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PubMed1980

Photorespiration and Oxygen Inhibition of Photosynthesis in Chlorella pyrenoidosa.

The inhibition of photosynthesis by O(2) in air-grown Chlorella pyrenoidosa was investigated using three experimental techniques (artificial leaf, aqueous method, and O(2) electrode) to measure carbon assimilation. CO(2) response curves were determined under different O(2), pH, and temperature conditions. Regardless of the experimental technique and condition, O(2) inhibition was not evident until a concentration of 50% was reached; V(max) values were reduced whereas K(m) (CO(2)) values were unaffected by the increasing O(2) concentration. The response of photosynthesis to O(2) was independent of CO(2) and HCO(3) (-) concentrations as well as temperature. Relative rates of photosynthesis showed a 4 to 5% stimulation in 2% O(2), a 12% inhibition in 50% O(2), and a 24% inhibition in 100% O(2). The inhibition by 50% O(2) was still reversible after 20 minutes exposure whereas 100% O(2) caused irreversible inhibition after only 4 minutes.The O(2) inhibition is discussed in terms of the oxygenase reaction and a Mehler reaction supporting pseudocyclic electron flow. The results are inconsistent with the proposals that photorespiration exists in these algae and that a CO(2)-concentrating mechanism suppresses the O(2) inhibition of photosynthesis.

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PubMed1981

Uncouplers stimulate photosynthesis in intact chloroplasts by enhancing light-activation of enzymes regulated by the ferredoxin-thioredoxin system.

Some uncouplers stimulate CO(2)-dependent O(2) evolution by intact spinach chloroplasts at pH 8.6. This effect is not due to alkalinization of the stroma. The stimulation is observed only when photosynthesis has been partly inhibited by the presence of H(2)O(2), generated in a Mehler-type reaction by the broken chloroplasts which always contaminate the intact chloroplast preparations. The addition of methyl viologen increases the Mehler-type reaction and results in greater inhibition of photosynthesis. The addition of excess catalase stimulates photosynthesis by preventing accumulation of H(2)O(2). The uncouplers stimulate photosynthesis primarily by enhancing the light-activation of enzymes that are regulated by the ferredoxin-thioredoxin system, and this effect results from the influence of the uncouplers on the redox poising of the ferredoxin in the intact chloroplasts.

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PubMed1982

Influence of Varying CO(2) and Orthophosphate Concentrations on Rates of Photosynthesis, and Synthesis of Glycolate and Dihydroxyacetone Phosphate by Wheat Chloroplasts.

Intact chloroplasts of wheat (Triticum aestivum) were isolated from mesophyll protoplasts. With decreasing concentrations of bicarbonate from 10 to 0.3 millimolar (pH 8.0), the optimal concentration of orthophosphate (Pi) for photosynthetic O(2) evolution decreased from a value of 0.1 to 0.2 millimolar to 0 to 0.025 millimolar. The extremely low Pi optimum for photosynthesis at the low bicarbonate levels of 0.3 millimolar was increased by lowering the O(2) concentration from 253 (21% gas phase) to 72 micromolar (6% gas phase). The relative amount of glycolate and dihydroxyacetone phosphate (DHAP) synthesized under high and low levels of bicarbonate and varying levels of Pi was determined. At low levels of bicarbonate, glycolate was the main product, whereas at high bicarbonate levels, DHAP was the main product. Most of the DHAP and glycolate was found in the extrachloroplastic fraction.The rate of photosynthesis at low levels of bicarbonate (0.3 millimolar) was as high as 75 to 95% of that at high levels of bicarbonate (10 millimolar) at the respective optimal levels of Pi. At low bicarbonate levels, and without Pi, there was little lag in photosynthetic O(2) evolution upon illumination in comparison to that of high bicarbonate levels and optimal levels of Pi. It is proposed that conditions which favor glycolate synthesis allow photosynthesis to continue without a depletion of internal Pi, whereas consumption of Pi may occur in the chloroplast during the net synthesis of organic phosphates under high levels of bicarbonate and without addition of Pi. At low bicarbonate levels, the extreme susceptibility of photosynthesis to inhibition by Pi may be due to excessive export of carbon from the chloroplast in the form of both glycolate and triose phosphate.

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PubMed1982

Effects of aminoacetonitrile on net photosynthesis, ribulose-1,5-bisphosphate levels, and glycolate pathway intermediates.

