Fraction Of Open Reaction Centers Research Articles

Enhanced photochemical efficiency of PSII in Prosopis juliflora suggests contribution to invasion advantage over native C3 xero-halophytes under salt stress.

Chlorophyll a fluorescence parameters related to PSII photochemistry, photoprotection and photoinhibition were investigated in four C3 plant species growing in their natural habitat: Prosopis juliflora ; Abutilon indicum ; Salvadora persica ; and Phragmites karka . This study compared the light reaction responses of P. juliflora , an invasive species, with three native co-existing species, which adapt to varying water deficit and high salt stress. Chlorophyll a fluorescence quenching analyses revealed that P. juliflora had the highest photochemical quantum efficiency and yield, regulated by higher fraction of open reaction centres and reduced photoprotective energy dissipation without compromising the integrity of photosynthetic apparatus due to photoinhibition. Moreover, the elevated values of parameters obtained through polyphasic chlorophyll a fluorescence induction kinetics, which characterise the photochemistry of PSII and electron transport, highlighted the superior performance index of energy conservation in the transition from excitation to the reduction of intersystem electron carriers for P. juliflora compared to other species. Enhanced pigment contents and their stoichiometry in P. juliflora apparently contributed to upregulating fluxes and yields of energy absorbance, trapping and transport. This enhanced photochemistry, along with reduced non-photochemical processes, could explain the proclivity for invasion advantage in P. juliflora across diverse stress conditions.

Validating and Developing Hyperspectral Indices for Tracing Leaf Chlorophyll Fluorescence Parameters under Varying Light Conditions

Chlorophyll a fluorescence (ChlFa) parameters provide insight into the physiological and biochemical processes of plants and have been widely applied to monitor and evaluate the photochemical process and photosynthetic capacity of plants in a variety of environments. Recent advances in remote sensing provide new opportunities for the detection of ChlFa at large scales but demand further tremendous efforts. Among such efforts, application of the hyperspectral index is always possible, but the performance of hyperspectral indices in detecting ChlFa parameters under varying light conditions is much less investigated. The objective of this study is to investigate the performance of reported hyperspectral indices for tracking ChlFa parameters under different light conditions and to develop and evaluate novel spectral indices. Therefore, an experiment was conducted to simultaneously measure ChlFa parameters and spectral reflectance of sunlit and shaded leaves under varying light conditions, and 28 reported hyperspectral indices were examined for their performance in tracking the ChlFa parameters. Furthermore, we developed novel hyperspectral indices based on various spectral transformations. The results indicated that the maximum quantum efficiency of photosystem II (PSIImax), the cumulative quantum yield of photochemistry (ΦP), and the fraction of open reaction centers in photosystem II (qL) of sunlit leaves were significantly higher than those of shaded leaves, while the cumulative quantum yield of regulated thermal dissipation (ΦN) and fluorescence (ΦF) of shaded leaves was higher than that of sunlit leaves. Efficient tracing of ChlFa parameters could not be achieved from previously published spectral indices. In comparison, all ChlFa parameters were well quantified in shaded leaves when using novel hyperspectral indices, although the hyperspectral indices for tracing the non-photochemical quenching (NPQ) and ΦF were not stable, especially for sunlit leaves. Our findings justify the use of hyperspectral indices as a practical approach to estimating ChlFa parameters. However, caution should be used when using spectral indices to track ChlFa parameters based on the differences in sunlit and shaded leaves.

Early Drought Stress Warning in Plants: Color Pictures of Photosystem II Photochemistry

