Influence of carbon supply on the circadian rhythmicity of photosynthesis and its stimulation by blue light in Ectocarpus siliculosus: clues to the mechanism of inorganic carbon acquisition in lower brown algae

Stimulation or light-saturated rates of photosynthesis in Ectocarpus siliculosus (Dillwyn) Lyngb. by blue light was eliminated by increasing dissolved inorganic carbon (DIC) or by lowering pH in natural seawater. The amplitude of the circadian rhythm of photosynthesis was also diminished under these conditions, and the pH compensation points in a closed system were higher in the presence of blue light and during the circadian day. These observations suggest that blue light and the circadian clock regulate the activity of a carbon acquisition system in these plants. The inhibitor of external carbonic anhydrase, acetazolamide, reduced overall rates of photosynthesis by only about 30%, but ethoxyzolamide suppressed the circadian rhythm of photosynthesis almost completely and markedly reduced the duration of responses to blue light pulses. Similar patterns were obtained when photosynthesis was measured in strongly limiting DIC concentrations (0–0.5 mol m^?3). Since blue light stimulated photosynthesis under these conditions of strong carbon limitation, we suggest that blue light activates the release of CO₂ from an internal CO₂ store. We propose a metabolic pathway with similarities to that of CAM plants. Non-photosynthetic fixation leads to the accumulation of a storage metabolite. The circadian clock and blue light control the mobilization of CO₂ at the site of decarboxylation of this metabolite. In the presence of continuous blue light the pathway is proposed to cycle and act as a pump for CO₂ into the chloroplasts. This hypothesis helps to explain a number of previously reported peculiarities of brown algal photosynthesis.     

DIEL PERIODICITY OF PHOTOSYNTHESIS AND CELL DIVISION COMPARED IN THALASSIOSIRA WEISSFLOGII (BACILLARIOPHYCEAE)1

Diel patterns of photosynthesis and cell division were examined in Thalassiosira weissflogii Grun. (clone Actin) grown in nitrogen-limited cyclostat culture. Ammonia (NH₄+) was either supplied continuously or as a daily pulse to cultures grown in constant light or in a light: dark cycle. When either nitrogen or light was supplied periodically, both cell division and photosynthetic capacity were periodic. When both nitrogen and light were supplied periodically, cell division was coupled to the N-pulse whereas periodicities of photosynthetic capacity were modified but remained coupled to the light-dark cycle. Diel oscillations in photosynthesis were i) largely independent of cellular pigmentation and ii) similar for light-limiting and saturating irradiances. Periodicity in photosynthetic capacity also persisted following transfer of non-dividing batch cultures to constant light. Results suggest that photosynthesis but not cell division was coupled to a circadian clock in T. weissflogii. A circadian rhythm of photosynthesis may optimize carbon assimilation in phytoplankton exposed to intermittent nutrient supply by ensuring that maximum photosynthetic capacity occurs during the day.     

Influence of carbon supply on the stimulation of light-saturated photosynthesis by blue light in Laminaria saccharina: implications for the mechanism of carbon acquisition in higher brown algae

In saturating irradiances of red light, photosynthesis of Laminaria saccharina (L.) Lamouroux was stimulated by low irradiances of continuous blue light only when the supply of dissolved inorganic carbon (DIC) was limiting. The degree of this stimulation was inversely proportional to the logarithm of the concentration of free CO₂, whether this was adjusted by varying the total DIC or the pH at a given DIC concentration. The final pH reached in a closed system was higher in blue light than in red light. Both acetazolamide and ethoxyzolamide suppressed the responses to blue light almost completely, but reduced photosynthesis in red light by only 30%. Buffering the pH of the seawater also suppressed the stimulation of photosynthesis by blue light without affecting the photosynthetic rate in red light. The transient stimulation of O₂ evolution by a blue light pulse was not accompanied by a corresponding increase in CO₂ consumption. These observations could be explained if, in analogy to the mechanism proposed for Ectocarpus (Schmid, Mills & Dring 1996, Plant Cell and Environment 19,373–382, this issue, accompanying paper), photosynthesis was supported by a blue-light-activated release of CO₂ from an internal store. We suggest that the store is located in the vacuoles of the cortical tissue of the blades. The main photosynthetic tissue, however, is in the overlying meristoderm, and blue-light-activated mobilization of the store could stimulate O₂ evolution only if periplasmic carbonic anhydrase was available to facilitate CO₂ uptake from the cortex.     