The effects of aminoacetonitrile (a competitive inhibitor of glycine oxidation) on net photosynthesis, glycolate pathway intermediates, and ribulose-1,5-bisphosphate (RuBP) levels have been investigated at different O(2) and CO(2) concentrations with soybean (Glycine max)[L] Merr. cv Pioneer 1677) leaf discs floated on 25 millimolar aminoacetonitrile (AAN) for 50 minutes prior to assay.At 2% O(2) and 200 or 330 microliters per liter CO(2), the inhibitor had no effect on the rate of net photosynthesis and RuBP levels when compared with the control levels. At 11% to 60% O(2), AAN caused a decrease in net photosynthesis in addition to the inhibition by O(2). This extra inhibition ranged from 22% to 59% depending on the O(2) and CO(2) concentrations. The levels of RuBP, however, were 1.3 to 2.7 times higher than in the control plants at the same O(2) concentrations. At 40% O(2) and 200 microliters per liter CO(2), the inhibitor caused a 6-fold increase in glycine and more than 2-fold increase in glyoxylate levels, whereas those of glycolate decreased by approximately one-half.The decrease in net photosynthesis observed with AAN is not the result of the depletion of the RuBP pool due to the lack of recycling of carbon from the glycolate pathway to the Calvin cycle. The higher levels of RuBP caused by AAN in photorespiratory conditions, suggest that RuBP carboxylase was inhibited. Glyoxylate could be a possible candidate for the inhibition of the enzyme but what is known so far about its inhibitory properties in vitro may not fit the existing in vivo conditions. An alternative explanation for the inhibition is proposed.

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PubMed1983

Nuclear Suppressors of the Photosensitivity Associated with Defective Photosynthesis in Chlamydomonas reinhardii.

Several nuclear mutations were recovered that suppress the photosensitivity associated with the Chamydomonas reinhardii chloroplast mutant rcl-u-1-10-6C, which is defective in ribulose-1,5-bisphosphate carboxylase/oxygenase. Two of the suppressor mutations affect other components of photosynthesis. These results show that suppressors of photosensitivity are sufficiently common to permit the recovery of photosensitive, photosynthesis-deficient mutants in bright light, and indicate that photosynthesis-deficient mutants selected and maintained in the light may accumulate suppressors which can confuse the biochemical analysis of lesions in photosynthesis. One of the suppressor mutations inhibits photosystem II activity, indicating that photosensitivity can be mediated by partial reactions of the photosynthetic electron transport chain.

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PubMed1983

Relationship between Photosynthesis and Respiration: The Effect of Carbohydrate Status on the Rate of CO(2) Production by Respiration in Darkened and Illuminated Wheat Leaves.

The rate of dark CO(2) efflux from mature wheat (Triticum aestivum cv Gabo) leaves at the end of the night is less than that found after a period of photosynthesis. After photosynthesis, the dark CO(2) efflux shows complex dependence on time and temperature. For about 30 minutes after darkening, CO(2) efflux includes a large component which can be abolished by transferring illuminated leaves to 3% O(2) and 330 microbar CO(2) before darkening. After 30 minutes of darkness, a relatively steady rate of CO(2) efflux was obtained. The temperature dependence of steady-state dark CO(2) efflux at the end of the night differs from that after a period of photosynthesis. The higher rate of dark CO(2) efflux following photosynthesis is correlated with accumulated net CO(2) assimilation and with an increase in several carbohydrate fractions in the leaf. It is also correlated with an increase in the CO(2) compensation point in 21% O(2), and an increase in the light compensation point. The interactions between CO(2) efflux from carbohydrate oxidation and photorespiration are discussed. It is concluded that the rate of CO(2) efflux by respiration is comparable in darkened and illuminated wheat leaves.

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PubMed1983

Analysis of the role of the phosphate translocator and external metabolites in steady-state chloroplast photosynthesis.

