Light,genotype, and abscisic acid affect chloroplast positioning in guard cells of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> leaves in distinct ways

The goal of this study was to investigate the effects of light intensity, genotype, and various chemical treatments on chloroplast movement in guard cells of Arabidopsis thaliana leaves. After treatment at various light intensities (dark, low, and high light), leaf discs were fixed with glutaraldehyde, and imaged using confocal laser microscopy. Each chloroplast was assigned a horizontal (close to pore, center, or epidermal side) and vertical (outer, middle, inner) position. White light had a distinct effect on chloroplast positioning, most notably under high light (HL) when chloroplasts on the upper leaf surface of wild-type (WT) moved from epidermal and center positions toward the pore. This was not the case for phot1-5/phot2-1 or phot2-1 plants, thus phototropins are essential for chloroplast positioning in guard cells. In npq1-2 mutants, fewer chloroplasts moved to the pore position under HL than in WT plants, indicating that white light can affect chloroplast positioning also in a zeaxanthin-dependent way. Cytochalasin B inhibited the movement of chloroplasts to the pore under HL, while oryzalin did not, supporting the idea that actin plays a role in the movement. The movement along actin cables is dependent on CHUP1 since chloroplast positioning in chup1 was significantly altered. Abscisic acid (ABA) caused most chloroplasts in WT and phot1-5/phot2-1 to be localized in the center, middle part of the guard cells irrespective of light treatment. This indicates that not only light but also water stress influences chloroplast positioning.     

Chloroplast avoidance movement is not functional in plants grown under strong sunlight

Chloroplast movement in nine climbing plant species was investigated. It is thought that chloroplasts generally escape from strong light to avoid photodamage but accumulate towards weak light to perform photosynthesis effectively. Unexpectedly, however, the leaves of climbing plants grown under strong sunlight showed very low or no chloroplast photorelocation responses to either weak or strong blue light when detected by red light transmittance through leaves. Direct observations of Cayratia japonica leaves, for example, revealed that the average number of chloroplasts in upper periclinal walls of palisade tissue cells was only 1.2 after weak blue‐light irradiation and almost all of the chloroplasts remained at the anticlinal wall, the state of chloroplast avoidance response. The leaves grown under strong light have thin and columnar palisade tissue cells comparing with the leaves grown under low light. Depending on our analyses and our schematic model, the thinner cells in a unit leaf area have a wider total plasma membrane area, such that more chloroplasts can exist on the plasma membrane in the thinner cells than in the thicker cells in a unit leaf‐area basis. The same strategy might be used in other plant leaves grown under direct sunlight.     

Photoprotective function of betacyanin in leaves of <Emphasis Type="Italic">Amaranthus cruentus</Emphasis> L. under water stress

The photoprotective function of leaf betacyanin in water-stressed Amaranthus cruentus plants was examined by comparing leaves of two strains which differ significantly in the amount of betacyanin. At 0, 1, and 2 days after the imposed water stress, leaves were subjected to high-light (HL) treatment to assess their photosynthetic capacity and photoinhibition susceptibility. The water stress equally reduced leaf relative water content (RWC), gas-exchange rate and chlorophyll (Chl) contents in both leaves, indicating that the severity of water stress was comparable between the strains. Consequently, the extent of photoinhibition after the HL treatment increased in both strains as water stress developed; however, it was significantly greater in acyanic leaves than in betacyanic leaves, suggesting lower photoinhibition susceptibility in the betacyanic strain. The betacyanic leaves also exhibited approximately 30% higher values for photochemical quenching coefficient (q_P) during the period of water stress despite the nonphotochemical quenching coefficient (q_N) did not differ significantly between the strains. These results may be partially explained by the increased amount of leaf betacyanin under water stress. Moreover, a decrease in Chl content in betacyanic leaves might have enhanced light screening effect of betacyanin by increasing relative abundance of betacyanin to Chl molecule. In addition, reduced Chl content increased light penetrability of leaves. As a result, the extent of photoinhibition at the deeper tissue was exacerbated and the Chl fluorescence emitted from these tissues was more readily detected, facilitating assessment of photoinhibition at deeper tissues where the effect of betacyanic light screening is considered to be most apparent. Our results demonstrated that leaf betacyanin contributes to total photoprotective capacity of A. cruentus leaves by lowering excitation pressure on photosystem II (PSII) via attenuation of potentially harmful excess incident light under water stress.     

