Kok Effect Research Articles

Importance of the cytochrome pathway of mitochondrial electron transport over the alternative pathway during the Kok effect in leaf discs of pea (Pisum sativum).

The Kok effect refers to the progressive light-induced inhibition of dark respiration at low light intensities, which saturates around the light compensation point. This appears as a sudden break around the light compensation point in the plot of photosynthesis versus light intensity. The magnitude of the break can be considered as a measure of the Kok effect. In the present work, the importance of different components of dark respiration during the Kok effect was investigated by using low concentrations of mitochondrial inhibitors in leaf discs of pea (Pisum sativum L. cv. Azad P1). The effects of glucose (stimulates respiration) and 0.8 M sorbitol (imposes osmotic stress and inhibits photosynthesis) were also studied for comparison. The magnitude of the break decreased significantly in the presence of antimycin A or oligomycin (inhibitors of cytochrome pathway of mitochondrial electron transport and ATP synthase, respectively). In contrast, there was no significant change with salicylhydroxamic acid (SHAM; an inhibitor of alternative pathway of mitochondrial electron transport). The magnitude of the break increased significantly with glucose, and decreased on exposure to osmotic stress. Our results suggest that the Kok effect (inhibition of dark respiration in light) is modulated by inhibitors of cytochrome pathway and ATP synthesis, but not that of the alternative pathway.

Chlororespiration: an adaptation to nitrogen deficiency in Chlamydomonas reinhardtii.

When grown under nitrogen limitation, pronounced chlororespiratory activity develops together with an altered composition of thylakoid membranes in Chlamydomonas reinhardtii. Relative to control cultures, the flash-inhibited, chlororespiration-dependent O2 consumption signal increases 10-fold. Also augmented is the light-sensitive respiratory activity responsible for the "Kok effect," reflecting competitive inhibition of chlororespiratory electron transport by photosystem I. Fluorescence measurements show that the thylakoid plastoquinone pool is extensively reduced in dark-adapted, N-limited cells. Thylakoids of N-limited cells have reduced amounts of cytochrome b6, cytochrome f, and light-harvesting complexes. However, thylakoid-bound NADH-PQ oxidoreductase, with major subunits of 51 kDa and 17 kDa, is increased 7-fold and two novel cytochromes of 34 and 12.5 kDa are highly abundant. Thus, components of photosynthetic and chlororespiratory electron transport pathways are differentially regulated by N availability.

Automated measurement of leaf photosynthetic O 2 evolution as a function of photon flux density

An automated procedure is described that allows the rate of photosynthesis, as a function of photon flux density (PFD), to be determined and plotted within 30 minutes. The method is based on polarographic measurement of O 2 evolution from a piece of leaf enclosed in a chamber and illuminated from above by an array of light-emitting diodes. The light emitted from these diodes is altered by a computer which also facilitates analyses of the data so derived. Applications of the procedure to leaves of shade and sun plants, to studies of photoinhibition and to analysis of the Kok effect, are described.

The Kok effect and the light-inhibition of chlororespiration in Chlamydomonas reinhardtii

Measurements of O 2 exchange have been performed as a function of light intensity in the green alga Chlamydomonas reinhardtii by using 18O 2, and mass spectrometry. The Kok effect (i.e. a decrease in the slope of net O 2 exchange curve) was observed at a light intensity of about 15 μE·m −2·s −1. This effect was due to the inhibition by light of O 2 uptake. In conditions where mitochondrial respiration was inhibited by antimycin A and salicyl hydroxamic acid, illumination was found to reduce O 2 uptake to the same extent as in the control experiment. We conclude that the Kok effect in Chlamydomonas is not the consequence of the inhibition of mitochondrial respiration, but is due to the inhibition of chlororespiration induced by photosystem I activity.

THE MINIMUM PHOTON REQUIREMENT FOR PHOTOSYNTHESIS: AN ANALYSIS OF THE DATA OF WARBURG & BURK (1950) AND YUAN, EVANS & DANIELS (1955).

Variations in the apparent photon requirement for photosynthesis (Φ-1 co 2 ) or (Φ-1 co 2 ) in the data of Warburg & Burk (1950) and Yuan, Evans & Daniels (1955) can be ascribed to changes in O2 uptake and energy-dependent processes which result in aberrant photon requirements in organisms subjected to non-optimal conditions. The increase in Φ-1 co 2 with increases in the gas exchange quotient (γ) in the observations of Yuan et al. (1955) is consistent with increases in photorespiratory production of glycollate, whilst changes in Φ-1 co 2 and Φ-1 co 2 in the results of Warburg & Burk (1950) can be explained by a variable Kok effect associated with nitrate assimilation at low light levels. When these O2 and energy-dependent processes are minimal, the lowest values should be observed. The minimum value obtained when Chlorella is photosynthesizing under optimal conditions is 6 mol photons mol-1 O2 . These results provide direct independent evidence for a photon requirement for photosynthesis of less than 8 mol photons mol-1 O2 . Such a value is not consistent with the Z scheme of photosynthesis.

