A two-stage solar photobioreactor for cultivation of microalgae based on solar concentrators

A novel two-stage experimental photobioreactor (PBR) with a total volume of 450 L and based uniquely on solar concentrators—linear Fresnel lenses—has been constructed and tested. Daily courses of irradiance, and also its distribution inside cultivation tubes, were studied in two unit types. The supra-high irradiance units in the ‘roof’ achieved a maximum summer value above 6 mmol photon m⁻² s⁻¹, while irradiance in the vertical-facade units was lower than ‘ambient’. In model cultivations, cultures of the cyanobacterium Arthrospira platensis were cultivated at much higher solar irradiances than those usually recorded outdoors in summer, indicating that this organism is resilient to high-irradiance (photoinhibition). Starting from a biomass density of 0.5 g L⁻¹ at optimum temperature, the cultures grew exponentially. A two-stage cultivation process of the green microalga Haematococcus pluvialis was investigated with respect to correlations between photochemical activities and astaxanthin production. The culture was first grown in low-irradiance units, and then exposed to supra-high irradiance when the rate of astaxanthin production was 30–50% higher than in the culture exposed to ‘ambient’ irradiance. Within 4 days, the astaxanthin content reached 3% of dry weight, whereas under ambient irradiance the astaxanthin content was 25% lower.     

A novel photobioreactor with transparent rectangular chambers for cultivation of microalgae

An open tank photobioreactor containing transparent rectangular chambers (TRCs) was developed to improve the photosynthetic efficiency of microalgal cultivation. The TRCs, made of transparent acrylic, conducted light deep into the photobioreactor, especially at high cell concentrations. The average irradiance, I_av, was calculated by Lambert–Beer's law, and was used to determine the light conditions in the cultivation system. The photobioreactor provided large areas of illumination that improved the effective utilization of light energy for microalgae growth and created a good artificial environment for a high rate of cell growth, even at low I_av. The biomass concentration of Chlorella sp. reached 3.745 g L⁻¹ on the 13th day, with biomass productivity of 0.340 g L⁻¹ d⁻¹. The total biomass obtained was 56% more than that of similar culture systems without TRCs.     

Massive cultivation of microalgae: Results and prospects

An account is given of the development of the utilization of microalgae for food and feed with special emphasis on the advantages of algal technologies for tropical and subtropical countries. The present status of microalgae mass production is characterized with respect to technology, product properties, yields, nutrition, toxicology and economics. As a multipurpose operation, the treatment of liquid wastes with algae-bacteria systems is the most promising microalgal technology. It yields proteinaceous microbial biomass as a comparatively inexpensive by-product of the operation of high-rate algal ponds, either at the simplified rural level or at the technically more elaborate industrial level. The aspect of hard-currency saving by employing algae-bacteria systems in sewage treatment for animal feed production is stressed.     

Use of photoacclimation in the design of a novel photobioreactor to achieve high yields in algal mass cultivation

The concept of a completely new and novel photobioreactor consisting of various compartments each with a specific light regime is described. This is in response to the debate and development which have taken place in recent years concerning photobioreactor design and closed systems. It is well known that algae can photo-acclimate to various light intensities. At the extremes, they can be high light (HL) or low light (LL) acclimated. Both HL and LL acclimated algae typically have very specific characteristics indicating the plasticity of the organisms, which have developed specific strategies during evolution to cope with continuous and dynamic light fields. Not only are these considerations important in photobioreactor design, but also for the production of certain biocompounds, whose synthesis has specific light requirements. In the continuous flow photobioreactor described here, algal cells acclimated to different light conditions together permit utilization of the entire light gradient found in an optically dense medium, such as in a high-density culture. Compared to a single compartment vertical flat-plate photobioreactor, the multicompartment reactor yielded a 37% higher productivity rate. This is a significant improvement in photobioreactor performance.     

