Changes In Chloroplast Structure Research Articles

Special issue in honour of Prof. Reto J. Strasser - Phosphorus deficiency affects the I-step of chlorophyll a fluorescence induction curve of radish

Phosphorus (P) is an essential nutrient for plant development and its fertilisation efficiency is an important issue in agriculture. Difficulties in diagnosing P deficiency in plants have created a demand for a quick and noninvasive method/tool for assessing plant P status on-site. It has been well documented that chlorophyll fluorescence measurement is a very useful and sensitive tool for screening nutrients deficiency in plants. Although impact of P deficiency on photosynthesis process has been widely examined, its effect/s on photosynthetic electron transport between the two photosystems is still not clear. In this work, we studied the response of radish plants to P deficiency throughout stress conditions and recovery period. Leaf phosphorus content, growth rate, and photosynthetic activity and efficiency were monitored. Under P deficiency, a decrease in all of these physiological features was observed. Moreover, some changes in chloroplast structure were explored, including a reduction in chloroplast grana amount and loose arrangement. Additionally, the I-step disappeared from chlorophyll fluorescence induction curves. This can be linked to the inactivation of PSI and suppression of the cyclic phosphorylation. The observed decreases in quantum yield and the efficiency of electron transport chain in P-deficient plants could be due to downregulation mechanisms in the photosynthetic apparatus. However, both the inhibition of net photosynthetic rate, as well as the severe effect on the I-step were reversible when plants were resupplied with phosphorus. Loss of I-step can be used as a specific bioindicator for early detection of phosphorus deficiency in radish plants.

Chlorophyll catabolism precedes changes in chloroplast structure and proteome during leaf senescence.

The earliest visual changes of leaf senescence occur in the chloroplast as chlorophyll is degraded and photosynthesis declines. Yet, a comprehensive understanding of the sequence of catabolic events occurring in chloroplasts during natural leaf senescence is still missing. Here, we combined confocal and electron microscopy together with proteomics and biochemistry to follow structural and molecular changes during Arabidopsis leaf senescence. We observed that initiation of chlorophyll catabolism precedes other breakdown processes. Chloroplast size, stacking of thylakoids, and efficiency of PSII remain stable until late stages of senescence, whereas the number and size of plastoglobules increase. Unlike catabolic enzymes, whose level increase, the level of most proteins decreases during senescence, and chloroplast proteins are overrepresented among these. However, the rate of their disappearance is variable, mostly uncoordinated and independent of their inherent stability during earlier developmental stages. Unexpectedly, degradation of chlorophyll‐binding proteins lags behind chlorophyll catabolism. Autophagy and vacuole proteins are retained at relatively high levels, highlighting the role of extra‐plastidic degradation processes especially in late stages of senescence. The observation that chlorophyll catabolism precedes all other catabolic events may suggest that this process enables or signals further catabolic processes in chloroplasts.

Ascorbate-Glutathione Cycle and Ultrastructural Analyses of Two Kenaf Cultivars (Hibiscus cannabinus L.) under Chromium Stress.

Kenaf (Hibiscus cannabinus L.) with high tolerance to chromium (Cr) can be used in the phytoremediation of chromium-contaminated soil. However, the mechanisms of chromium accumulation and tolerance in kenaf are still unclear. A hydroponic experiment was taken to screen two kenaf cultivars with Cr tolerance among nine kenaf cultivars via a tolerance index. This is first time the ascorbate-glutathione (AsA-GSH) cycle and chloroplast structural changes involved in Cr tolerance of two kenaf cultivars are explored. This study indicated that enhancement of chromium concentrations reduced nine kenaf growth rates and plant biomass. In addition, in all the nine cultivars, the roots had higher Cr accumulation than the shoots. Cr-tolerant cultivar Zhe70-3 with the maximum tolerant index had the significantly higher enzymatic activities of ascorbate peroxidase (APX), glutathione reductase (GR), dehydroascorbate reductase (DHAR) and mono- dehydroascorbate reductase (MDHAR) in non-enzymatic antioxidant system compared to Cr-sensitive cultivar Zhe77-1. In addition, higher GSH and AsA contents and lower damages of chloroplast ultrastructure were observed in Zhe70-3 under Cr treatment. In conclusion, Cr stress can cause less oxidative stress and destruction of chloroplast ultrastructure in Cr-tolerant cultivar Zhe70-3, and the AsA-GSH cycle may play a crucial role in kenaf Cr tolerance.