Drought, the major limiting factor for plant growth and crop productivity, affecting several physiological and biochemical processes, is expected to increase in duration, intensity, and frequency as a consequence of climate change. Plants have developed several approaches to either avoid or tolerate water deficit. Plants as a response to drought stress (DS), close stomata, reducing carbon dioxide (CO2) entry in the leaf, thus decreasing photosynthesis which results in reduced synthesis of essential organic molecules that sustain the life on earth. The reduced CO2 fixation, decreases electron transport rate (ETR), while the absorbed light energy overdoes what can be used for photochemistry resulting in excess reactive oxygen species (ROS) and oxidative stress. Current imaging techniques allow non-destructive monitoring of changes in the physiological state of plants under DS. Thermographic visualization, near-infrared imaging, and chlorophyll a fluorescence imaging are the most common verified imaging techniques for detecting stress-related changes in the display of light emission from plant leaves. Chlorophyll a fluorescence analysis, by use of the pulse amplitude modulation (PAM) method, can principally calculate the amount of absorbed light energy that is directed for photochemistry in photosystem II (PSII) (ΦPSII), dissipated as heat (ΦNPQ), or dissipated by the non-radiative fluorescence processes (ΦNO). The method of chlorophyll a fluorescence imaging analysis by providing colour pictures of the whole leaf PSII photochemistry, can successfully identify the early drought stress warning signals. Its implementation allowed visualization of the leaf spatial photosynthetic heterogeneity and discrimination between mild drought stress (MiDS), moderate drought stress (MoDS), and severe drought stress (SDS). The fraction of open reaction centers of PSII (qp) is suggested as the most sensitive and suitable indicator of an early drought stress warning and also for selecting drought tolerant cultivars.

The Protective Role of Non-Photochemical Quenching in PSII Photo-Susceptibility: A Case Study in the Field.

Non-photochemical quenching (NPQ) has been regarded as a safety valve to dissipate excess absorbed light energy not used for photochemistry. However, there exists no general consensus on the photoprotective role of NPQ. In the present study, we quantified the Photosystem II (PSII) photo-susceptibilities (mpi) in the presence of lincomycin, under red light given to five shade-acclimated tree species grown in the field. Photosynthetic energy partitioning theory was applied to investigate the relationships between mpi and each of the regulatory light-induced NPQ [Y(NPQ)], the quantum yield of the constitutive nonregulatory NPQ [Y(NO)] and the PSII photochemical yield in the light-adapted state [Y(PSII)] under different red irradiances. It was found that in the low to moderate irradiance range (50-800 μmol m-2 s-1) when the fraction of open reaction centers (qP) exceeded 0.4, mpi exhibited no association with Y(NPQ), Y(NO) and Y(PSII) across species. However, when qP < 0.4 (1,500 μmol m-2 s-1), there existed positive relationships between mpi and Y(NPQ) or Y(NO) but a negative relationship between mpi and Y(PSII). It is postulated that both Y(NPQ) and Y(NO) contain protective and damage components and that using only Y(NPQ) or Y(NO) metrics to identify the photo-susceptibility of a species is a risk. It seems that qP regulates the balance of the two components for each of Y(NPQ) and Y(NO). Under strong irradiance, when both protective Y(NPQ) and Y(NO) are saturated/depressed, the forward electron flow [i.e. Y(PSII)] acts as the last defense to resist photoinhibition.

Harnessing Chlorophyll Fluorescence for Phenotyping Analysis of Wild and Cultivated Tomato for High Photochemical Efficiency under Water Deficit for Climate Change Resilience

Fluctuations of the weather conditions, due to global climate change, greatly influence plant growth and development, eventually affecting crop yield and quality, but also plant survival. Since water shortage is one of the key risks for the future of agriculture, exploring the capability of crop species to grow with limited water is therefore fundamental. By using chlorophyll fluorescence analysis, we evaluated the responses of wild tomato accession Solanum pennellii LA0716, Solanum lycopersicum cv. Μ82, the introgression line IL12-4 (from cv. M82 Χ LA0716), and the Greek tomato cultivars cv. Santorini and cv. Zakinthos, to moderate drought stress (MoDS) and severe drought stress (SDS), in order to identify the minimum irrigation level for efficient photosynthetic performance. Agronomic traits (plant height, number of leaves and root/shoot biomass), relative water content (RWC), and lipid peroxidation, were also measured. Under almost 50% deficit irrigation, S. pennellii exhibited an enhanced photosynthetic function by displaying a hormetic response of electron transport rate (ETR), due to an increased fraction of open reaction centers, it is suggested to be activated by the low increase of reactive oxygen species (ROS). A low increase of ROS is regarded to be beneficial by stimulating defense responses and also triggering a more oxidized redox state of quinone A (QA), corresponding in S. pennellii under 50% deficit irrigation, to the lowest stomatal opening, resulting in reduction of water loss. Solanumpennellii was the most tolerant to drought, as it was expected, and could manage to have an adequate photochemical function with almost 30% water regime of well-watered plants. With 50% deficit irrigation, cv. Μ82 and cv. Santorini did not show any difference in photochemical efficiency to control plants and are recommended to be cultivated under deficit irrigation as an effective strategy to enhance agricultural sustainability under a global climate change. We conclude that instead of the previously used Fv/Fm ratio, the redox state of QA, as it can be estimated by the chlorophyll fluorescence parameter 1 - qL, is a better indicator to evaluate photosynthetic efficiency and select drought tolerant cultivars under deficit irrigation.

