Activation of a Reserve Pool of Photosystem II in Chlamydomonas reinhardtii Counteracts Photoinhibition

The effect of strong irradiance (2000 micromole photons per square meter per second) on PSII heterogeneity in intact cells of Chlamydomonas reinhardtii was investigated. Low light (LL, 15 micromole photons per square meter per second) grown C. reinhardtii are photoinhibited upon exposure to strong irradiance, and the loss of photosynthetic functioning is due to damage to PSII. Under physiological growth conditions, PSII is distributed into two pools. The large antenna size (PSII_α) centers account for about 70% of all PSII in the thylakoid membrane and are responsible for plastoquinone reduction (Q_b-reducing centers). The smaller antenna (PSII_β) account for the remainder of PSII and exist in a state not yet able to photoreduce plastoquinone (Q_b-nonreducing centers). The exposure of C. reinhardtii cells to 60 minutes of strong irradiance disabled about half of the primary charge separation between P680 and pheophytin. The PSII_β content remained the same or slightly increased during strong-irradiance treatment, whereas the photochemical activity of PSII_α decreased by 80%. Analysis of fluorescence induction transients displayed by intact cells indicated that strong irradiance led to a conversion of PSII_β from a Q_b-nonreducing to a Q_b-reducing state. Parallel measurements of the rate of oxygen evolution revealed that photosynthetic electron transport was maintained at high rates, despite the loss of activity by a majority of PSII_α. The results suggest that PSII_β in C. reinhardtii may serve as a reserve pool of PSII that augments photosynthetic electron-transport rates during exposure to strong irradiance and partially compensates for the adverse effect of photoinhibition on PSII_α.     

Compensatory Alterations in the Photochemical Apparatus of a Photoregulatory, Chlorophyll b-Deficient Mutant of Maize

Characterization of the functional organization of the photochemical apparatus in the light sensitive chlorophyll b-deficient oil yellow-yellow green (OY-YG) mutant of maize (Zea mays) is presented. Spectrophotometric and kinetic analysis revealed substantially lower amounts of the light harvesting complex of photosystem II (LHCII-peripheral) in high light-grown OY-YG thylakoids. However, accumulation of a tightly bound LHCII appears unaffected by the lesion. Changes in photosystem (PS) stoichiometry include lower amounts of PSII with characteristic fast kinetics (PSII_α) and a substantial accumulation of PSII centers with characteristic slow kinetics (PSII_β) in the thylakoid membrane of the OY-YG mutant. Thus, PSII_β is the dominant photosystem in the mutant chloroplasts. In contrast to wild type, roughly 80% of the mutant PSII_β centers are functionally coupled to the plastoquinone pool and are probably localized in the appressed regions of the thylakoid membrane. These centers, designated PSII_β-Q_B-reducing (Q_B being the secondary electron quinone acceptor of PSII), are clearly distinct from the typical PSII_β-Q_B-nonreducing centers found in the stroma lamellae of wild-type chloroplasts. It is concluded that the observed changes in the stoichiometry of electron-transport complexes reflect the existence of a regulatory mechanism for the adjustment of photosystem stoichiometry in chloroplasts designed to correct any imbalance in light absorption by the two photosystems.     

Response of the Photosynthetic Apparatus in Dunaliella salina (Green Algae) to Irradiance Stress

The response of the photosynthetic apparatus in the green alga Dunaliella salina, to irradiance stress was investigated. Cells were grown under physiological conditions at 500 millimoles per square meter per second (control) and under irradiance-stress conditions at 1700 millimoles per square meter per second incident intensity (high light, HL). In control cells, the light-harvesting antenna of photosystem I (PSI) contained 210 chlorophyll a/b molecules. It was reduced to 105 chlorophyll a/b in HL-grown cells. In control cells, the dominant form of photosystem II (PSII) was PSII_α(about 63% of the total PSII) containing >250 chlorophyll a/b molecules. The smaller antenna size PSII_β centers (about 37% of PSII) contained 135 ± 10 chlorophyll a/b molecules. In sharp contrast, the dominant form of PSII in HL-grown cells accounted for about 95% of all PSII centers and had an antenna size of only about 60 chlorophyll a molecules. This newly identified PSII unit is termed PSII_γ. The HL-grown cells showed a substantially elevated PSII/PSI stoichiometry ratio in their thylakoid membranes (PSII/PSI = 3.0/1.0) compared to that of control cells (PSII/PSI = 1.4/1.0). The steady state irradiance stress created a chronic photoinhibition condition in which D. salina thylakoids accumulate an excess of photochemically inactive PSII units. These PSII units contain both the reaction center proteins and the core chlorophyll-protein antenna complex but cannot perform a photochemical charge separation. The results are discussed in terms of regulatory mechanism(s) in the plant cell whose function is to alleviate the adverse effect of irradiance stress.     