Leaf inner structure and immunogold localization of some key enzymes involved in carbon metabolism in CAM plants

There are relatively few reports on the leaf structure and in situ immunolocalization of carbon metabolism enzymes in crassulacean acid metabolism (CAM) plants, compared with reports on C4 plants. The leaf inner structure and the subcellular location of some key CAM enzymes for a phosphoenolpyruvate carboxykinase (PCK) CAM species, Ananas comosus, and three malic enzyme (ME) CAM species, Mesembryanthemum crystallinum, Kalanchoe daigremontiana, and K. pinnata, was investigated by immunogold labelling and electron microscopy in this study. The leaves of these species had few intercellular air spaces in the mesophyll. A large vacuole occupied the mesophyll cells, and many vesicles of various sizes occurred in the cytosol. Immunocytochemical study revealed that labelling was present for phosphoenolpyruvate carboxylase in the cytosol and for ribulose-1,5-bisphosphate carboxylase/oxygenase in the chloroplasts of the mesophyll cells in all species. No specific labelling for pyruvate orthophosphate dikinase (PPDK) was observed in the PCK-CAM species. In the ME-CAM species, the patterns of labelling for PPDK differed. In M. crystallinum labelling for PPDK was present only in the chloroplasts, whereas in the two Kalanchoe species it occurred in the cytosol as well as in the chloroplasts. These results suggest that the subcellular localization of PPDK varies with ME-CAM species, in contrast to the conventional belief that it is localized in the chloroplasts.Key words: Crassulacean acid metabolism, immunolocalization, leaf inner structure, phosphoenolpyruvate carboxylase, pyruvate orthophosphate dikinase.      

A comparative immunocytochemical localization study of phosphoenolpyruvate carboxylase in leaves of higher plants

The intracellular localization of phosphoenolpyruvate (PEP) carboxylase in plants belonging to the C₄, Crassulacean acid metabolism (CAM) and C₃ types was invetigated using an immunocytochemical method with an immune serum raised against the sorghum leaf enzyme. The plants studied were sorghum, maize (C₄ type), kalanchoe (CAM type), french bean, and spinach (C₃ type). In the green leaves of C₄ plants, it was shown that the carboxylase was located in the mesophyll and stomatic cells, being largely cytosolic in the mesophyll cells. Similarly, in CAM plants, the enzyme was found mainly outside the chloroplasts. In contrast, in C₃ plants, the PEP carboxylase appeared to be distributed between the cytosol and the chloroplasts of foliar parenchyma. Examination of sections from etiolated leaves showed fluorescence emission from etioplasts and cytosol for the parenchyma of french bean as well as for the bundle sheath and mesophyll of sorghum leaves. This data indicated that during the greening process photoregulation and evolution of PEP carboxylase is dependent on the tissue and on the metabolic type of the plant considered.Abbreviations CAM Crassulacean acid metabolism - PEP phosphoenolpyruvate     

Influence of Photoperiod and Leaf Age on Crassulacean Acid Metabolism in Portulacaria afra (L.) Jacq

The possibility that Crassulacean acid metabolism (CAM) is subject to long day photoperiodic control in Portulacaria afra (L.) Jacq., a facultative CAM plant, was studied. Periodic measurements of ¹⁴CO₂ uptake, stomatal resistance, and titratable acidity were made on plants exposed to long and short day photoperiods. Results indicates that waterstressed P. afra had primarily nocturnal CO₂ uptake, daytime stomatal closure, and a large diurnal acid fluctuation in either photoperiod. Mature leaf tissue from nonstressed plants under long days exhibited a moderate diurnal acid fluctuation and midday stomatal closure. Under short days, there was a reduced diurnal acid fluctuation in mature leaf tissue. Young leaf tissue taken from nonstressed plants did not utilize the CAM pathway under either photoperiod as indicated by daytime CO₂ uptake, lack of diurnal acid fluctuation, and incomplete daytime stomatal closure.

The induction of CAM in P. afra appears to be related to the water status of the plant and the age of the leaf tissue. The photosynthetic metabolism of mature leaves may be partly under the control of water stress and of photoperiod, where CAM is favored under long days.

