteins per square meter per second). Under subsaturating light intensities (<165 microeinsteins per square meter per second) NH₄⁺ addition resulted in O₂ consumption in the light which was alleviated by the presence of the tricarboxylic acid cycle inhibitor fluoroacetate. Addition of NO₃⁻ or NH₄⁺ resulted in a large stimulation of dark respiration (67 and 129%, respectively) and dark carbon fixation (360 and 2080%, respectively). The duration of N-induced perturbations was dependent on the concentration of added N. Inhibition of glutamine 2-oxoglutarate aminotransferase by azaserine alleviated all these effects. It is proposed that suppression of photosynthetic carbon fixation in response to N pulsing was the result of a competition for metabolites between the Calvin cycle and nitrogen assimilation. Carbon skeletons required for nitrogen assimilation would be derived from tricarboxylic acid cycle intermediates. To maintain tricarboxylic acid cycle activity triose phosphates would be exported from the chloroplast. This would decrease the rate of ribulose bisphosphate regeneration and consequently decrease net photosynthetic carbon accumulation. Stoichiometric calculations indicate that the Calvin cycle is one source of triose phosphates for N assimilation; however, during transient N resupply the major demand for triose phosphates must be met by starch or sucrose breakdown. The effects of N-pulsing on O₂ evolution, dark respiration, and dark C-fixation are shown to be consistent with this model.     

Carbon Dioxide Fixation in Soybean Roots and Nodules: I. CHARACTERIZATION AND COMPARISON WITH N(2) FIXATION AND COMPOSITION OF XYLEM EXUDATE DURING EARLY NODULE DEVELOPMENT

These studies demonstrate that soybean (Merr) roots and nodules possess an active system for fixing CO₂. The maximum rates of CO₂ fixation observed for roots and nodules of intact plants were 120 and 110 nanomoles CO₂ fixed per milligram dry weight per hour, respectively. Results of labeling studies suggest a primary role for phosphoenolpyruvate carboxylase in CO₂ assimilation in these tissues. After pulse-labeling with ¹⁴CO₂ for 2 minutes, 70% of the total radioactivity was lost within 18 minutes via respiration and/or translocation out of nodules. During the vegetative stages of growth of soybeans grown symbiotically, CO₂ fixation in nodules increased at the onset of N₂ fixation but declined to a lower level prior to the decrease in N₂ fixation. This decrease coincided with a decrease in the transport of amino acids, especially asparagine, and an increase in the export of ureides. These findings are consistent with a dual role for CO₂ fixation, providing substrates for energy-yielding metabolism and supplying carbon skeletons for NH₄⁺ assimilation and amino acid biosynthesis.     

Demonstration of Both a Photosynthetic and a Nonphotosynthetic CO(2) Requirement for NH(4) Assimilation in the Green Alga Selenastrum minutum

Nitrogen-limited and nitrogen-sufficient cell cultures of Selenastrum minutum (Naeg.) Collins (Chlorophyta) were used to investigate the dependence of NH₄⁺ assimilation on exogenous CO₂. N-sufficient cells were only able to assimilate NH₄⁺ maximally in the presence of CO₂ and light. Inhibition of photosynthesis with 3-(3,4-dichlorophenyl)-1,1-dimethylurea, diuron also inhibited NH₄⁺ assimilation. These results indicate that NH₄⁺ assimilation by N-sufficient cells exhibited a strict requirement for photosynthetic CO₂ fixation. N-limited cells assimilated NH₄⁺ both in the dark and in the light in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, diuron, indicating that photosynthetic CO₂ fixation was not required for NH₄⁺ assimilation. Using CO₂ removal techniques reported previously in the literature, we were unable to demonstrate CO₂-dependent NH₄⁺ assimilation in N-limited cells. However, employing more stringent CO₂ removal techniques we were able to show a CO₂ dependence of NH₄⁺ assimilation in both the light and dark, which was independent of photosynthesis. The results indicate two independent CO₂ requirements for NH₄⁺ assimilation. The first is as a substrate for photosynthetic CO₂ fixation, whereas the second is a nonphoto-synthetic requirement, presumably as a substrate for the anaplerotic reaction catalyzed by phosphoenolpyruvate carboxylase.     

