Photorespiration is More Effective than the Mehler Reaction in Protecting the Photosynthetic Apparatus against Photoinhibition

I. Isolated intact chloroplasts: Photosystem II, but not photosystem I, of the electron transport chain is rapidly photoinactivated even by very low intensities of red light when no large proton gradient can be formed and the electron transport chain becomes over-reduced in the absence of oxygen and other reducable substrates. Electron acceptors including oxygen provide protection against photoinactivation. Nevertheless, photosystem II is rapidly, and photosystem I more slowly, photoinactivated by high intensities of red light when oxygen is the only electron acceptor available. Increased damage is observed at increased oxygen concentrations although catalase is added to destroy H₂O₂ formed during oxygen reduction in the Mehler reaction. Photoinactivation can be decreased, but not prevented by ascorbate which reduces hydrogen peroxide inside the chloroplasts and increases coupled electron flow. II. Leaves: Simple measurements of chlorophyll fluorescence permit assessment of damage to photosystem II after exposure of leaves to high intensity illumination. In contrast to isolated chloroplasts, chloroplasts suffer more damage in situ at reduced than at elevated oxygen concentrations. The difference in the responses is due to photorespiration which is active in leaves, but not in isolated chloroplasts. After photosynthesis and photorespiration are inhibited by feeding glyceraldehyde to leaves, photoinactivation is markedly increased, although oxygen reduction in the Mehler reaction is not affected by glyceraldehyde. In the presence of reduced CO₂ levels, photorespiratory reactions, but not the Mehler reaction, can prevent the overreduction of the electron transport chain. Over-reduction indicates ineffective control of photosystem II activity. Effective control is needed for protection of the electron transport chain against photoinactivation. It is suggested to be made possible by coupled cyclic electron flow around photosystem I which is facilitated by the redox poising resulting from the interplay between photorespiratory carbohydrate oxidation and the refixation of evolved CO₂.     

Light-induced changes in chlorophyll a fluorescence and cytochrome f in intact spinach chloroplasts: The site of light-dependent regulation of electron transport

Dark-adapted intact spinach chloroplasts exhibited two peaks,P and M₁, at the early phase of fluorescence induction and atransient reduction of cytochrome f shortly after its initialphotooxidation and in parallel to the appearance of P. Analysisof the peak P and the transient reduction of cytochrome f indicatedthat electron transport in intact spinach chloroplasts was regulatedby light: electron transport was inactivated at the reducingside of photosystem I in the dark-adapted chloroplasts but rapidlyreactivated by illumination. The fluorescence peak M₁ was correlatedto the proton gradient formed across the thylakoid membrane. Effects on P and transient reduction of cytochromef of NO₂^–,3-phosphoglycerate (PGA) and oxalacetate (OAA), which can penetrateinto intact chloroplasts and accept electrons at different sitesafter photosystem I, were studied to determine the site of thelight regulation. NC₂^–, which receives electrons fromreduced ferredoxin, markedly diminished both P and the transientreduction of cytochrome.f, whereas PGA and OAA, the reductionsof which are NADP-dependent, failed to affect the two transients.The ineffectiveness of PGA and OAA could not be attributed tothe dark inactivation of glyceraldehyde-3-phosphate and malicdehydrogenases, because dark-adapted chloroplasts still retainedsufficiently high levels of the enzyme activities. The resultsindicate that electron transport in intact spinach chloroplastsis regulated by light after ferredoxin but before NADP, i.e.,at the reducing terminal of the electron transport chain. (Received May 29, 1980; )     

Photosynthetic reactions of chloroplasts with unusual structures

Photosynthetic reactions of whole leaves and isolated chloroplasts from various mutants of Nicotiana tabacum have been correlated to the lamellar structure seen in electron micrographs of the chloroplasts. In this way it could be established that a fully active photosystem I can be associated with single unfolded thylakoids. The complete photosynthetic electron transport system including the oxygen evolving apparatus of photosystem II, on the other hand, appears to require a close packing of at least 2 thylakoids. The unusual high capacity for photosynthesis observed earlier for leaves of certain aurea mutants is reflected by a correspondingly high activity of the isolated chloroplasts in the Hill reaction. These chloroplasts contain extended areas where 2 thylakoids touch by forming simple lamellar overlappings instead of the familiar stacks of lamellar discs.     

