Photosynthetic responses to light variation in rainforest species

Summary The dependence of net carbon gain during lightflecks (artificial sunflecks) on leaf induction state, lightfleck duration, lightfleck photosynthetic photon flux density (PFD), and the previous light environment were investigated in A. macrorrhiza and T. australis, two Australian rainforest species. The photosynthetic efficiency during lightflecks was also investigated by comparing observed values of carbon gain with predicted values based on steady-state CO₂ assimilation rates. In both species, carbon gain and photosynthetic efficiency increased during a series of five 30-or 60-s lightflecks that followed a long period of low light; efficiency was linearly related to leaf induction state.In fully-induced leaves of both species, efficiency decreased and carbon gain increased with lightfleck duration. Low-light grown A. macrorrhiza had greater efficiency than predicted based on steady-state rates (above 100%) for lightflecks less than 40 s long, whereas leaves grown in high light had efficiencies exceeding 100% only during 5-s lightflecks. The efficiency of leaves of T. australis ranged from 58% for 40-s lightflecks to 96% for 5-s lightflecks.In low-light grown leaves of A. macrorrhiza, photosynthetic responses to lightflecks below 120 mol m^-2 s^-1 were not affected significantly by the previous light level. However, during lightflecks at 530 mol m^-2 s^-1, net carbon gain and photosynthetic efficiency of leaves previously exposed to low light levels were significantly reduced relative to those of leaves previously exposed to 120 and 530 mol m^-2 s^-1.These results indicate that, in shade-tolerant species, net carbon gain during sunflecks can be enhanced over values predicted from steady-state CO₂ assimilation rates. The degree of enhancement, if any, will depend on sunfleck duration, previous light environment, and sunfleck PFD. In forest understory environments, the temporal pattern of light distribution may have far greater consequences for leaf carbon gain than the total integrated PFD.Supported by National Science Foundation Grant BSR 8217071 and USDA Grant 85-CRCR-1-1620     

Photosynthesis in flashing light in soybean leaves grown in different conditions. II. Lightfleck utilization efficiency

Leaves of soybean plants grown in contrasting light and nutrient availability conditions were exposed to constant and to flashing light regimes with lightflecks of different frequencies, durations and photon flux density (PFD). The lightfleck characteristics were selected to be representative of the range of variation found for sunflecks in a soybean canopy. CO₂ fixation rates were measured using a fast-response gas-exchange apparatus. The net CO₂ fixation due to 1-s-duration lightflecks was 1·3 times higher than predicted from steady-state measurements in constant light at the lightfleck and background PFD. This lightfleck utilization efficiency (LUE) was somewhat higher at a high than at a low frequency of one second lightflecks. LUE in flashing light with very short lightflecks (0·2s) and single 1 s lightflecks was as high as 2, but decreased sharply with increasing duration of lightflecks. This decrease occurred because CO₂ fixation rates during lightflecks were constrained by carbon metabolism and induction limitations, and because the contribution of post-illumination CO₂ fixation to total CO₂ fixation decreased with increased duration of lightflecks. LUE increased with increased PFD during the lightflecks, particularly in leaves from plants grown in high-light, high-nutrient conditions. Saturation PFDs were much higher in flashing light than in constant light. Only small differences in LUE were apparent between leaves from the three growth conditions.     

Characteristics of transient photosynthesis in Quercus serrata seedlings grown under lightfleck and constant light regimes

Photosynthetic-induction response and light-fleck utilization were investigated for the current-year seedlings of Quercus serrata, a deciduous tree found in temperate regions of Japan. The tree seedlings were grown under three light regimes: a constant low photosynthetic photon flux density (PFD) regime of 50 mol m^–2 s^–1, a constant high PFD regime of 500 mol m^–2 s^–1, and a lightfleck regime with alternated low (lasting 5 s) and high (lasting 35 s) PFD. The photosynthetic-induction response following a sudden increase of PFD from 50 to 500 mol m^–2 s^–1 exhibited two phases: an initial fast increase complete within 3–5 s, and a second slow increase lasting for 15–20 min. Induction times required to reach 50% and 90% of steady-state assimilation rates were significantly shorter in leaves from the constant low PFD than those from the high PFD regime. During the first 60–100 s, the ratio of observed integrated CO₂ uptake to that predicted by assuming that a steady-state assimilation would be achieved instantaneously after the light increase was significantly higher for leaves from the low PFD regime than from the high PFD regime. Lightfleck utilization was examined for various durations of PFD of 500 mol m^–2 s^–1 on a background PFD of 50 mol m^–2 s^–1. Lightfleck utilization efficiency was significantly higher in low PFD leaves than in the high PFD leaves for 5-s and 10-s lightflecks, but showed no difference among different light regimes for 100-s lightflecks. The contribution of post-illumination CO₂ fixation to total carbon gain decreased markedly with increasing lightfleck durations, but exhibited no significant difference among growth regimes. Photosynthetic performances of induction response and lightfleck utilization in leaves from the lightfleck regime were more similar to those in leaves from the low PFD regime. It may be the total daily PFD rather than PFD dynamics in light regimes that affects the characteristics of transient photosynthesis in Q. serrata seedlings.     

