A MAJOR POLYPEPTIDE OF CHLOROPLAST MEMBRANES OF CHLAMYDOMONAS REINHARDI : Evidence for Synthesis in the Cytoplasm as a Soluble Component

Electrophoresis of thylakoid membrane polypeptides from Chlamydomonas reinhardi revealed two major polypeptide fractions. But electrophoresis of the total protein of green cells showed that these membrane polypeptides were not major components of the cell. However, a polypeptide fraction whose characteristics are those of fraction c (a designation used for reference in this paper), one of the two major polypeptides of thylakoid membranes, was resolved in the electrophoretic pattern of total protein of green cells. This polypeptide could not be detected in dark-grown, etiolated cells. Synthesis of the polypeptide occurred during greening of etiolated cells exposed to light. When chloramphenicol (final concentration, 200 µg/ml) was added to the medium during greening to inhibit chloroplastic protein synthesis, synthesis of chlorophyll and formation of thylakoid membranes were also inhibited to an extent resulting in levels of chlorophyll and membranes 20–25% of those found in control cells. However, synthesis of fraction c was not affected by the drug. This polypeptide appeared in the soluble fraction of the cell under these conditions, indicating that this protein was synthesized in the cytoplasm as a soluble component. When normally greening cells were transferred from light to dark, synthesis of the major membrane polypeptides decreased. Also, it was found that synthesis of both subunits of ribulose 1, 5-diphosphate carboxylase was inhibited by chloramphenicol, and that synthesis of this enzyme stopped when cells were transferred from light to dark.     

Accumulation of Chlorophyll a/b-Binding Polypeptides in Chlamydomonas reinhardtii y-1 in the Light or Dark at 38 degrees C : Evidence for Proteolytic Control

The kinetics of accumulation of light harvesting chlorophyll (Chl) a/b-binding polypeptides (LHCPs) in thylakoid membranes were analyzed during greening of Chlamydomonas reinhardtii y-1 at 38°C. Initial accumulation of LHCPs in thylakoid membranes was linear; LHCP precursors or polypeptides in transit within the chloroplast stroma were not detected. The rate of accumulation in the light was at least five-fold greater than that in the dark. The relatively small amount of LHCPs that accumulated in the dark was integrated properly in the membrane, as judged by the pattern of cleavage in vitro by exogenous proteases, and did not turn over at a significant rate in vivo. The kinetic data suggested that in y-1 cells either translation of LHCP mRNA was inhibited in the dark or newly synthesized polypeptides were degraded concurrently with transport into the chloroplast unless rescued by Chl. LHCPs accumulated in cells of the Chl b-deficient strain pg-113 at the same rate in the dark or the light at 38°C, an indication that light did not affect translation of LHCP mRNA. Membrane-associated LHCPs in pg-113 cells were completely degraded, in contrast to those in y-1 cells, by exogenous proteases, which suggested that pg-113 cells are deficient in a proteolytic activity. A peptidase was recovered from y-1 cells in a membrane fraction with a buoyant density slightly less than that of thylakoid membranes. Although a role for this activity in degradation of LHCPs has not been established, the specific activity of this peptidase in pg-113 cells was only 10 to 15% of the level in y-1 cells.     

Photosynthetic Membranes of Porphyridium cruentum: An Analysis of Chlorophyll-Protein Complexes and Heme-Binding Proteins

Three chlorophyll-protein complexes (CP I, CP III, CP IV) were electrophoretically separated from thylakoids of the eukaryotic red alga Porphyridium cruentum. CP I contained the primary photochemical reaction center of photosystem I as judged by its light-induced reversible absorbance change at 700 nanometers, by its fluorescence emission maximum at 720 nanometers (−196°C), and by the molecular weight of its apoprotein (68,000 daltons). CP III and CP IV appeared to belong with photosystem II as suggested by the absence of light-reversible absorbance at 700 nanometers, by their fluorescence maximum at 690 nanometers (−196°C), and by the presence of a chlorophyll-binding polypeptide with a molecular weight of about 52,000 daltons. CP IV when completely denatured had two additional polypeptides of about 40,000 and 48,000 daltons. All three chlorophyll-protein complexes contained carotenoids: the chlorophyll/carotenoid molar ratio of 15:1 for CP I, and 20:1 for CP III and CP IV. The thylakoid membranes of P. cruentum contained four cytochromes, detected by heme-dependent peroxidase activity, but there was no observed association with the electrophoretically separated chlorophyll-protein complexes.     

