ACCLIMATION OF EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) TO PHOTON FLUX DENSITY1

Growth rate, pigment composition, and noninvasive chl a fluorescence parameters were assessed for a noncalcifying strain of the prymnesiophyte Emiliania huxleyi Lohman grown at 50, 100, 200, and 800 μmol photons·m^?2·s^?1. Emiliania huxleyi grown at high photon flux density (PFD) was characterized by increased specific growth rates, 0.82 d^?1 for high PFD grown cells compared with 0.38 d^?1 for low PFD grown cells, and higher in vivo chl a specific attenuation coefficients that were most likely due to a decreased pigment package, consistent with the observed decrease in cellular photosynthetic pigment content. High PFD growth conditions also induced a 2.5‐fold increase in the pool of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin responsible for dissipation of excess energy. Dark‐adapted maximal photochemical efficiency (F_v/F_m) remained constant at around 0.58 for cells grown over the range of PFDs, and therefore the observed decline, from 0.57 to 0.33, in the PSII maximum efficiency in the light‐adapted state, (F_v′/F_m′), with increasing growth PFD was due to increased dissipation of excess energy, most likely via the xanthophyll cycle and not due to photoinhibition. The PSII operating efficiency (F_q′/F_m′) decreased from 0.48 to 0.21 with increasing growth PFD due to both saturation of photochemistry and an increase in nonphotochemical quenching. The changes in the physiological parameters with growth PFD enable E. huxleyi to maximize rates of photosynthesis under subsaturating conditions and protect the photosynthetic apparatus from excess energy while supporting higher saturating rates of photosynthesis under saturating PFDs.     

Brassinosteroids accelerate recovery of photosynthetic apparatus from cold stress by balancing the electron partitioning,carboxylation and redox homeostasis in cucumber

The aim of this study was to examine the role of brassinosteroids (BRs) in protecting the photosynthetic apparatus from cold‐induced damage in cucumber (Cucumis sativus) plants. Recovery at both high light (HL) and low light (LL) after a cooling at 10/7°C induced irreversible inhibition of CO₂ assimilation, photoinhibition at photosystem I (PSI) and inhibition of enzyme activities of Calvin cycle and ascorbate (AsA)‐reduced glutathione (GSH) cycle, followed by accumulation of H₂O₂ and malondialdehyde. However, cold‐induced photoinhibition at PSII was fully recovered at LL but not at HL. Meanwhile, recovery at HL increased electron flux to O₂‐dependent alternative pathway [Ja(O₂‐dependent)]. Foliar application of 24‐epibrassinolide (EBR) accelerated recovery from photoinhibition of PSII but not of PSI. EBR also significantly increased CO₂ assimilation, activity of Calvin cycle enzymes and electron flux to carbon reduction [Je(PCR)], with a concomitant decrease in Ja(O₂‐dependent); meanwhile EBR increased the activity of enzymes in AsA‐GSH cycle and cellular redox states. However, the positive effect of EBR on plant recovery was observed only at HL, but not LL. These results indicate that BR accelerates the recovery of photosynthetic apparatus at HL by activation of enzymes in Calvin cycle and increasing the antioxidant capacity, which in turn mitigate the photooxidative stress and the inhibition of plant growth during the recovery.     

LIGHT‐INDUCED RESPONSES OF OXYGEN PHOTOREDUCTION,REACTIVE OXYGEN SPECIES PRODUCTION AND SCAVENGING IN TWO DIATOM SPECIES1

Diatoms are frequently exposed to high light (HL) levels, which can result in photoinhibition and damage to PSII. Many microalgae can photoreduce oxygen using the Mehler reaction driven by PSI, which could protect PSII. The ability of Nitzschia epithemioides Grunow and Thalassiosira pseudonana Hasle et Heimdal grown at 50 and 300 μmol photons · m^?2 · s^?1 to photoreduce oxygen was examined by mass spectrometric measurements of ¹⁸O₂. Both species exhibited significant rates of oxygen photoreduction at saturating light levels, with cells grown in HL exhibiting higher rates. HL‐grown T. pseudonana had maximum rates of oxygen photoreduction five times greater than N. epithemoides, with 49% of electrons transported through PSII being used to reduce oxygen. Exposure to excess light (1,000 μmol photons · m^?2 · s^?1) produced similar decreases in the operating quantum efficiency of PSII (F_q′/F_m′) of low light (LL)‐ and HL‐grown N. epithemoides, whereas HL‐grown T. pseudonana exhibited much smaller decreases in F_q′/F_m′ than LL‐grown cells. HL‐grown T. pseudonana and N. epithemioides exhibited greater superoxide and hydrogen peroxide production, higher activities (in T. pseudonana) of superoxide dismutase (SOD) and ascorbate peroxidase (APX), and increased expression of three SOD‐ and one APX‐encoding genes after 60 min of excess light compared to LL‐grown cells. These responses provide a mechanism that contributes to the photoprotection of PSII against photodamage.     

