Tansley Review No. 36 Excited leaves

Photosynthesis of the Cyanidioschyzon merolae cells in blue, red, and white light

Photosynthesis and respiration rates, pigment contents, CO2 compensation point, and carbonic anhydrase activity in Cyanidioschizon merolae cultivated in blue, red, and white light were measured. At the same light quality as during the growth, the photosynthesis of cells in blue light was significantly lowered, while under red light only slightly decreased as compared with white control. In white light, the quality of light during growth had no effect on the rate of photosynthesis at low O2 and high CO2 concentration, whereas their atmospheric level caused only slight decrease. Blue light reduced markedly photosynthesis rate of cells grown in white and red light, whereas the effect of red light was not so great. Only cells grown in the blue light showed increased respiration rate following the period of both the darkness and illumination. Cells grown in red light had the greatest amount of chlorophyll a, zeaxanthin, and β-carotene, while those in blue light had more phycocyanin. The dependence on O2 concentration of the CO2 compensation point and the rate of photosynthesis indicate that this alga possessed photorespiration. Differences in the rate of photosynthesis at different light qualities are discussed in relation to the content of pigments and transferred light energy together with the possible influence of related processes. Our data showed that blue and red light regulate photosynthesis in C. merolae for adjusting its metabolism to unfavorable for photosynthesis light conditions.

Evolution of the Z-scheme of photosynthesis: a perspective

The concept of the Z-scheme of oxygenic photosynthesis is in all the textbooks. However, its evolution is not. We focus here mainly on some of the history of its biophysical aspects. We have arbitrarily divided here the 1941-2016 period into three sub-periods: (a) Origin of the concept of two light reactions: first hinted at, in 1941, by James Franck and Karl Herzfeld; described and explained, in 1945, by Eugene Rabinowitch; and a clear hypothesis, given in 1956 by Rabinowitch, of the then available cytochrome experiments: one light oxidizing it and another reducing it; (b) Experimental discovery of the two light reactions and two pigment systems and the Z-scheme of photosynthesis: Robert Emerson's discovery, in 1957, of enhancement in photosynthesis when two light beams (one in the far-red region, and the other of shorter wavelengths) are given together than when given separately; and the 1960 scheme of Robin Hill & Fay Bendall; and

Chlorophyll fluorescence varies more across seasons than leaf water potential in drought-prone plants

Among the effects of environmental change, the intensification of drought events is noteworthy, and tropical vegetation is predicted to be highly vulnerable to it. However, it is not clear how tropical plants in drought-prone habitats will respond to this change. In a coastal sandy plain environment, we evaluated the response of six plant species to water deficits across seasons, the relationship between their morpho-physiological traits, and which traits would be the best descriptors of plants' response to drought. Regardless of leaf succulence and phenology, responses between seasons were most strongly related to chlorophyll fluorescence. In this study we have demonstrated that a better comprehension of how tropical species from drought-prone habitats cope with changes in water availability can be based on seasonal variation in leaf water potential and chlorophyll fluorescence. Temporal variation in leaf water potential and chlorophyll fluorescence was found useful for differentiating between groups of sandy soil species that are responsive or unresponsive to water availability. However, chlorophyll fluorescence appeared to be a more sensitive descriptor of their seasonal and short-term responses.

Modification of the biochemical pathways of plants induced by ozone: what are the varied routes to change?

When plants are observed under a low dose of ozone, some physiological and metabolic shifts occur. Barring extreme injury such as tissue damage or stomata closure, most of these disruptive changes are likely to have been initiated at the level of gene expression. The belief is oxidative products formed in ozone exposed leaves, e.g. hydrogen peroxide, are responsible for much of the biochemical adjustments. The first line of defense is a range of antioxidants, such as ascorbate and glutathione, but if this defense is overwhelmed, subsequent actions occur, similar to systemic acquired resistance or general wounding. Yet there are seemingly unrelated metabolic responses which are also triggered, such as early senescence. We discuss here the current understanding of gene control and signal transduction/control in order to increase our comprehension of how ozone alters the basic metabolism of plants and how plants counteract or cope with ozone.

