On the significance of photoinhibition of photosynthesis in the field and its generality among species

Photoinhibition was examined in naturally exposed willow leaves in the field. In the afternoon on clear and warm days, the quantum yield of electron transport, derived from gas exchange data, was decreased by 28%. Besides this photoinhibition, decreases in the photosynthetic capacity and in the stomatal conductance were also observed. Of these three limitations of carbon assimilation, photoinhibition was the major one at limiting light only.To investigate the generality of photoinhibition, shade- and sun-acclimated leaves of fourteen different species were compared in a laboratory study. Photoinhibition was quantified by fluorescence measurements following exposure to moderate and high light for 30 min. The extent of photoinhibition was inversely related to the photochemical quenching, q_p, during exposure (the proportion of open PS II traps). This relationship was the same independent of the species, the light-acclimation state of the leaf and the light intensity. However, sun- and shade-acclimated leaves occupied opposite sides of the relationship: the sun-leaves showed lower photoinhibition and higher q_p. The sun-leaves were also more competent than shade-leaves by showing faster recovery from a given level of photoinhibition.Abbreviations F₀, F_V, F_M, F_S minimal, variable, maximal and steady-state fluorescence - q_N, q_i total and photoinhibitory non-photochemical quenching of variable fluorescence - q_p photochemical quenching     

Photochemical and Non-Photochemical Quenching Coefficients of the Chlorophyll Fluorescence: Comparison of Variation and Limits

There are several types of quenching coefficients currently in use which describe the decrease of the chlorophyll fluorescence: the photochemical quenching coefficients q_P and q_(P)rel and the non-photochemical quenching coefficients q_N, q_(N)rel, and NPQ. These five coefficients were calculated for a broad variety of cases of the fluorescence signals in a normal, realistic range and for determining the limits in a range with extremely low and high fluorescence values. The calculations showed that the quenching coefficients currently in use are not only numbers between 0 and 1 as one would expect when taking them as a relative measure of the quenching process. Most quenching coefficients must be regarded and interpreted carefully separated from each other. Each photochemical quenching coefficient and each non-photochemical quenching coefficient describe the same fluorescence signal in a different way. Only the relative quenching coefficients q_(P)rel and q_(N)rel match together and can be used to demonstrate a shift of the energy de-excitation from the photochemical to the non-photochemical route. This revised version was published online in August 2006 with corrections to the Cover Date.     

How to correctly determine the different chlorophyll fluorescence parameters and the chlorophyll fluorescence decrease ratio R<Subscript>Fd</Subscript> of leaves with the PAM fluorometer

This contribution is a practical guide to the measurement of the different chlorophyll (Chl) fluorescence parameters and gives examples of their development under high-irradiance stress. From the Chl fluorescence induction kinetics upon irradiation of dark-adapted leaves, measured with the PAM fluorometer, various Chl fluorescence parameters, ratios, and quenching coefficients can be determined, which provide information on the functionality of the photosystem 2 (PS2) and the photosynthetic apparatus. These are the parameters F_v, F_m, F₀, F_m′, F_v′, NF, and ΔF, the Chl fluorescence ratios F_v/F_m, F_v/F₀, ΔF/F_m′, as well as the photochemical (q_P) and non-photochemical quenching coefficients (q_N, q_CN, and NPQ). q_N consists of three components (q_N = q_E + q_T + q_I), the contribution of which can be determined via Chl fluorescence relaxation kinetics measured in the dark period after the induction kinetics. The above Chl fluorescence parameters and ratios, many of which are measured in the dark-adapted state of leaves, primarily provide information on the functionality of PS2. In fully developed green and dark-green leaves these Chl fluorescence parameters, measured at the upper adaxial leaf side, only reflect the Chl fluorescence of a small portion of the leaf chloroplasts of the green palisade parenchyma cells at the upper outer leaf half. Thus, PAM fluorometer measurements have to be performed at both leaf sides to obtain information on all chloroplasts of the whole leaf. Combined high irradiance (HI) and heat stress, applied at the upper leaf side, strongly reduced the quantum yield of the photochemical energy conversion at the upper leaf half to nearly zero, whereas the Chl fluorescence signals measured at the lower leaf side were not or only little affected. During this HL-stress treatment, q_N, q_CN, and NPQ increased in both leaf sides, but to a much higher extent at the lower compared to the upper leaf side. q_N was the best indicator for non-photochemical quenching even during a stronger HL-stress, whereas q_CN and NPQ decreased with progressive stress even though non-photochemical quenching still continued. It is strongly recommended to determine, in addition to the classical fluorescence parameters, via the PAM fluorometer also the Chl fluorescence decrease ratio R_Fd (F_d/F_s), which, when measured at saturation irradiance is directly correlated to the net CO₂ assimilation rate (_{P N}) of leaves. This R_Fd-ratio can be determined from the Chl fluorescence induction kinetics measured with the PAM fluorometer using continuous saturating light (cSL) during 4–5 min. As the R_Fd-values are fast measurable indicators correlating with the photosynthetic activity of whole leaves, they should always be determined via the PAM fluorometer parallel to the other Chl fluorescence coefficients and ratios.     