ALGAL PHOTOSYNTHETIC MEMBRANE COMPLEXES AND THE PHOTOSYNTHESIS-IRRADIANCE CURVE: A COMPARISON OF LIGHT-ADAPTATION RESPONSES IN CHLAMYDOMONAS REINHARDTII (CHLOROPHYTA)1

Chlamydomonas reinhardtii was grown at photon flux densities (PFDs) ranging from 47 to 400 μE.m^-2 s^-1. The total cellular content of chlorophyll (Chl) was twice as high in the low light (LL) versus high light (HL) grown cells. On an equal Chl basis, photosystem II (PSII) and cytochrome f (Cyt f) content was higher in HL cells, but photosystem I (PSI) concentration displayed little variation with the light intensity during cell growth. Consequently, there was a shift in the ratio of PSII / PSI and Cyt / PSI from near unity in LL cells to greater than two in HL cells. The functional Chl antenna size of PSII and PSI ranged from 460 and 170 Chl (a + b)in HL-grown cells to 620 and 370 Chl (a+ b)in LL-grown cells, respectively. The initial slope of the Chl-specific photosyn-thesis-irradiance (P-I) curve was similar in LL- and HL-grown cells, but the light saturated rate of photosynthesis was lower under LL. The response to low light was beneficial at the cellular level, since there was an enhancement of photosynthesis in LL. The PFD for the onset of light saturation, 1 was a factor of 2 lower in LL- relative to HL-grown photosythetic membranes. Since growth PFD varied by a factor of ten, photosynthesis shifted from being light-limited in the LL regime to light-saturated in the HL regime. The requirement for balanced absorption of light by the two photosystems constrains the PSII / PSI ratio to near unity when growth is light-limited, but such a constraint does not apply in HL conditions. Instead the concentration of individual electron transport complexes way be related to the pool size necessary for maximum rates of steady-state electron transport. Thus the stoichiometry of electron transport complexes changes in response to growth PFD and this change is correlated with the response flexlbility of algal photosynthesis in diverse light environments.     

Different strategies of photoacclimation by two strains of Emiliania huxleyi (Haptophyta)1

Photoacclimation involves the modification of components of the light and dark reactions to optimize photosynthesis following changes in available light. All of the energy required for photosynthesis comes from linear electron transport through PSII and PSI and is dependent upon the amount of light harvested by PSII relative to PSI (a*_PSII and a*_PSI). The amount of light harvested is determined by the effective absorption cross‐sections (σ_PSII, σ_PSI) and cellular contents of the PSII and PSI reaction center complexes (RCII, RCI). Here, we examine the effective absorption cross‐sections and reaction center contents for calcifying (B11) and noncalcifying (B92) strains of the globally important coccolithophorid Emiliania huxleyi (Lohmann) W. H. Hay et H. Mohler when grown under various photon flux densities (PFDs). The two strains displayed different “strategies” of acclimation. As growth PFD increased, B11 preferentially changed σ and the cellular content of chl a per cell over PSU “size” (the total cellular chl a content associated with the reaction center complexes); strain B92 preferentially changed PSU size over the cellular content of reaction complexes. Neither strategy was specifically consistent with the majority of previous studies from other microalgal species. For both strains, cellular light absorption for PSII and PSI was maintained close to unity across the range of growth PFDs since changes of σ_PSII and σ_PSI were reciprocated by those of RCIIs and RCIs per cell. Our results demonstrate a significant adaptive flexibility of E. huxleyi to photoacclimate. Finally, we calculated the amount of chl a associated with either photosystem to consider our interpretations of photoacclimation based on conventional determinations of PSU size.     