Paul Henry Latimer (1925–2011): discoverer of selective scattering in photosynthetic systems

The response of marine phytoplankton to the ongoing increase in atmospheric pCO₂ reflects the consequences of both increased CO₂ concentration and decreased pH in surface seawater. In the model diatom Thalassiosira weissflogii, we explored the effects of varying pCO₂ and pH, independently and in concert, on photosynthesis and respiration by incubating samples in water enriched in H₂ ¹⁸O. In long-term experiments (~6-h) at saturating light intensity, we observed no effects of pH or pCO₂ on growth rate, photosynthesis or respiration. This absence of a measurable response reflects the very small change in energy used by the carbon concentrating mechanism (CCM) compared to the energy used in carbon fixation. In short-term experiments (~3 min), we also observed no effects of pCO₂ or pH, even under limiting light intensity. We surmise that in T. weissflogii, it is the photosynthetic production of NADPH and ATP, rather than the CO₂-saturation of Rubisco that controls the rate of photosynthesis at low irradiance. In short-term experiments, we observed a slightly higher respiration rate at low pH at the onset of the dark period, possibly reflecting the energy used for exporting H⁺ and maintaining pH homeostasis. Based on what is known of the biochemistry of marine phytoplankton, our results are likely generalizable to other diatoms and a number of other eukaryotic species. The direct effects of ocean acidification on growth, photosynthesis and respiration in these organisms should be small over the range of atmospheric pCO₂ predicted for the twenty-first century.     

Relationship between Photosynthesis and Respiration: The Effect of Carbohydrate Status on the Rate of CO(2) Production by Respiration in Darkened and Illuminated Wheat Leaves

The rate of dark CO₂ efflux from mature wheat (Triticum aestivum cv Gabo) leaves at the end of the night is less than that found after a period of photosynthesis. After photosynthesis, the dark CO₂ efflux shows complex dependence on time and temperature. For about 30 minutes after darkening, CO₂ efflux includes a large component which can be abolished by transferring illuminated leaves to 3% O₂ and 330 microbar CO₂ before darkening. After 30 minutes of darkness, a relatively steady rate of CO₂ efflux was obtained. The temperature dependence of steady-state dark CO₂ efflux at the end of the night differs from that after a period of photosynthesis. The higher rate of dark CO₂ efflux following photosynthesis is correlated with accumulated net CO₂ assimilation and with an increase in several carbohydrate fractions in the leaf. It is also correlated with an increase in the CO₂ compensation point in 21% O₂, and an increase in the light compensation point. The interactions between CO₂ efflux from carbohydrate oxidation and photorespiration are discussed. It is concluded that the rate of CO₂ efflux by respiration is comparable in darkened and illuminated wheat leaves.     

The role of dark carbon dioxide fixation in root nodules of soybean

The magnitude and role of dark CO₂ fixation were examined in nodules of intact soybean plants (Harosoy 63 × Rhizobium japonicum strain USDA 16). The estimated rate of nodule dark CO₂ fixation, based on a 2 minute pulse-feed with ¹⁴CO₂ under saturating conditions, was 102 micromoles per gram dry weight per hour. This was equivalent to 14% of net nodule respiration. Only 18% of this CO₂ fixation was estimated to be required for organic and amino acid synthesis for growth and export processes. The major portion (75-92%) of fixed label was released as CO₂ within 60 minutes. The labeling pattern during pulse-chase experiments was consistent with CO₂ fixation by phosphoenolpyruvate carboxylase. During the chase, the greatest loss of label occurred in organic acids. Exposure of nodulated roots to Ar:O₂ (80:20) did not affect dark CO₂ fixation, while exposure to O₂:CO₂ (95:5) resulted in 54% inhibition. From these results, it was concluded that at least 66% of dark CO₂ fixation in soybean may be involved with the production of organic acids, which when oxidized would be capable of providing at least 48% of the requirement for ATP equivalents to support nitrogenase activity.     

