Light-independent accumulation of chlorophyll a and b and protochlorophyllide in green barley (Hordeum vulgare)

Barley ( Hordeum vulgare L. cvs Clipper, Procter, Astrix) seedlings were transferred from daylight to darkness and changes in chlorophyll a , chlorophyll b , protochlorophyllide and chlorophyllide (μ leaf⁻¹) in either the first or second leaf determined spectrophotometrically after separating the esterified from unesterified pigments by partitioning between ammoniacal acetone and light petroleum ether. Chlorophyll a and b as well as protochlorophyllide accumulated in the dark. The ratio of chlorophyll to protochlorophyllide formed in the absence of light was 18:1. 5-aminolevulinic acid (10 m M ) promoted the synthesis of chlorophyll a and b and protochlorophyllide. Pigment synthesis and response to 5-aminolevulinic acid addition was related to tissue age. Mature tissue in the apical third of the leaf accumulated most chlorophyll, but per μg chlorophyll present at the time of transfer to darkness, was less efficient than immature tissue towards the base of the leaf. Immature tissue was also most responsive to added 5-aminolevulinic acid. Chlorophyll synthesis in the dark was accompanied by chloroplast development. Chloroplasts in immature leaf tissue increased in size and extent of thylakoid development when transferred from daylight to darkness. The results indicate that chlorophyll synthesis and chloroplast membrane development in light-grown barley continue into the dark phase of the diurnal cycle. A light-independent protochlorophyllide reductase in light-grown barley seedlings is postulated.     

Photocontrol of anthocyanin formation in turnip seedlings

Summary As measured by in vivo spectrophotometry the phytochrome content in etiolated turnip seedlings was higher in cotyledons than in hypocotyls; in the latter, it is confined to the apical part. During early growth in darkness the amount increased in both tissues to a maximum, reached about 40 hours after sowing; the levels then gradually declined. Separation of seedlings into hypocotyl and cotyledons increased the rate of phytochrome loss in the former, but not in the latter.Following 5 minutes of red light P _frdecayed very rapidly in darkness; after 1.5 hours all of the phytochrome was present as P _r, which was presumably not converted initially. In continuous red light the total phytochrome was reduced to below the detection level within 3 hours. Seedling age markedly affected the loss of phytochrome following red light; more was destroyed in older than in younger hypocotyls and apparent new synthesis occurred only in young seedlings. The capacity to synthesise phytochrome differed in cotyledons and hypocotyl. In cotyledons, synthesis occurred following shots of red light varying from 10 seconds, to 6×I minute, but the amount of newly formed phytochrome was not related to the amount destroyed: after 5 hours of continuous red light no new synthesis occurred. In hypocotyls, the amount of phytochrome synthesised was related to the amount previously destroyed, and the phytochrome content after 24 hours of darkness was similar following all red light treatments of 1 minute or longer: new synthesis occurred following 5 hours of continuous red light.In far-red light phytochrome decayed very slowly, approaching the limit of detection after 48 hours. In cotyledons some loss was already observed after 5 hours of far-red and, in hypocotyls, after about 10 hours.These results are discussed in relation to the possible role of phytochrome as the pigment mediating anthocyanin synthesis in prolonged far-red light.     

Glucose and δ-Aminolevulinic Acid Stimulate the Dark Chlorophyll Synthesis of Rice Seedlings

This research was to examine if rice (Oryza sativa L.), a monocotyledon of angiosperm, was able to synthesize chlorophyll (Chl) in complete darkness. Five-cm-tall etiolated seedlings of rice were used as starting materials and treated with or without various concentrations of glucose and/or δ-aminolevulinic acid (ALA) in the dark. Leaves harvested at the indicated time were determined for their contents of Chl, protoporphyrin Ⅸ(Proto), Mg-protoporphyrin Ⅸ(Mg-Proto) and protochlorophyllide (Pchlide). The mole percentage of porphyrin was calculated. The Chl content in the etiolated rice seedlings slightly increased from about 2.5 μg/g to 7.5 μg/g within 12 d in the dark, but the total Chl of dark-grown rice increased from 0.36 μg/g to 3.6 μg/g. While the mole percentages of Proto, Mg-Proto and Pchlide in the dark-grown seedlings without any treatment were about 65%, 27.5% and 7.5% at the beginning, respectively, those in the light-grown seedlings were about 42.5%, 35% and 22.5%, respectively. The mole percentage of porphyrin of etiolated seedlings resumed its normal ratio within 2 d after treatment with glucose. While the Chl content of etiolated seedlings grown in culture solution with 3% and 6% glucose increased 2.5 and 4.0 folds, respectively, those with 3% and 6% glucose and 1 mmol/L ALA increased 22 and 24 folds, respectively. It is concluded that angiosperm might be able to synthesize a small amount of Chl in complete darkness, that either glucose or ALA could stimulate dark Chl synthesis in angiosperm, and that a combination of glucose and ALA exhibited an additional effect. It is still unknown and remains to be further explored what is the mechanism of the effect of glucose and ALA on the Chl synthesis of rice in the dark. Key words: angiosperm; rice; dark chlorophyll synthesis; glucose; δ-aminolevulinic acid; protoporphyrin Ⅸ; Mg-protoporphyrin Ⅸ; protochlorophyllide     