Low temperature development of winter rye leaves alters the detergent solubilization of thylakoid membranes

Thylakoids isolated from leaves of winter rye (Secale cereale L. cv Puma) grown at either 20 or 5°C were extracted with the nonionic detergents Triton X-100 and octyl glucoside. Less total chlorophyll was extracted from 5°C thylakoids by these detergents under all conditions, including pretreatment with cations. Thylakoids from either 20 or 5°C leaves were solubilized in 0.7% Triton X-100 and centrifuged on sucrose gradients to purify the light harvesting complex (LHCII). Greater yields of LHCII were obtained by cation precipitation of particles derived from 20°C thylakoids than from 5°C thylakoids. When 20 and 5°C thylakoids were phosphorylated and completely solubilized in sodium dodecyl sulfate, no differences were observed in the ³²Pi-labeling characteristics of the membrane polypeptides. However, when phosphorylated thylakoids were extracted with octyl glucoside, extraction of LHCII associated with the 5°C thylakoids was markedly reduced in comparison with the extraction of LHCII from 20°C membranes. Since 20 and 5°C thylakoids exhibited significant differences in the Chl content and Chl a/b ratios of membrane fractions produced after solubilization with either Triton X-100 or octyl glucoside, and since few differences between the proteins of the two membranes could be observed following complete denaturation in sodium dodecyl sulfate, we conclude that the integral structure of the thylakoid membrane is affected during rye leaf development at low temperature.     

Greening under High Light or Cold Temperature Affects the Level of Xanthophyll-Cycle Pigments, Early Light-Inducible Proteins, and Light-Harvesting Polypeptides in Wild-Type Barley and the Chlorina f2 Mutant

Etiolated seedlings of wild type and the chlorina f2 mutant of barley (Hordeum vulgare) were exposed to greening at either 5°C or 20°C and continuous illumination varying from 50 to 800 μmol m⁻² s⁻¹. Exposure to either moderate temperature and high light or low temperature and moderate light inhibited chlorophyll a and b accumulation in the wild type and in the f2 mutant. Continuous illumination under these greening conditions resulted in transient accumulations of zeaxanthin, concomitant transient decreases in violaxanthin, and fluctuations in the epoxidation state of the xanthophyll pool. Photoinhibition-induced xanthophyll-cycle activity was detectable after only 3 h of greening at 20°C and 250 μmol m⁻² s⁻¹. Immunoblot analyses of the accumulation of the 14-kD early light-inducible protein but not the major (Lhcb2) or minor (Lhcb5) light-harvesting polypeptides demonstrated transient kinetics similar to those observed for zeaxanthin accumulation during greening at either 5°C or 20°C for both the wild type and the f2 mutant. Furthermore, greening of the f2 mutant at either 5°C or 20°C indicated that Lhcb2 is not essential for the regulation of the xanthophyll cycle in barley. These results are consistent with the thesis that early light-inducible proteins may bind zeaxanthin as well as other xanthophylls and dissipate excess light energy to protect the developing photosynthetic apparatus from excess excitation. We discuss the role of energy balance and photosystem II excitation pressure in the regulation of the xanthophyll cycle during chloroplast biogenesis in wild-type barley and the f2 mutant.     