Studies on the Mechanism of Photoinhibition in Higher Plants: I. EFFECTS OF HIGH LIGHT INTENSITY ON CHLOROPLAST ACTIVITIES IN CUCUMBER ADAPTED TO LOW LIGHT

Cucumber plants (Cucumis sativus L.), grown at low quantum flux density (120-150 microeinsteins per square meter per second) were photoinhibited by a three-hour exposure in air to ten times the light intensity experienced during growth. Chloroplasts were isolated from photoinhibited and control leaves and the following activities determined: O₂ evolution in the presence of ferricyanide, photosystem I activity, noncyclic and cyclic photophosphorylation, and light-induced proton uptake. Chlorophyll and chloroplast absorbance spectra, and chloroplast fluorescence were also measured. It was found that photosystem II electron transport and non-cyclic photophosphorylation were inhibited by about 50%, while cyclic photophosphorylation was less inhibited and photosystem I electron transport and light-induced proton uptake were unaffected. Electron transport to methylviologen could not be fully restored by electron donation to photosystem II. Chloroplast fluorescence induction at room temperature was strongly reduced following photoinhibition. There was no difference in the absorption spectra of the extracted chlorophylls from control and photoinhibited chloroplasts, but an increase of the absorption in the blue wavelength region was observed in the photoinhibited chloroplasts. It is suggested that high light stress does not result in alteration of the membrane properties, as is the case in low-temperature stress for example, but affects directly the photosynthetic reaction centers, primarily of photosystem II.     

Rates of vectorial proton transport supported by cyclic electron flow during oxygen reduction by illuminated intact chloroplasts

The light-dependent quenching of 9-aminoacridine fluorescence was used to monitor the state of the transthylakoid proton gradient in illuminated intact chloroplasts in the presence or absence of external electron acceptors. The absence of appreciable light-dependent fluorescence quenching under anaerobic conditions indicated inhibition of coupled electron transport in the absence of external electron acceptors. Oxygen relieved this inhibition. However, when DCMU inhibited excessive reduction of the plastoquinone pool in the absence of oxygen, coupled cyclic electron transport supported the formation of a transthylakoid proton gradient even under anaerobiosis. This proton gradient collapsed in the presence of oxygen. Under aerobic conditions, and when KCN inhibited ribulose bisphosphate carboxylase and ascorbate peroxidase, fluorescence quenching indicated the formation of a transthylakoid proton gradient which was larger with oxygen in the Mehler reaction as electron acceptor than with methylviologen at similar rates of linear electron transport. Apparently, cyclic electron transport occured simultaneously with linear electron transport, when oxygen was available as electron acceptor, but not when methylviologen accepted electrons from Photosystem I. The ratio of cyclic to linear electron transport could be increased by low concentrations of DCMU. This shows that even under aerobic conditions cyclic electron transport is limited in isolated intact chloroplasts by excessive reduction of electron carriers. In fact, P₇₀₀ in the reaction center of Photosystem I remained reduced in illuminated isolated chloroplasts under conditions which resulted in extensive oxidation of P₇₀₀ in leaves. This shows that regulation of Photosystem II activity is less effective in isolated chloroplasts than in leaves. Assuming that a Q-cycle supports a H⁺/e ratio of 3 during slow linear electron transport, vectorial proton transport coupled to Photosystem I-dependent cyclic electron flow could be calculated. The highest calculated rate of Photosystem I-dependent proton transport, which was not yet light-saturated, was 330 mol protons (mg chlorophyll h)^–1 in intact chloroplasts. If H⁺/e is not three but two proton transfer is not 330 but 220 mol (mg Chl H)^–1. Differences in the regulation of cyclic electron transport in isolated chloroplasts and in leaves are discussed.     

