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1.
Analyses of chlorophyll fluorescence and photosynthetic oxygen evolution were conducted to understand why cold-hardened winter rye (Secale cereale L.) is more resistant to photoinhibition of photosynthesis than is non-hardened winter rye. Under similar light and temperature conditions, leaves of cold-hardened rye were able to keep a larger fraction of the PS II reaction centres in an open configuration, i.e. a higher ratio of oxidized to reduced QA (the primary, stable quinone acceptor of PSII), than leaves of non-hardened rye. Three fold-higher photon fluence rates were required for cold-hardened leaves than for non-hardened leaves in order to establish the same proportion of oxidized to reduced QA. This ability of cold-hardened rye fully accounted for its higher resistance to photoinhibition; under similar redox states of qa cold-hardened and non-hardened leaves of winter rye exhibited similar sensitivities to photoinhibition. Under given light and temperature conditions, it was the higher capacity for light-saturated photosynthesis in cold-hardened than in non-hardened leaves, which was responsible for maintaining a higher proportion of oxidized to reduced QA. This higher capacity for photosynthesis of cold-hardened leaves also explained the increased resistance of photosynthesis to photoinhibition upon cold-hardening.Abbreviations Fm and F'm fluorescence when all PSII reaction centres are closed in dark- and light-acclimated leaves, respectively - Fo and F'o fluorescence when all PSII reaction centres are open in darkness and steady-state light, respectively - Fv variable fluorescence (F'm-F'o) under steady-state light conditions - Fv/Fm the ratio of variable to maximum fluorescence as an expression of the maximum photochemical yield of PSII in dark-acclimated leaves - QA the primary, stable, quinone electron acceptor of PSII - qN non-photochemical quenching of fluorescence due to high energy state (pH) - qp photochemical quenching of fluorescence - RH cold-hardened rye - RNH non-hardened rye This work was supported by a Natural Sciences and Engineering Research Council of Canada (NSERCC) Operating Grant to N.P.A.H. G.Ö. was supported by an NSERCC International Exchange Award and by the Swedish Natural Science Research Council.  相似文献   

2.
Light- and CO2-saturated photosynthesis of nonhardened rye (Secale cereale L. cv. Musketeer) was reduced from 18.10 to 7.17 mol O2·m–2·s–1 when leaves were transferred from 20 to 5°C for 30 min. Following cold-hardening at 5°C for ten weeks, photosynthesis recovered to 15.05 mol O2·m–2·s–1,comparable to the nonhardened rate at 20°C. Recovery of photosynthesis was associated with increases in the total activity and activation of enzymes of the photosynthetic carbon-reduction cycle and of sucrose synthesis. The total hexose-phosphate pool increase by 30% and 120% for nonhardened and cold-hardened leaves respectively when measured at 5°C. The large increase in esterified phosphate in coldhardened leaves occurred without a limitation in inorganic phosphate supply. In contrast, the much smaller increase in esterified phosphate in nonhardened leaves was associated with an inhibition of ribulose-1,5-bisphosphate carboxylase/oxygenase and sucrose-phosphate synthase activation. It is suggested that the large increases in hexose phosphates in cold-hardened leaves compensates for the higher substrate threshold concentrations needed for enzyme activation at low temperatures. High substrate concentrations could also compensate for the kinetic limitations imposed by product inhibition from the accumulation of sucrose at 5°C. Nonhardened leaves appear to be unable to compensate in this fashion due to an inadequate supply of inorganic phosphate.Abbreviations DHAP dihydroxyacetone phosphate - Fru6P fructose-6-phosphate - Fru 1,6BP fructose-1,6-bisphosphate - Fru1,6BPase fructose-1,6-bisphosphatase - Glc6P glucose-6-phosphate - PGA 3-phosphoglycerate - PPFD photosynthetic photon flux density - CH cold-hardened rye grown at 5°C - NH nonhardened rye grown at 24°C - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - SPS sucrose-phosphate synthase - UDPGlc uridine 5-diphosphoglucose This work was supported by operating grants from the Swedish Natural Sciences Research Council to G.Ö. and P.G.  相似文献   