Three-Dimensional Analysis of Chloroplast Structures Associated with Virus Infection.

Chloroplasts are multifunctional organelles whose morphology is affected by environmental stresses. Although the three-dimensional (3D) architecture of thylakoid membranes has been reported previously, a 3D visualization of chloroplast under stress has not been explored. In this work, we used a positive-strand RNA ((+)RNA) virus, barley stripe mosaic virus (BSMV) to observe chloroplast structural changes during infection by electron tomography. The analyses revealed remodeling of the chloroplast membranes, characterized by the clustering of outer membrane-invaginated spherules in inner membrane-derived packets. Diverse morphologies of cytoplasmic invaginations (CIs) were evident with spherules at the periphery and different sized openings connecting the CIs to the cytoplasm. Immunoelectron microscopy of these viral components verified that the aberrant membrane structures were sites for BSMV replication. The BSMV αa replication protein localized at the surface of the chloroplasts and played a prominent role in eliciting chloroplast membrane rearrangements. In sum, our results have revealed the 3D structure of the chloroplasts induced by BSMV infection. These findings contribute to our understanding of chloroplast morphological changes under stress conditions and during assembly of plant (+)RNA virus replication complexes.

Differential Effects of Cold Stress on Chloroplasts Structures and Photosynthetic Characteristics in Cold-Sensitive and Cold-Tolerant Cultivars of Sugarcane

Freezing often results in a significant loss of sugarcane production. An investigation was conducted in two sugarcane cultivars, GT28 (Guitang 28, cold tolerant) and YL6 (Yuanlin 6, cold sensitive) to understand the cold tolerance mechanisms. The plants at early ripening stage, 245 days after planting, were exposed to cold stress (0 °C) for 0, 2, 4, and 6 days. The changes in chloroplast structure, microtubule, and physiological parameters under cold stress were determined. The morphology and chloroplast ultrastructure of the cultivar GT28 were observed normal while those of the cultivar YL6 was severely damaged and accumulation of starch grains in chloroplasts was evident under cold stress. At the beginning of cold stress, the microtubules in the cultivar GT28 was more obviously depolymerized than those in the cultivar YL6. With extended cold stress, the microtubules in the cultivar YL6 decreased noticeably and the chloroplast membrane became indistinct, while the microtubule fluorescence in the cultivar GT28 was intensified, indicating that the structure of microtubules was reorganized. Consequently, the cultivar GT28 showed a greater photosynthetic activity than YL6 under cold stress. The cold tolerance of the cultivar GT28 appeared to be related to the integrity of chloroplast structure, the stability of microtubule structure, and balanced physiological metabolisms. These results revealed that more stable chloroplast structure and microtubule structure under cold stress are the important physiological foundation of cold-tolerant sugarcane cultivars.

Protection of thylakoids against combined light and drought by a lumenal substance in the resurrection plant Haberlea rhodopensis.

