Chlorophyll Synthesis in Dark-Grown Pine Primary Needles

The pigment content of dark-grown primary needles of Pinus jeffreyi L. and Pinus sylvestris L. was determined by high-performance liquid chromatography. The state of protochlorophyllide a and of chlorophylls during dark growth were analyzed by in situ 77 K fluorescence spectroscopy. Both measurements unambiguously demonstrated that pine primary needles are able to synthesize chlorophyll in the dark. Norflurazon strongly inhibited both carotenoid and chlorophyll synthesis. Needles of plants treated with this inhibitor had low chlorophyll content, contained only traces of xanthophylls, and accumulated carotenoid precursors. The first form of chlorophyll detected in young pine needles grown in darkness had an emission maximum at 678 nm. Chlorophyll-protein complexes with in situ spectroscopic properties similar to those of fully green needles (685, 695, and 735 nm) later accumulated in untreated plants, whereas in norflurazon-treated plants the photosystem I emission at 735 nm was completely lacking. To better characterize the light-dependent chlorophyll biosynthetic pathway in pine needles, the 77 K fluorescence properties of in situ protochlorophyllide a spectral forms were studied. Photoactive and nonphotoactive protochlorophyllide a forms with emission properties similar to those reported for dark-grown angiosperms were found, but excitation spectra were substantially red shifted. Because of their lower chlorophyll content, norflurazon-treated plants were used to study the protochlorophyllide a photoreduction process triggered by one light flash. The first stable chlorophyllide photoproduct was a chlorophyllide a form emitting at 688 nm as in angiosperms. Further chlorophyllide a shifts usually observed in angiosperms were not detected. The rapid regeneration of photoactive protochlorophyllide a from nonphotoactive protochlorophyllide after one flash was demonstrated.     

Phytochrome Regulation of Greening in Pisum: Chlorophyll Accumulation and Abundance of mRNA for the Light-Harvesting Chlorophyll a/b Binding Proteins

A brief pulse of red light eliminates or reduces the lag in chlorophyll accumulation that occurs when dark-grown pea seedlings are transferred to continuous white light. The red light pulse also induces the accumulation of specific mRNAs. We compared time courses, escape from reversal by far-red light, and fluence-response behavior for induction of mRNA for the light-harvesting chlorophyll a/b binding proteins (Cab mRNA) with those for induction of rapid chlorophyll accumulation in seedlings of Pisum sativum cv Alaska. In both cases the time courses of low fluence and very low fluence responses diverged from each other in a similar fashion: the low fluence responses continued to increase for at least 24 hours, while the very low fluence responses reached saturation by 8 to 16 hours. Both responses escaped from reversibility by far-red slowly, approaching the red control level after 16 hours. The fluence-response curve for the Cab mRNA increase, on the other hand, showed threshold and saturation at fluences 10-fold lower than threshold and saturation values for the greening response. Therefore, the level of Cab mRNA, as measured by the presence of sequences hybridizing to a cDNA probe, does not limit the rate of chlorophyll accumulation after transfer of pea seedlings to white light. The Cab mRNA level in the buds of seedlings grown under continuous red light remained high even when the red fluence rate was too low to allow significant greening. In this case also, abundance of Cab mRNA cannot be what limits chlorophyll accumulation.     

Chloroplast Biogenesis: XX. Accumulation of Porphyrin and Phorbin Pigments in Cucumber Cotyledons during Photoperiodic Greening

A study of greening in cucumber (Cucumis sativus L.) cotyledons grown under a light (14-hour) dark (10-hour) photoperiodic regime was undertaken. The pools of protoporphyrin IX, Mg-protoporphyrin IX monoester, protochlorophyllide, and protochlorophyllide ester were determined spectrofluorometrically. Chlorophyll a and b were monitored spectrophotometrically. Pigments were extracted during the 3rd hour of each light period and at the end of each subsequent dark period during the first seven growth cycles. Protoporphyrin IX did not accumulate during greening. Mg-protoporphyrin IX monoester and longer wavelength metalloporphyrins accumulated during the light cycles and disappeared in the dark. Their disappearance was accompanied by the accumulation of protochlorophyll. Higher levels of protochlorophyll were observed in the dark than in the light, and the greatest accumulation occurred during the third and fourth dark cycles. Protochlorophyllide was present in 3- to 10-fold excess over protochlorophyllide ester; it was detectable during the period of net chlorophyll accumulation as well as afterward. In contrast, protochlorophyllide ester was observable only during the first four photoperiodic cycles, suggesting that it was a metabolic intermediate only during the early stages of chlorophyll accumulation. Between the third and fourth growth cycles, a rapid increase in area and fresh weight per cotyledon began. This was accompanied by a 250-fold increase in the level of chlorophyll a + b during the three subsequent growth cycles. No lag period in the accumulation of chlorophyll b was observed, and at all stages of greening, the chlorophyll a/b ratio was approximately 3.     