Rearrangement of the chloroplast thylakoid at chilling temperature in the light

The leaves of chilling-sensitive pumpkin (Cucurbita pepo L.) showed symptoms reminiscent of photoinhibition when kept for 4 days at 5°C in moderate light. A decrease was observed in the variable part of chlorophyll α fluorescence, apparent quantum yield, and maximum rate of O₂ evolution. Chloroplast whole-chain electron transport activity measured from chloroplast thylakoids had decreased to 51% of the control value. Photosystem II (PSII) activity decreased by only 9%, suggesting that photoinhibition was not responsible for the loss of electron transport activity. An increase in the proportion of PSII_β (measured as a β_max value) was observed after the chilling treatment. Fractionation of thylakoid membranes showed a 42% increase in PSII activity in the nonappressed region while that in the appressed region decreased slightly. This was accompanied by a decrease in the ratio of the length of appressed to nonappressed thylakoid membranes. Leaf photosynthesis largely recovered within 24 hours of returning to the original growth conditions. We suggest that the increase in the proportion of PSII_β during chilling in light plays a role in protecting PSII from photoinhibitory damage.     

The Nature of Light-Induced Inhibition of Photosystem II in Pumpkin (Cucurbita pepo L.) Leaves Depends on Temperature

Attached leaves of pumpkin (Cucurbita pepo L.) were treated in high or moderate light at room temperature or a 1°C. The symptoms of photoinhibition appearing during light treatments at room temperature could be attributed to a decrease in the primary activity of PSII. However, when the light treatment was given at 1°C, the quantum yield of photosynthetic oxygen evolution decreased much more than would be expected from the decrease in the ratio of variable to maximum fluorescence at 77°K. Also, light treatment at 1°C lowered the chloroplast wholechain electron transfer capacity much more than it affected PSII electron transport (H₂O to paraphenylbenzoquinone). Light treatments at both room temperature and 1°C led to an increase in B_max, which indicates an increase in the proportion of PSII_β centers. PSI was not affected by the light treatments, and the treatments in the dark at 1°C caused only minor changes in the measured properties of the leaves. We conclude that high light always inhibits the primary activity of PSII, but at low temperature there is greater inhibition of electron transfer from primary electron accepting plastoquinone of PSII to the plastoquinone pool, which leads to a drastic decrease in the quantum yield of oxygen evolution in the chilling-sensitive pumpkin.     

Photosystem electron-transport capacity and light-harvesting antenna size in maize chloroplasts

Spectrophotometric and kinetic measurements were applied to yield photosystem (PS) stoichiometries and the functional antenna size of PSI, PSII_α, and PSII_β in Zea mays chloroplasts in situ. Concentrations of PSII and PSI reaction centers were determined from the amplitude of the light-induced absorbance change at 320 and 700 nm, which reflect the photoreduction of the primary electron acceptor Q of PSII and the photooxidation of the reaction center P700 of PSI, respectively. Determination of the functional chlorophyll antenna size (N) for each photosystem was obtained from the measurement of the rate of light absorption by the respective reaction center. Under the experimental conditions employed, the rate of light absorption by each reaction center was directly proportional to the number of light-harvesting chlorophyll molecules associated with the respective photosystem. We determined N_P700 = 195, N_α = 230, N_β = 50 for the number of chlorophyll molecules in the light-harvesting antenna of PSI, PSII_α, and PSII_β, respectively. The above values were used to estimate the PSII/PSI electron-transport capacity ratio (C) in maize chloroplasts. In mesophyll chloroplasts C > 1.4, indicating that, under green actinic excitation when Chl a and Chl b molecules absorb nearly equal amounts of excitation, PSII has a capacity to turn over electrons faster than PSI. In bundle sheath chloroplasts C < 1, suggesting that such chloroplasts are not optimally poised for linear electron transport and reductant generation.     

