Effect Of Photoinhibition Research Articles

Spectroscopic analysis of sequestered chloroplasts in Elphidium williamsoni (Foraminifera).

Foraminifera are unicellular, marine organisms that occur worldwide. A very common species in the German Wadden Sea is Elphidium williamsoni. Some foraminifera (such as elphidia) are able to use kleptoplastidy, which allows them to incorporate chloroplasts from their algal food source into their own cell body. The experiments reported here are based on the fact that chlorophyll (a and c) can be detected in the intact cells with spectroscopic methods in the visible spectral range, which allows an indirect investigation of the presence of sequestered chloroplasts. Starving experiments of E. williamsoni in the light (24h continuous) showed that the greatest decrease in chlorophyll content was recorded within the first 20-30days. From day 60 on, chlorophyll was hardly detectable. Through subsequent feeding on a renewed algal food source a significant increase in the chlorophyll content in foraminifera was noticed. The degradation of chlorophyll in the dark (24h continuous darkness) during the starving period was much more complex. Chlorophyll was still detected in the cells after 113days of starving time. Therefore, we hypotheses that the effect of photoinhibition applies to chloroplasts in foraminifera under continuous illumination.

Bifurcation analysis of solutions to a nonlocal phytoplankton model under photoinhibition

<p style='text-indent:20px;'>We investigate the effect of photoinhibition in a nonlocal reaction-diffusion-advection equation, which models the dynamics of a single phytoplankton species in a water column where the growth of the species depends solely on light. First, for <inline-formula><tex-math id="M1">\begin{document}$ k_0 = 0 $\end{document}</tex-math></inline-formula>, we proved that system (1)-(3) forms a strongly monotone dynamical system with respect to a non-standard cone related to the cumulative distribution function. Second, local and global bifurcation theory are used to show that the model with photoinhibition possesses multiple steady-states with the change of parameter ranges.</p>

What causes the great green tide disaster in the South Yellow Sea of China in 2021?

The largest green tide in the South Yellow Sea (SYS) of China broke out in 2021 since the first outbreak in 2007. What causes the great outbreak has caused widespread concern. In this study, we monitor the green tide in the SYS in 2021 using Normalized Difference Vegetation Index and Google Earth Engine and then identify the main controlling factors from environmental change and human activity. In terms of human activity, Pyropia yezoensis (P. yezoensis) cultivation in Northern Jiangsu shoal of China has an impact on the scale of green tide. Compared with the same period in previous years, the early recycling of P. yezoensis cultivation rafts in 2021 provided more floating seed for green tide. The human activity intensity index in coastal cities in Jiangsu and Shandong provinces of China has been increasing for the past nine years, leading to large amounts of nutrients being released into seawater, providing green tide with the nutrients needed for growth. From the perspective of environmental changes, the sea surface temperature was higher in June and July in the SYS, which was favorable for green tide growth. The abundant solar radiation in May and July is conducive to photosynthesis of Ulva prolifera. The lower photosynthetically active radiation in August is conducive to reduce the effect of photoinhibition and promote algal growth. These findings are useful for further understanding the rules of green tide outbreaks and controlling green tide disasters.

Hydrogen peroxide can be a plausible biomarker in cyanobacterial bloom treatment

The effect of combined stresses, photoinhibition, and nutrient depletion on the oxidative stress of cyanobacteria was measured in laboratory experiments to develop the biomass prediction model. Phormidium ambiguum was exposed to various photosynthetically active radiation (PAR) intensities and phosphorous (P) concentrations with fixed nitrogen concentrations. The samples were subjected to stress assays by detecting the hydrogen peroxide (H2O2) concentration and antioxidant activities of catalase (CAT) and superoxide dismutase (SOD). H2O2 concentrations decreased to 30 µmol m−2 s−1 of PAR, then increased with higher PAR intensities. Regarding P concentrations, H2O2 concentrations (nmol L−1) generally decreased with increasing P concentrations. SOD and CAT activities were proportionate to the H2O2 protein−1. No H2O2 concentrations detected outside cells indicated the biological production of H2O2, and the accumulated H2O2 concentration inside cells was parameterized with H2O2 concentration protein−1. With over 30 µmol m−2 s−1 of PAR, H2O2 concentration protein−1 had a similar increasing trend with PAR intensity, independently of P concentration. Meanwhile, with increasing P concentration, H2O2 protein−1 decreased in a similar pattern regardless of PAR intensity. Protein content decreased with gradually increasing H2O2 up to 4 nmol H2O2 mg−1 protein, which provides a threshold to restrict the growth of cyanobacteria. With these results, an empirical formula—protein (mg L−1) = − 192*Log((H2O2/protein)/4.1), where H2O2/protein (nmol mg−1) = − 0.312*PAR2/(502 + PAR2)*((25/PAR)4 + 1)*Log(P/133,100), as a function of total phosphorus concentration, P (µg L−1)—was developed to obtain the cyanobacteria biomass.