Chloroplast culture: The chlorophyll repair potential of mature chloroplasts incubated in a simple medium

The chlorophyll repair potential of mature Cucumis chloroplasts incubated in a simple Tris-HCI/sucrose medium is described. The chloroplasts were isolated from green, fully expanded Cucumis cotyledons which were capable of chlorophyll repair. This was evidenced by a functional chlorophyll biosynthetic pathway in the mature tissue. The biosynthesis of protochlorophyllide from exogenous δ-aminolevulinic acid was used as a marker for the operation of the chlorophyll biosynthetic chain between δ-aminolevulinic acid and protochlorophyllide. The conversion of exogenous protochlorophyllide into chlorophyll a was used as a marker for the operation of the chlorophyll pathway beyond protochlorophyllide. It appeared from these studies that contrary to published reports, unfortified fully developed Cucumis chloroplasts incubated in Tris-HCl/sucrose without the addition of cofactors exhibited a partial and limited chlorophyll repair capability. Their net tetrapyrrole biosynthetic competence from δ-aminolevulinic acid was confined to the accumulation of coproporphyrin. No net tetrapyrrole biosynthesis beyond coproporphyrin was observed. However, the plastids were capable of incorporating small amounts of δ-amino-[4-¹⁴C]levulinic acid into [¹⁴C] protochlorophyllide but were incapable of converting exogenous protochlorophyllide into chlorophyll. After prolonged incubation of the unfortified chloroplasts in the dark, a fluorescent protochlorophyllide-like compound accumulated. This compound [Cp (E₄₃₀-F₆₃₁)] exhibited a soret excitation maximum at 430 nm (E₄₃₀) and a fluorescence emission maximum at 631 nm (F₆₃₁) in methanol/acetone (4 : 1, v/v). Cp (E₄₃₀-F₆₃₁) was shown to be neither protochlorophyllide nor zinc-protochlorophyllide but an enzymatic degradation product of chlorophyll. The exact chemical identity of this compound has not yet been determined.     

Photocontrol of the Accumulation of Plastid Polypeptides during Greening of Tomato Cotyledons : Potentiation by a Pulse of Red Light

A pulse of red light acting through phytochrome accelerates the formation of chlorophyll upon subsequent transfer of dark-grown seedlings to continuous white light. Specific antibodies were used to follow the accumulation of representative subunits of the major photosynthetic complexes during greening of seedlings of tomato (Lycopersicon esculentum). The time course for accumulation of the various subunits was compared in seedlings that received a red light pulse 4 h prior to transfer to continuous white light and parallel controls that did not receive a red light pulse. The light-harvesting chlorophyll-binding proteins of photosystem II (LHC II), the 33-kD extrinsic polypeptide of the oxygen-evolving complex (OEC1), and subunit II of photosystem I (psaD gene product) all increased in the light, and did so much faster in seedlings that received the inductive red light pulse. The red light pulse had no significant effect on the abundance of the small subunit of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), nor on several plastid-encoded polypeptides: the large subunit of Rubisco, the β subunit of the CF₁ complex of plastid ATPase, and the 43- and 47-kD subunits of photosystem II (CP43, CP47). Subunits I (cytochrome b₆f) and III (Rieske Fe-S protein) of the cytochrome b₆f complex showed a small or no increase as a result of the red pulse. The potentiation of greening by a pulse of red light, therefore, is not expressed uniformly in the abundance of all the photosynthetic complexes and their subunits.     