The roles of photochemical and non-photochemical quenching in regulating photosynthesis depend on the phases of fluctuating light conditions.

The induction and relaxation of photochemistry and non-photochemical quenching (NPQ) are not instantaneous and require time to respond to fluctuating environments. There is a lack of integrated understanding on how photochemistry and NPQ influence photosynthesis in fluctuating environments. We measured the induction and relaxation of chlorophyll a fluorescence and gas exchange in poplar and cotton at varying temperatures under saturating and fluctuating lights. When the light shifted from dark to high, the fraction of open reaction centers in photosystem II (qL) gradually increased while NPQ increased suddenly and then remained stable. Temperature significantly changed the response of qL but not that of NPQ during the dark to high light transition. Increased qL led to higher photosynthesis but their precise relationship was affected by NPQ and temperature. qL was significantly related to biochemical capacity. Thus, qL appears to be a strong indicator of the activation of carboxylase, leading to the similar dynamics between qL and photosynthesis. When the light shifted from high to low intensity, NPQ is still engaged at a high level, causing a stronger decline in photosynthesis. Our finding suggests that the dynamic effects of photochemistry and NPQ on photosynthesis depend on the phases of environmental fluctuations and interactive effects of light and temperature. Across the full spectra of light fluctuation, the slow induction of qL is a more important limiting factor than the slow relaxation of NPQ for photosynthesis in typical ranges of temperature for photosynthesis. The findings provided a new perspective to improve photosynthetic productivity with molecular biology under natural fluctuating environments.

Changes in Light Energy Utilization in Photosystem II and Reactive Oxygen Species Generation in Potato Leaves by the Pinworm Tuta absoluta.

We evaluated photosystem II (PSII) functionality in potato plants (Solanum tuberosum L.) before and after a 15 min feeding by the leaf miner Tuta absoluta using chlorophyll a fluorescence imaging analysis combined with reactive oxygen species (ROS) detection. Fifteen minutes after feeding, we observed at the feeding zone and at the whole leaf a decrease in the effective quantum yield of photosystem II (PSII) photochemistry (ΦPSII). While at the feeding zone the quantum yield of regulated non-photochemical energy loss in PSII (ΦNPQ) did not change, at the whole leaf level there was a significant increase. As a result, at the feeding zone a significant increase in the quantum yield of non-regulated energy loss in PSII (ΦNO) occurred, but there was no change at the whole leaf level compared to that before feeding, indicating no change in singlet oxygen (1O2) formation. The decreased ΦPSII after feeding was due to a decreased fraction of open reaction centers (qp), since the efficiency of open PSII reaction centers to utilize the light energy (Fv′/Fm′) did not differ before and after feeding. The decreased fraction of open reaction centers resulted in increased excess excitation energy (EXC) at the feeding zone and at the whole leaf level, while hydrogen peroxide (H2O2) production was detected only at the feeding zone. Although the whole leaf PSII efficiency decreased compared to that before feeding, the maximum efficiency of PSII photochemistry (Fv/Fm), and the efficiency of the water-splitting complex on the donor side of PSII (Fv/Fo), did not differ to that before feeding, thus they cannot be considered as sensitive parameters to monitor biotic stress effects. Chlorophyll fluorescence imaging analysis proved to be a good indicator to monitor even short-term impacts of insect herbivory on photosynthetic function, and among the studied parameters, the reduction status of the plastoquinone pool (qp) was the most sensitive and suitable indicator to probe photosynthetic function under biotic stress.

