Resistance to low temperature photoinhibition is not associated with isolated thylakoid membranes of winter rye

In vivo measurements of chlorophyll a fluorescence indicate that cold-hardened winter rye (Secale cereale L. cv Musketeer) develops a resistance to low temperature-induced photoinhibition compared with nonhardened rye. After 7.2 hours at 5°C and 1550 micromoles per square meter per second, the ratio of variable fluorescence/maximum fluorescence was depressed by only 23% in cold-hardened rye compared with 46% in nonhardened rye. We have tested the hypothesis that the principal site of this resistance to photoinhibition resides at the level of rye thylakoid membranes. Thylakoids were isolated from cold-hardened and nonhardened rye and exposed to high irradiance (1000-2600 micromoles per square meter per second) at either 5 or 20°C. The photoinhibitory response measured by room temperature fluorescence induction, photosystem II electron transport, photoacoustic spectroscopy, or [¹⁴C]atrazine binding indicates that the differential resistance to low temperature-induced photoinhibition in vivo is not observed in isolated thylakoids. Similar results were obtained whether isolated rye thylakoids were photoinhibited or thylakoids were isolated from rye leaves preexposed to a photoinhibitory treatment. Thus, we conclude that increased resistance to low temperature-induced photoinhibition is not a property of thylakoid membranes but is associated with a higher level of cellular organization.     

Photosynthetic control, “energy-dependent” quenching of chlorophyll fluorescence and photophosphorylation under influence of tertiary amines

The effects of the tertiary amines tetracaine, brucine and dibucaine on photophosphorylation and control of photosynthetic electron transport in isolated chloroplasts of Spinacia oleracea were investigated. Tertiary amines inhibited photophosphorylation while the related electron transport decreased to the rates, observed under non-phosphorylating conditions. Light induced quenching of 9-aminoacridine fluorescence and uptake of ¹⁴C-labelled methylamine in the thylakoid lumen declined in parallel with photophosphorylation, indicating a decline of the transthylakoid proton gradient. In the presence of ionophoric uncouplers such as nigericin, no effect of tertiary amines on electron transport was seen in a range of concentration where photophosphorylation was inhibited. Under the influence of the tertiary amines tested, pH-dependent feed-back control of photosystem II, as indicated by energy-dependent quenching of chlorophyll fluorescence, was unaffected or even increased in a range of concentration where 9-aminoacridine fluorescence quenching and photophosphorylation were inhibited. The data are discussed with respect to a possible involvement of localized proton flow pathways in energy coupling and feed-back control of electron transport.Abbreviations 9-AA 9-aminoacridine - J _e flux of photosynthetic electron transport - PC photosynthetic control - pH₁ H⁺ concentration in the thylakoid lumen - pmf proton motive force - _P potential quantum yield of photochemistry of photosystem II (with open reaction centers) - Q _A primary quinone-type electron acceptor of photosystem II - q _Q photochemical quenching of chlorophyll fluorescence - q _E energy-dependent quenching of chlorophyll fluorescence - q _AA light-induced quenching of 9-amino-acridine fluorescence     

Influence of photoinhibition on electron transport and photophosphorylation of isolated chloroplasts

The effects of a photoinhibition treatment (PIT) on electron transport and photophosphorylation reactions were measured in chloroplasts isolated from triazine-resistant and susceptible Chenopodium album plants grown under high and low irradiance. Electron transport dependent on photosystem I (PSI) alone was much less affected by PIT than that dependent on both photosystem II (PSII) and PSI. There was a smaller difference in susceptibility to PIT between the photophosphorylation activitity dependent on PSI alone and that dependent on both PSII and PSI. Because in all cases photophosphorylation activity decreased faster upon PIT than the rate of electron transport, we conclude that photoinhibition causes a gradual uncoupling of electron transport with phosphorylation. Since the extent of the light-induced proton gradient across the thylakoid membrane decreased upon PIT, it is suggested that photoinhibiton causes a proton leakiness of the membrane. We have found no significant differences to PIT of the various reactions measured in chloroplasts isolated from triazine-resistant and susceptible plants. We have also not observed any significant differences to PIT of the photophosphorylation reactions in chloroplasts of plants grown under low irradiance, compared with those grown under high irradiance. However, the electron transport reactions in chloroplasts from plants grown under low irradiance appeared to be somewhat less sensitive to PIT than those grown under high irradiance.     