The role of enzyme activation state in limiting carbon assimilation under variable light conditions

The mechanisms regulating transient photosynthesis by soybean (Glycine max) leaves were examined by comparing photosynthetic rates and carbon reduction cycle enzyme activities under flashing (saturating 1 s lightflecks separated by low photon flux density (PFD) periods of different durations) and continuous PFD. At the same mean PFD, the mean photosynthetic rates were reduced under flashing as compared to continuous light. However, as the duration of the low PFD period lengthened, the CO₂ assimilation attributable to a lightfleck increased. This enhanced lightfleck CO₂ assimilation was accounted for by a greater postillumination CO₂ fixation occurring after the lightfleck. The induction state of photosynthesis, ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco), fructose 1,6-bisphosphatase (FBPase) and ribulose 5-phosphate kinase (Ru5P kinase) activities all responded similarly and were all lower under flashing as compared to constant PFD of the same integrated mean value. However, the fast phase of induction and FBPase and Ru5P kinase activities were reduced more than were the slow phase of induction and rubisco activity. This was consistent with the role of the former enzymes in the fast induction component that limited RuBP regeneration. Competition for reducing power between carbon metabolism and thioredoxin-mediated enzyme activation may have resulted in lower enzyme activation states and hence lower induction states under flashing than continuous PFD, especially at low lightfleck frequencies (low mean PFD).Abbreviations FBPase fructose 1,6-bisphosphatase (EC 3.1.3.11) - LUE lightfleck use efficiency - P-glycerate 3-phosphoglycerate - PICF post-illumination CO₂ fixation - Ru5P kinase ribulose 5-phosphate kinase (EC 2.7.1.19) - RuBP ribulose 1,5-bisphosphate - rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) - SBpase sedoheptulose 1,7-bisphosphatase (EC 3.1.3.37)     

Concurrent Measurements of Oxygen and Carbon Dioxide Exchange during Lightflecks in Maize (Zea mays L.)

Leaves of maize (Zea mays L.) were enclosed in a temperature-controlled cuvette under 35 Pa (350 [mu]bars) CO2 and 0.2 kPa (0.2%)O2 and exposed to short periods (1-30 s) of illumination (light-flecks). The rate and total amount of CO2 assimilated and O2 evolved were measured. The O2 evolution rate was taken as an indicator of the rate of photosynthetic noncyclic electron transport (NCET). In this C4 species, the response of electron transport during the lightflecks qualitatively mimicked that of C3 species previously tested, whereas the response of CO2 assimilation differed. Under short-duration lightflecks at high photon flux density (PFD), the mean rate of O2 evolution was greater than the steady-state rate of O2 evolution under the same PFD due to a burst of O2 evolution at the beginning of the lightfleck. This O2 burst was taken as indicating a high level of NCET involved in the buildup of assimilatory charge via ATP, NADPH, and reduced or phosphorylated metabolites. However, as lightfleck duration decreased, the amount of CO2 assimilated per unit time of the lightfleck (the mean rate of CO2 assimilation) decreased. There was also a burst of CO2 from the leaf at the beginning of low-PFD lightflecks that further reduced the assimilation during these lightflecks. The results are discussed in terms of the buildup of assimilatory charge through the synthesis of high-energy metabolites specific to C4 metabolism. It is speculated that the inefficiency of carbon uptake during brief light transients in the C4 species, relative to C3 species, is due to the futile synthesis of C4 cycle intermediates.     

Light and the maintenance of photosynthetic competence in leaves of Populus balsamifera L. during short-term exposures to high concentrations of sulfur dioxide

