Acclimation of Two Distinct Plant Species,Spring Barley and Norway Spruce,to Combined Effect of Various Irradiance and CO2 Concentration During Cultivation in Controlled Environment

The short-term acclimation (10-d) of Norway spruce [Picea abies (L.) Karst] to elevated CO₂ concentration (EC) in combination with low irradiance (100 mol m^–2 s^–1) resulted in stimulation of CO₂ assimilation (by 61 %), increased total chlorophyll (Chl) content (by 17 %), significantly higher photosystem 2 (PS2) photochemical efficiency (F_v/F_m; by 4 %), and reduced demand on non-radiative dissipation of absorbed excitation energy corresponding with enhanced capacity of photon utilisation within PS2. On the other hand, at high cultivation irradiance (1 200 mol m^–2 s^–1) both Norway spruce and spring barley (Hordeum vulgare L. cv. Akcent) responded to EC by reduced photosynthetic capacity and prolonged inhibition of F_v/F_m accompanied with enhanced non-radiative dissipation of absorbed photon energy. Norway spruce needles revealed the expressive retention of zeaxanthin and antheraxanthin (Z+A) in darkness and higher violaxanthin (V) convertibility (yielding even 95 %) under all cultivation regimes in comparison with barley plants. In addition, the non-photochemical quenching of minimum Chl a fluorescence (SV₀), expressing the extent of non-radiative dissipation of absorbed photon energy within light-harvesting complexes (LHCs), linearly correlated with V conversion to Z+A very well in spruce, but not in barley plants. Finally, a key role of the Z+A-mediated non-radiative dissipation within LHCs in acclimation of spruce photosynthetic apparatus to high irradiance alone and in combination with EC was documented by extremely high SV₀ values, fast induction of non-radiative dissipation of absorbed photon energy, and its stability in darkness.     

Photoinhibition in the Antarctic moss Grimmia antarctici Card when exposed to cycles of freezing and thawing

Freezing and thawing of the endemic moss species Grimmia antarctici Card, caused photoinhibition. When snow cover was removed from moss in the field, resulting in exposure to fluctuating temperatures and light conditions, photoinhibition, measured as a reduction in the ratio of variable to maximum chlorophyll a fluorescence (F_v/F_m), was observed. The extent of photoinhibition was highly variable and appeared to be reversible during periods of warmer temperatures. A series of controlled laboratory studies found that the light conditions that prevail between freezing and thawing events influenced the recovery from photoinhibition observed during freezing and thawing, with low light conditions facilitating the greatest rates of recovery. After four cycles of freezing and thawing, recovery from photoinhibition in hydrated moss was achieved within 12 h of transfer to 5°C and 15 μmol quanta m^?2 s^?1. These results favour the hypothesis that photoinhibition observed during freezing represents a protective process involving the down-regulation of photo-system II when photosynthetic carbon assimilation is limited by low temperatures.     

Response of Photosynthetic Apparatus of Spring Barley (Hordeum vulgare L.) to Combined Effect of Elevated CO2 Concentration and Different Growth Irradiance

The response of barley (Hordeum vulgare L. cv. Akcent) to various photosynthetic photon flux densities (PPFDs) and elevated [CO₂] [700 μmol (CO₂) mol⁻¹; EC] was studied by gas exchange, chlorophyll (Chl) a fluorescence, and pigment analysis. In comparison with barley grown under ambient [CO₂] [350 μmol (CO₂) mol⁻¹; AC] the EC acclimation resulted in a decrease in photosynthetic capacity, reduced stomatal conductance, and decreased total Chl content. The extent of acclimation depression of photosynthesis, the most pronounced for the plants grown at 730 μmol m⁻² s⁻¹ (PPFD₇₃₀), may be related to the degree of sink-limitation. The increased non-radiative dissipation of absorbed photon energy for all EC plants corresponded to the higher de-epoxidation state of xanthophylls only for PPFD₇₃₀ barley. Further, a pronounced decrease in photosystem 2 (PS2) photochemical efficiency (given as F_V/F_M) for EC plants grown at 730 and 1 200 μmol m⁻² s⁻¹ in comparison with AC barley was related to the reduced epoxidation of antheraxanthin and zeaxanthin back to violaxanthin in darkness. Thus the EC conditions sensitise the photosynthetic apparatus of high-irradiance acclimated barley plants (particularly PPFD₇₃₀) to the photoinactivation of PS2. This revised version was published online in August 2006 with corrections to the Cover Date.     