Chlorophyll fluorescence as a tool for evaluation of drought stress in strawberry

The effect of water deficit on chlorophyll fluorescence, sugar content, and growth parameters of strawberry (Fragaria×ananassa Duch. cv. Elsanta) was studied. Drought stress caused significant reductions in leaf water potential, fresh and dry masses, leaf area, and leaf number. A gradual reduction of photochemical quenching (q_P) and quantum efficiency (Φ_PS2) was observed under drought stress while non-photochemical quenching (q_N) increased. Maximum efficiency of photosystem 2 (F_v/F_m) was not affected by drought stress.     

Photoadaptation in the Green Alga Spongiochloris Sp. A Three-Fluorometer Study

The dark-adapted cells of the green alga Spongiochloris sp. were exposed to "white light" of 1000 μmol(photon) m⁻² s⁻¹ for 2 h and then dark adapted for 1.5 h. Changes of photochemical activities during photoadaptation were followed by measurement of chlorophyll (Chl) fluorescence kinetics, 77 K emission spectra, photosynthetic oxygen evolution, and pigment composition. We observed a build-up of slowly-relaxing non-photochemical quenching which led to a decrease of the F_v/F_m parameter and the connectivity. In contrast to the depression of F_v/F_m (35 %) and the rise of non-photochemical quenching (∼ 1.6), we observed an increase in effective absorption cross-section (20 %), Hill reaction (30 %), photosynthetic oxygen evolution (80 %), and electron transport rate estimated from the Chl fluorescence analysis (80 %). We showed an inconsistency in the presently used interpretation schemes, and ascribe the discrepancy between the increase of effective absorption cross-section and the photosynthetic activities on one side and the effective non-photochemical quenching on the other side to the build-up of a quenching mechanism which dissipates energy in closed reaction centres. Such a type of quenching changes the ratio between thermal dissipation and fluorescence without any effect on photochemical yield. In this case the F_v/F_m ratio cannot be used as a measure of the maximum photochemical yield of PS2. This revised version was published online in August 2006 with corrections to the Cover Date.     

Diurnal cycle of chlorophyll fluorescence in <Emphasis Type="Italic">Phalaenopsis</Emphasis>

Chlorophyll (Chl) fluorescence of warm day/cool night temperature exposed Phalaenopsis plants was measured hourly during 48 h to study the simultaneous temperature and irradiance response of the photosynthetic physiology. The daily pattern of fluorescence kinetics showed abrupt changes of photochemical quenching (q_P), non-photochemical quenching (NPQ) and quantum yield of photosystem II electron transport (Φ_PSII) upon transition from day to night and vice versa. During the day, the course of Φ_PSII and NPQ was related to the air temperature pattern, while maximum quantum efficiency of PSII photochemistry (F_v/F_m) revealed a rather light dependent response. Information on these daily dynamics in fluorescence kinetics is important with respect to meaningful data collection and interpretation.     