Temperature Dependence of Photosynthesis in Agropyron smithii Rydb. : II. CONTRIBUTION FROM ELECTRON TRANSPORT AND PHOTOPHOSPHORYLATION

As part of an analysis of the factors regulating photosynthesis in Agropyron smithii Rydb., a C₃ grass, the response of electron transport and photophosphorylation to temperature in isolated chloroplast thylakoids has been examined. The response of the light reactions to temperature was found to depend strongly on the preincubation time especially at temperatures above 35°C. Using methyl viologen as a noncyclic electron acceptor, coupled electron transport was found to be stable to 38°C; however, uncoupled electron transport was inhibited above 38°C. Photophosphorylation became unstable at lower temperatures, becoming progressively inhibited from 35 to 42°C. The coupling ratio, ATP/2e⁻, decreased continuously with temperature above 35°C. Likewise, photosystem I electron transport was stable up to 48°C, while cyclic photophosphorylation became inhibited above 35°C. Net proton uptake was found to decrease with temperatures above 35°C supporting the hypothesis that high temperature produces thermal uncoupling in these chloroplast thylakoids. Previously determined limitations of net photosynthesis in whole leaves in the temperature region from 35 to 40°C may be due to thermal uncoupling that limits ATP and/or changes the stromal environment required for photosynthetic carbon reduction. Previously determined limitations to photosynthesis in whole leaves above 40°C correlate with inhibition of photosynthetic electron transport at photosystem II along with the cessation of photophosphorylation.     

Modification of excitation energy distribution to photosystem I by protein phosphorylation and cation depletion during thylakoid biogenesis in wheat

The effects of protein phosphorylation and cation depletion on the electron transport rate and fluorescence emission characteristics of photosystem I at two stages of chloroplast development in light-grown wheat leaves are examined. The light-harvesting chlorophyll a/b protein complex associated with photosystem I (LHC I) was absent from the thylakoids at the early stage of development, but that associated with photosystem II (LHC II) was present. Protein phosphorylation produced an increase in the light-limited rate of photosystem I electron transport at the early stage of development when chlorophyll b was preferentially excited, indicating that LHC I is not required for transfer of excitation energy from phosphorylated LHC II to the core complex of photosystem I. However, no enhancement of photosystem I fluorescence at 77 K was observed at this stage of development, demonstrating that a strict relationship between excitation energy density in photosystem I pigment matrices and the long-wavelength fluorescence emission from photosystem I at 77 K does not exist. Depletion of Mg²⁺ from the thylakoids produced a stimulation of photosystem I electron transport at both stages of development, but a large enhancement of the photosystem I fluorescence emission was observed only in the thylakoids containing LHC I. It is suggested that the enhancement of PS I electron transport by Mg²⁺-depletion and phosphorylation of LHC II is associated with an enhancement of fluorescence at 77 K from LHC I and not from the core complex of PS I.     

Freezing of isolated thylakoid membranes in complex media

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.Abbreviations CF₁ chloroplast coupling factor - DCCD N,N-dicyclohexylcarbodiimide - Hepes 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid I=Santarius 1986 b     

Effect of osmotic stress on photosynthesis studied with the isolated spinach chloroplast : generation and use of reducing power

The effect of increasing assay medium sorbitol concentration from 0.33 to 1.0 molar on the photosynthetic reactions of intact and broken spinach (Spinacia oleracea L. var. Long Standing Bloomsdale) chloroplasts was investigated by monitoring O₂ evolution supported by the addition of glyceric acid 3-phosphate (PGA), oxaloacetic acid (OAA), 2,5-dimethyl-p-benzoquinone, and 2,6-dichlorophenolindophenol or as O₂ uptake with methyl viologen as acceptor.