The quantum yield of CO2 fixation is reduced for several minutes after prior exposure to darkness. Exploration of the underlying causes

Previous work has shown that the apparent quantum yield of CO2 fixation can be reduced for up to several minutes after prior exposure to darkness. In the work reported here, we investigated this phenomenon more fully and have deduced information about the underlying processes. This was done mainly by concurrent measurements of O2 and CO2 exchange in an oxygen-free atmosphere. Measurements of O2 evolution indicated that photochemical efficiency was not lost through dark adaptation, and that O2 evolution could proceed immediately at high rates provided that there were reducible pools of photosynthetic intermediates. Part of the delay in reaching the full quantum yield of CO2 fixation could be attributed to the need to build up pools of photosynthetic intermediates to high enough levels to support steady rates of CO2 fixation. There was no evidence that Rubisco inactivation contributed towards delayed CO2 uptake (under measurement conditions of low light). However, we obtained evidence that an enzyme in the reaction path between triose phosphates and RuBP must become completely inactivated in the dark. As a consequence, in dark-adapted leaves, a large amount of triose phosphates were exported from the chloroplast over the first minute of light rather than being converted to RuBP for CO2 fixation. That pattern was not observed if the pre-incubation light level was increased to just 3-5 micromol quanta m(-2) s(-1). The findings from this work underscore that there are fundamental differences in enzyme activation between complete darkness and even a very low light level of only 3-5 micromol quanta m(-2) s(-1) which predispose leaves to different gas exchange patterns once leaves are transferred to higher light levels.

Estimating photosynthetic electron transport via chlorophyll fluorometry without Photosystem II light saturation

Estimates of thylakoid electron transport rates (J(e)) from chlorophyll fluorometry are often used in combination with leaf gas exchange measurements to provide detailed information about photosynthetic activity of leaves in situ. Estimating J(e) requires accurate determination of the quantum efficiency of Photosystem II (Phi(P)), which in turn requires momentary light saturation of the Photosystem II light harvesting complex to induce the maximum fluorescence signal (F(M)'). In practice, full saturation is often difficult to achieve, especially when incident photosynthetic photon flux density (Q) is high and energy is effectively dissipated by non-photochemical quenching. In the present work, a method for estimating the true F(M)' under high Q was developed, using multiple light pulses of varying intensity (Q'). The form of the expected relationship between the apparent F(M)' and Q' was derived from theoretical considerations. This allowed the true F(M)' at infinite Q' to be estimated from linear regression. Using a commercially available leaf gas exchange/ chlorophyll fluorescence measurement system, J(e) was compared to gross photosynthetic CO(2) assimilation (A(G)) under conditions where the relationship between J(e) and A(G) was expected to be linear. Both in C(4) leaves (Zea mays) in ambient air and also in C(3) leaves (Gossypium hirsutum) under non-photorespiratory conditions the apparent ratio between J(e) and A(G) declined at high Q when Phi(P) was calculated from F(M)' measured simply using the highest available saturating pulse intensity. When F(M)' was determined using the multiple pulse / linear regression technique, the expected relationship between J(e) and A(G) at high Q was restored, indicating that the Phi(P) estimate was improved. This method of determining F(M)' should prove useful for verifying when saturating pulse intensities are sufficient, and for accurately determining Phi(P) when they are not.

The Z-scheme--down hill all the way

The Z-scheme is considered in the context of personal recollections of events during the time that it was conceived and an evaluation of its enduring importance.

Physiological response of Pinus halepensis needles under ozone and water stress conditions

The aim of this study was to evaluate how physiological processes of potted Pinus halepensis plants, grown under controlled conditions, were affected by ozone (O3) and/or water stress, integrating the gas exchange and biochemical data with fluorescence OJIP polyphasic transient data. Plants submitted to only water stress (T1) and with ozone (T3) showed a strong decrease in stomatal conductance and gas exchange, coinciding with a reduction of maximum yield of photochemistry (varphipo) and very negative values of leaf water potential. Simultaneously, a great increase of both PSII antenna size, indicated by absorption per reaction centre, and electron transport per reaction centre were found. The reduction of photosynthesis in the O3-treated plants (T2) by a slowing down of the Calvin cycle was supported by the increase of related fluorescence parameters such as relative variable fluorescence, heat de-excitation constant, energy de-excitation by spillover, and the decrease of varphipo. We suggest an antagonistic effect between the two stresses to explain the delayed ozone-induced decrease of stomatal conductance values for T3 with respect to T1 plants, by an alteration of the physiological mechanisms of stomatal opening, which involve the increase of intra-cellular free-calcium induced by ABA under co-occurring water shortage. We emphasise the importance of considering the intensity of the individual stress factor in studies concerning the interaction of stresses.