High photosynthetic efficiency of a rice (Oryza sativa L.) xantha mutant

Comparative analysis revealed that a xantha rice mutant (cv. Huangyu B) had higher ratios of chlorophyll (Chl) a/b and carotenoids/Chl, and higher photosynthetic efficiency than its wild type parent (cv. II32 B). Unexpectedly, the mutant had higher net photosynthetic rate (P _N) than II32 B. This might have resulted from its lower non-photochemical quenching (q_N) but higher maximal photochemical efficiency (F_V/F_M), higher excitation energy capture efficiency of photosystem 2 (PS2) reaction centres (F_V′/F_M′), higher photochemical quenching (q_P), higher effective PS2 quantum yield (Φ_PS2), and higher non-cyclic electron transport rate (ETR). This is the first report of a chlorophyll mutant that has higher photosynthetic efficiency and main Chl fluorescence parameters than its wild type. This mutant could become a unique material both for the basic research on photosynthesis and for the development of high yielding rice cultivars.     

On the relationship between the quantum yield of Photosystem II electron transport,as determined by chlorophyll fluorescence and the quantum yield of CO2-dependent O2 evolution

We tested the two empirical models of the relationship between chlorophyll fluorescence and photosynthesis, previously published by Weis E and Berry JA 1987 (Biochim Biophys Acta 894: 198–208) and Genty B et al. 1989 (Biochim Biophys Acta 990: 87–92). These were applied to data from different species representing different states of light acclimation, to species with C₃ or C₄ photosynthesis, and to wild-type and a chlorophyll b-less chlorina mutant of barley. Photosynthesis measured as CO₂-saturated O₂ evolution and modulated fluorescence were simultaneously monitored over a range of photon flux densities. The quantum yields of O₂ evolution (ØO₂) were based on absorbed photons, and the fluorescence parameters for photochemical (q_p) and non-photochemical (q_N) quenching, as well as the ratio of variable fluorescence to maximum fluorescence during steady-state illumination (F'_v/F'_m), were determined. In accordance with the Weis and Berry model, most plants studied exhibited an approximately linear relationship between ØO₂/q_p (i.e., the yield of O₂ evolution by open Photosystem II reaction centres) and q_N, except for wild-type barley that showed a non-linear relationship. In contrast to the linear relationship reported by Genty et al. for q_p×F'_v/F'_m (i.e., the quantum yield of Photosystem II electron transport) and ØCO₂, we found a non-linear relationship between q_p×F'_v/F'_m and ØO₂ for all plants, except for the chlorina mutant of barley, which showed a largely linear relationship. The curvilinearity of wild-type barley deviated somewhat from that of other species tested. The non-linear part of the relationship was confined to low, limiting photon flux densities, whereas at higher light levels the relationship was linear. Photoinhibition did not change the overall shape of the relationship between q_p×F'_v/F'_m and ØO₂ except that the maximum values of the quantum yields of Photosystem II electron transport and photosynthetic O₂ evolution decreased in proportion to the degree of photoinhibition. This implies that the quantum yield of Photosystem II electron transport under high light conditions may be similar for photoinhibited and non-inhibited plants. Based on our experimental results and theoretical analyses of photochemical and non-photochemical fluoresce quenching processes, we conclude that both models, although not universal for all plants, provide useful means for the prediction of photosynthesis from fluorescence parameters. However, we also discuss that conditions which alter one or more of the rate constants that determine the various fluorescence parameters, as well as differential light penetration in assays for oxygen evolution and fluorescence emission, may have direct effect on the relationships of the two models.Abbreviations F₀ and F'₀ fluorescence when all Photosystem II reaction centres are open in dark- and light-acclimated leaves, respectively - F_m and F'_m fluorescence when all Photosystem II reaction centres are closed in dark and light, respectively - F_v variable fluorescence equal to F_m-F₀ - F_s steady state level of fluorescence in light - F'_v and F'_m variable (F'_m-F'₀) and maximum fluorescence under steady state light conditions - HEPES N-2-hydroxyethylpiperazine-N-2-ethane-sulphonic acid - Q_A the primary, stabile quinone acceptor of Photosystem II - q_N non-photochemical quenching of fluorescence - q_p photochemical quenching of fluorescence - ØO₂ quantum yield of CO₂-saturated O₂ evolution based on absorbed photons     

Chlorophyll fluorescence in the resurrection plant Selaginella lepidophylla (Hook. & Grev.) Spring during high-light and desiccation stress,and evidence for zeaxanthin-associated photoprotection