Potential Impact of Photoinhibition on Microphytobenthic Primary Production on a Large Intertidal Mudflat

AbstractMicrophytobenthos (MPB) are a key primary producer of intertidal mudflats. MPB face strong variability in incident irradiance during low tides. Despite photoprotection and photoacclimation, such variations can translate into the photoinhibition of MPB cells. This study explores the effect of photoinhibition on MPB primary production (PP) over a large and productive temperate mudflat (Brouage mudflat, NW France). We used a regional and high‐resolution tri‐dimensional hydrodynamic model coupled to an MPB model with or without photoinhibition. Photoinhibition leads to a 20% (−0.79 × t C) decrease of the simulated MPB PP over the entire mudflat. As the upper shore is exposed to light more frequently and longer than the lower shore, the decrease of MPB PP is higher on the upper shore (−29%) than on the lower shore (−5%). With the highest photosynthetically active radiation cumulated over the mudflat, the decrease of MPB PP due to photoinhibition is the highest during spring and spring tides (−22% and −23%, respectively). The model suggests MPB photoinhibition is sensitive to the photoacclimation status of MPB cells through the light saturation parameter. This first modeling attempt to account for MPB photoinhibition is highly constrained by our current theoretical knowledge and limitations on the MPB growth physiology, but it suggests that this process can have a substantial impact on the MPB PP. As such, assessing the MPB photosynthetic response to the highly variable environmental conditions that prevail in large and productive intertidal mudflats is a real challenge for quantifying MPB PP from a synoptic to inter‐annual time scale.

Lipid profiling of symbiosomes in scleractinian coral in response to herbicide-induced photoinhibition

Photosystem II herbicides, such as Irgarol 1051, are frequently found as common contaminants in coral reefs and represent a particular threat to coral health due to the effect of photoinhibition on symbiotic algae. Membrane lipids form dynamic structures depending on cell requirements, offering a distinct lipid profile for characterizing ambient condition-induced cellular alterations. To obtain fundamental insight into the effect of herbicides on coral health, the glycerophosphocholine profiles of symbiosomes in a coral, Seriatopora caliendrum, were observed during 4 days of Irgarol exposure (50–200 ng/L). On day 1 of exposure, the predominant changes were an increase in phosphatidylcholines with 2 unsaturated chains, especially PC(22:6/22:6), a saturated plasmanylcholine PC(O-18:0/16:0) and a decrease in polyunsaturated plasmanylcholines. These changes indicated an upregulation of membrane-related functions to cope with physiological disturbances, such as increased H2O2 levels, induced by photoinhibition. However, these changes were mostly inverted or insignificant after day 1of exposure. A decrease in lyso-glycerophosphocholines and an increase in phosphatidylcholines with a polyunsaturated chain were also induced as the exposure time increased. Based on the physicochemical properties, these lipid changes can be considered to counteract lipid oxidation-induced perturbations in membrane structure and to protect against oxidative attack. In addition, the effect of fat-soluble antioxidants on lipid peroxidation was observed in the high-dose (200 ng/L)-treated group, resulting in a change in the lipid profile disproportionate to the dose. This study concludes that coral accommodates the membrane lipid profile of symbiosomes to herbicide-induced photoinhibition dependent on the duration of exposure.

Moderate Photoinhibition of Photosystem II Significantly Affects Linear Electron Flow in the Shade-Demanding Plant Panax notoginseng.

