Verteilung und Wanderung von Phosphoglycerat zwischen den Chloroplasten und dem Cytoplasma während der Photosynthese

The distribution of phosphoglyceric acid (PGA) * between chloroplasts and cytoplasm of leaf cells during transients from dark to light and vice versa has been investigated. The data indicate that pools of PGA in the chloroplasts and cytoplasm are interchangeable and that PGA may function as a transport metabolite in actively metabolising leaf cells. These views are supported by the following results:

1. In the presence of 14CO2 and light, labelled PGA rapidly appears in the cytoplasm, even though the carboxydismutase reaction, in which 14C enters into PGA, proceeds in the chloroplasts.

2. After less than 1 min. illumination in the presence of 14CO2, the distribution of labelled PGA between chloroplasts and cytoplasm reaches an equilibrium, which is then maintained. The same distribution is to be found by enzymatic analyses of the total pools of PGA in chloroplasts and cytoplasm. When equilibrium is reached, the percentages of both 14C labelled and of total PGA to be found in the chloroplasts of Spinach and Elodea are approximately 75% and 35 —40% respectively.

3. In both the chloroplasts and the cytoplasm, the levels of PGA first decrease after illumination to a fraction of the original dark levels and then show a concomitant slow increase. On darkening a further very rapid increase in PGA occurs in chloroplasts and cytoplasm.

4. In photosynthetically active leaf material the rate of decrease in the level of cytoplasmic PGA, as observed after 12 —15 secs. illumination, is higher than the turnover rate of PGA in respiration.

5. Upon illumination, aqueously isolated chloroplasts, suspended in isotonic sucrose buffer, reduce added PGA to dihydroxyacetone phosphate and other products far faster than they reduce added NADP. Whereas PGA reduction is not increased by ultrasonic disintegration of the chloroplasts, the reduction of NADP is stimulated. This indicates that whereas the movement of NADP is prevented by a permeability barrier, the transferance of PGA across the chloroplast membrane occurs easily.

6. In illuminated Elodea shoots the inhibition of metabolism by cyanide after 15 secs. photosynthesis in the presence of Η14CO3 ⊖ leads to a rapid decrease in PGA. This applies to both the 14C labelled PGA and the total PGA to a similar extent. The decrease in PGA amounts from 70 — 85% of the original dark levels. Since the chloroplasts of Elodea contain only 35—40% of the total PGA of the cell, a fall in the level of PGA as a result of the cyanide poisoning obviously occurs not only in the chloroplasts, but also in the cytoplasm. Since cyanide effectively inhibits cytochrome oxidase, while PGA reduction in the chloroplasts is relatively resistant, the large decrease in PGA suggests that part of the cytoplasmic PGA is transferred into the chloroplasts and reduced there.

Die Verteilung des Orthophosphates auf Plastiden, Cytoplasma und Vacuole in der Blattzelle und ihre Veränderung im Licht-Dunkel-Wedisel

The distribution of orthophosphate between chloroplasts and the nonchloroplast parts of leaf cells has been investigated. Upon illumination, the percentage of the total orthophosphate located in the chloroplasts decreases rapidly, while darkening results in a slow increase of that percentage to the original dark value. If, in the dark, comparable concentrations of orthophosphate in the chloroplasts and in the cytoplasm are assumed, the concentration of orthophosphate in the vacuole can be calculated from the available data. Values are then obtained, which are considerably lower than the corresponding values in the protoplasm; this indicates that a concentration gradient for orthophosphate is maintained between the cytoplasm and the vacuole.

Leaves of spinach and Elodea, which were fed 32P for 2 to 4 hours and then left to stand for two or three days, showed higher specific activities of orthophosphate in the chloroplasts than in the nonchloroplast parts of the cells. Since there is reasn to assume a fairly rapid exchange of orthophosphate between chloroplasts and cytoplasm, the differences in the specific activities which were observed several days after feeding with 32P, point to a very slow exchange of orthophosphate across the tonoplast membrane.

I. Intrazellulärer Transport von Zwischenprodukten der Photosynthese im Photosynthese-Gleichgewicht und im Dunkel-Licht-Dunkel-Wechsel

1. The distribution — before, during and after photosynthesis — of different phosphate esters in chloroplasts and cytoplasm of leaf cells of spinach and Elodea has been investigated. In steady state experiments intact leaves were fed, during illumination, with 14CO2. The kinetics of the distribution of labelled phosphate esters between chloroplasts and cytoplasm were determined. In further experiments with intact leaves fluctuations in the pool sizes of phosphate esters in chloroplasts and in the cytoplasm were recorded in the dark/light and the light/dark transient. Independent fluctuations served as an indication that little or no exchange between chloroplasts and cytoplasm takes place. Concomitant fluctuations suggest rapid exchange.

2. Although labelling takes place in the chloroplasts, a number of labelled phosphorylated intermediates appear rapidly in the cytoplasm during steady state photosynthesis of intact leaves in the presence of 14CO2. This is particularly true for phosphoglyceric acid, glucose-6-phosphate, fructose-6-phosphate and fructose-1,6-diphosphate. On the other hand, labelled ribulosediphosphate, sedoheptulosediphosphate and sedoheptulosemonophosphate remain largely in the chloroplasts. This agrees with earlier work on the behaviour of phosphorylated intermediates during the induction period of photosynthesis.