PSII photochemistry, thermal energy dissipation, and the xanthophyll cycle in Kalanchoë daigremontiana exposed to a combination of water stress and high light

Kalanchoë daigremontiana, a CAM plant grown in a greenhouse, was subjected to severe water stress. The changes in photosystem II (PSII) photochemistry were investigated in water‐stressed leaves. To separate water stress effects from photoinhibition, water stress was imposed at low irradiance (daily peak PFD 150 μmol m^?2 s^?1). There were no significant changes in the maximal efficiency of PSII photochemistry (F_v/F_m), the traditional fluorescence induction kinetics (OIP) and the polyphasic fluorescence induction kinetics (OJIP), suggesting that water stress had no direct effects on the primary PSII photochemistry in dark‐adapted leaves. However, PSII photochemistry in light‐adapted leaves was modified in water‐stressed plants. This was shown by the decrease in the actual PSII efficiency (Φ_PSII), the efficiency of excitation energy capture by open PSII centres (F_v′/F_m′), and photochemical quenching (q_P), as well as a significant increase in non‐photochemical quenching (NPQ) in particular at high PFDs. In addition, photoinhibition and the xanthophyll cycle were investigated in water‐stressed leaves when exposed to 50% full sunlight and full sunlight. At midday, water stress induced a substantial decrease in F_v/F_m which was reversible. Such a decrease was greater at higher irradiance. Similar results were observed in Φ_PSII, q_P, and F_v′/F_m′. On the other hand, water stress induced a significant increase in NPQ and the level of zeaxanthin via the de‐epoxidation of violaxanthin and their increases were greater at higher irradiance. The results suggest that water stress led to increased susceptibility to photoinhibition which was attributed to a photoprotective process but not to a photodamage process. Such a photoprotection was associated with the enhanced formation of zeaxanthin via de‐epoxidation of violaxanthin. The results also suggest that thermal dissipation of excess energy associated with the xanthophyll cycle may be an important adaptive mechanism to help protect the photosynthetic apparatus from photoinhibitory damage for CAM plants normally growing in arid and semi‐arid areas where they are subjected to a combination of water stress and high light.     

Winter photoinhibition in needles of <Emphasis Type="Italic">Taxus baccata</Emphasis> seedlings acclimated to different light levels

Seasonal variability of maximum quantum yield of PSII photochemistry (F_v/F_m) was studied in needles of Taxus baccata seedlings acclimated to full light (HL, 100% solar irradiance), medium light (ML, 18% irradiance) or low light (LL, 5% irradiance). In HL plants, F_v/F_m was below 0.8 (i.e. state of photoinhibition) throughout the whole experimental period from November to May, with the greatest decline in January and February (when F_v/F_m value reached 0.37). In ML seedlings, significant declines of F_v/F_m occurred in January (with the lowest level at 0.666), whereas the decline in LL seedlings (down to 0.750) was not significant. Full recovery of F_v/F_m in HL seedlings was delayed until the end of May, in contrast to ML and LL seedlings. F_v/F_m was significantly correlated with daily mean (T _mean), maximal (T _max) and minimal (T _min) temperature and T _min was consistently the best predictor of F_v/F_m in HL and ML needles. Temperature averages obtained over 3 or 5 days prior to measurement were better predictors of F_v/F_m than 1- or 30-day averages. Thus our results indicate a strong light-dependent seasonal photoinhibition in needles of T. baccata as well as suggest a coupling of F_v/F_m to cumulative temperature from several preceding days. The dependence of sustained winter photoinhibition on light level to which the plants are acclimated was further demonstrated when plants from the three light environments were exposed to full daylight over single days in December, February and April and F_v/F_m was followed throughout the day to determine residual sensitivity of electron transport to ambient irradiance. In February, the treatment revealed a considerable midday increase in photoinhibition in ML plants, much less in HL (already downregulated) and none in LL plants. This suggested a greater capacity for photosynthetic utilization of electrons in LL plants and a readiness for rapid induction of photoinhibition in ML plants. Further differences between plants acclimated to contrasting light regimes were revealed during springtime de-acclimation, when short term regeneration dynamics of F_v/F_m and the relaxation of nonphotochemical quenching (NPQ) indicated a stronger persistent thermal mechanism for energy dissipation in HL plants. The ability of Taxus baccata to sustain winter photoinhibition from autumn until late spring can be beneficial for protection against an excessive light occurring together with frosts but may also restrict photosynthetic carbon gain by this shade-tolerant species when growing in well illuminated sites.     