Although photoinhibition of photosystem II (PSII) frequently occurs under natural growing conditions, knowledge about the effect of moderate photoinhibition on linear electron flow (LEF) remains controversial. Furthermore, mechanisms underlying the decrease in LEF upon PSII photoinhibition are not well clarified. We examined how selective PSII photoinhibition influenced LEF in the attached leaves of shade-demanding plant Panax notoginseng. After leaves were exposed to a high level of light (2258 μmol photons m-2 s-1) for 30 and 60 min, the maximum quantum yield of PSII (Fv/Fm) decreased by 17 and 23%, respectively, whereas the maximum photo-oxidizable P700 (Pm) remained stable. Therefore, this species displayed selective PSII photodamage under strong illumination. After these treatments, LEF was significantly decreased under all light levels but acidification of the thylakoid lumen changed only slightly. Furthermore, the decrease in LEF under low light was positively correlated with the extent of PSII photoinhibition. Thus, the decline in LEF was not caused by the enhancement of lumenal acidification, but was induced by a decrease in PSII activity. These results indicate that residual PSII activity is an important determinant of LEF in this shade-adapted species, and they provide new insight into how strong illumination affects the growth of shade-demanding plants.

Modelling the Effect of Photoinhibition on Microalgal Production Potential in Fixed and Trackable Photobioreactors in Odisha, India

Microalgae have received a great deal of attention among researchers in recent decades in the production of sustainable bioenergy due to the limitations of second-generation biofuels. However, scalability and economics are the two key challenges that need to be overcome for sustainable biofuel production at field level, and mathematical modelling can be utilized as an effective tool to evaluate the influencing factors. This article focuses on the mathematical modelling of microalgal growth and carbon dioxide sequestration potential of a fixed photobioreactor (PBR) at 25° inclination facing south and a two-axis trackable PBR in Odisha, India. The total geographic area of Odisha has been represented in 1195 spatial sites, each site representing around 130 sq. km of the real scale dimensions approximately. The model incorporates site-specific data of solar radiation, climatic conditions and PBR configurations to derive the bioenergy content of microalgal biomass by photon energy balance. The effect of photoinhibition was also studied, and the outputs from the mathematical modelling, such as daily microalgal lipid production and carbon dioxide sequestration potential were plotted for the whole of Odisha using QGIS software. The net microalgal biomass production rate drastically reduced to around 30% and 40% due to the effect of photoinhibition in the case of fixed and trackable PBR systems respectively. The outcome of the present study could influence the policy-makers for selecting suitable sites for the implementation of microalgal-based biofuel production facility.

Repetitive light pulse-induced photoinhibition of photosystem I severely affects CO2 assimilation and photoprotection in wheat leaves.

It was previously found that photosystem I (PSI) photoinhibition represents mostly irreversible damage with a slow recovery; however, its physiological significance has not been sufficiently characterized. The aim of the study was to assess the effect of PSI photoinhibition on photosynthesis in vivo. The inactivation of PSI was done by a series of short light saturation pulses applied by fluorimeter in darkness (every 10s for 15min), which led to decrease of both PSI (~60%) and photosystem II (PSII) (~15%) photochemical activity. No PSI recovery was observed within 2days, whereas the PSII was fully recovered. Strongly limited PSI electron transport led to an imbalance between PSII and PSI photochemistry, with a high excitation pressure on PSII acceptor side and low oxidation of the PSI donor side. Low and delayed light-induced NPQ and P700(+) rise in inactivated samples indicated a decrease in formation of transthylakoid proton gradient (ΔpH), which was confirmed also by analysis of electrochromic bandshift (ECSt) records. In parallel with photochemical parameters, the CO2 assimilation was also strongly inhibited, more in low light (~70%) than in high light (~45%); the decrease was not caused by stomatal closure. PSI electron transport limited the CO2 assimilation at low to moderate light intensities, but it seems not to be directly responsible for a low CO2 assimilation at high light. In this regard, the possible effects of PSI photoinhibition on the redox signaling in chloroplast and its role in downregulation of Calvin cycle activity are discussed.

Collective Calculation of Actual Values of Non-Photochemical Quenching from Their Apparent Values after Chloroplast Movement and Photoinhibition