UV-B response of green and etiolated barley seedlings

7-d-old etiolated and green barley seedlings (Hordeum vulgare L. cv. Alfa) were irradiated with UV-B for 30 min and then kept for 24 h in light or darkness. Chlorophyll (Chl) synthesis was inhibited by about 30 % as a result of UV-B irradiation, but there were no significant changes in photochemical activity measured by variable to maximum fluorescence ratio (F_v/F_m), quantum yield (Φ_PS2) and oxygen evolution rate. Electron transport of etiolated seedlings was similar to that of green ones, nevertheless, the Chl content was more then 2-fold lower. Ribulose-1,5-bisphosphate carboxylase/oxygenase large and small subunits were diminished as a result of UV-B irradiation in etiolated and green plants, especially in those kept in the darkness. Catalase activity decreased and total superoxide dismutase activity increased in green and etiolated plants following UV-B treatment. When benzidine was used as a substrate, an isoform located between guaiacol peroxidases 2 and 3 (guaiacol peroxidase X) appeared, which was specific for UV-B treatment. As a result of irradiation, the contents of UV-B absorbing and UV-B induced compounds increased in green seedlings but not in etiolated seedlings.     

Chlorophyll turnover in etiolated greening barley transferred to darkness: Isotopic (1-14C glutamic acid) evidence of dark chlorophyll synthesis in the absence of chlorophyll accumulation

Synthesis of chlorophyll was initiated in 5- to 6-day-old dark-grown barley (Hordeum vulgare L. cv. Clipper)seedlings by exposing them to light in the presence of 1-¹⁴ C glutamic acid supplied via the roots.The plants were then returned to darkness. At the end of light treatment (_T) and after 7 or 18 h dark treatment chlorophylls a and b were extracted, quantified (μgleaf¹). purified by HPLC to their magnesium-free derivatives (pheophytin a and b) and their molar radioactivities determined. After 2 h exposure to light followed by 6 h illumination in the presence of 1-¹⁴ C glutamic acid, seedlings had accumulated 4-7 nmol chlorophyll leaf¹ and had incorporated between 900-1 350 Bq (g fresh weight)¹ of radioactive label into the chlorophyll pool. When seedlings were transferred to darkness, label continued to be incorporated and after 18 h the radioactivity of the chlorophyll pool had increased by 300-700 Bq (g fresh weight)¹. Net chlorophyll content, however, remained constant during dark treatment. The increase in radioactivity of the chlorophyll pool in darkness represented the difference between a net increase of label incorporated into chlorophyll a and a small loss of label from chlorophyll b. The absence of measurable radioactivity in the phytol moiety of labelled chlorophyll a, extracted at the endof dark treatment, demonstrated thatincorporation of label was into the tetrapyrrole moiely of chlorophyll and not into the phytol chain. Light-independent incorporation of 1-¹⁴ C glutamic acid into chlorophyll of greening barley seedlings transferred to darkness indicates that chlorophyll synthesis continues when light is withheld. We interpret the net gain in radioactivity of chlorophyll in darkness, in the absence of a net gain in chlorophyll content, to chlorophyll turnover i.e. to simultaneous synthesis and breakdown of chlorophyll when etiolated greening barley seedlings are transferred to darkness.     