Uncoupled 2,6-dichlorophenolindophenol-supported whole chain electron transport (photosystems I and II) was inhibited from the 0.33 molar rate by 14% and 48.6% at 0.67 and 1.0 molar sorbitol in the intact chloroplast and by only 0.4% and 25.0% in the broken chloroplast preparation. Whole chain electron flow from water to other oxidants (OAA, methyl viologen) was also inhibited at increased osmoticum in intact preparations while electron flow from water to methyl viologen, ferricyanide, and NADP in broken preparations did not demonstrate the osmotic response. Electron transport to 2,5-dimethyl-p-benzoquinone (photosystem II) from H₂O and to methyl viologen (photosystem I) from 3,3′-diaminobenzidine were found to be unaffected by osmolarity in both intact and broken preparations.

The stress response was more pronounced (26-38%) with PGA as substrate in the presence of 0.67 molar sorbitol than the inhibition found with uncoupled and coupled linear electron flow. In addition, substrate availability and ATP generated by cyclic photophosphorylation evaluated by addition of Antimycin A were found not to be mediating the full osmotic inhibition of PGA-supported O₂ evolution. In a reconstituted (thylakoids plus stromal protein) chloroplast system to which a substrate level of PGA was added, O₂ evolution was only slightly (7.8%) inhibited by increased osmolarity (0.33-0.67 molar sorbitol) indicating that the level of osmotic inhibition above that contributed by adverse effects on electron flow can be attributed to the functioning of the photosynthetic carbon reduction cycle within the intact chloroplasts.

     

The flexibility of metabolic interactions between chloroplasts and mitochondria in Nicotiana tabacum leaf

To examine the effect of mitochondrial function on photosynthesis, wild-type and transgenic Nicotiana tabacum with varying amounts of alternative oxidase (AOX) were treated with different respiratory inhibitors. Initially, each inhibitor increased the reduction state of the chloroplast electron transport chain, most severely in AOX knockdowns and least severely in AOX overexpressors. This indicated that the mitochondrion was a necessary sink for photo-generated reductant, contributing to the ‘P700 oxidation capacity’ of photosystem I. Initially, the Complex III inhibitor myxothiazol and the mitochondrial ATP synthase inhibitor oligomycin caused an increase in photosystem II regulated non-photochemical quenching not evident with the Complex III inhibitor antimycin A (AA). This indicated that the increased quenching depended upon AA-sensitive cyclic electron transport (CET). Following 12 h with oligomycin, the reduction state of the chloroplast electron transport chain recovered in all plant lines. Recovery was associated with large increases in the protein amount of chloroplast ATP synthase and mitochondrial uncoupling protein. This increased the capacity for photophosphorylation in the absence of oxidative phosphorylation and enabled the mitochondrion to act again as a sink for photo-generated reductant. Comparing the AA and myxothiazol treatments at 12 h showed that CET optimized photosystem I quantum yield, depending upon the P700 oxidation capacity. When this capacity was too high, CET drew electrons away from other sinks, moderating the P700⁺ amount. When P700 oxidation capacity was too low, CET acted as an electron overflow, moderating the amount of reduced P700. This study reveals flexible chloroplast–mitochondrion interactions able to overcome lesions in energy metabolism.     

Changes in the structure and the functional state of thylakoids under the conditions of osmotic shock

The effect of osmotic shock on the ultrastructure and functions of C-class pea chloroplasts has been examined. When incubated in a non-sucrose medium for 30 s or more, thylakoids were found to pass to a stable deformed state. This state was characterized by an altered orientation of thylakoids to each other with the lumen thickness remaining the same as in the normal state. Experiments with shorter incubation periods (10–20 s) revealed a swelling of thylakoids, which probably represented an intermediate stage. The deformation of the thylakoid system was accompanied by a decrease in the non-cyclic ATP synthesis but by an increase in the rate of cyclic photophosphorylation. Besides, the deformed thylakoids demonstrated an acceleration of the basal electron transport, as well a rise in the light-induced H⁺ and imidazol uptake. The data obtained are discussed in the light of membrane interactions fixing the configuration of a thylakoid.     