NaCl-induced photoinhibition and recovery of the photosynthetic activity of a <Emphasis Type="Italic">katG</Emphasis><Superscript>−</Superscript> mutant of cyanobacterium <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803

The joint effects of 0.5 M NaCl and light of different intensities on the activity of the photosynthetic apparatus and ATP content in cells of the katG ⁻ mutant of cyanobacterium Synechocystis sp. PCC 6803 have been studied. The mutant demonstrated a higher photoinhibition rate and a slower rate of recovery compared with the wild type, as shown by measurements of the CO₂-dependent O₂ production and delayed fluorescence of Chl a. The presence of 0.5 M NaCl in the incubation medium caused equal photoinhibition of the photosynthetic apparatus at I = 1200 μE m⁻² s⁻¹ in the mutant and wild-type cells. At I = 2400 μE m⁻² s⁻¹, we observed stronger inhibition and slower recovery of the photosynthetic apparatus in the katG ⁻ mutant than in wild-type cells. The data obtained evidence an important role of catalase-peroxidase in the system of reparation of the photosynthetic apparatus damaged by high-intensity light, especially at the background of NaCl stress.     

Analysis of Qualitative Contribution of Assimilatory and Non-Assimilatory De-Excitation Processes to Adaptation of Photosynthetic Apparatus of Barley Plants to High Irradiance

The adaptation of barley (Hordeum vulgare L. cv. Akcent) plants to low (LI, 50 µmol m^–2 s^–1) and high (HI, 1000 µmol m^–2 s^–1) growth irradiances was studied using the simultaneous measurements of the photosynthetic oxygen evolution and chlorophyll a (Chl a) fluorescence at room temperature. If measured under ambient CO₂ concentration, neither increase of the oxygen evolution rate (P) nor enhancement of non-radiative dissipation of the absorbed excitation energy within photosystem 2 (PS2) (determined as non-photochemical quenching of Chl a fluorescence, NPQ) were observed for HI plants compared with LI plants. Nevertheless, the HI plants exhibited a significantly higher proportion of Q_A in oxidised state (estimated from photochemical quenching of Chl a fluorescence, q_P), by 49–102 % at irradiances above 200 µmol m^–2 s^–1 and an about 1.5 fold increase of irradiance-saturated PS2 electron transport rate (ETR) as compared to LI plants. At high CO₂ concentration the degree of P stimulation was approximately three times higher for HI than for LI plants, and the irradiance-saturated P values at irradiances of 2 440 and 2 900 µmol m^–2 s^–1 were by 130 and 150 % higher for HI plants than for LI plants. We suggest that non-assimilatory electron transport dominates in the adaptation of the photosynthetic apparatus of barley grown at high irradiances under ambient CO₂ rather than an increased NPQ or an enhancement of irradiance-saturated photosynthesis.     

Photosynthetic activity of homoiochlorophyllous desiccation tolerant plant <Emphasis Type="Italic">Haberlea rhodopensis</Emphasis> during dehydration and rehydration