Investigation of the CO2 Dependence of Quantum Yield and Respiration in Eucalyptus pauciflora

In leaves of C(3) plants, the rate of nonphotorespiratory respiration appears to be higher in darkness than in the light. This change from a high to a low rate of carbon loss with increasing photon flux density leads to an increase in the apparent quantum yield of photosynthetic CO(2) assimilation at low photon flux densities (Kok effect). The mechanism of this suppression of nonphotorespiratory respiration is not understood, but biochemical evidence and the observation that a Kok effect is often not observed under low O(2), has led to the suggestion that photorespiration might be involved in some way. This hypothesis was tested with snowgum (Eucalyptus pauciflora Sieb. ex Spreng.) using gas exchange methods. The test was based on the assumption that if photorespiration were involved, then it would be expected that the intercellular partial pressure of CO(2) would also have an influence on the Kok effect. Under normal atmospheric levels of CO(2) and O(2), a Kok effect was found. Changing the intercellular partial pressure of CO(2), however, did not affect the estimate of nonphotorespiratory respiraton, and it was concluded that its decrease with increasing photon flux density did not involve photorespiration. Concurrent measurements showed that the quantum yield of net assimilation of CO(2) increased with increasing intercellular partial pressure of CO(2), and this increase agreed closely with predictions based on recent models of photosynthesis.

Assimilatory power as a driving force in photosynthesis

Intact spinach chloroplasts were permitted to photoreduce added 3-phosphoglycerate until oxygen evolution was replaced by oxygen uptake. The chloroplast suspensions were then analyzed for dihydroxyacetone phosphate and residual 3-phosphoglycerate. Ratios of dihydroxyacetone phosphate to phosphoglycerate served to calculate assimilatory power ([ ATP] [ ADP][ P i ]) × ([ NADPH] [ NADP]) . Extrapolation yielded maximum values of assimilatory power P A of about 4000 (M −1) as long as the chloroplasts continued to oxidize dihydroxyacetone phosphate via ribulose bisphosphate oxygenase. When the oxygen concentration was reduced to 25 μM, maximum values of P A approached 25 000 (M −1) at light saturation of phosphoglycerate reduction. Maximum P A declined as chloroplasts aged. When the light intensity was reduced, P A decreased and intact chloroplasts oxidized dihydroxyacetone phosphate which had previously been exported into the medium. This observation explains the transient inhibition of photosynthesis of leaves after a sudden reduction in light intensity. Maximum P A calculated for broken chloroplasts by multiplication of NADPH NADP ratios and maximum phosphorylation potentials [ ATP] [ ADP][ P i] measured separately in thylakoid suspensions at light saturation was as high as 2.5·10 6 ( M −1). The discrepancy between P A in the stroma of intact chloroplasts and the values calculated for thylakoid suspensions is explained by inefficient cyclic electron flow which is incapable of raising phosphorylation potentials to high levels when NADP is reduced in chloroplasts. In leaves, maximum P A in chloroplasts was even lower than in isolated chloroplasts. Turnover of ATP and NADPH in situ prevents P A from reaching high levels even when net assimilation is zero. P A was higher in leaves at low light intensities when carbon reduction was slow than at high light intensities when it was fast. This can explain the Kok effect. The apparent paradox that photosynthetic flux is increased as the driving force P A is decreased is explained by regulation of enzymes of the Calvin cycle. Maximum rates of electron flow and phosphorylation and therefore also of photosynthesis are possible only when levels of P A are kept low. Rapid use of P A requires high activities of the enzymes of the Calvin cycle and may explain the necessity of enzyme activation.

Premières données sur l'activité photosynthétique de quelques algues subaériennes vivant en milieux très peu éclairés

The photosynthetic activity of three sciaphilous algae (including two cavernicolous ones) was measured with a Clark electrode in the air, by light minus dark differences in the rates of [Formula: see text] under laboratory and near natural physical conditions. Photosynthesis versus light intensity curves showed a sharp slope at low light and a suddenly lower slope at medium intensities; this is interpretable as the Kok effect. This phenomenon is stronger with cavernicolous algae than with less sciaphilous plants. High yield activity of the three studied algae was observed only in the naturally encountered range of light intensity. A seasonal adaptation of light activity was shown but not a daily one.