ClpB1 Overproduction in Synechocystis sp. Strain PCC 6803 Increases Tolerance to Rapid Heat Shock

ClpB1 is a heat shock protein known to disaggregate large protein complexes. Constitutive, 16-fold ClpB1 overproduction in the cyanobacterium Synechocystis sp. strain PCC 6803 increased cell survival by 20-fold when cultures were heated quickly (1°C/s) to 50°C and delayed cell death by an average of 3 min during incubation at high temperatures (>46°C). Cooverexpression of ClpB1 and another heat shock protein, DnaK2, further increased cell survival. According to immunocytochemistry results, ClpB1 is dispersed throughout the cytoplasm but is concentrated in specific areas and is more prevalent near thylakoid membranes. However, ClpB1 overproduction does not lead to a change in the morphology, chlorophyll content, or photosystem ratio. Whereas electron microscopy demonstrated that apparent protein aggregation occurred after heat treatment in the control strain, protein aggregate size was maintained in the ClpB1 overexpresser. Constitutive ClpB1 overproduction allows an earlier response to heat shock and protects from rapid heating of cultures.     

Short term acclimation of spinach to high temperatures: effect on chlorophyll fluorescence at 293 and 77 Kelvin in intact leaves

Using intact leaves of Spinacia oleracea (L.), reversible temperature-induced changes in chlorophyll fluorescence emitted at room temperature and at 77K were studied. Interpretation of fluorescence at 77K was largely facilitated by developing a new method to minimize reabsorption artifacts (`diluted leaf-powder'). Leaves of plants grown at 15 to 20°C were exposed for several hours to different temperatures. Upon incubation at 35°C in the dark or in the light, the following changes in 77K fluorescence occurred with a half-time of less than 1 hour: (a) the initial fluorescence (F₀) of photosystem I increased by 15%, while that one of photosystem II somewhat decreased; (b) although variable fluorescence declined in both photosystems, the decrease in photosystem II (40%) was more severe; (c) the changes were less significant after 480-nanometer excitation light was replaced by 430-nanometer light. The data were interpreted in terms of a reversible, temperature-induced change in thylakoid structure and related change in the distribution of the absorbed energy in favor of photosystem I, at the expense of photosystem II excitation, probably accompanied by an increase in the rate of thermal deactivation of excited states. The considerable decrease in the variable part of room temperature fluorescence gives rise to the suggestion that this transition has lowered the reduction level of plastoquinone, i.e. has increased electron flow through photosystem I, relative to photosystem II. Possible physiological and mechanistic analogies between this temperature-induced state transition and the light-dependent state 1-state 2 regulation has been discussed.     

Heterogeneity and Photoinhibition of Photosystem II Studied with Thermoluminescence

Thermoluminescence (TL) signals were recorded from grana stacks, margins, and stroma lamellae from fractionated, dark-adapted thylakoid membranes of spinach (Spinacia oleracea L.) in the absence and in the presence of 2,6-dichlorphenylindophenol (DCMU). In the absence of DCMU, the TL signal from grana fractions consisted of a homogenous B-band, which originates from recombination of the semi-quinone Q_B⁻ with the S₂ state of the water-splitting complex and reflects active photosystem II (PSII). In the presence of DCMU, the B-band was replaced by the Q-band, which originates from an S₂Q_A⁻ recombination. Margin fractions mainly showed two TL-bands, the B- and C-bands, at approximately 50°C in the absence of DCMU, and Q- and C-bands in the presence of DCMU. The C-band is ascribed to a Tyr_D⁺-Q_A⁻ recombination. In the absence of DCMU, the fractions of stromal lamellae mainly gave rise to a TL emission at 42°C. The intensity of this band was independent of the number of excitation flashes and was shifted to higher temperatures (52°C) after the addition of DCMU. Based on these observations, this band was considered to be a C-band. After photoinhibitory light treatment of uncoupled thylakoid membranes, the TL intensities of the B- and Q-bands decreased, whereas the intensity at 45°C (C-band) slightly increased. It is proposed that the 42 to 52°C band that was observed in marginal and stromal lamellae and in photoinhibited thylakoid membranes reflects inactive PSII centers that are assumed to be equivalent to inactive PSII Q_B-nonreducing centers.     

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.     