Alterations in Rubisco activity and in stomatal behavior induce a daily rhythm in photosynthesis of aerial leaves in the amphibious-plant Nuphar lutea

Nuphar lutea is an amphibious plant with submerged and aerial foliage, which raises the question how do both leaf types perform photosynthetically in two different environments. We found that the aerial leaves function like terrestrial sun-leaves in that their photosynthetic capability was high and saturated under high irradiance (ca. 1,500 μmol photons m⁻² s⁻¹). We show that stomatal opening and Rubisco activity in these leaves co-limited photosynthesis at saturating irradiance fluctuating in a daily rhythm. In the morning, sunlight stimulated stomatal opening, Rubisco synthesis, and the neutralization of a night-accumulated Rubisco inhibitor. Consequently, the light-saturated quantum efficiency and rate of photosynthesis increased 10-fold by midday. During the afternoon, gradual closure of the stomata and a decrease in Rubisco content reduced the light-saturated photosynthetic rate. However, at limited irradiance, stomatal behavior and Rubisco content had only a marginal effect on the photosynthetic rate, which did not change during the day. In contrast to the aerial leaves, the photosynthesis rate of the submerged leaves, adapted to a shaded environment, was saturated under lower irradiance. The light-saturated quantum efficiency of these leaves was much lower and did not change during the day. Due to their low photosynthetic affinity for CO₂ (35 μM) and inability to utilize other inorganic carbon species, their photosynthetic rate at air-equilibrated water was CO₂-limited. These results reveal differences in the photosynthetic performance of the two types of Nuphar leaves and unravel how photosynthetic daily rhythm in the aerial leaves is controlled.     

KINETICS OF BLUE-LIGHT STIMULATION AND CIRCADIAN RHYTHMICITY OF LIGHT-SATURATED PHOTOSYNTHESIS IN BROWN ALGAE: A SPECIES COMPARISON1

The time courses of photosynthetic rates in red light, with and without additional blue light, were investigated and compared in 20 species of brown algae. Species could be separated into two groups on the basis of the rhythmicity of their photosynthesis in red light and the kinetics of their responses to blue-light pulses. One group, which consisted of members of the Ectocarpales, Chordariales, and Dictyosiphonales, was characterized by strong and persistent circadian rhythmicity in red light. The photosynthetic responses of these species to blue-light pulses started within 10–30 s of the beginning of blue-light treatment and mostly contained at least two distinct kinetic components. An early component, which reached a maximum about 5–10 min after the blue-light pulse, was always detectable. Later components were seen as separate peaks or shoulders after an additional 10–20 min. The decay of the response in this group of species was mostly slow, with half-lives of between 0.5 and 1.5 h. In the second group of species, consisting of members of the Dictyotales, Laminariales, and Fucales, photosynthesis in red light was usually non-rhythmic, although circadian rhythms with a weak amplitude or of transient occurrence were observed in some plants of some species. The increase in photosynthesis in response to a blue-light pulse was not detectable until 70–330 s after the start of blue-light treatment, and the response itself had only a single component, with a maximum after about 10 min and half-life of 10–20 min. The lengths of the lag-phases were positively correlated with the times taken to reach the peak in this group, although the lag-phases and the half lives sometimes varied with time in individual plants. Two members of the Sphacelariales (Sphacelaria, Cladostephus) did not fit into either of the two groups because their photosynthesis was rhythmic, but their responses had long lag-phases, a single component, and moderately long half-lives. The differences in the kinetics of the photosynthetic response to blue-light pulses, which have been described for the two main groups of species, are thought to indicate that there are two distinct mechanisms by which light-saturated photosynthesis responds to blue light in brown algae. Since in some species the maximal photosynthesis after a blue-light pulse and the rate of photosynthesis in continuous blue light also varied in a circadian pattern, the response to blue light itself may be under circadian control.     

