Effects of growth irradiance on the photosynthetic action spectra of the marine dinoflagellate,Glenodinium sp.

Summary Whole cell absorption curves of the marine dinoflagellate Glenodinium sp., cultured at irradiances of 250W/cm² (low light) and 2500W/cm² (high light), were measured and their difference spectrum determined. Absorption by low light grown cells exceeded that of high light grown cells throughout the visible spectrum by a factor which ranged from 2 to 4. The difference spectrum supported the view that increased pigmentation, resulting from low light conditions, was largely due to an increase in cell content of a peridinin-chlorophyll a-protein (PCP) and an unidentified chlorophyll a component of the chloroplast membrane. Photosynthetic action spectrum measurements indicated that chlorophyll a, peridinin, and very likely chlorophyll c, were effective light-harvesting pigments for photosynthesis in both high and low light grown cultures of Glenodinium sp. Comparison of action spectra and absorption spectra suggested that low light grown cells selectively increased cellular absorption in the 480 nm to 560 nm region, and effectively utilized this spectral region for the promotion of oxygen evolution.Abbreviations PCP peridinin-chlorophyll a-protein - SIO (F.T. Haxo) Scripps Institution of Oceanography collection     

Distribution of Chlorophyll between the Two Photoreactions in Photosynthesis of the Blue-Green Alga Anacystis nidulans Grown at Two Different Light Intensities

Anacystis nidulans was grown in white light of two different intensities, 7 and 50 W ·m^?2. The in vivo pigmentations of the two cultures were compared. The ratio phycocyanin/chlorophyll a was 0.96 for cells grown at 7 W · m^?2 and 0.37 for cells grown at 50 W · m^?2. Phycocyanin-free photosynthetic lamellae (PSI-particles) were prepared, using French press treatment and fractionated centrifugation. Algae grown in the irradiance of 50 W · m^?2 showed a chlorophyll a/P700 ratio of 260, while algae grown at 7 W · m^?2 had a value of 140. Corresponding PSI-particles showed values of 122 and 109 respectively. Light-induced absorption difference spectra measured between 400–450nm indicated different ratios between cytochrome f and P700 in the two algal cultures. Enhancement studies of photosynthetic oxygen evolution were carried out. When a background beam of 691 nm was superimposed upon a signal beam of 625 nm, good enhancement was observed for both cultures. With the wavelengths 675 and 691 nm together a pronounced enhancement could be detected only in algae grown at the higher light level. Absorption spectra recorded on whole cells at 77°K revealed a small shift of the main red chlorophyll a absorption peak caused by light intensity. It is proposed that the reduction of the phycocyanin/chlorophyll a ratio in high light-grown cells is accompanied by an increased energy distribution by chlorophyll a into PSII.     

Evaluation of the Effect of Light Intensity on the Measurement of the Photosynthetic Rate in Plankton Microalgae by the Chlorophyll Fluorescence Method

The use of relative variable fluorescence (RVF) of chlorophyll, as measured in the presence of Diuron, an inhibitor of electron transfer, for the estimation of the photosynthetic activity of plankton microalgae was analyzed under a wide range of light intensities in the PAR region. Oxygen evolution rates (estimated by the method of light and dark bottles and the amperometric method), RVF, and chlorophyll a concentration were measured in parallel in natural algal cenoses and microecosystems. When the previously used regression equation, in the form A = b(F/F _d)C _chl I, where A is O₂ evolution rate (g/(m³ h), F/F _d is RVF (relative units), C _chl is chlorophyll a concentration (mg/m³), and I is light intensity (W/m²), was verified in the PAR region, we observed a nonlinear dependence of the correction coefficient b on I, which can be described by the formula b = 6.227 × 10³I. This result agrees with the hypothesis that chlorophyll a fluorescence quenching comprises photochemical (qQ) and energy (qE) components. On the basis of the energy model, we determined the upper limit b _max = 0.003 for light intensity range I< 4.4 W/m² and the lower limit b _min = 0.0003 for I = 400 W/m².     