The functional state of the photosynthetic apparatus of flowering homoiochlorophyllous desiccation tolerant plant Haberlea rhodopensis during dehydration and subsequent rehydration was investigated in order to characterize some of the mechanisms by which resurrection plants survive drought stress. The changes in the CO₂ assimilation rate, chlorophyll fluorescence parameters, thermoluminescence, fluorescence imaging and electrophoretic characteristics of the chloroplast proteins were measured in control, moderately dehydrated (50% water content), desiccated (5% water content) and rehydrated plants. During the first phase of desiccation the net CO₂ assimilation decline was influenced by stomatal closure. Further lowering of net CO₂ assimilation was caused by both the decrease in stomatal conductance and in the photochemical activity of photosystem II. Severe dehydration caused inhibition of quantum yield of PSII electron transport, disappearance of thermoluminescence B band and mainly charge recombination related to S₂Q_A⁻ takes place. The blue and green fluorescence emission in desiccated leaves strongly increased. It could be suggested that unchanged chlorophyll content and amounts of chlorophyll–proteins, reversible modifications in PSII electron transport and enhanced probability for non-radiative energy dissipation as well as increased polyphenolic synthesis during desiccation of Haberlea contribute to drought resistance and fast recovery after rehydration.     

A few molecules of zeaxanthin per reaction centre of photosystem II permit effective thermal dissipation of light energy in photosystem II of a poikilohydric moss

The relationship between thermal dissipation of light energy (as indicated by the quenching of chlorophyll fluorescence), zeaxanthin availability and protonation reactions was investigated in the moss Rhytidiadelphus squarrosus (Hedw.) Warnst. In the absence of zeaxanthin and actinic illumination, acidification by 20% CO₂ in air was incapable of quenching basal, so-called F ₀ fluorescence either in the moss or in spinach (Spinacia oleracea L.) leaves. However, 1-s light pulses given either every 40, 60 or 200 s increased thermal dissipation as indicated by F ₀ and F _m quenching in the presence of 20% CO₂ in air in the moss, but not in spinach while reaction centres of photosystem II (PSII) were photochemically open. In the moss, a few short light pulses, which were separated by prolonged dark times, were sufficient to raise zeaxanthin levels in the presence of 20% CO₂ in air. Simultaneously, quantum efficiency of charge separation in PSII was decreased. Increasing the CO₂ concentration beyond 20% further decreased quantum efficiency even in the absence of short light pulses. Under conditions optimal for fluorescence quenching, one molecule of zeaxanthin per reaction centre of PSII was sufficient to decrease quantum efficiency of charge separation in PSII by 50%. Thus, in combination with a protonation reaction, one molecule of zeaxanthin was as efficient at capturing excitation energy as a photochemically open reaction centre. The data are discussed in relation to the interaction between zeaxanthin and thylakoid protonation, which enables effective thermal dissipation of light energy in the antennae of PSII in the moss but not in higher plants when actinic illumination is absent. Received: 8 April 2000 / Accepted: 31 August 2000     

Photosynthetic acclimation to low temperature by western red cedar seedlings

Imposition of low, but above freezing, temperatures resulted in a gradual increase in the cold hardiness of western red cedar seedlings. This was associated with a decrease in the maximum rates of photosynthetic CO₂ fixation and O₂ evolution, and changes in chlorophyll a fluorescence transients which indicated that photoinhibition had occurred. Maximum photosynthetic rates declined approximately 40% during cold hardening. The leaves changed colour from green to red-brown during the hardening process. The colour change was due to the synthesis of large amounts of the carotenoid rhodoxanthin. Lutein levels doubled, while chlorophyll declined slightly. Dehardening resulted in the rapid recovery of photosynthesis to control levels, the rapid disappearance of rhodoxanthin, and the return of lutein levels to control. It is suggested that rhodoxanthin accumulation at low temperature functions to decrease the light intensity reaching the photosynthetic apparatus. The combination of photoinhibition and rhodoxanthin synthesis probably serves to protect the photosynthetic capacity of the seedlings at low temperature.     

Changes in photosynthetic apparatus during dark incubation of detached leaves from control and ultraviolet-B treatedVigna seedlings

Changes in various components of photosynthetic apparatus during the 4 d dark incubation at 25°C of detached control and ultraviolet-B (UV-B) treatedVigna unguiculata L. leaves were examined. The photosynthetic apparatus was more degraded in younger control seedlings and for a longer time UV-B treated seedlings than in the older or for a shorter time UV-B treated seedlings. This was shown by determining the losses in chlorophyll (Chl) and protein contents, variable fluorescence yield, photosystem (PS) 2, PS1 and ribulose-1,5-bisphosphate carboxylase (RuBPC) activities, and photosynthetic¹⁴CO₂ fixation. In contrast, the Car/Chl ratio increased during the dark incubation due to less expressed degradation of Car.     