3. In the dark the level of the bulk of the sugar diphosphates is lower in the chloroplasts than in the cytoplasm of intact leaves. On illumination, a large accumulation occurs only in the chloroplasts. This behaviour suggests impermeability of the chloroplast membrane towards at least some of the sugar diphosphates. In contrast, concomitant large fluctuations in the levels of dihydroxyacetonephosphate and fructose-1,6-diphosphate have been observed during the transients from dark to light and vice versa in chloroplasts and cytoplasm alike indicating that at least one of these compounds functions as a transport metabolite. Changes in the concentrations of glucose-6-phosphate and fructose-6-phosphate were much smaller under the influence of light than those of other sugar phosphates.

4. The results demonstrate the role of phosphorylated transport metabolites in carbon metabolism in chloroplasts and cytoplasm. Implications of these findings in relation to photosynthesis, respiration and the regulation of metabolism are discussed.

Age-related differences in frost sensitivity of the photosynthetic apparatus of two Plagiomnium species

Shoots of two species of moss, Plagiomnium undulatum (Hedw.) Kop. and Plagiomnium affine (Funck) Kop., were subjected to freezing at various temperatures. After thawing, the activities of different photosynthetic reactions were determined in relation to the ages of the leaves. Analysis of the fast kinetics of chlorophyll-a fluorescence of individual leaves showed that young and old tissues were considerably less frost tolerant than mature ones. In principle, the pattern of freeze inactivation of photosynthetic reactions resembles that observed in higher plants. The decreases in the amplitude of Fv (variable fluorescence) and the ratio of Fv to Fm (maximum fluorescence) with increasing freezing stress reflect a progressive inactivation of photosystem II (PSII)-mediated electron transport, i.e. inhibition of photoreaction to photochemistry and-or electron donation to the photochemical reaction, and thus a decline in the potential photochemical efficiency of PSII. The insignificant change in the F0 (constant fluorescence) level during progressive decline of Fv indicates that the excitation-energy transfer between antenna pigments and from those to reaction centres of PSII was little impaired by lethal freezing stress. Sugar analyses of various stem sections showed that ontogenetic variation in the frost tolerance of leaves cannot be attributed to differences in the cellular levels of sucrose, glucose and fructose.

Freezing of isolated thylakoid membranes in complex media. VI. The effect of pH

Thylakoid membranes isolated from spinach leaves (Spinacia oleracea L. cv. Monatol) were used as a model biomembrane system for evaluating the significance of the hydrogen ion activity for cryoprotection. After freeze-thaw treatment in a buffered complex medium adjusted to various pH, light-induced photosynthetic membrane reactions were determined at optimum proton concentration. When thylakoids were suspended at hydrogen ion activities above and below the physiologically important pH range, irreversible inhibition of membrane functions was significantly less distinct after freezing at -15 degrees C than after storage for the same time at 0 degree C. It is suggested that thylakoid preservation at subfreezing temperatures could be due to temperature- and concentration-induced changes of the proton activity in the unfrozen part of the system and retardation of the temperature-dependent aging processes of the isolated membranes. In addition, the increase in the concentration of cryoprotective compounds during freezing could stabilize chloroplast membranes against the deleterious effect of unfavorable high and low proton concentrations. Thylakoid injury brought about by lowering the pH was primarily due to dissociation of the chloroplast coupling factor (CF1), which increased the proton permeability of the membranes and caused inhibition of photophosphorylation. In media adjusted to more alkaline pH, inactivation of the water oxidation system was an initial result of membrane damage. Then, noncyclic photophosphorylation was limited by photosystem II-mediated electron flow. Photosystem I-driven electron transport was substantially more stable over a wide pH range.

Freezing of isolated thylakoid membranes in complex media. V. Inactivation and protection of electron transport reactions

When chloroplast thylakoid membranes isolated from spinach leaves (Spinacia oleracea L. cv. Monatol) were frozen in media containing the predominant inorganic electrolytes of the chloroplast stroma, linear photosynthetic electron transport became progressively inhibited. After onset of freezing, both PSII- and PSI-mediated electron flow were inactivated almost to the same extent. Prolonged storage of the membranes in the frozen state increased damage to PSII relative to PSI activity. Under these conditions, a preferential injury of the water oxidation system was not observed. In thylakoids stored at 0 °C, PSI activity remained fairly unimpaired but inactivation of PSII occurred with strongest inhibition at the oxidizing side.The addition of low-molecular-weight cryoprotectants such as glycerol, sugars, certain amino acids and carbonic acids to thylakoid suspensions prior to freezing provided almost complete preservation of PSI activity and considerable but incomplete stabilization of PSII.

