УДК 581.1

CYCLIC ELECTRON FLOW PLAYS AN IMPORTANT ROLE IN PROTECTION OF SPINACH LEAVES UNDER HIGH TEMPERATURE STRESS 1

© 2016 D. Agrawal*, S. I. Allakhverdiev**, ***, ****, A. Jajoo*

*School of Life science, Devi Ahilya University, Indore, India

**Timiryazev Institute of Plant Physiology, Russian Academy of Sciences, Moscow

***Institute of Basic Biological Problems, Russian Academy of Sciences, Pushchino, Moscow oblast

****Department of Plant Physiology, Faculty of Biology, Moscow State University, Moscow

Received July 11, 2015

Heat stress is one of the major abiotic stresses and affects plant productivity in a negative manner. Photosynthetic processes are largely influenced by heat stress. In spinach (Spinacia oleracea L.) leaves at 40°C the decrease in PSII activity was mainly due to the decreased efficiency to capture excitation energy, increased yield of regulatory energy dissipation mechanism Y(NPQ), and decreased quantum yield Y(II). According to the results till 45°C PSI is stable and protective while at still higher temperature stability of PSI was reduced and protection was not sufficient. Therefore, we conclude that cyclic electron flow plays an important role in protection for PSI from heat stress.

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1 This text was submitted by the authors in English.

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Abbreviations: Chl  chlorophyll; CEF – cyclic electron flow; ETR(II) – electron transport rate of PSII; qP – photochemical fluorescence quenching; NPQ – non-photochemical quenching; Y(I) – quantum yield of PSI; Y(II) – quantum yield of PSII; Y(NA) – quantum yield of non-photochemical quenching in PSI acceptor side limitation; Y(ND) – quantum yield of non-photochemical quenching due to donor side limitation; Y(NO) – quantum yield of non-regulated energy dissipation; Y(NPQ) – quantum yield of regulated energy dissipation; Y(CEF) – quantum yield of cyclic electron flow around PSI.

Corresponding author: Anjana Jajoo. School of Life Science, Devi Ahilya University, Indore 452017, M.P., India; fax: �731-4263453; e-mail: anjanajajoo@hotmail.com

Keywords: Spinacia oleracea – chlorophyll fluorescence – cyclic electron flow – energy dissipation – high temperature – quantum yield – photosystem I (PSI) – photosystem II (PSII)

 

INTRODUCTION

Heat stress is one of the main abiotic stresses that limit plant biomass production and productivity, especially, in tropical and subtropical countries. Photosynthesis is one of the important processes which gets affected by any kind of environmental stress [1]. There are, at least, three major stress-sensitive sites in the photosynthetic machinery: the photosystems, mainly photosystem II (PSII) with its oxygen-evolving complex (OEC), the ATP generating system, and the carbon assimilation processes. PSII is the critical site of damage by low and high temperatures together with various stress factors such as drought, salinity, UV radiation, and environmental pollutants [2–5].

High temperature induces significant changes in the composition of chloroplast membrane lipids and proteins that results in structural modifications of the thylakoid membrane [6]. Los and Murata [7] reported that high temperatures cause the fluidization of the thylakoid membrane. Due to high temperature the oxygen-evolving/water splitting complex is disrupted, with functional manganese ions and extrinsic proteins being released [8] and the peripheral light-harvesting system separates from the PSII complexes [9] with the reduced size PSII migrating from the appressed regions of the thylakoid membranes to the non-appressed regions [10]. Kreslavski et al. [11] have shown that exposure to short-term heat stress (42°C) induces a progressive destacking of the wheat thylakoid membranes.

High temperature causes a large decrease in the rate of carbon dioxide fixation. CO2 assimilation is the major consumer of photon energy absorbed by the antenna pigments and any decrease in the rate of photosynthesis inevitably results in accumulation of excess photon energy under high temperature. Plants have evolved a large variety of mechanisms to protect their photosynthetic apparatus against damage resulting from high temperature. Heat stress can induce the antioxidant enzymes to overcome the increased oxidative stress [12]. In spite of these findings, it remains unclear how heat-treated plants regulate the dissipation of excess photon energy when low CO2 assimilation requires less chemical energy than that provided by the absorbed photons.