The function of photosystem (PS)II during desiccation and exposure to high photon flux density (PFD) was investigated via analysis of chlorophyll fluorescence in the desert resurrection plant Selaginella lepidophylla (Hook. and Grev.) Spring. Exposure of hydrated, physiologically competent stems to 2000 mol · m^–2 · s^–1 PFD caused significant reductions in both intrinsic fluorescence yield (F_O) and photochemical efficiency of PSII (F_V/F_M) but recovery to pre-exposure values was rapid under low PFD. Desiccation under low PFD also affected fluorescence characteristics. Both F_V/F_M and photochemical fluorescence quenching remained high until about 40% relative water content and both then decreased rapidly as plants approached 0% relative water content. In contrast, the maximum fluorescence yield (F_M) decreased and non-photochemical fluorescence quenching increased early during desiccation. In plants dried at high PFD, the decrease in F_V/F_M was accentuated and F_O was reduced, however, fluorescence characteristics returned to near pre-exposure values after 24-h of rehydration and recovery at low PFD. Pretreatment of stems with dithiothreitol, an inhibitor of zeaxanthin synthesis, accelerated the decline in F_V/F_M and significantly increased F_O relative to controls at 925 mol · m^–2 · s^–1 PFD, and the differences persisted over a 3-h low-PFD recovery period. Pretreatment with dithiothreitol also significantly decreased non-photochemical fluorescence quenching, increased the reduction state of Q_A, the primary electron acceptor of PSII, and prevented the synthesis of zeaxanthin relative to controls when stems were exposed to PFDs in excess of 250 mol · m^–2 · s^–1. These results indicate that a zeaxanthin-associated mechanism of photoprotection exists in this desert pteridophyte that may help to prevent photoinhibitory damage in the fully hydrated state and which may play an additional role in protecting PSII as thylakoid membranes undergo water loss.Abbreviations and Symbols DTT dithiothreitol - EPS epoxidation state - F_O yield of instantaneous fluorescence at open PSII centers - F_M maximum yield of fluorescence at closed PSII centers induced by saturating light - F_M F_M determined during actinic illumination - F_V yield of variable fluorescence (F_M-F_O) - F_V/F_M photochemical efficiency of PSII - q_P photochemical fluorescence quenching - q_NP non-photochemical fluorescence quenching of Schreiber et al. (1986) - NPQ non-photochemical fluorescence quenching from the Stern-Volmer equation - PFD photon flux density - RWC relative water content This paper is based on research done while W.G.E. was on leave of absence at Duke University during the fall of 1990. We would like to thank Dan Yakir, John Skillman, Steve Grace, and Suchandra Balachandran and many others at Duke University for their help and input with this research. Dr. Barbara Demmig-Adams provided zeaxanthin for standard-curve purposes.     

Effect of Different Sugars on Photosynthesis and Chlorophyll Fluorescence in Photoautotrophic Tomato Suspension Cell Cultures

The effects of metabolisable sugars sucrose and glucose along with non-metabolisable isomers of sucrose palatinose and turanose were tested. Rate of oxygen evolution (P), electron transport rate (ETR), and photochemical quenching (q_p) showed substantial decrease after 24 and 48 h by glucose and sucrose treatments, whereas there was no effect on all these parameters by the treatment with palatinose and turanose. Also the F_v/F_m ratio remained constant through the time of studies revealing that the maximal photochemical capacity of the cells was unchanged. Non-photochemical quenching (q_N) showed a decrease compared to the control values by all the treatments. Hence P and Chl fluorescence parameter were affected only by those sugars which are used in the metabolic pathways and not by sugar analogues.     