     

Diurnal changes in photosystem II photochemistry, photoprotective compounds and stress-related phytohormones in the CAM plant, Aptenia cordifolia

Acclimation of photosynthetic light reactions to daily changes in solar radiation requires adjustments in photosystem II photochemistry and may be affected by environmental stresses, such as drought. In this study, we examined the effects of a short-term, severe water deficit on diurnal variations in photosystem II photochemistry, photoprotective compounds (tocopherols and carotenoids, including the xanthophyll cycle) and stress-related phytohormones (abscisic acid and salicylic acid) in the CAM plant, Aptenia cordifolia L. f. Schwantes. Violaxanthin was rapidly converted to zeaxanthin under high light, the de-epoxidation state of the xanthophyll cycle reaching maximum levels of 0.95 at midday in irrigated plants. Under a higher photoprotective demand caused by water deficit, plants showed significant increases in abscisic acid and γ-tocopherol levels, which were followed by decreases in β-carotene and the F_v/F_m ratio at later stages of stress. Decreases in this ratio below 0.70 correlated with sustained increases in the de-epoxidation state of the xanthophyll cycle, which kept above 0.90 at night after 15 days of water deficit. In contrast to abscisic acid, salicylic acid levels kept constant under water deficit and showed a sharp decrease during the day both under irrigated and water stress conditions. We conclude that the CAM plant, A. cordifolia showed several strategies of acclimation to short-term water deficit, including abscisic acid and γ-tocopherol accumulation, as well as sustained increases in the de-epoxidation state of the xanthophyll cycle, which was tightly coupled to daily variations in photosystem II photochemistry. The differential accumulation of tocopherol homologues under water deficit and the diurnal fluctuations of salicylic acid levels in this CAM plant will also be discussed.     

Aggregative movement of C4 mesophyll chloroplasts is promoted by low CO2 under high intensity blue light

• C₄ plants supply concentrated CO₂ to bundle sheath (BS) cells, improving photosynthetic efficiency by suppressing photorespiration. Mesophyll chloroplasts in C₄ plants are redistributed toward the sides of the BS cells (aggregative movement) in response to environmental stresses under light. Although this chloroplast movement is common in C₄ plants, the significance and mechanisms underlying the aggregative movement remain unknown.
• Under environmental stresses, such as drought and salt, CO₂ uptake from the atmosphere is suppressed by closing stomata to prevent water loss. We hypothesized that CO₂ limitation may induce the chloroplast aggregative movement. In this study, the mesophyll chloroplast arrangement in a leaf of finger millet, an NAD-malic enzyme type C₄ plant, was examined under different CO₂ concentrations and light conditions.
• CO₂ limitation around the leaves promoted the aggregative movement, but the aggregative movement was not suppressed, even at the higher CO₂ concentration than in the atmosphere, under high intensity blue light. In addition, mesophyll chloroplasts did not change their arrangement under darkness or red light.
• From these results, it can be concluded that CO₂ limitation is not a direct inducer of the aggregative movement but would be a promoting factor of the movement under high intensity blue light.

     

Effects of salt-tolerant rootstock grafting on ultrastructure,photosynthetic capacity,and H<Subscript>2</Subscript>O<Subscript>2</Subscript>-scavenging system in chloroplasts of cucumber seedlings under NaCl stress

Plant growth, photosynthetic parameters, chloroplast ultrastructure, and the ascorbate-glutathione cycle system in chloroplasts of self-grafted and rootstock-grafted cucumber leaves were investigated. Grafted plants were grown hydroponically and were exposed to 0, 50, and 100 mM NaCl concentrations for 10 days. Under NaCl stress, the hydrogen peroxide (H₂O₂) content in cucumber chloroplasts increased, the chloroplast ultrastructure was damaged, and the gas stomatal conductance, intercellular CO₂ concentration, as well as shoot dry weight, plant height, stem diameter, leaf area, and leaf relative water content were inhibited, whereas these changes were less severe in rootstock-grafted plants. The activities of ascorbate peroxidase (APX; EC 1.11.1.11), glutathione reductase (GR; EC 1.6.4.2), and dehydroascorbate reductase (DHAR EC 1.8.5.1) were higher in the chloroplasts of rootstock-grafted plants compared with those of self-grafted plants under 50 and 100 mM NaCl. Similar trends were shown in leaf net CO₂ assimilation rate and transpiration rate, as well as reduced glutathione content under 100 mM NaCl. Results suggest that rootstock grafting enhances the H₂O₂-scavenging capacity of the ascorbate–glutathione cycle in cucumber chloroplasts under NaCl stress, thereby protecting the chloroplast structure and improving the photosynthetic performance of cucumber leaves. As a result, cucumber growth is promoted.     