Haberlea rhodopensis is a perennial, herbaceous, saxicolous, poikilohydric flowering plant that is able to survive desiccation to air-dried state under irradiance below 30 micromol m-2 s-1. However, desiccation at irradiance of 350 micromol m-2 s-1 induced irreversible changes in the photosynthetic apparatus, and mature leaves did not recover after rehydration. The aim here was to establish the causes and mechanisms of irreversible damage of the photosynthetic apparatus due to dehydration at high irradiance, and to elucidate the mechanisms determining recovery. Changes in chloroplast structure, CO2 assimilation, chlorophyll fluorescence parameters, fluorescence imaging and the polypeptide patterns during desiccation of Haberlea under medium (100 micromol m-2 s-1; ML) irradiance were compared with those under low (30 micromol m-2 s-1; LL) irradiance. Well-watered plants (control) at 100 micromol m-2 s-1 were not damaged. Plants desiccated at LL or ML had similar rates of water loss. Dehydration at ML decreased the quantum efficiency of photosystem II photochemistry, and particularly the CO2 assimilation rate, more rapidly than at LL. Dehydration induced accumulation of stress proteins in leaves under both LL and ML. Photosynthetic activity and polypeptide composition were completely restored in LL plants after 1 week of rehydration, but changes persisted under ML conditions. Electron microscopy of structural changes in the chloroplast showed that the thylakoid lumen is filled with an electron-dense substance (dense luminal substance, DLS), while the thylakoid membranes are lightly stained. Upon dehydration and rehydration the DLS thinned and disappeared, the time course largely depending on the illumination: whereas DLS persisted during desiccation and started to disappear during late recovery under LL, it disappeared from the onset of dehydration and later was completely lost under ML. Accumulation of DLS (possibly phenolics) in the thylakoid lumen is demonstrated and is proposed as a mechanism protecting the thylakoid membranes of H. rhodopensis during desiccation and recovery under LL. Disappearance of DLS during desiccation in ML could leave the thylakoid membranes without protection, allowing oxidative damage during dehydration and the initial rehydration, thus preventing recovery of photosynthesis.

Changes in chloroplast ultrastructure in some high-alpine plants: adaptation to metabolic demands and climate?

The cytology of leaf cells from five different high-alpine plants was studied and compared with structures in chloroplasts from the typical high-alpine plant Ranunculus glacialis previously described as having frequent envelope plus stroma protrusions. The plants under investigation ranged from subalpine/alpine Geum montanum through alpine Geum reptans, Poa alpina var. vivipara, and Oxyria digyna to nival Cerastium uniflorum and R. glacialis. The general leaf structure (by light microscopy) and leaf mesophyll cell ultrastructure (by transmission electron microscopy [TEM]) did not show any specialized structures unique to these mountain species. However, chloroplast protrusion formation could be found in G. reptans and, to a greater extent, in O. digyna. The other species exhibited only a low percentage of such chloroplast structural changes. Occurrence of protrusions in samples of G. montanum and O. digyna growing in a mild climate at about 50 m above sea level was drastically reduced. Serial TEM sections of O. digyna cells showed that the protrusions can appear as rather broad and long appendices of plastids, often forming pocketlike structures where mitochondria and microbodies are in close vicinity to the plastid and to each other. It is suggested that some high-alpine plants may form such protrusions to facilitate fast exchange of molecules between cytoplasm and plastid as an adaptation to the short, often unfavorable vegetation period in the Alps, while other species may have developed different types of adaptation that are not expressed in ultrastructural changes of the plastids.

Early chloroplastic alterations analysed by optical coherence tomography during a harpin‐induced hypersensitive response

The hypersensitive response has been mostly studied by molecular and biochemical methods after sample destruction. The development of imaging techniques allows the monitoring of physiological changes before any signs of cell death. Here, we follow the early steps of a hypersensitive-like response induced by the bacterial elicitor harpin in Nicotiana sp. We describe cytological modifications after inoculation of the harpin protein, using confocal fluorescence microscopy (CFM) and optical coherence tomography (OCT), an interferometric-based microscopy. The changes detected by CFM occurred 5 h after harpin infiltration and corresponded to a redistribution of the chloroplasts from the upper to the inner regions of the palisade mesophyll cells which could be related to a perturbation in the microtubule network. Using OCT, we were able to detect a decrease in chloroplast backscattered signal as early as 30 min after harpin infiltration. A simple physical model, which accounted for the structure and distribution of thylakoid membranes, suggested that this loss of scattering could be associated with a modification in the refractive index of the thylakoid membranes. Our OCT observations were correlated with a decrease in photosynthesis, emphasizing changes in chloroplast structure as one of the earliest hallmarks of plant hypersensitive cell death.