Evaluation of the spatiotemporal effects of bisphenol A on the leaves of the seagrass Cymodocea nodosa

The organic pollutant bisphenol A (BPA) causes adverse effects on aquatic biota. The present study explored the toxicity mechanism of environmentally occurring BPA concentrations (0.03–3 μg L−1) on the seagrass Cymodocea nodosa intermediate leaf photosynthetic machinery. A "mosaic" type BPA effect pattern was observed, with "unaffected" and "affected”" leaf areas. In negatively affected leaf areas cells had a dark appearance and lost their chlorophyll auto-fluorescence, while hydrogen peroxide (H2O2) content increased time-dependently. In the "unaffected" leaf areas, cells exhibited increased phenolic compound production. At 1 μg L−1 of BPA exposure, there was no effect on the fraction of open reaction centers (qP) compared to control and also no significant effect on the quantum yield of non-regulated non-photochemical energy loss in PSII (ΦΝΟ). However, a 3 μg L−1 BPA application resulted in a significant ΦΝΟ increase, even from the first exposure day. Ultrastructural observations revealed electronically dense damaged thylakoids in the plastids, while effects on Golgi dictyosomes and the endoplasmic reticulum were also observed at 3 μg L−1 BPA. The up-regulated H2O2 BPA-derived production seems to be a key factor causing both oxidative damages but probably also triggering retrograde signalling, conferring tolerance to BPA in the "unaffected" leaf areas.

Arbuscular Mycorrhizal Symbiosis Enhances Photosynthesis in the Medicinal Herb Salvia fruticosa by Improving Photosystem II Photochemistry.

We investigated the influence of Salvia fruticosa colonization by the arbuscular mycorrhizal fungi (AMF) Rhizophagus irregularis on photosynthetic function by using chlorophyll fluorescence imaging analysis to evaluate the light energy use in photosystem II (PSII) of inoculated and non-inoculated plants. We observed that inoculated plants used significantly higher absorbed energy in photochemistry (ΦPSII) than non-inoculated and exhibited significant lower excess excitation energy (EXC). However, the increased ΦPSII in inoculated plants did not result in a reduced non-regulated energy loss in PSII (ΦNO), suggesting the same singlet oxygen (1O2) formation between inoculated and non-inoculated plants. The increased ΦPSII in inoculated plants was due to an increased efficiency of open PSII centers to utilize the absorbed light (Fv’/Fm’) due to a decreased non-photochemical quenching (NPQ) since there was no difference in the fraction of open reaction centers (qp). The decreased NPQ in inoculated plants resulted in an increased electron-transport rate (ETR) compared to non-inoculated. Yet, inoculated plants exhibited a higher efficiency of the water-splitting complex on the donor side of PSII as revealed by the increased Fv/Fo ratio. A spatial heterogeneity between the leaf tip and the leaf base for the parameters ΦPSII and ΦNPQ was observed in both inoculated and non-inoculated plants, reflecting different developmental zones. Overall, our findings suggest that the increased ETR of inoculated S. fruticosa contributes to increased photosynthetic performance, providing growth advantages to inoculated plants by increasing their aboveground biomass, mainly by increasing leaf biomass.

Spatial Heterogeneity of Cadmium Effects on Salvia sclarea Leaves Revealed by Chlorophyll Fluorescence Imaging Analysis and Laser Ablation Inductively Coupled Plasma Mass Spectrometry.

In this study, for a first time (according to our knowledge), we couple the methodologies of chlorophyll fluorescence imaging analysis (CF-IA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), in order to investigate the effects of cadmium (Cd) accumulation on photosystem II (PSII) photochemistry. We used as plant material Salvia sclarea that grew hydroponically with or without (control) 100 μM Cd for five days. The spatial heterogeneity of a decreased effective quantum yield of electron transport (ΦPSΙΙ) that was observed after exposure to Cd was linked to the spatial pattern of high Cd accumulation. However, the high increase of non-photochemical quenching (NPQ), at the leaf part with the high Cd accumulation, resulted in the decrease of the quantum yield of non-regulated energy loss (ΦNO) even more than that of control leaves. Thus, S. sclarea leaves exposed to 100 μM Cd exhibited lower reactive oxygen species (ROS) production as singlet oxygen (1O2). In addition, the increased photoprotective heat dissipation (NPQ) in the whole leaf under Cd exposure was sufficient enough to retain the same fraction of open reaction centers (qp) with control leaves. Our results demonstrated that CF-IA and LA-ICP-MS could be successfully combined to monitor heavy metal effects and plant tolerance mechanisms.