CHANGES IN CHEMICAL COMPOSITION OF THYLAKOID MEMBRANES DURING GREENING OF THE y-1 MUTANT OF CHLAMYDOMONAS REINHARDI

Two fractions of thylakoid membranes (TMF) have been isolated from disrupted (French press) algal cells by using a discontinuous sucrose gradient. TMF-II consists mostly of thylakoid membranes still partially organized in grana; it contains also fragments of chloroplast envelope, pyrenoid tubules, and starch granules; thus it amounts to a fraction of chloroplast fragments which have lost practically all matrix components. TMF-I consists of smaller chloroplast fragments and is contaminated to a larger extent than TMF-II by other subcellular components, primarily mitochondria. TMF-II accounts for about 12% of the protein and 30% of the chlorophyll of the whole cell; it contains cytochrome 554 and carotenoids in the same ratio to chlorophyll as the latter, and shows photosystems I and II activities but lacks enzymatic activities characteristic of the dark reactions. During the greening of the y-1 mutant of Chlamydomonas, TMF's have been isolated over a range of chlorophyll concentrations from 5 to 25 µg/10⁷ cells. The results showed that during this period the ratios of chlorophyll to cytochrome 554 and of chlorophyll to carotenoids, and the relative concentrations of individual carotenoids were continuously changing. The findings support the view that during greening, thylakoid membranes are produced by multistep assembly.     

Influence of light on the heat sensitivity of the photosynthetic apparatus in isolated spinach chloroplasts

The most heat-sensitive functions of chloroplasts in Spinacia oleracea L. including the stromal carboxylation reaction, the light-induced electrical field gradient across the thylakoid membrane, as well as the overall photosynthetic CO₂ fixation were less affected by heat if chloroplasts were heated in the light: 50% inactivation occurred around 35°C in the dark and around 40°C in the light. Relative low light intensities were sufficient to obtain optimal protection against heat. In contrast, the light-induced ΔpH across the thylakoid membrane, the photophosphorylation, and the photochemical activity of photosystem II which were less sensitive to heat in the dark (50% inactivation above 40°C) were not protected by light. Photosystem II even was destabilized somewhat by light.

The effect of light on the heat sensitivity of the water-splitting reaction was dependent on the pH in the medium. Protection by light only occurred at alkaline pH, in which case heat sensitivity was high (50% inactivation at 33°C in the dark and at 38°C in the light). Protection was prevented by uncouplers. At pH 6.8 when the heat sensitivity was low in any case (50% inactivation at 41°C in the dark), light had no further protecting effect.

Protection by light has been discussed in terms of light-induced transport of protons from the stroma to the thylakoid space and related ion fluxes.

     

Photochemical Apparatus Organization in the Chloroplasts of Two Beta vulgaris Genotypes

The composition and structural organization of thylakoid membranes of a low chlorophyll mutant of Beta vulgaris was investigated using spectroscopic, kinetic and electrophoretic techniques. The data obtained were compared with those of a standard F1 hybrid of the same species. The mutant was depleted in chlorophyll b relative to the hybrid and it had a higher photosystem II/photosystem I reaction center (Q/P₇₀₀) ratio and a smaller functional chlorophyll antenna size. Analysis of thylakoid membranes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the mutant lacked a portion of the chlorophyll a/b light-harvesting complex but was enriched in the photosystem II reaction center chlorophyll protein complex. Comparison of functional antenna sizes and of photosystem stoichiometries determined electrophoretically were in good agreement with those determined spectroscopically. Both approaches indicated that about 30% of the total chlorophyll was associated with photosystem I and about 70% with photosystem II. A greater proportion of photosystem II_β was detected in the mutant. The results suggest that a higher photosystem II to photosystem I ratio in the sugar beet mutant has apparently compensated for the smaller photosystem II chlorophyll light-harvesting antenna in its chloroplasts. Moreover, a lack of chlorophyll a/b light-harvesting complex correlates with the abundance of photosystem II_β. It is proposed that a developmental relationship exists between the two types of photosystem II where photosystem II_β is a precursor form of photosystem II_α occurring prior to the addition of the chlorophyll a/b light-harvesting complex and grana formation.     

