Light-enhanced dark respiration in leaves, isolated cells and protoplasts of various types of C4 plants

The rate of respiratory CO2 evolution from the leaves of Zea mays, Panicum miliaceum, and Panicum maximum, representing NADP-ME, NAD-ME, and PEP-CK types of C4 plants, respectively, was increased by approximately two to four times after a period of photosynthesis. This light-enhanced dark respiration (LEDR) was a function of net photosynthetic rate specific to plant species, and was depressed by 1% O2. When malate, aspartate, oxaloacetate or glycine solution at 50 mM concentration was introduced into the leaves instead of water, the rate of LEDR was enhanced, far less in Z. mays (by 10-25%) than in P. miliaceum (by 25-35%) or P. maximum (by 40-75%). The enhancement of LEDR under glycine was relatively stable over a period of 1 h, whereas the remaining metabolites caused its decrease following a transient increase. The metabolites reduced the net photosynthesis rate in the two Panicum species, but not in Z. mays, where this process was stimulated by glycine. The bundle sheath cells from P. miliaceum exhibited a higher rate of LEDR than those of Z. mays and P. maximum. Glycine had no effect on the respiration rate of the cells, but malate increased in cells of Z. mays and P. miliaceum by about 50% and 30%, respectively. With the exception of aspartate, which stimulated both the O2 evolution and O2 uptake in P. maximum, the remaining metabolites reduced photosynthetic O2 evolution from bundle sheath cells in Panicun species. The net O2 exchange in illuminated cells of Z. mays did not respond to CO2 or metabolites. Leaf mesophyll protoplasts of Z. mays and P. miliaceum, and bundle sheath protoplasts of Z. mays, which are unable to fix CO2 photosynthetically, also produced LEDR, but the mesophyll protoplasts, compared with bundle sheath protoplasts, required twice the time of illumination to obtain the maximal rate. The results suggest that the substrates for LEDR in C4 plants are generated during a period of illumination not only via the Calvin cycle reactions, but also by the conversion of endogenous compounds present in leaf cells. The stimulation of LEDR under glycine is discussed in relation to its direct or indirect effect on mitochondrial respiration.     

16O2/18O2 analysis of oxygen exchange in Dunaliella tertiolecta. Evidence for the inhibition of mitochondrial respiration in the light

A mass spectrometric ¹⁶O₂/¹⁸O₂-isotope technique was used to analyse the rates of gross O₂ evolution, net O₂ evolution and gross O₂ uptake in relation to photon fluence rate by Dunaliella tertiolecta adapted to 0.5, 1.0, 1.5, 2.0 and 2.5 M NaCl at 25°C and pH 7.0.At concentrations of dissolved inorganic carbon saturating for photosynthesis (200 M) gross O₂ evolution and net O₂ evolution increased with increasing salinity as well as with photon fluence rate. Light compensation was also enhanced with increased salinities. Light saturation of net O₂ evolution was reached at about 1000 mol m^-2s^-1 for all salt concentrations tested. Gross O₂ uptake in the light was increased in relation to the NaCl concentration but it was decreased with increasing photon fluence rate for almost all salinities, although an enhanced flow of light generated electrons was simultaneously observed. In addition, a comparison between gross O₂ uptake at 1000 mol photons m^-2s^-1, dark respiration before illumination and immediately after darkening of each experiment showed that gross O₂ uptake in the light paralleled but was lower than mitochondrial O₂ consumption in the dark.From these results it is suggested that O₂ uptake by Dunaliella tertiolecta in the light is mainly influenced by mitochondrial O₂ uptake. Therefore, it appears that the light dependent inhibition of gross O₂ uptake is caused by a reduction in mitochondrial O₂ consumption by light.Abbreviations DCMU 3-(3, 4-dichlorophenyl)-1, 1-dimethylurea - DHAP dihydroxy-acetonephosphate - DIC dissolved inorganic carbon - DR_a rate of dark respiration immediately after illumination - DR_b rate of dark respiration before illumination - E₀ rate of gross oxygen evolution in the light - NET rate of net oxygen evolution in the light - PFR photon fluence rate - RubP rubulose-1,5-bisphosphate - SHAM salicyl hydroxamic acid - U₀ rate of gross oxygen uptake in the light     