Temperature and Light Dependence of Photosynthetic Activities in Wheat Seedlings Grown in the Presence of DCMU [3-(3,4-Dichlorophenyl)-1,1-Dimethylurea]

Photosynthetic electron transfer was studied in thylakoids isolated from control and DCMU-grown wheat (Triticum aestivum L.) seedlings. When exposed to high temperature (HT) and high iradiance (HI), thylakoids showed large variations in the photosynthetic electron transport activities and thylakoid membrane proteins. A drastic reduction in the rate of whole electron transport chain (H₂O MV) was envisaged in control thylakoids when exposed to HT and HI. Such reduction was mainly due to the loss of photosystem 2, PS2 (H₂O DCBQ) activity. The thylakoids isolated from seedlings grown in the presence of DCMU showed greater resistance to HT and HI treatment. The artificial exogenous electron donors MnCl₂, DPC, and NH₂OH failed to restore the HI induced loss of PS2 activity in both control and DCMU thylakoids. In contrast, addition of DPC and NH₂OH significantly restored the HT induced loss of PS2 activity in control thylakoids and partially in DCMU thylakoids. Similar results were obtained when F_v/F_m was evaluated by chlorophyll fluorescence measurements. The marked loss of PS2 activity in control thylakoids was evidently due to the loss of 33, 23, and 17 kDa extrinsic polypeptides and 28-25 kDa LHCP polypeptides.     

Influence of thylakoid protein phosphorylation on photosynthetic electron transport and photophosphorylation

Data are reported which show that thylakoid protein phosphorylation decreases photosystem II fluorescence yield and enhances the photosystem I dependent photophosphorylation catalyzed by phenazinemethosulphate in the presence of DCMU. The stimulation is larger at low light intensity, but is still observed at high intensity. These observations are interpreted to demonstrate that thylakoid protein phosphorylation causes a transfer of excitation energy from PS II to PS I, but may also have an independent stimulatory effect on PS I dependent photophosphorylation.     

Role of a novel photosystem II-associated carbonic anhydrase in photosynthetic carbon assimilation in Chlamydomonas reinhardtii

Intracellular carbonic anhydrases (CA) in aquatic photosynthetic organisms are involved in the CO2-concentrating mechanism (CCM), which helps to overcome CO2 limitation in the environment. In the green alga Chlamydomonas reinhardtii, this CCM is initiated and maintained by the pH gradient created across the chloroplast thylakoid membranes by photosystem (PS) II-mediated electron transport. We show here that photosynthesis is stimulated by a novel, intracellular alpha-CA bound to the chloroplast thylakoids. It is associated with PSII on the lumenal side of the thylakoid membranes. We demonstrate that PSII in association with this lumenal CA operates to provide an ample flux of CO2 for carboxylation.     

The cause of PSI photoinhibition at low temperatures in leaves of Cucumis sativus, a chilling‐sensitive plant

In a chilling-sensitive plant, cucumber, chilling of leaves in the light results in irreversible damage to PSI. Recent in vitro studies suggested that hydroxyl radicals, which are formed in the presence of H₂O₂ and reduced Fe-S centers, are involved in the PSI inhibition. We therefore examined this possibility in vivo. Chilling of leaves at 5°C in the light caused a temporary increase in H₂O₂ concentration, which was probably due to the net H₂O₂ production in vivo. The activity, measured at 5°C, of the thylakoid ascorbate peroxidase (APX), a key enzyme of the H₂O₂-scavenging system, was about 20% of that measured at 25°C. The isolated thylakoids retaining high thylakoid APX activity did not show light-dependent net H₂O₂ production at 25°C. However, at 5°C, net production of H₂O₂ was observed. Since the rate of electron flow to molecular oxygen in the isolated thylakoids was ca 5 mmol e^? mol^?1 Chl s^?1 at 5°C, the H₂O₂-scavenging capacity was below this level. When intact leaves were illuminated at 5°C at an irradiance of 100 µmol m^?2 s^?1, the rate of electron transport through PSII was ca 20 mmol e^? mol^?1 Chl s^?1 and more than 80% of Q_A was in the reduced state. Since thylakoids are uncoupled in cucumber leaves at 5°C in the light. ATP is not formed and energy dissipation in the form of heat is suppressed. Therefore, the electron flow to molecular oxygen would be greater than 5 mmol e^? mol^?1 Chl s^?1. Moreover, under such conditions, components in the electron transport chain, including Fe-S centers in PSI, were probably reduced. These features indicate that, when cucumber leaves are chilled in the light, hydroxyl radicals can be produced by the Fenton reaction and cause damage to PSI.     