不同光照条件下喜阴植物谢君魔芋对稳态和动态光源的响应特征

谢君魔芋(Amorphophallus xiei)是起源于云南西南地区热带雨林的典型喜阴植物,近年来得到了广泛种植和推广,在种植过程中,谢君魔芋需要采用遮荫栽培模式。为了揭示谢君魔芋对光照强度的适应策略,该研究探讨了生长在不同光照强度下(透光率为50%、29%、17%、7%)谢君魔芋叶片的光合作用特征、光合诱导特征、光合色素含量以及叶片氮素(N)含量和N分配。结果表明:随着生长环境光照强度的降低,单位叶面积和单位叶质量最大净光合速率、光合色素含量、最大羧化速率、最大电子传递速率及比叶面积均增大,而暗呼吸和光补偿点均减小。在光合诱导过程中,生长在透光率为17%光环境中的谢君魔芋完成50%光合诱导所需的时间最短,约为81.4 s;在光诱导进行10 min时,诱导状态最高,为87.3%。完成50%和90%光合诱导所需的时间与低光下初始气孔导度呈负相关关系。随着生长光照强度降低,叶片中的N分配到羧化组分和生物能转化组分中的比例先增大后减小,在透光率为17%的光环境下具有最大值;而叶片中的N分配到捕光色素组分中的比例随着生长环境光照强度降低而增加。该研究结果表明,喜阴植物谢君魔芋通过加强对低光和动态光源的利用能力及有效的N资源分配策略来适应低光照环境。     

Dynamics of the xanthophyll cycle and non-radiative dissipation of absorbed light energy during exposure of Norway spruce to high irradiance

The response of Norway spruce saplings (Picea abies [L.] Karst.) was monitored continuously during short-term exposure (10 days) to high irradiance (HI; 1000mumolm(-2)s(-1)). Compared with plants acclimated to low irradiance (100mumolm(-2)s(-1)), plants after HI exposure were characterized by a significantly reduced CO(2) assimilation rate throughout the light response curve. Pigment contents varied only slightly during HI exposure, but a rapid and strong response was observed in xanthophyll cycle activity, particularly within the first 3 days of the HI treatment. Both violaxanthin convertibility under HI and the amount of zeaxanthin pool sustained in darkness increased markedly under HI conditions. These changes were accompanied by an enhanced non-radiative dissipation of absorbed light energy (NRD) and the acceleration of induction of both NRD and de-epoxidation of the xanthophyll cycle pigments. We found a strong negative linear correlation between the amount of sustained de-epoxidized xanthophylls and the photosystem II (PSII) photochemical efficiency (F(V)/F(M)), indicating photoprotective down-regulation of the PSII function. Recovery of F(V)/F(M) at the end of the HI treatment revealed that Norway spruce was able to cope with a 10-fold elevated irradiance due particularly to an efficient NRD within the PSII antenna that was associated with enhanced violaxanthin convertibility and a light-induced accumulation of zeaxanthin that persisted in darkness.     

Activation of non-photochemical quenching in thylakoids and leaves

The mechanism of rapidly-relaxing non-photochemical quenching in two plant species,Chenopodium album L. andDigitalis purpurea L., that differ considerably in their capacity for such quenching has been investigated (Johnson G.N. et al. 1993, Plant Cell Environ.16, 673–679). Illumination of leaves of both species in the presence of 2% O₂ balance N₂ led to the formation of zeaxanthin. When thylakoids were isolated from leaves of each species that had been so treated it was found that inD. purpurea non-photochemical quenching was “activated” relative to the control; a higher level of quenching was found for a given trans-thylakoid pH gradient. No such activation of non-photochemical quenching was observed inC. album. Similar conclusions were drawn when comparing quenching in intact leaves. It is concluded that light activation of quenching is a process that cannot readily be induced inC. album. Measurement of the sensitivity of non-photochemical quenching in leaves ofC. album andD. purpurea to dithiothreitol (DTT; a reagent that inhibits formation of zeaxanthin) showed differences between the two species. In both cases, feeding leaves with DTT inhibited the light-induced formation of zeaxanthin. InC. album this was accompanied by complete inhibition of reversible non-photochemical quenching, whereas inD. purpurea this inhibition was only partial. Data are discussed in relation to studies on the mechanism of quenching and the role of zeaxanthin in this process.     