The Formation of Terpenoids and Their Role in the Metabolism of Pinus halepensis Mill

The results of chemical and ultrastructural research show thatin seedlings of Pinus halepensis growing on vermiculite andwater, nourished by the endosperm, the synthesis of resin proceedsas well in darkness as in light. Starved seedlings, which hadused all the endosperm, were grown without nutrients for tenadditional days in darkness. In these seedlings the quantityof terpenes was greater than in seedlings which were still nourishedby the endosperm. In spite of the long period of starvation,which lowered the density of the cytoplasm and of the numberof the organelles (in epithelial and other cells), the plantsdid not use the terpenoid constituents of the resin as substrates. Pinus halapensis Mill., resin synthesis, terpenoids     

Endogenous Phytohormones and Regulation of Morphogenesis of Arabidopsis thalianaby Blue Light

We studied the effects of blue light (BL) on the levels of endogenous phytohormones (IAA, ABA, gibberellins, and cytokinins) and morphogenesis of the 7-day-old Arabidopsis thaliana(L.) Heynh seedlings of wild type (Ler) and its hy4mutant with a disturbed synthesis of cryptochrome 1 (CRY1), which is a receptor for BL. In darkness, the mutant contained considerably less free IAA and zeatin, but much more ABA as compared to the wild-type seedlings. BL retarded the hypocotyl growth in the wild-type seedlings but stimulated it in the mutant. Elongation of mutant hypocotyls was accompanied by accumulation of free IAA and a decrease in the content of free ABA; the level of cytokinins did not change. We believe that the response of the hy4hypocotyls to BL is mediated by a BL receptor distinct from cryptochrome 1. The conclusion is that light and hormonal signals interact in the control of the hypocotyl growth in A. thalianaseedlings.     

Comparative uptake of nitrate by intact seedlings of C3 (barley) and C4 (corn) plants: Effect of light and exogenously supplied sucrose

The effect of light and exogenously supplied sucrose on NO₃ ⁻ uptake was studied in 9-day-old intact C₃ (barley) and C₄ (corn) seedlings. The seedlings used were uninduced for nitrate uptake system (i.e. had never seen nitrogen during germination and growth) and were exposed to continuous light for 3 days to avoid any diurnal variation and to load the seedlings fully with photosynthates. The uptake assay was conducted either in light or in darkness. Prior to assay, seedlings were treated with darkness or light for 24 h. Accordingly, four sets of seedlings, i.e. pretreated with light and assayed in light (LL); pretreated and assayed in darkness (DD); pretreated with light and assayed in darkness (LD); and pretreated with darkness and assayed in light (DL) were formed. Barley exhibited 55% higher NO₃ ⁻ uptake than corn during light (LL) and 91% higher during darkness (DD). Shifting barley seedlings from light to dark (LD) or dark to light (DL) for uptake assay, did not affect NO₃ ⁻ uptake, i.e. in LD the uptake was similar to LL and in DL it was similar to DD. However, in corn, the light conditions during the assay determined the uptake regardless of the conditions during the period preceding the assay. One percent sucrose in the medium increased NO₃ ⁻ uptake by 31% in barley and 70% in corn during light (LL). The corresponding increase during darkness (DD) was 38% in both barley and corn. Removal of the corn residual endosperm decreased NO₃ ⁻ uptake by 40% during darkness. Etiolated seedlings (those having never seen light) of both barley and corn were able to take up significant amount of NO₃ ⁻ during darkness. Externally supplied sucrose in the assay medium of etiolated seedlings increased the NO₃ ⁻ uptake to about 4 and 2 fold in barley and corn, respectively. The data presented here provide evidence that: 1. In intact seedlings, light per se is not obligatory for NO₃ ⁻ uptake and that the carbohydrate supply may mimic light. 2. Light affected the NO₃ ⁻ uptake differently in barley and corn. This revised version was published online in June 2006 with corrections to the Cover Date.     

