The fast and slow kinetics of chlorophyll a fluorescence induction in plants, algae and cyanobacteria: a viewpoint

The light-induced/dark-reversible changes in the chlorophyll (Chl) a fluorescence of photosynthetic cells and membranes in the mus-to-several min time window (fluorescence induction, FI; or Kautsky transient) reflect quantum yield changes (quenching/de-quenching) as well as changes in the number of Chls a in photosystem II (PS II; state transitions). Both relate to excitation trapping in PS II and the ensuing photosynthetic electron transport (PSET), and to secondary PSET effects, such as ion translocation across thylakoid membranes and filling or depletion of post-PS II and post-PS I pools of metabolites. In addition, high actinic light doses may depress Chl a fluorescence irreversibly (photoinhibitory lowering; q(I)). FI has been studied quite extensively in plants an algae (less so in cyanobacteria) as it affords a low resolution panoramic view of the photosynthesis process. Total FI comprises two transients, a fast initial (OPS; for Origin, Peak, Steady state) and a second slower transient (SMT; for Steady state, Maximum, Terminal state), whose details are characteristically different in eukaryotic (plants and algae) and prokaryotic (cyanobacteria) oxygenic photosynthetic organisms. In the former, maximal fluorescence output occurs at peak P, with peak M lying much lower or being absent, in which case the PSMT phases are replaced by a monotonous PT fluorescence decay. In contrast, in phycobilisome (PBS)-containing cyanobacteria maximal fluorescence occurs at M which lies much higher than peak P. It will be argued that this difference is caused by a fluorescence lowering trend (state 1 --> 2 transition) that dominates the FI pattern of plants and algae, and correspondingly by a fluorescence increasing trend (state 2 --> 1 transition) that dominates the FI of PBS-containing cyanobacteria. Characteristically, however, the FI pattern of the PBS-minus cyanobacterium Acaryochloris marina resembles the FI patterns of algae and plants and not of the PBS-containing cyanobacteria.

Relations between electron transport and carbon assimilation; simultaneous measurement of chlorophyll fluorescence, transthylakoid pH gradient and O 2 evolution in isolated chloroplasts

An apparatus is described that allows simultaneous measurement of photosynthetic O 2 evolution, chlorophyll fluorescence and the trans­thylakoid pH gradient (∆pH) in isolated chloroplasts irradiated with light sufficient to saturate photosynthesis. In intact chloroplasts, quenching of chlorophyll fluorescence due to both oxidation of the primary electron acceptor of photosystem II (Q) and formation of ∆pH was seen. The relative proportions of the two kinds of quenching varied in response to ( a ) the light intensity, ( b ) the presence of phosphoglycerate and ( c ) whether or not the chloroplasts were in the induction period or in a period of linear photosynthetic O 2 evolution. In broken chloroplasts reconstituted for CO 2 fixation, transient changes in the rates of O 2 evolution, ∆pH, the redox state of Q and chlorophyll fluorescence were observed as a result of changes in ( a ) the availability of electron acceptor as determined by the additions of NADP and phosphoglycerate and ( b ) the ratio of ATP to ADP, as manipulated by addition of ribose 5-phosphate. The changes in chlorophyll fluorescence in this system could be manipulated to show a pattern very similar to that observed in leaves.

Some effects of CO 2 concentration and decreased O 2 concentration on induction fluorescence in leaves

A procedure is described that permits spinach leaves to display secondary fluorescence kinetics when illuminated in air at 20 °C. The initial peak in chlorophyll a fluorescence is then followed by a fall to a quasi-steady state (S), a rise to a new peak (M) and a final fall to a terminal steady-state value (T). These kinetics can be modified by changing the periods of light and darkness before measurement. The M peak is abolished by exposure to CO 2 -free air and greatly modified by exposure to 5 % CO 2 . In 2% O 2 the period of darkness immediately before illumination needs to be lengthened if secondary kinetics are to be observed. The results are discussed in relation to the probable impact of photosynthetic carbon assimilation on fluorescence-quenching mechanisms.

Slow secondary fluorescence kinetics associated with the onset of photosynthetic carbon assimilation in intact isolated chloroplasts

Experiments with carefully isolated, largely intact chloroplasts, capable of fast rates of CO 2 -dependent O 2 evolution, show that the fall in chlorophyll a fluorescence (from the early maxima reached immediately after illumination) is interrupted by a ‘shoulder’ which is associated with the exponential increase in the rate of O 2 evolution. The length of this induction period was increased by storage, by decreased temperature, by increased orthophosphate concentration in the assay medium or by the presence of D, L-glyceraldehyde. It could also be shortened by the addition of 3-phosphoglycerate or dihydroxyacetonephosphate. In each treatment the shoulder in fluorescence shifted so that the association with the period of exponential increase was maintained. When illumination was re-started after a short dark interval, induction was minimal and no shoulder could be discerned, but both the lag in the onset of O 2 evolution and the shoulder were restored when the chloroplasts were resuspended in fresh assay medium during the period of darkness. The relation between chlorophyll a fluorescence and the onset of photosynthetic carbon assimilation is discussed.