Response of the diatom Phaeodactylum tricornutum to photooxidative stress resulting from high light exposure

The response of microalgae to photooxidative stress resulting from high light exposure is a well-studied phenomenon. However, direct analyses of photosystem II (PSII) D1 protein (the main target of photoinhibition) in diatoms are scarce. In this study, the response of the diatom model species Phaeodactylum tricornutum to short-term exposure to high light was examined and the levels of D1 protein determined immunochemically. Low light (LL) acclimated cells (40 μmol photons m(-2) s(-1)) subjected to high light (HL, 1,250 μmol photons m(-2) s(-1)) showed rapid induction of non-photochemical quenching (NPQ) and ca. 20-fold increase in diatoxanthin (DT) concentration. This resulted from the conversion of diadinoxanthin (DD) to DT through the activation of the DD-cycle. D1 protein levels under LL decreased about 30% after 1 h of the addition of lincomycin (LINC), a chloroplast protein synthesis inhibitor, showing significant D1 degradation and repair under low irradiance. Exposure to HL lead to a 3.2-fold increase in D1 degradation rate, whereas average D1 repair rate was 1.3-x higher under HL than LL, leading to decreased levels of D1 protein under HL. There were significant effects of both HL and LINC on P. tricornutum maximum quantum yield of PSII (F(v)/F(m)), showing a reduction of active PSII reaction centres. Partial recovery of F(v)/F(m) in the dark demonstrates the photosynthetic resilience of this diatom to changes in the light regime. P. tricornutum showed high allocation of total protein to D1 and an active D1-repair cycle to limit photoinhibition.     

Biooptical characteristics of PSII and PSI in 33 species (13 pigment groups) of marine phytoplankton,and the relevance for pulse‐amplitude‐modulated and fast‐repetition‐rate fluorometry1

We studied the variability of in vivo absorption coefficients and PSII‐scaled fluorescence excitation (fl‐ex) spectra of high light (HL) and low light (LL) acclimated cultures of 33 phytoplankton species that belonged to 13 different pigment groups (PGs) and 10 different phytoplankton classes. By scaling fl‐ex spectra to the corresponding absorption spectra by matching them in the 540–650 nm range, we obtained estimates for the fraction of total chl a that resided in PSII, the absorption of light by PSII, PSI, and photoprotective carotenoids. The in vivo red peak absorption maxima ranged from 673 to 679 nm, reflecting bonding of chl a to different pigment proteins. A simple approach is presented for quantifying intracellular self‐shading and evaluating the impact of photoacclimation on biooptical characteristics of the different PGs examined. In view of these results, parameters used in the calculation of oxygenic photosynthesis based on pulse‐amplitude‐modulated (PAM) and fast‐repetition‐rate (FRR) fluorometers are discussed, showing that the ratio between light available to PSII and total absorption, essential for the calculation of the oxygen release rate (using the PSII‐scaled fluorescence spectrum as a proxy) was dependent on species and photoacclimation state. Three subgroups of chromophytes exhibited 70%–80%, 60%–80%, and 50%–60% chl a in PSII‐LHCII; the two subgroups of chlorophytes, 70% or 80%; and cyanobacteria, only 12%. In contrast, the mean fraction for chromo‐ and chlorophytes of quanta absorbed by PSII was 73% in LL‐ and 55% in HL‐acclimated cells; thus, the corresponding ratios 0.55 and 0.73 might be used as correction factors adjusting for quanta absorbed by PSII for PAM and FRR measurements.     

COMPARISON OF RESISTANCE TO LIGHT STRESS IN TOXIC AND NON‐TOXIC STRAINS OF MICROCYSTIS AERUGINOSA (CYANOPHYTA)1

Blooms of Microcystis aeruginosa (Kützing) Kützing occur frequently in many freshwater ecosystems around the world, but the role of environmental factors in promoting the growth and determining the proportion of toxic and non‐toxic strains still requires more investigation. In this study, four strains (toxic CPCC299 & FACHB905 and non‐toxic CPCC632 & FACHB315) were exposed to high light (HL) condition, similar to light intensity found at the surface of a bloom, to evaluate their sensitivity to photoinhibition. We also estimated their capacity to recover from this HL stress. For all strains, our results showed an increased inhibition of the photosynthetic activity with HL treatment time. When comparing the extent of photoinhibition between strains, both toxic strains were more resistant to the treatment and recovered completely their photosynthetic capacity after 3 h, while non‐toxic strains needed more time to recover. For toxic strains, the rETR under HL was higher compared to the rETR under low light (LL) control condition despite 50% photoinhibition. This suggests that the detrimental effect of high light (HL; up to 2 h) is outweighed by their higher photosynthetic potential. This conclusion did not stand for non‐toxic strains, and indicates their preference for LL environment. We also demonstrated that a LL/HL cycle induced a 259% increase in cell yield for a toxic strain and a decrease by 22% for a non‐toxic strain. This also indicates that toxic strains have higher tolerance to HL in a fluctuating light environment. Our data demonstrated that difference of sensitivity to HL between strains can modify the competitive outcome between toxic and non‐toxic strains and may affect bloom toxicity.     