Effects of the sesquiterpene lactone tetraesters thapsigargicin and thapsigargin, from roots of Thapsia garganica L., on isolated spinach chloroplasts

The effect of thapsigargicin and thapsigargin, extracted from the roots of Thapsia garganica L., on isolated photosynthetic membranes (thylakoids) and intact chloroplasts from spinach leaves (Spinacia oleracea L.) was investigated. Both sesquiterpene lactone tetraesters impair membranes and organelles in an identical, chlorophyll-dependent manner. In thylakoids these compounds primarily act as inhibitors of photophosphorylation. At lower sesquiterpene lactone tetraester/chlorophyll ratios, cyclic and non-cyclic photophosphorylation, ADP-stimulated electron transport and the photosynthetic control ratio progressively decreased with increasing concentrations of thapsigargicin and thapsigargin, whereas the state 4 electron flow, the coupling efficiency of photophosphorylation, the light-induced proton gradient, and the H+ flux across the membranes remained nearly unaffected. Half-maximal inhibition of photophosphorylation was obtained with 4-5 X 10(-7) moles sesquiterpene lactone tetraesters per mg chlorophyll. At higher sesquiterpene lactone tetraester/chlorophyll ratios, uncoupling of photophosphorylation from electron transport occurred. This was evident from stimulation of the state 4 electron flow, decline in the ADP/2e- ratio, increase in proton permeability and decrease in delta pH, whereas the uncoupled electron transport was only little impaired. In intact chloroplasts inhibition of HCO-3, 3-phosphoglycerate and oxaloacetate reduction by thapsigargicin and thapsigargin was not caused by inactivation of the photochemical reactions of the thylakoid membranes but were rather due to alterations in the permeability properties of the chloroplast envelope. This was concluded from similarities in the kinetics of these reactions. It is suggested that the highly lipid soluble sesquiterpene lactone tetraesters effectively disrupt the lipid-protein associations of biomembranes.

Freezing of isolated thylakoid membranes in complex media : III. Differences in the pattern of inactivation of photosynthetic reactions

Chloroplast thylakoid membranes isolated from spinach leaves (Spinacia oleracea L. cv. Monatol) were subjected to a freeze-thaw treatment in a buffered medium containing 70 mM KCl, 30 mM NaNO3 and 20 mM K2SO4 in different combinations. In the presence of the three predominant inorganic electrolytes, inactivation of photophosphorylation was mainly caused by a decrease in the capacity of the photosynthetic electron transport; release of proteins from the membranes was not manifest and light-induced H(+) gradient and proton permeability were largely unaffected. Omission of nitrate from the medium had little effect. When either sulfate or chloride or both were omitted prior to freezing, inactivation of photophosphorylation was correlated with stimulation of the phosphorylating electron flow, marked increase in H(+) permeability and loss of the ability of the thylakoids to accumulate protons in the light. In the absence of sulfate, uncoupling was mainly a consequence of the dissociation of chloroplast coupling factor (CF1). Partial restoration of proton impermeability and pH gradient occurred upon the addition of N,N'-dicyclohexylcarbodiimide (DCCD). When sulfate was present but chloride omitted, CF1 remained attached to the membranes and the addition of DCCD had no effect, indicating that the increase in proton efflux was caused by a different mechanism. It is concluded that sulfate stabilizes the CF1 and prevents its release from the membranes, but KCl is also necessary for maintaining the low permeability of the membranes to protons. The importance of complex media for investigations on isolated biomembrane systems is stressed.

Effective cryoprotection of thylakoid membranes by ATP

Freezing of isolated spinach thylakoids in the presence of NaCl uncoupled photophosphorylation from electron flow and increased the permeability of the membranes to protons. Addition of ATP prior to freezing diminished membrane inactivation. On a molar basis, ATP was at least 100 times more effective in protecting thylakoids from freezing damage than low-molecularweight carbohydrates such as sucrose and glucose. The cryoprotective effectiveness of ATP was increased by Mg(2+). In the absence of carbohydrates, preservation of thylakoids during freezing in 100 mM NaCl was saturated at about 1-2 mM ATP, but under these conditions membranes were not fully protected. However, in the presence of small amounts of sugars which did not significantly prevent thylakoid inactivation during freezing, ATP concentrations considerably lower than 0.5 mM caused nearly complete membrane protection. Neither ADP nor AMP could substitute for ATP. These findings indicate that cryoprotection by ATP cannot be explained by a colligative mechanism. It is suggested that ATP acts on the chloroplast coupling factor, either by modifying its conformation or by preventing its release from the membranes. The results are discussed in regard to freezing injury and resistance in vivo.

Factors contributing to inactivation of isolated thylakoid membranes during freezing in the presence of variable amounts of glucose and NaCl

During freezing of isolated spinach thylakoids in sugar/salt solutions, the two solutes affected membrane survival in opposite ways: membrane damage due to increased electrolyte concentration can be prevented by sugar. Calculation of the final concentrations of NaCl or glucose reached in the residual unfrozen portion of the system revealed that the effects of the solutes on membrane activity can be explained in part by colligative action. In addition, the fraction of the residual liquid in the frozen system contributes to membrane injury. During severe freezing in the presence of very low initial solute concentrations, membrane damage drastically increased with a decrease in the volume of the unfrozen solution. Freezing injury under these conditions is likely to be due to mechanical damage by the ice crystals that occupy a very high fraction of the frozen system. At higher starting concentrations of sugar plus salt, membrane damage increased with an increase in the amount of the residual unfrozen liquid. Thylakoid inactivation at these higher initial solute concentrations can be largely attributed to dilution of the membrane fraction, as freezing damage at a given sugar/salt ratio decreased with increasing the thylakoid concentration in the sample. Moreover, membrane survival in the absence of freezing decreased with lowering the temperature, indicating that the temperature affected membrane damage not only via alterations related to the ice formation. From the data it was evident that damage of thylakoid membranes was determined by various individual factors, such as the amount of ice formed, the final concentrations of solutes and membranes in the residual unfrozen solution, the final volume of this fraction, the temperature and the freezing time. The relative contribution of these factors depended on the experimental conditions, mainly the sugar/salt ratio, the initial solute concentrations, and the freezing temperature.

