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

Thylakoid membrane stability in drought-tolerant and drought-sensitive plants

The stress stability of membranes from two drought-tolerant plants (Craterostigma plantagineum andCeterach officinarum) was compared with that of a drought-sensitive plant (Spinacia oleracea) in model experiments. Thylakoids from these plants were exposed to excessive sugar or salt concentrations or to freezing. All stresses caused loss of membrane function as indicated by the loss of cyclic photophosphorylation or the inability of the membranes to maintain a large proton gradient in the light. However, loss of membrane functions caused by osmotic dehydration in the presence of sugars was reversible. Irreversible membrane damage during freezing or exposure to salt was attributed mainly to chaotropic solute effects. The sensitivity to different stresses was comparable in thylakoid membranes from tolerant and sensitive plants indicating that the stress tolerance of a plant can hardly be attributed to specific membrane structures which would increase membrane stability. Levels of membrane-compatible solutes such as sugars or amino acids, among them proline, were much higher in the drought-tolerant plants than in spinach. Isolated thylakoids suspended in solutions containing an excess of sugars remained functional after dehydration by freeze-drying. This indicates that membrane-compatible solutes are important in preventing membrane damage during dehydration of poikilohydric plants.

Photosynthetic characteristics of chloroplasts isolated fromMesembryanthemum crystallinum L., a halophilic plant capable of Crassulacean acid metabolism

Photosynthetically highly active chloroplasts were routinely obtained by rupture of leaf protoplasts from the halophyteMesembryanthemum crystallinum which exhibited the photosynthetic characteristics of either a C3 plant when grown with 20 mmol l(-1) NaCl in the rooting medium, or a Crassulacean-acid-metabolism (CAM) plant when grown with 400 mmol l(-1) NaCl. Photosynthesis rates of C3 and CAM chloroplasts were 150-250 and 90-150 μmol mg(-1) chlorophyll h(-1), respectively. Because of osmotic adjustment, CAM chloroplasts required higher sorbitol concentrations (0.7-0.8 mol l(-1)) in the assay medium than C3 chloroplasts (0.3-0.4 mol l(-1)) for optimum activity. Substitution of sorbitol by NaCl as the osmoticum strongly reduced photosynthesis of CAM chloroplasts. Rates of electron transport (ferricyanide reduction, uncoupled) remained unaffected over a range of sorbitol concentrations (0 to 1 mol l(-1)). Sensitivity of electron transport to increasing levels of NaCl was less pronounced than the NaCl-sensitivity of CO2 fixation by intact chloroplasts. The CAM chloroplasts showed a broad pH optimum of photosynthesis between pH 7.0 and 8.2; photosynthesis of C3 chloroplasts dropped markedly below pH 7.6. The CAM chloroplasts maintained a higher transenvelope proton gradient than C3 chloroplasts both in the light and dark. External pyruvate (5 mmol l(-1)) inhibited photosynthesis of CAM chloroplasts, but not of C3 chloroplasts. Inhibition was reduced by increased external concentrations of orthophosphate.

Photosynthesis under osmotic stress : Inhibition of photosynthesis of intact chloroplasts, protoplasts, and leaf slices at high osmotic potentials

1. Photosynthesis of leaf slices, mesophyll protoplasts, and intact chloroplasts of spinach was inhibited in hypertonic sorbitol solutions. Sorbitol could be replaced by other nonpenetrating osmotica such as sucrose or glycinebetaine. As a penetrating solute, ethyleneglycol was also inhibitory, but osmolarities required for inhibition of photosynthesis were considerably higher than in the case of non-penetrating osmotica.-2. With leaf slices and protoplasts, 50% inhibition by sorbitol was usually observed at osmotic potentials between 25 and 40 bar. With isolated intact chloroplasts, the osmotic potentials producing 50% inhibition varied considerably. Depending on the growth conditions of the plant material, 50% inhibition occurred between 14 and 40 bar. The integrity of the chloroplast envelope as measured by the accessibility of the thylakoid system for ferricyanide was not affected by osmotic stress.-3. Quantum requirements for CO2 assimilation and reduction of 3-phosphoglycerate or nitrite by intact chloroplasts increased under osmotic stress. The increase was larger for CO2 reduction than for reduction of 3-phosphoglycerate or nitrite.-4. In intact chloroplasts, electron transport to methylviologen was not much affected by osmotic stress. Basal electron transport was not stimulated, suggesting absence of uncoupling.-5. The increase in ATP/ADP ratios on illumination of intact chloroplasts was slower at an osmotic potential of 36 bar than at 11 bar.-6. The results indicate that inhibition of photosynthesis is not caused by the sensitivity of a single photosynthetic reaction to increased osmotic potentials. Rather, several reactions are sensitive to water stress. Osmotic stress acts on the photosynthetic apparatus mainly at the level of dark reactions and ATP synthesis, and much less on primary photoreactions or electron transport, between water and the primary oxidant of photosystem I.-7. The different sensitivity of chloroplasts to penetrating and non-penetrating solutes and the observed variability of chloroplast sensitivity to stress suggests that the reduction in water potential is not directly responsible for damage to the photosynthetic apparatus during osmotic stress. Rather, the composition of the chloroplasts appears to be a decisive factor which determines sensitivity or resistance to osmotic stress.