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.     

Calcium and magnesium effect on divalent cation deficientHydrilla verticillata thylakoid electron transport activity

The role of divalent cations like magnesium (Mg²⁺) and calcium (Ca²⁺) was irrvestigated on energy distribution process ofHydrilla verticillata thylakoids. Effect of these cations was tested on relative quantum yield of photosystem (PS) II catalyzed electron transport activity, room and liquid nitrogen temperature fluorescence emission properties and thylakoid light scattering characteristics. The electron transport activity was found to be stimulated in the presence of these cations in a light intensity independent manner. The concentration of cation required for maximum stimulation was nearly 10–12 mM. Comparatively, Ca²⁺ was more effective than Mg²⁺. Cation induced stimulation in electron transport activity was not accompanied by increase in chlorophylla fluorescence intensity either at room (25°C) or liquid nitrogen (77°K) temperatures. Furthermore, 540 nm absorption and 90° light scattering properties of thylakoids remained insensitive towards divalent cations. These facts together suggest that divalent cations inHydrilla thylakoids are not effective in supporting the excitation distribution between the interacting photosystem complexes.     

A rapid technique for isolating chloroplasts with high rates of endogenous photophosphorylation

The main features of the procedure developed for rapid chloroplast isolation are: 1) gentle grinding of the plant material in a special nylon bag which retains nearly all whole cells and large debris, 2) osmoticum concentration chosen on the basis of the measured endogenous photophosphorylation, 3) a single, brief, low-speed centrifugation, 4) pellet resuspension by means of a vortex mixer, and 5) a total elapsed time from harvesting the plants to the obtaining of a resuspended chloroplast pellet of only 2 minutes. The usual isolation medium consists of an osmoticum (0.2 m sucrose) and a buffer (0.02 m N-tris-(hydroxymethyl) methyl-2-aminoethanesulfonate-NaOH, pH 7.9). In addition to these, the incubation medium contains only 200 μm ADP and 200 μm phosphate. Photophosphorylation rates of 24 μmoles ATP formed per mg chlorophyll per hour are consistently obtained using chloroplasts isolated from peas (Pisum sativum var. Laxton's Superb). The rate of endogenous photophosphorylation is maximal when the isolation and incubation media have an osmolarity of about 0.19 made up either with sucrose or with NaCl. The high rates and ease of measurement of endogenous photophosphorylation may facilitate the study of certain soluble components of chloroplasts as well as the general state of the photosynthetic ability of the plant.     

ATP formation onset lag and post-illumination phosphorylation initiated with single-turnover flashes. III. Characterization of the ATP formation onset lag and post-illumination phosphorylation for thylakoids exhibiting localized or bulk-phase delocalized energy coupling

When 100 mM KCl replaced sucrose in a chloroplast thylakoid stock suspension buffer, the membranes were converted from a localized proton gradient to a delocalized proton gradient energy coupling mode. The KCl-suspended but not the sucrose-suspended thylakoids showed pyridine-dependent extensions of the ATP onset lag and pyridine effects on post-illumination phosphorylation. The ATP formation assays were performed in a medium of identical composition, using about a 200-fold dilution of the stock thylakoid suspension; hence the different responses were due to the pretreatment, and not the conditions present in the phosphorylation assay. Such permeable buffer effects on ATP formation provide a clear indicator of delocalized proton gradients as the driving force for phosphorylation. The pyridine-dependent increases in the onset lags (and effects on post-illumination phosphorylation) were not due to different ionic conductivities of the membranes (measured by the 515 nm electrochromic absorption change), H⁺/e ^– ratios, or electron transport capacities for the two thylakoid preparations. Thylakoid volumes and [ 14C]pyridine equilibration were similar with both preparations. The KCl-induced shift toward a bulk-phase delocalized energy coupling mode was reversed when the thylakoids were placed back in a low-salt medium.Proton uptake, at the ATP-formation energization threshold flash number, was much larger in the KCl-treated thylakoids and they also had a longer ATP formation onset lag, when no pyridine was present. These results are consistent with the salt treatment exposing additional endogenous buffering groups for interaction with the proton gradient. The concomitant appearance of the pyridine buffer effects implies that the additional endogenous buffering groups must be located on proteins directly exposed in the aqueous lumen phase.Kinetic analysis of the decay of the post-illumination phosphorylation in the two thylakoid preparations showed different apparent first-order rate constants, consistent with there being two different compartments contributing to the proton reservoirs that energize ATP formation. We suggest that the two compartments are a membrane-phase localized compartment operative in the sucrose-treated thylakoids and the bulk lumen phase into which protons readily equilibrate in the KCl-treated thylakoids.     