Modifications to Thylakoid Composition during Development of Maize Leaves at Low Growth Temperatures

The effects of reductions in growth temperature on the development of thylakoids of maize (Zea mays var LG11) leaves are examined. Thylakoids isolated from mesophyll cells of leaves grown at 17° and 14°C, compared with 25°C, exhibited a decreased accumulation of many polypeptides, which was accompanied by a loss of activity of photosystems (PS) I and II. Probing the polypeptide profiles with a range of antibodies specific for thylakoid proteins demonstrated that a number of polypeptides encoded by the chloroplast genome failed to accumulate at low temperatures. Although thylakoid protein synthesis was reduced severely at 14°C compared with 25°C, major synthesis of both chloroplast and nuclear encoded polypeptides was detected. It is suggested that the lack of accumulation of some thylakoid proteins at low temperatures may be due to an inability to stabilize the proteins in the membranes. A number of thylakoid polypeptides were found to appear as the growth temperature was decreased. Analyses of pigments and polypeptides demonstrated that decreases in the photosystem reaction center core complexes occur relative to the light harvesting complex associated with PS II at reduced growth temperatures. Differential effects on the development of PSI and PSII were also observed, with PSII activity being preferentially reduced. Reductions in PSII content and activity occurred in parallel with decreases in the quantum yield and light-saturated rate of CO₂ assimilation. Fractionation of thylakoid pigment-protein complexes showed that the ratio of monomeric:oligomeric form of the light harvesting complex associated with PSII increased at low growth temperature, which is consistent with a chill-induced modification of thylakoid organization. Many, but not all, of the characteristic changes in thylakoid protein metabolism, which were observed when leaves were grown at low temperatures in controlled environments, were identified in leaves of a field maize crop during the early growing season when low temperatures were experienced by the crop. Chill-induced perturbations of thylakoid development can occur in the field in temperate regions and may have implications for the photosynthetic productivity of the crop.     

BIOGENESIS OF CHLOROPLAST MEMBRANES : IV. Lipid and Pigment Changes during Synthesis of Chloroplast Membranes in a Mutant of Chlamydomonas reinhardi y-1

The glycolipid, phospholipid, pigment, and fatty acid content in whole y-1 cells during the greening process have been investigated. The time course of their changes indicates that phosphatidyl glycerol and glycolipids are the main lipids synthesized specifically during illumination of dark-grown cells, concomitant with an increase in the polyunsaturated C18:2 and C18:3 fatty acids. The pigment complex of light-grown cells consists mainly of chlorophylls a and b, lutein, β-carotene, violaxanthin, and neoxanthin. During the greening process, chlorophylls a and b are synthesized in constant proportions (ratio a/b equals 2.6), β-carotene and violaxanthin do not change significantly, and lutein and neoxanthin increase. The molar ratios of the different lipids and pigment to total chlorophyll during greening has been calculated. It was found that during the initial phase of greening when chlorophyll is synthesized at increasing rates, the molar ratios of various lipids and pigments to chlorophyll decrease and tend to become constant when chlorophyll and membrane synthesis proceed at constant rates. The implication of these findings with respect to the concept of membrane assembly through a spontaneous single step process is discussed     

Response of the Photosynthetic Apparatus in Dunaliella salina (Green Algae) to Irradiance Stress

The response of the photosynthetic apparatus in the green alga Dunaliella salina, to irradiance stress was investigated. Cells were grown under physiological conditions at 500 millimoles per square meter per second (control) and under irradiance-stress conditions at 1700 millimoles per square meter per second incident intensity (high light, HL). In control cells, the light-harvesting antenna of photosystem I (PSI) contained 210 chlorophyll a/b molecules. It was reduced to 105 chlorophyll a/b in HL-grown cells. In control cells, the dominant form of photosystem II (PSII) was PSII_α(about 63% of the total PSII) containing >250 chlorophyll a/b molecules. The smaller antenna size PSII_β centers (about 37% of PSII) contained 135 ± 10 chlorophyll a/b molecules. In sharp contrast, the dominant form of PSII in HL-grown cells accounted for about 95% of all PSII centers and had an antenna size of only about 60 chlorophyll a molecules. This newly identified PSII unit is termed PSII_γ. The HL-grown cells showed a substantially elevated PSII/PSI stoichiometry ratio in their thylakoid membranes (PSII/PSI = 3.0/1.0) compared to that of control cells (PSII/PSI = 1.4/1.0). The steady state irradiance stress created a chronic photoinhibition condition in which D. salina thylakoids accumulate an excess of photochemically inactive PSII units. These PSII units contain both the reaction center proteins and the core chlorophyll-protein antenna complex but cannot perform a photochemical charge separation. The results are discussed in terms of regulatory mechanism(s) in the plant cell whose function is to alleviate the adverse effect of irradiance stress.     