3.
The effects of oxygen concentration and light intensity on the rates of apparent photosynthesis, true photosynthesis, photorespiration and dark respiration of detached spruce twigs were determined by means of an infra-red carbon dioxide analyzer (IRCA). A closed circuit system IRCA was filled with either 1 per cent of oxygen in nitrogen, air (21 % O2) or pure oxygen (100 % O2). Two light intensities 30 × 103 erg · cm ?2· s?1 and 120 × 103 erg · cm?2· s?1 were applied. It has been found that the inhibitory effect of high concentration of oxygen on the apparent photosynthesis was mainly a result of a stimulation of the rate of CO2 production in light (photorespiration). In the atmosphere of 100 % O2, photorespiration accounts for 66–80 per cent of total CO2 uptake (true photosynthesis). Owing to a strong acceleration of photorespiration by high oxygen concentrations, the rate of true photosynthesis calculated as the sum of apparent photosynthesis and photorespiration was by several times less inhibited by oxygen than the rate of apparent photosynthesis. The rates of dark respiration were essentially unaffected by the oxygen concentrations used in the experiments. An increase in the intensity of light from 30 × 103 erg · cm?3· s?1 to 120 · 103 erg · cm?2· s?1 enhanced the rate of photorespiration in the atmospheres of 21 and 100 % oxygen but not in 1 % O2. The rate of apparent photosynthesis, however, was little affected by light intensity in an atmosphere of 1 % oxygen.  相似文献   

4.
Cold-hardened rye leaves have been shown to be more resistant to low temperature photoinhibition than non-hardened rye leaves. Isolated mesophyll cells from winter rye (Secale cereale L. cv. Musketeer) were exposed to photoinhibitory light conditions to estimate the importance of leaf morphology and leaf optical properties in the resistance of cold-hardened rye leaves to photoinhibition. Cold-hardened rye cells showed more resistance to photoinhibition than non-hardened rye cells when monitored with chlorophyll a variable to maximal fluorescence ratio (Fv/Fm). Thus, leaf morphology does not contribute to the resistance of cold-hardened rye leaves to low temperature photoinhibition. However, cold-hardened and non-hardened rye cells showed a similar extent of photoinhibition when photsynthetic CO2 fixation rates were measured. They also showed the same capacity to recover from photoinhibition. During both photoinhibition and recovery, Fv/Fm and light limited CO2 fixation rates showed different kinetics. We propose that inactivation and subsequent reactivation during recovery of some light activated Calvin cycle enzymes explain the greater extent of photoinhibition of light limited CO2 fixation and its faster recovery compared to Fv/Fm kinetics during photoinhibition.  相似文献   

5.
Chloroplasts developed at cold-hardening (5°C) and non-hardening temperatures (20°C) were compared with respect to the stability of photosynthetic electron transport activities, the capacity to produce and maintain a H+ gradient and the capacity fat photophosphorylation as a function of resuspension in the presence or absence of osmoticum. The results for electron transport indicate that whole chain, photosystem I and pfaotosystem II activities in non-hardened chloroplast thyalkoids were unaffected by resuspension in the presence of high or low osmoticum. In contrast, the same electron transport activities in cold-hardened chloroplast thylakoids exhibited a 3- to 4-fold decrease in activity when resuspended in the presence of low osmoticum. Impairment of electron transport through photosystem II of cold-hardened thylakoids resuspended in the presence of low osmoticum was supported by room temperature fluorescence induction kinetics. Since the presence of Mn2+ partially overcame this inhibition, it is concluded that this osmotically-induced inhibition of PSII activity in cold-hardened chloroplast thylakoids may, in part, be due to damage to the H2O-splitting side of photosystem II. Both the initial rate and the maximum capacity for cyclic photophosphorylation were significantly inhibited in cold-hardened as compared to non-hardened thylakoids upon resuspension in the presence of low concentrations of osmoticum. This was correlated with an inability of the cold-hardened chloroplast thylakoids to maintain a significant transrnembrane H+ gradient. The results indicate that cold-hardened thylakoid membranes required an osmotic concentration (0.8 M) twice as high as non-hardened thylakoids (0.4 M) to produce the same initial rate of H+ uptake. In addition, the capacity to produce a proton gradient in cold-hardened thylakoids was less stable than that in non-hardened thylakoids regardless of the osmotic concentration tested. It is concluded that development of rye thylakoid membranes at low temperature results in a differential sensitivity to low osmoticum and thus extreme caution should be exercised when comparing the structure and function of isolated thylakoids developed under contrasting thermal regimes.  相似文献   