Kok Effect and the Quantum Yield of Photosynthesis

The linear response of photosynthesis to light at low photon flux densities is known to change abruptly in the vicinity of the light compensation point so that the quantum yield seems to decrease as radiation increases. We studied this ;Kok effect' in attached sunflower (Helianthus annuus L. cv IS894) leaves using gas exchange techniques. The effect was present even though respiration was constant in the dark. It was observed at a similar photon flux density (7 to 11 micromole photons per square meter per second absorbed photosynthetically active radiation) despite a wide range of light compensation points as well as rates of photosynthesis. The effect was not apparent when photorespiration was inhibited at low pO(2) (1 kilopascal), but this result was complicated because dark respiration was quite O(2)-sensitive and was partially suppressed under these conditions. The Kok effect was observed at saturating pCO(2) and, therefore, could not be explained by a change in photorespiration. Instead, the magnitude of the effect varied as dark respiration varied in a single leaf, and was minimized when dark respiration was minimized, indicating that a partial suppression of dark respiration by light is responsible. Quantum yields measured at photon flux densities between 0 and 7 to 11 micromole photons per square meter per second, therefore, represent the combined yields of photosynthesis and of the suppression of a component of dark respiration by light. This leads to an overestimate of the quantum yield of photosynthesis. In view of these results, quantum yields of photosynthesis must be measured (a) when respiration is constant in the dark, and (b) when dark respiration has been inhibited either at low pO(2) to eliminate most of the light-induced suppression of dark respiration or at photon flux densities above that required to saturate the light-induced suppression of dark respiration. Significant errors in quantum yields of photosynthesis can result in leaves exhibiting this respiratory behavior if these principles are not followed.

On the O 2 flash yields of two cyanophytes

We explored O 2 flash yield in two cyanophytes, Anacystis nidulans and Agmenellum quadruplicatum. On a rate-measuring electrode, a single flash gave a contour of O 2 evolution with a peak at about 10 ms which was maximum (100) for 680 nm background light. On 625 nm illumination the peak was smaller (62) but was followed by an increased tail of O 2 attributed to enhancement of the background. After a period of darkness, repetitive flashes (5 Hz) gave a highly damped initial oscillation in individual flash yields which finally reached steady state at 94% of the yield for 680 nm illumination. When O 2 of repetitive flashes was measured as an integrated flash yield the results was distinctive and similar to that for a continuous light 1 (680 nm). An apparent inhibition of respiration which persisted into the following dark period was taken as evidence for the Kok effect. With a concentration-measuring electrode, integrated flash yield vs. flash rate showed the same nonlinear behavior as O 2 rate vs. intensity of light 1. We draw three conclusions about the two cyanophytes. (a) The plastoquinone pool is substantially reduced in darkness. (b) Because of a high ratio of reaction centers, reaction center 1 / reaction center 2, for the two photoreactions, saturating flashes behave as light 1. (c) Because repetitive flashes are light 1, they also give a Kok effect which must be guarded against in measurements designed to count reaction centers.

Transport as the basis of the kok effect. Levels of some photosynthetic intermediates and activation of light-regulated enzymes during photosynthesis of chloroplasts and green leaf protoplasts

Experiments were performed with intact chloroplasts and leaf cell protoplasts isolated from spinach. The light-dependent decrease in (H +) in the chloroplast stroma counteracts carbon reduction and is offset at low light intensities by a large decrease in NADP and a significant increase in [ ATP] [ ADP] ratios. Excess accumulation of NADPH and/or ATP permits 3-phosphogly cerate reduction to occur. With increasing light intensity, NADP levels and [ ATP] [ ADP] ratios increased. High rates of photosynthesis were observed at high and at low [ ATP] [ ADP] ratios. Levels of dihydroxyacetone phosphate were dramatically increased in the light. In chloroplasts, this permitted conversion to ribulose bisphosphate which on carboxylation yields 3-phosphoglycerate. The light-dependent alkalization of the chloroplast stroma is known to be responsible for phosphogly cerate retention in the chloroplasts. A high chloroplast ratio of phosphogly cerate to dihydroxyacetone phosphate aids carbon reduction. Measured ratios of dihydroxyacetone phosphate to phosphogly cerate were averages between low chloroplast ratios and high cytosolic ratios. They were far higher, even under low-intensity illumination, than dark ratios. Since cytosolic NADH levels are known to increase much less in the light than cytosolic dihydroxyacetone phosphate levels, the large increase in the ratio of didydroxyacetone phosphate to phosphogly cerate must considerably increase cytosolic phosphorylation potentials even at very low light intensities. It is proposed that this increase is communicated to the mitochondrial adenylate system, and inhibits dark respiratory activity, giving rise to the Kok effect. The extent of stroma alkalization, the efficiency of metabolite shuttles across the chloroplast envelope, and rates of cytosolic ATP consumption are proposed to be factors determining whether and to what extent the Kok effect can be observed. Light activation of chloroplast enzymes was slow at low and fast at high light intensities. This contrasts to low NADP levels at low and usually higher levels at high light intensities. Maximum enzyme activation was observed far below light saturation of photosynthesis, and light activation of enzymes was often less pronounced at very high than at intermediate light intensities.