Leaves of Populus balsamifera grown under full natural sunlight were treated with 0, 1, or 2 l SO₂·1^-1 air under one of four different photon flux densities (PFD). When the SO₂ exposures took place in darkness or at 300 mol photons·m^-2·s^-1, sulfate accumulated to the levels predicted by measurements of stomatal conductance during SO₂ exposure. Under conditions of higher PFD (750 and 1550 mol·m^-2·s^-1), however, the predicted levels of accumulated sulfate were substantially higher than those obtained from anion chromatography of the leaf extracts. Light-and CO₂-saturated capacity as well as the photon yield of photosynthetic O₂ evolution were reduced with increasing concentration of SO₂. At 2 l SO₂·1^-1 air, the greatest reductions in both photosynthetic, capacity and photon yield occurred when the leaves were exposed to SO₂ in the dark, and increasingly smaller reductions in each occurred with increasing PFD during SO₂ exposure. This indicates that the inhibition of photosynthesis resulting from SO₂ exposure was reduced when the exposure occurred under conditions of higher light. The ratio F _v/F _M (variable/maximum fluorescence emission) for photosyntem II (PSII), a measure of the photochemical efficiency of PSII, remained unaffected by exposure of leaves to SO₂ in the dark and exhibited only moderate reductions with increasing PFD during the exposure, indicating that PSII was not a primary site of damage by SO₂. Pretreatment of leaves with SO₂ in the dark, however, increased the susceptibility of PSII to photoinhibition, as such pretreated leaves exhibited much greater reductions inF _V/F _M when transferred to moderate or high light in air than comparable control leaves.Abbreviations and symbols A₁₂₀₀ photosynthetic capacity (CO₂-saturated rate of O₂ evolution at 1200 mol photons·m^-2·s^-1) - F_o instantaneous fluorescence emission - F_M maximum fluorescence emission - F_V variable fluorescence emission - PFD photon flux density (400–700 nm) - PSII photosystem II     

Analysis of oxygen evolution during photosynthetic induction and in multiple-turnover flashes in sunflower leaves

Exchange of CO₂ and O₂ and chlorophyll fluorescence were measured in the presence of 360 1 · 1^–1 CO₂ in nitrogen in Helianthus annuss L. leaves which had been preconditioned in the dark or at a photon flux density (PFD) of 24 mol · m^–2 · s^–1 either in 21 or 0% O₂. An initial light-dependent O₂ outburst of 6 mol · m^–2 was measured after aerobic dark incubation. It was attributed to the reduction of electron carriers, predominantly plastoquinone. The maximum initial rate of O₂ evolution at PFD 8000 mol · m^–2 · s^–1 was 170 mol · m^–2 · s^–2 or about four times the steady CO₂-and light-saturated rate of photosynthesis. Fluorescence measurements showed that the rate was still acceptor-limited. Fast O₂ evolution ceased after electron carriers were reduced in the dark-adapted leaf, but continued for a short time at the lower rate of 62 mol · m^–2 · s^–1 in the light-adapted leaf. The data are interpreted to show that enzymes involved in 3-phosphoglycerate reduction are dark-inhibited, but were fully active in low light. In a dark-adapted leaf, respiratory CO₂ evolution continued under nitrogen; it was partially inhibited by illumination. Prolonged exposure of a leaf to anaerobic conditions caused reducing equivalents to accumulate. This was shown by a slowly increasing chlorophyll fluorescence yield which indicated the reduction of the PSII acceptor Q_A in the dark. When the leaf was illuminated, no O₂ evolution was detected from short light pulses, although transient O₂ production was appreciable during longer light pulses. This indicates that an electron donor (pool size about 2–3 e/PSII reaction center) became reduced in the dark and the first photons were used to oxidise this donor instead of water.Abbreviations Chl chlorophyll - CRC carbon reduction cycle - GAPDH NADP-glyceraldehyde-phosphate dehydrogenase - PFD photon flux density - PGA 3-phosphoglycerate - RuBP ribulose bisphosphate - TCA tricarboxylic acid cycle To whom correspondence should be addressedThis work received support by the Estonian Academy of Sciences, the Gottfried-Wilhelm-Leibniz Program of the Deutsche For-schungsgemeinschaft and the Sonderforschungsbereich 251 of the University of Würzburg.     

Photosynthesis in enzymatically isolated leaf cells from the CAM plant Sedum telephium L.

A technique has been developed for the enzymatic isolation of leaf cells from the Crassulacean acid-metabolism plant Sedum telephium. The cells exhibited high activity in both ¹⁴CO₂ incorporation (30–70 mol CO₂ mg^-1 chlorophyll h^-1) and O₂ evolution in the presence of bicarbonate (60–110 mol O₂ mg^-1 chlorophyll h^-1). Half-maximum saturation of ¹⁴CO₂ incorporation occurred at a bicarbonate concentration of ca. 2 mM (20 M CO₂) at pH 8.4 and 30°C. Two types of light-dependent O₂ evolution are reported: O₂ evolution in the absence of exogenously supplied bicarbonate (endogenous O₂ evolution), and bicarbonate-stimulated O₂ evolution. Oxygen evolution in the presence of approximately ambient concentrations of CO₂ appeared to be a combination of the endogenous O₂ evolution and O₂ evolution from fixation of the exogenously supplied CO₂.Abbreviations CAM Crassulacean acid metabolism - cirlo chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - PEP phosphoenolpyruvate - RuDP ribulose-1,5-diphosphate     