Chlorophyll fluorescence is an important tool to measure photosynthetic efficiency of plants [13]. The light curve response (LCR) is an analytical tool that provides valuable information on the efficiency with which photosynthetically active radiation is used by plants [14]. It is a plot of photosynthetic parameters against photon flux density. The quantum yields of PSII and PSI are calculated by measuring light curve response at different temperatures. These quantum yields are actually energy conversion parameters, which comprehensively describe the fate of excitation energy in PSII and allow deep insight into the plant's capacity to cope with excess excitation energy.

In the present study, the effect of high temperature on energy distribution in PSII and the redox state of P700 was investigated in spinach leaves. An important role of cyclic electron flow (CEF) around PSI is proposed to protect the photosynthetic apparatus from heat-induced damage.

 

MATERIALS AND METHODS

High temperature treatment to spinach leaf discs. Fresh spinach (Spinacia oleracea L.) leaves were brought from the market and cut into small discs (2.6 cm diameter) which were transferred to the Petri plates containing distilled water. Leaf discs were then incubated for 1 h in dark. Then these leaf discs were given heat treatment in dark in water-bath at respective temperature (25, 30, 35, 40, and 45°C for 5 min).

Measurement of chlorophyll fluorescence and P700 absorbance change. Light curve response was measured in heat treated leaf discs using DUAL PAM 100 by providing a range of light intensities. Fluorescence parameters were recorded after 30-s exposure to each of the photon flux density (PFD) (11, 18, 27, 58, 100, 131, 221, 344, 536, and 830 μmol/(m2 s)).

Chlorophyll (Chl) fluorescence and the redox state of P700 were measured simultaneously with a dual wavelength (830/875 nm) unit connected to a computer with control software. Actinic illumination of constant intensity was started and SP was supplied every 30 s for fluorescence and P700 analyses.

In case of measurement of Chl fluorescence, measuring light determines the F0 and F0' while Fm and Fm′ were determined by giving a saturating light pulse of 300 ms duration and 10 000 μmol/(m2 s) intensity. Same saturating light pulse was used to measure P700 parameters. Complementary quantum yields of PSII and PSI were automatically calculated by the dual PAM software. Description of various parameters of PSII and PSI obtained from LCR curves are summarized in the table.

Statistical treatment. The results are expressed as mean values and standard deviation (SD) and all the assays were carried out in replicates (3–6 sets of each analysis). The values presented are the averages of 3–4 samples in each case (temperature).

 

RESULTS AND DISCUSSION

Heat-induced changes in quantum yields of energy conversions in PSII at different temperatures

To know the effect of different temperatures (25, 35, 40, 45, 50, and 55°C) on PSII and PSI, various LCR parameters have been studied in spinach leaf discs. Y(II) is the effective quantum yield of PSII and is the product of photochemical quenching coefficient (qP) and the efficiency of excitation capture by open PSII reaction centers (Fv′/Fm′) [15–17]. Y(II) undergoes temporal changes throughout the illumination period and all the values were recorded. Here we are providing values obtained at light intensity of 100 μmol/(m2 s) where significant changes were observed. Y(II) decreased from 0.57 to 0 as the temperature stress was increased from 25 to 55°C (fig. 1). This decrease in Y(II) represents the reduced activity of PSII. The component qP was affected only slightly (up to 4%) at 40°C (5 min) and was decreased by 76, 99, and 100% at 45, 50, and 55°C respectively in comparison to control (fig. 1). The decrease in qP indicated that the number of open reaction centers of PSII was decreased. It may reflect that some of the open (active) reaction centers are converted to inactive or closed reaction centers due to high temperature stress and hence primary photochemistry is affected. With the increase in temperature stress there occur reduction in the quantum yield of PSII photochemistry which was also accompanied by an enhancement in the relative proportion of thermal energy dissipation in the antenna of PSII (1 – Fv′/Fm′) [15, 18] as indicated in the fig. 1. Fv′/Fm′ is the light-adapted maximum quantum yield of PSII [16, 17, 19] and also indicates the capacity of open PSII reaction centers to capture excitation energy. Fv′/Fm′ decreased by 13% at 40°C while at 45, 50, and 55°C a drastic decline by 63, 68, and 72% was observed respectively (fig. 1). The decreased value of Fv/Fm′ indicated that the capacity of open PSII reaction centers to capture excitation energy was low at high temperature. It was found that there was only slight change in qP at 40°C, which indicated that the decrease in Y(II) at this temperature was mainly due to the decrease in Fv′/Fm′. After treatments with temperatures above 40°C, Fv′/Fm′ as well as qP both contributed for the low effective quantum yield of PSII. The decreased effective quantum yield of PSII at high temperature stress indicated down regulation of PSII activity.