Photoperiodism and Crassulacean acid metabolism

Measurements of net CO₂ exchange, malate accumulation, properties and capacity of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) in leaves of different ages of two short-day dependent Crassulacean acid metabolism (CAM) plants (Kalanchoe blossfeldiana v. Poelln. Tom thumb and K. velutina Welw.) show that, in both species: a) young leaves from plants grown under long days display a CO₂ exchange pattern typical of C₃ plants; b) leaf aging promotes CAM under long-day conditions; c) short-day treatment induces CAM in young leaves to a higher degree than aging under long days; d) at least in K. blossfeldiana, the PEPC form developed with leaf aging under long days and the enzyme form synthetized de novo in young leaves grown under short days were shown to have similar properties. Short days also promote CAM in older leaves though at a lesser extent than in young leaves: The result is that this photoperiodic treatment increases the general level of CAM performance by the whole plant. The physiological meaning of the control of PEPC capacity by photoperiodism could be to afford a precisely timed seasonal increase in CAM potentiality, enabling the plant to immediately optimize its response to the onset of drought periods.Abbreviations CAM Crassulacean acid metabolism - PEP phosphoenolpyruvate - PEPC phosphoenolpyruvate carboxylase (EC 4.1.1.31) - LD long day - SD short day     

ANATOMICAL AND PHYSIOLOGICAL ACCLIMATION OF FATSIA JAPONICA LEAVES TO IRRADIANCE

Anatomical and physiological leaf characteristics and biomass production of Fatsia japonica plants were studied. Plants were grown in a growth chamber at 300 μmol m^-2 s^-1 (high light) and 50 μmol m^-2 s^-1 (low light) photosynthetic photon flux density. Plants grown under high light showed a net maximum photosynthetic rate 44% higher than plants grown under low light; the light compensation point and the light saturation point were also higher in high-light plants. Photosynthetic oxygen evolution in isolated chloroplasts was about 40% higher in high-light plants. However, chlorophyll content on a dry weight basis, on a leaf area basis, and per chloroplast was greater in plants grown under low light. Leaf thickness in high-light plants was 13% higher than in low-light plants. The number of chloroplasts was 30% higher in high-light leaves, while chloroplast size was only slightly higher. Chloroplast ultrastructure was also affected by light. Leaf dry weight, leaf area, and biomass production per plant were drastically reduced under low light. Thus, F. japonica is a plant that is able to acclimate to different photosynthetic photon flux density by altering its anatomical and physiological characteristics. However, low-light acclimation of this plant has a considerable limiting effect on biomass production.     

Evidence that the induction of crassulacean acid metabolism by water stress in Mesembryanthemum crystallinum (L.) involves root signalling

The aim of this study was to investigate whether the root system of Mesembryanthemum crystallinum (L.) plays a role in triggering the induction of crassulacean acid metabolism (CAM) during water stress. Depriving well-irrigated plants of water, by allowing the soil surrounding the roots to dry, caused increased daily losses in leaf relative water content (RVVC) and mesophyll cell turgor pressure. The RWC of the roots also declined. Subsequently plants exhibited physiological characteristics of CAM photosynthesis (i.e. diurnal fluctuations in leaf titratable acidity and nocturnal net CO₂ fixation). When the root system of plants was divided equally between two soil compartments and one half deprived of water, plants exhibited physiological characteristics of CAM without prior changes in leaf RWC content or mesophyll cell turgor pressure. Only the RWC of the water-stressed portion of the roots was reduced. These data suggest that in water-stressed plants daily changes in leaf water relations greater than those observed in well-irrigated plants, are not essential to trigger CAM expression. It is probable that a reduction in soil water availability can be perceived by the roots of M. crystallinum and that this information is conveyed to the leaves triggering the transition from C₃ to CAM photosynthesis.     