RELATIONS BETWEEN SUNFLECK SEQUENCES AND PHOTOINHIBITION OF PHOTOSYNTHESIS IN A TROPICAL RAIN FOREST UNDERSTORY HERB

We report the photosynthetic characteristics of a C₃ shade plant native to the tropical rain forest understory. It was shown that Elatostema repens Lour. (Hall) f. (Urticaceae) presents a large light adjustment capacity. The effects of several lightfleck sequences on photoinhibition of photosynthesis and carbon gain are analyzed. Photoinhibition is measured both as a decrease in leaf net CO₂ uptake in limiting light (shown to be linearly correlated to quantum yield of O₂ evolution measured at saturating CO₂) and as a decrease of the ratio of variable fluorescence (Fv) to maximum fluorescence (Fmax) measured in liquid nitrogen. It is shown that lightflecks (from 10 to 30 min in duration) of 700 μmol m^–2 s^–1 (high light) induce photoinhibition, and that the effects of those successive high light periods are additive; there is apparently no recovery from photoinhibition during the low light periods (from 10 to 45 min in duration). In contrast, the Fv/Fmax ratio, though decreasing similarly to quantum yield of net CO₂ uptake on leaves submitted to a continuous illumination of 700 μmol m^–2 s^–1, is only decreased a little on leaves submitted to lightfleck sequences of the same photon flux density. Lightflecks of 250 μmol m^–2 s^–1 are not photoinhibitory. Compared to the control maintained under light growth condition (40 μmol m^–2 s^–1) carbon gain is increased on leaves submitted to lightflecks; this gain remains high throughout the light cycles on leaves submitted to nonphotoinhibitory lightflecks and to the photoinhibitory lightflecks followed by the shortest low light period. In the other cases, carbon gain, higher than that of the control at the beginning of the treatments, decreases and becomes lower than the control carbon gain. Finally, the relevance of photoinhibition in the tropical rain forest understory environment is discussed.     

Photoinhibition of photosynthesis in intact bean leaves: role of light and temperature,and requirement for chloroplast-protein synthesis during recovery

Photoinhibition of photosynthesis was induced in intact leaves of Phaseolus vulgaris L. grown at a photon flux density (PFD; photon fluence rate) of 300 mol·m^-2·s^-1, by exposure to a PFD of 1400 mol·m^-2·s^-1. Subsequent recovery from photoinhibition was followed at temperatures ranging from 5 to 35°C and at a PFD of either 20 or 140 mol·m^-2·s^-1 or in complete darkness. Photoinhibition and recovery were monitored mainly by chlorophyll fluorescence emission at 77K but also by photosynthetic O₂ evolution. The effects of the protein-synthesis inhibitors, cycloheximide and chloramphenicol, on photoinhibition and recovery were also determined. The results demonstrate that recovery was temperature-dependent with rates slow below 15°C and optimal at 30°C. Light was required for maximum recovery but the process was light-saturated at a PFD of 20 mol·m^-2·s^-1. Chloramphenicol, but not cycloheximide, inactivated the repair process, indicating that recovery involved the synthesis of one or more chloroplast-encoded proteins. With chloramphenicol, it was shown that photoinhibition and recovery occurred concomitantly. The temperature-dependency of the photoinhibition process was, therefore, in part determined by the effect of temperature on the recovery process. Consequently, photoinhibition is the net difference between the rate of damage and the rate of repair. The susceptibility of chilling-sensitive plant species to photoinhibition at low temperatures is proposed to result from the low rates of recovery in this temperature range.Abbreviations and symbols Da Dalton - Fo, Fm, Fv instantaneous, maximum, variable fluorescence emission - PFD photon flux density - PSII photosystem II - photon yield C.I.W.-D.P.B. Publication No. 871     

The kinetics of ribulose-1,5-bisphosphate carboxylase/oxygenase in vivo inferred from measurements of photosynthesis in leaves of transgenic tobacco