Zinc Uptake, Photosynthetic Efficiency and Oxidative Stress in the Seagrass Cymodocea nodosa Exposed to ZnO Nanoparticles.

We characterized zinc oxide nanoparticles (ZnO NPs) by dynamic light scattering (DLS) measurements, and transmission electron microscopy (TEM), while we evaluated photosystem II (PSII) responses, Zn uptake kinetics, and hydrogen peroxide (H2O2) accumulation, in C. nodosa exposed to 5 mg L−1 and 10 mg L−1 ZnO NPs for 4 h, 12 h, 24 h, 48 h and 72 h. Four h after exposure to 10 mg L−1 ZnO NPs, we noticed a disturbance of PSII functioning that became more severe after 12 h. However, after a 24 h exposure to 10 mg L−1 ZnO NPs, we observed a hormetic response, with both time and dose as the basal stress levels needed for induction of the adaptive response. This was achieved through the reduced plastoquinone (PQ) pool, at a 12 h exposure, which mediated the generation of chloroplastic H2O2; acting as a fast acclimation signaling molecule. Nevertheless, longer treatment (48 h and 72 h) resulted in decreasing the photoprotective mechanism to dissipate excess energy as heat (NPQ) and increasing the quantum yield of non-regulated energy loss (ΦNO). This increased the formation of singlet oxygen (1O2), and decreased the fraction of open reaction centers, mostly after a 72-h exposure at 10 mg L−1 ZnO NPs due to increased Zn uptake compared to 5 mg L−1.

Copper bioaccumulation, photosystem II functioning, and oxidative stress in the seagrass Cymodocea nodosa exposed to copper oxide nanoparticles.

Photosynthetic activity, oxidative stress, and Cu bioaccumulation in the seagrass Cymodocea nodosa were assessed 4, 12, 24, 48, and 72h after exposure to two copper oxide nanoparticle (CuO NP) concentrations (5 and 10mgL-1). CuO NPs were characterized by scanning electron microscopy (SEM) and dynamic light scattering measurements (DLS). Chlorophyll fluorescence analysis was applied to detect photosystem II (PSII) functionality, while the Cu accumulation kinetics into the leaf blades was fitted to the Michaelis-Menten equation. The uptake kinetics was rapid during the first 4h of exposure and reached an equilibrium state after 10h exposure to 10mgL-1 and after 27h to 5mgL-1 CuO NPs. As a result, 4-h treatment with 5mgL-1 CuO NPs, decreased the quantum yield of PS II photochemistry (Φ PSΙΙ ) with a parallel increase in the regulated non-photochemical energy loss in PSII (Φ NPQ ). However, the photoprotective dissipation of excess absorbed light energy as heat, through the process of non-photochemical quenching (NPQ), did not maintain the same fraction of open reaction centers (q p ) as in control plants. This reduced number of open reaction centers resulted in a significant increase of H2O2 production in the leaf veins serving possibly as an antioxidant defense signal. Twenty-four-hour treatment had no significant effect on Φ PSΙΙ and q p compared to controls. However, 24h exposure to 5mgL-1 CuO NPs increased the quantum yield of non-regulated energy loss in PSII (Φ NO ), and thus the formation of singlet oxygen (1O2) via the triplet state of chlorophyll, possible because the uptake kinetics had not yet reached the equilibrium state as did 10mgL-1. Longer-duration treatment (48 and 72h) had less effect on the allocation of absorbed light energy at PSII and the fraction of open reaction centers, compared to 4-h treatment, suggesting the function of a stress defense mechanism. The response of C. nodosa leaves to CuO NPs fits the "Threshold for Tolerance Model" with a threshold time (more than 4h) required for induction of a stress defense mechanism, through H2O2 production.

Molecular iodine emission rates and photosynthetic performance of different thallus parts of Laminaria digitata (Phaeophyceae) during emersion.