Effects of Glucose Feeding on Respiration and Photosynthesis in Photoautotrophic Dianthus caryophyllus Cells: Mass Spectrometric Determination of Gas Exchange

When glucose (20 millimolar) was added to photoautotrophic cell suspension cultures of Dianthus caryophyllus, there was during the first 10 hours an accumulation of carbohydrates and phosphorylated compounds. These biochemical changes were accompanied by a progressive decrease of net photosynthesis and a twofold increase of the dark respiratory rate. The rise of respiration was associated with a rise of fumarase and cytochrome c oxidase activities, two mitochondrial markers. Gas exchange of illuminated cells were performed with a mass spectrometry technique and clearly established that during the first hours of glucose feeding, the decrease of net photosynthesis was essentially due to an increase of respiration in light, whereas the photosynthetic processes (gross O₂ evolution and gross CO₂ fixation) were almost not affected. However, after 24 hours of experiment, O₂ evolution and CO₂ fixation started to decline in turn. While ribulose-1,5-bisphosphate carboxylase activity was little affected during the first 48 hours of the experiment, the maximal light-induced phosphoribulokinase activity dramatically decreased with time and represented after 48 hours only 30% of its initial activity. It is postulated that the decrease in phosphoribulokinase activity was at least partially responsible for the decrease of CO₂ fixation and the metabolic events involved in this regulation are discussed.     

PHOTOSYNTHESIS AND PHOTOSYNTHESIS-COUPLED RESPIRATION IN NATURAL BIOFILMS QUANTIFIED WITH OXYGEN MICROSENSORS1

Photosynthesis and respiration were analyzed in natural biofilms by use of O₂ microsensors. Depth profiles of gross photosynthesis were obtained from the rate of decrease in O₂ concentration during the first few seconds following extinction of light, and net photosynthesis of the photic zone was calculated from O₂ concentration gradients measured at steady state. Respiration within the photic zone was calculated as the difference between gross and net photosynthesis. Two types of biofilms were investigated: one dominated by diatoms, and one dominated by cyanobacteria. High O₂/CO₂ ratios caused increased respiration especially within the diatom biofilm, which could indicate that photorespiration was a dominant O₂-consuming process. The rate of respiration was constant within both biofilms during the first 4.6 s following extinction of light, even when respiration was stimulated by high O₂/CO₂ ratio. The assumption of a constant rate of respiration during the dark period is an essential one for the determination of gross photosynthetic activity by use of O₂ microsensors. We here present the first evidence to substantiate this assumption. The results strongly suggest that gross photosynthesis as measured by use of O₂ microsensors may include carbon equivalents that are subsequently lost through photorespiration. Computer modeling of photosynthesis profiles measured after 1.1, 1.6, and 2.6 s of dark incubation illustrated how the actual photosynthesis profile could have appeared if it had been possible to do the determination at time 0. Diffusion of O₂ during the up to 4.6-s long dark incubations did not affect gross photosynthetic rate when integrated over all depths, but the apparent vertical distribution of the photosynthetic activity was strongly affected.     

Light-enhanced dark respiration in mesophyll protoplasts from leaves of pea

The respiratory oxygen uptake by mesophyll protoplasts of pea (Pisum sativum cv Arkel) was stimulated up to threefold after 15 minutes of illumination at an intensity of 1250 microeinsteins per square meter per second in the presence of 5 millimolar bicarbonate at 30°C. The extent of light-enhanced dark respiration (LEDR) increased progressively with duration of preillumination. The LEDR exhibited two phases. The initial high rate of respiration decreased in about 10 minutes to a lower steady value similar to that before illumination. The promotion of LEDR by the presence of bicarbonate and inhibition by glyceraldehyde or 3-(3,4-dichlorophenyl)-1,1-dimethylurea suggested that LEDR was dependent on products of photosynthetic carbon assimilation/electron transport. Thus, the photosynthetic products exert a markedly quick influence on dark respiration in mesophyll protoplasts.     