Transgenic tobacco (Nicotiana tabacum L. cv. W38) with an antisense gene directed against the mRNA of the ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit was used to determine the kinetic properties of Rubisco in vivo. The leaves of these plants contained only 34% as much Rubisco as those of the wild type, but other photosynthetic components were not significantly affected. Consequently, the rate of CO₂ assimilation by the antisense plants was limited by Rubisco activity over a wide range of CO₂ partial pressures. Unlike in the wild-type leaves, where the rate of regeneration of ribulose bisphosphate limited CO₂ assimilation at intercellular partial pressures above 400 ubar, photosynthesis in the leaves of the antisense plants responded hyperbolically to CO₂, allowing the kinetic parameters of Rubisco in vivo to be inferred. We calculated a maximal catalytic turnover rate, k_cat, of 3.5+0.2 mol CO₂·(mol sites)^–1·s^–1 at 25° C in vivo. By comparison, we measured a value of 2.9 mol CO₂·(mol sites)^–1·^–1 in vitro with leaf extracts. To estimate the Michaelis-Menten constants for CO₂ and O₂, the rate of CO₂ assimilation was measured at 25° C at different intercellular partial pressures of CO₂ and O₂. These measurements were combined with carbon-isotope analysis (¹³C/¹²C) of CO₂ in the air passing over the leaf to estimate the conductance for transfer of CO₂ from the substomatal cavities to the sites of carboxylation (0.3 mol·m^–2·s^–1·bar^–1) and thus the partial pressure of CO₂ at the sites of carboxylation. The calculated Michaelis-Menten constants for CO₂ and O₂ were 259 ±57 bar (8.6±1.9M) and 179 mbar (226 M), respectively, and the effective Michaelis-Menten constant for CO₂ in 200 mbar O₂ was 549 bar (18.3 M). From measurements of the photocompensation point (_* = 38.6 ubar) we estimated Rubisco's relative specificity for CO₂, as opposed to O₂ to be 97.5 in vivo. These values were dependent on the size of the estimated CO₂-transfer conductance.Abbreviations and Symbols A CO₂-assimilation rate - g_w conductance for CO₂ transfer from the substomatal cavities to the sites of carboxylation - K_c, K_o Michaelis-Menten constants for carboxylation, oxygenation of Rubisco - k_cat V_cmax/[active site] - O partial pressure of O₂ at the site of carboxylation - p_c partial pressure of CO₂ at the site of carboxylation - p_i intercellular CO₂ partial pressure - R_d day respiration (non-photorespiratory CO₂ evolution) - Rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - S_c/o relative specificity factor for Rubisco - SSu small subunit of Rubisco - V_cmax, V_omax maximum rates of Rubisco carboxylation, oxygenation - _* partial pressure of CO₂ in the chloroplast at which photorespiratory CO₂ evolution equals the rate of carboxylation     

Effect of dissolved inorganic carbon on oxygen evolution and uptake by Chlamydomonas reinhardtii suspensions adapted to ambient and CO2-enriched air

Mass spectrometric measurements of ¹⁶O₂ and ¹⁸O₂ isotopes were used to compare the rates of gross O₂ evolution (E₀), O₂ uptake (U₀) and net O₂ evolution (NET) in relation to different concentrations of dissolved inorganic carbon (DIC) by Chlamydomonas reinhardtii cells grown in air (air-grown), in air enriched with 5% CO₂ (CO₂-grown) and by cells grown in 5% CO₂ and then adapted to air for 6h (air-adapted).At a photon fluence rate (PFR) saturating for photosynthesis (700 mol photons m^-2 s^-1), pH=7.0 and 28°C, U₀ equalled E₀ at the DIC compensation point which was 10M DIC for CO₂-grown and zero for air-grown cells. Both E₀ and U₀ were strongly dependent on DIC and reached DIC saturation at 480 M and 70 M for CO₂-grown and air-grown algae respectively. U₀ increased from DIC compensation to DIC saturation. The U₀ values were about 40 (CO₂-grown), 165 (air-adapted) and 60 mol O₂ mg Chl^-1 h^-1 (air-grown). Above DIC compensation the U₀/E₀ ratios of air-adapted and air-grown algae were always higher than those of CO₂-grown cells. These differences in O₂ exchange between CO₂- and air-grown algae seem to be inducable since air-adapted algae respond similarly to air-grown cells.For all algae, the rates of dark respiratory O₂ uptake measured 5 min after darkening were considerably lower than the rates of O₂ uptake just before darkening. The contribution of dark respiration, photorespiration and the Mehler reaction to U₀ is discussed and the energy requirement of the inducable CO₂/HCO₃ ^- concentrating mechanism present in air-adapted and air-grown C. reinhardtii cells is considered.Abbreviations DIC dissolved inorganic carbon - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - E₀ rate of photosynthetic gross O₂ evolution - PCO photosynthetic carbon oxidation - PFR photon fluence rate - PS I photosystem I - PS II photosystem II - U₀ rate of O₂ uptake in the light - MS mass spectrometer     

Photosynthetic gas exchange response of poplars to steady-state and dynamic light environments

The steady-state and dynamic photosynthetic response of two poplar species (Populus tremuloides and P. fremontii) to variations in photon flux density (PFD) were observed with a field portable gas exchange system. These poplars were shown to be very shade intolerant with high light saturation (800 to 1300 mol photons m^–2 s^–1) and light compensation (70 to 100 mol m^–2 s^–1) points. Understory poplar leaves showed no physiological acclimation to understory light environments. These plants become photosynthetically induced quickly (10 min). Activation of Rubisco was the primary limitation for induction, with stomatal opening playing only a minor role. Leaves maintained high stomatal conductances and stomata were unresponsive to variations in PFD. Leaves were very efficient at utilizing rapidly fluctuating light environments similar to those naturally occurring in canopies. Post-illumination CO₂ fixation contributed proportionally more to the carbon gain of leaves during short frequent lightflecks than longer less frequent ones. The benefits of a more dynamic understory light environment for the carbon economy of these species are discussed.     