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.

Loss of function of biomembranes and solubilization of membrane proteins during freezing

Isolated thylakoid membranes are damaged during freezing in dilute salt solutions, as shown by the inactivation of photochemical thylakoid reactions. After freezing, a number of membrane proteins were found in the particle-free supernatant. Up to 5% of the total membrane protein was solubilized by freezing, and the pattern of released proteins as seen in sodium dodecyl sulfate gel electrophoretograms was influenced by the nature of the solutes present. Membranes protected by sucrose did not release much protein during freezing. Concentrated salt solutions caused protein release also in the absence of freezing. Among the proteins released were ferredoxin--NADP+ reductase, plastocyanin and coupling factor CF1. Subunits of CF1 were found in different proportions in the supernatants of thylakoid suspensions after freezing in the presence of different salts. Cyclic photophosphorylation was largely inactivated before significant protein release could be detected. It is suggested that protein release is the final consequence of the nonspecific suppression of intramembrane ionic interactions by the high ionic strength created in the vicinity of the membranes by the accumulation of salts during slow freezing. Salt effects on water structure and alterations of nonpolar membrane interactions by the incorporation of (protonated) lipophilic anions from organic salts into the membrane phase during freezing may also be involved.

Cryoprotective leaf proteins

Leaves of frost-resistant plants contain a number of soluble proteins which are capable of protecting isolated biomembranes against inactivation during freezing. Such proteins have not been found in non-hardy summer material. The pattern of protective proteins was not uniform in hardy material of different origin and appeared to change with the season. Cryoprotective proteins were isolated by preparative gel electrophoresis. Molecular weights of different proteins as determined by their electrophoretic mobility in sodium dodecyl sulfate gels were between 10000 and 20000. Circular dichroism measurements failed to indicate helical structures. The amino acid composition of 2 active proteins revealed a high content of polar amino acids. The proteins were heat-stable. They were, on a molar basis, more than 1000 times as effective in protecting thylakoid membranes against freezing damage as low-molecular-weight cryoprotectants such as sucrose, glycerol or dimethylsulfoxide. Very low concentrations of the proteins increased cryoprotection provided by sucrose. Of a number of oligopeptides of known composition, only a few were cryoprotective. Their activity was very small as compared with that of the active proteins. The concentration of the cryoprotective proteins in hardy leaves appeared to be high enough for a significant contribution of the proteins to the frost tolerance of resistant plants.

[Investigations on the effect of blue light on the photosynthetic O2 exchange]

1. In cells of Chlorella fusca darkened for 12 and more hours and preilluminated with red light, irradiation with blue light of low intensity (5×10(-10) einstein cm(-2) sec(-1), 465 nm) for 2 minutes results in a substantial enhancement (up to 80%) of the "gross O2 evolution" (difference between the rate of O2 exchange in dark and light) in a subsequent red light period (5×10(-10) einstein cm(-2) sec(-1), 670 nm). 2. By analysing the kinetics of O2 exchange after the light has been switched on and off it is possible to distinguish between true photosynthetic O2 evolution and a photosynthetic inhibition of respiratory O2 uptake. Only the latter component is strongly enhanced by blue light, whereas O2 evolution is almost unchanged. This is also shown when both components are separated by using various inhibitors. 3. It was shown in different ways that the inhibitory effect of photosynthesis on the respiratory O2 uptake is strongly dependent on the respiratory activity. This inhibition is most pronounced at medium activity, but nearly zero at very low respiratory activity. Therefore it is concluded that blue light works primarily only by enhancing the respiratory activity, which declined to very low values during the preceeding long lasting dark period. 4. This interpretation is strongly supported by the finding that any acceleration of the respiratory dark turnover by uncouplers or by glucose at low concentrations produces quite the same effect on the photosynthetic O2 exchange as blue light does.

[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.

Carbon dioxide exchange in cotton: some anomalous fluctuations

Anomalous depressions in carbon dioxide exchange were observed in cotton leaves that were exhibiting oscillations in transpiration under controlled conditions of environment. The depressions occurred only when leaf temperature exceeded 37.5 degrees C and when the leaf diffusive resistance was minimum. Stomatal control of the supply of carbon dioxide to the leaf does not seem to be implicated in the effect.

[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.