The effect of light quality and gibberellic acid (GA3) on photosynthesis and respiration rates of pea seedlings

CO₂ exchange were measured on pea seedlings (Pisum sativum L. var. Bördi) cultivated from seeds imbibed either in water (C-plants) or in gibberellic acid (GA₃) at the concentration of 25 g/1 (GA-plants), and then grown under 17 W/m² blue light (B-plants) or 11 W/m² red light (R-plants).When measured under the same light conditions as during growth the net photosynthesis (APS) rate in B-plants was about twice higher than that in R-plants. Dark respiration (DR) rate was 70% higher in B- than in R-plants. Red light retarded the development of photosynthetic activity, but GA₃ suppressed this effect. The hormone enhanced net photosynthesis and dark respiration to the same extent.When measured under saturating white light net photosynthesis rate of C-plants was also two times higher in B-plants than in R-plants. Growth conditions had only a slight effect on the APS of GA-plants under white light. APS rates of GA-plants grown under red light were higher under white light than those of C-plants, but lower than those of plants grown under blue light.We assume that blue light induced formation of plants that were adapted to higher light intensity: red light had an opposite effect, whereas gibberellic acid induced formation of plants that were adapted to medium light intensity.     

Excretion of photosynthetically fixed organic carbon by metalimnetic phytoplankton

The effects of light intensity, oxygen concentration, and pH on the rates of photosynthesis and net excretion by metalimnetic phytoplankton populations of Little Crooked Lake, Indiana, were studied. Photosynthetic rates increased from 1.42 to 3.14 mg C·mg^–1 chlorophylla·hour^–1 within a range of light intensities from 65 to 150E·m^–2·sec^–1, whereas net excretion remained constant at 0.05 mg C·mg^–1 chlorophylla·hour^–1. Bacteria assimilated approximately 50% of the carbon released by the phytoplankton under these conditions. Excreted carbon (organic compounds either assimilated by bacteria or dissolved in the lake water) was produced by phytoplankton at rates of 0.02–0.15 mg C·mg^–1 chlorophylla·hour^–1. These rates were 6%–13% of the photosynthetic rates of the phytoplankton. Both total excretion of carbon and bacterial assimilation of excreted carbon increased at high light intensities whereas net excretion remained fairly constant. Elevated oxygen concentrations in samples incubated at 150E· m^–2·sec^–1 decreased rates of both photosynthesis and net excretion. The photosynthetic rate increased from 3.0 to 5.0 mg C·mg^–1 chlorophylla· hour^–1 as the pH was raised from 7.5 to 8.8. Net excretion within this range decreased slightly. Calculation of total primary production using a numerical model showed that whereas 225.8 g C·m^–2 was photosynthetically fixed between 12 May and 24 August 1982, a maximum of about 9.3 g C·m^–2 was released extracellularly.     

Culture of the marine phytoplankter,Dunaliella tertiolecta,with light-dark cycles

Summary Dunaliella tertiolecta was grown in continuous culture, maintained by a single daily dilution to a constant cell concentration, with photoperiods of duration 3 to 18 hours. Illumination was provided with filtered tungsten light having a maximum intensity at 580–590 m. Average intensity at the culture surface was maintained at 0.05 cal/cm²xmin. Temperature was regulated at 20° C. Daily cell production and the fraction of cells dividing daily are reported for each of the photoperiods. Cyclic diurnal variations were noted in cell pigment content, cell volume and photosynthetic rate. Corresponding variations in cell carbon/chlorophyll a ratios were small and may be unimportant for the estimation of biomass from chlorophyll a in field work. An equation for calculating daily growth rate in continuous light, from light absorption by cell chlorophyll a, derived previously, was modified to include the influence of providing light in discrete photoperiods.Supported by U. S. Atomic Energy Commission, Contract AT(11-1)-34 Project 108.     

Light-Induced Adaptive Responses under Greenhouse and Controlled Conditions in the Fern Pteris cretica var. ouvrardii

The photosynthetic capabilities of the fern Pteris cretica var. ouvrardii were analysed by means of the light response curves of CO₂ exchange. In control growth conditions (greenhouse, low-light: 20–32 W m^?2); photosynthesis was shown to be saturated for low irradiance (20–25 W m^?2); the saturating photosynthetic rate, very low as compared to higher plants, was due to an extremely high intracellular resistance. When irradiance during the photosynthesis measurement was higher than 60–80 W m^?2, a constant decline of net CO₂ exchange as a function of time was observed. When irradiance during growth was enhanced, whether in greenhouse (20–250 W m^?2) or controlled (62 W m^?2) conditions, the first fronds that had developed in the new condition from the crosier stage exhibited decreased net maximal photosynthesis and a decreased efficiency in low light, but saturating irradiance was unmodified. However, the fronds whose entire differentiation (from meristem) occurred under these moderate irradiances (plants defoliated of all fronds and crosiers at the time of transfer), possessed more efficient photosynthetic characteristics than control plants. Pteris is able to grow under extreme shade conditions (4–8 W m^?2); light saturating photosynthesis and efficiency are higher under extreme shade than under control conditions. These adaptive characteristics indicate that Pteris is a well-adapted shade species.     