Photosystem II Cyclic Heterogeneity and Photoactivation in the Diazotrophic, Unicellular Cyanobacterium Cyanothece Species ATCC 51142

The unicellular, diazotrophic cyanobacterium Cyanothece sp. ATCC 51142 demonstrated important modifications to photosystem II (PSII) centers when grown under light/dark N₂-fixing conditions. The properties of PSII were studied throughout the diurnal cycle using O₂-flash-yield and pulse-amplitude-modulated fluorescence techniques. Nonphotochemical quenching (q_N) of PSII increased during N₂ fixation and persisted after treatments known to induce transitions to state 1. The q_N was high in cells grown in the dark, and then disappeared progressively during the first 4 h of light growth. The photoactivation probability, ε, demonstrated interesting oscillations, with peaks near 3 h of darkness and 4 and 10 h of light. Experiments and calculations of the S-state distribution indicated that PSII displays a high level of heterogeneity, especially as the cells prepare for N₂ fixation. We conclude that the oxidizing side of PSII is strongly affected during the period before and after the peak of nitrogenase activity; changes include a lowered capacity for O₂ evolution, altered dark stability of PSII centers, and substantial changes in q_N.     

Photosynthetic Light Utilization Efficiency, Photosystem II Heterogeneity, and Fluorescence Quenching in Chlamydomonas reinhardtii during the Induction of the CO(2)-Concentrating Mechanism

The photosynthetic light-response curve, the relative amounts of the different photosystem II (PSII) units, and fluorescence quenching were altered in an adaptive manner when CO₂-enriched wild-type Chlamydomonas reinhardtii cells were transferred to low levels of CO₂. This treatment is known to result in the induction of an energy-dependent CO₂-concentrating mechanism (CCM) that increases the internal inorganic carbon concentration and thus the photosynthetic CO₂ utilization efficiency. After 3 to 6 h of low inorganic carbon treatment, several changes in the photosynthetic energy-transducing reactions appeared and proceeded for about 12 h. After this time, the fluorescence parameter variable/maximal fluorescence yield and the amounts of both PSIIα and PSIIβ (secondary quinone electron acceptor of PSII-reducing) centers had decreased, whereas the amount of PSIIβ (secondary quinone electron acceptor of PSII-nonreducing) centers had increased. The yield of noncyclic electron transport also decreased during the induction of the CCM, whereas both photochemical and nonphotochemical quenching of PSII fluorescence increased. Concurrent with these changes, the photosynthetic light-utilization efficiency also decreased significantly, largely attributed to a decline in the curvature parameter θ, the convexity of the photosynthetic light-response curve. Thus, it is concluded that the increased CO₂ utilization efficiency in algal cells possessing the CCM is maintained at the cost of a reduced light utilization efficiency, most probably due to the reduced energy flow through PSII.     

The Regulation of Photosynthetic Electron Transport during Nutrient Deprivation in Chlamydomonas reinhardtii

The light-saturated rate of photosynthetic O₂ evolution in Chlamydomonas reinhardtii declined by approximately 75% on a per-cell basis after 4 d of P starvation or 1 d of S starvation. Quantitation of the partial reactions of photosynthetic electron transport demonstrated that the light-saturated rate of photosystem (PS) I activity was unaffected by P or S limitation, whereas light-saturated PSII activity was reduced by more than 50%. This decline in PSII activity correlated with a decline in both the maximal quantum efficiency of PSII and the accumulation of the secondary quinone electron acceptor of PSII nonreducing centers (PSII centers capable of performing a charge separation but unable to reduce the plastoquinone pool). In addition to a decline in the light-saturated rate of O₂ evolution, there was reduced efficiency of excitation energy transfer to the reaction centers of PSII (because of dissipation of absorbed light energy as heat and because of a transition to state 2). These findings establish a common suite of alterations in photosynthetic electron transport that results in decreased linear electron flow when C. reinhardtii is limited for either P or S. It was interesting that the decline in the maximum quantum efficiency of PSII and the accumulation of the secondary quinone electron acceptor of PSII nonreducing centers were regulated specifically during S-limited growth by the SacI gene product, which was previously shown to be critical for the acclimation of C. reinhardtii to S limitation (J.P. Davies, F.H. Yildiz, and A.R. Grossman [1996] EMBO J 15: 2150–2159).     