Events Surrounding the Early Development of Euglena Chloroplasts: VI. Action Spectra for the Formation of Chlorophyll, Lag Elimination in Chlorophyll Synthesis, and Appearance of TPN-dependent Triose Phosphate Dehydrogenase and Alkaline DNase Activities

Action spectra derived from dose-response curves measured for various processes associated with chloroplast development in Euglena gracilis var. bacillaris are presented. The action spectrum for chlorophyll synthesis during the first 36 hours of continuous illumination of dark-grown resting cells resembles the absorption spectrum of protochlorophyll(ide). The action spectrum for the preillumination phase of potentiation, during which preillumination followed by a dark period brings about lag elimination in chlorophyll synthesis when the cells are subsequently exposed to postilluminating light, shows a high peak in the blue region (at about 433 nm) with a small peak in the yellow-orange region (at about 597 nm); the postillumination phase yields an action spectrum very similar to that obtained for chlorophyll synthesis in continuous light in normal, unpotentiated cells, with peaks at 433 and 631 nm. Alkaline DNase and TPN-linked triose phosphate dehydrogenase, two plastid enzymes which are synthesized outside the chloroplast, yield action spectra which are consistent with protochlorophyll(ide) being the major light receptor. The action spectra which implicate pigments resembling protochlorophyll(ide) holochrome have blue to red peak ratios in the vicinity of 5:1 as does the absorption spectrum of the protochlorophyllide holochrome from beans; the action spectrum is not identical with the holochrome spectrum indicating that the Euglena holochrome may differ from the bean pigment in details of its absorption spectrum. The action spectrum for preillumination, shows a ratio of the blue peak to the red effectiveness of about 24:1. This suggests that preillumination is controlled by a photoreceptor different from the protochlorophyll(ide) holochrome.     

Induction of delta-Aminolevulinic Acid Formation in Etiolated Maize Leaves Controlled by Two Light Systems

A short illumination of etiolated maize (Zea mays) leaves with red light causes a protochlorophyll(ide)-chlorophyll(ide) conversion and induces the synthesis of δ-aminolevulinic acid (ALA) during a subsequent dark period. In leaves treated with levulinic acid, more ALA is formed in the dark than in control leaves. Far red light does not cause a conversion of protochlorophyll(ide) into chlorophyll(ide) and does not induce accumulation of ALA in the dark. Both red and far red preilluminations cause a significant potentiation of ALA synthesis during a period of white light subsequent to the dark period. The results indicate a dual light control of ALA formation. The possible role of phytochrome and protochlorophyllide as photoreceptors in this control system is discussed.     

Altered Phytochrome Regulation of Greening in an aurea Mutant of Tomato

A brief pulse of red light accelerates chlorophyll accumulation upon subsequent transfer of dark-grown tomato (Lycopersicon esculentum) seedlings to continuous white light. Such potentiation of greening was compared in wild type and an aurea mutant W616. This mutant has been the subject of recent studies of phytochrome phototransduction; its dark-grown seedlings are deficient in phytochrome, and light-grown plants have yellow-green leaves. The rate of greening was slower in the mutant, but the extent (relative to the dark control) of potentiation by the red pulse was similar to that in the wild type. In the wild type, the fluence-response curve for potentiation of greening indicates substantial components in the VLF (very low fluence) and LF (low fluence) ranges. Far-red light could only partially reverse the effect of red. In the aurea mutant, only red light in the LF range was effective, and the effect of red was completely reversed by far-red light. When grown in total darkness, aurea seedlings are also deficient in photoconvertible PChl(ide). Upon transfer to white light, the aurea mutant was defective in both the abundance and light regulation of the light-harvesting chlorophyll a/b binding polypeptide(s) [LHC(II)]. The results are consistent with the VLF response in greening being mediated by phytochrome. Furthermore, the data support the hypothesis that light modulates LHC(II) levels through its control of the synthesis of both chlorophyll and its LHC(II) apoproteins. Some, but not all, aspects of the aurea phenotype can be accounted for by the deficiency in photoreception by phytochrome.     

The sites of photoconversion of protochlorophyllide to chlorophyllide in barley seedlings

The photoreduction of protochlorophyllide a to chlorophyllide a in intact 6-day-old seedlings of etiolated barley (Hordeum vulgare) exhibits a small initial phase, followed by an induction period of about 1 hour before a rapid phase of additional chlorophyll formation begins. Cycloheximide, an inhibitor of protein synthesis, has no effect on the initial phase of conversion of preformed protochlorophyllide, but it either abolishes or severely inhibits the subsequent phase of rapid chlorophyll synthesis within 45 minutes of its application to the seedlings. An analysis of the biphasic inhibition process suggests that the lifetime of the enzyme controlling protochlorophyllide synthesis (probably δ-amino-levulinic acid synthetase) is not longer than 10 minutes.     