Cooperation of xanthophyll cycle with water-water cycle in the protection of photosystems 1 and 2 against inactivation during chilling stress under low irradiance

The xanthophyll cycle and the water-water cycle had different functional significance in chilling-sensitive sweet pepper upon exposure to chilling temperature (4 °C) under low irradiance (100 µmol m⁻² s⁻¹) for 6 h. During chilling stress, effects of non-photochemical quenching (NPQ) on photosystem 2 (PS2) in dithiothreitol (DTT) fed leaves remained distinguishable from that of the water-water cycle in diethyldithiocarbamate (DDTC) fed leaves. In DTT-fed leaves, NPQ decreased greatly accompanied by visible inhibition of the de-epoxidized ratio of the xanthophyll cycle, and maximum photochemical efficiency of PS2 (F_v/F_m) decreased markedly. Thus the xanthophyll cycle-dependent NPQ could protect PS2 through energy dissipation under chilling stress. However, NPQ had a slighter effect on photosystem 1 (PS1) in DTT-fed leaves than in DDTC-fed leaves, whereas effects of the water-water cycle on PS1 remained distinguishable from that of NPQ. Inhibiting superoxide dismutase (SOD) activity increased the accumulation of , the oxidation level of P700 (P700⁺) decreased markedly relative to the control and DTT-fed leaves. Both F_v/F_m and NPQ changed little in DDTC-fed leaves accompanied by little change of (A+Z)/(V+A+Z). This is the active oxygen species inducing PS1 photoinhibition in sweet pepper. The water-water cycle can be interrupted easily at chilling temperature. We propose that during chilling stress under low irradiance, the xanthophyll cycle-dependent NPQ has the main function to protect PS2, whereas the water-water cycle is not only the pathway to dissipate energy but also the dominant factor causing PS1 chilling-sensitivity in sweet pepper.This research was supported by the State Key Basic Research and Development Plan of China (G1998010100), the Natural Science Foundation of China (30370854), and the open project from Key Lab of Crop Biology of Shandong Province.     

Photoinhibition of photosystem II in vivo is preceded by down-regulation through light-induced acidification of the lumen: Consequences for the mechanism of photoinhibition in vivo

The mechanism of photoinhibition of photosystem II (PSII) was studied in intact leaf discs of Spinacia oleracea L. and detached leaves of Vigna unguiculata L. The leaf material was exposed to different photon flux densities (PFDs) for 100 min, while non-photochemical (q_N) and photochemical quenching (q_p) of chlorophyll fluorescence were monitored. The ‘energy’ and redox state of PSII were manipulated quite independently of the PFD by application of different temperatures (5–20° C), [CO₂] and [O₂] at different PFDs. A linear or curvilinear relationship between q_p and photoinhibition of PSII was observed. When [CO₂] and [O₂] were both low (30 μl · l^?1 and 2%, respectively), PSII was less susceptible at a given q_p than at ambient or higher [CO₂] and photoinhibition became only substantial when q_p decreased below 0.3. When high levels of energy-dependent quenching (q_E) (between 0.6 and 0.8) were reached, a further increase of the PFD or a further decrease of the metabolic demand for ATP and NADPH led to a shift from q_E to photoinhibitory quenching (q_I). This shift indicated that photoinhibition was preceded by down-regulation through light-induced acidification of the lumen. We propose that photoinhibition took place in the centers down-regulated by q_E. The shift from q_E to q_I occurred concomitant with q_P decreasing to zero. The results clearly show that photoinhibition does not primarily depend on the photon density in the antenna, but that photoinhibition depends on the energy state of the membrane in combination with the redox balance of PSII. The results are discussed with regard to the mechanism of photoinhibition of PSII, considering, in particular, effects of light-induced acidification on the donor side of PSII. Interestingly, cold-acclimation of spinach leaves did not significantly affect the relationship between q_P, q_E and photoinhibition of PSII at low temperature.     

