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1.
Rao IM  Sharp RE  Boyer JS 《Plant physiology》1987,84(4):1214-1219
We grew sunflower (Helianthus annuus L.) plants in nutrient solutions having nutritionally adequate but low or high Mg2+ concentrations and determined whether photosynthesis was effected as leaf water potentials (ψw) decreased. Leaf Mg contents were 3- to 4-fold higher in the plants grown in high Mg2+ concentrations (10 millimolar) than in those grown in low concentrations (0.25 millimolar). These contents were sufficient to support maximum growth, plant dry weight, and photosynthesis, and the plants appeared normal. As low ψw developed, photosynthesis was inhibited but moreso in high Mg leaves than in low Mg leaves. The effect was particularly apparent under conditions of light- and CO2-saturation, indicating that the chloroplast capacity to fix CO2 was altered. The differential inhibition observed in leaves of differing Mg contents was not observed in leaves having differing K contents, suggesting that the effect may have been specific for Mg. Because Mg2+ inhibits photophosphorylation and coupling factor activities at concentrations likely to occur as leaves dehydrate, Mg may play a role in the inhibition of chloroplast reactions at low ψw, especially in leaves such as sunflower that markedly decrease in water content as ψw decreases.  相似文献   

2.
Inhomogeneous photosynthetic activity has been reported to occur in drought-stressed leaves. In addition, it has been suggested that these water stress-induced nonuniformities in photosynthesis are caused by “patchy” stomatal closure and that the phenomenon may have created the illusion of a nonstomatal component to the inhibition of photosynthesis. Because these earlier studies were performed with nonacclimated growth chamber-grown plants, we sought to determine whether such “patches” existed in drought-treated, field-grown plants or in chamber-grown plants that had been acclimated to low leaf water potentials (ψleaf). Cotton (Gossypium hirsutum L.) was grown in the field and subjected to drought by withholding irrigation and rain from 24 d after planting. The distribution of photosynthesis, which may reflect the stomatal aperture distribution in a heterobaric species such as cotton, was assayed by autoradiography after briefly exposing attached leaves of field-grown plants to 14CO2. A homogeneous distribution of radioactive photosynthate was evident even at the lowest ψleaf of −1.34 MPa. “Patchiness” could, however, be induced by uprooting the plant and allowing the shoot to air dry for 6 to 8 min. In parallel studies, growth chamber-grown plants were acclimated to drought by withholding irrigation for three 5-d drought cycles interspersed with irrigation. This drought acclimation lowered the ψleaf value at which control rates of photosynthesis could be sustained by approximately 0.7 MPa and was accompanied by a similar decline in the ψleaf at which patchiness first appeared. Photosynthetic inhomogeneities in chamber-grown plants that were visible during moderate water stress and ambient levels of CO2 could be largely removed with elevated CO2 levels (3000 μL L−1), suggesting that they were stomatal in nature. However, advanced dehydration (less than approximately 2.0 MPa) resulted in “patches” that could not be so removed and were probably caused by nonstomatal factors. The demonstration that patches do not exist in drought-treated, field-grown cotton and that the presence of patches in chamber-grown plants can be altered by treatments that cause an acclimation of photosynthesis leads us to conclude that spatial heterogeneities in photosynthesis probably do not occur frequently under natural drought conditions.  相似文献   