Regulation of photosynthetic electron-transport in Phaseolus vulgaris L., as determined by room-temperature chlorophyll a fluorescence

The regulation of photosystem II (PSII) by light-, CO2-, and O2-dependent changes in the capacity for carbon metabolism was studied. Estimates of the rate of electron transport through PSII were made from gas-exchange data and from measurements of chlorophyll fluorescence. At subsaturating photon-flux density (PFD), the rate of electron transport was independent of O2 and CO2. Feedback on electron transport was observed under two conditions. At saturating PFD and low partial pressure of CO2, p(CO2), the rate of electron transport increased with p(CO2). However, at high p(CO2), switching from normal to low p(O2) did not affect the net rate of photosynthetic CO2 assimilation but the rate of electron-transport decreased by an amount related to the change in the rate of photorespiration. We interpret these effects as 1) regulation of ribulose-1,5-bisphosphatecarboxylase (RuBPCase, EC 4.1.1.39) activity to match the rate of electron transport at limiting PFD, 2) regulation of electron-transport rate to match the rate of RuBPCase at low p(CO2), and 3) regulation of the electron-transport rate to match the capacity for starch and sucrose synthesis at high p(CO2) and PFD. These studies provide evidence that PSII is regulated so that the capacity for electron transport is matched to the capacity for other processes required by photosynthesis, such as ribulose-bisphosphate carboxylation and starch and sucrose synthesis. We show that at least two mechanisms contribute to the regulation of PSII activity and that the relative engagement of these mechanisms varies with time following a step change in the capacity for ribulose-bisphosphate carboxylation and starch and sucrose synthesis. Finally, we take advantage of the relatively slow activation of deactivated RuBPCase in vivo to show that the activation level of this enzyme can limit the rate of electron transport as evidenced by increased feedback on PSII following a step change in p(CO2). As RuBPCase as activated, the feedback on PSII declined.

Photorespiratory N donors, aminotransferase specificity and photosynthesis in a mutant of barley deficient in serine: glyoxylate aminotransferase activity

A mutant of Hordeum vulgare L. (LaPr 85/84) deficient in serine: glyoxylate aminotransferase (EC 2.6.1.45) activity has been isolated. The plant also lacks serine: pyruvate aminotransferase and asparagine: glyoxylate aminotransferase activities. Genetic analysis of the mutation strongly indicates that these three activities are all carried on the same enzyme protein. The mutant is incapable of normal rates of photosynthesis in air but can be maintained at 0.7% CO2. The rate of photosynthesis cannot be restored by supplying hydroxypyruvate, glycerate, glutamate or ammonium sulphate through the xylem stream. This photorespiratory mutant demonstrates convincingly that photorespiration still occurs under conditions in which photosynthesis becomes insensitive to oxygen levels. Two major peaks and one minor peak of serine: glyoxylate aminotransferase activity can be separated in extracts of leaves of wild-type barley by diethylaminoethyl-sephacel chromatography. All three peaks are missing from the mutant, LaPr 85/84. The mutant showed the expected rate (50%) of ammonia release during photorespiration but produced CO2 at twice the wild-type rate when it was fed [(14)C]glyoxylate. The large accumulation of serine detected in the mutant under photorespiratory conditions shows the importance of the enzyme activity in vivo. The effect of the mutation on transient changes in chlorophyll a fluorescence initiated by changing the atmospheric CO2 concentration are presented and the role of the enzyme activity under nonphotorespiratory conditions is discussed.

The role of carbon dioxide and oxygen in determining chlorophyll fluorescence quenching during leaf development

Chlorophyll fluorescence emission at 680 nm (F680) and the rate of CO2 fixation were measured simultaneously in sections along the length of wheat and maize leaves. These leaves possess a basal meristem and show a gradation in development towards the leaf tip. The redox state of the primary electron acceptor, Q, of photosystem II was estimated using a non-invasive method. Distal mature leaf sections displayed typical F680 induction curves which were generally anti-parallel with CO2 fixation and during which Q became gradually oxidised. In leaf-base sections net assimilation of CO2 was not detectable, F680 quenched slowly and monotonously without displaying any of the oscillations typical of mature tissue and Q remained relatively reduced. Sections cut from mid-regions of the leaf showed intermediate characteristics. There were no major differences between the wheat and maize leaf in the parameters measured. The results support the hypothesis that generation of the transthylakoid proton gradient and associated ATP production is not a major limitation to photosynthesis during leaf development in either C3 or C4 plants. Removal of CO2 from the mature leaf sections caused little change in steady-state F680 and produced about 50% reduction of Q. When O2 was then removed, F680 rose sharply and Q became almost totally reduced. In immature tissue unable to assimilate CO2, removal of O2 alone caused a similar large rise in F680 and reduction of Q whilst removal of CO2 had negligible effects on F680 and the redox state of Q. It is concluded that in leaf tissue unable to assimilate CO2, either because CO2 is absent or the tissue is immature, O2 acts as an electron acceptor and maintains Q in a partially oxidised state. The important implication that O2 may have a role in the prevention of photoinhibition of the photochemical apparatus in the developing leaf is discussed.