Analysis of non-photochemical energy dissipating processes in wild type <Emphasis Type="Italic">Dunaliella salina</Emphasis> (green algae) and in <Emphasis Type="Italic">zea1</Emphasis>, a mutant constitutively accumulating zeaxanthin

Generally there is a correlation between the amount of zeaxanthin accumulated within the chloroplast of oxygenic photosynthetic organisms and the degree of non-photochemical quenching (NPQ). Although constitutive accumulation of zeaxanthin can help protect plants from photo-oxidative stress, organisms with such a phenotype have been reported to have altered rates of NPQ induction. In this study, basic fluorescence principles and the routinely used NPQ analysis technique were employed to investigate excitation energy quenching in the unicellular green alga Dunaliella salina, in both wild type (WT) and a mutant, zea1, constitutively accumulating zeaxanthin under all growth conditions. The results showed that, in D. salina, NPQ is a multi-component process consisting of energy- or ΔpH-dependent quenching (qE), state-transition quenching (qT), and photoinhibition quenching (qI). Despite the vast difference in the amount of zeaxanthin in WT and the zea1 mutant grown under low light, the overall kinetics of NPQ induction were almost the same. Only a slight difference in the relative contribution of each quenching component could be detected. Of all the NPQ subcomponents, qE seemed to be the primary NPQ operating in this alga in response to short-term exposure to excessive irradiance. Whenever qE could not operate, i.e., in the presence of nigericin, or under conditions where the level of photon flux is beyond its quenching power, qT and/or qI could adequately compensate its photoprotective function.     

Non-photochemical quenching of chlorophyll a fluorescence in isolated chloroplasts under conditions of stressed photosynthesis

Non-photochemical quenching of chlorophyll a fluorescence after short-time light, heat and osmotic stress was investigated with intact chloroplasts from Spinacia oleracea L. The proportions of non-photochemical fluorescence quenching (q _N ) which are related (q _E ) and unrelated (q _I ) to the transthylakoid proton gradient (pH) were determined. Light stress resulted in an increasing contribution of q _Ito total q _N.The linear dependence of q. _Eand pH, as seen in controls, was maintained. The mechanisms underlying this type of quenching are obviously unaffected by photoin-hibition. In constrast, q _Ewas severely affected by heat and osmotic stress. In low light, the response of q _Eto changes in pH was enhanced, whereas it was reduced in high light. The data are discussed with reference to the hypothesis that q _Eis related to thermal dissipation of excitation energy from photosystem II. It is shown that q _Eis not only controlled by pH, but also by external factors.Abbreviations and symbols 9-AA 9-aminoacridine - F _o basic chlorophyll fluorescence - F _o variable chlorophyll fluorescence - L ₂ saturating light pulse - PS photosystem - q _E pH-dependent, non-photochemical quenching of fluorescence - q _I pH-independent, non-photochemical quenching - q _N entire non-photochemical quenching - q _Q photochemical quenching     

Photoinhibition of photosynthesis: effect on chlorophyll fluorescence at 77K in intact leaves and in chloroplast membranes of Nerium oleander