3.
Wise RR  Ort DR 《Plant physiology》1989,90(2):657-664
The response of in situ photophosphorylation in attached cucumber (Cucumis sativus L. cv Ashley) leaves to chilling under strong illumination was investigated. A single-beam kinetic spectrophotometer fitted with a clamp-on, whole leaf cuvette was used to measure the flash-induced electrochromic absorbance change at 518 minus 540 nanometers (ΔA518−540) in attached leaves. The relaxation kinetics of the electric field-indicating ΔA518−540 measures the rate of depolarization of the thylakoid membrane. Since this depolarization process is normally dominated by proton efflux through the coupling factor during ATP synthesis, this technique can be used, in conjuction with careful controls, as a monitor of in situ ATP formation competence. Whole, attached leaves were chilled at 5°C and 1000 microeinsteins per square meter per second for up to 6 hours then rewarmed in the dark at room temperature for 30 minutes and 100% relative humidity. Leaf water potential, chlorophyll content, and the effective optical pathlength for the absorption measurements were not affected by the treatment. Light- and CO2-saturated leaf disc oxygen evolution and the quantum efficiency of photosynthesis were inhibited by approximately 50% after 3 hours of light chilling and by approximately 75% after 6 hours. Despite the large inhibition to net photosynthesis, the measurements of ΔA518−540 relaxation kinetics showed photophosphorylation to be largely unaffected by the chilling and light exposure. The amplitude of the ΔA518-540 measures the degree of energization of the photosynthetic membranes and was reduced significantly by chilling in the light. The cause of the decreased energization was traced to impaired turnover of photosystem II. Our measurements showed that the chilling of whole leaves in the light caused neither an uncoupling of photophosphorylation from photosynthetic electron transport nor any irreversible inhibition of the chloroplast coupling factor in situ. The sizeable inhibition in net photosynthesis observed after chilling in the light cannot, therefore, be attributed to any direct effect on photophosphorylation competence.  相似文献   

4.
Lauer MJ  Boyer JS 《Plant physiology》1992,98(4):1310-1316
Observations of nonuniform photosynthesis across leaves cast doubt on internal CO2 partial pressures (pi) calculated on the assumption of uniformity and can lead to incorrect conclusions about the stomatal control of photosynthesis. The problem can be avoided by measuring pi directly because the assumptions of uniformity are not necessary. We therefore developed a method that allowed pi to be measured continuously in situ for days at a time under growth conditions and used it to investigate intact leaves of sunflower (Helianthus annuus L.), soybean (Glycine max L. Merr.), and bush bean (Phaseolus vulgaris L.) subjected to high or low leaf water potentials (ψw) or high concentrations of abscisic acid (ABA). The leaves maintained a relatively constant differential (Δp) between ambient CO2 and measured pi throughout the light period when water was supplied. When water was withheld, ψw decreased and the stomata began to close, but measured pi increased until the leaf reached a ψw of −1.76 (bush bean), −2.12 (sunflower) or −3.10 (soybean) megapascals, at which point Δp = 0. The increasing pi indicated that stomata did not inhibit CO2 uptake and a Δp of zero indicated that CO2 uptake became zero despite the high availability of CO2 inside the leaf. In contrast, when sunflower leaves at high ψw were treated with ABA, pi did not increase and instead decreased rapidly and steadily for up to 8 hours even as ψw increased, as expected if ABA treatment primarily affected stomatal conductance. The accumulating CO2 at low ψw and contrasting response to ABA indicates that photosynthetic biochemistry limited photosynthesis at low ψw but not at high ABA.  相似文献   

5.
Acclimation of photosynthesis to low leaf water potentials   总被引:21,自引:9,他引:12       下载免费PDF全文
Photosynthesis is reduced at low leaf water potentials (Ψl) but repeated water deficits can decrease this reduction, resulting in photosynthetic acclimation. The contribution of the stomata and the chloroplasts to this acclimation is unknown. We evaluated stomatal and chloroplast contributions when soil-grown sunflower (Helianthus annuus L.) plants were subjected to water deficit pretreatments for 2 weeks. The relationship between photosynthesis and Ψl, determined from gas-exchange and isopiestic thermocouple psychometry, was shifted 3 to 4 bars towards lower Ψl, in pretreated plants. Leaf diffusive resistance was similarly affected. Chloroplast activity, demonstrated in situ with measurements of quantum yield and the capacity to fix CO2 at all partial pressures of CO2, and in vitro by photosystem II activity of isolated organelles, was inhibited at low Ψl but less in pretreated plants than in control plants. The magnitude of this inhibition indicated that decreases in chloroplast activity contributed more than closure of stomata both to losses in photosynthesis and to the acclimation of photosynthesis to low Ψl.  相似文献   