A theoretical and experimental analysis of the qP and qN coefficients of chlorophyll fluorescence quenching and their relation to photochemical and nonphotochemical events

The initial (F₀), maximal (F_M) and steady-state (F_S) levels of chlorophyll fluorescence emitted by intact pea leaves exposed to various light intensities and environmental conditions, were measured with a modulated fluorescence technique and were analysed in the context of a theory for the energy fluxes within the photochemical apparatus of photosynthesis. The theoretically derived expressions of the fluorescence signals contain only three terms, X=J₂p_2F/(1–G), Y=T/(1–G) and V, where V is the relative variable fluorescence, J₂ is the light absorption flux in PS II, p_2F is the probability of fluorescence from PS II, G and T are, respectively, the probabilities for energy transfer between PS II units and for energy cycling between the reaction center and the chlorophyll pool: F₀=X, F_M=X/(1–Y) and F_S=X(1+(YV/(1–Y))). It is demonstrated that the amplitudes of the previously defined coefficients of chlorophyll fluorescence quenching, q_P and q_N, reflect, not just photochemical (q_P) or nonphotochemical (q_N) events as implied in the definitions, but both photochemical and nonphotochemical processes of PS II deactivation. The coefficient q_P is a measure of the ratio between the actual macroscopic quantum yield of photochemistry in PS II (41-1) in a given light state and its maximal value measured when all PS II traps are open (41-2) in that state, with 41-3 and 41-4. When the partial connection between PS II units is taken into consideration, 1-q_P is nonlinearily related to the fraction of closed reaction centers and is dependent on the rate constants of all (photochemical as well as nonphotochemical) exciton-consuming processes in PS II. On the other hand, 1-q_N equals the (normalized) ratio of the rate constant of photochemistry (k_2b) to the combined rate constant (k_N) of all the nonphotochemical deactivation processes excluding the rate constant k₂₂ of energy transfer between PS II units. It is demonstrated that additional (qualitative) information on the individual rate constants, k_N-k₂₂ and k_2b, is provided by the fluorescence ratios 1/F_M and (1/F₀)–(1/F_M), respectively. Although, in theory, 41-5 is determined by the value of both k_2b and k_N-k₂₂, experimental results presented in this paper show that, under various environmental conditions, 41-6 is modulated largely through changes in k _N, confirming the idea that PS II quantum efficiency is dynamically regulated in vivo by nonphotochemical energy dissipation.Abbreviations Chl chlorophyll - F₀, F_M and F_S initial, maximal and steady-state levels of modulated Chl fluorescence emitted by light-adapted leaves - PS I and II photosystem I and II - q_P and q_N (previously defined) photochemical and nonphotochemical components of Chl fluorescence quenching     

Changes in Chlorophyll Fluorescence During the Course of Photoperiod and in Response to Drought in Casuarina equisetifolia Forst. and Forst.

The effects of drought and the diurnal changes in photosynthetic electron transport were studied in non-nodulated plants of Casuarina equisetifolia. The induction of fluorescence showed a slightly higher I step in water-stressed than control plants, and the time from the start of irradiation to the P step of induction was significantly shortened by drought. The quantum efficiency of photosystem 2 (PS2) in the dark-adapted state (F_v/F_m) was generally not affected by drought, whereas it decreased during the central hours of the day. The decrease in quantum yield of PS2 electron transport (₂) in water-stressed plants was associated with decreases in the photochemical efficiency of open (oxidised) PS2 centres (F_v'/F_m') and increases in non-photochemical quenching (q_N) rather than with increased closure of PS2 centres (lowered photochemical quenching, q_P). In contrast, the changes in quantum yield of electron transport during the day were related to changes in q_P rather than in F_v'/F_m'. When chlorophyll fluorescence was measured at the same irradiance during the day, a greater q_N was observed at the end of the drying cycle than after watering, and early and late in the photoperiod than in the central hours of the day. The greater q_N at the beginning and end of the day did not prevent an increase in energy not used photochemically nor dissipated non-photochemically. Drought did not affect this excess of photon energy.     

Non-photochemical quenching of Fo in leaves is emission wavelength dependent: consequences for quenching analysis and its interpretation

The effects of light-induced non-photochemical quenching on the minimal F_o, and variable F_v, fluorescence emissions at 690 and 730 nm in leaves were determined. Non-photochemical quenching of F_o, but not F_v, was found to be dependent upon the wavelength of emission, and was greater at 690 nm than at 730 nm. For emission at 730, compared to at 690 nm, approx. 30% of F_o was not affected by non-photochemical quenching processes in leaves of C3 plants; in maize leaves this was found to be approx. 50%. The data indicate that a substantial proportion of the pigments contributing to F_o emission at 730 nm are not quenched by light-induced, non-photochemical quenching processes and that there are large differences in the pigment matrices contributing to F_o and F_v emissions at 730 nm, compared to those at 690 nm. These findings have important implications for the accurate estimation and interpretation of non-photochemical quenching of fluorescence parameters and their use in the calculation of photochemical efficiencies in leaves. Measurements of fluorescence emissions at wavelengths above 700 nm are likely to give rise to significant errors when used for determinations of photochemical and non-photochemical quenching parameters.     