Effects of growth-light quantity,growth-light quality and CO2 concentration on Rubisco deactivation during low PFD or darkness

The effects of CO₂ concentration and the effects of growth-light conditions on Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) deactivation were examined for Spinacea oleracea (spinach). Rubisco deactivation kinetics and the degree that Rubisco activation limited the rise in photosynthesis following an increase in photon flux density (PFD) were determined from gas-exchange time courses. There were no significant differences in the apparent relaxation time for Rubisco deactivation among leaves exposed to high or low CO₂ (50 or 1000 mol mol^-1) and low PFD (170 mol m^-2 s^-1) or darkness. However, when PFD was increased to 1700 mol m^-2 s^-1 following a period of low PFD or darkness, leaves exposed to low CO₂ × low PFD showed a lower contribution to the photosynthetic induction process by the activation of Rubisco than leaves exposed to the other treatments. For the growth-light experiments, spinach was grown under high PFD × high red:far-red ratio (R:FR), low PFD × high R:FR, or low PFD × low R:FR light environments. Leaves that matured under the low PFD × low R:FR treatment showed a lower percent change in photosynthesis due to Rubisco activation than leaves exposed to the other growth-light treatments. However, there were no significant differences among the growth-light treatments in the maximum contribution of Rubisco activation to the induction response or in the apparent relaxation time for Rubisco deactivation during shade events.     

Modelling photosynthesis in fluctuating light with inclusion of stomatal conductance, biochemical activation and pools of key photosynthetic intermediates

Photosynthetic carbon gain in rapidly fluctuating light is controlled by stomatal conductance, activation of ribulose-1,5-bisphosphate carboxylase-oxygenase, a fast induction step in the regeneration of ribulose-1,5-bisphosphate, and the build-up of pools of photosynthetic intermediates that allow post-illumination CO₂ fixation. Experimental work over recent years has identified and characterised these factors. A physiologically-based dynamic model is described here that incorporates these factors and allows the simulation of carbon gain in response to any arbitrary sequence of light levels. The model output is found to conform well to previously reported plant responses of Alocasia macrorrhiza (L.) G. Don. observed under widely differing conditions. The model shows (i) responses of net assimilation rate and stomatal conductance to constant light levels and different CO₂ concentrations that are consistent with experimental observations and predictions of a steady-state model; (ii) carbon gain to continue after the end of lightflecks, especially in uninduced leaves; (iii) carbon gain to be only marginally reduced during low-light periods of up to 2 s; (iv) a fast-inducing component in the regeneration of ribulose-1,5-bisphosphate to be limiting for up to 60 s after an increase in light in uninduced leaves: the duration of this limitation lengthens with increasing CO₂ concentration and is absent at low CO₂ concentration; (v) oxygen evolution to exceed CO₂ fixation during the first few seconds of a lightfleck, but CO₂ fixation to continue after the end of the lightfleck whereas oxygen evolution decreases to low-light rates immediately. The model is thus able to reproduce published responses of leaves to a variety of perturbations. This provides good evidence that the present formulation of the model includes the essential rate-determining factors of photosynthesis under fluctuating light conditions. Received: 27 January 1997 / Accepted: 15 April 1997     

Inhibition of photosynthesis by chilling in moderate light: a comparison of plants sensitive and insensitive to chilling