6.
Plant water status, leaf tissue pressure-volume relationships, and photosynthetic gas exchange were monitored in five coffee (Coffea arabica L.) cultivars growing in drying soil in the field. There were large differences among cultivars in the rates at which leaf water potential (ΨL) and gas exchange activity declined when irrigation was discontinued. Pressure-volume curve analysis indicated that increased leaf water deficits in droughted plants led to reductions in bulk leaf elasticity, osmotic potential, and in the ΨL at which turgor loss occurred. Adjustments in ΨL at zero turgor were not sufficient to prevent loss or near loss of turgor in three of five cultivars at the lowest values of midday ΨL attained. Maintenance of protoplasmic volume was more pronounced than maintenance of turgor as soil drying progressed. Changes in assimilation and stomatal conductance were largely independent of changes in bulk leaf turgor, but were associated with changes in relative symplast volume. It is suggested that osmotic and elastic adjustment contributed to maintenance of gas exchange in droughted coffee leaves probably through their effects on symplast volume rather than turgor.  相似文献   

7.
The losses in chloroplast capacity to fix CO2 when photosynthesis is reduced at low leaf water potential (ψ1) have been proposed to result from photoinhibition. We investigated this possibility in soil-grown sunflower (Helianthus annuus L. cv IS894) using gas exchange techniques to measure directly the influence of light during dehydration on the in situ chloroplast capacity to fix CO2. The quantum yield for CO2 fixation as well as the rate of light- and CO2-saturated photosynthesis were strongly inhibited at low ψ1. The extent of inhibition was the same whether the leaves were exposed to high or to low light during dehydration. When intercellular partial pressures of CO2 were decreased to the compensation point, which was lower than the partial pressures resulting from stomatal closure, the inhibition of the quantum yield was also unaffected. Photoinhibition could be observed only after high light exposures were imposed under nonphysiological low CO2 and O2 where both photosynthesis and photorespiration were suppressed. The experiments are the first to test whether gas exchange at low ψ1 is affected by potentially photoinhibitory conditions and show that the loss in chloroplast capacity to fix CO2 was entirely the result of a direct effect of water availability on chloroplast function and not photoinhibition.  相似文献   

8.
Stomatal conductance is coupled to leaf photosynthetic rate over a broad range of environmental conditions. We have investigated the extent to which chloroplasts in guard cells may contribute to this coupling through their photosynthetic activity. Guard cells were isolated by sonication of abaxial epidermal peels of Vicia faba. The electrochromic band shift of isolated guard cells was probed in vivo as a means of studying the electric field that is generated across the thylakoid membranes by photosynthetic electron transport and dissipated by photophosphorylation. Both guard cells and mesophyll cells exhibited fast and slow components in the formation of the flash-induced electrochromic change. The spectrum of electrochromic absorbance changes in guard cells was the same as in the leaf mesophyll and was typical of that observed in isolated chloroplasts. This observation indicates that electron transport and photophosphorylation occur in guard cell chloroplasts. Neither the fast nor the slow component of the absorbance change was observed in the presence of the uncoupler carbonylcyanide p-trifluoromethoxy-phenylhydrazone which confirms that the absorbance change was caused by the electric field across the thylakoid membranes. The magnitude of the fast rise was reduced by half in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Therefore, photosystem II is functional and roughly equal in concentration to photosystem I in guard cell chloroplasts. The slow rise was abolished by 2,5-dibromo-3-methyl-6-isopropyl-1,4-benzoquinone indicating the involvement of the cytochrome b6/f complex in electron transport between the two photosystems. Relaxation of the absorbance change was irreversibly retarded in cells treated with the energy transfer inhibitor, N,N′-dicyclohexylcarbodiimide. The slowing of the rapid decay kinetics by N,N′-dicyclohexylcarbodiimide confirms that the electrical potential across the thyalkoid membrane is dissipated by photophosphorylation. These results show that guard cell chloroplasts conduct photosynthetic electron transport in a manner similar to that in mesophyll cells and provide the first evidence that photophosphorylation occurs in guard cells in vivo.  相似文献   

