Energy transfer in the photochemical apparatus of flashed bean leaves

Frequently asked questions about in vivo chlorophyll fluorescence: practical issues

The aim of this educational review is to provide practical information on the hardware, methodology, and the hands on application of chlorophyll (Chl) a fluorescence technology. We present the paper in a question and answer format like frequently asked questions. Although nearly all information on the application of Chl a fluorescence can be found in the literature, it is not always easily accessible. This paper is primarily aimed at scientists who have some experience with the application of Chl a fluorescence but are still in the process of discovering what it all means and how it can be used. Topics discussed are (among other things) the kind of information that can be obtained using different fluorescence techniques, the interpretation of Chl a fluorescence signals, specific applications of these techniques, and practical advice on different subjects, such as on the length of dark adaptation before measurement of the Chl a fluorescence transient. The paper also provides the physiological background for some of the applied procedures. It also serves as a source of reference for experienced scientists.

The energy flux theory 35 years later: formulations and applications

Several models have been proposed for the energetic behavior of the photosynthetic apparatus and a variety of experimental techniques are nowadays available to determine parameters that can quantify this behavior. The Energy Flux Theory (EFT) developed by Strasser 35 years ago provides a straightforward way to formulate any possible energetic communication between any complex arrangement of interconnected pigment systems and any energy transduction by these systems. We here revisit the EFT, starting from the basic general definitions and equations and presenting applications in formulating the energy distribution in photosystem (PS) II units with variable connectivity, as originally derived, where certain simplifications were adopted. We then proceed to the derivation of equations for a PSII model of higher complexity, which corresponds, from the formalistic point of view, to the later formulated and now broadly accepted exciton-radical-pair model. We also compare the formulations derived with the EFT with those obtained, by different approaches, in the classic papers on energetic connectivity. Moreover, we apply the EFT for the evaluation of the excitation energy distribution between PSII and PSI and the distinction between state transitions and PSII to PSI excitation energy migration. Our analysis demonstrates that the EFT is a powerful approach for the formulation of any possible model, at any complexity level, even of models that may be proposed in the future, with the advantage that any possible energetic communication or energy transduction can be easily formulated mathematically by trivial algebraic equations. Moreover, the biophysical parameters introduced by the EFT and applicable for any possible model can be linked with obtainable experimental signals, provided that the theoretical resolution of the model does not go beyond the experimental resolution.

Quantification of stress adaptation by laser-induced fluorescence spectroscopy of plants exposed to engine exhaust emission and drought

The effects of drought and petrol engine exhaust pollutants, such as SO₂ and NO₂ and suspended particulate matter (SPM), on the photosynthetic activity of colocasia [Colocasia esculenta (L.) Schott], kacholam (Kaempferia galanga L.) and tapioca (Manihot esculenta Crantz) plants were studied from in vivo laser-induced chlorophyll fluorescence (LICF) spectra. An open-top chamber (OTC) of 2.5 m diameter and 3 m height incorporating an air-filtering unit was developed for this study. Plants grown inside the OTC were exposed to exhaust emissions from a two-stroke Birla Yamaha genset for 10 d, while a control group was maintained outside. Gaseous pollutants and SPM present inside the OTC during the exposure period were measured with a high-volume air sampler. The steady-state LICF spectra of the control and treated plants were recorded in the 650-750-nm region. Fluorescence induction kinetics (Kautsky effect) was also recorded during the stress period from dark-adapted intact plant leaves at the chlorophyll bands of 685 and 730 nm. The vitality indexes (Rfd-685 and Rfd-730) and stress adaptation index (Ap) derived from the induction kinetics were utilised along with the chlorophyll fluorescence intensity ratio (F685 /  F730) for evaluation of stress-induced changes in plants.

Violaxanthin de-epoxidase in etiolated leaves

In etiolated leaves the occurrence of the enzymatic violaxanthin de-epoxidation to zeaxanthin is shown. The carotenoid transformation is provoked by the infiltration of whole leaves with ascorbate at pH 5 and is susceptible to DTT. Identification of the de-epoxidase activity is achieved by in vivo spectroscopy and pigment analysis (TLC).

Mono- bi- tri- and polypartite models in photosynthesis

It is shown how energy fluxes in mono-bi-tri- and polypartite photosystems can be described. The derivation of the energy distribution term α and the probability of spill over p21 as proposed by W.L. BUTLER are reviewed.