Inorganic Phosphate (Pi) Enhancement of Dark Respiration in the Pi-Limited Green Alga Selenastrum minutum (Interactions between H+/Pi Cotransport,the Plasmalemma H+-ATPase,and Dark Respiratory Carbon Flow)

Inorganic phosphate (Pi) enrichment of the Pi-limited green alga Selenastrum minutum in the dark caused a 2.5-fold increase in the rate of O2 consumption. Alkalization of the media during Pi assimilation was consistent with a H+/Pi cotransport mechanism with a stoichiometry of at least 2 H+ cotransported per Pi. Dark O2 consumption remained enhanced beyond the period of Pi assimilation and did not recover until the medium was reacidified. This result, coupled with an immediate decrease in adenylate energy charge following Pi enrichment, suggested that respiration is regulated by the ATP requirements of a plasmalemma H+-ATPase that is activated to maintain intracellular pH and provide proton motive force to power Pi uptake. Concentrations of tricarboxylic acid cycle intermediates decreased following Pi enrichment and respiratory CO2 efflux increased, indicating that the tricarboxylic acid cycle was activated to supply reductant to the mitochondrial electron transport chain. These results are consistent with direct inhibition of electron transport by ADP limitation. Enhanced rates of starch breakdown and increases in glycolytic metabolites indicated that respiratory carbon flow was activated to supply reductant to the electron transport chain and to rapidly assimilate Pi into metabolic intermediates. The mechanism that initiates glycolytic carbon flow could not be clearly identified by product:substrate ratios due to the complex nature of Pi assimilation. High levels of triose-P and low levels of phosphoenolpyruvate were the primary regulators of pyruvate kinase and phosphofructokinase, respectively.     

Relationship between postillumination burst of CO2 and enhancement of respiration in tall fescue leaves

The postillumination burst (PIB) of CO₂ and light-enhanced dark respiration (LEDR) depending on oxygen concentration, temperature, respiratory substrates and photorespiratory inhibitor aminoacetonitrile (AAN) were investigated in detached leaves of tall fescue (Festuca arundinacea) using a closed circuit system with an infrared gas analyzer. No PIB was observed in 1 % O₂ under temperature over the range from 15 °C to 35 °C. The rate of LEDR was about twice as low in 1 % O₂ as that in 21 and 50 % O₂ under all temperatures applied. The PIB was absent and LEDR decreased at 21 % O₂ following illumination of leaves for 1 hour at 1 % O₂. When 200 mM glycine or malate solutions were introduced into the leaves of tall fescue, the magnitudes of PIB increased by about 60 and 40 % and rate of LEDR by about 70 % and 40 %, respectively. Pyruvate and succinate were less effective in promotion of PIB and LEDR. AAN had a small stimulatory effect on PIB and LEDR (about 20 % and 10 %, respectively). The dependences between magnitudes of PIB and rates of LEDR were highly correlated (r=0.94). The results presented indicate that atmospheric concentration of oxygen during the period of photosynthesis of tall fescue leaves was necessary not only for occurrence of PIB and LEDR but also for production of substrate(s) (glycine and/or malate) for these phenomena.     

Diel variations in carbon isotopic composition and concentration of organic acids and their impact on plant dark respiration in different species

Leaf respiration in the dark and its C isotopic composition (δ¹³C_R) contain information about internal metabolic processes and respiratory substrates. δ¹³C_R is known to be less negative compared to potential respiratory substrates, in particular shortly after darkening during light enhanced dark respiration (LEDR). This phenomenon might be driven by respiration of accumulated ¹³C‐enriched organic acids, however, studies simultaneously measuring δ¹³C_R during LEDR and potential respiratory substrates are rare. We determined δ¹³C_R and respiration rates (R) during LEDR, as well as δ¹³C and concentrations of potential respiratory substrates using compound‐specific isotope analyses. The measurements were conducted throughout the diel cycle in several plant species under different environmental conditions. δ¹³C_R and R patterns during LEDR were strongly species‐specific and showed an initial peak, which was followed by a progressive decrease in both values. The species‐specific differences in δ¹³C_R and R during LEDR may be partially explained by the isotopic composition of organic acids (e.g., oxalate, isocitrate, quinate, shikimate, malate), which were ¹³C‐enriched compared to other respiratory substrates (e.g., sugars and amino acids). However, the diel variations in both δ¹³C and concentrations of the organic acids were generally low. Thus, additional factors such as the heterogeneous isotope distribution in organic acids and the relative contribution of the organic acids to respiration are required to explain the strong ¹³C enrichment in leaf dark‐respired CO₂.     