9.
Needles from phosphorus deficient seedlings of Pinus radiata D. Don grown for 8 weeks at either 330 or 660 microliters CO2 per liter displayed chlorophyll a fluorescence induction kinetics characteristic of structural changes within the thylakoid chloroplast membrane, i.e. constant yield fluorescence (FO) was increased and induced fluorescence ([FP-FI]/FO) was reduced. The effect was greatest in the undroughted plants grown at 660 μl CO2 L−1. By week 22 at 330 μl CO2 L−1 acclimation to P deficiency had occurred as shown by the similarity in the fluorescence characteristics and maximum rates of photosynthesis of the needles from the two P treatments. However, acclimation did not occur in the plants grown at 660 μl CO2 L−1. The light saturated rate of photosynthesis of needles with adequate P was higher at 660 μl CO2 L−1 than at 330 μl CO2 L−1, whereas photosynthesis of P deficient plants showed no increase when grown at the higher CO2 concentration. The average growth increase due to CO2 enrichment was 14% in P deficient plants and 32% when P was adequate. In drought stressed plants grown at 330 μl CO2 L−1, there was a reduction in the maximal rate of quenching of fluorescence (RQ) after the major peak. Constant yield fluorescence was unaffected but induced fluorescence was lower. These results indicate that electron flow subsequent to photosystem II was affected by drought stress. At 660 μl CO2 L−1 this response was eliminated showing that CO2 enrichment improved the ability of the seedlings to acclimate to drought stress. The average growth increase with CO2 enrichment was 37% in drought stressed plants and 19% in unstressed plants.  相似文献   

10.
Richard Wagner  Wolfgang Junge 《BBA》1977,462(2):259-272
The membrane bound coupling factor of photophosphorylation is studied after pretreatment of broken chloroplasts with the bifunctional N,N-orthophenyldimaleimide under energization of the thylakoid membrane by mild flashing light. The proton conduction of the membrane is monitored both via the electrochromic absorption changes and via selective pH-indicating dyes. It is found that the coupling factor, after interaction with N,N-orthophenyldimaleimide during the preillumination period, shortcircuits one of the two protons pumped inside after excitation of chloroplasts with one short flash of light. In contrast to the low proton conductivity of the unperturbed thylakoid membrane (relaxation time for a proton gradient > 5s), this extra proton channel leads to a partial relaxation of a proton gradient within a few ms. Although limited to only one proton per electron, this extra proton conducting pathway is not otherwise specific. It operates with protons resulting from both Photosystem I and Photosystem II activity. In addition it operates with protons already present in the internal phase before firing of the exciting light flash. These effects are prevented by the presence of ATP (but not GTP) during the preillumination period. It is suggested that the modified coupling factor is gated open by the light induced electric field across the thylakoid membrane while self closing after passage of one proton per activated coupling factor.  相似文献   

11.
At low water potential (ψw), dehydration reduces the symplast volume of leaf tissue. The effect of this reduction on photosynthetic capacity was investigated. The influence of osmotic adjustment on this relationship was also examined. To examine these relationships, comparative studies were undertaken on two wheat cultivars, one that osmotically adjusts in response to water deficits (`Condor'), and one that lacks this capacity (`Capelle Desprez'). During a 9-day stress cycle, when water was withheld from plants grown in a growth chamber, the relative water content of leaves declined by 30% in both cultivars. Leaf osmotic potential (ψs) declined to a greater degree in Condor plants. Measuring ψs at full turgor indicated that osmotic adjustment occurred in stressed Condor, but not in Capelle plants. Two methods were used to examine the degree of symplast (i.e. protoplast) volume reduction in tissue rapidly equilibrated to increasingly low ψw. Both techniques gave similar results. With well-watered plants, symplast volume reduction from the maximum (found at high ψw for each cultivar) was the same for Condor and Capelle. After a stress cycle, volume was maintained to a greater degree at low ψw in Condor leaf tissue than in Capelle. Nonstomatally controlled photosynthesis was inhibited to the same degree at low ψw in leaf tissue prepared from well-watered Condor and Capelle plants. However, photosynthetic capacity was maintained to a greater degree at low ψw in tissue prepared from stressed Condor plants than in tissue from stressed Capelle plants. Net CO2 uptake in attached leaves was monitored using an infrared gas analyzer. These studies indicated that in water stressed plants, photosynthesis was 106.5% higher in Condor than Capelle at ambient [CO2] and 21.8% higher at elevated external [CO2]. The results presented in this report were interpreted as consistent with the hypothesis that there is a causal association between protoplast (and presumably chloroplast) volume reduction at low ψw and low ψw inhibition of photosynthesis. Also, the data indicate that osmotic adjustment allows for maintenance of relatively greater volume at low ψw, thus reducing low ψw inhibition of chloroplast photosynthetic potential.  相似文献   