Light-dependent reversal of dark-chilling induced changes in chloroplast structure and arrangement of chlorophyll–protein complexes in bean thylakoid membranes

Changes in chloroplast structure and rearrangement of chlorophyll–protein (CP) complexes were investigated in detached leaves of bean ( Phaseolus vulgaris L. cv. Eureka), a chilling-sensitive plant, during 5-day dark-chilling at 1 °C and subsequent 3-h photoactivation under white light (200 μmol photons m −2 s −1) at 22 °C. Although, no change in chlorophyll (Chl) content and Chl a/ b ratio in all samples was observed, overall fluorescence intensity of fluorescence emission and excitation spectra of thylakoid membranes isolated from dark-chilled leaves decreased to about 50%, and remained after photoactivation at 70% of that of the control sample. Concomitantly, the ratio between fluorescence intensities of PSI and PSII (F736/F681) at 120 K increased 1.5-fold upon chilling, and was fully reversed after photoactivation. Moreover, chilling stress seems to induce a decrease of the relative contribution of LHCII fluorescence to the thylakoid emission spectra at 120 K, and an increase of that from LHCI and PSI, correlated with a decrease of stability of LHCI–PSI and LHCII trimers, shown by mild-denaturing electrophoresis. These effects were reversed to a large extent after photoactivation, with the exception of LHCII, which remained partly in the aggregated form. In view of these data, it is likely that dark-chilling stress induces partial disassembly of CP complexes, not completely restorable upon photoactivation. These data are further supported by confocal laser scanning fluorescence microscopy, which showed that regular grana arrangement observed in chloroplasts isolated from control leaves was destroyed by dark-chilling stress, and was partially reconstructed after photoactivation. In line with this, Chl a fluorescence spectra of leaf discs demonstrated that dark-chilling caused a decrease of the quantum yield PSII photochemistry ( F v/ F m) by almost 40% in 5 days. Complete restoration of the photochemical activity of PSII required 9 h post-chilling photoactivation, while only 3 h were needed to reconstruct thylakoid membrane organization and chloroplast structure. The latter demonstrated that the long-term dark-chilled bean leaves started to suffer from photoinhibition after transfer to moderate irradiance and temperature conditions, delaying the recovery of PSII photochemistry, independently of photo-induced reconstruction of PSII complexes.

The phytotoxin coronatine and methyl jasmonate impact multiple phytohormone pathways in tomato

Coronatine (COR) is a phytotoxin produced by several pathovars of Pseudomonas syringae and consists of coronafacic acid (CFA), an analog of methyl jasmonic acid (MeJA), and coronamic acid (CMA), which resembles 1-aminocyclopropane-1-carboxylic acid (ACC), a precursor to ethylene. An understanding of how COR functions, is perceived by different plant tissues, and the extent to which it mimics MeJA remain unclear. In this study, COR and related compounds were examined with respect to structure and function. The results indicate that conjugation of CFA to an amino acid is required for optimal activity in tomato, including chlorosis, changes in chloroplast structure, cell wall thickening, accumulation of proteinase inhibitors, induction of anthocyanins, and root growth inhibition. cDNA microarrays were utilized to understand the molecular processes that are regulated by MeJA, COR, CFA and CMA in tomato leaves. A comparison of COR- and MeJA-regulated transcriptomes revealed that COR regulated 35% of the MeJA-induced genes. There was significant overlap in the number of COR and CFA-regulated genes with CFA impacting the expression of 39.4% of the COR-regulated genes. Taken together, the results of biological assays, ultrastructural studies, and gene expression profiling demonstrate that: (1) the intact COR molecule impacts signaling in tomato via the jasmonic acid, ethylene, and auxin pathways; (2) CMA does not function as a structural analog of ACC; (3) COR has a broader range of functions than either CFA or CMA; and (4) COR and MeJA share similar, but not identical activities and impact multiple phytohormone pathways in tomato.