The emission of molecular iodine (I(2)) from the stipe, the meristematic area and the distal blade of the brown macroalga Laminaria digitata (Hudson) Lamouroux (Phaeophyceae) was monitored under low light and dark conditions. Photosynthetic parameters were determined to investigate both the extent of stress experienced by different thallus parts and the effects of emersion on photosynthesis. Immediately after air exposure, intense I(2) emission was detectable from all thallus parts. I(2) emission declined continuously over a period of 180 min following the initial burst, but was not affected by the light regime. The total number of mole of I(2) emitted by stipes was approximately 10 times higher than those emitted from other thallus parts. Initial I(2) emission rates (measured within 30 min of exposure to air) were highest for stipes (median values: 2,999 and 5,222 pmol g(-1) dw min(-1) in low light and dark, respectively) and lower, by one order of magnitude, for meristematic regions and distal blades. After exposure to air for between 60 and 180 min, I(2) emission rates of all thallus parts were reduced by 70-80%. Air exposure resulted in a decrease of the maximum photosystem II (PSII) efficiency (F(v)/F(m)) by 3%, and in a 25-55% increase of the effective PSII quantum efficiency (F(v)/F'(m)); this was caused by a higher fraction of open reaction centres (q(P)), whereas the efficiency of the latter in capturing energy (F'(v)/F'(m)) remained constant. The results indicate the presence of an iodine pool which is easily volatilised and depleted due to air exposure, even under apparently low stress conditions.

Influenza della luce e dell’umidità del suolo sulla crescita e fisiologia della rinnovazione di farnia nelle radure di querceti misti

In “S. Rossore, Migliarino, Massaciuccoli” Natural Park (Pisa, I) six-month-old pedunculate oak seedlings (Quercus robur L.) were transplanted within natural gaps of a mixed oak forest. Micro-environmental variability for radiation and water soil content were measured for 145 seedlings during the year. Irradiation relative to the open field (IR) ranged from 5% to 57%. Seven classes of IR each with 20 seedlings were selected. Leaf mass per area was strongly influenced by IR. In the first 3 years survival was high (95, 76 and 75%, respectively) and seedling reached 14±6 cm, 27±13 cm and 39±19 (sd) cm of height. Even if IR and soil water content (SWC) were negatively associated, indicating a lower SWC at the centre of the gaps, height and its relative growth rate increased with IR (explored range: 8-40%) with a significant interaction with SWC in the 1st year, indicating the positive effect of soil moisture. In the 3rd year dimensional traits were higher in L+W+ (high light and humidity) followed by L-W+ (low light and high humidity), L+W- and finally by L-W-. Summer drought typical of the Mediterranean climate was evaluated by chlorophyll fluorescence of PSII on apical leaves of seedlings and mature trees at the beginning (21 June) and in mid-summer (20 July). While in June physiological traits did not differ between low and high IR, in mid-summer (at the peak of water-stress) seedlings of the two highest light classes showed chronic photoinhibition (Fv/Fm<0.75) and an increase in thermal dissipation (D) by constitutive term (Dc=1-Fv/Fm) and by regulated mechanisms of dissipation through xanthophyll-cycle term (Dx). Moreover, in July seedling leaf physiology largely differed with IR: leaves acclimated to high IR have higher photosynthetic potentialities, as shown by electron transport rate (ETR) and quantum yield (P) at saturating light maintained by an increase of the fraction of open reaction centres (qP), counterbalancing the efficiency decrease of the single reaction centres (Fv’/Fm’) dynamically protected via xanthophyll-cycle (Dx). Tree apical leaf physiology behaved differently from seedling leaves both in June (except Fv/Fm) and in July (except Dx). In June photochemistry was higher in tree apical leaves with higher ETR, P, qP and Fv’/Fm’ with a lower fraction of energy dissipated and in particular through lower loss by photoprotection through Dx as expected for high light conditions without stress; in July no chronic photoinhibition was observed in tree light leaves, the efficiency of single reaction centres (Fv’/Fm’) remained high but a drop in the fraction of open centre (qP) decreased significantly P and thus ETR. Consequently the highest potentialities in photochemistry (P) were observed in large gaps (at 40% IR).

Leaf Chlorophyll Fluorescence: Background and Fundamentals for Plant Biologists

This review on chlorophyll a fluorescence starts with an overview of the primary photochemistry occurring at PSII and a characterization of the so-called “open” and “closed” states of its reaction centers. This provides the theoretical background for understanding the origin of PSII-emitted fluorescence and how its yield varies with the fraction of open reaction centers. The review proceeds to discuss the changes in fluorescence emission following illumination of a dark-adapted leaf and to define the PSII intrinsic quantum yield of photochemistry, which in turn provides an indication of PSII capacity. In light-adapted leaves, it is discussed how the use of modulated fluorometers and the double lighting technique allow an evaluation of photochemical and non-photochemical quenching, two parameters that give useful information about the plant’s photosynthetic performance under field conditions. Finally, it is described how the PSII operational efficiency can be used to calculate the photosynthetic electron transport rate and the conditions under which this is linearly related to the CO2 assimilation rate. Some requirements for a valid application of the technique as well as some limitations in interpreting its results are discussed.