The role of the phosphate translocator and the importance of the extrachloroplastic concentrations of phosphate, 3-phosphoglycerate, and dihydroxyacetone phosphate in steady-state photosynthesis is examined with a kinetic model. The steady-state stromal concentrations of these compounds are calculated as a function of the rate of the various partial reactions of photosynthesis, at various external concentrations which span those likely to occur in vivo. It is shown how the net transport requirements of the various reactions necessitate different adjustments in the stromal concentrations of these compounds, away from the equilibrium values expected in the absence of metabolism. Under most circumstances, the high exchange capacity of the phosphate translocator relative to the transport requirements of CO(2) fixation limits the extent of these displacements, but conditions when the phosphate translocator is limiting photosynthesis are observed and discussed. The model provides a basis for a more quantitative understanding of the role of the phosphate translocator and the external concentrations of phosphate, 3-phosphoglycerate, and dihydroxyacetone phosphate in photosynthesis.

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PubMed1983

Stromal acidification mediates in vivo water stress inhibition of nonstomatal-controlled photosynthesis.

Stromal acidification has been reported to mediate reduced osmotic potential (psi(pi)) effects on photosynthesis in the isolated spinach chloroplast (Berkowitz, Gibbs 1983 Plant Physiol 72: 1100-1109). To determine if stromal acidification mediates osmotic dehydration inhibition of photosynthesis in vivo, the effects of a weak base (NH(4)Cl), which raises stromal pH, on CO(2) fixation of vacuum-infiltrated spinach leaf slices, Chlamydomonas reinhardii cells and Aphanocapsa 6308 cells under isotonic and dehydrating conditions were investigated. Five millimolar NH(4)Cl stimulated spinach leaf slice CO(2) fixation by 43% under stress (0.67 molar sorbitol) conditions, and had little effect on fixation under isotonic (0.33 molar sorbitol) conditions. Chlamydomonas cells were found to be more sensitive to reduced psi(pi) than spinach leaf slices. CO(2) fixation in the cells of the green alga Chlamydomonas reinhardii was 99 and 17 micromoles per milligram chlorophyll per hour, respectively, at 0.1 molar mannitol and 0.28 molar mannitol. Five millimolar NH(4)Cl stimulated CO(2) fixation of Chlamydomonas cells by 147% under stress (0.28 molar mannitol) conditions. Aphanocapsa 6308 cells (blue-green alga) were also found to be sensitive to reduced psi(pi), and inhibitions in photosynthesis were partially reversed by NH(4)Cl. These data indicate that in vivo water stress inhibition of photosynthesis is facilitated by stromal acidification, and that this inhibition can be at least partially reversed in situ.

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PubMed1983

Effect of pod removal on leaf photosynthesis and soluble protein composition of field-grown soybeans.

Well nodulated, field-grown soybeans (Glycine max [L.] Merr. var Williams) were depodded just prior to seed development and near mid pod-fill. Both treatments caused a considerable increase in leaf dry weight, suggesting continued photosynthate production following pod removal. Moreover, depodding had a marked effect on leaf soluble protein without affecting total proteolytic activity. Early depodding caused a 50% increase in leaf protein, and both early and late depodding caused the retention of protein for several weeks following the decline in control leaves. But despite this retention of protein, leaves of depodded plants showed no difference in the onset of the irreversible decline in photosynthesis. Therefore, although depodding delayed the loss of leaf chlorophyll and protein, it did not delay the onset of functional leaf senescence and in fact, actually appeared to enhance the rate of decline in photosynthesis. There was a good correlation between the irreversible decline in ribulose bisphosphate carboxylase (activity and amount) and that of photosynthesis. In contrast, the correlation did not seem as good between stomatal closure and the onset of the irreversible decline in photosynthesis. The reason total soluble protein remained high following depodding while carboxylase, which normally comprised 40% of the soluble protein, declined was because several polypeptides increased in amounts sufficient to offset the loss of carboxylase. This change in leaf protein composition indicates a change in leaf function; this is discussed in terms of other recent findings.

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PubMed1984

Acclimation of photosynthesis to low leaf water potentials.

Photosynthesis is reduced at low leaf water potentials (Psi(l)) but repeated water deficits can decrease this reduction, resulting in photosynthetic acclimation. The contribution of the stomata and the chloroplasts to this acclimation is unknown. We evaluated stomatal and chloroplast contributions when soil-grown sunflower (Helianthus annuus L.) plants were subjected to water deficit pretreatments for 2 weeks. The relationship between photosynthesis and Psi(l), determined from gas-exchange and isopiestic thermocouple psychometry, was shifted 3 to 4 bars towards lower Psi(l), in pretreated plants. Leaf diffusive resistance was similarly affected. Chloroplast activity, demonstrated in situ with measurements of quantum yield and the capacity to fix CO(2) at all partial pressures of CO(2), and in vitro by photosystem II activity of isolated organelles, was inhibited at low Psi(l) but less in pretreated plants than in control plants. The magnitude of this inhibition indicated that decreases in chloroplast activity contributed more than closure of stomata both to losses in photosynthesis and to the acclimation of photosynthesis to low Psi(l).