Photosynthetic Decline from High Temperature Stress during Maturation of Wheat : I. Interaction with Senescence Processes

Photosynthetic capacity decreases rapidly when temperate species are exposed to heat stress during reproductive development. We investigated whether injury in wheat (Triticum aestivum L.) resulted from general acceleration of senescence processes or specific heat-induced lesions. In situ photosynthetic capacity of leaf discs and thylakoid reactions were measured using flag leaf tissue from two cultivars maintained at 20 and 35°C during maturation. Photosynthetic rates of leaf discs decreased faster at 35 than at 20°C and were more photolabile in cv Len than in cv Waverly at high temperature. Patterns of thylakoid breakdown also differed in the two wheat genotypes at 20°C: intersystem electron transport and photosystem II activity decreased linearly during postanthesis development in Len wheat, whereas coupling of photophosphorylation to electron transport declined late during senescence in Waverly wheat. Heat stress induced early loss of intersystem electron transport followed sequentially by decreased silicomolybdic acid, + 3-(3,4-dichlorophenyl)-1-dimethylurea-mediated photosystem II activity and 2,5-dichloro-p-benzoquinone-mediated photosystem II activity in Len. Stress accelerated the uncoupling process, but loss of intersystem electron transport and photosystem II activities was slower in Waverly than in Len. We conclude that high temperature initially accelerated thylakoid component breakdown, an effect similar to normal senescence patterns. Thylakoid breakdown may induce a destabilizing imbalance between component reaction rates; an imbalance between photosystem II and cytochrome f/b₆-mediated activities would be particularly damaging during heat stress.     

Biogenesis of Thylakoid Membranes in Chlamydomonas reinhardtii y1 (A Kinetic Study of Initial Greening)

Initiation of thylakoid membrane assembly was examined in degreened cells of Chlamydomonas reinhardtii y1 cells depleted of thylakoid membranes and photosynthetic activity by growth in the dark for 3 to 4 d. Photoreductive activities of photosystem II (PSII) and photosystem I (PSI) increased with no apparent lag when degreened cells were exposed to light at 38[deg]C. However, fluorescence transients induced by actinic light, which reflect the functional state of PSII, changed only slightly during the first 2 h of greening. When these cells were treated with 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) or saturating light, fluorescence increased commensurate with the cellular content of chlorophyll. In similar experiments with greening cells of C. reinhardtii CC-2341 (ac-u-g-2.3), a PSI-minus strain, fluorescence increased with chlorophyll without treatment with DCMU. These data suggested that fluorescence of initial PSII centers in greening y1 cells was quenched by activity of PSI. Continuous monitoring of fluorescence in the presence or absence of DCMU showed that assembly of quenched PSII centers occurred within seconds after exposure of y1 cells to light. These results are consistent with initial assembly of PSI and PSII within localized domains, where their proximity allows efficient energy coupling.     