Chlorophyll Formation in Greening Bean Leaves during the Early Stages

In the evolution of the absorption spectrum of etiolated bean leaves (Phaseolus vulgaris L.) following illumination, a rapid photoconversion of 50% or more of the active protochlorophyllide at room temperature is followed by a shift of the chlorophyll(ide) absorption maximum: C₆₇₈→ →C₆₈₄→C_{672 nm}. Kinetic studies at 2 C and the absence of an isosbestic point provide evidence for an intermediate between C₆₇₈ and C₆₈₄. A dramatically different evolution is observed following the photoconversion of only 5 to 30% of the active protochlorophyllide at room temperature. C₆₇₂ appears within 30 seconds, and no subsequent dark shift occurs during the following 90 minutes. At 0 C, conversion of 5% of the active protochlorophyllide produces a new species, C₆₇₆, which converts progressively to C₆₇₂ within 10 minutes. We interpret the results in terms of two photochemical steps operating in series for the complete conversion of active protochlorophyllide. Furthermore, there appears to be competition between an irreversible, terminal dark shift and the second light reaction. We propose a scheme based on dimers of protochlorophyllide reduced stepwise to dimers of chlorophyllide in two successive light reactions. The intermediate mixed protochlorophyllide-chlorophyllide dimer absorbs at 676 nm and displays a much faster dissociation to monomers than does the chlorophyllide-chlorophyllide dimer.     

Photoconversion of protochlorophyll to chlorophyll a in a mutant of Chlorella regularis

Dark-grown cells of a mutant strain of Chlorella regularis containedchlorophyll a and protochlorophyll, phytyl ester of protochlorophyllide.Under illumination, protochlorophyll was quantitatively anddirectly converted into chlorophyll a. The photoconversion wasdependent on light intensity and temperature and proceeded ina cell-free preparation. The pathway of chlorophyll formation found in the mutant cellsis entirely different from that from protochlorophyllide byway of chlorophyllide a, which is generally observed in greenplants. ¹Present address: Division of Biology, Medical College of Miyazaki,Miyazaki 889-16, Japan. ²Present address: Division of Environmental Biology, The NationalInstitute for Environmental Studies, Ibaragi 300-21, Japan. (Received October 24, 1975; )     

Time Dependence of Phytochrome-mediated Carotenoid and Chlorophyll Synthesis in Sorghum bicolor L

In 5-day-old etiolated Sorghum seedlings, red light irradiationfor 1 s enhanced carotenoid and chlorophyll accumulation, and5 min of red light treatment saturated the photoresponse. Thedegree of red/far-red photoreversibility of carotenoid accumulationwas dependent on the age of the plant. No significant escapefrom far-red reversibility was observed up to 30 min after theonset of a saturating red light pulse in 5-day-old etiolatedseedlings. Thereafter, the escape was relatively fast and completedwithin 180 min. Sorghum bicolor L, carotenogenesis, phytochrome, time dependence, chlorophyll synthesis     

The Photostability of Different Chlorophyll Forms in Dark Grown Leaves of Wheat

The stability against high intensity irradiation (red light, 700 W m²) was investigated for the chlorophyll(ide) pigments formed after photoreduction of the protochlorophyllide in dark grown leaves of wheat. Connections were found between changes in absorption spectrum in vivo (the Shibata shift and the late red-shift) and changes in photostability both in young (five-day) and old (12-day) leaves. The photostability of both the 684-form and the 673-form as well as the rate of the changes in photostability (the Shibata shift and the late red-shift) decreased with the age of the dark grown plants. It was concluded that the more pronounced decrease in the chlorophyll(ide) contents found at irradiation of older dark grown leaves mostly depended on the lower rate of the changes in the photostability of the pigment in old leaves. No resynthesis of protochlorophyllide occurred before the onset of the late red-shift. The results and their connection with the lag in chlorophyll formation are discussed. This lag is more pronounced in older dark grown wheat.     