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PubMed1984

Oxygen Uptake and Photosynthesis of the Red Macroalga, Chondrus crispus, in Seawater: Effects of Light and CO(2) Concentration.

With an experimental system using mass spectrometry techniques and infra-red gas analysis of CO(2) developed for aquatic plants, we studied the responses to various light intensities and CO(2) concentrations of photosynthesis and O(2) uptake of the red macroalga Chondrus crispus S. The CO(2) exchange resistance at air-water interface which could limit the photosynthesis was experimentally measured. It allowed the calculation of the free dissolved CO(2) concentration. The response to light showed a small O(2) uptake (37% of net photosynthesis in standard conditions) compared to C(3) plants; it was always higher than dark respiration and probably included a photoindependent part. The response to CO(2) showed: (a) an O(2) uptake relatively insensitive to CO(2) concentration and not completely inhibited with high CO(2), (b) a general inhibition of gas exchanges below 130 microliters CO(2) per liter (gas phase), (c) an absence of an inverse relationship between O(2) and CO(2) uptakes, and (d) a low apparent K(m) of photosynthesis for free CO(2) (1 micromolar). These results suggest that O(2) uptake in the light is the sum of different oxidation processes such as the glycolate pathway, the Mehler reaction, and mitochondrial respiration. The high affinity for CO(2) is discussed in relation to the use of HCO(3) (-) and/or the internal CO(2) accumulation.

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PubMed1985

Influence of photosynthesis and chlorophyll synthesis on polypeptide accumulation in greening euglena.

Two-dimensional gel electrophoresis resolves total cellular protein from Euglena gracilis klebs var bacillaris Cori into 640 polypeptides, 79 of which are induced by light exposure. The inhibition of chloroplast translation by streptomycin, the direct inhibition of photosynthesis as well as the indirect inhibition of chlorophyll synthesis by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and the specific inhibition of photosynthesis but not chlorophyll synthesis by DCMU in the presence of 17 millimolar ethanol failed to inhibit the accumulation of 40 polypeptides. These polypeptides appear to be synthesized on cytoplasmic ribosomes and their accumulation is independent of the developmental status of the chloroplast. Streptomycin but not DCMU completely inhibited the accumulation of six polypeptides which are undetectable in mutants lacking chloroplast DNA suggesting that these polypeptides are translated on chloroplast ribosomes. The accumulation of seven polypeptides which are detectable in mutants lacking chloroplast DNA was also inhibited by streptomycin but not by DCMU suggesting that the accumulation of these polypeptides is dependent upon stabilization by a chloroplast translation product. The accumulation of 12 polypeptides was inhibited by streptomycin and by DCMU under conditions in which chlorophyll synthesis was inhibited, but not under conditions in which chlorophyll synthesis was unaffected by DCMU. The inhibition by DCMU of the accumulation of these polypeptides appears to be due to the inhibition of chlorophyll synthesis suggesting that they are components of pigment protein complexes. The accumulation of six polypeptides was inhibited under all conditions in which photosynthesis was inhibited suggesting that the accumulation of these polypeptides is dependent upon a product of photosynthesis.

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PubMed1987

Sodium Requirement for Photosynthesis and Its Relationship with Dinitrogen Fixation and the External CO(2) Concentration in Cyanobacteria.

Cells of Anabaena PCC 7119 and of a mutant strain of Nostoc muscorum unable to fix dinitrogen, grown at pH 8 and under low CO(2) tension (air), showed a reduced capacity for photosynthesis when cultured in the absence of sodium, this inhibition being followed by symptoms of photooxidation, such as chlorosis, oxygen consumption in the light, and decrease of superoxide dismutase activity. The impairment of photosynthesis preceded that of nitrogenase activity, indicating that the requirement for sodium in photosynthesis was independent of its effects on nitrogen metabolism. However, when cyanobacteria were grown at pH 6.3 or under high CO(2) tensions, sodium was not required for photosynthesis and no symptoms of photooxidation were observed.