Chlorophyll Fluorescence Parameters: The Definitions,Photosynthetic Meaning,and Mutual Relationships

Chlorophyll fluorescence parameters (Chl FPs) derived from the slow (long-term) induction kinetics of modulated Chl a fluorescence are reviewed and analysed with respect to their application in photosynthesis research. Only four mutually independent Chl FPs, calculated from values of five essential Chl fluorescence (ChlF) yields, are distinguished as the basic ones. These are: the maximum quantum yield of PS2 photochemistry (_{P O}), the photochemical quenching of variable ChlF (q_P), the non-photochemical quenching of variable ChlF (q_N), and the relative change of minimum ChlF (q_O). _{P O} refers to the dark-adapted state of a thylakoid membrane, q_P, q_N and q_O characterise the light-adapted state. It is demonstrated that all other Chl FPs can be determined using this quartet of parameters. Moreover, three FPs related to the non-radiative energy dissipation within thylakoid membranes are evaluated, namely: the non-photochemical ChlF quenching (NPQ), the complete non-photochemical quenching of ChlF (q_CN), and the effective quantum yield of non-photochemical processes in PS2 (_N). New FPs, the total quenching of variable ChlF (q_TV) and the absolute quenching of ChlF (q_A) which allow to quantify co-action of the photochemical and non-photochemical processes during a light period are defined and analysed. The interpretation of Chl FPs and recommendations for their application in the photosynthesis research are also given. Some alternative FPs used in the laboratory practice have only an approximate character and can lead to incorrect conclusions if applied to stressed plants. They are reviewed and compared with the standard ones. All formulae and conclusions discussed herein are verified using experimental values obtained on young seedlings of the Norway spruce (Picea abies [L.] Karst.).     

Photosynthesis and chlorophyll fluorescence response to low sink demand of tubers and roots in <Emphasis Type="Italic">Dahlia pinnata</Emphasis> source leaves

Photosynthetic rate (P_N) and chlorophyll (Chl) fluorescence induction of source leaves in response to a low sink demand created by girdling the branch (GB) between the root-tuber-system and the leaves were studied in Dahlia pinnata L. cv. Rigolet during the stage of rapid tuber growth in the greenhouse. GB resulted in significantly lower values of P_N, stomatal conductance (g_s), and transpiration rate (E), but in higher leaf temperature (T_l) compared with those of controls. With exception of maximum quantum yield of photosystem 2 (PS 2) photochemistry (F_v/F_m) and maximum ratio of quantum yields of photochemical and concurrent non-photochemical processes in PS 2 (F_v/F₀), no significant differences were observed in Chl fluorescence parameters between girdled and control leaves on days 1 and 2 after GB, indicating no apparent damage in the photosynthetic apparatus. However, longer girdling duration resulted in higher non-photochemical Chl fluorescence quenching (NPQ), but lower F_v/F₀, actual efficiency of energy conversion in PS 2 under steady-state conditions (Φ_PS2), and photochemical quenching coefficient (qP) in comparison with controls from 10:00 to 16:00 or 15:00 on days 4 and 5, respectively, indicating reversible injury in the photosynthetic apparatus.     