Environmental effects on circadian rhythms in photosynthesis and stomatal opening

Persistent circadian rhythms in photosynthesis and stomatal opening occurred in bean (Phaseolus vulgaris L.) plants transferred from a natural photoperiod to a variety of constant conditions. Photosynthesis, measured as carbon assimilation, and stomatal opening, as conductance to water vapor, oscillated with a freerunning period close to 24 h under constant moderate light, as well as under light-limiting and CO₂-limiting conditions. The rhythms damped under constant conditions conducive to high photosynthetic rates, as did rates of carbon assimilation and stomatal conductance, and this damping correlated with the accumulation of carbohydrate. No rhythm in respiration occurred in plants transferred to constant darkness, and the rhythm in stomatal opening damped rapidly in constant darkness. Damping of rhythms also occurred in leaflets exposed to constant light and CO₂-free air, demonstrating that active photosynthesis and not simply light was necessary for sustained expression of these rhythms. This is CIWDPB Publication No. 1142 This research was supported by National Science Foundation grant BSR 8717422 (C.B.F.) and a U.S. Department of Agriculture training grant to Stanford University (T.L.H.).     

STIMULATION OF LIGHT-SATURATED PHOTOSYNTHESIS IN LAMINARIA (PHAEOPHYTA) BY BLUE LIGHT1

The light-saturated rate of photosynthesis in blue light was 50-100% higher than that in red light for young sporophytes of Laminaria digitata (Huds.) Lamour., although photosynthetic rates were slightly higher in red than in blue light at low irradiances. Short exposures to low irradiances (e.g. 2 min at 20 μmol · m^?2· s^?1) of blue light also stimulated the subsequent photosynthesis of Laminaria sporophytes in saturating irradiances of red light but had little effect on photosynthesis in low irradiances of red light. The full stimulatory effect of short exposures to blue light was observed within 5 min of the blue treatment and persisted for at least 15 min in red light or in darkness. Thereafter, the effect began to decline, but some stimulation was still detectable 45 min after the blue treatment. The degree of stimulation was proportional to the logarithm of the photon exposure to blue light over the range 0.15-2.4 mmol · m^?2, and the effectiveness of an exposure to 0.6 mmol · m^?2at different wavelengths was high at 402-475 nm (with a peak at 460-475 nm) but declined sharply at 475-497 nm and was minimal at 544-701 nm. Blue light appears, therefore, to exert a direct effect on the dark reaction of photosynthesis in brown algae, possibly by activating carbon-fixing enzymes or by stimulating the uptake or transport of inorganic carbon in the plants.     

Photosynthetic characteristics of charophytes from tropical lotic ecosystems

Responses of photosynthetic rates, determined by oxygen evolution using the light and dark bottles technique, to different temperatures, irradiances, pH, and diurnal rhythm were analyzed under laboratory conditions in four charophyte species (Chara braunii Gmelin, C. guairensis R. Bicudo, Nitella subglomerata A. Braun and Nitella sp.) from lotic habitats in southeastern Brazil. Parameters derived from the photosynthesis versus irradiance curves indicated affinity to low irradiances for all algae tested. Some degree of photoinhibition, [β= ‐(0.30–0.13) mg O2 g^?1 dry weight Ir¹ (μmol photons m^?2 s^?1)^?1], low light compensation points (I_c= 4–20 μmol photons m^?2 s^?1) were found for all species analyzed, as well as low values of light saturation parameter (I_k) and saturation (I_s) 29–130 and 92–169 μmol photons m^?2 s^?1, respectively. Photoacclimation was observed in two populations of N. subglomerata collected from sites with different irradiances, consisting of variations in photosynthetic parameters (higher values of a, and lower of I_k and maximum photosynthetic rate, P_max, in the population under lower irradiance). The highest photosynthetic rates for Chara species were observed at 10–15°C, while for Nitella the highest photosynthetic rate was observed at 20–25°C, despite the lack of significant differences among most levels tested. Rates of dark respiration significantly increase with temperature, with the highest values at 25°C. The results from pH experiments showed highest photosynthetic rates under pH 4.0 for all algae, suggesting higher affinity for inorganic carbon in the form of carbon dioxide, except in one population of N. subglomerata, with similar rates under the three levels, suggesting indistinct use of bicarbonate and carbon dioxide. Diurnal changes in photosynthetic rates revealed a general pattern for most algae tested, which was characterized by two peaks: the first (higher) during the morning (07.00–11.00) and the second (lower) in the afternoon (14.00–17.00). This suggests an endogenous rhythm determining the daily variations in photosynthetic rates.     