STRATEGIES AND RATES OF PHOTOACCLIMATION IN TWO MAJOR SOUTHERN OCEAN PHYTOPLANKTON TAXA: PHAEOCYSTIS ANTARCTICA (HAPTOPHYTA) AND FRAGILARIOPSIS CYLINDRUS (BACILLARIOPHYCEAE)1

We investigated rates and mechanisms of photoacclimation in cultures of Phaeocystis antarctica G. Karst. and Fragilariopsis cylindrus (Grunow) Willi Krieg, phytoplankton taxa that each dominate distinct areas of the Ross Sea, Antarctica. Both P. antarctica and F. cylindrus acclimated to increases in irradiance by reducing the effective size of the pigment antenna (σPSII) via xanthophyll‐cycle activity and reductions in chl. While enhanced photoprotection facilitated increases in specific growth rate and eventually led to higher light‐saturated photosynthetic rates (P^cell_m) in P. antarctica, increases in those variables were much smaller in F. cylindrus. In response to a lower irradiance, relaxation of xanthophyll‐cycle activity led to an increase in σPSII in both taxa, which occurred much more slowly in F. cylindrus. A surprising increase in specific growth rate over the first 36 h of acclimation in P. antarctica may have facilitated the significant reductions in P^cell_m observed in that taxon. In general, P. antarctica acclimated more quickly to changes in irradiance than F. cylindrus, exhibited a wider range in photosynthetic rates, but was more susceptible to photoinhibition. This acclimation strategy is consistent with growth in deeply mixed water columns with variations in irradiance that allow time for repair. In contrast, the slower acclimation rates, extensive photoprotection, and low photoinhibition exhibited by F. cylindrus suggest that it does not require the same period for repair as P. antarctica and is best suited for growth in habitats with relatively uniform irradiance, such as shallow mixed layers or sea ice.     

Interaction between photon flux density and elevated temperatures on photoinhibition in Alocasia macrorrhiza

The effects of light and elevated temperatures on the efficiency of energy conversion in PSII [?_PSII = (Fm′−Fs)/Fm′], pigment composition and heat tolerance of shade-acclimated Alocasia macrorrhiza were investigated. Leaf discs were exposed for 3 h to high light (HL; 1600 μmol photons · m⁻² · s⁻¹) or low light (LL; 20 μmol photons · m⁻² · s⁻¹) and a series of constant temperatures ranging from 30 to 49 °C. All HL treatments led to rapid and severe decreases in ?_PSII. During the 2-h recovery period (LL, 25 °C) following the HL treatments, fast and slow recovery phases could be distinguished. Leaf discs that had experienced HL and 30 °C recovered completely while no recovery of ?_PSII was seen after a 3-h exposure to HL and 45 °C. A 3-h exposure to 45 °C at LL led to a less severe decrease in ?_PSII and complete recovery was accomplished after less than 1 h. Under LL conditions a temperature of 49 °C was necessary to cause an irreversible decrease in ?_PSII, followed by necrosis the next day. Streptomycin had no effect on the degree of reduction and recovery in ?_PSII discs exposed to HL and 35–45 °C, but partially inhibited recovery in discs exposed to HL and 30 °C. Streptomycin led to a more severe decrease in ?_PSII at LL and 49 °C and completely inhibited recovery. Streptomycin had no effect on the conversion of the xanthophyll-cycle pigments during the treatment or the recovery. The epoxidation state was roughly the same in all leaf discs after a 3-h HL treatment (0.270–0.346) irrespective of the exposure temperature. The back-conversion of zeaxanthin into violaxanthin after a 2-h recovery period was only seen in leaf discs that had been exposed to HL and 30 °C. The thermotolerance of shade A. macrorrhiza leaves of 49.0 ± 0.7 °C (determined by fluorescence) coincided with the temperature at which damage occurred in leaf discs exposed to LL. However, under HL the critical temperature under which necrosis occurred was much lower (42 °C). The thermotolerance of A. macrorrhiza shade leaves could be increased by a short exposure (<20 min) to slightly elevated temperatures. Received: 11 June 1997 / Accepted: 9 September 1997     