Investigating energy partitioning during photosynthesis using an expanded quantum yield convention

In higher plants, regulation of excess absorbed light is essential for their survival and fitness, as it enables avoidance of a build up of singlet oxygen and other reactive oxygen species. Regulation processes (known as non-photochemical quenching; NPQ) can be monitored by steady-state fluorescence on intact plant leaves. Pulse amplitude modulated (PAM) measurements of chlorophyll a fluorescence have been used for over 20 years to evaluate the amount of NPQ and photochemistry (PC). Recently, a quantum yield representation of NPQ ( Φ NPQ), which incorporates a variable fraction of open reaction centers, was proposed by Hendrickson et al. [L. Hendrickson, R.T. Furbank, W.S. Chow, Photosynth. Res. 82 (2004) 73]. In this work we extend the quantum yield approach to describe the yields of reversible energy-dependent quenching ( Φ qE), state transitions to balance PC between photosystems II and I ( Φ qT), and photoinhibition quenching associated with damaged reaction centers ( Φ qI). We showed the additivity of the various quantum yield components of NPQ through experiments on wild-type and npq1 strains of Arabidopsis thaliana. The quantum yield approach enables comparison of Φ qE with data from a variety of techniques used to investigate the mechanism of qE. We showed that Φ qE for a series of A. thaliana genotypes scales linearly with the magnitude of zeaxanthin cation formation, suggesting that charge-transfer quenching is largely responsible for qE in plants.

Differences in thermal acclimation of chloroplast functioning in two ecotypes of Valonia utricularis (Chlorophyta)

Chloroplast functioning in two temperature ecotypes of the tropical to warm-temperate green macrophyte Valonia utricularis was monitored by measuring chlorophyll a fluorescence parameters. One ecotype from the Mediterranean Sea is, with respect to growth and survival, more cold-adapted and eurythermal than the cold-sensitive and stenothermal ecotype from the Indian Ocean. Short-term temperature responses of the effective quantum yield (ΦDPSII) between 5 and 35°C and the potential for acclimation of photosystem II to suboptimal growth temperatures (15, 18, 20°C) were investigated. When grown at optimal temperature (25°C), the two ecotypes did not differ to a great extent in the short-term temperature response of OPSII, even though the temperature optimum was lower in the Mediterranean compared with the Indian Ocean isolate (26 vs 33°C). In both ecotypes, decreases in ΦPSII were primarily caused by decreases in the efficiency of excitation capture (Fv′/Fm′) and by the decreased fraction of open reaction centres (qP). ΦDPSII declined in both isolates to 20–30% of the maximum when measured at 5°C. However, the ecotypes showed distinct differences in their potential to acclimate to suboptimal growth temperatures. The Indian Ocean ecotype was not capable of cold acclimation and suffered from photoinhibition when grown at 15–20°C, even though it was capable of adjusting its pigment composition, i.e. by decreasing light absorption combined with increasing the capacity for zeaxanthin-induced energy dissipation. The Mediterranean ecotype was capable of acclimating its photosynthetic efficiency to temperatures of 15–20°C, achieving optimal photosynthetic activity at 18–20°C. Changes in Fv′/Fm′ were mainly responsible for the acclimation response of ΦDPSII. It is suggested that acquisition of cold adaptation and the potential for temperature acclimation in populations of the northeast Atlantic/Mediterranean Sea have enabled V. utricularis, a species with a tropical origin, to extend its biogeographic range into warm-temperate regions.