Photosynthetic activity, in leaf slices and isolated thylakoids, was examined at 25° C after preincubation of the slices at either 25° C or 4° C at a moderate photon flux density (PFD) of 450 mol·m^–2·s^–1, or at 4° C in the dark. The plants used wereSpinacia oleracea L.,Cucumis sativus L. andNerium oleander L. which was acclimated to growth at 20° C or 45° C. The plants were grown at a PFD of 550 mol·m^–2·s^–1. Photosynthesis, measured as CO₂-dependent O₂ evolution, was not inhibited in leaf slices from any plant after preincubation at 25° C at a moderate PFD or at 4° C in the dark. However, exposure to 4° C at a moderate PFD induced an inhibition of CO₂-dependent O₂ evolution within 1 h inC. sativus, a chilling-sensitive plant, and in 45° C-grownN. oleander. The inhibition in these plants after 5 h reached 80% and 40%, respectively, and was independent of the CO₂ concentration but was reduced at O₂ concentrations of less than 3%. Methyl-viologen-dependent O₂ exchange in leaf slices from these plants was not inhibited. There was no photoxidation of chlorophyll, in isolated thylakoids, or any inhibition of electron transport at photosystem (PS)II, PSI or through both photosystems which would account for the inhibition of photosynthesis. The conditions which inhibit photosynthesis in chilling-sensitive plants do not cause inhibition inS. oleracea, a chilling-insensitive plant, or in 20° C-grownN. oleander. The CO₂-dependent photosynthesis, measured at 5° C, was reduced to about 3% of that recorded at 25° C in chilling-sensitive plants but only to about 30% in the chilling-insensitive plants. Methyl-viologen-dependent O₂ exchange, measured at 5° C, was greater than 25% of the activity at 25° C in all the plants. The results indicate that the mechanism of the chilling-induced inhibition of photosynthesis does not involve damage to PSII. That inhibition of photosynthesis is observed only in the chilling-sensitive plants indicates it is related, in some way, to the disproportionate decrease in photosynthetic activity in these plants at chilling temperatures.Abbreviations Chl chlorophyll - DPIPH reduced form of 2,6-dichlorophenol-indophenol - DMQ 2,5-dimethyl-p-benzoquinone - MV methyl viologen - 20°-oleander Nerium oleander grown at 20° C - 45°-oleander N. oleander grown at 45° C - PFD photon flux density (photon fluence rate) - PSI and PSII photosystem I and II, respectively     

Stimulation of photosynthesis by 2% oxygen at low temperatures is restored by phosphate

The effect of phosphate feeding on the influence of low (2%) oxygen on photosynthetic carbon assimilation has been investigated in leaf discs of spinach (Spinacia oleracea L.) at 12°C. The following observations were made. First, after the transition from 20% O₂ to 2% O₂, the rate of CO₂ uptake was inhibited at CO₂ concentrations between about 250 and about 800 l CO₂·l^-1. Second, phosphate feeding stimulated the rate of CO₂ uptake in 20% O₂ at higher concentrations of CO₂ (500–900 l·l^-1). Third, phosphate feeding stimulated the rate of CO₂ uptake in 2% O₂ at all but the highest (900 l·l^-1) and lowest 74 (l·l^-1) concentrations of CO₂ employed. Phosphate thereby restored the stimulation of photosynthesis by 2% O₂ and it did so over a wide range of lower temperatures. Fourth, oscillatory behaviour, however generated, was dampened by phosphate feeding, even at very low concentrations of CO₂. Contents of leaf metabolites were measured during the transition to 2% O₂ in control and phosphate-fed leaf discs. During this period the ratio glycerate-3-phosphate/triose phosphate rose steeply, but fell again only in the phosphate-treated leaf discs. These data, taken together with measured ATP/ADP ratios, showed that assimilatory power, the ratio [ATP]·[NAD(P)H]/[ADP]·[Pi]·[NAD(P)], decreased when leaves were exposed to 2% O₂, but that this decrease was minimised by previous feeding of phosphate. The mechanism of phosphate limitation is discussed in the light of the results.Abbreviations C_i intercellular concentration of CO₂ - RuBP ribulose-1,5-bisphosphate     

Chlorophyll fluorescence in the resurrection plant Selaginella lepidophylla (Hook. & Grev.) Spring during high-light and desiccation stress,and evidence for zeaxanthin-associated photoprotection