12.
Studies were undertaken to examine the relationship between water deficit effects on photosynthesis and the extent of protoplast volume reduction which occurs in leaves at low water potential (Ψw). This relationship was monitored in two cultivars (`Condor' and `Capelle Desprez') of cultivated wheat (Triticum aestivum) that differed in sensitivity to drought, and in a wild relative of cultivated wheat (Triticum kotschyi) that has been previously found to be `drought resistant.' When subjected to periods of water stress, Condor and T. kotschyi plants underwent osmotic adjustment; Capelle plants did not. Photosynthetic capacity was maintained to different extents in the three genotypes as leaf Ψw declined during stress; Capelle plants were most severely affected. Calculations of internal leaf [CO2] and stomatal conductance from gas exchange measurements indicated that differences in photosynthetic inhibition at low Ψw among the genotypes were primarily due to nonstomatal effects. The extent of protoplast volume reduction that occurred in leaves at low Ψw was also found to be different in the three genotypes; maintenance of protoplast volume and photosynthetic capacity in stressed plants of the genotypes appeared to be correlated. When the extent of water stress-induced inhibition of photosynthesis was plotted as a function of declining protoplast volume, this relationship appeared identical for the three genotypes. It was concluded that there is a correlative association between protoplast volume and photosynthetic capacity in leaves of wheat plants subjected to periods of water stress.  相似文献   

13.
The relative importance of stomatal and nonstomatal limitations to net photosynthesis (A) and possible signals responsible for stomatal limitations were investigated in unhardened Pinus taeda seedlings at low soil temperatures. After 2 days at soil temperatures between 13 and 7°C, A was reduced by 20 to 50%, respectively. The reduction in A at these moderate root-chilling conditions appeared to be the result of stomatal limitations, based on the decrease in intercellular CO2 concentrations (ci). This conclusion was supported by A versus ci analysis and measurements of O2 evolution at saturating CO2, which suggested increases in stomatal but not biochemical limitations at these soil temperatures. Nonuniform stomatal apertures, which were demonstrated with abscisic acid, were not apparent 2 days after root chilling, and results of our A versus ci analysis appear valid. Bulk shoot water potential (ψ) declined as soil temperature dropped below 16°C. When half the root system of seedlings was chilled, shoot ψ and gas-exchange rates did not decline. Thus, nonhydraulic root-shoot signals were not implicated in stomatal limitations. The initial decrease in leaf conductance to water vapor after root chilling appeared to precede any detectable decrease in bulk fascicle ψ, but may be in response to a decrease in turgor of epidermal cells. These reductions in leaf conductance to water vapor, which occurred within 30 minutes of root chilling, could be delayed and temporarily reversed by reducing the leaf-to-air vapor-pressure deficit, suggesting that hydraulic signals may be involved in initiating stomatal closure. By independently manipulating the leaf-to-air vapor-pressure deficit of individual fascicles, we could induce uptake of water vapor through stomata, suggesting that nonsaturated conditions occur in the intercellular airspaces. There was an anomaly in our results on seedlings maintained for 2 days at soil temperatures below 7°C. Lower A appeared primarily the result of nonstomatal limitations, based on large increases in calculated ci and A versus ci analysis. In contrast, measurements of O2 evolution at saturating CO2 concentrations implied nonstomatal limitations per se did not increase at these temperatures. One explanation for this paradox is that calculations of ci are unreliable at very low gas-exchange rates because of inadequate measurement resolution, and limitations of A are predominantly stomatal. An alternative interpretation is that increases in ci are real and the results from O2-evolution measurements are in error. The high CO2 concentration used in O2-evolution measurements (15%) may have overcome nonstomatal limitations by enzymes that were down-regulated by a feedback mechanism. In this scenario, carbohydrate feedback limitations may be responsible for nonstomatal reductions in A after 2 days at soil temperatures below 7°C.  相似文献   