Absence of Variable Fluorescence from Guard Cell Chloroplasts of Stenotaphrum secundatum

The lack of detectable variable fluorescence from guard cell chloroplasts in both the albino and green portions of variegated leaves of St. Augustine grass (Stenotaphrum secundatum var variegatum A.S. Hitchc.) is reported. Fluorescence was measured either with a highly sensitive, modified fluorescence microscope which was capable of recording fluorescence induction curves from single chloroplasts, or with a spectrofluorometer. Both fast and slow fluorescence transients from S. secundatum guard cells showed a rapid rise and then remained at a steady level. Neither variable fluorescence increase (induction) nor decrease (quenching), properties normally associated with photosystem II, was observed from these chloroplasts. These fluorescence kinetics did not change either with alterations of the specimen preparation procedure or with alterations of the excitation light intensities and wavelengths. These results indicate that guard cell chloroplasts in this variety of S. secundatum do not conduct normal photosystem II electron transport. Light regulation of stomatal conductance in intact leaves of this plant did occur, however, and was similar to light regulation observed in other species. The conductance of the green portion of the leaves was much greater in the light than in the dark, and was much greater than the conductance of the albino portion of the leaves. Stomata in the green portion of the leaves also showed greater opening in blue light than in red light. These results provide evidence that stomatal regulation in this variety of S. secundatum does not rely on photosystem II electron transport in guard cell chloroplasts.     

Chloroplast Reactions of Photosynthetic Mutants in Zea mays

Three seedling lethal mutants of Zea mays with impaired photosynthesis are described. These recessive mutants were selected on the basis of high chlorophyll fluorescence. They have normal chlorophyll pigmentation but are unable to fix CO₂ fully. Evidence is presented from fluorescence characteristics of isolated chloroplasts that both photosystem I and II mutants were isolated. Using conventional measures of photosynthetic electron transport, we suggest that the photosystem I mutant has limited ability to reduce NADP. The other two mutants are clearly blocked in photosystem II, one possibly lacking the primary electron acceptor.     

Oxygen Evolution and Oxygen Uptake by Isolated Chloroplasts of Wheat Irradiated with Monochromatic Light, without the Addition of an Oxidant

The oxygen exchange obtained when isolated chloroplasts of wheat are irradiated, without the addition of a Hill oxidant, has been investigated. Depending on the wavelength, two types of oxygen exchange are obtained. In light absorbed by both photosystems an oxygen gush appears directly upon irradiation. This oxygen evolving reaction is soon replaced by an oxygen uptake which is present until the end of the irradiation period. In light absorbed mainly in photosystem I, no oxygen gush can be observed, instead an oxygen uptake appears directly upon irradiation. An oxygen evolving process can also be observed in irradiations performed with photo-system I light, but this process appears after 10–15 seconds of irradiation. The influence of various external factors on the oxygen gush and the oxygen uptake, e.g. different wavelengths, light intensity, length of the dark periods between irradiations, was studied. The results show that the oxygen evolving reaction appearing upon irradiation with light absorbed by photosystem II and I, reflect the reduction of an oxidant, probably plasto-quinone, in the electron transport chain between the two photosystems. The reoxidation of this oxidant can be brought about after irradiating with light absorbed in photosystem I, or by prolonging the dark period between irradiations, or through some unknown process connected to photosystem II. The oxygen uptake which consists of two components, one appearing directly upon irradiation and the other one appearing after about 10 seconds of irradiation, confirms earlier observations that oxygen can be reduced in photosystem I. The electrons for the oxygen uptake appearing directly upon irradiation, are obtained from the reduced intermediates in the electron transport chain between the two photosystems. The electrons for the other oxygen uptake process are obtained from a reductant in the chloroplasts with access to the carrier chain between the photosystems. Whether the two oxygen uptake reactions reflect two sites of interaction of oxygen with the electron transport chain or only one site is discussed.     