Inhibition of photosynthesis by viral infection: Effect on PSII structure and function

The effect of viral infection on photosynthesis was investigated in Nicotiana benthamiana Gray plants infected with different strains of pepper and paprika mild mottle viruses (PMMoV and PaMMoV) and chimeric viral genomes derived from them. In both symptomatic and asymptomatic leaves of virus‐infected plants, photosynthetic electron transport in photosystem II (PSII) was reduced. In all cases analyzed, viral infection affected the polypeptide pattern of the oxygen‐evolving complex (OEC) in thylakoid membranes. The levels of both the 24 and 16 kDa proteins were reduced to a differing extent when compared with the levels in healthy control. This loss of the OEC extrinsic proteins affected the oxygen evolution rates of thylakoid membranes and leaves from infected plants. Additionally, viral coat protein (CP) was found associated with the chloroplasts and the thylakoid membranes of the infected plants. The CP accumulation level was dependent upon both the post‐infection time and the virus analyzed, but independent of the CP itself since hybrid viruses did not behave as their parental viruses with the same CP, with respect to PSII inhibition, CP accumulation rates and OEC protein levels. Modulated chlorophyll (Chl) fluorescence and oxygen evolution measurements carried out in both types of leaves showed that the quantum yield of PSII electron transport was diminished in infected plants with respect to those of control plants. The decrease in electron transport efficiency was mainly caused by a reduction in the fraction of open reaction centers. The infected plants also showed a reduction in the efficiency of excitation capture in PSII by photoprotective thermal dissipation of excess excitation energy.

Correlations between chlorophyll fluorescence quenching parameters and photosynthesis in a segregating Lycopersicon esculentum × L. peruvianum population as measured under constant conditions

Chl fluorescence of mature leaves in low-temperature treated plants was studied under identical measuring conditions in a segregating population of the F3 offspring of a cross between a chilling-tolerant and a chilling-sensitive tomato species. Through recombination of genes involved in photosynthesis, the population revealed a wide, continuous variability of photosynthetic capacity from plants performing much worse to those performing better than the parental lines of the cross. In the parental species, a nearly linear correlation was observed between photochemical chl fluorescence quenching (qP) and O2 evolution over a wide temperature range. Across the F3 generation, still a weak correlation between the two parameters was found at 20 °C, but not at 10 °C, when measured under identical conditions. This indicates that the fraction of open reaction centres could at least in part be adjusted to the photosynthetic capacity of the individual genotype. However, the correlation was so weak, that the previously suggested use of qP as a selection criterion for chilling tolerance of photosynthesis in breeding programs is regarded as doubtful, as long as photosynthesis rates are not measured in addition. Quantum efficiency of Photosystem II (ΦPSII) was strongly dependent on qP both at 20 and at 10 °C measuring temperature and depended on the quantum efficiency of open reaction centres (F′v/F′m) at 20, but not at 10 °C. F′v/F′m, in turn, correlated negatively with the processes of energy dissipation by the mechanisms of non-photochemical quenching (qN), i.e. its fast-relaxing component (qF) and photoinhibitory quenching (qI).

Theory of fluorescence induction in photosystem II: derivation of analytical expressions in a model including exciton-radical-pair equilibrium and restricted energy transfer between photosynthetic units

The theoretical relationships between the fluorescence and photochemical yields of PS II and the fraction of open reaction centers are examined in a general model endowed with the following features: i) a homogeneous, infinite PS II domain; ii) exciton-radical-pair equilibrium; and iii) different rates of exciton transfer between core and peripheral antenna beds. Simple analytical relations are derived for the yields and their time courses in induction experiments. The introduction of the exciton-radical-pair equilibrium, for both the open and closed states of the trap, is shown to be equivalent to an irreversible trapping scheme with modified parameters. Variation of the interunit transfer rate allows continuous modulation from the case of separated units to the pure lake model. Broadly used relations for estimating the relative amount of reaction centers from the complementary area of the fluorescence kinetics or the photochemical yield from fluorescence levels are examined in this framework. Their dependence on parameters controlling exciton decay is discussed, allowing assessment of their range of applicability. An experimental induction curve is analyzed, with a discussion of its decomposition into alpha and beta contributions. The sigmoidicity of the induction kinetics is characterized by a single parameter J related to Joliot's p, which is shown to depend on both the connectivity of the photosynthetic units and reaction center parameters. On the other hand, the relation between J and the extreme fluorescence levels (or the deviation from the linear Stern-Volmer dependence of 1/phi f on the fraction of open traps) is controlled only by antenna connectivity. Experimental data are consistent with a model of connected units for PS II alpha, intermediate between the pure lake model of unrestricted exciton transfer and the isolated units model.