Chlorophyll a fluorescence and light-scattering kinetics displayed by leaves during induction of photosynthesis

Light-scattering, which can be taken as an indicator of the transthylakoid proton-gradient, and chlorophyll a fluorescence, have been followed simultaneously during re-illumination of spinach leaves at different energy fluence rates and carbon dioxide concentrations. The slow fluorescence transient ("M peak"), which has been associated with photosynthetic induction, was observed in air only at the lower fluence rates used. Data are presented that indicate that M peaks in chlorophyll fluorescence kinetics can only be observed if there is also a simultaneous transient in light-scattering and that these transients are observed when the dark period is relatively long, fluence rate relatively low, and CO2 concentration relatively high.The results are discussed in relation to the varying demands on ATP by carbon assimilation during induction of photosynthesis at different carbon dioxide concentrations and the manner in which these variations influence the quenching of chlorophyll a fluorescence.

Steady-state room temperature fluorescence and CO2 assimilation rates in intact leaves

Steady-state room temperature variable fluorescence from leaves was measured as a function of CO2 pressure in Xanthium strumarium L. and Phaseolus vulgaris L. Measurements were made in a range of light intensities, at normal and low O2 parital pressure and over a range of temperatures.At low CO2 pressure fluorescence increased with increasing CO2. At higher CO2 pressure fluorescence usually decreased with increasing CO2 but occasionally increased slightly. The transition CO2 pressure between the responses could be changed by changing light, O2 pressure, or temperature. This breakpoint in the fluorescence-CO2 curve was a reliable indicator of the transition between ribulose 1,5-bisphosphate (RuBP) saturated assimilation and RuBP regeneration limited assimilation. The fluorescence signal was not a reliable indicator of O2-insensitive assimilation in these C3 species.

Photosynthetic induction in C4 leaves : An investigation using infra-red gas analysis and chlorophyll a fluorescence

Simultaneous measurements of CO2 uptake, transpiration rate, and chlorophyll a fluorescence in leaf strips of C4 plants during the induction phase of photosynthesis are described. The timecourse of CO2 fixation is biphasic with the initial phase occurring within the first 1 to 5 min and the secondary phase consisting of a slow rise to the steady-state rate of photosynthesis. Transpiration rate follows the CO2-fixation timecourse closely but the intercellular CO2 concentration never falls below saturation for C4 plants. Chlorophyll a fluorescence quenching occurs exclusively during the initial fast phase of the CO2-fixation timecourse. The effect of duration of dark pretreatment of leaves on these parameters and the effects of light intensity and CO2 concentration are examined. These results are discussed with respect to the C4 cycle and photochemical and non-photochemical chlorophyll fluorescence quenching.

Carbon metabolism and gas exchange in leaves of Zea mays L. : Interaction between the C3 and C 4 pathways during photosynthetic induction

The aim of this work was to investigate the mechanism of formation of triose phosphates and 3-phosphoglycerate during photosynthetic induction in leaves of Zea mays. Simultaneous measurements of gas exchange, chlorophyll a fluorescence and metabolite contents of maize leaves were made. Leaves illuminated in the absence of CO2 showed a build-up of triose phosphates during the first 2 min of illumination which was comparable to the build-up observed in the presence of CO2. Isolated mesophyll protoplasts, which lack the Calvin cycle, also showed a build-up of triose phosphates upon illumination. Leaves contained amounts of phosphoglycerate mutase and enolase adequate to account for the formation of triose phosphates and 3-phosphoglycerate from intermediates of the C4 cycle and their precursors.

The relationship between carbon dioxide fixation and chlorophyll a fluorescence during induction of photosynthesis in maize leaves at different temperatures and carbon dioxide concentrations

The rate of CO2 fixation (Fc) and 680 nm chlorophyll fluorescence emission (F680) were measured simultaneously during induction of photosynthesis in Zea mays L. leaves under varying experimental conditions in order to assess the validity of fluorescence as an indicator of in vivo photosynthetic carbon assimilation. Z. mays leaves showed typical 'Kautsky' fluorescence induction curves consisting of a fast rise in emission (O to P) followed by a slow quenching via a major transient (S-M) to a steady-state (T). After an initial lag, net CO2 assimilation commenced at a point corresponding to the onset of the S-M transient on the F680 induction curve. Subsequently, Fc and F680 always arrived at a steady-state simultaneously. Decreasing the dark-adaption period increased the rate of induction of both parameters. Alteration of leaf temperature produced anti-parallel changes in induction characteristics of Fc and F680. Reducing the CO2 level to below that required for saturation of photosynthesis also produced anti-parallel changes during induction, however, at CO2 concentrations tenfold greater than the atmospheric level the rate of F680 quenching from P to T was appreciably reduced without a similar change in the induction of Fc. Removal of CO2 at steady-state produced only a small increase in F680 and a correspondingly small decrease in F680 occurred when CO2 was re-introduced. The complex relationship between chlorophyll fluorescence and carbon assimilation in vivo is discussed and the applicability of fluorescence as an indicator of carbon assimilation is considered.