Regulation of excitation energy in a wheat mutant deficient in light-harvesting pigment protein complex

Chloroplasts of the CD3 wheat mutant were deficient primarily in chlorophyll of light harvesting pigment proteins (LHPP) 1 and 2 and CP1a. The reduced level of protein associated with chlorophyll of LHPP1 and LHPP2 and the reduced level of low molecular weight polypeptides between 23 and 29 kilodaltons confirmed that the CD3 mutant was deficient in the LHPP complex. The high fluorescence emission ratio at 740 (F740) to 686 nanometers (F686) observed from chloroplasts of normal wheat following light induced phosphorylation of the LHPP complex was not noted from mutant chloroplasts. The long wavelength peak fluorescence emission (F740) was shifted to a shorter wavelength peak (F725) and was reduced in intensity compared to that of normal wheat thylakoids. The ratio of variable fluorescence to maximum fluorescence, a measure of PSII photochemical efficiency, was the same for the normal wheat and mutant leaves. The ratios of uncoupled photosystem I/photosystem II electron transport rates for mutant and normal wheat chloroplasts were similar at saturating light suggesting that absorbed excitation energy was distributed to the two photosystem reaction centers of the mutant in a similar manner as in the normal wheat. Proteins of the LHPP complex were differentially phosphorylated by action of a membrane protein kinase when both normal wheat and CD3 mutant thylakoids were irradiated without an electron transport chain acceptor. Even though the F740/F686 ratio was low in mutant thylakoids, the phosphorylation of the 27-kilodalton LHPP polypeptide was consistent with the mutant being in a state II condition. The data gave rise to the suggestion that the F740/F686 ratio might not indicate excitation energy distribution to the two photosystems in the mutant.     

Photosynthetic activities in the petunia corolla

Pink Petuniahybrida (cv Hit Parade Rosa) corollas were found to contain photosynthetically active chloroplasts. The corolla chloroplasts were similar to those of green leaves in size and structure. The chlorophyll (Chl) content of Petunia corollas increased during early stages of flower development, reaching a maximum just before anthesis. Chloroplasts isolated from corollas at this stage, carried out photosystem I-dependent electron transport at rates which were two-thirds of those measured in chloroplasts from green leaves, but full chain electron transport at only one-quarter of the rate carried out by chloroplasts from green leaves. Both the light saturated rate and the quantum yield for electron transport were lower in corolla chloroplasts, which also required lower intensities for light saturation. Reduced efficiency of photosystem II photoreactions in the corolla was also indicated by the ratio between variable and constant components of Chl fluorescence, which was lower in corollas compared to green leaves. The induction time of Chl fluorescence was at least three times shorter in corollas compared to green leaves, indicating a smaller number of functional photosystem II centers (per Chl) in the corolla. It is suggested that corolla chloroplasts of Petunia might have a role in flower developmental processes.     

Photoinhibition of photosynthesis in Lemna gibba as induced by the interaction between light and temperature. II. Photosynthetic electron transport

Photoinhibition of photosynthesis in Lemna gibba L. was induced by exposing intact plants to a high photosynthetic photon flux density of 1 750 μmol m⁻² s⁻¹ at a low temperature of 3°C. Subsequently isolated chloroplasts showed pronounced reductions in the capacity of whole chain electron transport, measured as Hill activity, and in the efficiency of electron transport to the primary electron acceptor Q of photosystem II, measured as variable chlorophyll fluorescence at 20°C. These changes proceeded with similar kinetics (probably of the first-order reaction), suggesting that the site of photoinhibition is in the electron transfer to Q. A partial uncoupling of the whole chain electron transport also occured. The capacity of electron transport mediated by photosystem I was unaffected. The extent of photoinhibition of photosynthetic electron transport, as produced by a 2 h exposure of L. gibba to three different combinations of photon flux density and temperature was studied. It was shown that intrinsically similar states of photoinhibition, on the evidence of their time courses of recovery, were induced by either a high photon flux density and 25°C or by a moderate photon flux density and 3°C.     