6.
为了进一步了解光照下植物呼吸作用的内在机理以及呼吸作用和光合作用的关系,该文研究了在光照下菜豆(Phaseolus vulgaris)叶片抗氰呼吸与光合作用的关系。研究发现,将黑暗下生长的菜豆幼苗叶片转到光照下10 h,总呼吸、抗氰呼吸以及抗氰呼吸在总呼吸中的比例均逐步上升;光照也导致了叶片叶绿体光合放氧和CO2固定的出现及其速率的增加,但光合放氧和CO2固定速率的增加均滞后于抗氰呼吸的增加。将黑暗下生长的叶片转到光照下之前用抗氰呼吸的抑制剂水杨基氧肟酸(SHAM)处理叶片,发现用SHAM处理并没有导致叶片在光照下光合放氧和CO2固定速率的明显变化,这也提示了黑暗下生长的叶片转至光照的过程中,抗氰呼吸和光合作用没有产生偶联。进一步研究发现,在黑暗中对叶片施加短时间的光照能够增加抗氰呼吸在总呼吸中的比例,但短时间的光照对叶片光合CO2固定速率没有影响。这些结果表明了光照对抗氰呼吸的诱导可以不依赖于光合作用,光照可能是作为一种直接的信号去诱导抗氰呼吸。  相似文献   

7.
The effect of O2 on the CO2 exchange of detached soybean leaves was measured with a Clark oxygen electrode and infrared carbon dioxide analysers in both open and closed systems.

The rate of apparent photosynthesis was inhibited by O2 while the steady rate of respiration after a few minutes in the dark was not affected. Part of the inhibition of apparent photosynthesis was shown to be a result of increased photorespiration. This stimulation of photorespiration by O2 was manifested by an increase in the CO2 compensation point.

The differential effects of O2 on dark respiration (no effect) and photorespiration (stimulation) indicated that these were 2 different processes.

Moreover the extrapolation of the CO2 compensation point to zero at zero O2 indicated that dark respiration was suppressed in the light at least at zero O2 concentration.

  相似文献   

8.
Measurements of photosynthesis and respiration were made on leaves in summer in a Quercus rubra L. canopy at approximately hourly intervals throughout 5 days and nights. Leaves were selected in the upper canopy in fully sunlit conditions (upper) and in the lower canopy (lower). In addition, leaves in the upper canopy were shaded (upper shaded) to decrease photosynthesis rates. The data were used to test the hypothesis that total night‐time respiration is dependent on total photosynthesis during the previous day and that the response is mediated through changes in storage in carbohydrate pools. Measurements were made on clear sunny days with similar solar irradiance and air temperature, except for the last day when temperature, especially at night, was lower than that for the previous days. Maximum rates of photosynthesis in the upper leaves (18.7 μmol m?2 s?1) were approximately four times higher than those in the lower leaves (4.3 μmol m?2 s?1) and maximum photosynthesis rates in the upper shaded leaves (8.0 μmol m?2 s?1) were about half those in the upper leaves. There was a strong linear relationship between total night‐time respiration and total photosynthesis during the previous day when rates of respiration were normalized to a fixed temperature of 20°C, removing the effects of temperature from this relationship. Measurements of specific leaf area, nitrogen and chlorophyll concentration and calculations of the maximum rate of carboxylation activity, Vcmax, were not significantly different between upper and upper shaded leaves 5 days after the shading treatment was started. There were small, but significant decreases in the rate of apparent maximum electron transport at saturating irradiance, Jmax (P>0.05), and light use efficiency, ? (P<0.05), for upper shaded leaves compared with those for upper leaves. This suggests that the duration of shading in the experiment was sufficient to initiate changes in the electron transport, but not the carboxylation processes of photosynthesis. Support for the hypothesis was provided from analysis of soluble sugar and starch concentrations in leaves. Respiration rates in the upper shaded leaves were lower than those expected from a relationship between respiration and soluble sugar concentration for fully exposed upper and lower leaves. However, there was no similar difference in starch concentrations. This suggests that shading for the duration of several days did not affect sugar concentrations but reduced starch concentrations in leaves, leading to lower rates of respiration at night. A model was used to quantify the significance of the findings on estimated canopy CO2 exchange for the full growing season. Introducing respiration as a function of total photosynthesis on the previous day resulted in a decrease in growing season night‐time respiration by 23% compared with the value when respiration was held constant. This highlights the need for a process‐based approach linking respiration to photosynthesis when modelling long‐term carbon exchange in forest ecosystems.  相似文献   