The effect of exposing intact leaves and isolated chloroplast membranes of Nerium oleander L. to excessive light levels under otherwise favorable conditions was followed by measuring photosynthetic CO₂ uptake, electron transport and low-temperature (77K=-196°C) fluorescence kinetics. Photoinhibition, as manifested by a reduced rate and photon (quantum) yield of photosynthesis and a reduced electron transport rate, was accompanied by marked changes in fluorescence characteristics of the exposed upper leaf surface while there was little effect on the shaded lower surface. The most prominent effect of photoinhibitory treatment of leaves and chloroplasts was a strong quenching of the variable fluorescence emission at 692 nm (F_v,692) while the instantaneous fluorescence (F_o,692) was slightly increased. The maximum and the variable fluorescence at 734 nm were also reduced but not as much as F_M,692 and F_v,692. The results support the view that photoinhibition involves an inactivation of the primary photochemistry of photosystem II by damaging the reaction-center complex. In intact leaves photoinhibition increased with increased light level, increased exposure time, and with decreased temperature. Increased CO₂ pressure or decreased O₂ pressure provided no protection against photoinhibition. With isolated chloroplasts, inhibition of photosystem II occurred even under essentially anaerobic conditions. Measurements of fluorescence characteristics at 77K provides a simple, rapid, sensitive and reproducible method for assessing photoinhibitory injury to leaves. The method should prove especially useful in studies of the occurrence of photoinhibition in nature and of interactive effects between high light levels and major environmental stress factors.Abbreviations and symbols PFD photon flux area density - PSI, PSII photosystem I, II - F_M, F_O, F_V maximum, instantaneous, variable fluorescence emission C.I.W.-D.P.B. Publication No. 773     

Inhibition of photosynthetic reactions under water stress: interaction with light level

When the shrub Nerium oleander L., growing under full natural daylight outdoors, was subjected to water stress, stomatal conductance declined, and so did non-stomatal components of photosynthesis, including the CO₂-saturated rate of CO₂ uptake by intact leaves and the activity of electron transport by chloroplasts isolated from stressed plants. This inactivation of photosynthetic activity was accompanied by changes in the fluorescence characteristics determined at 77 K (-196°C) for the upper leaf surface and from isolated chloroplasts. The maximum (F _M) and the variable (F _V) fluorescence yield at 692 nm were strongly quenched but there was little effect on the instantaneous (F _O) fluorescence. There was a concomitant quenching of the maximum and variable fluorescence at 734 nm. These results indicate an inactivation of the primary photochemistry associated with photosystem II. The lower, naturally shaded surfaces of the same leaves were much less affected than the upper surfaces and water-stress treatment of plants kept in deep shade had little or no effect on the fluorescence characteristics of either surface, or of chloroplasts isolated from the water-stressed leaves. The effects of subjecting N. oleander plants, growing in full daylight, to water stress are indistinguishable from those resulting when plants, grown under a lower light regime, are exposed to full daylight (photoinhibition). Both kinds of stress evidently cause an inactivation of the primary photochemistry associated with photosystem II. The results indicate that water stress predisposes the leaves to photoinhibition. Recovery from this inhibition, following restoration of favorable water relations, is very slow, indicating that photoinhibition is an important component of the damage to the photosynthetic system that takes place when plants are exposed to water stress in the field. The underlying causes of this water-stress-induced susceptibility to photoinhibition are unknown; stomatal closure or elevated leaf temperature cannot explain the increased susceptibility.Abbreviations and symbols Chl chlorophyll - PFD photon flux area density - PSI, PSII photosystem I, II - F _M, F _O, F _V maximum, instantaneous, variable fluorescence emission - leaf water potential C.I.W.-D.P.B. Publication No. 775     

The synthesis of NPQ-effective zeaxanthin depends on the presence of a transmembrane proton gradient and a slightly basic stromal side of the thylakoid membrane