14CO 2-fixation under various limited light conditions in Porphyridium cruentum

Photosynthetic 14CO 2-fixation of Porphyridium cruentum showed under limited red light a Kok effect. Under various light conditions (20 or 35 β × cm −1) 14C-label was found in amino acids only. The main radioactivity could be measured in aspartate and glutamate under green light (547 nm) and in glycine/serine under white or far red light (693 nm). Under relatively high light conditions (200 or 300 μ W × cm −2) a few products of the Calvin cycle were labelled and 50–75% of 14C-labelling of total soluble photosynthetic products was obtained in amino acids only. Results were discussed with reference to CO 2-fixation via phosphoenolpyruvate (PEP) carboxylation.

The Kok Effect and Its Relationship to Photorespiration in Tobacco

Abstract The Kok effect of photosynthesis was investigated in different tobacco mutants. It was found that the breaks in the light intensity curve were always at or around 1000 lux in all plants tested regardless of the unit sizes which differed by a factor of 10. It was concluded that the photo­ receptor responsible for the effect must be present in the wild type and the chlorophyll deficient mutants in the same amount and is probably not chlorophyll. Due to the fact that the light dependency of the Hill reaction in isolated tobacco chloroplasts also shows a break at or around the “Kok intensity” it was concluded that probably a structural change of the photochemical apparatus around 1000 lux contributes to the effect. Measurement of 180 2-uptake by mass spectrometry at low light intensity shows at low CO2-concentration an enhancement of 180 2-uptake again at/around 1000 lux indicating that photorespiration starts to function at the “Kok intensity”. Due to the fact that 180 2-uptake remains constant at high CO2-concentrations the break in the photosynthetic light intensity curve cannot be due to an inhibition of “dark respiration” at low light intensities.

Effect of Light on the CO<SUB>2</SUB> Evolution of C<SUB>3</SUB> and C<SUB>4</SUB> Plant in Relation to the K<SUB>O</SUB>K Effect

Effect of Light on the CO<SUB>2</SUB> Evolution of C<SUB>3</SUB> and C<SUB>4</SUB> Plant in Relation to the K<SUB>O</SUB>K Effect

The Kok Effect in Chlamydomonas reinhardi

A Haxo-Blinks rate-measuring oxygen electrode together with a modulated light source gave an average current signal (change in net O(2) exchange) and a modulated current signal (photosynthetic O(2) evolution). Using this apparatus, net O(2) exchange and photosynthetic O(2) evolution at low intensities have been studied in the green alga, Chlamydomonas reinhardi. At both 645 nm and 695 nm, the curves of net O(2) exchange as a function of light intensity were steeper at lowest intensities than about compensation, indicative of the Kok effect. The effect was greater at 695 nm than at 645 nm. The corresponding curves of photosynthetic O(2) evolution, on the other hand, showed no Kok effect; here, the slope was lowest at lowest intensity. The absence of the Kok effect in O(2) evolution, together with its sensitivity to monofluoroacetic acid, show that it is due to an interaction of photosynthesis and respiration. The effect was exaggerated by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. In the presence of concentrations of this inhibitor sufficient to inhibit O(2) evolution completely, a light-induced change in net O(2) exchange remained. This was interpreted as a system I dependent depression of respiratory O(2) uptake. The Kok effect remained undiminished in concentrations of carbonyl cyanide m-chlorophenylhydrazone and 2,4-dinitrophenol which partially uncoupled either oxidative phosphorylation alone or both oxidative and photosynthetic phosphorylations. The above results can be explained within a model of the Kok effect in which O(2) uptake is depressed by diversion of reductant away from respiratory electron transport and into photosystem I. The same photodepression of O(2) uptake also appears to account for a transient in net O(2) exchange seen in several algae upon turning off the light.

Respiration and intensity dependence of photosynthesis in Chlorella.

1. Respiration changes as a result of illumination. 2. In the absence of glucose or other supply of substrate, respiration decays in the dark showing at least two types-a fast decay in a few minutes and a slow decay lasting hours. 3. Respiratory response to illumination is delayed. 4. Intermittent illumination (in the absence of glucose, etc.) produces a periodic variation in respiration with a delay or phase lag. 5. Periodic variation of respiration may produce a higher average value in the dark than in the light due to the lag and depending upon the period of intermittent illumination. 6. Based upon average respiration values our data confirm the Kok effect. 7. Interpolated values of respiration, however, result in photosynthetic rates which are linearly dependent upon intensity of illumination. 8. Thus the quantum efficiency is found to be independent of intensity, over the wide range of intensities investigated.