Chlorophyll fluorescence parameters such as Fv/Fm, NPQ and ΦII (YII) are widely used to estimate the fitness and photosynthetic activity of plant leaves and non-photochemical dissipation of excessive excitation energy in photosystem II. The effect of chloroplast movement on these fluorescence parameters reduces the accuracy of estimations of the size of de-excitation processes, but there is no method to calculate correct parameters from altered (fluctuated) parameters. Chloroplast movement was recently identified as the “middle” kinetic component of NPQ. In this paper, we devised a complex but reasonable mathematical method to remove the effect of chloroplast movement on fluorescence parameters, based on our previously reported fluorescence theory. The fraction of “S fluctuation” (designated as σ) was estimated from fluorescence observations and used to calculate the non-fluctuated Fs and F′m. fluorescence yields. From the σ values, the fractional change of light absorbance by a leaf caused by chloroplast movement was estimated at 70% - 100%, which varied according to the experimental conditions and plant species. The effect of photoinhibition on fluorescence parameters was also examined in this paper. The photochemical and non-photochemical de-excitation sizes during photoinhibition (measured by the parameters qPI and qSlow) changed on a single regression line. Using this correlation, qPI and qSlow can be predicted from Fv/Fm, and the non-fluctuated Fm and Fo values can be estimated from the fluctuated F″m and F″o values.

Multiple steady-states in phytoplankton population induced by photoinhibition

We study the effect of photoinhibition in a nonlocal reaction–diffusion–advection equation, which models the dynamics of a single phytoplankton species in a water column where the growth of the species depends solely on light. Our results show that, in contrast to the case of no photoinhibition, where at most one positive steady state can exist, the model with photoinhibition possesses at least two positive steady states in certain parameter ranges. Our approach involves bifurcation theory and perturbation–reduction arguments.

Growth Rate Estimation of algae in Raceway Ponds: A novel Approach

Microalgae mass cultiviation is a promising future source of biomass for energy and food production. In order to optimize productivity of large scale plants and to make them environmentally and economically sustainable, energy requirements have to be minimized. In particular, mixing of the growth medium is a major energy input, and its effect on overall productivity should be better understood. Several dynamic models have been developed to represent the effect of a rapidly varying light on the photosynthesis process especially for the effect of photoinhibition on growth. In order to assess the mixing effects in a complex hydrodynamic regime, we propose to reconstruct the light profile received by a single cell. A multi-layer Saint-Venant approach is used to simulate the hydrodynamics of the system. It allows for the computation of Lagrangian trajectories, and finally, when knowing the light distribution, the light pattern perceived by a cell. This pattern is then used with the dynamical model for photosynthesis. In a last step, the growth rate of the whole system is estimated as the average over a set of trajectories.

Implications of geometric plasticity for maximizing photosynthesis in branching corals

Reef-building corals are an example of plastic photosynthetic organisms that occupy environments of high spatiotemporal variations in incident irradiance. Many phototrophs use a range of photoacclimatory mechanisms to optimize light levels reaching the photosynthetic units within the cells. In this study, we set out to determine whether phenotypic plasticity in branching corals across light habitats optimizes potential light utilization and photosynthesis. In order to do this, we mapped incident light levels across coral surfaces in branching corals and measured the photosynthetic capacity across various within-colony surfaces. Based on the field data and modelled frequency distribution of within-colony surface light levels, our results show that branching corals are substantially self-shaded at both 5 and 18 m, and the modal light level for the within-colony surface is 50 mu mol photons m(-2) s(-1). Light profiles across different locations showed that the lowest attenuation at both depths was found on the inner surface of the outermost branches, while the most self-shading surface was on the bottom side of these branches. In contrast, vertically extended branches in the central part of the colony showed no differences between the sides of branches. The photosynthetic activity at these coral surfaces confirmed that the outermost branches had the greatest change in sun- and shade-adapted surfaces; the inner surfaces had a 50 % greater relative maximum electron transport rate compared to the outer side of the outermost branches. This was further confirmed by sensitivity analysis, showing that branch position was the most influential parameter in estimating whole-colony relative electron transport rate (rETR). As a whole, shallow colonies have double the photosynthetic capacity compared to deep colonies. In terms of phenotypic plasticity potentially optimizing photosynthetic capacity, we found that at 18 m, the present coral colony morphology increased the whole-colony rETR, while at 5 m, the colony morphology decreased potential light utilization and photosynthetic output. This result of potential energy acquisition being underutilized in shallow, highly lit waters due to the shallow type morphology present may represent a trade-off between optimizing light capture and reducing light damage, as this type morphology can perhaps decrease long-term costs of and effect of photoinhibition. This may be an important strategy as opposed to adopting a type morphology, which results in an overall higher energetic acquisition. Conversely, it could also be that maximizing light utilization and potential photosynthetic output is more important in low-light habitats for Acropora humilis.