Ultraviolet‐B radiation causes shade‐type ultrastructural changes in Brassica napus

Cell and chloroplast structural changes in palisade cells from mature leaves of Brassica napus L. cv. Paroll were quantified following exposure of plants to enhanced ultraviolet‐B (280–320 nm; 13 kJ m−2 day−1 biologically effective UV‐B) radiation at two different levels of photosynthetically active radiation (PAR, 400–700 nm; 200 and 700 μmol m−2 s−1). Short‐term changes in leaf ultrastructure after 30 min and longer term changes after one day and one week were analyzed using stereological techniques incorporating light and electron microscopy and mathematical reconstruction of a mean cell for each sample.Ultraviolet‐B together with either relatively high or low PAR resulted in cell structural changes resembling those typical of plants under shade conditions, with the most marked response occurring after 30 min of UV‐B radiation. The ultrastructural changes at the cellular level were generally similar in both the relatively high and low PAR plus UV‐B radiation treatments. The surface areas of all three thylakoid types, the appressed, non‐appressed and margin thylakoids increased in the palisade tissue under supplemental UV‐B irradiation.Although the appressed and non‐appressed thylakoids increased in surface area, they did not increase equally, leaving open the possibility that the two thylakoid types have independent regulatory systems or different sensitivity to UV‐B radiation. Increased thylakoid packing (mm2 thylakoid membrane per mm2 leaf surface) in UV‐B‐exposed plants may increase the statistical probability of photon interception. An increased level of UV‐absorbing pigments after one week of supplemental UV‐B radiation did not prevent or significantly ameliorate UV effects. Our data supported the assumption that UV‐B radiation may have a regulatory role besides damaging effects and that an increased UV‐B environment will likely increase this regulatory influence of UV‐B radiation.

Chloroplast Structure after Water and High-Temperature Stress in Two Lines of Maize that Differ in Endogenous Levels of Abscisic Acid

Chloroplast structure in mesophyll cells from the high-level abscisic acid (ABA) line of maize (Zea mays L.), ZPBL 1304, and the low-level ABA line, ZPL 389, was studied in response to leaf dehydration and high temperature (45 C). Thirteen-day-old seedlings were exposed to 7-d soil drying followed by either 6-h or 24-h heat stress. Seven-day soil drying followed by 6-h high-temperature stress did not affect chloroplasts in line ZPBL 1304 but damaged many chloroplasts of line ZPL 389; chloroplast-envelope membranes were broken and not distinct, and many thylakoids were swollen. Seven-day soil drying followed by 24-h heat stress affected chloroplasts in both lines, but changes in chloroplast structure were greater in ZPL 389 than in ZPBL 1304. The main damage to the chloroplasts in ZPBL 1304 included thylakoid swelling, disruption of the chloroplast envelope, and distortion of chloroplast shape. Chloroplasts in ZPL 389 suffered severe damage: their shape was irregular, envelope membranes were not visible, g...

Regulation of 5-Aminolevulinic Acid Synthesis in Developing Chloroplasts

Intact chloroplasts isolated from greening cucumber (Cucumis sativus L. var Beit Alpha) cotyledons regenerated protochlorophyllide (Pchlide) in the dark with added cofactors from either exogenous glutamate or endogenous substrates. No other intermediates of the chlorophyll biosynthetic pathway accumulated. When inhibitors of 5-aminolevulinic acid (ALA) dehydratase were added, the Pchlide that failed to form was replaced by an excessive amount of ALA. When greening seedlings were returned to the dark, ALA-synthesizing activity in the isolated chloroplasts decreased dramatically and recovered if the dark-treated seedlings were again exposed to continuous white light prior to chloroplast isolation. Both the decline and the recovery of ALA-synthesizing activity were complete in approximately 50 minutes. Changes in chloroplast structure during in vivo light to dark and dark to light transitions (as evidenced by electron microscopy) were much slower. Exposing isolated chloroplasts from dark-treated seedlings to short white flashes before incubation transformed nearly all the endogenous Pchlide, but hardly stimulated ALA synthesis, suggesting that Pchlide does not act as a feed-back inhibitor on ALA synthesis. Chloroplasts isolated from dark-treated tissue did not form Pchlide from glutamate when incubated in the dark with added cofactors; moreover, the endogenous Pchlide did not turn over in organello. However, these chloroplasts did synthesize Pchlide from added ALA at the normal rate and synthesized ALA from glutamate at a reduced, but still significant, rate. Mg chelation was not affected by in vivo dark treatment.