Effects of glyoxylate on photosynthesis by intact chloroplasts

Because glyoxylate inhibits CO₂ fixation by intact chloroplasts and purified ribulose bisphosphate carboxylase/oxygenase, glyoxylate might be expected to exert some regulatory effect on photosynthesis. However, ribulose bisphosphate carboxylase activity and activation in intact chloroplasts from Spinacia oleracea L. leaves were not substantially inhibited by 10 millimolar glyoxylate. In the light, the ribulose bisphosphate pool decreased to half when 10 millimolar glyoxylate was present, whereas this pool doubled in the control. When 10 millimolar glyoxylate or formate was present during photosynthesis, the fructose bisphosphate pool in the chloroplasts doubled. Thus, glyoxylate appeared to inhibit the regeneration of ribulose bisphosphate, but not its utilization.

The fixation of CO₂ by intact chloroplasts was inhibited by salts of several weak acids, and the inhibition was more severe at pH 6.0 than at pH 8.0. At pH 6.0, glyoxylate inhibited CO₂ fixation by 50% at 50 micromolar, and glycolate caused 50% inhibition at 150 micromolar. This inhibition of CO₂ fixation seems to be a general effect of salts of weak acids.

Radioactive glyoxylate was reduced to glycolate by chloroplasts more rapidly in the light than in the dark. Glyoxylate reductase (NADP⁺) from intact chloroplast preparations had an apparent K_m (glyoxylate) of 140 micromolar and a V_max of 3 micromoles per minute per milligram chlorophyll.

     

PHOTOCONTROL OF STOMATAL MOVEMENTS

1. Opening in light is a feature common to the majority of functional stomata, but the current argument is against the traditional view that light is the principal environmental promoter of opening, because stomata can open in the dark in response to CO₂ removal and/or temperature increase. In this review, evidence is provided that light is more efficient and effective than other physical factors in both producing and maintaining wide opening. However, light acts on stomata both directly and indirectly, in conjunction with changes in, for example, CO₂ balance, water regime and temperature of the leaf tissue. 2. Three general categories of light effects on stomata are recognized: (a) photosynthetic effects driven by metabolic processes, induced or enhanced by light, (b) hydrophotic effects mediating through light-induced changes in epidermal turgor, and (c) photothermal effects arising from light-dependent changes in leaf temperature. 3. Photosynthetic effects involve both CO₂ depletion, and starch mobilization, malate synthesis, H⁺ extrusion, and accumulation of K⁺ and C1^- in guard cells; these processes are triggered by light of different qualities: (a) Both blue and red light are involved in photosynthetic CO₂ fixation, utilizing energy from photosynthetic light reaction(s), which provides C precursors for synthesis of stornatal starch. (b) Blue light, but not red, enhances starch mobilization, PEP carboxylase activity and respiration. Accordingly, blue light is postulated to enhance hydrolysis of stornatal starch providing C₃ precursors for malate synthesis via PEP-fixation of endogenous CO₂; the active extrusion of H⁺, derived from malate, is coupled with K⁺ influx to guard cells. Malate and C1^- are competitive anions, for K⁺, and one begins to play a progressively more important role as the other becomes limiting; in intact leaves, however, malate plays a more decisive role. These processes are driven by the energy from blue-light-enhanced respiration. (c) Both photosynthetic fixation and PEP carboxylation act as CO₂ sensors, but the exact role of CO₂ in the stornatal mechanism has yet to be determined. 4. Hydrophotic and photothermal effects facilitate guard cell expansion by releasing epidermal pressure through enhanced evaporative water loss, and are, therefore, indirect effects of light; photothermal effects may also contribute to metabolic processes outlined in paragraph 3. 5. Stomatal closure in the dark accompanies starch synthesis, malate reduction, efflux of K⁺ and C1^- from guard cells, and accumulation of CO₂ in substomatal cavities. Malate may be converted to starch via C₂ compounds. Guard cells release K⁺ and C1- into apoplastic space, from which they are removed by neighbouring cells. The entry of K⁺ into neighbouring cells is supposed to be coupled with H⁺ extrusion. These processes are dependent on respiratory energy. 6. The differential abaxial and adaxial stomatal light responses are related to inherent metabolic differences between the two epidermes, but the biochemical basis is not known.     