9.
Rates of net photosynthesis and respiration were determined for Pithophora oedogonia (Mont.) Wittr. acclimatized to 56 combinations of light (7–1200 μE m?2 s?1) and temperature (5–35°C). Conditions for maximum net photosynthesis were estimated to be 26°C and 970 μE m?2 s?1. The rate of net photosyntheses varied considerably with temperature, with the maximum measured value (9.67 mg O2 h?1 g dry wt.?1) occurring at 25°C. Respiration rate increased with temperature and the light received just prior to measurement. The maximum respiration rate (7.05 mg O2 g?1 h?1) occurred at 30°C and 1200 μE m?2 s?1. Exposure of Pithophora to light levels of 600 or 1200 μE m?2 s?1 prior to determination of the respiration rate resulted in significantly elevated levels of oxygen consumption at temperatures ≥ 15°C. The relationship between light, temperature and photosynthesis and respiration were summarized as three-dimensional response surfaces.  相似文献   

10.
The homogeneous distribution of the phytoplankton in a shallow (mean depth 8·6 m) unstratified lake, L. Neagh, Northern Ireland, facilitated the study of the interaction of components controlling gross photosynthesis per unit area. These included the photosynthetic capacity, the phytoplankton content of the euphotic zone, and a logarithmic function describing the effective radiation input. These factors were analysed for two sites, the open lake and Kinnego Bay, which respectively had standing crops of up to 90 and 300 mg chlorophyll a m?3 and maximum daily rates of gross integral photosynthesis of 11·7 and 15·6 g O2 m?2 day?1. Values are reduced by the high contribution to light attenuation by non-algal sources, which increases at low standing crops particularly in winter, when values of integral photosynthesis decrease to 0·5 g O2 m?2 day?1. This relative change is the result of self-shading behaviour of the phytoplankton altering the crop content of the euphotic zone at different population densities. Changes in the irradiance function, incorporating day length, are largely responsible for the changes in daily rates of integral gross photosynthesis; as daily irradiance is also a determinant of water temperature, it exerts further influence through the photosynthetic capacity which was strongly correlated with temperature. Much of the gain in gross photosynthesis resulting from higher photosynthetic capacity may not be reflected in a higher net column photosynthesis, because of the greater proportional rise in respiration with temperature. The balance in the water column between respiration losses and photosynthetic input may frequently alter since the ratio of illuminated to dark zones is between 1/4 to 1/5 in the open lake, and small shifts in any of the controlling features may result in conditions unfavourable for growth. This is analysed especially for the increase of diatoms in spring, when small modifications of the underwater light field can delay growth.  相似文献   

11.
Bown AW 《Plant physiology》1982,70(3):803-810
Aerated and stirred suspensions of mechanically isolated Asparagus sprengeri Regel mesophyll cells were used to investigate the roles of respiration and photosynthesis in net H+ efflux. Rates varied between 0.12 and 1.99 nanomoles H+ per 106 cells per minute or 3 and 40 nanomoles H+ per milligram chlorophyll per minute. The mean rate of H+ efflux was 10% greater in the dark. 3-(3,4-Dichlorophenyl)-l,l-dimethylurea, an inhibitor of noncyclic photophosphorylation, did not inhibit H+ efflux from illuminated cells. Bubbling with N2 or addition of oligomycin, an inhibitor of mitochondrial ATP production, resulted in rapid and virtually complete inhibition of H+ efflux in light or dark. In the absence of aeration, H+ efflux came to a halt but resumed with aeration or illumination. When aeration was switched to CO2-free air, rates of H+ efflux were reduced 43% in the dark and 57% in the light. Oligomycin eliminated dark CO2 fixation but not photosynthetic CO2 fixation. It is suggested that H+ efflux is dependent on respiration and dark CO2 fixation, but independent of photosynthesis.  相似文献   