The function of photosystem (PS)II during desiccation and exposure to high photon flux density (PFD) was investigated via analysis of chlorophyll fluorescence in the desert resurrection plant Selaginella lepidophylla (Hook. and Grev.) Spring. Exposure of hydrated, physiologically competent stems to 2000 mol · m^–2 · s^–1 PFD caused significant reductions in both intrinsic fluorescence yield (F_O) and photochemical efficiency of PSII (F_V/F_M) but recovery to pre-exposure values was rapid under low PFD. Desiccation under low PFD also affected fluorescence characteristics. Both F_V/F_M and photochemical fluorescence quenching remained high until about 40% relative water content and both then decreased rapidly as plants approached 0% relative water content. In contrast, the maximum fluorescence yield (F_M) decreased and non-photochemical fluorescence quenching increased early during desiccation. In plants dried at high PFD, the decrease in F_V/F_M was accentuated and F_O was reduced, however, fluorescence characteristics returned to near pre-exposure values after 24-h of rehydration and recovery at low PFD. Pretreatment of stems with dithiothreitol, an inhibitor of zeaxanthin synthesis, accelerated the decline in F_V/F_M and significantly increased F_O relative to controls at 925 mol · m^–2 · s^–1 PFD, and the differences persisted over a 3-h low-PFD recovery period. Pretreatment with dithiothreitol also significantly decreased non-photochemical fluorescence quenching, increased the reduction state of Q_A, the primary electron acceptor of PSII, and prevented the synthesis of zeaxanthin relative to controls when stems were exposed to PFDs in excess of 250 mol · m^–2 · s^–1. These results indicate that a zeaxanthin-associated mechanism of photoprotection exists in this desert pteridophyte that may help to prevent photoinhibitory damage in the fully hydrated state and which may play an additional role in protecting PSII as thylakoid membranes undergo water loss.Abbreviations and Symbols DTT dithiothreitol - EPS epoxidation state - F_O yield of instantaneous fluorescence at open PSII centers - F_M maximum yield of fluorescence at closed PSII centers induced by saturating light - F_M F_M determined during actinic illumination - F_V yield of variable fluorescence (F_M-F_O) - F_V/F_M photochemical efficiency of PSII - q_P photochemical fluorescence quenching - q_NP non-photochemical fluorescence quenching of Schreiber et al. (1986) - NPQ non-photochemical fluorescence quenching from the Stern-Volmer equation - PFD photon flux density - RWC relative water content This paper is based on research done while W.G.E. was on leave of absence at Duke University during the fall of 1990. We would like to thank Dan Yakir, John Skillman, Steve Grace, and Suchandra Balachandran and many others at Duke University for their help and input with this research. Dr. Barbara Demmig-Adams provided zeaxanthin for standard-curve purposes.     

Photosynthetic activity of isolated chloroplasts from Euglena gracilis

Functional chloroplasts from photoheterotrophic Euglena gracilis can be isolated in isoosmotic gradients of 10–80% Percoll. The chloroplasts display rates of CO₂ dependent O₂ evolution and CO₂ fixation of 30–50 mol mg^-1 chlorophyll h^-1 or 25–35% of the net O₂ evolution by the whole cells and appear to be strikingly different from spinach chloroplasts in several respects: 1. tolerance to high concentration of orthophosphate in the assay medium; 2. inability to support oxaloacetate-dependent O₂ evolution; 3. ability to support only low to moderate rates of 3-phosphoglycerate-dependent O₂ evolution; 4. an apparent absence of a phosphate translocator in the terms described by Heldt and Rapley ([1970] FEBS Lett. 10, 143–148).University of California, Dept. of Plant and Soil Biology, 108 Hilgard Hall, Berkeley, CA 94720 USA     

On the use of Avena protoplasts to study chloroplast development

Different methods were tested to isolate protoplasts from etiolated, partially greened, and light-grown leaves of Avena sativa. Preparations with high yields and high photosynthetic capacities (time of illumination 4 h) were obtained when small transverse leaf segments were incubated for 2 h at 30°C in 2% cellulysin (Calbiochem), 0.6 M mannitol, and 0.5% bovine serum albumin (BSA) at pH 5.6, without shaking. As measured by light-dependent O₂ evolution or fixation of labeled bicarbonate, protoplasts exhibited rates of up to 124 mol per mg of chlorophyll per h at 20°C and saturating bicarbonate, which were nearly identical to those found with intact leaves. The assay conditions necessary for this activity were 0.6 M sorbitol, 50 mM N-2-hydroxy-ethylpiperazine-N-2-ethane sulfonic acid (pH 7.6), and 10 mM NaHCO₃. If plastids were isolated from these protoplasts, sorbitol was 0.45 M, including 10 mM ethylenediaminetetraacetate (EDTA). under these conditions, rates of photosynthesis were up to 125 (light-grown) and 71 (6 h illuminated) mol O₂ evolved or ¹⁴CO₂ fixed per mg of chlorophyll per h, compared to 3.5 mol·mg chl^-1·h^-1 obtained with mechanically isolated plastids. With this system, CO₂-dependent O₂ evolution was already detected after 3 h of illumination of etiolated tissue, but could only be observed at pH values between 7.6 and 8.6, in the presence of EDTA. At lower pH (7.3) or at pH 7.6 in the absence of EDTA, light-dependent O₂ evolution up to 24 h of greening was only measurable with 3-phosphoglycerate as the substrate. The possible effects of EDTA in this respect as well as the advantages of using protoplasts or plastids isolated from protoplasts for developmental studies are discussed.Abbreviations BSA bovine serum albumin - EDTA ethylenediamine tetraacetic acid - HEPES N-2-hydroxyethyl-piperazine-N-2-ethane-sulphonic acid - MES 2(N-morpholino) ethane sulphonic acid - PGA 3-phosphoglycerate