The physiological response of marine diatoms to ocean acidification: differential roles of seawater pCO2 and pH

Although increasing the pCO₂ for diatoms will presumably down‐regulate the CO₂‐concentrating mechanism (CCM) to save energy for growth, different species have been reported to respond differently to ocean acidification (OA). To better understand their growth responses to OA, we acclimated the diatoms Thalassiosira pseudonana, Phaeodactylum tricornutum, and Chaetoceros muelleri to ambient (pCO₂ 400 μatm, pH 8.1), carbonated (pCO₂ 800 μatm, pH 8.1), acidified (pCO₂ 400 μatm, pH 7.8), and OA (pCO₂ 800 μatm, pH 7.8) conditions and investigated how seawater pCO₂ and pH affect their CCMs, photosynthesis, and respiration both individually and jointly. In all three diatoms, carbonation down‐regulated the CCMs, while acidification increased both the photosynthetic carbon fixation rate and the fraction of CO₂ as the inorganic carbon source. The positive OA effect on photosynthetic carbon fixation was more pronounced in C. muelleri, which had a relatively lower photosynthetic affinity for CO₂, than in either T. pseudonana or P. tricornutum. In response to OA, T. pseudonana increased respiration for active disposal of H⁺ to maintain its intracellular pH, whereas P. tricornutum and C. muelleri retained their respiration rate but lowered the intracellular pH to maintain the cross‐membrane electrochemical gradient for H⁺ efflux. As the net result of changes in photosynthesis and respiration, growth enhancement to OA of the three diatoms followed the order of C. muelleri > P. tricornutum > T. pseudonana. This study demonstrates that elucidating the separate and joint impacts of increased pCO₂ and decreased pH aids the mechanistic understanding of OA effects on diatoms in the future, acidified oceans.     

Effect of long-term H2S fumigation on photosynthesis in spinach. Correlation between CO2 fixation and chlorophyll a fluorescence

Spinach plunts (Spinacia oleracea L. cv. Monosa) were exposed to air with and without 0.25 μl l^-1 H₂S. Effects of H₂S exposure for up to 18 days on photosynthesis, dark respiration and on chlorophyll a fluorescence were studied. Dark respiration was not affected by H₂S fumigation. Photosynthetic CO₂ fixation decreased linearly with time in both control and fumigated plants. The rate of decrease in CO₂ fixation was faster in the fumigated plants; after 14 days of exposure the fumigated plants showed a decrease in CO₂ fixation of 23%äs compared with the control plants. The H₂S-induced decrease in CO₂ fixation was accompanied by a decrease in quenching of the chlorophyll fluorescence. The most characteristic change in chlorophyll fluorescence was a decreased difference between maximum and steady-state fluorescence [(P-T)/P), suggesting a reduced efficiency in the use of photochemical energy in photosynthesis. Differences in CO₂ fixation were more pronounced whcn measured at high light intensity; the maximum rate of CO₂ fixation at light saturation decreased significantly with time in the H₂S-exposed plants; after 14 days of H₂S exposure a decrease of more than 70% was noted. The decrease in CO₂ fixation could not be attributed to a decreased chlorophyll content; on the contrary, chlorophyll content even slightly increased during fumigation. The initial increase in CO₂ fixation rate with increasing light intensity was also reduced by prolonged H₂S fumigation, indicating an effect of H₂S fumigation on photosynthetic electron transport. Finally, the phytotoxicity of H₂S is discusscd in relation to the H₂S-induced changes in photosynthetic CO₂ fixation and chlorophyll a fluorescence, and the effect of H₂S on leaf development observed in earlier studies.     

树干皮层光合作用--生理生态功能和测定方法

大部分植物的树干(枝条)等部位含有能进行光合作用的绿色组织,树皮叶绿素含量最高可达750 mg/m2。这些绿色组织能够再固定树干内部的CO2(来源于自身组织呼吸或者木质部液流运输),使树干向大气排放的CO2量减少60%—90%皮层光合作用是树干生理活动的重要组成部分,其与树干呼吸和液流速率之间均有密切关系,对植物的碳平衡有重要作用。概述了皮层光合作用的生理生态功能;介绍了皮层光合作用测定和计算方法;讨论了皮层光合作用研究存在的问题;通过加入皮层光合作用的测量修正质量平衡法,以减少树干呼吸测定的不确定性。建议综合运用稳定碳同位素示踪、CO2和O2微传感器、树干液流技术等,准确地区分树干内部CO2的来源及比例,分析各个组分与影响因素的关系。同时,在微观上揭示皮层光合作用的基因组调控功能,在宏观上探讨尺度扩展、模型模拟,并与涡度协方差技术和遥感技术相融合以提高区域尺度估算的精度。     