The Psb32 Protein Aids in Repairing Photodamaged Photosystem II in the Cyanobacterium Synechocystis 6803

Photosystem II (PSII), a membrane protein complex, catalyzes the photochemical oxidation of water to molecular oxygen. This enzyme complex consists of approximately 20 stoichiometric protein components. However, due to the highly energetic reactions it catalyzes as part of its normal activity, PSII is continuously damaged and repaired. With advances in protein detection technologies, an increasing number of sub-stoichiometric PSII proteins have been identified, many of which aid in the biogenesis and assembly of this protein complex. Psb32 (Sll1390) has previously been identified as a protein associated with highly active purified PSII preparations from the cyanobacterium Synechocystis sp. PCC 6803. To investigate its function, the subcellular localization of Psb32 and the impact of deletion of the psb32 gene on PSII were analyzed. Here, we show that Psb32 is an integral membrane protein, primarily located in the thylakoid membranes. Although not required for cell viability, Psb32 protects cells from oxidative stress and additionally confers a selective fitness advantage in mixed culture experiments. Specifically, Psb32 protects PSII from photodamage and accelerates its repair. Thus, the data suggest that Psb32 plays an important role in minimizing the effect of photoinhibition on PSII.

The Function of Trichomes of an Amphibious Fern, Marsilea quadrifolia

Marsilea quadrifolia, an amphibious fern, has the ability to develop heterophyllous, aerial and submerged leaves. In contrast to submerged leaves, aerial leaves have trichomes on both surfaces. To examine if the presence of trichomes can reflect excess light and hence reduce the risk of being damaged by excess light, we compared the optical properties and chlorophyll a fluorescence, in particular the maximum PSII photochemical efficiency (Fv/Fm), of M. quadrifolia leaves with trichomes (intact) and those having trichomes removed (de-trichomed). Photosynthetic gas exchange measurements were also conducted to quantify transpirational water loss and instantaneous water use efficiency (WUE) of M. quadrifolia intact and de-trichomed leaves. The results showed that removal of trichomes neither affected the optical properties in the visible part of the solar spectrum nor the midday depression of Fv/Fm values of leaflets. In contrast, significantly increased in transpiration rates and decreased rates in WUE were found in de- trichomed leaflets in comparison to intact ones. These results imply that the presence of trichomes is of more importance in reducing water loss than in reflecting light and protecting M. quadrifolia against the potentially damaging effect of photoinhibition in aerial environments.

Effects of photoinhibition on whole‐plant carbon gain assessed with a photosynthesis model

ABSTRACTA canopy photosynthesis model was modified to assess the effect of photoinhibition on whole‐plant carbon gain. Photoinhibitory changes in maximum quantum yield of photosystem II (Fv/Fm) could be explained solely from a parameter (Lflux) calculated from the light micro‐environment of the leaves. This relationship between Fv/Fm and the intercepted cumulative light dose, integrated and equally weighted over several hours was incorporated into the model. The effect of photoinhibition on net photosynthesis was described through relationships between photoinhibition and the shaping parameters of the photosynthetic light‐response curve (quantum use efficiency, convexity, and maximum capacity). This new aspect of the model was then validated by comparing measured field data (diurnal courses of Fv/Fm) with simulation results. Sensitivity analyses revealed that the extent of photoinhibitory reduction of whole‐plant photosynthesis was strongly dependent on the structural parameters (LAI and leaf angle). Simulations for a Mediterranean evergreen oak, Quercus coccifera, under climatic conditions which cause mild photoinhibition revealed a daily loss of 7·5–8·5% of potential carbon gain in the upper sunlit canopy layers, a 3% loss in the bottom canopy, and an overall loss of 6·1%. Thus, this canopy photoinhibition model (CANO‐PI) allows the quantitative evaluation of photoinhibition effects on primary production.

Do oxidative stress conditions impairing photosynthesis in the light manifest as photoinhibition?

We compared the effect of photoinhibition by excess photosynthetically active radiation (PAR), UV-B irradiation combined with PAR, low temperature stress and paraquat treatment on photosystem (PS) II. Although the experimental conditions ensured that the four studied stress conditions resulted in approximately the same extent of PS II inactivation, they clearly followed different molecular mechanisms. Our results show that singlet oxygen production in inactivated PS II reaction centres is a unique characteristic of photoinhibition by excess PAR. Neither the accumulation of inactive PS II reaction centres (as in UV-B or chilling stress), nor photo-oxidative damage of PS II (as in paraquat stress) is able to produce the special oxidizing conditions characteristic of acceptor-side-induced photoinhibition.