Electron transport to oxygen mitigates against the photoinactivation of Photosystem II in vivo

The role of electron transport to O2 in mitigating against photoinactivation of Photosystem (PS) II was investigated in leaves of pea (Pisum sativum L.) grown in moderate light (250 μmol m(-2) s(-1)). During short-term illumination, the electron flux at PS II and non-radiative dissipation of absorbed quanta, calculated from chlorophyll fluorescence quenching, increased with increasing O2 concentration at each light regime tested. The photoinactivation of PS II in pea leaves was monitored by the oxygen yield per repetitive flash as a function of photon exposure (mol photons m(-2)). The number of functional PS II complexes decreased nonlinearly with increasing photon exposure, with greater photoinactivation of PS II at a lower O2 concentration. The results suggest that electron transport to O2, via the twin processes of oxygenase photorespiration and the Mehler reaction, mitigates against the photoinactivation of PS II in vivo, through both utilization of photons in electron transport and increased nonradiative dissipation of excitation. Photoprotection via electron transport to O2 in vivo is a useful addition to the large extent of photoprotection mediated by carbon-assimilatory electron transport in 1.1% CO2 alone.

An active Mehler-peroxidase reaction sequence can prevent cyclic PS I electron transport in the presence of dioxygen in intact spinach chloroplasts

Simultaneous measurements of 9-aminoacridine (9-AA) fluorescence quenching, O2-uptake and chlorophyll fluorescence of intact spinach chloroplasts were carried out to assess the relationship between the transthylakoidal ΔpH and linear electron flux passing through Photosystem II. Three different types of O2-dependent electron flow were investigated: (1) Catalysed by methyl viologen; (2) in the absence of a catalyst and presence of an active ascorbate peroxidase (Mehler-peroxidase reaction); (3) in the absence of a catalyst and with the ascorbate peroxidase being inhibited by KCN (Mehler reaction). The aim of this study was to assess the relative contribution of ΔpH-formation which is not associated with electron flow through Photosystem II and, which should reflect Photosystem I cyclic flow under the different conditions. The relationship between the extent of 9-AA fluorescence quenching and O2-uptake rate was found to be almost linear when methyl viologen was present. In the absence of methyl viologen (Mehler reaction) an increase of 9-AA fluorescence quenching to a value of 20% at low light intensities was associated with considerably less O2-uptake than in the presence of methyl viologen, indicating the involvement of cyclic flow. These findings are in agreement with a preceding study of Kobayashi and Heber (1994). However, when no KCN was added, such that the complete Mehler-peroxidase reaction sequence was operative, the relationship between 9-AA fluorescence quenching and the flux through PS II, as measured via the chlorophyll fluorescence parameter ΔF/Fm' × PAR, was identical to that observed in the presence of methyl viologen. Under the assumption that methyl viologen prevents cyclic flow, it is concluded that there is no significant contribution of cyclic electron flow to ΔpH-generation in intact spinach chloroplasts.

On the relationship between chlorophyll fluorescence quenching and the quantum yield of electron transport in isolated thylakoids

The relationship between the empirical fluorescence index ΔF/Fm' and the quantum yield of linear electron flow, Φ(s), was investigated in isolated spinach thylakoids. Conditions were optimised for reliable determination of ΔF/Fm' and Φ(s) with methyl viologen or ferricyanide as electron acceptors under coupled and uncoupled conditions. Ascorbate in combination with methyl viologen was found to stimulate light-induced O2-uptake which is not reflected in ΔF/Fm' and interpreted to reflect superoxide reduction by ascorbate. In the absence of ascorbate, the plot of ΔF/Fm' vs. Φ(s) was mostly linear, except for the range of high quantum yields, i.e. at rather low photon flux densities. With ferricyanide as acceptor, use of relatively low concentrations (0.1-0.3 mM) was essential for correct Fm'-determinations, particularly under uncoupled conditions. Under coupled and uncoupled conditions the same basic relationship between ΔF/Fm' and Φ(s) was observed, irrespective of Φ(s) being decreased by increasing light intensity or by DCMU-addition. The plots obtained with methyl viologen and ferricyanide as acceptors were almost identical and similar to corresponding plots reported previously by other researchers for intact leaves. It is concluded that the index ΔF/Fm' can be used with isolated chloroplasts for characterisation of such types of electron flow which are difficult to assess otherwise, as e.g. O2 dependent flux. The origin of the 'non-linear' part of the relationship is discussed. An involvement of 'inactive' PS II centers with separate units and inefficient QA-QB electron transfer is considered likely.