The Role of Cytokinins in the Development and Metabolism of Barley Leaves III. The Effect on the RNA Metabolism in Various Cell Compartments During Senescence

Summary The degradation of RNA in excised barley leaves in darkness mainly concerned the 25 S and 18 S rRNAs, whereas the 23 S and 16 S rRNAs were. rehtively stable. The cytokinin prevented an increase in the RNase activity and caused an increase in the RNA level and in the incorporation of 14 C-uracil into the cytoplasmic rRNAs, their precursors and the low molecular RNAs. There was no incorporation of the precursor into the 23 S and 16 S RNAs. Changes in RNA resulting from the senescence and from the cytokinin treatment occurred mainly in the cytoplasmic and nuclear cell fractions; they were relatively small or non-existent in the chloroplast fraction. It is concluded that changes in chloroplast structure during senescence of barley leaves are rather secondary results of processes occuring in other cell compartments. Also the retarding effect of cytokinin on the chloroplast senescence seems to be rather indirect and resulting inter alia from a protection of the RNA and a stimulation of its synthesis in the cytoplasm and nucleus.

The effect of kinetin and auxin on the chloroplast structure and chlorophyll content in wheat coleoptiles

Kinetin and auxin when applied to excised segments of wheat coleoptiles bring about changes in chloroplast structure and chlorophyll content of parenchymatous cells. Auxin (IAA) alone at a concentration of 10-5M stimulated the elongation (growth), but the chloroplast membrane system was less developed and the chlorophyll content was lowest in comparison with control and other variants. Kinetin (KIN) exhibited various effects depending on the concentration used. 10-6M KIN somehow stimulated the elongation and enhanced the amount of grana coming to one chloroplast section, but the individual grana were relatively small and the chlorophyll content a little higher than in the control. On the other hand 10-5M KIN which did not promote the elongation of wheat coleoptiles, had the maximum stimulatory effect on the chloroplast membrane system, especially on the occurrence of large grana, and the chlorophyll content was highest in comparison with the other variants. The occurrence of starch grains in chloroplasts was lower than in the other variants. The effect of a joint treatment of KIN and IAA did not exceed that of KIN (10-5M) alone. Thus the development of chloroplasts and the accumulation of chlorophyll in wheat coleoptiles are stimulated by the concentration of KIN which does not promote the elongation of coleoptiles.

Changes in chloroplast structure during autospore formation inScenedesmus obtusiusculus

Changes in chloroplast structure of synchronous cells ofScenedesmus obtusiusculus in the late stages of the 15-hours light period are described. The mature cells contain highly lobed chloroplasts packed with starch grains which increase in the chloroplasts with the progress of cell age. The chloroplasts appear to contain closely apposed thylakoid membranes in parallel arrays. Chloroplast division occurs by constriction usually caused by the development of the cell plates of the dividing cell.

The effect of 1-ethyl-3(3-dimethylaminopropyl)carbodiimide on calcium binding and associated changes in chloroplast structure and chlorophyll a fluorescence in spinach chloroplasts

1. 1. Chemical modification of carboxyl groups on the chloroplast membrane with a water-soluble carbodiimide plus a nucleophile caused inhibition of Ca 2+ binding. 2. 2. Both binding sites were affected and showed a decrease in the number of binding sites and an increase in the dissociation constant. 3. 3. Cation-induced changes in chlorophyll a fluorescence and structural changes ( ΔA 540) were inhibited at the same carbodiimide concentrations as Ca 2+ binding, emphasizing the relationship between these processes. 4. 4. Chloroplasts that were illuminated with high intensity light for short time periods showed a decrease in the carbodiimide-mediated inhibition of Ca 2+ binding.