Localization of the Meler's reaction with ethanol catalase trap in the chain of photosynthetic electron transport]

The common view of photosystem I as the action site of catalase and ethanol at oxygen uptake in chloroplasts are based on indirect data on this reaction. That is why the question on Mehler reaction localization in electron transport chain with ethanolcatalase trap has been investigated anew. It has been demonstrated that oxygen uptake with catalase and ethanol does not decrease in presence of dibromothymoquinone (2,5-dibromo-3-methyl-6 isopropyl-p-benzoquinone--DBTQ) which blocks electron transfer to photosystem I at plastoquinones level. The summation of oxygen uptake activities is observed on the combined action of catalase and ethanol with any of the Mehler reagents functioning in photosystem I (methylviologen,FMN, epinephrine, ferredoxin). Catalase and ethanol in contrast to methylviologen have no effect on photooxidation rate of reduced dichlorphenolindophenol in photosystem I. The quatum yield of oxygen uptake with catalase and ethanol versus wave length of actinic light shows a distinct maximum in the photosystem II absorption area and a "red drop" in the longware area. The obtained data show that the Mehler reaction with catalase and ethanol takes place in photosystem II only.     

Effects of winter stress on photosynthetic electron transport and energy distribution between the two photosystems of pine as assayed by chlorophyll fluorescence kinetics

The fluorescence kinetics of both intact needles and isolated chloroplasts of summer active and winter stressed Pinus sylvestris were measured at both room temperature and 77 K. It was confirmed that winter stress inhibited the photochemical capacity of photosystem II but also that winter stress caused the strongest inhibition of the electron transport at the site where the plastoquinone pool is reduced. Parallel analyses of the fluorescence characteristics of photosystem II (F₆₉₃) and photosystem I (F₇₂₉) during photosystem II trap closure furthermore revealed that the yield of spillover of excitation energy from photosystem II to photosystem I decreased upon winter stress. We suggest that this is because of an increased radiationless decay of excitation energy both at the reaction center and antennae levels of photosystem II. There is, however, also a possibility that the decreased yield of spill-over is accentuated by a partial detachment of the light harvesting chlorophyll a/b complex from photosystem II upon winter stress.Paper presented at the FESPP meeting in Strasbourg (1984).     

Kinetics Studies on the Fluorescence Quencher in Isolated Chloroplasts

The chlorophyll fluorescence yield in isolated chloroplasts without an added electron acceptor is increased by actinic illumination. The decline in the fluorescence yield when the actinic illumination is extinguished can be accurately represented by three, independent, exponential decays with half-times of approximately 0.8, 5, and 30 sec. These results have been interpreted using Duysens' theory of fluorescence quenching by a compound (Q) on the reducing side of photosystem II. This theory states that changes in fluorescence yield are indicative of electron flow through Q. The most rapid decay is eliminated by an EDTA washing of the chloroplasts and the half-time is increased by uncoupling with ammonia and by added electron acceptors in suboptimal concentrations. Thus, this decay may represent electron flow from Q to intermediates on the oxidizing side of photosystem I. The decay with a half-time of 5 sec is affected in the same manner as the decay with the shortest half-time by the same procedures. However, electron donors to photosystem II lengthen the half-time of the 5 sec decay while eliminating the most rapid decay. This 5 sec decay can be interpreted as electron flow from Q to intermediates either on the reducing side of photosystem II or on the oxidizing side of photosystem I. The decay with the longest half-time is affected only by pH and electron donors to photosystem II. Therefore, this decay may indicate electron flow from Q to intermediates on the oxidizing side of photosystem II which may be connected to the regeneration of the oxygen burst.     