Cryopreservation of spinach chloroplast membranes by low-molecular-weight carbohydrates. I. Evidence for cryoprotection by a noncolligative-type mechanism

In freezing experiments with isolated spinach thylakoids (Spinacia oleracea L. cv. Monatol) the cryoprotective efficiency of various low-molecular-weight polyols was determined. The activity of cyclic photophosphorylation was used as an assay for the functional integrity of the membranes. The results were compared with the osmotic behavior of the cryoprotectants at high concentrations. Equimolal concentrations of polyols which exhibit nearly comparable freezing point depressions even at high concentrations differed considerably in their protective capacity during a freeze-thaw cycle. This was particularly distinct when glucose, galactose, and ethylene glycol monomethyl ether were compared, but was also evident when various pentoses and deoxy-hexoses were used as cryoprotectants. Even in the absence of freezing, carbohydrates exerted a stabilizing influence on biomembranes. From the data it is suggested that in addition to colligative action of the compounds, a specific noncolligative mechanism contributes to membrane protection during freezing.

Cryopreservation of spinach chloroplast membranes by low-molecular-weight carbohydrates. II. Discrimination between colligative and noncolligative protection

Thylakoid membranes isolated from spinach leaves (Spinacia oleracea L. cv. Monatol) were subjected to a freeze-thaw cycle in the presence of various concentrations of sugars, polyhydric alcohols, and NaCl. Functional integrity of the membranes was assayed by means of cyclic photophosphorylation. From the nonideal activity-concentration profiles of the carbohydrates the effective NaCl concentrations in the surroundings of the membranes at the respective freezing temperatures were calculated. Comparison of the cryoprotective efficiency of the various polyols revealed that cryopreservation by low-molecular-weight compounds is predominantly due to colligative action of the solutes. In addition, specific effects of carbohydrates which cannot be explained by the colligative concept are involved in cryoprotection. At NaCl concentrations exceeding 15 mm, the relative contribution of noncolligative membrane protection of a given polyol to overall cryopreservation was independent of the salt concentration. However, during freezing in the presence of very low salt concentrations, for instance 1-4 mm NaCl, cryoprotection due to colligative phenomena is reduced in favor of other mechanisms.

Effects of high-temperature stress on various biomembranes of leaf cells in situ and in vitro

The sensitivity of photosynthetic and respiratory functions to supraoptimal temperature stress was compared after heating of leaves, protoplasts and membrane systems of spinach (Spinacia oleracea L. cv. Monatol) and lettuce (Valerianella locusta [L.] Betcke) in situ and in vitro.After heating of whole leaves or protoplasts, endogenous respiration was not or only slightly affected at temperatures which caused a marked decrease of photosynthesis. This was manifested when mitochondria and thylakoids were isolated from heat-treated leaves. In the presence of exogenous substrates, mitochondrial electron transport and phosphorylation were even somewhat stimulated compared to the controls.Inactivation of net CO(2) uptake of whole leaves following heat stress and of the photochemical activities of chloroplast membranes isolated from heat-treated leaves of the same origin occurred nearly simultaneously. In protoplasts, photosynthesis was inactivated at temperatures far below those which caused drastic changes in the integrity of the tonoplast and the plasmalemma. This indicates that damage occurring within the chloroplasts rather than alterations in the compartmentation of the cell is responsible for the high sensitivity of photosynthesis to supraoptimal temperature stress.Mitochondria and thykaloids isolated from the same preparation of intact leaves under comparable conditions and subjected to heat treatment in vitro, however, were inactivated nearly in the same temperature range. Thus, mitochondria are much more stable within their cytoplasmic environment.

Effects of freezing on spinach leaf mitochondria and thylakoids in situ and in vitro

The sensitivity of spinach (Spinacia oleracea L.) leaf mitochondria and chloroplast membranes to subzero temperature stress was compared after freezing of the membrane systems in situ and in vitro. Respiratory and photosynthetic activities were measured polarographically.When leaves were frozen under controlled conditions for 2 hours to various minimum temperatures and mitochondria and chloroplasts isolated after thawing, the membrane systems showed a nearly simultaneous inactivation of respiratory and photosynthetic activities between -5 to -7 C. At that temperature range in both membrane systems phosphorylation became only slightly more affected than electron transport, i.e. after freezing in situ conspicuous uncoupling of phosphorylation from electron transport was not observed.In contrast, mitochondria and thylakoids isolated from the same preparation of intact leaves under comparable conditions using NaCl as osmoticum exhibited differences in sensitivity towards freezing for 2 to 4 hours at -25 C in vitro. In the absence of cryoprotectants, photophosphorylation of isolated thylakoids became completely uncoupled from electron transport which was increased several-fold compared with the unfrozen controls. Inactivation of respiratory functions of isolated mitochondria followed the same pattern as observed after freezing in situ. In the presence of sucrose for protection of thylakoids significantly higher concentrations of the cryoprotectant were necessary than for preservation of mitochondria. Thus, under the conditions used in this study chloroplast membranes proved to be more sensitive to freezing in vitro than mitochondria.