Characterization of photosynthesis, photoinhibition and the activities of C4 pathway enzymes in a superhigh-yield rice, Liangyoupeijiu

Characteristics of photosynthetic gas exchange, photoinhibition and C₄ pathway enzyme activities in both flag leaves and lemma were compared between a superhigh-yield rice (Oryza sativa L.) hybrid, Liangyoupeijiu and a traditional rice hybrid, Shanyou63. Liangyoupeijiu had a similar light saturated assimilation rate (Asat) to Shanyou63, but a much higher apparent quantum yield (AQY), carboxylation efficiency (CE) and quantum yield of CO₂ fixation (ΦCO₂). Liangyoupeijiu also showed a higher resistance to photoinhibition and higher non-radiative energy dissipation associated with the xanthophyll cycle than Shanyou63 when subjected to strong light. In addition, Liangyoupeijiu had higher activities of the C₄ pathway enzymes in both flag leaves and lemmas than Shanyou63. These results indicate that higher light and CO₂ use efficiency, higher resistance to photoinhibition and C₄ pathway in both flag leaf and lemma may contribute to the higher yield of the superhigh-yield rice hybrid, Liangyoupeijiu.     

Measurement of Differences in Red Chlorophyll Fluorescence and Photosynthetic Activity between Sun and Shade Leaves by Fluorescence Imaging

With a flash-lamp chlorophyll (Chl) fluorescence imaging system (FL-FIS) the photosynthetic activity of several thousand image points of intact shade and sun leaves of beech were screened in a non-destructive way within a few seconds. The photosynthetic activity was determined via imaging the Chl fluorescence at maximum F_p and steady state fluorescence F_s of the induction kinetics (Kautsky effect) and by a subsequent determination of the images of the fluorescence decrease ratio R_Fd and the ratio F_p/F_s. Both fluorescence ratios are linearly correlated to the photosynthetic CO₂ fixation rates. This imaging method permitted to detect the gradients in photosynthetic capacity and the patchiness of photosynthetic quantum conversion across the leaf. Sun leaves of beech showed a higher photosynthetic capacity and differential pigment ratios (Chl a/b and Chls/carotenoids) than shade leaves. Profile analysis and histogram of the Chl fluorescence yield and the Chl fluorescence ratios allow to quantify the differences in photosynthetic activity between different leaf parts and between sun and shade leaves with a high statistical significance.     

Changes in photosynthetic apparatus during dark incubation of detached leaves from control and ultraviolet-B treatedVigna seedlings

Changes in various components of photosynthetic apparatus during the 4 d dark incubation at 25°C of detached control and ultraviolet-B (UV-B) treatedVigna unguiculata L. leaves were examined. The photosynthetic apparatus was more degraded in younger control seedlings and for a longer time UV-B treated seedlings than in the older or for a shorter time UV-B treated seedlings. This was shown by determining the losses in chlorophyll (Chl) and protein contents, variable fluorescence yield, photosystem (PS) 2, PS1 and ribulose-1,5-bisphosphate carboxylase (RuBPC) activities, and photosynthetic¹⁴CO₂ fixation. In contrast, the Car/Chl ratio increased during the dark incubation due to less expressed degradation of Car.     