Adaptation to CO(2) Level and Changes in the Phosphorylation of Thylakoid Proteins during the Cell Cycle of Chlamydomonas reinhardtii

The photosynthetic performance of synchronously grown Chlamydomonas reinhardtii alternated rhythmically during the cell cycle. The activity of the “CO₂ concentrating mechanism” including the ability to accumulate CO₂ internally and the activity of carbonic anhydrase peaked after 6 to 9 hours of light and reached minimum after 6 to 9 hours of dark. Consequently, the apparent photosynthetic affinity to extracellular CO₂ alternated rhythmically. At the end of the dark period the cells behaved as if they were adapted to high CO₂ even though they were continuously aerated with air. Results from experiments in which the light or dark periods were extended bear on the interaction between the internal (cell cycle or biological clock) and the external (light) signal. The observed rhythmical alterations in photosynthetic V_max may result from changes in PSII activity. The latter may be partly explained by the capacity for phosphorylation of thylakoid proteins, which reached maximum after 9 hours of light and decreased toward the dark period.     

Photosynthetic performance of regenerated protoplasts from disintegrated cells of <Emphasis Type="Italic">Bryopsis hypnoides</Emphasis> (Chlorophyta)

The photosynthetic performances of regenerated protoplasts of Bryopsis hypnoides, which were incubated in seawater for 1, 6, 12, and 24 h, were studied using chlorophyll (Chl) fluorescence and oxygen measurements. Results showed that for the regenerated protoplasts, the pigment content, the ratios of photosynthetic rate to respiration rate, the maximal photosystem II (PSII) quantum yield (F_v/F_m), and the effective PSII quantum yield (Φ_PSII) decreased gradually along with the regeneration progress, indicated that during 24 h of regeneration there was a remarkable reduction in PSII activity of those newly formed protoplasts. We assumed that during the cultivation progress the regenerated protoplasts had different photosynthetic vigor, with only some of them able to germinate and develop into mature thalli. The above results only reflected the photosynthetic features of the regenerated protoplasts at each time point as a whole, rather than the actual photosynthetic activity of individual aggregations. Further investigation suggested a relationship between the size of regenerated protoplasts and their viability. The results showed that the middle-sized group (diameter 20–60 μm) retained the largest number of protoplasts for 24 h of growth. The changes in F_v/F_m and Φ_PSII of the four groups of differently sized protoplasts (i.e. < 20, 20–60, 60–100, and > 100 μm) revealed that the protoplasts 20–60 μm in diameter had the highest potential activity of the photosynthetic light energy absorption and conversion for several hours.     

Internal CO(2) Supply during Photosynthesis of Sun and Shade Grown CAM Plants in Relation to Photoinhibition

Leaves of Kalanchoë pinnata were exposed in the dark to air (allowing the fixation of CO₂ into malic acid) or 2% O₂, 0% CO₂ (preventing malic acid accumulation). They were then exposed to bright light in the presence or absence of external CO₂ and light dependent inhibition of photosynthetic properties assessed by changes in 77 K fluorescence from photosystem II (PSII), light response curves and quantum yields of O₂ exchange, rates of electron transport from H₂O through Q_B (secondary electron acceptor from the PSII reaction center) in isolated thylakoids, and numbers of functional PSII centers in intact leaf discs. Sun leaves of K. pinnata experienced greater photoinhibition when exposed to high light in the absence of CO₂ if malic acid accumulation had been prevented during the previous dark period. Shade leaves experienced a high degree of photoinhibition when exposed to high light regardless of whether malic acid had been allowed to accumulate in the previous dark period or not. Quantum yields were depressed to a greater degree than was 77 K fluorescence from PSII following photoinhibition.     

Photooxidative Damage in Photosynthetic Activities of Chromatium vinosum

The capacity of photosynthetic CO₂ fixation in the anaerobic purple-sulfur bacterium, Chromatium vinosum is markedly impaired by strong illumination (9 × 10⁴ lux) in the presence of 100% O₂. In the absence of HCO₃⁻, decline in activity occurred gradually, with about 40% of the initial activity remaining after a 1-hour incubation. The addition of 50 millimolar HCO₃⁻ to the incubation medium resulted in a measurable delay (about 30 minutes) of the inactivation process. Ribulose-1,5-bisphosphate carboxylase activity and light-dependent O₂ uptake (electron flow) or crude extracts prepared after pretreatment of the bacterial cells with O₂ and light were not affected but the photophosphorylation capacity of either bacterial cells or chromatophores was drastically reduced. The inhibition of photophos-phorylation in the chromatophore preparations was significantly reduced by the addition of either an O₂⁻ scavenger, Tiron, or an ¹O₂ scavenger, α-tocopherol. These results suggest that the active O₂ species, O₂⁻ or ¹O₂, might take part in the observed inactivation.