Effects of freezing on isolated plant mitochondria

Mitochondria isolated from spinach leaves (Spinacia oleracea L.) and potato tubers (Solanum tuberosum L.) were partly injured when subjected to freezing for 2 to 4 h at-25°C in salt solutions in the absence of cryoprotectants. Damage was manifested by the inactivation of respiratory properties and increase in the permeability of the mitochondrial membranes. Decrease in respiratory control indicated that the control mechanism of the electron transport chain was influenced by freezing. Oxidative phosphorylation was only slightly more affected than electron transport. The inactivation of the membrane systems was caused by an increase in the concentration of membrane-toxic solutes. This was confirmed by treatment of the organelles at 0°C in solutions of high salt concentrations. When sugar was present in the course of freezing, mitochondria were partly or completely protected. On a molar basis, sucrose was more effective in membrane protection than glucose. Under certain conditions amino acids, e.g., proline and hydroxyproline, also stabilized isolated mitochondria during freezing.

Investigations on heat resistance of spinach leaves

Exposure of spinach plants to high temperature (35° C) increased the heat resistance of the leaves by about 3° C. This hardening process occurred within 4 to 6 h, whereas dehardening at 20°/15° C required 1 to 2 days. At 5° C dehardening did not take place. Hardening and dehardening occurred in both the dark and the light. The hardiness was tested by exposure of the leaves to heat stress and subsequent measurements of chlorophyll fluorescence induction and light-induced absorbance changes at 535 nm on the leaves and of the photosynthetic electron transport in thylakoids isolated after heat treatment. Heat-induced damage to both heat-hardened and non-hardened leaves seemed to consist primarily in a breakdown of the membrane potential of the thylakoids accompanied by partial inactivation of electron transport through photosystem II. The increase in heat resistance was not due to temperature-induced changes in lipid content and fatty acid composition of the thylakoids, and no conspicuous changes in the polypeptide composition of the membranes were observed. Prolonged heat treatment at 35° C up to 3 days significantly decreased the total lipid content and the degree of unsaturation of the fatty acids of membrane lipids without further increase in the thermostability of the leaves. Intact chloroplasts isolated from heat-hardened leaves retained increased heat resistance. When the stroma of the chloroplasts was removed, the thermostability of the thylakoids was decreased and was comparable to the heat resistance of chloroplast membranes obtained from non-hardened control plants. Compartmentation studies demonstrated that the content of soluble sugars within the chloroplasts and the whole leaf tissue decreased as heat hardiness increased. This indicated that in spinach leaves, sugars play no protective role in heat hardiness. The results suggest that changes in the ultrastructure of thylakoids in connection with a stabilizing effect of soluble non-sugar stroma compounds are responsible for acclimatization of the photosynthetic apparatus to high temperature conditions. Changes in the chemical composition of the chloroplast membranes did not appear to play a role in the acclimatization.

Changes in Chloroplast Membrane Lipids during Adaptation of Barley to Extreme Salinity

During adaptation of barley (Hordeum vulgare L.) seedlings to extremely high concentrations of sodium chloride in the root space, the content of galactolipids of chloroplast membranes decreased considerably. Alterations in membrane lipids were due to the high concentration of ions rather than to the increase in the water potential. Sodium chloride was accumulated in the leaf cells and affected lipid-synthesizing enzymes such as galactosyl transferase and acylase which are attached to the chloroplast envelope. The return of salt-adapted barley seedlings to a nutrient solution with low salt concentration resulted in a reversal of the observed changes. It is suggested that the decrease in content of galactolipids in biomembranes is one of the factors causing increased salt resistance in barley plants which are adapted to extreme salinity.

Sugar compartmentation in frost-hardy and partially dehardened cabbage leaf cells

In frost-hardy and partially dehardened leaves of Brassica oleracea L. var. sabellica L. the distribution of cryoprotective sugars and of chloride between chloroplasts and the nonchloroplast part of leaf cells was investigated using the nonaqueous isolation technique as a means of cell fractionation. In chloroplasts of frost-hardy leaves high concentrations of sucrose and raffinose and comparatively low concentrations of chloride have been found. The ratios between sugars and chloride were so as to ascertain complete protection of the frost-sensitive thylakoid membranes during freezing. During dehardening, sugars decreased especially in the chloroplasts. There was a conversion of sucrose and raffinose into monosaccharides. This led to a large increase in the concentration of glucose and fructose in the nonchloroplast parts of the cells. There is evidence that the sugar concentration in the vacuole increased at the expense of sugars located in chloroplasts and cytoplasm. The quantity of sugars that remained in the chloroplasts did not appear to be sufficient for complete membrane protection at very low freezing temperatures.