14.
Factors that may influence the extent of thylakoid membrane appression have been examined using lettuce (Lactuca sativa cv. Celtuce) grown under different irradiances. Electron microscopy and salt-induced chlorophyll fluorescence suggest that the percentage of membrane appression is increased in plants grown in low light (20 Wm–2) compared with those grown in high light (150 Wm–2). In high light plants surface charge, as measured by 9-aminoacridine, was found to be twice that measured in low light plants. There was a similar difference in ATPase activity of CF1 and in light saturated photophosphorylation. The chlorophyll content of LHC-2 as a proportion of the total chlorophyll was greatest in thylakoids of low light plants. Measurement of non-cyclic photophosphorylation rates suggested that membrane appression has a stimulatory role in the photophosphorylation process. The importance of these inter-related factors for the mechanism of thylakoid appression is discussed.Abbreviations PS photosystem - chl chlorophyll - LHC-2 light harvesting chlorophyll-protein complex serving PS 2 - CF1 coupling factor 1 - NADP nicotinamide-adenine dinucleotide phosphate  相似文献   

15.
Boyer JS 《Plant physiology》1971,48(5):532-536
The inhibition of photosynthesis at low leaf water potentials was studied in soil-grown sunflower to determine the degree to which photosynthesis under high light was affected by stomatal and nonstomatal factors. Below leaf water potentials of −11 to −12 bars, rates of photosynthesis at high light intensities were insensitive to external concentrations of CO2 between 200 and 400 microliters per liter. Photosynthesis also was largely insensitive to leaf temperature between 10 and 30 C. Changes in CO2 concentration and temperature had negligible effect on leaf diffusive resistance. The lack of CO2 and temperature response for both photosynthesis and leaf diffuse resistance indicates that rates of photosynthesis were not limited by either CO2 diffusion or a photosynthetic enzyme. It was concluded that photosynthesis under high light was probably limited by reduced photochemical activity of the leaves at water potentials below −11 to −12 bars.  相似文献   

16.
Effect of nano-TiO2 on photochemical reaction of chloroplasts of spinach   总被引:1,自引:0,他引:1  
The effects of nano-TiO2 (rutile) on the photochemical reaction of chloroplasts of spinach were studied. The results showed that when spinach was treated with 0.25% nano-TiO2, the Hill reaction, such as the reduction rate of FeCy, and the rate of evolution oxygen of chloroplasts was accelerated and noncyclic photophosphorylation (nc-PSP) activity of chloroplasts was higher than cyclic photophosphorylation (c-PSP) activity, the chloroplast coupling was improved and activities of Mg2+-ATPase and chloroplast coupling factor I (CF1)-ATPase on the thylakoid membranes were obviously activated. It suggested that photosynthesis promoted by nano-TiO2 might be related to activation of photochemical reaction of chloroplasts of spinach.  相似文献   

17.
The proton motive force (pmf) across the thylakoid membrane couples photosynthetic electron transport and ATP synthesis. In recent years, the electrochromic carotenoid and chlorophyll absorption band shift (ECS), peaking ∼515 nm, has become a widely used probe to measure pmf in leaves. However, the use of this technique to calculate the parsing of the pmf between the proton gradient (ΔpH) and electric potential (Δψ) components remains controversial. Interpretation of the ECS signal is complicated by overlapping absorption changes associated with violaxanthin de-epoxidation to zeaxanthin (ΔA505) and energy-dependent nonphotochemical quenching (qE; ΔA535). In this study, we used Arabidopsis (Arabidopsis thaliana) plants with altered xanthophyll cycle activity and photosystem II subunit S (PsbS) content to disentangle these overlapping contributions. In plants where overlap among ΔA505, ΔA535, and ECS is diminished, such as npq4 (lacking ΔA535) and npq1npq4 (also lacking ΔA505), the parsing method implies the Δψ contribution is virtually absent and pmf is solely composed of ΔpH. Conversely, in plants where ΔA535 and ECS overlap is enhanced, such as L17 (a PsbS overexpressor) and npq1 (where ΔA535 is blue-shifted to 525 nm) the parsing method implies a dominant contribution of Δψ to the total pmf. These results demonstrate the vast majority of the pmf attributed by the ECS parsing method to Δψ is caused by ΔA505 and ΔA535 overlap, confirming pmf is dominated by ΔpH following the first 60 s of continuous illumination under both low and high light conditions. Further implications of these findings for the regulation of photosynthesis are discussed.