ETR(II) indicates the electron transport rate of PSII and it decreased by only 13% at 40°C and by 92, 96, and 96% at 45, 50, and 55°C, respectively (fig. 2) further indicating that the quantum yield of PSII was reduced. Another parameter Y(NPQ) is the quantum yield of regulated energy dissipation in PSII. With increasing temperature, Y(NPQ) increased from 0.18 (25°C) to 0.31, 0.24, 0.21, 0.20 at 40, 45, 50, and 55°C, respectively.

Y(NPQ) increased (fig. 1). The value of Y(NPQ) was the highest at 40°C. This dominant increase at 40°C of Y(NPQ) with respect to the control (fig. 1) is paralleled by decreases of Y(II) and Y(NO). This dominant increased value of Y(NPQ) suggested that the regulatory mechanism, i.e., the NPQ-generating photoprotective reactions were effective, and excess excitation energy was dissipated as heat [16, 20–22]. Increased Y(NPQ) also suggested that less energy was available for PSII photochemistry [16, 21]. But at 45, 50, and 55°C the increase in Y(NPQ) was low as compared to 40°C which indicated that damage to PSII was large, resulting in impaired protective regulatory mechanism. Y(NO) is the quantum yield of non-regulated energy dissipation in PSII [20, 23, 24]. As shown in fig. 1, Y(NO) decreased slightly at 40°C while further at high temperature stress from 45 to 55°C, Y(NO) increased drastically. This high value of Y(NO) indicated that plants passively dissipate a fraction of energy in the form of heat and fluorescence as protective regulatory mechanism was not much effective [16, 21]. It was supported by less increased value of Y(NPQ) from 45 to 55°C. The increase in Y(NO) due to heat stress indicates that PSII activity decreased due to increase in the number of closed reaction centers of PSII. It also indicates that both the photochemical energy conversion and the protective regulatory mechanisms are insufficient to process excitation energy. This high value of Y(NO) in the range from 45 to 55°C also suggests that increased Y(NO) was the main pathway to dissipate the excessive excitation energy. Excessive closure of PSII reaction centers due to blockage of PSII activity under heat stress may also help to protect PSI from heat induced damage [16].

Insufficient photoprotective reactions above 40°C can be understood by measuring Y(NPQ)/Y(NO) ratio which decreased from 0.72 to 0.34, 0.27, and 0.25 at 45, 50, and 55°C temperature (fig. 3). Such drop in Y(NPQ)/Y(NO) ratio is an indicative of severe damage of the photo-protective reactions. At the same time, an increase of Y(NPQ)/Y(NO) from 0.72 to 1.05 and 1.63 at 35 and 40°C indicated that protective regulatory mechanisms were effective so plant able to protect from heat induced damages through dissipating the excessive excitation energy in the form of heat.

According to the results, qP was more resistant at 40C while Fv′/Fm′ was sensitive to the temperature stress. Thus it can be said that the earlier damage in PSII was due to decreased Fv′/Fm′ (efficiency to capture excitation energy). Y(NO) was also more resistant at 40C, but at higher temperature higher than 40°C, a significant change in all the parameters resulted in decreased PSII efficiency, i.e. a decrease in open RC, reduced efficiency to capture excitation energy, less quantum yield and an enhancement in non regulatory energy dissipation, ultimately leading to impaired electron transport from PSII to PSI.