Relative thermostability of the chloroplast envelope

Intact isolated chloroplasts from leaves of Spinacia oleracea L. were subjected to heat treatment. After heating, the integrity of the chloroplast envelopes and the activities of various light-dependent chloroplast reactions were tested. The integrity of the chloroplast envelopes, as judged from rates of ferricyanide reduction, enzyme compartmentation and visual appearance of the chloroplasts in the light microscope with phase optics, was affected much less by heat stress than the photochemical reactions of thylakoids. This indicates a comparatively high thermostability of the chloroplast envelope membranes. It is also evidence of a differential thermostability of different biomembranes. Photophosphorylation was highly susceptible to thermal stress. Heat treatment that partly inactivated phosphorylation stimulated light-dependent quenching of 9-aminoacridine fluorescence, which served as an indicator of proton transfer from stroma to thylakoids in intact chloroplasts. Drastic changes in the characteristics of chlorophyll a fluorescence emission caused by heating were probably due to structural alterations of the thylakoid system.

The protective effect of sugars on chloroplast membranes during temperature and water stress and its relationship to frost, desiccation and heat resistance

Freezing, desiccation and high-temperature stress may under certain conditions result in inactivation of electron transport (DCIP reduction) and cyclic photophosphorylation of isolated chloroplast membranes of spinach (Spinacia oleracea L.). When sugars are present during temperature and water stress, the thylakoids may be partially or completely protected. This membrane stabilization depends on the concentration of sugars and their molecular size. The trisaccharide raffinose is, on a molar basis, more effective than the disaccharide sucrose and the latter more than the monosaccharide glucose. An uncoupling effect and a stimulation of electron transport can be observed during freezing, desiccation and heat treatment, e.g. electron transport reactions are less sensitive to temperature and water stress than is photophosphorylation. As sugars are known to accumulate in winter, unspecific membrane stabilization by sugars may help to explain the often reported parallel development of frost, drought and heat resistance in many plants during winter.

The effect of freezing on thylakoid membranes in the presence of organic acids

The effect of salts of organic acids on washed and non-washed chloroplast membranes during freezing was investigated. Thylakoids were isolated from spinach leaves (Spinacia oleracea L.) and, prior to freezing, salts of various organic acids or inorganic salts or both were added. Freezing occurred for 3 to 4 hours at -25 C. After thawing membrane integrity was investigated by measuring the activity of cyclic photophosphorylation.At very low NaCl levels (1 to 3 mm, washed thylakoids) salts of organic acids either could not prevent membrane inactivation in the course of freezing (succinate) or were effective only at relatively high concentrations (0.1 m or more of acetate, pyruvate, malate, tartrate, citrate). If NaCl was present at higher concentrations (e.g., 0.1 m) some organic acids, e.g. succinate, malate, tartrate, and citrate, were able to protect frost-sensitive thylakoids at surprisingly low concentrations (10 to 20 mm). Other inorganic salts such as KCl, MgCl(2), NaNO(3) could also induce protection by organic acids which otherwise were ineffective or poorly effective. For effective protection, a more or less constant ratio between inorganic salt and organic acid or between two or more organic acids had to be maintained. Departure to either side from the optimal ratio led to progressive inactivation.The unspecificity of the protective effect of organic acids suggests that these compounds protect colligatively. There are also indications that, in addition, more specific interaction with the membranes contributes to protection. At temperatures above the freezing point, the presence of salts of organic acids decreased the rate of membrane inactivation by high electrolyte concentrations.

Direct and indirect transfer of ATP and ADP across the chloroplast envelope

1. In intact leaves of Elodea densa illumination resulted in a large increase in the levels of chloroplastic and cytoplasmic ATP and a decrease in chloroplastic and cytoplasmic ADP. Reverse changes were observed on darkening. The kinetics of the fluctuations were similar in chloroplasts and cytoplasm suggesting effective transfer of ATP and ADP between chloroplasts and cytoplasm. Ratios of ATP to ADP were significantly lower in the chloroplasts than in the cytoplasm in the dark and in the light. This may indicate different phosphate potentials in chloroplasts and cytoplasm. Transfer of ATP across the chloroplast envelope as calculated from the light-dependent cytoplasmic ATP increase was 7 to 9 μmoles/mg chlorophyll per hour. Actual transfer rates are probably higher.

2. The determination of the rate of adenylate transfer across the envelope of intact isolated chloroplasts requires information on the composition of chloroplast preparations. Intact chloroplasts were quantitatively separated from envelope-free chloroplasts by density gradient centrifugation on Ludox gradients. The percentage of envelope-free chloroplasts in preparations of intact chloroplasts was also determined from measurements of light-dependent ferricyanide reduction.

3. During isolation in sorbitol buffer ca. 50% of the adenylates were lost from intact chloroplasts which were still capable of high rates of phosphoglycerate reduction and photosynthesis.

4. Adenylate transfer across the envelope of isolated chloroplasts as measured by the light-dependent phosphorylation of added ADP in the absence of cofactors was slow and occured at a rate of 0 to 4 µmoles/mg chloroplyll per hour. In the dark chloroplastic adenylate kinase reacted only very slowly with added AMP and ATP to form ADP. Breakage of the chloroplast envelope stimulated reaction rates.

5. Indirect transfer of ATP and ADP across the chloroplast envelope occurred via a shuttle transfer of phosphoglycerate and dihydroxyacetone phosphate. 3-phosphoglyceraldehyde is not involved as a transport metabolite. Maximum transfer rates of phosphoglycerate and dihydroxyacetone phosphate across the chloroplast envelope were higher than maximum rates of photosynthesis and reached 300 μmoles/mg chlorophyll per hour. Indirect transfer of ATP was somewhat slower than rates of phosphoglycerate reduction by isolated chloroplasts. In vivo transfer of phosphate energy by this transport metabolite system is under control of the redox state of pyridine nucleotides and of the phosphate potential.