Thermal Acclimation of Photosynthetic Electron Transport Activity by Thylakoids of Saxifraga cernua

Thermal acclimation by Saxifraga cernua to low temperatures results in a change in the optimum temperature for gross photosynthetic activity and may directly involve the photosynthetic apparatus. In order to test this hypothesis photosynthetic electron transport activity of S. cernua thylakoids acclimated to growth temperatures of 20°C and 10°C was measured in vitro. Both populations exhibited optimum temperatures for whole chain and PSII electron transport activity at temperatures close to those at which the plants were grown. Chlorophyll a fluorescence transients from 10°C-acclimated leaves showed higher rates in the rise and subsequent quenching of variable fluorescence at low measuring temperatures; 20°C-acclimated leaves showed higher rates of fluorescence rise at higher measuring temperatures. At these higher temperatures, fluorescence quenching rates were similar in both populations. The kinetics of State 1-State 2 transitions in 20°C- and 10°C-acclimated leaf discs were measured as changes in the magnitude of the fluorescence emission maxima measured at 77K. Leaves acclimated at 10°C showed a larger F730/F695 ratio at low temperatures, while at higher temperatures, 20°C-acclimated leaves showed a higher F730/F695 ratio after the establishment of State 2. High incubation temperatures also resulted in a decrease in the F695/F685 ratio for 10°C-acclimated leaves, suggesting a reduction in the excitation transfer from the light-harvesting complex of photosystem II to photosystem II reaction centers. The relative amounts of chlorophyll-protein complexes and thylakoid polypeptides separated electro-phoretically were similar for both 20°C- and 10°C-acclimated leaves. Thus, photosynthetic acclimation to low temperatures by S. cernua is correlated with an increase in photosynthetic electron transport activity but does not appear to be accompanied by major structural changes or different relative amounts in thylakoid protein composition.     

Altered chloroplast structure and function in a mutant of Arabidopsis deficient in plastid glycerol-3-phosphate acyltransferase activity

Mutants of Arabidopsis thaliana deficient in plastid glycerol-3-phosphate acyltransferase activity have altered chloroplast membrane lipid composition. This caused an increase in the number of regions of appressed membrane per chloroplast and a decrease in the average number of thylakoid membranes in the appressed regions. The net effect was a significant decrease in the ratio of appressed to nonappressed membranes. A comparison of 77 K fluorescence emission spectra of thylakoid membranes from the mutant and wild type indicated that the ultrastructural changes were associated with an altered distribution of excitation energy transfer from antenna chlorophyll to photosystem II and photosystem I in the mutant. The changes in leaf lipid composition did not significantly affect growth or development of the mutant under standard conditions. However, at temperatures above 28°C the mutant grew slightly more rapidly than the wild type, and measurements of temperature-induced fluorescence yield enhancement suggested an increased thermal stability of the photosynthetic apparatus of the mutant. These effects are consistent with other evidence suggesting that membrane lipid composition is an important determinant of chloroplast structure but has relatively minor direct effects on the function of the membrane proteins associated with photosynthetic electron transport.     

Temperature and plant adaptation. I. Interaction of temperature and light in the synthesis of chlorophyll in corn

The effect of temperature and light intensity have been studied in relation to the greening of etiolated corn (Zea mays cv. Pioneer 309-B) seedlings. Chlorophyll accumulation is rapid at high temperature (28°) under all conditions of light intensity. At low temperature (16°), and particularly in combination with high light intensity (3000-4500 ft-c), the accumulation of both chlorophyll and carotene is inhibited.

Low pigment content at 16° is not directly due to a block in the pigment synthesizing mechanism, but rather to the photodestruction of chlorophyll prior to its stabilization in the membrane structure of the chloroplast lamellae. The parallel reduction in carotene content at high light intensity is probably a contributing factor, because of its role in protecting chlorophyll from photodestruction. The greater severity of photo-oxidation of chlorophyll at low temperature in corn when compared with wheat, appears to be due to a slower rate of protochlorophyllide synthesis and subsequent esterification. Thus in corn at 16° there is a prolongation of the photosensitive stage during chlorophyll synthesis. Photo-oxidation at 16° has also been shown to be a function of the incident light energy, with the photosynthetic pigments acting as receptors for their own destruction.

In comparison with the behavior of corn, wheat seedlings green rapidly at high light intensity at both 16° and 28°. This contrasting temperature response with respect to chlorophyll synthesis may underlie a fundamental difference in adaptation of these 2 species to growth in the temperate zones of the world.