Two types of PS II centres as manifested by light saturation of delayed fluorescence from DCMU-treated chloroplasts

Intensity of 2 s delayed fluorescence (DF) as a function of steady-state actinic light intensity was investigated in pea chloroplasts in the presence of 10 M DCMU. The light saturation curve of DF was approximated by a sum of two hyperbolic components which differ by an order of magnitude in the half-saturating incident light intensity. The relative contribution of the amplitudes of the components was practically independent of cation (Na⁺ and Mg²⁺) concentration and a short-term heating of the chloroplasts at 45°C. The component saturating at low incident light intensity was selectively suppressed by 100 M DCMU or by 1 mol g^-1 Chl oleic acid. DF intensity following excitation by a single saturating 15 s flash was equal to the intensity of the component saturating at a low incident light intensity. Upon flash excitation, the maximum steady-state DF level was found to be attained only after a series of saturating flashes. It is concluded that the two components of the DF light saturation curves are related to PS II centres heterogeneity in quantum yield of stabilization of the reduced primary quinone acceptor.Abbreviations DF Delayed fluorescence - L₁- and L₂-components DF components saturating at low and high incident light intensity, respectively - I incident light intensity - L DF intensity - P₆₈₀ reaction centre chlorophyll of PS II - Q_A and Q_B primary and secondary quinone acceptors of PS II, respectively     

Foliar CO2photoassimilation and chloroplast linear electron transport rates in nitrogen-sufficient and nitrogen-limited soybean plants

Leaflets of soybean plants which are moderately inorganic nitrogen (N)-limited exhibit either no difference in the rate of net photosynthesis or as much as a 15–23% lower net photosynthesis rate per unit area than leaflets of N-sufficient plants [Robinson JM (1996) Photosynth Res 50: 133–148; Robinson JM (1997a) Int J Plant Sci 158: 32–43]. However, mature leaflets of N-limited soybean plants have a higher CO₂photoassimilation rate per unit chlorophyll than leaflets of N-sufficient soybean plants at both moderate light intensity (500 µmol m^-2s^-1) and saturating light intensity (1200 µmol m^-2s^-1) [Robinson JM (1996) Photosynth Res 50: 133–148]. This study was undertaken to determine whether chloroplast thylakoids isolated from the leaflets of nitrogen-limited soybean plants displayed similar or higher linear electron transport rates (H₂O ferredoxin NADP) per unit chlorophyll than thylakoids isolated from leaflets of N-sufficient plants. Chlorophyll concentration in reaction mixtures containing chloroplast thylakoids prepared from leaflets of N-limited plants was manipulated so that it was similar to the chlorophyll concentration in reaction mixtures of thylakoids prepared from leaflets of N-sufficient plants. Measurements of ferredoxin dependent, NADP dependent, O₂photo-evolution in thylakoid isolates were carried out in saturating light (1500 µmol m^-2s^-1) and with (an uncoupler) in the chloroplast reaction mixtures. Chloroplast thylakoids isolated from N-limited soybean plant leaflets routinely had a 1.5 to 1.7 times higher rate of uncoupled, whole chain electron transport per unit chlorophyll in saturating light than did chloroplast thylakoids isolated from leaflets of N-sufficient plants. The results suggest that the photosystems and photosynthetic electron transport chain components are more active per unit Chl in leaflet chloroplast thylakoids of N-limited soybean plants than in thylakoids of N-sufficient plants.     

Photosynthesis and growth rates of Laurencia brongniartii J. Agardh (Rhodophyta, Ceramiales) in preparation for cultivation