Intracellular localization of enzymes in leaves and chloroplast membrane permeability to compounds involved in amino acid syntheses

1. Enzyme distribution between chloroplasts and the nonchloroplast parts of green leaf cells of Spinacia oleracea, Nicotiana rustica, Vicia faba, and Phaseolus vulgaris have been investigated by use of the nonaqueous chloroplast isolation technique. Whereas pyruvate kinase and peroxidase were located only or mainly outside of the chloroplasts, the other enzymes studied, isocitric dehydrogenase, glutathione reductase, NAD- and NADP-dependent pyridine nucleotide quinone reductase, malic dehydrogenase, NAD- and NADP-dependent glyoxylate reductase, glutamate-oxaloacetate transaminase, NAD-dependent glutamic dehydrogenase, and NADP-dependent aspartic dehydrogenase were both inside and outside of the plastids. In contrast, NADP-dependent glyceraldehyde-3-phosphate dehydrogenase is located only within the chloroplasts.

2. Intact isolated spinach chloroplasts incorporated only a very small amount of labeled carbon from 14CO2 into amino acids in the light. The addition of NH4Cl did not increase the amount of labeled amino acids and had no effect on the total amount of 14C fixed during short time photosynthesis. However, NH4 ⊕ caused changes in the pathway of carbon during photosynthesis. In the presence of NH4 ⊕, more 14C was incorporated into sugar monophosphates and phosphoglyceric acid than in the absence of NH4 ⊕.

3. 14C-labeled glycine and serine fed to intact isolated spinach chloroplasts were neither accumulated nor transformed into other compounds, but 14C-labeled glutamic acid was converted into glutamine. This transformation took place only in the light in chloroplasts containing an intact outer envelope. The addition of NH4 ⊕ and certain substrates and cofactors did not increase the rate of transformation.

4. The penetration of some amino acids and substrates through the outer envelope of the chloroplasts was investigated on aqueously isolated spinach plastids. It was found that a-ketoglutarate, oxaloacetate, pyruvate, aspartate, and alanine are able to penetrate the envelope although at least for some of these compounds the outer membrane of the chloroplasts acts as a partial barrier.

5. From the experiments reported here and in connection with the results published by other investigators it can be concluded that the most common amino acids such as glutamic acid, aspartic acid, alanine, glycine, and serine are able to penetrate through the outer envelope of the chloroplasts and the synthesis of these amino acids can occur in the leaf cells inside as well as outside of the chloroplasts.

[The effect of freezing and desiccation of chloroplasts in the presence of electrolytes]

The effect of freezing, desiccation and various electrolytes on photophosphorylation, electron transport and some enzyme reactions of isolated spinach chloroplasts has been investigated. Freezing of broken chloroplasts took place at-25°C for 3 hrs; desiccation was performed at +2°C in vacuo over CaCl2 for 3 hrs. The influence of various electrolytes during freezing or drying or during incubation of thylakoids or stroma enzymes for 3 hrs at +2°C in electrolyte solutions was determined. After treatment, the activities of a number of enzymes and enzyme systems were measured under normal conditions, e. g. in the absence of elevated electrolyte levels in a reaction medium which contained only the substrates and cofactors which are necessary for the respective enzyme reactions.Only photophosphorylation and electron transport were affected by freezing, desiccation and high concentrations of electrolytes; various soluble enzymes investigated here were not inactivated under the same conditions. In general, mild dehydration and lower concentrations of electrolytes resulted in an irreversible inactivation of ATP synthesis but did not impair ferricyanide reduction. With increasing dehydration or at higher concentrations of electrolytes the Hill reaction was also inhibited. In a certain range of dehydration and electrolyte concentration uncoupling of photophosphorylation from electron transport took place. Sugar protects the sensitive structures against the deleterious effect of both dehydration and high concentration of electrolytes.Various electrolytes affected thylakoid membranes differently. Inactivation of the membranes increased with increasing ion radius and decreasing hydration envelope of univalent or divalent cations. Divalent cations were more destructive than univalent cations. Anions did not follow these rules. Within a group of similar anions (halides or organic anions) effectivity decreased with increasing hydration envelope. On a molar basis, polyvalent anions were less effective than univalent anions. Inactivation by anions followed Hofmeister's series in seversed order. However, exceptions were observed and it appears that various ions affect the membrane in a specific manner.Inactivation of photophosphorylation and electron transport due to freezing or desiccation is identical to that due to high concentrations of electrolytes. This suggests that during dehydration due to freezing or drying the concentration of electrolytes in the remaining solution is responsible for the inactivation of the sensitive membranes.