Prior illumination and the respiration of maize leaves in the dark

The course of respiration of attached maize (Zea mays L.) leaves was measured by infrared gas analysis of CO₂ efflux in the dark following illumination in atmospheres of 300 microliters of CO₂ per liter of air, CO₂-free air, and CO₂-free N₂ containing 400 microliters of O₂ per liter. CO₂ efflux from control leaves started 3 to 4 minutes after darkening, increased to a maximum after about 20 minutes, and returned to a steady minimum after 2 to 3 hours. Respiration was quantitatively related to prior illumination, independent of net CO₂ fixation in the light, and depressed by N₂. Light, but not air, was required to produce a substrate for respiration in the subsequent dark period; air was required for oxidation of the substrate to CO₂. The stimulation of respiration by prior illumination in maize leaves differs in its slower onset and greater duration from the postillumination burst of photorespiration.     

The Influence of Leaf Age on C(4) Photosynthesis and the Accumulation of Inorganic Carbon in Flaveria trinervia, a C(4) Dicot

Characteristics of C₄ photosynthesis were examined in young, mid-age, and mature leaves of Flaveria trinervia (an NADP-malic enzyme-type C₄ dicot). The turnover of [4-¹⁴C] (malate plus aspartate) following a pulse with ¹⁴CO₂ was similar in leaves of different ages (apparent half-time of 18-25 seconds). However, the rate of ¹⁴CO₂ incorporation in mid-age leaves was about 1.5-fold higher than in young leaves, and about 2.5-fold higher than in mature leaves. The rate of ¹⁴CO₂ fixation was proportional to the total active pool of malate plus aspartate but was not correlated with the total photosynthetically derived inorganic carbon pool. The leaf's ability to concentrate inorganic carbon photosynthetically declined during leaf expansion, from 29 down to 7 nanomoles per milligram chlorophyll. Similarly, the active aspartate pool also declined during leaf expansion, from about 123 down to 20 nanomoles per milligram chlorophyll. Enhanced metabolism of aspartate to CO₂ and pyruvate in young leaves is suggested to facilitate the maintenance of high CO₂ levels in bundle sheath cells which are thought to have a higher conductance to CO₂.     

Photosynthesis controls of CO2 efflux from maize rhizosphere

The effects of different shading periods of maize plants on rhizosphere respiration and soil organic matter decomposition were investigated by using a ¹³C natural abundance and ¹⁴C pulse labeling simultaneously. ¹³C was a tracer for total C assimilated by maize during the whole growth period, and ¹⁴C was a tracer for recently assimilated C. CO₂ efflux from bare soil was 4 times less than the total CO₂ efflux from planted soil under normal lighting. Comparing to the normal lighting control (12/12 h day/night), eight days with reduced photosynthesis (12/36 h day/night period) and strongly reduced photosynthesis (12/84 h day/night period) resulted in 39% and 68% decrease of the total CO₂ efflux from soil, respectively. The analysis of ¹³C natural abundance showed that root-derived CO₂ efflux accounted for 82%, 68% and 56% of total CO₂ efflux from the planted soil with normal, prolonged and strongly prolonged night periods, respectively. Clear diurnal dynamics of the total CO₂ efflux from soil with normal day-night period as well as its strong reduction by prolonged night period indicated tight coupling with plant photosynthetic activity. The light-on events after prolonged dark periods led to increases of root-derived and therefore of total CO₂ efflux from soil. Any factor affecting photosynthesis, or substrate supply to roots and rhizosphere microorganisms, is an important determinant of root-derived CO₂ efflux, and thereby, total CO₂ efflux from soils. ¹⁴C labeling of plants before the first light treatment did not show any significant differences in the ¹⁴CO₂ respired in the rhizosphere between different dark periods because the assimilate level in the plants was high. Second labeling, conducted after prolonged night phases, showed higher contribution of recently assimilated C (¹⁴C) to the root-derived CO₂ efflux by shaded plants. Results from ¹³C natural abundance showed that the cultivation of maize on Chromic Luvisol decreased soil organic matter (SOM) mineralization compared to unplanted soil (negative priming effect). A more important finding is the observed tight coupling of the negative rhizosphere effect on SOM decomposition with photosynthesis.