Enhanced NPQ affects long-term acclimation in the spring ephemeral Berteroa incana

The spring ephemeral Berteroa incana is a familial relative of Arabidopsis thaliana and thrives in a diverse range of terrestrial ecosystems. Within this study, the novel chlorophyll fluorescence parameter of photochemical quenching in the dark (qPd) was used to measure the redox state of the primary quinone electron acceptor (Q_A) in order to estimate the openness of photosystem II (PSII) reaction centres (RC). From this, the early onset of photoinactivation can be sensitively quantified alongside the light tolerance of PSII and the photoprotective efficiency of nonphotochemical quenching (NPQ). This study shows that, with regards to A. thaliana, NPQ is enhanced in B. incana in both low-light (LL) and high-light (HL) acclimation states. Moreover, light tolerance is increased by up to 500%, the rate of photoinactivation is heavily diminished, and the ability to recover from light stress is enhanced in B. incana, relative to A. thaliana. This is due to faster synthesis of zeaxanthin and a larger xanthophyll cycle (XC) pool available for deepoxidation. Moreover, preferential energy transfer via CP47 around the RC further enhances efficient photoprotection. As a result, a high functional cross-section of photosystem II is maintained and is not downregulated when B. incana is acclimated to HL. A greater capacity for protective NPQ allows B. incana to maintain an enhanced light-harvesting capability when acclimated to a range of light conditions. This enhancement of flexible short-term protection saves the metabolic cost of long-term acclimatory changes.     

STREAM PERIPHYTON PHOTOACCLIMATION RESPONSE IN FIELD CONDITIONS: EFFECT OF COMMUNITY DEVELOPMENT AND SEASONAL CHANGES1

The photochemical behavior of intact stream periphyton communities in France was evaluated in response to the time course of natural light. Intact biofilms grown on glass substrata were collected at three development stages in July and November, and structural parameters of the biofilms were investigated (diatom density and taxonomy). At each season, physiological parameters based on pigment analysis (HPLC) and pulse‐amplitude‐modulated (PAM) chl fluorescence technique were estimated periodically during a day from dawn to zenith. Regardless of the community studied, the optimal quantum yield of PSII (F_v/F_m), the effective PSII efficiency (Φ_PSII), the nonphotochemical quenching (NPQ), and the relative electron transport rate (rETR) exhibited clear dynamic patterns over the morning. Moreover, microalgae responded to the light increase by developing the photoprotective xanthophyll cycle. The analysis of P‐I parameters and pigment profiles suggests that July communities were adapted to higher light environments in comparison with November ones, which could be partly explained by a shift in the taxonomic composition. Finally, differences between development stages were significant only in July. In particular, photoinhibition was less pronounced in mature assemblages, indicating that self‐shading (in relation to algal biomass) could have influenced photosynthesis in older communities.     

Interactions between water stress, sun-shade acclimation, heat tolerance and photoinhibition in the sclerophyll Heteromeles arbutifolia

Gas exchange and chlorophyll fluorescence techniques were used to evaluate the acclimation capacity of the schlerophyll shrub Heteromeles arbutifolia M. Roem. to the multiple co-occurring summer stresses of the California chaparral. We examined the influence of water, heat and high light stresses on the carbon gain and survival of sun and shade seedlings via a factorial experiment involving a slow drying cycle applied to plants grown outdoors during the summer. The photochemical efficiency of PSII exhibited a diurnal, transient decrease (δF/F_m′) and a chronic decrease or photoinhibition (F_v/F_m) in plants exposed to full sunlight. Water stress enhanced both transient decreases of δF/F_m’and photoinhibition. Effects of decreased δF/F_m’and F_v/F_m on carbon gain were observed only in well-watered plants since in water-stressed plants they were overidden by stomatal closure. Reductions in photochemical efficiency and stomatal conductance were observed in all plants exposed to full sunlight, even in those that were well-watered. This suggested that H. arbutifolia sacrificed carbon gain for water conservation and photoprotection (both structurally via shoot architecture and physiologically via down-regulation) and that this response was triggered by a hot and dry atmosphere together with high PFD, before severe water, heat or high PFD stresses occur. We found fast adaptive adjustments of the thermal stability of PSII (diurnal changes) and a superimposed long-term acclimation (days to weeks) to high leaf temperatures. Water stress enhanced resistance of PSII to high temperatures both in the dark and over a wide range of PFD. Low PFD protected photochemical activity against inactivation by heat while high PFD exacerbated damage of PSII by heat. The greater interception of radiation by horizontally restrained leaves relative to the steep leaves of sun-acclimated plants caused photoinhibition and increased leaf temperature. When transpirational cooling was decreased by water stress, leaf temperature surpassed the limits of chloroplast thermostability. The remarkable acclimation of water-stressed plants to high leaf temperatures proved insufficient for the semi-natural environmental conditions of the experiment. Summer stresses characteristic of Mediterranean-type climates (high leaf temperatures in particular) are a potential limiting factor for seedling survival in H. arbutifolia, especially for shade seedlings lacking the crucial structural photoprotection provided by steep leaf angles.     