Freezing injury in cold-acclimated and unhardened spinach leaves

Spinach plants (Spinacia oleracea L.) were frost-hardened by cold-acclimation to 1° C or kept in an unhardy state at 20°/14° C in phytotrons. Detached leaves were exposed to temperatures below 0°C. Rates of photosynthetic CO₂ uptake by the leaves, recorded after frost treatment, served as a measure of freezing injury. Thylakoid membranes were isolated from frost-injured leaves and their photosynthetic activities tested. Ice formation occurred at about-4° to-5° C, both in unhardened and cold-acclimated leaves. After thawing, unhardened leaves appeared severely damaged when they had been exposed to-5° to-8° C. Acclimated leaves were damaged by freezing at temperatures between-10° to-14° C. The pattern of freezing damage was complex and appeared to be identical in hardened and unhardened leaves: 1. Inactivation of photosynthesis and respiration of the leaves occurred almost simultaneously. 2. When the leaves were partly damaged, the rates of photosynthetic electron transport and noncyclic photophosphorylation and the extent of light-induced H⁺ uptake by the isolated thylakoids were lowered at about the same degree. The dark decay of the proton gradient was, however, not stimulated, indicating that the permeability of the membrane to-ward protons and metal cations had not increased. 3. As shown by partial reactions of the electron transport system, freezing of leaves predominantly inhibited the oxygen evolution, but photosystem II and photosystem I-dependent electron transport were also impaired. 4. Damage of the chloroplast envelope was indicated by a decline in the percentage of intact chloroplasts found in preparations from injured leaves. The results are discussed in relation to earlier studies on freezing damage of thylakoid membranes occurring in vitro.Abbreviations Chl chlorophyll - DCPIP 2,6-dichlorophenol indophenol - HEPES N-2-hydroxyethylpiperazine-N-2-ethane sulfonic acid - MES 2(N-morpholino) ethane sulfonic acid     

A study of factors which regulate the membrane appression of lettuce thylakoids in relation to irradiance

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

Photosynthetic acclimation of Tradescantia albiflora to growth irradiance: morphological, ultrastructural and growth responses

Tradescantia albiflora (Kunth), a trailing ground species naturally occurring in deep shade in rainforests, has an unusual photosynthetic acclimation profile for growth irradiance. Although capable of increasing its capacity for electron transport, photophosphorylation and carbon fixation when grown in full sunlight, Tradescantia has constant chlorophyll alb ratios, photosystem reaction centre stoichiometry and pigment-protein composition at all growth irradiances (Chow et al. 1991. Physiol. Plant. 81: 175–182). To gain an insight into the compensatory strategies which allow Tradescantia to grow in both high and low lights, plants were grown under shade cloth (100 to 1.4% relative growth irradiance) and leaf and chloroplast attributes were compared. While shade Tradescantia chloroplasts had three times more chlorophyll per chloroplast and twice the length of thylakoid membranes compared to plants grown in full sunlight, the ratios of appressed to nonappressed thylakoid membranes were constant. The average net surface charge density of destacked thylakoids was the same for plants grown at moderate and low-irradiance, consistent with their similar stacking profiles. Tradescantia plants grown in direct sunlight had 10-times more fresh and dry weight per plant compared to plants grown in shade, despite a lower photosynthetic capacity on a leaf area basis with partial photoinhibition. We conclude that having a light-harvesting apparatus permanently locked into the "shade-plant mode " does not necessarily prevent a plant from thriving in high light. Analyses of leaf growth at different irradiances provide a partial explanation of the manner in which Tradescantia compensates for very low photosynthetic capacity per unit leaf in sunlight.