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PubMed1988

Salinity Effects on Photosynthesis, Carbon Allocation, and Nitrogen Assimilation in the Red Alga, Gelidium coulteri.

The long-term effects of altered salinities on the physiology of the intertidal red alga Gelidium coulteri Harv. were assessed. Plants were transfered from 30 grams per liter salinity to media with salinities from 0 to 50 grams per liter. Growth rate, agar, photosynthesis, respiration, and various metabolites were quantified after 5 days and 5 weeks adaptation. After 5 days, growth rates were lower for plants at all altered salinities. Growth rates recovered from these values with 5 weeks adaptation, except for salinities of 10 grams per liter and below, where tissues bleached and died. Photosynthetic O(2) evolution was lower than control values at both higher and lower salinities after 5 days and did not change over time. Carbon fixation at the altered salinities was unchanged after 5 days, but decreased below 25 grams per liter and above 40 grams per liter after 5 weeks. Respiration increased at lower salinities. Phycobili-protein and chlorophyll were lower for all altered salinities after 5 days. These decreases continued at lower salinities, then were stable after 5 weeks. Chlorophyll recovered over time at higher salinities. Decreases in protein at lower salinities were quantitatively attributable to phycobili-protein loss. Total N levels and C:N ratios were nearly constant across all salinities tested. Carbon flow into glutamate and aspartate decreased with both decreasing and increasing salinities. Glycine, serine, and glycolate levels increased with both increasing and decreasing salinity, indicating a stimulation of photorespiration. The cell wall component agar increased with decreasing salinity, although biosynthesis was inhibited at both higher and lower salinities. The storage compound floridoside increased with increasing salinity. The evidence suggests stress responses to altered salinities that directly affected photosynthesis, respiration, and nitrogen assimilation and indirectly affected photosynthate flow. At low salinities, respiration and photorespiration exceeded photosynthesis with lethal results. At higher salinities, although photosynthesis was inhibited, respiration was low and carbon fixation adequate to offset increased photorespiration.

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PubMed1989

Photosynthetic Plasticity in Flaveria brownii: Growth Irradiance and the Expression of C(4) Photosynthesis.

Photosynthesis was examined in leaves of Flaveria brownii A. M. Powell, grown under either 14% or 100% full sunlight. In leaves of high light grown plants, the CO(2) compensation point and the inhibition of photosynthesis by 21% O(2) were significantly lower, while activities of ribulose 1,5-bisphosphate carboxylase/oxygenase and various C(4) cycle enzymes were considerably higher than those in leaves grown in low light. Both the CO(2) compensation point and the degree of O(2) inhibition of apparent photosynthesis were relatively insensitive to the light intensity used during measurements with plants from either growth conditions. Partitioning of atmospheric CO(2) between Rubisco of the C(3) pathway and phosphoenolpyruvate carboxylase of the C(4) cycle was determined by exposing leaves to (14)CO(2) for 3 to 16 seconds, and extrapolating the labeling curves of initial products to zero time. Results indicated that approximately 94% of the CO(2) was fixed by the C(4) cycle in high light grown plants, versus approximately 78% in low light grown plants. Thus, growth of F. brownii in high light increased the expressed level of C(4) photosynthesis. Consistent with the carbon partitioning patterns, photosynthetic enzyme activities (on a chlorophyll basis) in protoplasts from leaves of high light grown plants showed a more C(4)-like pattern of compartmentation. Pyruvate, Pi dikinase and phosphoenolpyruvate carboxylase were more enriched in the mesophyll cells, while NADP-malic enzyme and ribulose 1,5-bisphosphate carboxylase/oxygenase were relatively more abundant in the bundle sheath cells of high light than of low light grown plants. Thus, these results indicate that F. brownii has plasticity in its utilization of different pathways of carbon assimilation, depending on the light conditions during growth.

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PubMed1990

Further studies on o(2)-resistant photosynthesis and photorespiration in a tobacco mutant with enhanced catalase activity.