12.
The possible protective role of endogenous isoprene against oxidative stress caused by singlet oxygen (1O2) was studied in the isoprene‐emitting plant Phragmites australis. Leaves emitting isoprene and leaves in which isoprene synthesis was inhibited by fosmidomycin were exposed to increasing concentrations of 1O2 generated by Rose Bengal (RB) sensitizer at different light intensities. In isoprene‐emitting leaves, photosynthesis and H2O2 and malonyldialdehyde (MDA) contents were not affected by low to moderate 1O2 concentrations generated at light intensities of 800 and 1240 µmol m?2 s?1, but symptoms of damage and reactive oxygen accumulation started to be observed when high levels of 1O2 were generated by very high light intensity (1810 µmol m?2 s?1). A dramatic decrease in photosynthetic performance and an increase in H2O2 and MDA levels were measured in isoprene‐inhibited RB‐fed leaves, but photosynthesis was not significantly inhibited in leaves in which the isoprene leaf pool was reconstituted by fumigating exogenous isoprene. The inhibition of photosynthesis in isoprene‐inhibited leaves was linearly associated with the light intensity and with the consequently formed 1O2. Hence, physiological levels of endogenous isoprene may supply protection against 1O2. The protection mechanisms may involve a direct reaction of isoprene with 1O2. Moreover, as it is a small lipophilic molecule, it may assist hydrophobic interactions in membranes, resulting in their stabilization. The isoprene‐conjugated double bond structure may also quench 1O2 by facilitating energy transfer and heat dissipation. This action is typical of other isoprenoids, but we speculate that isoprene may provide a more dynamic protection mechanism as it is synthesized promptly when high light intensity produces 1O2.  相似文献   

13.
Oxygen evolution and chlorophyll fluorescence were measured in cold-hardened and unhardened leaves of barley ( Hordeum vulgare L. cv. Asa) during the induction period of photosynthesis. The lag phase of light-saturated photosynthesis was increased and steady-state rates of photosynthesis were higher in cold-hardened than in unhardened barley leaves. Fluorescence was quenched more rapidly during the first minutes of induction in hardened than unhardened leaves, largely because of greater energy-dependent quenching (qE). Also, slow fluorescence transients through the M peak were delayed and less pronounced in cold-hardened than in unhardened leaves. Based upon the combined fluorescence and oxygen evolution data it was concluded that cold-hardening delayed light activation of the energy consuming carbon reduction cycle, thereby delaying the use of ATP and NADPH formed in the light reaction. Measurements of oxygen evolution and fluorescence kinetics during photosynthetic induction under oxygenic and anoxygenic conditions suggest that oxygen photoreduction is important for additional ATP generation during both the onset of photosynthetic carbon assimilation and during steady-state photosynthesis.  相似文献   

14.
Oxygen may enhance CO2-saturated photosynthesis in intact leaves, which display the Warburg effect when illuminated at the current atmospheric level of CO2 and O2, of about 350 μl l−1 and 21%, respectively. The magnitude of the stimulation depends on irradiance. The K M(O2) of the stimulation is 128 μM (10.6% O2). Maximum enhancement in wheat leaves is 6.1 and 5.3 μmol m−2 s−1 under 27.9 and 18.7 mW cm−2, respectively, corresponding to a 25–30% increase in the ribulose 1,5-bisphosphate (RuBP) turnover rate if compared with O2-free ambient gas phase. The stimulation appears in 5–10 s after a sharp increase in O2. In response to a decrease in O2, the new stabilized rate is reached in 5–7 min. The stimulation does not involve any increase in the activity of Rubisco. The effect correlates with increased concentration of RuBP. Oxygen enhances CO2-saturated photosynthesis by acting as a terminal electron acceptor in the photosynthetic electron transport. The magnitude of the effect may be adopted as an index of the pseudocyclic photophosphorylation in vivo.  相似文献   

15.
Leaks and isotopic disequilibria represent potential errors and artefacts during combined measurements of gas exchange and carbon isotope discrimination (Δ). This paper presents new protocols to quantify, minimize, and correct such phenomena. We performed experiments with gradients of CO2 concentration (up to ±250 μmol mol?1) and δ13CCO2 (34‰), between a clamp‐on leaf cuvette (LI‐6400) and surrounding air, to assess (1) leak coefficients for CO2, 12CO2, and 13CO2 with the empty cuvette and with intact leaves of Holcus lanatus (C3) or Sorghum bicolor (C4) in the cuvette; and (2) isotopic disequilibria between net photosynthesis and dark respiration in light. Leak coefficients were virtually identical for 12CO2 and 13CO2, but ~8 times higher with leaves in the cuvette. Leaks generated errors on Δ up to 6‰ for H. lanatus and 2‰ for S. bicolor in full light; isotopic disequilibria produced similar variation of Δ. Leak errors in Δ in darkness were much larger due to small biological : leak flux ratios. Leak artefacts were fully corrected with leak coefficients determined on the same leaves as Δ measurements. Analysis of isotopic disequilibria enabled partitioning of net photosynthesis and dark respiration, and indicated inhibitions of dark respiration in full light (H. lanatus: 14%, S. bicolor: 58%).  相似文献   