Response of photosynthetic machinery of field-grown kiwifruit under Mediterranean conditions during drought and re-watering

Groups of Actinidia deliciosa A. Chev. C.F. Liang et A.R. Ferguson var. deliciosa kiwifruit plants were subjected to soil water shortage (D), while other groups were well irrigated (I). Variations in chlorophyll (Chl) a fluorescence indices and leaf gas exchange were determined once plants were severely stressed (25 d after the beginning of the D-cycle). Daily maximum values of photosynthetic photon flux density (PPFD) were ca. 1 650 μmol(photon) m⁻² s⁻¹, while air temperatures peaked at 34.6 °C. High irradiance per se did not greatly affect the efficiency of photosystem (PS) 2, but predisposed its synergistic reduction by D co-occurrence. Fluorescence showed transient photodamage of PS2 with a complete recovery in the afternoon in both D and I plants. Upon re-watering the efficiency of PS2 was suboptimal (95 %) at day 2 after irrigation was reinitiated. At early morning of the day 5 of re-watering, photosynthesis and stomatal conductance recovered at about 95 and 80 % of I vines, respectively, indicating some after-stress effect on stomatal aperture. Once excessive photons reached PS2, the thermal dissipation of surplus excitation energy was the main strategy to save the photosynthetic apparatus and to optimize carbon fixation. The rather prompt recovery of both Chl a fluorescence indices and net photosynthetic rate during re-watering indicated that kiwifruit photosynthetic apparatus is prepared to cope with temporary water shortage under Mediterranean-type-climates.     

A two-photon excitation study on the role of carotenoid dark states in the regulation of plant photosynthesis

Plants are exposed to sun light intensities that vary rapidly over several orders of magnitude during a typical day. It is known that the regulation of photosynthetic activity under these circumstances is essential for the survival and fitness of natural and gene modified plants. A quick balancing between utilization and dissipation of absorbed light energy ensures optimized levels of CO₂ fixation and protection from photo damage by excessive light-irradiation. Despite intensive investigations the biophysical mechanisms of these regulation processes are still poorly understood. Potentially involved singlet states of carotenoids are optically “dark” and so far it was impossible to investigate their role directly in living plants by conventional absorption or fluorescence spectroscopy. Here, we show by selective two-photon excitation of the carotenoid dark states in planta that a dominant part of the regulation is correlated with a substantial change in the energy transfer between these states and the chlorophylls (Chl). The results support a considerable role of the molecular gear shift model in which a reversible and step-wise enzymatic modification of the electronic structure of xanthophyll carotenoids enables a switching between carotenoid-to-Chl light-harvesting and Chl-to-carotenoid quenching. The shifting can be observed in real time in any plant. Treatment with the xanthophyll cycle inhibitor dithiothreitol slowed down both the light adaptation and the carotenoid–Chl energy flow changes to the same extent. Based on these results, we propose a biophysical quenching model in which both carotenoid dark states and radical cations contribute to the dissipation of excessive excitation energy. An erratum to this article can be found at     

Non-Radiative Dissipation of Absorbed Excitation Energy Within Photosynthetic Apparatus of Higher Plants

The review deals with thermal dissipation of absorbed excitation energy within pigment-protein complexes of thylakoid membranes in higher plants. We focus on the de-excitation regulatory processes within photosystem 2 (PS2) that can be monitored as non-photochemical quenching of chlorophyll (Chl) a fluorescence consisting of three components known as energy-dependent quenching (q_E), state-transition quenching (q_T), and photoinhibitory quenching (q_I). We summarize the role of thylakoid lumen pH, xanthophylls, and PS2 proteins in q_E mechanism. Further, both the similarity between q_E and q_I and specific features of q_I are described. The other routes of thermal energy dissipation are also mentioned, that is dissipation within photosystem 1 and dissipation through the triplet Chl pathway. The significance of the individual de-excitation processes in protection against photo-oxidative damage to the photosynthetic apparatus under excess photon supply is stretched.     