Respiratory carbon metabolism in the high mountain plant species Ranunculus glacialis

Very little is known about the primary carbon metabolism of the high mountain plant Ranunculus glacialis. It is a species with C3 photosynthesis, but with exceptionally high malate content in its leaves, the biological significance of which remains unclear. 13C/12C-isotope ratio mass spectrometry (IRMS) and 13C-nuclear magnetic resonance (NMR) labelling were used to study the carbon metabolism of R. glacialis, paying special attention to respiration. Although leaf dark respiration was high, the temperature response had a Q10 of 2, and the respiratory quotient (CO2 produced divided by O2 consumed) was nearly 1, indicating that the respiratory pool is comprised of carbohydrates. Malate, which may be a large carbon substrate, was not respired. However, when CO2 fixed by photosynthesis was labelled, little labelling of the CO2 subsequently respired in the dark was detected, indicating that: (i) most of the carbon recently assimilated during photosynthesis is not respired in the dark; and (ii) the carbon used for respiration originates from (unlabelled) reserves. This is the first demonstration of such a low metabolic coupling of assimilated and respired carbon in leaves. The biological significance of the uncoupling between assimilation and respiration is discussed.     

Kok effect and the quantum yield of photosynthesis : light partially inhibits dark respiration

The linear response of photosynthesis to light at low photon flux densities is known to change abruptly in the vicinity of the light compensation point so that the quantum yield seems to decrease as radiation increases. We studied this `Kok effect' in attached sunflower (Helianthus annuus L. cv IS894) leaves using gas exchange techniques. The effect was present even though respiration was constant in the dark. It was observed at a similar photon flux density (7 to 11 micromole photons per square meter per second absorbed photosynthetically active radiation) despite a wide range of light compensation points as well as rates of photosynthesis. The effect was not apparent when photorespiration was inhibited at low pO₂ (1 kilopascal), but this result was complicated because dark respiration was quite O₂-sensitive and was partially suppressed under these conditions. The Kok effect was observed at saturating pCO₂ and, therefore, could not be explained by a change in photorespiration. Instead, the magnitude of the effect varied as dark respiration varied in a single leaf, and was minimized when dark respiration was minimized, indicating that a partial suppression of dark respiration by light is responsible. Quantum yields measured at photon flux densities between 0 and 7 to 11 micromole photons per square meter per second, therefore, represent the combined yields of photosynthesis and of the suppression of a component of dark respiration by light. This leads to an overestimate of the quantum yield of photosynthesis. In view of these results, quantum yields of photosynthesis must be measured (a) when respiration is constant in the dark, and (b) when dark respiration has been inhibited either at low pO₂ to eliminate most of the light-induced suppression of dark respiration or at photon flux densities above that required to saturate the light-induced suppression of dark respiration. Significant errors in quantum yields of photosynthesis can result in leaves exhibiting this respiratory behavior if these principles are not followed.     

Arbuscular mycorrhizal colonization on carbon economy in perennial ryegrass: quantification by 13CO2/12CO2 steady-state labelling and gas exchange

Effects of the arbuscular mycorrhizal fungus (AMF) Glomus hoi on the carbon economy of perennial ryegrass (Lolium perenne) were investigated by comparing nonmycorrhizal and mycorrhizal plants of the same size, morphology and phosphorus status. Plants were grown in the presence of CO2 sources with different C isotope composition (delta13C -1 or -44). Relative respiration and gross photosynthesis rates, and belowground allocation of C assimilated during one light period ('new C'), as well as its contribution to respiration, were quantified by the concerted use of 13CO2/12CO2 steady-state labelling and 13CO2/12CO2 gas-exchange techniques. AMF (G. hoi) enhanced the relative respiration rate of the root + soil system by 16%, inducing an extra C flow amounting to 3% of daily gross photosynthesis. Total C flow into AMF growth and respiration was estimated at < 8% of daily gross photosynthesis. This was associated with a greater amount of new C allocated belowground and respired in mycorrhizal plants. AMF colonization affected the sources supplying belowground respiration, indicating a greater importance of plant C stores in supplying respiration and/or the participation of storage pools within fungal tissues. When ontogenetic and nutritional effects were accounted for, AMF increased belowground C costs, which were not compensated by increased photosynthesis rates. Therefore the instantaneous relative growth rate was lower in mycorrhizal plants.     