In the present study we address the question which factors during the synthesis of zeaxanthin determine its capacity to act as a non-photochemical quencher of chlorophyll fluorescence. Our results show that zeaxanthin has to be synthesized in the presence of a transmembrane proton gradient. However, it is not essential that the proton gradient is generated by the light-driven electron transport. NPQ-effective zeaxanthin can also be formed by an artificial proton gradient in the dark due to ATP hydrolysis. Zeaxanthin that is synthesized in the dark in the absence of a proton gradient by the low pH-dependent activation of violaxanthin de-epoxidase is not able to induce NPQ. The second important factor during the synthesis of zeaxanthin is the pH-value of the stromal side of the thylakoid membrane. Here we show that the stromal side has to be neutral or slightly basic in order to generate zeaxanthin which is able to induce NPQ. Thylakoid membranes in reaction medium pH 5.2, which experience low pH-values on both sides of the membrane, are unable to generate NPQ-effective zeaxanthin, even in the presence of an additional light-driven proton gradient. Analysing the pigment contents of purified photosystem II light-harvesting complexes we are further able to show that the NPQ ineffectiveness of zeaxanthin formed in the absence of a proton gradient is not caused by changes in its rebinding to the light-harvesting proteins. Purified monomeric and trimeric light-harvesting complexes contain comparable amounts of zeaxanthin when they are isolated from thylakoid membranes enriched in either NPQ-effective or ineffective zeaxanthin.     

Use of simultaneous analysis of gas-exchange and chlorophyll fluorescence quenching for analysing the effects of water stress on photosynthesis in apple leaves

Summary A convenient system for the rapid simultaneous measurement of both chlorophyll fluorescence quenching using a modulated light system, and of CO₂, and water vapour exchange by leaves is described. The system was used in a study of the effects of water deficits on the photosynthesis by apple leaves (Malus x domestica Borkh.). Apple leaves were found to have low values of steady-state variable fluorescence, and the existence of significant fluorescence with open traps (F_o) quenching necessitated the measurement and use of a corrected F_o in the calculation of quenching components. Long-term water stress had a marked effect on both gas-exchange and chlorophyll fluorescence quenching. Non-photochemical quenching (qn) in particular was increased in water-stressed leaves, and it was particularly sensitive to incident radiation in such leaves. In contrast, rapid dehydration only affected gas exchange. Relaxation of qn quenching in the dark was slow, taking approximately 10 min for a 50% recovery, in well-watered and in draughted plants, and whether or not the plants had been exposed to high light.     

Paraheliotropic leaf movement in Siratro as a protective mechanism against drought-induced damage to primary photosynthetic reactions: damage by excessive light and heat

Damage to primary photosynthetic reactions by drought, excess light and heat in leaves of Macroptilium atropurpureum Dc. cv. Siratro was assessed by measurements of chlorophyll fluorescence emission kinetics at 77 K (-196°C). Paraheliotropic leaf movement protected waterstressed Siratro leaves from damage by excess light (photoinhibition), by heat, and by the interactive effects of excess light and high leaf temperatures. When the leaves were restrained to a horizontal position, photoinhibition occurred and the degree of photoinhibitory damage increased with the time of exposure to high levels of solar radiation. Severe inhibition was followed by leaf death, but leaves gradually recovered from moderate damage. This drought-induced photoinhibitory damage seemed more closely related to low leaf water potential than to low leaf conductance. Exposure to leaf temperatures above 42°C caused damage to the photosynthetic system even in the dark and leaves died at 48°C. Between 42 and 48°C the degree of heat damage increased with the time of exposure, but recovery from moderate heat damage occurred over several days. The threshold temperature for direct heat damage increased with the growth temperature regime, but was unaffected by water-stress history or by current leaf water status. No direct heat damage occurred below 42°C, but in water-stressed plants photoinhibition increased with increasing leaf temperature in the range 31–42°C and with increasing photon flux density up to full sunglight values. Thus, water stress evidently predisposes the photosynthetic system to photoinhibition and high leaf temperature exacerbates this photoinhibitory damage. It seems probable that, under the climatic conditions where Siratro occurs in nature, but in the absence of paraheliotropic leaf movement, photoinhibitory damage would occur more frequently during drought than would direct heat damage.Abbreviations and symbols PFD photon flux area density - PSI, PSII photosyntem I, II - F _M, F _O, F _V maximum, instantaneous, variable fluorescence emission - PLM paraheliotropic leaf movement; all data of parameter of variation are mean ± standard error     