Evidence for light-dependent recycling of respired carbon dioxide by the cotton fruit

Conservation of respired CO₂ by an efficient recycling mechanism in fruit could provide a significant source of C for yield productivity. However, the extent to which such a mechanism operates in cotton (Gossypium hirsutum L.) is unknown. Therefore, a combination of CO₂ exchange, stable C isotope, and chlorophyll (Chl) fluorescence techniques were used to examine the recycling of respired CO₂ in cotton fruit. Respiratory CO₂ losses of illuminated fruit were reduced 15 to 20% compared with losses for dark-incubated fruit. This light-dependent reduction in CO₂ efflux occurred almost exclusively via the fruit's outer capsule wall. Compared with the photosynthetic activity of leaves, CO₂ recycling by the outer capsule wall was 35 to 40% as efficient. Calculation of ¹⁴CO₂ fixation on a per Chl basis revealed that the rate of CO₂ recycling for the capsule wall was 62.2 micromoles ¹⁴CO₂ per millimole Chl per second compared with an assimilation rate of 64.6 micromoles ¹⁴CO₂ per millimole Chl per second for leaves. During fruit development, CO₂ recycling contributed more than 10% of that C necessary for fruit dry weight growth. Carbon isotope analyses (δ¹³C) showed significant differences among the organs examined, but the observed isotopic compositions were consistent with a C₃ pathway of photosynthesis. Pulse-modulated Chl fluorescence indicated that leaves and fruit were equally efficient in photochemical and nonphotochemical dissipation of light energy. These studies demonstrated that the cotton fruit possesses a highly efficient, light-dependent CO₂ recovery mechanism that aids in the net retention of plant C and, therein, contributes to yield productivity.     

Co<Subscript>2</Subscript> assimilation and chlorophyll fluorescence in green <Emphasis Type="Italic">versus</Emphasis> red <Emphasis Type="Italic">Berberis thunbergii</Emphasis> leaves measured with different quality irradiation

Photosynthesis, photorespiration, and chlorophyll (Chl) fluorescence in green and red Berberis thunbergii leaves were studied with two different measuring radiations, red (RR) and “white” (WR). The photosynthetic and photorespiration rates responded differently to the different radiation qualities, which indicate that the carboxylase and oxygenase activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) were affected. Differences in photosynthetic rate between the two color leaves were less under RR than under WR. However, this reduced difference in photosynthetic rate was not correlated with the stomatal response to the measuring radiation qualities. Compared with the WR, the RR reduced the differences in dark-adapted minimum and maximum fluorescence, steady-state fluorescence, light-adapted maximum fluorescence, and actual photochemical efficiency (Φ_PS2) of photosystem 2 (PS2), but enlarged the difference in non-photochemical quenching between the two color leaves. Differences in both maximum quantum yield of PS2 and ratio of Φ_PS2 to quantum yield of CO₂ fixation between the two color leaves were similar under the two measuring radiations. To exclude disturbance of radiation attenuation caused by anthocyanins, it is better to use RR to compare the photosynthesis and Chl fluorescence in green versus red leaves.     

Differential characteristics of photochemical acclimation to cold in two contrasting sweet sorghum hybrids

Sweet sorghum has a photosynthetic system which is highly sensitive to cold stress and hence strongly limits its development in temperate environments; therefore, the identification of key exploitable cold tolerance traits is imperative. From a preliminary field trial, two dissimilar sweet sorghum hybrids (ICSSH31 and Bulldozer), in terms of early vigor and productivity, were selected for a controlled‐environment trial aiming at identifying useful traits related to acclimation mechanisms to cold stress. The higher cold tolerance of Bulldozer was partially related to a more efficient photochemical regulation mechanism of the incoming light energy: the higher tolerance of photosystem II (PSII) to photo‐inactivation was because of a more effective dissipation capacity of the excess of energy and to a more balanced diversion of the absorbed energy into alternative energy sinks. ICSSH31 increased the dissipation and accumulation of a large amount of xanthophylls, as in Bulldozer, but, at the same time, inactivated the oxygen evolving complex and the re‐synthesis of chlorophyll (Chl) a and b, thus, leading to an overproduction of CO₂ fixation enzymes after re‐warming. In summary, in Bulldozer, the acclimation adjustments of the photosynthetic apparatus occurred through an efficient control of energy transfer toward the reaction centers, and this likely allowed a more successful seedling establishment; ICSSH31, conversely, exhibited a fast re‐synthesis of Chl pigments, which appears to divert photosynthates from dry matter accumulation. Such broad acclimation traits may constitute a source for selecting higher genetic gain traits relevant for enlarging the growing season of promising biomass sorghum ideotypes in temperate climates.     