Cost and benefit of the repair of photodamaged photosystem II in spinach leaves: roles of acclimation to growth light

When visible light is excess, the photosynthetic machinery is photoinhibited. The extent of net photoinhibition of photosystem II (PSII) is determined by a balance between the rate of photodamage to D1 and some other PSII proteins and the rate of the turnover cycle of these proteins. It is widely believed that the protein turnover requires much energy cost. The aims of this study are to (1) evaluate the energy cost of PSII repair, (2) measure the benefit in terms of photosynthetic gain realized by the repairing of the photodamaged PSII, and (3) know whether acclimation of photosynthesis to growth light affects the rates of the photodamage and repair. We grew spinach in high-light (HL) and low-light (LL) and measured the rates of D1 photodamage and repair in these leaves. We determined the rate constants of photodamage (k (pi)) and repair (k (rec)) by the PAM fluorometry in the presence or in the absence of lincomycin, an inhibitor of 70S protein synthesis. HL leaves showed smaller k (pi) and greater k (rec) than LL leaves. The energy cost of the repairing of the photodamaged D1 protein was <0.5?% of ATP produced by photophosphorylation at PPFDs ranging from 400 to 1600?μmol?m(-2)?s(-1) and was greater in HL leaves than in LL leaves. The benefits brought about by the repair were more than from 35 to 270 times the cost at PPFDs ranging from 400 to 1600?μmol?m(-2)?s(-1). The benefits of HL leaves were greater than those of LL leaves because of the higher photosynthesis rates in HL leaves. Running a simple simulation of daily photosynthesis using the parameters obtained in this study, we discuss why the plants need to pay the cost of D1 protein turnover to repair the photodamaged PSII.     

Far‐red light enhances photochemical efficiency in a wavelength‐dependent manner

Linear electron transport depends on balanced excitation of photosystem I and II. Far‐red light preferentially excites photosystem I (PSI) and can enhance the photosynthetic efficiency when combined with light that over‐excites photosystem II (PSII). The efficiency of different wavelengths of far‐red light exciting PSI was quantified by measuring the change in quantum yield of PSII (Φ_PSII) of lettuce (Lactuca sativa) under red/blue light with narrowband far‐red light added (from 678 to 752 nm, obtained using laser diodes). The Φ_PSII of lettuce increased with increasing wavelengths of added light from 678 to 703 nm, indicating longer wavelengths within this region are increasingly used more efficiently by PSI than by PSII. Adding 721 nm light resulted in similar Φ_PSII as adding 703 nm light, but Φ_PSII tended to decrease as wavelength increased from 721 to 731 nm, likely due to decreasing absorptance and low photon energy. Adding 752 nm light did not affect Φ_PSII. Leaf chlorophyll fluorescence light response measurements showed lettuce had higher Φ_PSII under halogen light (rich in far‐red) than under red/blue light (which over‐excites PSII). Far‐red light is more photosynthetically active than commonly believed, because of its synergistic interaction with light of shorter wavelengths.     

Can the progressive increase of C4 bundle sheath leakiness at low PFD be explained by incomplete suppression of photorespiration?