Electrochromic shift absorption kinetics show the steady-state transthylakoid proton motive force in plants is dominated by the proton concentration gradient under both low and high light conditions.  相似文献   

18.
Keck RW  Boyer JS 《Plant physiology》1974,53(3):474-479
Cyclic and noncyclic photophosphorylation and electron transport by photosystem 1, photosystem 2, and from water to methyl viologen (“whole chain”) were studied in chloroplasts isolated from sunflower (Helianthus annus L. var Russian Mammoth) leaves that had been desiccated to varying degrees. Electron transport showed considerable inhibition at leaf water potentials of −9 bars when the chloroplasts were exposed to an uncoupler in vitro, and it continued to decline in activity as leaf water potentials decreased. Electron transport by photosystem 2 and coupled electron transport by photosystem 1 and the whole chain were unaffected at leaf water potentials of −10 to −11 bars but became progressively inhibited between leaf water potentials of −11 and −17 bars. A low, stable activity remained at leaf water potentials below −17 bars. In contrast, both types of photophosphorylation were unaffected by leaf water potentials of −10 to −11 bars, but then ultimately became zero at leaf water potentials of −17 bars. Although the chloroplasts isolated from the desiccated leaves were coupled at leaf water potentials of −11 to −12 bars, they became progressively uncoupled as leaf water potentials decreased to −17 bars. Abscisic acid and ribonuclease had no effect on chloroplast photophosphorylation. The results are generally consistent with the idea that chloroplast activity begins to decrease at the same leaf water potentials that cause stomatal closure in sunflower leaves and that chloroplast electron transport begins to limit photosynthesis at leaf water potentials below about −11 bars. However, it suggests that, during severe desiccation, the limitation may shift from electron transport to photophosphorylation.  相似文献   

19.
The isolation of the chloroplast ATP synthase complex (CF0-CF1) and of CF1 from Dunaliella bardawil is described. The subunit structure of the D. bardawil ATPase differs from that of the spinach in that the D. bardawil α subunit migrates ahead of the β subunit and ε-migrates ahead of subunit II of CF0 when separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The CF1 isolated from D. bardawil resembles the CF1 isolated from Chladmydomonas reinhardi in that a reversible, Mg2+-dependent ATPase is induced by selected organic solvents. Glycerol stimulates cyclic photophosphorylation catalyzed by D. bardawil thylakoid membranes but inhibits photophosphorylation catalyzed by spinach thylakoid membranes. Glycerol (20%) also stimulates the rate of ATP-Pi exchange catalyzed by D. bardawil CF0-CF1 proteoliposomes but inhibits the activity with the spinach enzyme. The ethanol-activated, Mg2+-ATPase of the D. bardawil CF1 is more resistant to glycerol inhibition than the octylglucoside-activated, Mg2+-ATPase of spinach CF1 or the ethanol-activated, Mg2+-dependent ATPase of the C. reinhardi CF1. Both cyclic photophosphorylation and ATP-Pi exchange catalyzed by D. bardawil CF0-CF1 are more sensitive to high concentrations of NaCl than is the spinach complex.  相似文献   

20.
It is of theoretical as well as practical interest to identify the components of the photosynthetic machinery that govern variability in photosynthesis rate (A) and water-use efficiency (WUE), and to define the extent by which the component processes limit A and WUE during developing water-deficit stress. For that purpose, leaf exchange of CO2 and H2O was determined in two growth-chamber-grown wheat cultivars (Triticum aestivum L. cv TAM W-101 and cv Sturdy), and the capacity of A was determined and broken down into carboxylation efficiency (c.e.), light- and CO2-saturated A, and stomatal conductance (gs) components. The limitations on A measured at ambient CO2 concentration (A350) were estimated. No cultivar difference was observed when A350 was plotted versus leaf water potential (Ψw). Light- and CO2-saturated A, c.e., and gs decreased with decreasing leaf Ψw, but of the corresponding photosynthesis limitations only those caused by insufficient c.e. and gs increased. Thus, reduced stomatal aperture and Calvin cycle activity, but not electron transport/photophosphorylation, appeared to be major reasons for drought stress-induced inhibition of A350. WUE measured as A350/gs first increased with stomatal closure down to a gs of about 0.25 mol H2O m−2 s−1w = −1.6 MPa). However, it was predicted that A350/gs would decrease with more severe stress due to inhibition of c.e.  相似文献   

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