Correlation between monovalent cation-induced decreases in chlorophyll a fluorescence and chloroplast structural changes

Low concentrations (~ 3 m m) of salts of monovalent cations such as Na +, K +, and tetraethylammonium were found to decrease the turbidity of chloroplast suspensions. The turbidity changes (Δ 540) had the same kinetics, salt concentration dependence, and pH dependence as the monovalent cation-induced decreases in chlorophyll a fluorescence (9), suggesting that structural changes are the cause of the associated increases in spillover. Electron microscopy revealed that the grana are stacked when spillover is inhibited (in the absence of salts or the presence of divalent cations) and that monovalent cations cause the grana to unstack, thereby promoting spillover.

The effect of temperature of the rate of photosynthetic electron transfer in chloroplasts of chilling-sensitive and chilling-resistant plants

1. Photochemical activities as a function of temperature have been compared in chloroplasts isolated from chilling-sensitive (below approximately 12 °C) and chilling-resistant plants. 2. An Arrhenius plot of the photoreduction of NADP + from water by chloroplasts isolated from tomato ( Lycopersicon esculentum var. Gross Lisse), a chilling-sensitive plant, shows a change in slope at about 12 °C. Between 25 and 14 °C the activation energy for this reaction is 8.3 kcal·mole −1. Between 11 and 3 °C the activation energy increases to 22 kcal·mole −1. Photoreduction of NADP + by chloroplasts from another chilling-sensitive plant, bean ( Phaseolus vulgaris var. brown beauty), shows an increase in activation energy from 5.9 to 17.5 kcal·mole −1 below about 12 °C. 3. The photoreduction of NADP + by chloroplasts isolated from two chilling-resistant plants, lettuce ( Lactuca sativa var. winter lake) and pea ( Pisum sativum var. greenfeast), shows constant activation energies of 5.4 and 8.0 kcal·mole −1, respectively, over the temperature range 3–25 °C. 4. The effect of temperature on photosynthetic electron transfer in the chloroplasts of chilling-sensitive plants is localized in Photosystem I region of photosynthesis. Both the photoreduction of NADP + from reduced 2,6-dichlorophenol-indophenol and the ferredoxin-NADP + reductase (EC 1.6.99.4) activity of choroplasts of chilling-sensitive plants show increases in activation energies at approximately 12 °C whereas Photosystem II activity of chloroplasts of chilling-sensitive plants shows a constant activation energy over the temperature range 3–25 °C. The photoreduction of Diquat (1,1′-ethylene-2,2′-dipyridylium dibromide) from water by bean chloroplasts, however, does not show a change in activation energy over the same temperature range. The activation energies of each of these reactions in chilling-resistant plants is constant between 3 and 25 °C. 5. The effect of temperature on the activation energy of these reactions in chloroplasts from chilling-sensitive plants is reversible. 6. In chilling-sensitive plants, the increased activation energies below approximately 12 °C, with consequent decreased rates of reaction for the photoreduction of NADP +, would result in impaired photosynthetic activity at chilling temperatures. This could explain the changes in chloroplast structure and function when chilling-sensitive plants are exposed to chilling temperatures.

Changes in chloroplast structure during culture growth of Peridinium cinctum Fa. westii (Dinophyceae)

Abstract Studies of Peridinium cinctum Fa. westii showed that cell size and chlorophyll content increased during early growth and decreased with culture aging. Chloroplasts contained fewer thylakoids, were narrower, and showed greater association with endoplasmic reticulum and cytoplasmic ribosomes in cells at stationary phases than in those during exponential phase. Chloroplast divisions at early growth phases showed that bands comprising three thylakoids are continuous between daughter chloroplasts and may be synthesized prior to division.