 

Study of heat-induced changes on quantum yields of energy conversions in PSI at different temperatures

High temperature affected the light response curve of PSI. Here we have not shown the actual LCR but the parameters derived from them at 100 μmol/(m2 s) Y(I) represents the photochemical quantum yield of PSI [16, 25]. Till 45C, no significant change in Y(I) was seen (fig. 4). It indicates that the photochemistry of PSI was stable till 45C. Y(I) was decreased by 42 and 45% at 50 and 55C, respectively (fig. 4) which showed that PSI photochemistry declined because of unstabilized reaction centers [20]. Y(ND) is the non-photochemical quantum yield of PSI due to donor side limitation [16, 20]. Y(ND) significantly increased as the temperature was raised which indicates that PSII activity decreased due to damage at the donor side of PSII (fig. 4). This inhibition of PSII activity occured because of damage in the water splitting complex [16] and resulted in closed RCs. Y(ND) also represents the fraction of overall P700 that was oxidized at a given state due to a lack of donors. Thus, centers with oxidised P700 transform absorbed excitation energy quantitatively into heat. Y(NA) is the non-photochemical quantum yield of PSI due to acceptor side limitation. It was observed that with the increase in temperature Y(NA) decreased (fig. 4). Decreased value of Y(NA) indicates that the acceptor side limitation of PSI, i.e. the over-reduction of PSI was prevented (suppressed) under temperature stress and PSI was protected from heat induced damages. Decrease in the over-reduction of PSI may be due to the activation of cyclic electron flow (CEF) around PSI [16]. Till 45C stability in Y(I) and decrease in Y(NA) indicate that PSI was well protected probably due to strong stimulation of CEF around PSI. Furthermore, results also revealed that after 45C temperature, there occur decrease in Y(I) indicating damage to PSI, it suggested that although Y(NA) was low but unable to give sufficient protection to PSI from heat induced damages. Furthermore, results also indicated that PSI was stable till 45C while high value of Y(ND) due to high temperature stress indicated that donor side of PSII was damaged.

 

Study of heat-induced changes on cyclic electron flow at different temperatures

Analysis of the effect of high temperature was done on the role of quantum yield of cyclic electron flow (CEF) around PSI in spinach leaves treated at different temperature (2555C).

Y(CEF) is the effective quantum yield of cyclic electron flow and is calculated as Y(CEF) = Y(I)  Y(II) [16]. Y(CEF) increased with the increase in temperature (fig. 3). Maximum increase in the value of Y(CEF) from 0.21 to 0.65 was seen at 45°C. After 45°C the increase in Y(CEF) was lower. This stimulation of PSI mediated cyclic electron transport causes the protection of PSI from heat induced impairments. Enhanced CEF led to suppression in the over-reduction of the PSI acceptor side. Decrease in the over-reduction of the PSI acceptor side was also supported by decreased value of Y(NA). It was known that process of carbon fixation decreases at moderate thermal stress [26, 27]. The declined carbon fixation causes the over-reduction of the PSI acceptor side and excess accumulation of reducing power NADPH. Ultimately these conditions caused enhanced CEF around PSI. This enhanced CEF causes: 1) decreases in the over-reduction of the PSI acceptor side through transporting electrons from the PSI acceptor side to PQ, 2) also consumes excess reducing power NADPH through the NADPH dehydrogenase-dependent pathway [28]. Thus according to our results till 45°C PSI is stable and protective while at still higher temperature, some protection was provided by stimulation of CEF.