Abscisic acid immunolocalization and ultrastructural changes in water-stressed lavender (Lavandula stoechas L.) plants

The relationship between the bulk abscisic acid (ABA) content, ABA compartmental redistribution, and chloroplast ultrastructural changes was studied in leaves of lavender ( Lavandula stoechas L.) plants subjected to water stress. ABA was uniformly distributed in the cytosol, nucleus, chloroplasts, and cell walls of mesophyll cells in well-watered plants. In plants subjected to water stress (−2.6 MPa water potential) the bulk leaf ABA increased from 900 to 3 600 pmol g⁻¹ fresh weight. At the ultrastructural level, the first indication of this rise in ABA was a 4-fold increase in ABA immunolabeling in the cell wall in which the highest labeling values were recorded. This increase in apoplastic ABA in lavender was not attributable to ABA release from the chloroplast, because a simultaneous increase in ABA labeling was observed in both the chloroplast and nucleus (2- and 3-fold, respectively). Water stress induced a progressive increase in bulk leaf ABA concentration to 13 600 pmol g⁻¹ fresh weight coincident, with the highest immunolabeling of ABA in the nucleus and chloroplast. Under severe water stress, the chloroplast membrane broke down, resulting in leakage of ABA from the chloroplast. The stress-induced increase of ABA in chloroplasts and nuclei may serve a function other than affecting stomatal movement.     

SPAD chlorophyll meter reading can be pronouncedly affected by chloroplast movement

Non-destructive assessment of chlorophyll content has recently been widely done by chlorophyll meters based on measurement of leaf transmittance (e.g. the SPAD-502 chlorophyll meter measures the leaf transmittance at 650 and 940 nm). However, the leaf transmittance depends not only on the content of chlorophylls but also on their distribution in leaves. The chlorophyll distribution within leaves is co-determined by chloroplast arrangement in cells that depends on light conditions. When tobacco leaves were exposed to a strong blue light (about 340 μmol of photons m⁻² s⁻¹), a very pronounced increase in the leaf transmittance was observed as chloroplasts migrated from face position (along cell walls perpendicular to the incident light) to side position (along cell walls parallel to the incoming light) and the SPAD reading decreased markedly. This effect was more pronounced in the leaves of young tobacco plants compared with old ones; the difference between SPAD values in face and side position reached even about 35%. It is shown how the chloroplast movement changes a relationship between the SPAD readings and real chlorophyll content. For an elimination of the chloroplast movement effect, it can be recommended to measure the SPAD values in leaves with a defined chloroplasts arrangement.     

Spatial division of phosphoenolpyruvate carboxylase and nitrate reductase activity and its regulation by cytokinins in CAM-induced leaves of Guzmania monostachia (Bromeliaceae)

Crassulacean acid metabolism (CAM) is a physiological adaptation of plants that live in stress environment conditions. A good model of CAM modulation is the epiphytic bromeliad, Guzmania monostachia, which switches between two photosynthetic pathways (C₃–CAM) in response to different environmental conditions, such as light stress and water availability. Along the leaf length a gradient of acidity can be observed when G. monostachia plants are kept under water deficiency. Previous studies showed that the apical portions of the leaves present higher expression of CAM, while the basal regions exhibit lower expression of this photosynthetic pathway. The present study has demonstrated that it is possible to induce the CAM pathway in detached leaves of G. monostachia kept under water deficit for 7 d. Also, it was evaluated whether CAM expression can be modulated in detached leaves of Guzmania and whether some spatial separation between NO₃⁻ reduction and CO₂ fixation occurs in basal and apical portions of the leaf. In addition, we analyzed the involvement of endogenous cytokinins (free and ribosylated forms) as possible signal modulating both NO₃⁻ reduction and CO₂ fixation along the leaf blade of this bromeliad. Besides demonstrating a clear spatial and functional separation of carbon and nitrogen metabolism along G. monostachia leaves, the results obtained also indicated a probable negative correlation between endogenous free cytokinins – zeatin (Z) and isopentenyladenine (iP) – concentration and PEPC activity in the apical portions of G. monostachia leaves kept under water deficit. On the other hand, a possible positive correlation between endogenous Z and iP levels and NR activity in basal portions of drought-exposed and control leaves was verified. Together with the observations presented above, results obtained with exogenous cytokinins treatments, strongly suggest that free cytokinins might act as a stimulatory signal involved in NR activity regulation and as a negative regulator of PEPC activity in CAM-induced leaves of G. monostachia during a diel cycle.     