Nitrogen supply as a factor influencing photoinhibition and photosynthetic acclimation after transfer of shade-grown Solanum dulcamara to bright light

We have compared the ability of shadegrown clones of Solamum dulcamara L. from shade and sun habitats to acclimate to bright light, as a function of nitrogen nutrition before and after transfer to bright light. Leaves of S. dulcamara grown in the shade with 0.6 mM NO ₃ ^- have similar photosynthetic properties as leaves of plants grown with 12.0 mM NO ₃ ^- . When transferred to bright light for 1–2 d the leaves of these plants show substantial photoinhibition which is characterized by about 50% decrease in apparent quantum yield and a reduction in the rate of photosynthesis in air at light saturation. Photoinhibition of leaf photosynthesis is associated with reduction in the variable component of low-temperature fluorescence emission, and with loss of in-vitro electron transport, especially of photosystem II-dependent processes.We find no evidence for ecotypic differentiation in the potential for photosynthetic acclimation among shade and sun clones of S. dulcamara, or of differentiation with respect to nitrogen requirements for acclimation. Recovery from photoinhibition and subsequent acclimation of photosynthesis to bright light only occurs in leaves of plants provided with 12.0 mM NO ₃ ^- . In these, apparent quantum yield is fully restored after 14 d, and photosynthetic acclimation is shown by an increase in light-saturated photosynthesis in air, of light-and CO₂-saturated photosynthesis, and of the initial slope of the CO₂-response curve. The latter changes are highly correlated with changes in ribulose-bisphosphate-carboxylase activity in vitro. Plants supplied with 0.6 mM NO ₃ ^- show incomplete recovery of apparent quantum yield after 14 d, but CO₂-dependent leaf photosynthetic parameters return to control levels.Symbols and abbreviations F_o initial level of fluorescence at 77 K - F_m maximum level of fluorescence at 77 K - F_v variable components of fluorescence at 77 K (F_v=F_m-F_o) - PSI, PSII photosystem I and II, respectively - RuBP ribulose-1,5-bisphosphate - RuBPCase ribulose-1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39)     

Interactions of blue light and inorganic carbon supply in the control of light-saturated photosynthesis in brown algae

Photosynthetic capacities of five species of brown algae in red light were found to be strongly limited by the inorganic carbon supply of natural sea water. Under these conditions, pH 8·2 and dissolved inorganic carbon concentration (DIG) of 2·1 mol m^?3, a short pulse of blue light was found to increase the subsequent rate of photosynthesis in saturating red light. The degree of blue light stimulation varied between species, ranging from an increase of over 200% of the original rate in Colpomenia peregrins to only 10% in Dictyota dichotoma. Increasing the DIG concentration of sea water by bicarbonate addition resulted in carbon saturation of photosynthesis in all five species. Blue light stimulation was greatly reduced at these higher DIG concentrations. The response in Laminaria digitata was examined in more detail by manipulation of pH and DIG to produce solutions with different concentrations of dissolved CO₂. At a CO₂ concentration typical of normal sea water (12·4 mmol m^?3), blue light treatment increased photosynthetic rate by approximately 50%. Blue light stimulation was increased to over 150% at CO₂ concentrations below that of sea water, whereas at concentrations above that of sea water, the effect was diminished. Therefore, the effect of blue light on photosynthetic capacity appears to involve an increase in the rate of supply of carbon dioxide to the plant.     

Effect of photon flux density on inorganic carbon accumulation and net CO2 exchange in a high-CO2-requiring mutant of Chlamydomonas reinhardtii

The effect of photon flux density on inorganic carbon accumulation and photosynthetic CO₂ assimilation was determined by CO₂ exchange studies at three, limiting CO₂ concentrations with a ca-1 mutant of Chlamydomonas reinhardiii. This mutant accumulates a large internal inorganic carbon pool in the light which apparently is unavailable for photosynthetic assimilation. Although steady-state photosynthetic CO₂ assimilation did not respond to the varying photon flux densities because of CO₂ limitation, components of inorganic-carbon accumulation were not clearly light saturated even at 1100 mol photons m^-2 s^-1, indicating a substantial energy requirement for inorganic carbon transport and accumulation. Steady-state photosynthetic CO₂ assimilation responded to external CO₂ concentrations but not to changing internal inorganic carbon concentrations, confirming that diffusion of CO₂ into the cells supplies most of the CO₂ for photosynthetic assimilation and that the internal inorganic carbon pool is essentially unavailable for photosynthetic assimilation. The estimated concentration of the internal inorganic carbon pool was found to be relatively insensitive to the external CO₂ concentration over the small range tested, as would be expected if the concentration of this pool is limited by the internal to external inorganic carbon gradient. An attempt to use this CO₂ exchange method to determine whether inorganic carbon accumulation and photosynthetic CO₂ assimilation compete for energy at low photon flux densities proved inconclusive.     