16.
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.  相似文献   

17.
Biomass, akinete numbers, net photosynthesis, and respiration of Pithophora oedogonia were monitored over two growing seasons in shallow Surrey Lake, Indiana. Low rates of photosynthesis occurred from late fall to early spring and increased to maximum levels in late spring to summer (29–39 mgO2·g?1 dry wt·h?1). Areal biomass increased following the rise in photosynthesis and peaked in autumn (163–206g dry wt·m?2). Photosynthetic rates were directly correlated with temperature, nitrogen, and phosphorus over the entire annual cycle and during the growing season. Differences in photosynthetic activity and biomass between the two growing seasons (1980 and 1981) were apparently related to higher, early spring temperatures and higher levels of NO3-N and PO4-P in 1981. Laboratory investigations of temperature and light effects on Pithophora photosynthesis and respiration indicated that these processes were severely inhibited below 15°C. The highest Pmax value occurred at 35°C (0.602 μmol O2·mg?1 chl a·min?1). Rates of dark respiration did not increase above 25°C thus contributing to a favorable balance of photosynthetic production to respiratory utilization at high temperatures. Light was most efficiently utilized at 15°C as indicated by minimum values of Ik(47 μE·m?2·s?1) and Ic (6 μE·m?2·s?1). Comparison of P. oedogonia and Cladophora glomerata indicated that the former was more tolerant of temperatures above 30°C. Pithophora's tolerance of high temperature and efficient use of low light intensity appear to be adaptive to conditions found within the dense, floating algal mats and the shallow littoral areas inhabited by this filamentous alga.  相似文献   

18.
The respiration rate of leaves and mesophyll protoplasts of pea (Pisum sativum L.), from plants which were previously kept in darkness for 24 h was doubled following a period of photosynthesis at ambient level of O2 (21 %), whereas the low level of O2 (1 % and 4 % for leaves and protoplasts, respectively) reduced this light-enhanced dark respiration (LEDR) to the rate as noted before the illumination. Similarly to respiration rate, the oxygen at used concentrations had no effect on the ATP/ADP ratio in the dark-treated leaves. However, the ATP/ADP ratio in leaves photosynthesizing at 21 % O2 was higher (up to 40 %, dependence on CO2 concentration in the range 40–1600 1 dm−3) than in those photosynthesizing at 1 % O2 or darkened at air (21 % O2). Also, at 1 % O2 the accumulation of malate was suppressed (by about 40 %), to a value noted for leaves darkened at 21 % O2. The dark-treatment of leaves reduced the ability of isolated mitochondria to oxidize glycine (by about twofold) and succinate, but not malate. Mitochondria from both the light- and dark-treated leaves did not differ in qualitative composition of free amino acids, however, there were significant quantitative differences especially with respect to aspartate, alanine, glutamate and major intermediates of the photorespiratory pathway (glycine, serine). Our results suggest that accumulation of photorespiratory and respiratory metabolites in pea leaves during photosynthesis at 1 % O2 is reduced, hence the suppression of postillumination respiration rate.  相似文献   

19.
The present study shows the importance of alternative oxidase (AOX) pathway in optimizing photosynthesis under high light (HL). The responses of photosynthesis and respiration were monitored as O2 evolution and O2 uptake in mesophyll protoplasts of pea pre‐incubated under different light intensities. Under HL (3000 µmol m?2 s?1), mesophyll protoplasts showed remarkable decrease in the rates of NaHCO3‐dependent O2 evolution (indicator of photosynthetic carbon assimilation), while decrease in the rates of respiratory O2 uptake were marginal. While the capacity of AOX pathway increased significantly by two fold under HL, the capacity of cytochrome oxidase (COX) pathway decreased by >50% compared with capacities under darkness and normal light (NL). Further, the total cellular levels of pyruvate and malate, which are assimilatory products of active photosynthesis and stimulators of AOX activity, were increased remarkably parallel to the increase in AOX protein under HL. Upon restriction of AOX pathway using salicylhydroxamic acid (SHAM), the observed decrease in NaHCO3‐dependent O2 evolution or p‐benzoquinone (BQ)‐dependent O2 evolution [indicator of photosystem II (PSII) activity] and the increase in total cellular levels of pyruvate and malate were further aggravated/promoted under HL. The significance of raised malate and pyruvate levels in activation of AOX protein/AOX pathway, which in turn play an important role in dissipating excess chloroplastic reducing equivalents and sustenance of photosynthetic carbon assimilation to balance the effects of HL stress on photosynthesis, was depicted as a model.  相似文献   

20.
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