Laurencia brongniartii is usually found at depths below 4 m, but can be found in shallow subtidal areas in crevices and on the walls of a coral reef in Amami Oshima Island, Kagoshima Prefecture, Japan, where irradiances were significantly lower than those at similar depths in open water. In preparation for the possible cultivation of this species for its antibiotic compounds, the effects of temperature and irradiance on photosynthesis and growth were measured. Photosynthesis and growth rates of L. brongniartii explants were highest at 26 and 28 °C, which closely corresponded to temperatures found during August to late December when it was most abundant. The estimated maximum photosynthesis rate (P _max) was 4.41 mol photon m^–2 s^–1 at 26 °C and 4.07 mol photon m^–2 s^–1 at 28 °C. Saturating irradiance occurred at 95 mol photon m^–2 s^–1 at 26 °C and 65 mol photon m^–2 s^–1 at 28 °C. In contrast, growth experiments at 41.7 mol photon m^–2 s^–1 caused bleaching of explants and the maximum growth rate observed during the study was 3.02 ± 0.75% day^–1 at 28 °C and 25 mol photon m^–2 s^–1. The difference in the saturating irradiance for photosynthesis and the irradiance that caused bleaching in growth experiments suggests that long-term exposure to high irradiance was detrimental and should be addressed before the initiation of large scale cultivation.     

The analysis of photosynthesis in air and water of Ascophyllum nodosum (L.) Le Jol.

Summary The photosynthetic characteristics for the intertidal macroalga Ascophyllum nodosum were examined in air and water. Under ambient conditions of temperature (10° C) inorganic carbon concentrations (15.63 mmol CO₂ m^-3 or 2.0 mol TIC m^-3) and light (500 mol photons m^-2 s^-1) photosynthesis was slightly greater by the exposed alga than by the submerged alga. In both environments photosynthesis was light saturated at 200 mol photons m^-2 s^-1. The relationship between CO₂ concentration and photosynthesis in air could be accurately analysed using Michaelis-Menten kinetics, although the range of concentrations used were not saturating. In contrast the application of the Lineweaver-Burk and Woolf plots to aquatic photosynthesis was not suitable as the experimental data was similar to the Blackman type curves and not rectangular hyperbolae. This was reflected by the applicability of the Hill-Whittingham equation to describe the photosynthesis curves. The effect of unstirred layers and other limiting factors is discussed in relation to the kinetic parameters, V _max and K _m.     

Untersuchung der Beziehung zwischen Pigmentzusammensetzung und CO2-Gaswechsel bei der Blaualge Anacystis nidulans

Summary Changes in culture conditions caused strong changes in the pigment composition in the blue-green alga Anacystis nidulans. Under normal illumination (white light; 0.6·10³ erg/cm²·sec) the relation between the amounts of chlorophyll a and phycocyanin was 1:6.6. In a high light intensity (20.8·10³ erg/cm²·sec) the phycocyanin content was reduced and the relations thus changed to 1:1.9. Growing the algae in red light of high intensity (20·10³ erg/cm²·sec) increased the phycocyanin content; the chlorophyll a: phycocyanin relation was then 1:12.1.The action spectrum of apparent photosynthesis showed a minimum at 473 nm in all three cultures. The maximum of photosynthesis in low light cultures fell in the absorption region of phycocyanin at 621 nm. The action spectrum of the red light culture showed a reduced rate of photosynthesis in the same region. The strong light culture had an action spectrum similar to that of the red light culture with a maximum at 651 nm. The differing action spectrum of the low light culture may be a result of interruption in the energy transfer from phycocyanin to chlorophyll a within pigment system II.The transients of CO₂ exchange are independent of the pigment composition. Two different types of transients were found depending on the wavelength of the incident light. In red light of 550–650 nm a higher stationary rate was reached after a maximum of photosynthesis at the beginning of the illumination period. In blue and far red light a lower rate was found after the first maximum. Following a illumination period in blue or far red light a CO₂ evolution in the dark was observed. On the other hand, this CO₂ evolution was not found after illumination with red light. These effects are possiblt caused by a decarboxylation reaction (photorespiration) which occurs only in blue and far red light.     

PHOTOSYNTHESIS IN THE FEMALE GAMETOPHYTE OF GINKGO BILOBA

The female gametophyte of Ginkgo biloba is the only seed plant gametophyte known to contain chlorophyll. Measurements of photosynthetically active radiation (PAR) indicate that a gametophyte growing within an ovule can receive significant quantities of light (70 μmol photons m^–2 s^–1). Under the conditions of our experiments, whole gametophytes dissected free from ovules were capable of gross photosynthesis, but not net photosynthesis. On a dry wt basis, the maximum rate of carbon fixation under near saturating light intensities was 3.64 × 10^–3 μmol CO₂g^–1 s^–1. The unique ability of the female gametophyte of Ginkgo biloba to produce chlorophyll and engage in photosynthesis appears to result from its exposure to sufficient levels of light and a predisposition to react to this stimulus by the development of a functional photosynthetic apparatus.     