Modelling C3 photosynthesis: A sensitivity analysis of the photosynthetic carbon-reduction cycle

A model of the C ₃ photosynthetic system is developed which describes the sensitivity of the steadystate rate of carbon dioxide assimilation to changes in the activity of several enzymes of the system. The model requires measurements of the steady-state rate of carbon dioxide assimilation, the concentrations of several intermediates in the photosynthetic system, and the concentration of the active site of ribulose 1,5-bisphosphate carboxyalse/oxygenase (Rubisco). It is shown that in sunflowers (Helianthus annuus L.) at photon flux densities that are largely saturating for the rate of photosynthesis, the steady-stete rate of carbon dioxide assimilation is most sensitive to Rubisco activity and, to a lesser degree, to the activities of the stromal fructose, 6-bisphosphatase and the enzymes catalysing sucrose synthesis. The activities of sedoheptulose 1,7-bisphosphatase, ribulose 5-phosphate kinase, ATP synthase and the ADP-glucose pyrophosphorylase are calculated to have a negligible effect on the flux under the high-light conditions. The utility of this analysis in developing simpler models of photosynthesis is also discussed.Abbreviations c _i intercellular CO₂ concentration - C _infP ^supJ control coefficient for enzyme P with respect to flux J - DHAP dihydroxyacetonephosphate - E4P erythrose 4-phosphate - F6P fructose 6-phosphate - FBP fructose 1,6-bisphosphate - FBPase fructose 1,6-bisphosphatase - G3P glyceraldehyde 3-phosphate - G1P glucose 1-phosphate - G6P glucose 6-phosphate - Pi inorganic phosphate - PCR photosynthetic carbon reduction - PGA 3-phosphoglyceric acid - PPFD photosynthetically active photon flux density - R _n ^J response coefficient for effector n with respect to flux J - R5P ribose 5-phosphate - Rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase - Ru5P ribulose 5-phosphate - RuBP ribulose 1,5-bisphosphate - S7P sedoheptulose 7-phosphate - SBP sedoheptulose 1,7-bisphosphate - SBPase sedoheptulose 1,7-bisphosphatase - SPS sucrose-phosphate synthase - Xu5P xylulose 5-phosphate - _n ^P elasticity coefficient for effector n with respect to the catalytic velocity of enzyme P This research was funded by an Australian Research Council grant to I.E.W. and was undertaken during a visity by K.A.M. to the James Cook University of North Queensland. The expert help of Glenys Hanley and Mick Kelly is greatly appreciated.     

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.     

A rapid circadian rhythm of carbon-dioxide metabolism in Bryophyllum fedtschenkoi

Leaves of Bryophyllum fedtschenkoi Hamet et Perrier maintained in a stream of normal air and at 15° C exhibit a circadian rhythm of CO₂ uptake in continuous light but not in continuous darkness. The rhythm is unusual in that it persists for at least 10 d, and has a short period of approximately 18 h. The mechanism by which this rhythm is generated is discussed.Abbreviation PEPCase phosphoenolpyruvate carboxylase     

Circadian rhythms in Kalanchoë: effects of irradiance and temperature on gas exchange and carbon metabolism