Thus, it is concluded that at 40°C the decrease in PSII activity was mainly due to the decreased efficiency to capture excitation energy, increased yield of regulatory energy dissipation mechanism Y(NPQ), and decreased quantum yield Y(II). qP and Y(NO) are more resistant to 40°C (5 min) than at higher temperature. After 40°C a drastic enhancement in the non regulatory energy dissipation Y(NO) occurs due to less effective protective energy dissipation Y(NPQ). Along with this a significant decrease in the number of open RC, reduced efficiency to capture excitation energy and less quantum yield were seen. Ultimately it leads to impaired electron transport from PSII to PSI.

A gradual increase in the value of Y(ND) due to the high temperature indicates impairment at the donor side of PSII. Impairment in PSII activity may be due to slow rate of charge separation reaction of PSII. PSI is more stable than PSII under high temperature stress. Photochemistry of PSI was stable up to 45°C while it decreased at higher temperature (50 and 55°C). Due to high temperature stress the value of Y(NA) is low indicating suppression of the over-reduction of PSI acceptor side. This suppression of the over-reduction of PSI acceptor side is caused by increased Y(CEF) around PSI and helps to tolerate heat induced inhibitions of PSI. Thus due to high temperature, the decrease in Y(NA) should be mainly caused by the strong stimulation of CEF and it contributes to protect PSI from heat induced damage. According to the results till 45°C PSI is stable and protective while at still higher temperature stability of PSI was reduced and protection was not sufficient. Therefore, we conclude that CEF plays an important role in protection for PSI from heat stress.

Support from University Grant Commission (no. F.5-26/2007(BSR)) to D.A. and Joint Indo-Russian project from Department of Science and Technology (DST/RUS//RFBR/P-173) to A.J. and RFBR-DST (no. 14-04-92690) to S.I.A. is thankfully acknowledged.

 

REFERENCES

 