Photoinhibition of photosynthesis: effect on chlorophyll fluorescence at 77K in intact leaves and in chloroplast membranes of Nerium oleander

The effect of exposing intact leaves and isolated chloroplast membranes of Nerium oleander L. to excessive light levels under otherwise favorable conditions was followed by measuring photosynthetic CO₂ uptake, electron transport and low-temperature (77K=-196°C) fluorescence kinetics. Photoinhibition, as manifested by a reduced rate and photon (quantum) yield of photosynthesis and a reduced electron transport rate, was accompanied by marked changes in fluorescence characteristics of the exposed upper leaf surface while there was little effect on the shaded lower surface. The most prominent effect of photoinhibitory treatment of leaves and chloroplasts was a strong quenching of the variable fluorescence emission at 692 nm (F_v,692) while the instantaneous fluorescence (F_o,692) was slightly increased. The maximum and the variable fluorescence at 734 nm were also reduced but not as much as F_M,692 and F_v,692. The results support the view that photoinhibition involves an inactivation of the primary photochemistry of photosystem II by damaging the reaction-center complex. In intact leaves photoinhibition increased with increased light level, increased exposure time, and with decreased temperature. Increased CO₂ pressure or decreased O₂ pressure provided no protection against photoinhibition. With isolated chloroplasts, inhibition of photosystem II occurred even under essentially anaerobic conditions. Measurements of fluorescence characteristics at 77K provides a simple, rapid, sensitive and reproducible method for assessing photoinhibitory injury to leaves. The method should prove especially useful in studies of the occurrence of photoinhibition in nature and of interactive effects between high light levels and major environmental stress factors.Abbreviations and symbols PFD photon flux area density - PSI, PSII photosystem I, II - F_M, F_O, F_V maximum, instantaneous, variable fluorescence emission C.I.W.-D.P.B. Publication No. 773     

Inhibition of Photosystem II in Isolated Chloroplasts by Lead

Inhibition of photosynthetic electron transport in isolated chloroplasts by lead salts has been demonstrated. Photosystem I activity, as measured by electron transfer from dichlorophenol indophenol to methylviologen, was not reduced by such treatment. However, photosystem II was inhibited by lead salts when electron flow was measured from water to methylviologen and Hill reaction or by chlorophyll fluorescence. Fluorescence induction curves indicated the primary site of inhibition was on the oxidizing side of photosystem II. That this site was between the primary electron donor of photosystem II and the site of water oxidation could be demonstrated by hydroxylamine restoration of normal fluorescence following lead inhibition.     

Regulation of photosynthetic electron transport and photophosphorylation in intact chloroplasts and leaves of Spinacia oleracea L.

Oxygen ist reduced by the electron transport chain of chloroplasts during CO₂ reduction. The rate of electron flow to oxygen is low. Since antimycin A inhibited CO₂-dependent oxygen evolution, it is concluded that cyclic photophosphorylation contributes ATP to photosynthesis in chloroplasts which cannot satisfy the ATP requirement of CO₂ reduction by electron flow to NADP and to oxygen. Inhibition of photosynthesis by antimycin A was more significant at high than at low light intensities suggesting that cyclic photophosphorylation contributes to photosynthesis particularly at high intensities. Cyclic electron flow in intact chloroplasts is under the control of electron acceptors. At low light intensities or under far-red illumination it is decreased by substrates which accept electrons from photosystem I such as oxaloacetate, nitrite or oxygen. Obviously, the cyclic electron transport pathway is sensitive to electron drainage. In the absence of electron acceptors, cyclic electron flow is supported by far-red illumination and inhibited by red light. The inhibition by light exciting photosystem II demonstrated that the cyclic electron transport pathway is accessible to electrons from photosystem II. Inhibition can be relieved by oxygen which appears to prevent over-reduction of electron carriers of the cyclic pathway and thus has an important regulatory function. The data show that cyclic electron transport is under delicate redox control. Inhibition is caused both by excessive oxidation and by over-reduction of electron carriers of the pathway.     