Effect of photoinhibition on algal photosynthesis: a dynamic model

Recent evidence from algal physiology and molecular biology confirms that photoinhibi- tion is directly related to D1 protein damage and recovery, and D1 protein damage leads to a decrease in electron transfer or an increase in turnover time of the electron transfer chain. In this study, the turnover time of the electron transfer chain is defined as a function of the relative concentration of D1 protein in reaction centre II and the photoinhibition processes due to D1 protein degradation are incorporated into a model of photosynthesis, initiated by Dubinsky et al. (Plant Cell Physiol., 27, 1335-1349, 1986) and developed by Sakshaug et al. (Limnol. Oceanogr., 34, 198-205, 1989). D1 protein damage is assumed to be both light and D1 protein concentration dependent, and to be proportional to the cross-section of PSII (σ PSII). D1 protein recovery is only D1 protein concentration dependent. Two parameters, the damage constant (kd) and recovery rate (kr), are introduced to formulate the dynamics of D1 protein concentration. Setting minimal turnover time τm � 3 ms, maximal quantum yield φm � 0.1 mol O2 E-1, optical cross-section a* � 0.0075 m2 (mg Chl a)-1, σ PSII � 2n m 2, kd ranging from 0 to 10-7 and kr � 0.55 h-1, the dynamic effect of photoinhibition on photo- synthesis in natural conditions is simulated. Simulation results are consistent with observations. Some aspects of photosynthesis associated with photoinhibition, e.g. nutrient limitation and the interaction between photoinhibition and photoadaptation, are investigated through this dynamic model.

Photolimitation and photoinhibition as factors determining optimal dilution rate to produce eicosapentaenoic acid from cultures of the microalga Isochrysis galbana

Eicosapentaenoic acid (EPA) productivity from continuous cultures of the marine microalga Isochrysis galbana was studied, taking into account the irradiance on the reactor surface, that is, the photolimitation/photoinhibition regime to which the cells are exposed. Experiments were conducted under a wide variety of operating conditions. The dilution rate ranged from 0.005 h−1 to 0.040 h−1 at five external irradiances (820, 1620, 2050, 2450 and 3270 μmol photons m−2 s−1) covering photolimited to photoinhibited growth. Under these conditions, the specific growth rate (μ) was found to be the main factor influencing EPA content (ranging from 2.35% to 5.23% dryweight) and productivity (up to 0.88 mg l−1 h−1). The fatty acid content was not significantly affected by the external irradiance, but was influenced by the state of growth of the microalga, depending on whether the light regime was photolimiting or photoinhibiting. It might be suggested that light should no longer be considered an isolated factor affecting EPA synthesis, but an indirect influence through the photolimitation/photoinhibition regime and growth rate. At a given dilution rate, EPA content and biomass concentration are lower under photoinhibiting external irradiances than those corresponding to photolimiting conditions, and consequently EPA productivity decays. Since the effect of photoinhibition is less marked at high biomass concentration, a strategy to optimize EPA productivity from microalgal cultures could consist of reducing the dilution rate when the external irradiance increases above the phoinhibition threshold.

Water stress and photoinhibition in acclimatization ofRosa hybrida plantlets

MicropropagatedRosa hybrida plantlets were simultaneously rooted and acclimatized under 100 and 200 μmol m−2 s−1 light for 2 wk. At the end of the first week of acclimatization, the plantlets were transferred onto a low water potential medium (from −0.06 MPa to −0.3 MPa). Dry weight was decreased by increased hight and low water potential. Photoinhibition of photosynthesis, expressed as a decrease in Fv/Fm ratio and ΦPSII and an increase in 1 −qp, occurred in plants grown under 200 μmol m−2 s−1. When high light (200 μmol m−2 s−1) and water stress were applied simultaneously, their effects on chlorophyll fluorescence parameters depended on stress duration; after 1 d of water stress, photoinhibition was more pronounced; after 7 d of stress, Fv/Fm ratio and ΦPSII were higher than after 1 d of stress; photoinhibition was reduced. This suggests that after a 1-d stress, the effect of water stress alone included a superimposed effect of photoinhibition to which the water-stressed plants were sensitized; after 7 d, plantlets had adapted to water stress. The photoprotective effects under high light might result in energy dissipative mechanisms linked to photochemical and nonphotochemical quenching other than CO2 fixation.