  1. Mathur S., Agrawal A., Jajoo A. Photosynthesis: response to high temperature stress // J. Photochem. Photobiol. B: Biol. 2014. V. 137. P. 116-126.
  2. Mehta P., Jajoo A., Mathur S., Bharti S. Chlorophyll a fluorescence study revealing effects of high salt stress on photosystem II in wheat leaves // Plant Physiol. Biochem. 2010. V. 48. P. 16-20.
  3. Mathur S., Jajoo A., Mehta P., Bharti S. Analysis of elevated temperature-induced inhibition of photosystem II by using chlorophyll a fluorescence induction kinetics in wheat leaves (Triticum aestivum) // Plant Biol. 2010. V. 13. P. 1-6.
  4. Tomar R.S., Jajoo A. A quick investigation of the detrimental effects of environmental pollutant polycyclic aromatic hydrocarbon fluoranthene on the photosynthetic efficiency of wheat (Triticum aestivum) // Ecotoxicology. 2013. V. 22. P. 131-138.
  5. Mathur S., Jajoo A. Investigating deleterious effects of ultraviolet (UV) radiations on wheat by a quick method // Acta Physiol. Plant. 2015. V. 37. P. 121.
  6. Carpentier R. Effect of high-temperature stress on the photosynthetic apparatus // Handbook of Plant and Crop Stress / Ed. Pessarakli M. New York: Marcel Dekker, 1999. P. 337-348.
  7. Los D.A., Murata N. Membrane fluidity and its roles in the perception of environmental signals // Biochim. Biophys. Acta. 2004. V. 1666. P. 142-157.
  8. Tiwari A., Jajoo A., Bharti S. Heat-induced changes in the EPR signal of tyrosine D (YOX): a possible role of cytochrome b559 // J. Bioenerg. Biomembr. 2008. V. 40. P. 237-243.
  9. Gounaris K., Brain A.R.R., Quinn P.J., Williams W.P. Structural and functional changes associated with heat-induced phase separation of non-bilayer lipids in chloroplast thylakoid membranes // FEBS Lett. 1983. V. 153. P. 47-53.
  10. Sundby C., Melis A., Mдenpдд P., Andersson B. Temperature dependent changes in the antenna size of photosystem II. Reversible conversion of photosystem II?? to photosystem II?? // Biochim. Biophys. Acta. 1986. V. 851, P. 475-483.
  11. Kreslavski V., Tatarinzev N., Shabnova N., Semenovab G., Kosobryukhov A. Characterization of the nature of photosynthetic recovery of wheat seedlings from short-term dark heat exposures and analysis of the mode of acclimation to different light intensities // J. Plant Physiol. 2008. V. 165. P. 1592-1600.
  12. Guo Y.P., Zhou H.F., Zhang L.C. Photosynthetic characteristics and protective mechanisms against photooxidation during high temperature stress in two citrus species // Sci. Hortic. 2006. V. 108. P. 260-267.
  13. Lotfi R., Kouchebagh G., Khoshvaghti H. Biochemical and physiological responses of Brassica napus plants to humic acid under water stress // Физиология растений. 2015. Т. 62. С. 514-520.
  14. Klughammer C., Schreiber U. Complementary PSII quantum yields calculated from simple fluorescence parameters measured by PAM fluorometry and the Saturation Pulse method // PAM Application Notes. 2008. V. 1. P. 27-35.
  15. Georgieva K., Maslenkova L. Thermostability and photostabitity of PSII in leaves of resurrection plant Haberlea rhodopensis studied by means of chlorophyll fluorescence // Z. Naturforsh. 2006. V. 61c. P. 234-240.
  16. Huang W., Zhang S.B., Cao K.F. Cyclic electron flow plays an important role in photoprotection of tropical trees illuminated at temporal chilling temperature // Plant Cell Physiol. 2011. V. 52. P. 297-305.
  17. Huang W., Yang S.J., Zhang S.B., Zhang J.L., Cao K.F. Cyclic electron flow plays an important role in photoprotection for the resurrection plant Paraboea rufescens under drought stress // Planta. 2012. V. 235. P. 819-828.
  18. Demmig-Adams B., Adams W.W., Barker D.H., Logan B.A., Bowling B.A., Verhoeven A.S. Using chlorophyll fluorescence to assess the fraction of absorbed light allocated to thermal dissipation of excess excitation // Physiol. Plant. 1996. V. 98. P. 253-264.
  19. Kramer D.M., Johnson G., Kiirats O., Edwards G.E. New fluorescence parameters for the determination of QA redox state and excitation energy fluxes // Photosynth. Res. 2004. V. 79. P. 209-218.
  20. Essemine J., Govindachary S., Ammar S., Bouzid S., Carpentier R. Enhanced sensitivity of the photosynthetic apparatus to heat stress in digalactosyl-diacylglycerol deficient Arabidopsis // Environ. Exp. Bot. 2012. V. 80. P. 16-26.
  21. Hoogenboom M.O., Campbell D.A., Beraud E., DeZeeuw K., Ferrier-Pagйs C. Effects of light, food availability and temperature stress on the function of photosystem II and photosystem I of coral symbionts // PLoS one. 2012. V. 7(1): e30167. doi 10.1371/journal.pone.0030167
  22. Holzwarth A.R., Lenk D., Jahns P. On the analysis of non-photochemical chlorophyll fluorescence quenching curves. I. Theoretical considerations // Biochim. Biophys. Acta. 2013. V. 1827. P. 786-792.
  23. Ait A.N., Dewez D., Didur O., Popovic R. Effect of dichromate on photosystem II activity in xanthophyll-deficient mutants of Chlamydomonas reinhardtii // Photosynth. Res. 2008. V. 95. P. 45-53.
  24. Zivcak M., Brestic M., Kalaji H.M., Govindjee. Photosynthetic responses of sun- and shade-grown barley leaves to high light: is the lower PSII connectivity in shade leaves associated with protection against excess of light? // Photosynth. Res. 2014. V. 119. P. 339-354.
  25. Coopman R.E., Fuentes-Neira F.P., Briceсo V.F., Cabrera H.M., Corcuera L.J., Bravo L.A. Light energy partitioning in photosystems I and II during development of Nothofagus nitida growing under different light environments in the Chilean evergreen temperate rain forest // Trees. 2010. V. 24. P. 247-259.
  26. Feller U., Crafts-Brandner S.J., Salvucci M.E. Moderately high temperatures inhibit ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activase-mediated activation of Rubisco // Plant Physiol. 1998. V. 116. P. 539-546.
  27. Sharkey T.D. Effects of moderate heat stress on photosynthesis: importance of thylakoid reactions, Rubisco deactivation, reactive oxygen species, and thermotolerance provided by isoprene // Plant Cell Environ. 2005. V. 28. P. 269-277.
  28. Shikanai T. Cyclic electron transport around photosystem I: genetic approaches // Annu. Rev. Plant Biol. 2007. V. 58. P. 199-217.
  29. Summary of parameters, formulae and their description using data extracted from light curve responses [14, 16]