Some effects of 4-chloro-5-(dimethylamino)-2-phenyl-3(2H)-pyridazinone (San 9785) on the development of chloroplast thylakoid membranes in Hordeum vulgare L

Chloroplast ultrastructural and photochemical features were examined in 6-d-old barley (Hordeum vulgare L. cv. Sundance) plants which had developed in the presence of 4-chloro-5-(dimethylamino)-2-phenyl-3(2H)-pyridazinone (San 9785). In spite of a substantial modification of the fatty-acid composition of thylakoid lipids there were no gross abnormalities in chloroplast morphology, and normal amounts of membrane and chlorophyll were present. Fluorescence kinetics at 77K demonstrated considerable energetic interaction of photosystem (PS)I and PSII chlorophylls within the altered lipid environment. An interference with electron transport was indicated from altered room-temperature fluorescence kinetics at 20°C. Subtle changes in the arrangements of chloroplast membranes were consistently evident and the overall effects of these changes was to increase the proportion of appressed to nonappressed membranes. This correlated with a lower chlorophyll a/b ratio, an increase in the amount of light-harvesting chlorophylls as determined by gel electrophoresis and fluorescence emission spectra, and an increase in excitation-energy transfer from PSII to PSI, as predicted from current ideas on the organisation of photosystems in appressed and non-appressed thylakoid membranes.

Thylakoid protein kinase activity and associated control of excitation energy distribution during chloroplast biogenesis in wheat

The activity of thylakoid protein kinase and the regulation of excitation energy distribution between photosystems I and II was examined during chloroplast biogenesis in light-grown Triticum aestivum (wheat) leaves. The specific activity of the thylakoid protein kinase decreased some six-fold during development from the young plastids at the base of the 7-d-old leaf to the mature chloroplasts at the leaf tip. Appreciable activity was also detected in plastids isolated from etiolated leaves. In mature chloroplasts the majority of phosphate was incorporated into the Mr=26,000 apo-proteins of the light-harvesting chlorophyll a/b-protein complex (LHCP). However, at early stages of chloroplast development and in the etioplast, the phosphate was predominantly incorporated into a polypeptide of Mr=9,000 dalton. Immature thylakoids, isolated from the base of the leaf, had relatively low concentrations of LHCP and could perform a State 1-State 2 transition, as demonstrated by ATP-induced quenching of photosystem II fluorescence. Analyses of photosystem I and photosystem II fluorescence-induction curves from intact leaf tissue demonstrated that this transition occurs in vivo at early stages of leaf development and, therefore, may play an important role in regulating energy transduction during chloroplast biogenesis.

Connection between the rate of cooling and fluorescence properties at 77 K or isolated chloroplasts

Cooling of chloroplasts to--196 degrees C can under certain circumstances lead to an erroneous analysis of energy distribution. After minimizing influences of sample geometry and effects of plastid concentration it is shown that externally induced membrane change leads to an increase in the ratio F740/F687 of the fluorescence emission spectrum. Similar alterations can be observed by variation of the rate of cooling the plastids to 77 K, expecially if whole chloroplasts are used. The differences in emission ratios are indicative also of changes in initial energy distribution between the photosystems, given here by the value alphaN. This is inferred from experiments with either osmotically induced thylakoid disturbances or those effected through a slow cooling process. The circumstances and the significance of these observations are discussed.

Competition between the 735 nm fluorescence and the photochemistry of Photosystem I in chloroplasts at low temperature

Fluorescence emission spectra of chloroplasts, initially frozen to--196 degrees C, were measured at various temperatures as the sample was allowed to warm. The 735 nm emission band attributed to fluorescence from Photosystem I was approx. 10-fold greater at--196 degrees C than at--78 degrees C. The initial rate of photooxidation of P-700 was also measured at--196 degrees C and--78 degrees C and was found to be approximately twice as large at the higher temperature. It is proposed that the 735 nm emission band is fluorescence from a long wavelength form of chlorophyll, C-705, which acts as a trap for excitation energy in the antenna chlorophyl system of Photosystem I. Furthermore, it is proposed that C-705 only forms on cooling to low temperatures and that the temperature dependence of the 735 nm emission is the temperature dependence for the formation of C-705. C-705 and P-700 compete to trap the excitation energy in Photosystem I. It is estimated from the data that at--78 degrees C P-700 traps approx. 20 times more energy than C-705 while, at--196 degrees C, the two traps are approximately equally effective. By analogy, the 695 nm fluorescence which also appears on cooling to--196 degrees C is attributed to traps in Photosystem II which form only on cooling to temperatures near--196 degrees C.