Photosynthesis, Respiration and Post-illumination Fixation of CO2 by Corn Leaves as Influenced by Light and Oxygen Concentration

The effect of oxygen concentration on the rate of CO₂-uptake in continuous and intermittent light was studied as well as the CO₂-fixation during a short dark period after light was turned off. In addition the dark respiration and the CO₂-compensation point of attached and detached corn leaves were determined. Leaves of 4 to 22-day old plants were used as experimental material. A closed circuit system of an infrared carbon dioxide analyzer was employed to measure the rate of CO₂-exchange. It was found that in an atmosphere consisting of 100 % oxygen, there was about 50 per cent inhibition of the rate of CO₂-uptake in continuous and intermittent light compared to that in an atmosphere consisting of 21% oxygen. The same was true of the rate of CO₂-fixation in darkness during a short period after the light was turned off. Since the response to oxygen concentration of the CO₂-uptake in light and of the CO₂-fixation in darkness after the light was turned off were similar, it is concluded that the fixation of CO₂ in the short dark period represents an over- shoot of photosynthesis. The rate of dark respiration was little affected by the oxygen concentration in the ranges used in the experiments. The carbon dioxide compensation point which has been observed in leaves of 4 to 14-day old plants was not influenced by either oxygen concentration or light intensity. Since the changes in the rate of CO₂-uptake due to changes in the concentration of oxygen and light intensity had no effect on the CO₂-compensation point, it is concluded that a reabsorption of respiratory CO₂ by photosynthesis could not account for the low value of this point. These results are interpreted as a further corroboration of the statement that the leaves of corn lack the process of photorespiration and that dark respiration is inhibited in light. It was observed that the rate of the CO₂-uptake gradually increased in plants which were from 4 to 22-days old. The inhibitory effect of high concentration of oxygen on the rate of CO₂-uptake was relatively higher in old plants than in young ones.     

Significance of cold-season respiration and photosynthesis in a subarctic heath ecosystem in Northern Sweden

While substantial cold-season respiration has been documented in most arctic and alpine ecosystems in recent years, the significance of cold-season photosynthesis in these biomes is still believed to be small. In a mesic, subartic heath during both the cold and warm season, we measured in situ ecosystem respiration and photosynthesis with a chamber technique at ambient conditions and at artificially increased frequency of freeze–thaw (FT) cycles during fall and spring. We fitted the measured ecosystem exchange rates to respiration and photosynthesis models with R²-values ranging from 0.81 to 0.85. As expected, estimated cold-season (October, November, April and May) respiration was significant and accounted for at least 22% of the annual respiratory CO₂ flux. More surprisingly, estimated photosynthesis during this period accounted for up to 19% of the annual gross CO₂ uptake, suggesting that cold-season photosynthesis partly balanced the cold-season respiratory carbon losses and can be significant for the annual cycle of carbon. Still, during the full year the ecosystem was a significant net source of 120 ± 12 g C m⁻² to the atmosphere. Neither respiration nor photosynthetic rates were much affected by the extra FT cycles, although the mean rate of net ecosystem loss decreased slightly, but significantly, in May. The results suggest only a small response of net carbon fluxes to increased frequency of FT cycles in this ecosystem.     

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.     

Respiration in cell development

Summary Dark oxygen uptake was measured manometrically for cells of green high-temperature alga, Chlorella 7-11-05, separated from nonsynchronized populations by centrifugation into fractions of predominantly small or large cells. In the presence of exogenous glucose, respiration activity of the smaller (younger) cell fraction was invariably higher than that of the larger (older) cell fraction. In the absence of exogenous substrate, the difference in respiration rates in two fractions of cells was inconsistent from one experiment to another both in size and in sign. The dependence of dark respiration on the amount of available substrate makes the endogenous respiration rate unsuitable as an indicator of the inherent capacity of respiratory mechanisms.In observations on synchronized heterotrophically grown cells, the glucose respiration rate expressed per dry weight of cells gradually declined over the developmental period irrespective of the adequate exogenous supply of glucose or illumination by weak light. Observations on synchronized heterotrophically grown Chlorella cells thus corroborated studies of glucose respiration in cells separated into are groups by centrifugation.The decline in metabolic activity in the course of cell development previously established for growth and photosynthesis extends to include respiration activity. Disagreements among several investigators in regard to the course of respiration during cell development are probably due to the effects of accessory factors such as strong light during the preceding growth period or the scarcity of respiratory substrate during respiration measurements which affect and distort changes in the inherent capacity of metabolic mechanisms in the course of cell development.     