 

F0 and F0

Minimum fluorescence in the dark and light adapted state respiration.

Fm and Fm

Maximum fluorescence in the dark and light adapted state respiration.

Fv/Fm

Maximum quantum yield of PSII after dark adaptation = (FmF0)/Fm.

Fv/Fm

Maximum quantum yield of PSII after light adaptation = (Fm′ − F0′)/Fm′.

qP

Photochemical quenching = (Fm′ − Fs)/(Fm′ − F0′), where Fs represents the steady state fluorescence in light adapted state.

ETR(II)

Electron transport rate of PSII. The ETR(II) is a relative measure of the rate of electron transport (rate of charge separation at PSII reaction centers).

Y(II)

Effective quantum yield of PSII = (Fm′ − Fs)/Fm′.

Y(NO)

Quantum yield of non-regulated energy dissipation of PSII = Fs/Fm.

 

These complementary quantum yields add up to one: Y(II) + Y(NPQ) + Y(NO) = 1.

ETR(I)

Electron transport rate of PSI.

Y(I)

Effective quantum yield of PSI = 1 − Y(ND) − Y(NA). Y(I) is the fraction of overall P700 that in a given state is reduced and not limited by the acceptor side.

Y(ND)

Non-photochemical quantum yield of PSI due to donor side limitation (fraction of overall P700 that is oxidized in a given state) = 1 − P700red. P700red was determined by saturation pulse. It represents the fraction of overall P700 that is reduced in a given state.

Y(NA)

Non-photochemical quantum yield of PSI due to acceptor side limitation (fraction of overall P700 that cannot be oxidized by a saturation pulse in a given state due to a lack of acceptors = (PmPm′)/Pm). The sum of these three complementary quantum yields is unity: Y(I) + Y(ND) + Y(NA) = 1.

Y(CEF)

Y(CEF) around PSI was estimated from the difference between Y(I) and Y(II).

FIGURE CAPTIONS

 

Fig. 1. Effects of different temperatures on spinach leaf discs in relation to various parameters obtained from light response curves (LCR) at 100 μmol/(m2 s) light intensity.

1 – Y(II), quantum yield of PSII; 2  qP, photochemical fluorescence quenching; 3Fv'/Fm', efficiency of excitation capture by open PSII reaction centers; 4  (1  Fv'/Fm'), the relative proportion of excitation energy dissipated as heat in the PSII antenna; 5  Y(NO), non-regulated and 6  Y(NPQ) regulated energy dissipation.

 

Fig. 2. Effects of different temperatures on spinach leaf discs in relation to the electron transport rate of PSII, ETR (II) at 100 μmol/(m2 s) light intensity.

 

Fig. 3. Effects of different temperatures on (a) the ratio of the effective quantum yield of PSI and PSII (ΦPSI/ΦPSII), non-photochemical quenching (NPQ) of Chl fluorescence and (b) quantum yield of cyclic electron flow around PSI Y(CEF) in spinach leaf discs at 100 μmol/(m2 s) light intensity.

 

Fig. 4. Changes in the quantum yield of PSI, Y(I) (1); the quantum yield of non-photochemical quenching due to donor side limitation Y(ND) (2) and the quantum yield of non-photochemical quenching due to acceptor side limitation Y(NA) (3) in spinach leaf discs treated at different temperatures at 100 μmol/(m2 s) light intensity.