The Long Wave Length Forms of Chlorophyll a

When Euglena gracilis is cultured with light of low intensity (ca. 250 ft-c), an absorption band at 695 mμ is formed in an amount equal to about 20 per cent of the total chlorophyll absorption in this red region. An equally large proportion of C_a695 is observed in Ochromonas danica, irrespective of light intensity. Other algae tested appear to contain approximately 3 to 5 per cent of their chlorophyll as C_a695; this proportion does not increase as strikingly with lowering of the light intensity as it does in Euglena. C_a695 bleaches more readily than the other chlorophyll forms both reversibly, in whole cells, and irreversibly, in homogenates. Cells containing a large proportion of C_a695 have a fluorescence maximum at 708 mμ, as contrasted to the 687 mμ maximum in other algae. Occasionally, old cultures of Euglena contain cells with an absorption band at approximately 710 mμ. This absorption band is quite stable in aqueous extracts; when the pigment is transferred to ether an equivalent amount of pheophytin a is found to be present. Conditions leading to the formation of the 710 mμ absorption band are not yet known.     

Changes in Chlorophyll a/b Ratio and Products of CO(2) Fixation by Algae Grown in Blue or Red Light

Chlamydomonas and Chlorella were grown for 10 days in white light. 955 μw/cm² blue light (400-500 mμ) or 685 μw/cm² red light (above 600 mμ). Rates of growth in blue or red light were initially slow, but increased over a period of 5 days until normal growth rates were reestablished. During this adaptation period in blue light, total chlorophyll per volume of algae increased 20% while the chlorophyll a/b ratio decreased. In red light no change was observed in the total amount of chlorophyll or in the chlorophyll a/b ratio. After adaptation to growth in blue light and upon exposure to ¹⁴CO₂ with either blue or white light for 3 to 10 minutes, 30 to 36% of the total soluble fixed ¹⁴C accumulated in glycolate-¹⁴C which was the major product. However, with 1 minute experiments, it was shown that phosphate esters of the photosynthetic carbon cycle were labeled before the glycolate. Glycolate accumulation by algae grown in blue light occurred even at low light intensity. After growth of the algae in red light, ¹⁴C accumulated in malate, aspartate, glutamate and alanine, whereas glycolate contained less than 3% of the soluble ¹⁴C fraction.     

Analysis of the Heat Stress Induced Quenching of Variable Chlorophyll a Fluorescence in Beet Spinach Leaves: Possible Accumulation of Oxidized Quencher P680+ under High Illumination

Effect of preheating of beet spinach leaves on chlorophyll a fluorescence yield was analyzed with the help of additional high intensity illumination pulses using a pulse modulated fluorometer. Preheating at mildly elevated temperature (35–45°C) causes a shift in the redox state of secondary donor of photosystem II, possibly due to uncoupling of phosphorylation because of thermal induced membrane disorganization and associated alkalinization of intra thylakoid space. Also, at these preheating temperatures, a rise in photosystem I catalyzed electron transfer has been shown to occur. These two effects induce rapid quenching of Chi a fluorescence, which drops even in the presence of actinic light, below the level of initial fluorescence (F_o′ monitored by the weak modulated probing light. Preheating of leaf segments induces an increase in fluorescence in the presence of dluron, which blocks electron flow between two photosystems, and thus this increases in fluorescence yield (F_o′ as monitored by weak modulated light, is not solely due to disorganization of light harvesting Chi-protein complex but also due to a shift in the redox equilibrium of the donor at the oxidizing side of photosystem II resulting in rapid reduction of Q_A the stable primary acceptor of photosystem II. In 50°C preheated DCMU treated samples, the fluorescence yield increases in weak modulated light and it approaches that of maximal steady state (F_max) level. At preheating temperature of 48°–50°C, the inactivation of enzymes in the reducing side of photosystem I, causes an impairment of the reoxidation of QA and under this condition, a strong illumination causes quenching of Chi a fluorescence. This quenching seems to arise because of accumulation of the P680⁺, the oxidized physiological donor of photosystem which is a quencher of Chi a fluorescence. This quenching depended on the pulse intensity and duration which saturates P680⁺ accumulation and is greatly manifested when water oxidation complex is damaged.     