Effect of photosynthesis on pH variation in cyanobacterial biofilms from Roman catacombs

Cyanobacterial biofilms present on stone surfaces inRoman hypogea were studied with the aim of assessingtheir deteriogenic activity on the colonisedsubstrata. In order to achieve this, non-destructivemethods were developed and applied to measure pHvariation induced via photosynthesis and respirationin representative cyanobacteria from Roman catacombs.Amperometric and potentiometric microsensors were alsoused on Scytonema biofilms in culture in orderto measure photosynthesis and assess pH decreases andincreases during dark–light periods. Measurementsof pH showed that, starting with values slightly belowneutral, the pH in Scytonema biofilms increasedby 0.24–0.77 units in the transition from dark to1000 mol photon m^-2 s^-1 irradiance.Comparison of photosynthesis and pH curves recordedsimultaneously on the same artificial biofilm showeda maximum increase in pH value at irradiances higherthan those saturating photosynthesis. Alkalinisationof the substrate during illumination occurred to asufficient extent to induce precipitation of mineralcompounds, especially on calcareous substrates such asthose present in Roman hypogea.     

Adaptation of the photosynthetic apparatus of Nitellopsis obtusa to changing light intensity at the molecular level: different pathways of a singlet excitation quenching

The effect of prolonged illumination (60 min) with photosynthetically active monochromatic radiation of low intensity (3 μmol m⁻² s⁻¹) and high intensity (60 μmol m⁻² s⁻¹), corresponding to the physiological conditions and light stress conditions, respectively, was studied in the algae Nitellopsis obtusa. Illumination of Nitellopsis obtusa cells with strong light was associated with activation of the xanthophyll cycle, manifested by the deepoxidation of violaxanthin and accumulation of antheraxanthin and zeaxanthin. At the same time, the efficient singlet excitation quenching in the photosynthetic apparatus was activated, as demonstrated by the decrease in the intensity of the chlorophyll a fluorescence emission by ca 50 %. The difference of the fluorescence excitation spectra recorded before and after the light treatment match the difference absorption spectrum of the xanthophyll cycle pigments. The illumination with low light intensity resulted also in the chlorophyll a fluorescence quenching but the effect was very small (less than 10 %). The fluorescence quenching is interpreted in terms of the energy transfer between the Q_y energy level of chlorophyll a and the 2¹ A_g ⁻ energy level of zeaxanthin. The singlet energy levels of carotenoids, corresponding to the green spectral region, are also taken into consideration in the interpretation of the excitation energy exchange between the carotenoids and chlorophylls. Possible molecular mechanisms involved in the activation of the strong and the weak excitation quenching, including violaxanthin isomerization, and possible physiological functions of such pathways of energy transfer are discussed.     

Photooxidative damage to the photosynthetic apparatus during chilling

Short periods of chilling in the presence of light (up to 6 h: 1°C; 270 W/m²) decreased the subsequent apparent photosynthesis and chlorophyll fluorescence of leaf discs of Cucumis sativus L. cv. Kleine Groene Scherpe. The extent of the injury depended on the duration of the chilling pretreatment. After 6 h the subsequent apparent photosynthesis even reached a negative value, and it increased only slightly during the next 2 1/2 h. The decrease of apparent photosynthesis was not a consequence of increased dark respiration but was of photooxidative origin since the presence of both light and oxygen was required. Preincubation in the light for 2 h at 20°C sensitized leaf discs to subsequent photooxidation during chilling. Prompt and delayed chlorophyll fluorescence decreased simultaneously after chilling and light treatment. The corresponding decrease of photosynthesis and chlorophyll fluorescence is discussed in relation to primary photooxidative damage to the photosystems in the chloroplast membrane.     

Recovery from photoinhibition: effect of light and inhibition of protein synthesis of 32-kD chloroplast protein

Recovery from photoinhibition of photosynthesis in intact Lemna gibba was studied in presence of the protein synthesis inhibitors chloramphenicol and cycloheximide. Exposure to an irradiance of 1000 mol m^-2s^-1 in N₂ for 90 min induced 80% photoinhibition. The plants recovered photosynthesis when transfered to normal irradiances (210 mol m^-2s^-1) and air. Chloramphenicol added to the medium was taken up by the plant and reduced photosynthesis slightly. Recovery from photoinhibition was more inhibited than photosynthesis. Cycloheximide was also taken up by the plants and reduced synthesis of light harvesting chlorophyll protein: however, neither photosynthesis nor recovery were much affected. Synthesis of 32-kD chloroplast protein during recovery was inhibited by chloramphenicol, but not by cycloheximide. Synthesis of 32-kD protein was enhanced by 20–210 mol m^-2s^-1 light. The results support the hypothesis that synthesis of 32-kD protein is important for recovery of photosynthesis after photoinhibition.     