The pretreatment of the bacteria with O₂ and light inhibited CO₂ assimilation through the Calvin-Benson cycle, while relatively stimulating the formation of aspartate and glutamate. It also inhibited the conversion of glycolate to glycine, resulting in a sustained extracellular excretion of glycolate. The inactivation of photosynthetic CO₂ fixation by intact cells was enhanced by low temperature, KCN, or methylviologen addition during the pretreatment with O₂ and light. The mechanism(s) of O₂-dependent photoinactivation of photosynthetic activities in Chromatium are discussed in relation to the possible role of photorespiration as a means of producing CO₂ in the photosynthetic system.

     

Activities of Photosystems I and II in Chlamydomonas segnis Adapted and Adapting to Air and Air-enriched with Carbon Dioxide*

Activities of photosystems I and II were compared at a saturating irradiance in air- and 5% CO₂-adapted and adapting Chlamydomonas segnis at the active phase of photosynthesis during the cell cycle. PSII activity was 200% greater in air- than in 5% CO₂-adapted cells, while PSI activity was similar in both types of cells and matched the level of PSII activity in air-adapted cells. As a result, air- and 5% CO₂-adapted cells were characterized by low and high PSI/PSII ratios, respectively. In air-adapted cells, the greater PSII activity (rate of O₂ evolved) exceeded that of photosynthetic (Ps) O₂ evolution, resulting in a Ps/PSII ratio below unity. This was associated with higher levels of catalase activity, lower l -ascorbate content, and higher dehydro-l -ascorbate content than in 5% CO₂-adapted cells. During adaptation to air or 5% CO₂ for 6 h in light, PSI rather than PSII was sensitive to changes in the concentration of CO₂, and the adapting cells acquired the characteristics of air- and 5% CO₂-adapted cells as indicated by PSI/PSII, Ps/PSII, catalase activity, l -ascorbate and dehydro-l -ascorbate contents. The results are discussed in the light of changes in the molecular organization of the thylakoid membranes and enhanced non-cyclic electron transport coupled with O₂-uptake (Mehler reaction) for the generation of the ATP required for CO₂/HCO^?₃-transport in air-adapted and adapting cells.     

Diurnal cycle of chlorophyll fluorescence in <Emphasis Type="Italic">Phalaenopsis</Emphasis>

Chlorophyll (Chl) fluorescence of warm day/cool night temperature exposed Phalaenopsis plants was measured hourly during 48 h to study the simultaneous temperature and irradiance response of the photosynthetic physiology. The daily pattern of fluorescence kinetics showed abrupt changes of photochemical quenching (q_P), non-photochemical quenching (NPQ) and quantum yield of photosystem II electron transport (Φ_PSII) upon transition from day to night and vice versa. During the day, the course of Φ_PSII and NPQ was related to the air temperature pattern, while maximum quantum efficiency of PSII photochemistry (F_v/F_m) revealed a rather light dependent response. Information on these daily dynamics in fluorescence kinetics is important with respect to meaningful data collection and interpretation.     

Effects of groundwater depth variation on photosynthesis and photoprotection of <Emphasis Type="Italic">Elaeagnus angustifolia</Emphasis> L.

Physiological and photosynthetic responses were investigated at three different depths of groundwater (DGW: 1.4, 2.4, and 3.4 m) in Elaeagnus angustifolia L., a locally adapted tree to the arid region in northwest China. Predawn leaf water potential and chlorophyll content declined gradually with the increasing DGW, whereas there was little effect on predawn variable-to-maximum chlorophyll fluorescence ratio F _v/F _m and leaf carotenoid compositions (xanthophyll cycle pool, neoxanthin, lutein, and β-carotene). Net photosynthetic rate (P _n), quantum yield of PSII electron transport (Φ_PSII), stomatal conductance (Gs), and intercellular CO₂ concentration (Ci) declined obviously; however, P _n decreased more than Φ_PSII at deeper DGW. The photoinhibition of PSII at all three DGW occurred at midday in summer and increased as DGW increased. The ΔpH-dependent thermal dissipation and the level of de-epoxidation of the xanthophyll cycle at all three DGW reached their maxima at midday with the increase of light intensity. However, the fraction of functional PSII and light intensity at deeper DGW (2.4, 3.4 m) showed a negative correlation. This correlation suggested that most of violaxanthin was converted into zeaxanthin at midday, and the reversible inactivation of partial PSII reaction centers took place at deeper DGW. These results together suggest that both the xanthophyll cycle-dependent thermal dissipation and the reversible inactivation of partial PSII might have played important roles in avoiding the excess light-induced energy damage in leaves of this tree species at deeper DGW.     