Photoinhibition of photosynthesis in intact kiwifruit (Actinidia deliciosa) leaves: Recovery and its dependence on temperature

Recovery of photoinhibition in intact leaves of shade-grown kiwifruit was followed at temperatures between 10° and 35° C. Photoinhibition was initially induced by exposing the leaves for 240 min to a photon flux density (PFD) of 1 500 mol·m^-2·s^-1 at 20° C. In additional experiments to determine the effect of extent of photoinhibition on recovery, this period of exposure was varied between 90 and 400 min. The kinetics of recovery were followed by chlorophyll fluorescence at 77K. Recovery was rapid at temperatures of 25–35° and slow or negligible below 20° C. The results reinforce those from earlier studies that indicate chilling-sensitive species are particularly susceptible to photoinhibition at low temperatures because of the low rates of recovery. At all temperatures above 15° C, recovery followed pseudo first-order kinetics. The extent of photoinhibition affected the rate constant for recovery which declined in a linear fashion at all temperatures with increased photoinhibition. However, the extent of photoinhibition had little effect on the temperature-dependency of recovery. An analysis of the fluorescence characteristics indicated that a reduction in non-radiative energy dissipation and repair of damaged reaction centres contributed about equally to the apparent recovery though biochemical studies are needed to confirm this. From an interpretation of the kinetics of photoinhibition, we suggest that recovery occurring during photoinhibition is limited by factors different from those that affect post-photoinhibition recovery.Abbreviations and symbols F _o, F _m, F _v instantaneous, maximum, variable fluorescence - K _D, K _F, K _P, K _T rate constants for non-radiative energy dissipation, fluorescence, photochemistry, transfer to photosystem I - K(PI), k(R) rate constants for photoinhibition and recovery - PFD photon flux density - PSI, II photosystem I, II - _i photon yield of photosynthesis (incident light)     

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     

Two mechanisms of recovery from photoinhibition in vivo: Reactivation of photosystem II related and unrelated to D1-protein turnover

Recovery (at 20° C) of spinach (Spinacia oleracea L.) leaf sections from photoinhibition of photosynthesis was monitored by means of the fluorescence parameter F_V/F_M of intact leaf tissue and of PSII-driven electron-transport activity of isolated thylakoids. Different degrees of photoinactivation of PSII were obtained by preillumination in ambient air (at 4 or 20° C), CO₂-free air or at low and high O₂ levels (2 or 41 %) in N₂. The kinetics of recovery exhibited two distinct phases. The first phase usually was completed within about 20-60 min and was most pronounced after preillumination in low O₂. The slow phase proceeded for several hours leading to almost complete reactivation of PSII. Preincubation of the leaves with streptomycin (SM), which inhibits chloroplast-encoded protein synthesis, inhibited the slow recovery phase only, indicating the dependence of this phase on resynthesis of the reaction-centre protein, D1. The fast recovery phase remained largely unaffected by SM. Both phases were strongly but not totally dependent on irradiation of the leaf with low light. When SM was absent, net degradation of the D1 protein could neither be detected upon photoinhibitory irradiation nor during following incubation of the leaf sections in low light or darkness. In the presence of SM, net D1 degradation was seen and tended to increase with O₂ concentration during photoinhibition treatment. Based on these data, we suggest that photoinactivation of PSII in vivo occurs in at least two steps. From the first step, reactivation appears possible in low light without D1 turnover (fast recovery phase). Action of oxygen then may lead to a second step, in which the D1 protein is affected and reactivation requires its removal and replacement (slow phase).Abbreviations Chl chlorophyll - F₀, F_M and F_V initial, maximum total and maximum variable chlorophyll fluorescence yield, respectively - PFD photon flux density - SM streptomycin We thank Professor P. Böger (Department of Plant Physiology and Biochemistry, University of Konstanz, Germany) for a gift of D1-specific antibodies. The paper contains part of the thesis work of J.L. The study was supported by the Deutsche Forschungs-gemeinschaft (SFB 189).