Action spectra for photosynthetic adaptation in Scenedesmus obliquus

Photosynthetic adaptation of the unicellular green alga Scenedemus obliquus to different light conditions was investigated with respect to chlorophyll synthesis. Cultures were grown under white light (20 W · m^–2) from fluorescent lamps and were then transferred and subjected to the actual adaptation regime which consisted of a 24-h irradiation by different fluence rates and wavelengths. Fluence rate-response curves for chlorophyll synthesis were measured between 4 · 10^–2 and 1 · 10² W · m^–2. In white light from incandescent lamps, in blue and red light the fluence rate-response curves for chlorophyll (Chl) a and also for Chl b were bell-shaped. In red light the threshold was about the same as under blue light. The maximal amounts of Chl a and b were about twofold increased under blue light relative to the values obtained with red light. Action spectra for the stimulation of chlorophyll synthesis (Chl a + Chl b) as well as those for the separate chlorophylls showed two maxima near 450 and 500 nm. However, the action spectrum for Chl b synthesis demonstrated a considerably higher value in the 450-nm peak. Experiments with the photosynthesis inhibitor 3-(3,4-dichlorphenyl)-1,1-dimethylurea (DCMU) indicated that photosynthetic energy supply supported the photostimulation of chlorophyll synthesis. The action spectra indicate the cooperation of two photoreceptors. The 460-nm peak is attributed to the typical blue-light receptor, being more active in Chl b formation. The peak at 500 nm may represent carotenoproteins acting as an accessory pigment system.Abbreviations PCV packed cell volume - Chl total amount of chlorophyll - Chl a, b chlorophyll a, b - DCMU 3-(3,4-dichlorphenyl)-1,1-dimethylurea This project was supported by the Deutsche Forschungsgemeinschaft. We thank Ms. K. Bölte for technical assistance.     

Structural and functional properties of the coleoptile chloroplast: Photosynthesis and photosensory transduction

Recent studies have shown that guard cell and coleoptile chloroplasts appear to be involved in blue light photoreception during blue light-dependent stomatal opening and phototropic bending. The guard cell chloroplast has been studied in detail but the coleoptile chloroplast is poorly understood. The present study was aimed at the characterization of the corn coleoptile chloroplast, and its comparison with mesophyll and guard cell chloroplasts. Coleoptile chloroplasts operated the xanthophyll cycle, and their zeaxanthin content tracked incident rates of solar radiation throughout the day. Zeaxanthin formation was very sensitive to low incident fluence rates, and saturated at around 800–1000 mol m^–2 s^–1. Zeaxanthin formation in corn mesophyll chloroplasts was insensitive to low fluence rates and saturated at around 1800 mol m^–2 s^–1. Quenching rates of chlorophyll a fluorescence transients from coleoptile chloroplasts induced by saturating fluence rates of actinic red light increased as a function of zeaxanthin content. This implies that zeaxanthin plays a photoprotective role in the coleoptile chloroplast. Addition of low fluence rates of blue light to saturating red light also increased quenching rates in a zeaxanthin-dependent fashion. This blue light response of the coleoptile chloroplast is analogous to that of the guard cell chloroplast, and implicates these organelles in the sensory transduction of blue light. On a chlorophyll basis, coleoptile chloroplasts had high rates of photosynthetic oxygen evolution and low rates of photosynthetic carbon fixation, as compared with mesophyll chloroplasts. In contrast with the uniform chloroplast distribution in the leaf, coleoptile chloroplasts were predominately found in the outer cell layers of the coleoptile cortex, and had large starch grains and a moderate amount of stacked grana and stroma lamellae. Several key properties of the coleoptile chloroplast were different from those of mesophyll chloroplasts and resembled those of guard cell chloroplasts. We propose that the common properties of guard cell and coleoptile chloroplasts define a functional pattern characteristic of chloroplasts specialized in photosensory transduction.Abbreviations Ant or A antheraxanthin - dv/dt fluorescence quenching rate - Fm maximum yield of fluorescence with all PS II reaction centers closed - Fo yield of instantaneous fluorescence with all PS II reaction centers open - Vio or V violaxanthin - Zea or Z zeaxanthin