Chlorophyll a Fluorescence Yield as a Monitor of Both Active CO(2) and HCO(3) Transport by the Cyanobacterium Synechococcus UTEX 625

Simultaneous measurements have been made of inorganic carbon accumulation (by mass spectrometry) and chlorophyll a fluorescence yield of the cyanobacterium Synechococcus UTEX 625. The accumulation of inorganic carbon by the cells was accompanied by a substantial quenching of chlorophyll a fluorescence. The quenching occurred even when CO₂ fixation was inhibited by iodoacetamide and whether the accumulation of inorganic carbon resulted from either active CO₂ or HCO₃⁻ transport. Measurement of chlorophyll a fluorescence yield of cyanobacteria may prove to be a rapid and convenient means of screening for mutants of inorganic carbon accumulation.     

Effects of Cations and Abscisic Acid on Chlorophyll a Fluorescence in Guard Cells of Vicia faba

The effects of cations and abscisic acid on chloroplast activity in guard cells of Vicia faba were investigated by analysis of the transient of chlorophyll a fluorescence. When epidermal strips containing guard cells as the only living cells were incubated in water and illuminated with strong light, chlorophyll a fluorescence rose rapidly to a high intensity and then declined slowly to a stationary level. The rate of this decline was enhanced by K⁺ or Na⁺, and the effect of these cations was greater when added with phosphate than with chloride as the anion. Ca²⁺ suppressed the enhancement by Na⁺ and, to a lesser extent, that by K⁺. Abscisic acid also suppressed the enhancement by K⁺ and Na⁺. Since the fluorescence decline reflects the increase of intrathylakoid H⁺ concentration necessary for photophosphorylation, the acceleration of the decline by K⁺ (or Na⁺ in the absence of Ca²⁺) implicates chloroplast activity in ion accumulation by guard cells in the light. The differential effects of phosphate and chloride suggest that chloroplast activity may be involved in malate formation in guard cells in the light.     

Chlorophyll analysis by high-performance liquid chromatography

The separation and determination of chlorophylls by high-performance liquid chromatography (HPLC) is described. Chlorophylls and their derivatives were separated by reversed-phase HPLC based on hydrophobic interaction between solute and support, using an octadecyl silica column and elution with 100% methanol. Separated pigments were detected fluorometrically with a sensitivity in the picomole range: the fluorescence response was linear over a wide pigment concentration range. Resolution of five chlorophylls a and four protochlorophyll species esterified with different alcohols was achieved within 22 min in a single experiment. This method can be used for the determination of chlorophyll b, bacteriochlorophyll a esters and products synthesized from chlorophyll, but not for nonesterified pigments, i.e., chlorophyllide, protochlorophyllide and chlorophyll c. The chromatographic mobility of chlorophyll a esterified with different alcohols increases with increasing number of carbon atoms in the esterifying alcohols. The plots obtained from the logarithm of the capacity factor (k′) of these pigments versus the numbers of carbon atoms of the alcohol molecule gave a straight line, thus permitting the estimation of the chain length of unknown pigment esterifying alcohols. This HPLC separation technique did not cause the formation of artifacts. The deviation of the individual retention time for each pigment is less than ±0.5%, thus making this method suitable for the rapid identification and quantification of unknown pigments.     

Protein synthesis and phosphorylation of light-harvesting apoproteins in normal and chlorophyll b-deficient Chlamydomonas reinhardii

Phosphorylation of polypeptides in whole cells and in chloroplasts of different strains of Chlamydomonas reinhardii was studied. Phosphorylation in vivo was strongly reduced when cytoptasmic protein synthesis was inhibited either by anisomycin or by cycloheximide. In isolated chloroplasts these two inhibitors had no effect on labelling. The incorporation of [³²P]-phosphate into one of the apoproteins of the light-harvesting chlorophyll a/b -protein complex (LHC 2) was also studied in relation to its synthesis. In vivo, in a chlorophyll b -deficient mutant and in its parent strain we found a pronounced relationship between synthesis and phosphorylation of this LHC 2-apoprotein. Our results suggest that LHC 2-apoproteins, newly synthesized in the cytoplasm, are preferentially phosphorylated after synthesis. Together with the observation that phosphorylation still occurs in isolated chloroplasts we conclude that in vivo at least two levels of phosphorylation of the LHC 2-apoproteins have to be clearly differentiated. One level involves the phosphorylation of existing and the other of newly synthesized polypeptides. The biological significance of phosphorylation of the LHC 2-apoproteins in vivo and probably also of other thylakoid polypeptides is complex and not restricted to regulation of energy distribution between photosystems 1 and 2.