Chloroplast movements in the field

An ecophysiological understanding of chloroplast movements in leaves requires measurement of these movements under field conditions. A field‐portable instrument was constructed, based on a pulsed measuring beam and lock‐in detection that measures chloroplast movements in attached leaves by sensing the resultant changes in leaf transmittance. In the instrument and generally in nature, leaves are illuminated obliquely, in contrast with the perpendicular illumination used in most laboratory experiments on chloroplast movement. Microscopic analysis of cells illuminated obliquely with bright light verified that chloroplasts move out of the light path, and transmittance changes in response to oblique light were robust. Chloroplast movements in Alocasia brisbanensis under natural sunlight express kinetics and light requirements expected from laboratory observations: chloroplasts were in the periclinal position at dawn and dusk, anticlinal position in full sunlight midday, and in an intermediate position at night. Movement in response to bright light was rapid allowing responses to brief sunflecks. Movements in Helianthus tuberosum, Eustrephus latifolius and Cissus hypoglauca were qualitatively similar with differing kinetics and magnitude. In all four species, chloroplasts were in motion most of the time, rarely achieving the extreme anticlinal or periclinal positions.     

Characterization of a zebra mutant of rice with increased susceptibility to light stress

The rice zebra mutant TCM248 is a single recessive mutant. This mutant develops transverse-striped leaves with green and white sectors under alternate light/dark growth conditions. Mutants that were grown under a higher light intensity during the light period showed a more intense striped phenotype. The white tissues contained abnormal chloroplasts with few internal membrane structures, while the green tissues in the mutants contained normal chloroplasts. The white tissue contained only trace amounts of Chls and carotenoids, and mRNA accumulation of nuclear genes encoding chloroplast proteins (rbcS, cab) was strongly suppressed compared to that in the wild type plants. A series of growth condition shift experiments demonstrated that the mutant displayed the striped phenotype only if it was exposed to the alternate light/dark growth conditions during a limited stage of early leaf development. These data suggest that the zebra gene is involved in the acquisition of photoprotective capacity of the plants and that this gene functions at an early stage of chloroplast differentiation.     

Aquaporin Content in Cell Membranes of Mesembryanthemum crystallinum as Affected by Plant Transition from C3 to CAM Type of Photosynthesis

Western-blot analysis was used to determine the contents of aquaporin isoforms MIP A, MIP B, and MIP C in cell membranes isolated from roots and leaves of Mesembryanthemum crystallinum plants with C₃ and Crassulacean acid metabolism (CAM) types of photosynthesis. These membrane preparations were also used to assess osmotic water permeability; to this end, the rate of osmotic vesicle shrinking was registered as the light scattering intensity by the method of stopped flow. The cell membranes represented by the plasmalemma and the tonoplast-enriched fraction were obtained by separating the microsomes in a two-phase polymer system. Plant transition from C₃ to CAM-photosynthesis occurred in the course of plant development or was induced by salinization. All three isoforms under study were found in the plasma membranes of roots and leaves of the C₃ plants, whereas in the CAM plants, independent of the transition-inducing factor, the aquaporin contents notably decreased in the leaf membranes and remained unchanged in the roots. In the membranes isolated from roots and leaves of the C₃ plants, the values of osmotic water permeability exceeded two–threefold the corresponding indices characteristic of the CAM plants. The authors believe that aquaporin isoforms in M. crystallinum are under the organ- and tissue-specific control.     

Fine Chloroplast Structure in Cucumber and Pea Leaves Developed under Red Light

Photosynthetic activity, the content of various photosynthetic pigments, and the chloroplast ultrastructure were examined in the leaves of cucumber (Cucumis sativus L.) and pea (Pisum sativum L.) plants of different ages grown under red light (600–700 nm, 100 W/m²). In pea leaves tolerant to red-light irradiation, chloroplast ultrastructure did not essentially change. In the first true leaves of cucumber plants susceptible to red-light irradiation, we observed a considerable increase in the number and size of plastoglobules, the appearance of chloroplasts lacking grana or containing only infrequent grana, and stromal thylakoids. In the upper leaves of 22-day-old cucumber plants, the chloroplast structure was essentially similar to that of the control chloroplasts in white light, and we therefore suppose that these plants have acclimated to red light.