Gas exchange in K. blossfeldiana shows a circadian rhythm in net CO₂ uptake and transpiration when measured under low and medium irradiances. The period length varies between 21.4 h at 60 W m^-2 and 24.0 h at 10 W m^-2. In bright light (80 W m^-2) or darkness there are no rhythms. High leaf temperatures result in a fast dampening of the CO₂-uptake rhythm at moderate irradiances, but low leaf temperatures can not overcome the dampening in bright light. The rhythm in CO₂ uptake is accompanied by a less pronounced and more rapidly damped rhythm in transpiration and by oscillations in malate levels with the amplitude being highly reduced. The oscillations in starch content, usually observed to oscillate inversely to the acidification in light-dark cycles, disappear after the first cycle in continuous light. The balance between starch and malate levels depends in continuous light on the irradiance applied. Leaves show high malate and low starch content at low irradiance and high starch and low malate in bright light. During the first 12 h in continuous light replacing the usual dark period, malate synthesis decreases with the increasing irradiance. Up to 50 W m^-2 starch content decreases; at higher irradiances it increases above the values usually measured at the end of the light period of the 12:12 h light-dark cycle.Abbreviations CAM Crassulacean acid metabolism - FW fresh weight - PEP phosphoenolpyruvate     

Phosphate sequestration by glycerol and its effects on photosynthetic carbon assimilation by leaves

Glycerol induced a limitation on photosynthetic carbon assimilation by phosphate when supplied to leaves of barley (Hordeum vulgare L.) and spinach (Spinacia oleracea L.). This limitation by phosphate was evidenced by (i) reversibility of the inhibition of photosynthesis by glycerol by feeding orthophosphate (ii) a decrease in light-saturated rates of photosynthesis and saturation at a lower irradiance, (iii) the promotion of oscillations in photosynthetic CO₂ assimilation and in chlorophyll fluorescence, (iv) decreases in the pools of hexose monophosphates and triose phosphates and increases in the ratio of glycerate-3-phosphate to triose phosphate, (v) decreased photochemical quenching of chlorophyll fluorescence, and increased non-photochemical quenching, specifically of the component which relaxed rapidly, indicating that thylakoid energisation had increased. In barley there was a massive accumulation of glycerol-3-phosphate and an increase in the period of the oscillations, but in spinach the accumulation of glycerol-3-phosphate was comparatively slight. The mechanism(s) by which glycerol feeding affects photosynthetic carbon assimilation are discussed in the light of these results.Abbreviations Chl chlorophyll - C _i intercellular concentration of CO₂ - P phosphate - PGA glycerate-3-phosphate - Pi orthophosphate - triose-P sum of glyceraldehyde-3-phosphate and dihydroxyacetone phosphate     

Fractional control of photosynthesis by the QB protein,the cytochrome f/b 6 complex and other components of the photosynthesic apparatus

In order to obtain information on fractional control of photosynthesis by individual catalysts, catalytic activities in photosynthetic electron transport and carbon metabolism were modified by the addition of inhibitors, and the effect on photosynthetic flux was measured using chloroplasts of Spinacia oleracea L. In thylakoids with coupled electron transport, light-limited electron flow to ferricyanide was largely controlled by the Q_B protein of the electron-transport chain. Fractional control by the cytochrome f/b ₆ complex was insignificant under these conditions. Control by the cytochrome f/b ₆ complex dominated at high energy fluence rates where the contribution to control of the Q_B protein was very small. Uncoupling shifted control from the cytochrome f/b ₆ complex to the Q_B protein. Control of electron flow was more complex in assimilating chloroplasts than in thylakoids. The contributions of the cytochrome f/b ₆ complex and of the Q_B protein to control were smaller in intact chloroplasts than in thylakoids. Thus, even though the transit time for an electron through the electron-transport chain may be below 5 ms in leaves, oxidation of plastohydroquinone was only partially responsible for limiting photosynthesis under conditions of light and CO₂ saturation. The energy fluence rate influenced control coefficients. Fractional control of photosynthesis by the ATP synthetase, the cytochrome f/b ₆ complex and by ribulose-1,5-bisphosphate carboxylase increased with increasing fluence rates, whereas the contributions of the Q_B protein and of enzymes sensitive to SH-blocking agents decreased. The results show that the burdens of control are borne by several components of the photosynthetic apparatus, and that burdens are shifted as conditions for photosynthesis change.Abbreviations Chl chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DNP-INT 2,4-dinitro phenylether of 2-iodo-4-nitrothymol - pCMBS p-chloromercuribenzosulfonate     

Fluorescence quenching during photosynthesis and photoinhibition of Ulva rotundata blid.