The increase in net photosynthesis in M(4) progeny of an O(2)-resistant tobacco (Nicotiana tabacum) mutant relative to wild-type plants at 21 and 42% O(2) has been confirmed and further investigated. Self-pollination of an M(3) mutant produced M(4) progeny segregating high catalase phenotypes (average 40% greater than wild type) at a frequency of about 60%. The high catalase phenotype cosegregated precisely with O(2)-resistant photosynthesis. About 25% of the F(1) progeny of reciprocal crosses between the same M(3) mutant and wild type had high catalase activity, whether the mutant was used as the maternal or paternal parent, indicating nuclear inheritance. In high-catalase mutants the activity of NADH-hydroxypyruvate reductase, another peroxisomal enzyme, was the same as wild type. The mutants released 15% less photorespiratory CO(2) as a percent of net photosynthesis in CO(2)-free 21% O(2) and 36% less in CO(2)-free 42% O(2) compared with wild type. The mutant leaf tissue also released less (14)CO(2) per [1-(14)C]glycolate metabolized than wild type in normal air, consistent with less photorespiration in the mutant. The O(2)-resistant photosynthesis appears to be caused by a decrease in photorespiration especially under conditions of high O(2) where the stoichiometry of CO(2) release per glycolate metabolized is expected to be enhanced. The higher catalase activity in the mutant may decrease the nonenzymatic peroxidation of keto-acids such as hydroxypyruvate and glyoxylate by photorespiratory H(2)O(2).

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PubMed1990

Physiological investigations of a tobacco mutant with o(2)-resistant photosynthesis and enhanced catalase activity.

Experiments are described further indicating that O(2)-resistant photosynthesis observed in a tobacco (Nicotiana tabacum) mutant with enhanced catalase activity is associated with decreased photorespiration under conditions of high photorespiration relative to net photosynthesis. The effects on net photosynthesis of (a) increasing O(2) concentrations from 1% to 42% at low CO(2) (250 microliters CO(2) per liter), and (b) of increasing O(2) concentrations from 21% to 42% at high CO(2) (500 microliters CO(2) per liter) were investigated in M(6) progeny of mutant and wild-type leaf discs. The mutant displayed a progressive increase in net photosynthesis relative to wild type with increasing O(2) and the faster rate at 42% O(2) was completely reversed on returning to 21% O(2). The photosynthetic rate by the mutant was similar to wild type in 21% and 42% O(2) at 500 microliters CO(2) per liter, and a faster rate by the mutant was restored on returning to 250 microliters CO(2) per liter. The results are consistent with a lowered release of photorespiratory CO(2) by the mutant because greater catalase activity inhibits the chemical decarboxylation of alpha-keto acids by peroxisomal H(2)O(2). Higher catalase activity was observed in the tip and middle regions of expanding leaves than in the basal area. On successive selfing of mutant plants with enhanced catalase activity, the percent of plants with this phenotype increased from 60% in M(4) progeny to 85% in M(6) progeny. An increase was also observed in the percent of plants with especially high catalase activity (averaging 1.54 times wild type) on successive selfings suggesting that homozygosity for enhanced catalase activity was being approached.

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PubMed1991

Nonsteady-State Photosynthesis following an Increase in Photon Flux Density (PFD) : Effects of Magnitude and Duration of Initial PFD.

The response of photosynthesis to an increase in photon flux density (PFD) from low to higher PFD was investigated using spinach (Spinacia oleracea L.). The time-course for this response was qualitatively similar to that observed for a dark-to-high-PFD transition, showing an initial, rapid increase in photosynthesis over the first minute or so, followed by a slower increase lasting 5 to 10 minutes. This slow increase was approximately exponential and could be linearized using a semilogarithmic plot. The relaxation time (tau) for this slow phase was found to be a function of the starting PFD value. At starting PFD values below approximately 135 micromoles per square meter per second (including darkness), tau for the slow phase was approximately twice that observed for starting PFD values above 135 micromoles per square meter per second. This indicates a slower approach to steady state for leaves starting at PFD values below this threshold and a greater loss of potential photosynthesis. tau was relatively insensitive to starting PFD values below or above this transition value. The contribution of the slow phase to the total increase in photosynthesis following a low-to-high-PFD transition increased approximately exponentially with time at the lower PFD. The tau for the increase in the contribution of slow phase was determined to be 10.1 minutes. The implications of these data for activation and deactivation of ribulose-1,5-bisphosphate carboxylase/oxygenase and for the functioning of the leaf in a fluctuating light environment are discussed.

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