Diurnal changes in photochemical efficiency,the reduction state of Q,radiationless energy dissipation,and non-photochemical fluorescence quenching in cacti exposed to natural sunlight in northern Venezuela

Summary Diurnal measurements of chlorophyll a fluorescence from cacti (Nopalea cochenillifera, Opuntia ficus-indica, and Opuntia wentiana) growing in northern Venezuela were used to determine photochemical fluorescence quenching related to the reduction state of the primary electron acceptor of PS II as well as non-photochemical fluorescence quenching which reflects the fraction of energy going primarily into radiationless deexcitation. The cladodes used in this study were oriented such that one surface received direct sunlight in the morning and the other one during the afternoon. Both surfaces exhibited large increases in radiationless energy dissipation from the photochemical system accompanied by decreases in PS II photochemical efficiency during direct exposure to natural sunlight. During exposure to sunlight in the morning, dissipation of absorbed light energy through photosynthesis and radiationless energy dissipation was sufficient to maintain Q, the primary electron acceptor for PS II, in a low reduction state. During exposure to sunlight in the afternoon, however, the reduction state of Q rose to levels greater than 50%, presumably due to a decrease in photosynthetic electron transport as the decarboxylation of the nocturnally accumulated malic acid was completed. Exposure to direct sunlight in the afternoon also led to more sustained increases in radiationless energy dissipation. Furthermore, the increases in radiationless energy dissipation during exposure of a water-stressed cladode of O. wentiana to direct sunlight were much greater than those from other well-watered cacti, presumably due to sustained stomatal closure and decreased rates of photosynthetic electron transport. These results indicate that the radiationless dissipation of absorbed light is an important process in these CAM plants under natural conditions, and may reflect a protective mechanism against the potentially damaging effects of the accumulation of excessive energy, particularly under conditions where CO₂ availability is restricted.Abbreviations CAM crassulacean acid metabolism - F _o instantaneous fluorescence emission - F _M maximum fluorescence emission - F _v variable fluorescence emission - K _D rate constant for radiationless energy dissipation in the antenna chlorophyll - PFD photon flux density - PS I photosystem I - PS II photosystem II - Q primary electron acceptor of photosystem II - q _NP non-photochemical fluorescence quenching - q _P photochemical fluorescence quenching - T _C cladode temperature     

Simultaneous gas exchange and fluorescence measurements indicate differences in the response of sunflower,bean and maize to water stress

Gas exchange and fluorescence measurements of attached leaves of water stressed bean, sunflower and maize plants were carried out at two light intensities (250 mol quanta m^-2s^-1 and 850 mol quanta m^-2s^-1). Besides the restriction of transpiration and CO₂ uptake, the dissipation of excess light energy was clearly reflected in the light and dark reactions of photosynthesis under stress conditions. Bean and maize plants preferentially use non-photochemical quenching for light energy dissipation. In sunflower plants, excess light energy gave rise to photochemical quenching. Autoradiography of leaves after photosynthesis in ¹⁴CO₂ demonstrated the occurrence of leaf patchiness in sunflower and maize but not in bean. The contribution of CO₂ recycling within the leaves to energy dissipation was investigated by studies in 2.5% oxygen to suppress photorespiration. The participation of different energy dissipating mechanisms to quanta comsumption on agriculturally relevant species is discussed.Abbreviations F_o minimal fluorescence - F_m maximal fluorescence - F_p peak fluorescence - g leaf conductance - P_N net CO₂ uptake - q_N coefficient of non-photochemical quenching - q_P coefficient of photochemical quenching     

The efficiency of electron transfer from QA − to the donor side of Photosystem II decreases during induction of photosynthesis: Evidences from chlorophyll fluorescence and photoacoustic techniques