The role of calcium in the pH-dependent control of Photosystem II

pH-dependent inactivation of Photosystem (PS) II and related quenching of chlorophyll-a-fluorescence have been investigated in isolated thylakoids and PS II-particles and related to calcium release at the donor side of PS II. The capacity of oxygen evolution (measured under light saturation) decreases when the ΔpH is high and the pH in the thylakoid lumen decreases below 5.5. Oxygen evolution recovers upon uncoupling. The pH-response of inactivation can be described by a 1 H(+)-transition with an apparent pK-value of about 4.7. The yield of variable fluorescence decreases in parallel to the inactivation of oxygen evolution. pH-dependent quenching requires light and can be inhibited by DCMU. In PS II-particles, inactivation is accompanied by a reversible release of Ca(2+)-ions (one Ca(2+) released per 200 Chl). In isolated thylakoids, where a ΔpH was created by ATP-hydrolysis, both inactivation of oxygen evolution (and related fluorescence quenching) by internal acidification and the recovery of that inactivation can be suppressed by calcium-channel blockers. In the presence of the Ca(2+)-ionophore A23187, recovery of Chl-fluorescence (after relaxation of the ΔpH) is stimulated by external Ca(2+) and retarded by EGTA. As shown previously (Krieger and Weis 1993), inactivation of oxygen evolution at low pH is accompanied by an upward shift of the midpoint redox-potential, Em, of QA. Here, we show that in isolated PS II particles the pH-dependent redox-shift (about 160 mV, as measured from redox titration of Chl-fluorescence) is suppressed by Ca(2+)-channel blockers and DCMU. When a redox potential of -80 to -120mV was established in a suspension of isolated thylakoids, the primary quinone acceptor, QA, was largely reduced in presence of a ΔpH (created by ATP-hydrolysis) but oxidized in presence of an uncoupler. Ca(2+)-binding at the lumen side seems to control redox processes at the lumen- and stroma-side of PS II. We discuss Ca(2+)-release to be involved in the physiological process of 'high energy quenching'.

Intact chloroplasts display pH 5 optimum of O2-reduction in the absence of methyl viologen: Indirect evidence for a regulatory role of superoxide protonation

The pH-dependence of light-driven O2-reduction in intact spinach chloroplasts is studied by means of chlorophyll fluorescence quenching analysis and polarographic O2-uptake measurements. Most experiments are carried out in presence of KCN, which blocks activities of Calvin cycle, ascorbate peroxidase and superoxide dismutase. pH is varied by equilibration with external buffers in presence of nigericin. Vastly different pH-optima for O2-dependent electron flow are observed in the presence and absence of the redox catalyst methyl viologen. Both fluorescence quenching analysis and O2-uptake reveal a distinct pH 5 optimum of O2-reduction in the absence of methyl viologen. In the presence of this catalyst, O2-reduction is favoured in the alkaline region, with an optimum around pH 8, similar to other types of Hill reaction. It is suggested that in the absence of methyl viologen the extent of irreversibility of O2-reduction is determined by the rate of superoxide protonation. This implies that O2-reduction takes place within the aprotic phase of the thylakoid membrane and that superoxide-reoxidation via oxidized PS I donors competes with protonation. Superoxide protonation is proposed to occur at the internal surface of the thylakoid membrane. There is no competition between superoxide reoxidation and protonation when in the presence of methyl viologen the site of O2-reduction is shifted into the protic stroma phase. In confirmation of this interpretation, fluorescence measurements in the absence of KCN reveal, that non-catalysed O2-dependent electron flow is unique in beingstimulated by the transthylakoidal pH-gradient. On the basis of these findings a major regulatory role of O2-dependent electron flow under excess light conditions is postulated.

PAM fluorometer based on medium-frequency pulsed Xe-flash measuring light: A highly sensitive new tool in basic and applied photosynthesis research

A newly developed modulation fluorometer is described which employs repetitive 1 μs Xe-flashes for excitation light. Similar to the standard PAM Chlorophyll Fluorometer, which uses 1 μs LED pulses for measuring light, the integrated measuring light intensity is sufficiently low to monitor the dark-fluorescence level, Fo. The maximal fluorescence yield, Fm, can be determined with high selectivity upon application of a saturating light pulse. The Xe-PAM displays exceptionally high sensitivity, enabling quenching analysis at chlorophyll concentrations as low as 1 μg/l, thus allowing to assess photosynthesis of phytoplankton in natural waters like lakes, rivers and oceans. Due to high flexibility in the choice of excitation and emission wavelengths, this system also provides the experimental basis for a thorough study of fluorescence and photosynthesis properties of various algae classes with differing antenna organisation. By appropriate modifications, the instrument may as well be used to measure with great sensitivity and selectivity other types of fluorescence (e.g. NADPH-fluorescence), as well as light-scattering and absorbance changes.