Seasonal and diurnal monitoring of leaf polyamine content in Quercus ilex L. resprouts after fire in relation to changes in irradiation and photosynthetic parameters

Seasonal variations in free putrescine, spermidine and spermine content, gas-exchange and chlorophyll fluorescence parameters were followed during winter and summer on leaves of a similar age from undisturbed holm oak trees (control, C) and resprouts (R) originated after fire. We observed a general trend of putrescine content decrease with increasing irradiance. Putrescine content decreased markedly from winter to summer, especially in R, which were located on a site with much higher irradiation. Daily summer variations in putrescine showed a decline at midday from morning values, and they were also more accentuated in R. Measurement of gas-exchange and chlorophyll fluorescence parameters showed marked differences between C and R under their respective light conditions. R showed higher values of PSII quantum yield (Φ_PSII), photochemical quenching (qP) and intrinsic efficiency of open PSII centres () The Φ_PSII/PPFD response curve showed that under the same irradiance, Φ_PSII was enhanced in R and mainly under high light conditions. In spite of increasing irradiance from winter to summer, and especially in burned areas, the mentioned chlorophyll fluorescence parameters were maintained indicating the adaptation of the photosynthetic apparatus. Results derived from A/C _i and A/PPFD response curves showed enhanced photosynthetic capacity and lower non-stomatal limitation of photosynthesis in R during summer stress. The contribution of putrescine decline in the photoadaptation of the photosynthetic apparatus of species growing in natural forest habitats is considered.     

Far‐red light enhances photochemical efficiency in a wavelength‐dependent manner

Linear electron transport depends on balanced excitation of photosystem I and II. Far‐red light preferentially excites photosystem I (PSI) and can enhance the photosynthetic efficiency when combined with light that over‐excites photosystem II (PSII). The efficiency of different wavelengths of far‐red light exciting PSI was quantified by measuring the change in quantum yield of PSII (Φ_PSII) of lettuce (Lactuca sativa) under red/blue light with narrowband far‐red light added (from 678 to 752 nm, obtained using laser diodes). The Φ_PSII of lettuce increased with increasing wavelengths of added light from 678 to 703 nm, indicating longer wavelengths within this region are increasingly used more efficiently by PSI than by PSII. Adding 721 nm light resulted in similar Φ_PSII as adding 703 nm light, but Φ_PSII tended to decrease as wavelength increased from 721 to 731 nm, likely due to decreasing absorptance and low photon energy. Adding 752 nm light did not affect Φ_PSII. Leaf chlorophyll fluorescence light response measurements showed lettuce had higher Φ_PSII under halogen light (rich in far‐red) than under red/blue light (which over‐excites PSII). Far‐red light is more photosynthetically active than commonly believed, because of its synergistic interaction with light of shorter wavelengths.     

Development of the Two Heterogeneous Photosystem II Units in Etiolated Bean Leaves

The development of the photosystem II units in relation to the heterogeneity of their photochemical centers was studied in etiolated bean leaves (Phaseolus vulgaris var. red kidney) greened under continuous or intermittent light. The study was done in order to see whether grana are the loci of the units with the efficient photosystem II activity (α units), while the stroma thylakoids are the loci of the units with the less efficient photosystem II activity (β units), as it has been proposed. In addition, the interrelations between α and β centers have been investigated. It was found that the α and the β centers of photosystem II were present in the first photosynthetic membranes irrespective of the mode of greening of the leaves. The magnitude of their respective photochemical rate constants, K′_α and K_β, increased with time in continuous light and it reached the steady-state values of the mature chloroplasts within 16 hours, while in intermittent light it remained smaller. The differentiation of the system II units in α and β centers containing units is more evident under conditions of intermittent illumination, i.e. when the rate of chlorophyll biosynthesis is the limiting step for chloroplast development.