The amplitudes ratio of the fast and slow phases (A_fast/A_slow) in the kinetics of the dark relaxation of variable chlorophyll fluorescence (F_V) was studied after various periods of illumination of dark-adapted primary barley leaves. Simultaneously, photosynthetic activity was monitored using the photoacoustic technique and the photochemical and non-photochemical fluorescence quenching parameters. The ratio A_fast/A_slow changed with the preceding illumination time in a two-step manner. During the first stage of photosynthetic induction (0–20 s of illumination), characterized by a drop in O₂-dependent photoacoustic signal following an initial spike and by a relatively stable small value of photochemical F_V quenching, the ratio A_fast/A_slow remained practically unaltered. During the second stage (20–60 s of illumination), when both the rate of O₂ evolution and the photochemical F_V quenching were found to be sharply developed, a marked increase in the above ratio was also observed. A linear correlation was found between the value of the photochemical quenching and the ratio A_fast/A_slow during the second phase of photosynthetic induction. It is concluded that the slow phase appearing in the kinetics of F_V dark relaxation is not due to the existence of Photosystem II reaction centres lacking the ability to reduce P700⁺ with high rates, but is instead related to the limitation of electron release from Photosystem I during the initial stage of the induction period of photosynthesis. This limitation keeps the intersystem electron carriers in the reduced state and thus increases the probability of back electron transfer from Q_A ^– to the donor side of Photosystem II.Abbreviations A_fast/A_slow the ratio of magnitudes between the fast and slow phases of dark relaxation of variable fluorescence - F_O initial level of chlorophyll fluorescence - F_V variable chlorophyll fluorescence (F-F_O) - (F_V)_S the yield of variable chlorophyll fluorescence under saturating pulse in illuminated leaves - (F_V)_M the yield of variable chlorophyll fluorescence under saturating pulse in dark-adapted leaves - PA photoacoustic - PSI Photosystem I - PS II Photosystem II - qN non-photochemical quenching - qQ photochemical quenching     

The quantum efficiency of photosystem II and its relation to non-photochemical quenching of chlorophyll fluorescence; the effect of measuring-and growth temperature

The relation between the quantum yield of oxygen evolution of open photosystem II reactions centers (_p), calculated according to Weis and Berry (1987), and non-photochemical quenching of chlorophyll fluorescence of plants grown at 19°C and 7°C was measured at 19°C and 7°C. The relation was linear when measured at 19°C, but when measured at 7°C a deviation from linearity was observed at high values of non-photochemical quenching. In plants grown at 7°C this deviation occurred at higher values of non-photochemical quenching than in plants grown at 19°C. The deviations at high light intensity and low temperature are ascribed to an increase in an inhibition-related, non-photochemical quenching component (q_I).The relation between the quantum yield of excitation capture of open photosystem II reaction centers (_exe), calculated according to Genty et al. (1989), and non-photochemical quenching of chlorophyll fluorescence was found to be non-linear and was neither influenced by growth temperature nor by measuring temperature.At high PFD the efficiency of overall steady state electron transport measured by oxygen-evolution, correlated well with the product of q _N and the efficiency of excitation capture (_exe) but it deviated at low PFD. The deviations at low light intensity are attributed to the different populations of chloroplasts measured by gas exchange and chlorophyll fluorescence and to the light gradient within the leaf.Abbreviations F₀ basic fluorescence - F₀ basic fluorescence, thylakoid in energized state - F_m maximal fluorescence - F_m maximum fluorescence in energized state - F_s steady state fluorescence - F_v maximal variable fluorescence - PFD photon flux density - PS II_rc Photosystem II reaction center - q_F0 quenching of basic fluorescence - q_E energy related quenching - q_N non-photochemical quenching:-q_f-total quenching - q_I inhibition-related quenching - q_p photochemical quenching - q_r quenching due to state transition - R_d dark respiration - _p PS II efficiency of excitation capture of open PS II_rc - _pe extrapolated minimal value of _p - _p0 extrapolated maximal value of _p - _si quantum efficiency of linear electron transport, calculated from gas exchange measurements based on incident light - _sf quantum efficiency of linear electron transport, calculated from fluorescence measurements, based on incident measuring light     

A quantitative determination of photochemical and non-photochemical quenching during the slow phase of the chlorophyll fluorescence induction curve of bean leaves

Estimations of the changes in the reduction-oxidation state of Photosystem II electron acceptors in Phaseolus vulgaris leaves were made during the slow decline in chlorophyll fluorescence emission from the maximal level at P to the steady-state level at T. The relative contributions of photochemical and non-photochemical processes to the fluorescence quenching were determined from these data. At a low photon flux density of 100 μmol · m^?2 · s^?1, non-photochemical quenching was the major contributor to the fluorescence decline from P to T, although large charges were observed in photochemical quenching immediately after P. On increasing the light intensity 10-fold, the contribution of photochemical processes to fluorescence quenching was markedly diminished, with nearly all the P-to-T fluorescence decline being attributable to changes in non-photochemical quenching. The possible factors responsible for changes in non-photochemical quenching within the leaves are discussed.