Growth and Maintenance Respiration in Whole Plants, Tops, and Roots of Lolium multiflorum

Continuous measurements of CO₂-exchange were separately carried out on tops and roots of small swards of Lolium multiflorum grown in nutrient solution in growth chamber during 3–4 weeks. From these measurements, a daily carbon balance and accumulated dry matter could be established. The data were used to distinguish between two components of respiration, one proportional to growth or photosynthesis (growth respiration), the other proportional to plant dry weight (maintenance respiration). The separation of respiration in the two components was made by multiple regression analyses with daily photosynthesis or growth rate and accumulated dry matter as the independent variables. To ensure independency between the independent variables during the growth period, photosynthesis was varied by application of alternate three-day periods of high and low irradiance. From the two regression coefficients, the efficiency of converting assimilates into constructive growth (Y_G) and the maintenance coefficient (M) could be derived. Three experiments with varying length of photoperiod and dark period were carried out. The analyses were carried out for whole-plant respiration, respiration of tops and respiration of roots separately. Growth respiration for whole plants as well as for tops and for roots was lower — and hence the efficiencies higher — the longer the photoperiods were. Growth respiration and maintenance respiration were higher for roots than for tops. The high rate of root respiration may originate from release of HCO₃^? in exchange for NO₃^?. The parameters found can be utilized quantitatively in computer models of crop photosynthesis and respiration.     

Responses of Photosynthesis and Dark Respiration to Temperature

This analysis suggests that a model of the temperature dependenceof carbon exchange by a plant can be developed based upon absolutereaction rate theory. Component temperature-dependent physiologicalprocesses necessary to describe net photosynthesis over thebiological temperature range include the light reaction, darkreaction (carboxylase CO₂ uptake, oxygenase photorespiration)and mitochondrial dark respiration. An essential assumptionof the model is the reversibility of thermal inhibition. Supportingevidence for this assumption is provided within the biologicalrange. Thermodynamic constants were found to be strongly correlatedwith the thermal environment to which they were adapted. Therewas little difference in non-photorespiration thermodynamicconstants between C₂ andC₄species within thermal habitat types.The model shows the observed shift in temperature optima withlight intensity as a natural consequence of enzyme kineticsand absolute reaction rate theory. photosynthesis, photorespiration, dark respiration, temperature response, carbon exchange, mathematical model     

Studies on the respiratory properties of mitochondria isolated from developing winter wheat seedlings

Mitochondria isolated from shoots of 2 days, light- and dark-grown winter wheat (Triticum aestivum L. cv. Rideau) seedlings oxidize alpha-ketoglutarate and l-malate with good respiratory control and ADP: O ratios. The efficiency of oxidative phosphorylation, and respiratory control are both reduced significantly when succinate or NADH is employed as substrate. Respiratory control values and ADP: O ratios show a general decline in mitochondria from seedlings of increasing age, whether grown in light or dark. In light-grown seedlings, the decrease in respiratory control with aging is due principally to a decrease in the rate of state 3 respiration, while in dark-grown material, the decrease appears to be due mainly to an increased rate of state 4 respiration. In both light- and dark-grown seedlings, oxygen consumption during state 3 respiration is severely inhibited by oligomycin. During state 4 respiration, 2,4-dinitrophenol stimulates oxygen uptake to a level approximately two-thirds the normal ADP-stimulated rate.     

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.     

Photosynthesis, Dark Respiration, and Growth of Rumex patientia L. Exposed to Ultraviolet Irradiance (288 to 315 Nanometers) Simulating a Reduced Atmospheric Ozone Column

Net photosynthesis, dark respiration, and growth of Rumex patientia L. exposed to a ultraviolet irradiance (288-315 nanometers) simulating a 0.18 atm·cm stratospheric ozone column were determined. The ultraviolet irradiance corresponding to this 38% ozone decrease from normal was shown to be an effective inhibitor of photosynthesis and leaf growth. The repressive action on photosynthesis accumulated through time whereas leaf growth was retarded only during the initial few days of exposure. Small increases in dark respiration rates occurred but did not continue to increase with longer exposure periods. A reduction in total plant dry weight and leaf area of approximately 50% occurred after 22 days of treatment, whereas chlorophyll concentrations remained unaltered.