The ability to concentrate CO₂ around Rubisco allows C₄ crops to suppress photorespiration. However, as phosphoenolpyruvate regeneration requires ATP, the energetic efficiency of the C₄ pathway at low photosynthetic flux densities (PFD) becomes a balancing act between primary fixation and concentration of CO₂ in mesophyll (M) cells, and CO₂ reduction in bundle sheath (BS) cells. At low PFD, retro‐diffusion of CO₂ from BS cells, relative to the rate of bicarbonate fixation in M cells (termed leakiness φ), is known to increase. This paper investigates whether this increase in ? could be explained by incomplete inhibition of photorespiration. The PFD response of φ was measured at various O₂ partial pressures in young Zea mays plants grown at 250 (LL) and 750 µmol m^?2 s^?1 PFD (HL). φ increased at low PFD and was positively correlated with O₂ partial pressure. Low PFD during growth caused BS conductance and interveinal distance to be lower in the LL plants, compared to the HL plants, which correlated with lower φ. Model analysis showed that incomplete inhibition of photorespiration, especially in the HL plants, and an increase in the relative contribution of mitochondrial respiration at low PFD could explain the observed increases in φ.     

Reversible photoinhibition of unhardened and cold-acclimated spinach leaves at chilling temperatures

The photoinhibition of photosynthesis at chilling temperatures was investigated in cold-acclimated and unhardened (acclimated to +18° C) spinach (Spinacia oleracea L.) leaves. In unhardened leaves, reversible photoinhibition caused by exposure to moderate light at +4° C was based on reduced activity of photosystem (PS) II. This is shown by determination of quantum yield and capacity of electron transport in thylakoids isolated subsequent to photoinhibition and recovery treatments. The activity of PSII declined to approximately the same extent as the quantum yield of photosynthesis of photoinhibited leaves whereas PSI activity was only marginally affected. Leaves from plants acclimated to cold either in the field or in a growth chamber (+1° C), were considerably less susceptible to the light treatment. Only relatively high light levels led to photoinhibition, characterized by quenching of variable chlorophyll a fluorescence (F_V) and slight inhibition of PSII-driven electron transport. Fluorescence data obtained at 77 K indicated that the photoinhibition of cold-acclimated leaves (like that of the unhardened ones) was related to increased thermal energy dissipation. But in contrast to the unhardened leaves, 77 K fluorescence of cold-acclimated leaves did not reveal a relative increase of PSI excitation. High-light-treated, cold-acclimated leaves showed increased rates of dark respiration and a higher light compensation point. The photoinhibitory fluorescence quenching was fully reversible in low light levels both at +18° C and +4° C; the recovery was much faster than in unhardened leaves. Reversible photoinhibition is discussed as a protective mechanism against excess light based on transformation of PSII reaction centers to fluorescence quenchers.Abbreviations F_O initial fluorescence - F_M maximal fluorescence - F_V devariable fluorescence (f_m-f_o) - PFD photon flux density - PS photosystem - SD standard deviation The authors thank the Deutsche Forschungsgemeinschaft and the Academy of Finland for financial support.     

Response of the Photosynthetic Apparatus in Dunaliella salina (Green Algae) to Irradiance Stress

The response of the photosynthetic apparatus in the green alga Dunaliella salina, to irradiance stress was investigated. Cells were grown under physiological conditions at 500 millimoles per square meter per second (control) and under irradiance-stress conditions at 1700 millimoles per square meter per second incident intensity (high light, HL). In control cells, the light-harvesting antenna of photosystem I (PSI) contained 210 chlorophyll a/b molecules. It was reduced to 105 chlorophyll a/b in HL-grown cells. In control cells, the dominant form of photosystem II (PSII) was PSII_α(about 63% of the total PSII) containing >250 chlorophyll a/b molecules. The smaller antenna size PSII_β centers (about 37% of PSII) contained 135 ± 10 chlorophyll a/b molecules. In sharp contrast, the dominant form of PSII in HL-grown cells accounted for about 95% of all PSII centers and had an antenna size of only about 60 chlorophyll a molecules. This newly identified PSII unit is termed PSII_γ. The HL-grown cells showed a substantially elevated PSII/PSI stoichiometry ratio in their thylakoid membranes (PSII/PSI = 3.0/1.0) compared to that of control cells (PSII/PSI = 1.4/1.0). The steady state irradiance stress created a chronic photoinhibition condition in which D. salina thylakoids accumulate an excess of photochemically inactive PSII units. These PSII units contain both the reaction center proteins and the core chlorophyll-protein antenna complex but cannot perform a photochemical charge separation. The results are discussed in terms of regulatory mechanism(s) in the plant cell whose function is to alleviate the adverse effect of irradiance stress.