Tripartite and bipartite models of the photochemical apparatus of photosynthesis

Tripartite and bipartite models for the photochemical apparatus of photosynthesis are presented and examined. It is shown that the equations for the yields of fluorescence from the different parts of the photochemical apparatus of the tripartite model transform into the simple equations of the bipartite formulation when the probability for energy transfer from the light-harvesting chlorophyll a/b complex to photosystem II is unity. The nature of the 695 and 735 nm fluorescence bands which appear in the emission spectrum of chloroplasts at low temperature is examined. It is proposed that these bands are due to fluorescence from energy-trapping centres which form in the antenna chlorophyll of photosystem II and photosystem I on cooling to low temperature. Even though these fluorescence emissions can be regarded as low temperature artifacts since they are not present at physiological temperatures, they nevertheless are proportional to the excitation energy in the two photosystems and can be used to monitor energy distribution in the photochemical apparatus. However, the question of their artifactual nature is crucial to the interpretation of fluorescence-lifetime measurements at low temperature.

Fluorescence emission spectra of photosystem I, photosystem II and the light-harvesting chlorophyll a/b complex of higher plants

Fluorescence emission spectra excited at 514 and 633 nm were measured at -196 degrees C on dark-grown bean leaves which had been partially greened by a repetitive series of brief xenon flashes. Excitation at 514 nm resulted in a greater relative enrichment of the 730 nm emission band of Photosystem I than was obtained with 633 nm excitation. The difference spectrum between the 514 nm excited fluorescence and the 633 nm excited fluorescence was taken to be representative of a pure Photosystem I emission spectrum at -196 degrees C. It was estimated from an extrapolation of low temperature emission spectra taken from a series of flashed leaves of different chlorophyll content that the emission from Photosystem II at 730 nm was 12% of the peak emission at 694 nm. Using this estimate, the pure Photosystem I emission spectrum was subtracted from the measured emission spectrum of a flashed leaf to give an emission spectrum representative of pure Photosystem II fluorescence at -196 degrees C. Emission spectra were also measured on flashed leaves which had been illuminated for several hours in continuous light. Appreciable amounts of the light-harvesting chlorophyll a/b protein, which has a low temperature fluorescence emission maximum at 682 nm, accumulate during greening in continuous light. The emission spectra of Photosystem I and Photosystem II were subtracted from the measured emission spectrum of such a leaf to obtain the emission spectrum of the light-harvesting chlorophyll a/b protein at -196 degrees C.

Energy transfer from photosystem II to photosystem I in Porphyridium cruentum

Rates of photooxidation of P-700 by green (560 nm) or blue (438 nm) light were measured in whole cells of porphyridium cruentum which had been frozen to -196 degrees C under conditions in which the Photosystem II reaction centers were either all open (dark adapted cells) or all closed (preilluminated cells). The rate of photooxidation of P-700 at -196 degrees C by green actinic light was approx. 80% faster in the preilluminated cells than in the dark-adapted cells. With blue actinic light, the rates of P-700 photooxidation in the dark-adapted and preilluminated cells were not significantly different. These results are in excellent agreement with predictions based on our previous estimates of energy distribution in the photosynthetic apparatus of Porphyridium cruentum including the yield of energy transfer from Photosystem II to Photosystem I determined from low temperature fluorescence measurements.

Does the rate of cooling affect fluorescence properties of chloroplasts at -196 degrees C?

The question addressed in the title was examined by measuring fluorescence emission spectra and light-induced fluorescence-yield changes of chloroplasts which had been frozen to -196 degrees C rapidly, as very thin samples adsorbed into substrates whick were plunged directly into liquid nitrogen, or slowly by the cooling action of liquid nitrogen through the wall of the cuvette. Contrary to previous reports, we found that the rate of cooling had no influence on the shape of the emission spectrum, the extent of the variable fluorescence or the fraction of the absorbed quanta which are delivered initially to Photosystem I.

Energy transfer and the distribution of excitation energy in the photosynthetic apparatus of spinach chloroplasts

Equations are derived from our model of the photochemical apparatus of photosynthesis to show that the yield of energy transfer from Photosystem II to Photosystem I, phi T(II leads to I), can be obtained from measurements on an individual sample of chloroplasts frozen to -196 degrees C by comparing the sum of two specifically defined fluorescence excitation spectra with the absorption spectrum of the sample. Then, given that value of phiT(II leads to I), the fraction of the quanta absorbed by the photochemical apparatus which is distributed initially to Photosystem I, alpha, can be determined as a function of the wavelength of excitation from the same fluorescence excitation spectra. The results obtained in this study of individual samples of chloroplasts frozen to -196 degrees C in the absence of divalent cations, namely, that phi T(II leads to I)varies from a minimum value of 0.10 when the Photosystem II reaction centers are all open to a maximum value of 0.25 when the centers are all closed and that alpha has a value of about 0.30 which is almost independent of wavelength for wavelength shorter than 675 nm (alpha increases rapidly toward unity at wavelength longer than 675 nm), agrees quite well with results obtained previously from comparative measurements of chloroplasts frozen to -196 degrees C in the presence and absence of divalent cations.