The relationships between photoinhibition and photoprotection in high and low-light-grown Ulva were examined by a combination of chlorophyll-fluorescence-monitoring techniques. Tissues were exposed to a computer-controlled sequence of 5-min exposures to red light, followed by 5-min darkness, with stepwise increases in photon flux. Coefficients of chlorophyll fluorescence quenching (1?q_P and NPQ) were calculated following a saturating pulse of white light near the end of each 5-min light treatment. Dark-adapted chlorophyll fluorescence parameters (F₀ and F_V/F_M) were calculated from a saturating pulse at the end of each 5-min dark period. Low-light-grown Ulva showed consistently higher 1?q_P, i.e. higher reduction status of Q (high primary acceptor of photosystem II), and lower capacity for nonphotochemical quenching (NPQ) at saturating light than did high-light-grown plants. Consequently, low-light plants rapidly displayed photoinhibitory damage (increased F₀) at light saturation in seawater. Removal of dissolved inorganic carbon from seawater also led to photoinhibitory damage of high-light-grown Ulva at light saturation, and addition of saturating amounts of dissolved inorganic carbon protected low-light-grown plants against photoinhibitory damage. A large part of NPQ was abolished by treatment with 3 mM dithiothreitol and the processes so inhibited were evidently photoprotective, because dithiothreitol treatment accelerated photoinhibitory damage in both low- and high-light-grown Ulva. The extent of photoinhibitory damage in Ulva was exacerbated by treatment with chloramphenicol (1 mM) without much effect on chlorophyll-quenching parameters, evidently because this inhibitor of chloroplast protein synthesis reduced the rate of repair processes.     

Light- and temperature-entrained circadian regulation of activity and mRNA accumulation of 1-aminocyclopropane-1-carboxylic acid oxidase in Stellaria longipes

Stem and leaf tissues of Stellaria longipes Goldie (prairie ecotype) exhibit circadian rhythmicity in the activity and mRNA abundance for 1-aminocyclopropane-1-carboxylic acid oxidase (EC 1.4.3). The steady-state mRNA levels and enzymatic activity levels fluctuated with a period of approximately 24 h and reached their maxima by the middle of the light phase and minima by the middle of the dark phase. The oscillations showed damping under constant light, constant dark and constant temperature conditions, indicating that the rhythm is entrained by an external signal. The results indicate that light/dark cycles have greater entraining effects than temperature cycles. A 15-min red light pulse, but not a blue light pulse, could reset rhythm in continuous darkness, suggesting the possible role of a red-light signal transduction pathway in the circadian regulation of 1-aminocyclopropane-1-carboxylic acid oxidase.Abbreviations ACC 1-aminocyclopropane-1-carboxylic acid - DD continuous dark - LD light-dark - LL continuous light - ZT Zeitgeber time (start of light period for circadian entrainment) This study was supported by operating grants to C.C.C., and D.M.R. from the Natural Sciences and Engineering Research Council of Canada.The authors gratefully acknowledge the award of a Bettina Bahlsen memorial Graduate Scholarship by University of Calgary to A.K. We are grateful to Dr. M.M. Moloney for allowing the use of his laboratory facilities.     

Metabolic energy for stomatal opening. Roles of photophosphorylation and oxidative phosphorylation

The supply of energy for stomatal opening was investigated with epidermal peels of Commelina communis L. and Vicia faba L., under white, blue and red irradiation or in darkness. Fluencerate response curves of stomatal opening under blue and red light were consistent with the operation of two photosystems, one dependent on photosynthetic active radiation (PAR) and the other on blue light, in the guard cells. The PAR-dependent system was 3(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-sensitive and KCN-resistant and showed a relatively high threshold irradiance for its activation; its activity was most prominent at moderate to high irradiances. The blue-light-dependent photosystem was KCN-sensitive, was active at low irradiances, and interacted with the PAR-dependent photosystem at high blue irradiances. Stomatal opening in darkness, caused by CO₂-free air, fusicoccin or high KCl concentrations, was KCN-sensitive and DCMU-resistant. These data indicate that stomatal opening in darkness depends on oxidative phosphorylation for the supply of high-energy equivalents driving proton extrusion. Light-dependent stomatal opening appears to require photophosphorylation from guard-cell chloroplasts and the activation of the blue-light photosystem which could rely either on oxidative phosphorylation or a specific, membrane-bound electron-transport carrier.Abbreviations DCMU 3(3,4-dichlorophenyl)-1-1-dimethylurea - FC fusicoccin - KCN potassium cyanide - PAR photosynthetic active radiation - WL white light