[Assimilation of CO2, NADP and PGA reduction and ATP synthesis in intact leaf cells in relation to water content]

1. Photosynthesis, NADP and PGA reduction and ATP synthesis in strongly dehydrated intact leaves or parts of leaves of fodder beet were investigated. 2. Water stress leads to inhibition of photosynthesis. However, even loss of a major proportion of the water content of cells (up to 80%) does not completely suppress photosynthesis. Under these conditions the rate of photosynthesis is still comparable to that of respiration. Loss of water exceeding about 80% of the total water of leaf cells depresses photosynthesis to a level lower than that of respiration, but respiration is also reduced. Photosynthesis of leaf cells which survived strong dehydration is not restored parallel to rehydration and exhibits a pronounced lag phase in restoration. 3. In vivo in the light, NADP and PGA reduction and ATP synthesis are observed even if up to nearly 85% of the total water content of the leaf cells is removed by wilting. This finding is consistent with the results obtained in vitro with isolated chloroplasts which were dehydrated by exposure to solutions of different tonicity, and it agrees with measurements of photosynthesis in intact leaves. 4. The significance of the results in relation to drought resistance of plants is discussed.

[Hill reaction and photophosphorylation of isolated chloroplasts in relation to water content : II. Removal of water by CaCl2]

1. Isolated chloroplasts from leaves of spinach and beets were dehydrated by drying for 3 hours in vacuo over CaCl2 at +2°C in the absence and in the presence of different substances. After rehydration ferricyanide reduction, cyclic photophosphorylation with PSM as cofactor, noncyclic photophosphorylation and the level of free SH groups were investigated. Furthermore, the quantity of water bound under the conditions of the test by the chloroplast lamellae and by the different substances was determined. 2. Isolated chloroplasts, which were dehydrated for 3 hours over CaCl2 lost 98-99% of their water content. Under these conditions a sharp increase of SH groups occurred indicating protein denaturation. In addition Hill reaction and photophosphorylation were inactivated. The presence of sugars, soluble proteins and polypeptides during dehydration protected chloroplasts, fully or in part, against denaturation. At low concentrations of the protective substances preservation increased more or less linearly with increasing concentration. Inorganic and organic salts could not prevent the destruction of the system during dehydration. On the contrary, salts abolish the protection afforded by sugars. More sugar was required to give protection for photophosphorylation than for the electron transfer reactions of the Hill reaction. Uncoupling of photophosphorylation from electron transport therefore precedes the destruction of electron transfer due to dehydration. In principle, cyclic and noncyclic photophosphorylation showed the same behaviour. - In spinach and bett leaves, the critical limit for the dehydration of the protoplasmic structures seemed to be nearly 10-15% of the total water content. Removal of the "critical" water leads to injury. 3. The protective action of sugars and, at least in part, of peptone and bovine albumin may be explained by their ability to retain water during the drying. Under specified conditions 1 mol of sucrose binds twice as much water as the same amount of glucose. On a molar basis sucrose is twice as effective as glucose in protecting the Hill reaction. On the other hand it is also possible that sugars protect the sensitive proteins directly and specifically. - Accumulation of ions, even though these may bind as much water as neutral solutes such as sugars, is destructive. 4. No change in the SH content of the chloroplasts was obtained during dehydration in the presence of very small amounts of sugar, which is not sufficient to protect Hill reaction and photophosphorylation. In the absence of sugar a considerable increase in SH groups is observed on drying. No obvious correlation exists between the liberation of SH groups and the inactivation of Hill reaction and photophosphorylation. 5. The results demonstrate that plants resistant to high dehydration can increase their desiccation resistance through mobilisation of sugars and soluble proteins during the water loss. These substances can protect the sensitive protein structures during the dehydration. 6. The results obtained when isolated chloroplats were dehydrated with CaCl2 are consistent with those obtained in freezing experiments. In other words, the response of chloroplasts to dehydration is identical whatever the mode of dehydration is. The findings explain the similarities between frost and drought resistance observed by many different authors.

[Hill reaction and photophosphorylation of isolated chloroplasts in relation to water content : I. Removal of water by means of concentrated solutions]

1. Water was removed by means of concentrated solutions from chloroplasts which were isolated from leaves of spinach and beets. During and after the dehydration Hill reaction and cyclic photophosphorylation with PMS as a cofactor were investigated. As osmotic amterial glucose, sucrose, lutrol and NaCl were used. 2. No depression of ferricyanide reduction was obtained in 3 M sugar solution and in 2.5 M lutrol solution. These concentrations correspond to a loss of water amounting to 90% of the total water of leaf cells. In contrast, cyclic photophosphorylation was already decreased in 1-2 M solutions of sugar or lutrol, that means by much less dehydration. In 3 M solutions only 5-25% of the activity of the water saturated controls remained. However, this decrease in cyclic photophosphorylation occurred only when chloroplasts were kept dehydrated during the light reaction. When chloroplasts were permitted to return to optimal water conditions photophosphorylation was no longer inhibited. Therefore, extensive loss of water leads to reversible uncoupling of photophosphorylation from electron transport. 3. Relatively low concentrations of NaCl (as compared with sugar concentrations) damage the ability of chloroplasts to perform Hill reaction and photophosphorylation. Inactivation of the reactions is partly reversible at low concentrations of NaCl and irreversible at high concentrations. 4. The osmotic potential of leaves of sugar beet increased with increasing dehydration. Within a limited range the osmotic behaviour of the cell sap of leaf cells during dehydration was identical with that of NaCl solutions. 5. The possibility of correlating in vitro experiments in which dehydration is simulated by exposure of chloroplasts to various solutions with in vivo experiments using intact leaves which are dehydrated to different degrees is demonstrated.

Loss of Adenosine Triphosphate Synthesis Caused by Freezing and Its Relationship to Frost Hardiness Problems

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