Phototolerance of lichens, mosses and higher plants in an alpine environment: analysis of photoreactions

 Adaptation to excessive light is one of the requirements of survival in an alpine environment particularly for poikilohydric organisms which in contrast to the leaves of higher plants tolerate full dehydration. Changes in modulated chlorophyll fluorescence and 820-nm absorption were investigated in the lichens Xanthoria elegans (Link) Th. Fr. and Rhizocarpon geographicum (L.) DC, in the moss Grimmia alpestris Limpr. and the higher plants Geum montanum L., Gentiana lutea L. and Pisum sativum L., all collected at altitudes higher than 2000 m above sea level. In the dehydrated state, chlorophyll fluorescence was very low in the lichens and the moss, but high in the higher plants. It increased on rehydration in the lichens and the moss, but decreased in the higher plants. Light-induced charge separation in photosystem II was indicated by pulse-induced fluorescence increases only in dried leaves, not in the dry moss and dry lichens. Strong illumination caused photodamage in the dried leaves, but not in the dry moss and dry lichens. Light-dependent increases in 820-nm absorption revealed formation of potential quenchers of chlorophyll fluorescence in all dehydrated plants, but energy transfer to quenchers decreased chlorophyll fluorescence only in the moss and the lichens, not in the higher plants. In hydrated systems, coupled cyclic electron transport is suggested to occur concurrently with linear electron transport under strong actinic illumination particularly in the lichens because far more electrons became available after actinic illumination for the reduction of photo-oxidized P700 than were available in the pool of electron carriers between photosystems II and I. In the moss Grimmia, but not in the lichens or in leaves, light-dependent quenching of chlorophyll fluorescence was extensive even under nitrogen, indicating anaerobic thylakoid acidification by persistent cyclic electron transport. In the absence of actinic illumination, acidification by ca. 8% CO₂ in air quenched the initial chlorophyll fluorescence yield F_o only in the hydrated moss and the lichens, not in leaves of the higher plants. Under the same conditions, 8% CO₂ reduced the maximal fluorescence yield F_m strongly in the poikilohydric organisms, but only weakly or not at all in leaves. The data indicate the existence of deactivation pathways which enable poikilohydric organisms to avoid photodamage not only in the hydrated but also in the dehydrated state. In the hydrated state, strong nonphotochemical quenching of chlorophyll fluorescence indicated highly sensitive responses to excess light which facilitated the harmless dissipation of absorbed excitation energy into heat. Protonation-dependent fluorescence quenching by cyclic electron transport, P700 oxidation and, possibly, excitation transfer between the photosystems were effectively combined to produce phototolerance. Received: 10 December 1999 / Accepted: 13 April 2000     

Distinct physiological responses to a high light and low CO2 environment revealed by fluorescence quenching in photoautotrophically grown Chlamydomonas reinhardtii

Mechanisms for countering environmental stress are essential to photosynthetic organisms. Alteration of the photosynthetic apparatus, a mechanism for balancing the flux of light energy and carbon fixation, can be characterized by fluorescence properties. In this study, we have established a simple protocol to determine the extent of energy-dependent quenching (qE) and quenching by state transition (qT) in Chlamydomonas cells by examining their fluorescence properties under light fluctuations. We identified qE as the uncoupler-sensitive NPQ component that was rapidly relaxed upon transition to dark conditions. We characterized the qT component by determining low-temperature fluorescence spectra and analyzing a state-transition-less mutant. By these methods, we observed that similar abiotic stresses—high light conditions (where excess energy is supplied) and low CO₂ conditions (where energy utilization is limited)—induced different types of NPQ. High light conditions induced mainly qE-quenching that increased gradually while low CO₂ conditions induced mainly qT-quenching that peaked in 20 min and then decreased gradually. That high light and low carbon signals induced different physiological responses suggests that they triggered different genetic responses, which altered protein expression under each of the conditions.