The synergistic effect of drought and light stresses in sorghum and pearl millet

The effects of drought stress and high irradiance and their combination were studied under laboratory conditions using young plants of a very drought-resistant variety, ICMH 451, of pearl millet (Pennisetum glaucum) and three varieties of sorghum (Sorghum bicolor)—one drought-resistant from India, one drought-tolerant from Texas, and one drought-sensitive variety from France. CO₂ assimilation rates and photosystem II fluorescence in leaves were analyzed in parallel with photosynthetic electron transport, photosystem II fluorescence, and chlorophyll-protein composition in chloroplasts isolated from these leaves. High irradiance slightly increased CO₂ assimilation rates and electron transport activities of irrigated plants but not fluorescence. Drought stress (less than −1 megapascal) decreased CO₂ assimilation rates, fluorescence, and electron transport. Under the combined effects of drought stress and high irradiance, CO₂ assimilation rates and fluorescence were severely inhibited in leaves, as were the photosynthetic electron transport activities and fluorescence in chloroplasts (but not photosystem I activity). The synergistic or distinctive effect of drought and high irradiance is discussed. The experiments with pearl millet and three varieties of sorghum showed that different responses of plants to drought and light stresses can be monitored by plant physiological and biochemical techniques. Some of these techniques may have a potential for selection of stress-resistant varieties using seedlings.     

Absorption and Transfer of Light and Photoreduction Activities of Spinach Chloroplasts under Calcium Deficiency: Promotion by Cerium

Chloroplasts were isolated from spinach cultured in calcium-deficient, cerium-chloride-administered calcium-present Hoagland’s media or that of calcium-deficient Hoagland’s media and demonstrated the effects of cerium on distribution of light energy between photosystems II and I and photochemical activities of spinach chloroplast grown in calcium-deficient media. It was observed that calcium deprivation significantly inhibited light absorption, energy transfer from LHCII to photosystemII, excitation energy distribution from PSI to PSII, and transformation from light energy to electron energy and oxygen evolution of chloroplasts. However, cerium treatment to calcium-deficient chloroplasts could obviously improve light absorption and excitation energy distribution from photosystem I to photosystem II and increase activity of whole chain electron transport, photosystems II and I DCPIP photoreduction, and oxygen evolution of chloroplasts. The results suggested that cerium under calcium deficiency condition could substitute for calcium in chloroplasts, maintain the stability of chloroplast membrane, and improve photosynthesis of spinach chloroplast, but the mechanisms still need further study.     

Fluorescence Properties of Guard Cell Chloroplasts: EVIDENCE FOR LINEAR ELECTRON TRANSPORT AND LIGHT-HARVESTING PIGMENTS OF PHOTOSYSTEMS I AND II

The presence of chloroplasts in guard cells from leaf epidermis, coleoptile, flowers, and albino portions of variegated leaves was established by incident fluorescence microscopy, thus confirming the notion that guard cell chloroplasts are remarkably conserved. Room temperature emission spectra from a few chloroplasts in a single guard cell of Vicia faba showed one major peak at around 683 nanometers. Low-temperature (77 K) emission spectra from peels of albino portions of Chlorophytum comosum leaves and from mesophyll chloroplasts of green parts of the same leaves showed major peaks at around 687 and 733 nanometers, peaks usually attributed to photosystem II and photosystem I pigment systems, respectively. Spectra of peels of V. faba leaves showed similar peaks. However, fluorescence microscopy revealed that the Vicia peels, as well as those from Allium cepa and Tulipa sp., were contaminated with non-guard cell chloroplasts which were practically undetectable under bright field illumination. These observations pose restrictions on the use of epidermal peels as a source of isolated guard cell chloroplasts. Studies on the 3-(3,4-dichlorophenyl)-1,1-dimethylurea-sensitive variable fluorescence kinetics of uncontaminated epidermal peels of C. comosum indicated that guard cell chloroplasts operate a normal, photosystem II-dependent, linear electron transport. The above properties in combination with their reported inability to fix CO₂ photosynthetically may render the guard cell chloroplasts optimally suited to supply the reducing and high-energy phosphate equivalents needed to sustain active ion transport during stomatal opening in daylight.