Accumulation of photosynthetic pigments in Larix decidua Mill. and Picea abies (L.) Karst. cotyledons treated with 5-aminolevulinic acid under different irradiation

European larch (Larix decidua Mill.) and Norway spruce [Picea abies (L.) Karst.] synthesize chlorophyll (Chl) in darkness. This paper compares Chl accumulation in 14-d-old dark-grown seedlings of L. decidua and P. abies after shortterm (24 h) feeding with 5-aminolevulinic acid (ALA). We used two ALA concentrations (1 and 10 mM) fed to cotyledons of both species in darkness and in continuous light. The dark-grown seedlings of L. decidua accumulated Chl only in trace amounts and the seedlings remained etiolated. In contrast, P. abies seedlings grown in darkness were green and had significantly higher Chl content. After ALA feeding, higher protochlorophyllide (Pchlide) content was observed in L. decidua than in P. abies cotyledons incubated in darkness. Although short-term ALA feeding stimulated the synthesis of Pchlide, Chl content did not change significantly in cotyledons incubated in darkness. The Chl accumulation in cotyledons fed with ALA was similar to the rate of Chl accumulation in the controls. Higher Chl accumulation was reported in control samples after illumination: 86.9% in L. decidua cotyledons and 46.4% in P. abies cotyledons. The Chl content decreased and bleaching occurred in cotyledons incubated with ALA in light due to photooxidation. Analyses of Chlbinding proteins (D1 and LHCIIb) by Western blotting proved differences between Chl biosynthesis in L. decidua and P. abies seedlings in the dark and in the light. No remarkable increase was found in protein accumulation (D1 and LHCIIb) after ALA application. Our results showed interspecific difference in Chl synthesis between two gymnosperms. Shortterm ALA feeding did not stimulate Chl synthesis, thus ALA synthesis was not the rate-limiting step in Chl synthesis in the dark.     

Phytochrome A enhances the promotion of hypocotyl growth caused by reductions in levels of phytochrome B in its far-red-light-absorbing form in light-grown Arabidopsis thaliana.

We sought to determine if phytochrome B (phyB)-mediated responses to the red light (R)/far-red light (FR) ratio are affected by phytochrome A (phyA) activity in light-grown seedlings of Arabidopsis thaliana. Pulses of FR delayed into the dark period were less effective than end-of-day (EOD) FR in promoting hypocotyl growth over a given period in darkness. White light minus blue light interposed instead of darkness between the end of the white-light photoperiod and the FR pulse was sufficient to maintain responsivity to the decrease in phyB in FR-light-absorbing form in wild-type (WT) seedlings, but not in the phyA mutant. Compared with EOD R, hourly R+FR pulses provided throughout the night caused a stronger promotion of stem growth than a single EOD R+FR pulse in WT Arabidopsis, cucumber, mustard, sunflower, tobacco, and tomato, but not in phyA Arabidopsis or in the aurea mutant of tomato. WT seedlings of Arabidopsis responded to a range of high EOD R/FR ratios, whereas the phyA mutant required stronger reductions in the EOD R/FR ratio. In sunlight, phyA seedlings of Arabidopsis showed no response to the "early warning" signals of neighboring vegetation, and hypocotyl-growth promotion occurred at higher plant densities than in the WT. Thus, under a series of light conditions, the sensitivity or responsivity to reductions in the R/FR ratio were larger in WT than in phyA seedlings. A product of phyA is therefore proposed to enhance the hypocotyl-growth response to decreases in phyB in FR-light-absorbing form in light grown seedlings.     

Pigment Accumulation and Photosynthesis in Developing Rye Coleoptiles

Rye seedlings (Secale cereale L.) were grown in darkness or irradiated with white light (WL) of medium intensity (100 μmol quanta . m^?2 . s^?1). In the coleoptile and primary leaf of irradiated plants synthesis of chlorophylls a and b with average ratios of 2.4 and 2.7, respectively, were measured. The chlorophyll content of the fully green primary leaf was about 10-fold larger than that of the pale green coleoptile. In darkness a large increase in carotenoid content occurred in the primary leaf, but in the coleoptile the level of carotenoids remained very low. In both organs a WL-induced increase in carotenoid synthesis was observed. The level of anthocyanin was likewise enhanced by WL. In the coleoptile and primary leaf of irradiated seedlings light-dependent oxygen evolution was measured. The specific activity of the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase was enhanced after WL treatment. The results indicate that the mature rye coleoptile is a photosynthetically active plant organ.     

Mechanism of phytochrome action in the control of biosynthesis of anthocyanin in Brassica oleracea

-The synthesis of anthocyanin in red-cabbage is very sensitive to control by light, R/FR reversibility being effected by exposures of 5 min duration, The demonstration of this control does not depend upon a preceding irradiation of high-intensity light but depends upon the duration of incubation in darkness subsequent to irradiation. R/FR reversibility is well shown in seedlings kept in darkness for 48 hr after exposure but after 120 hr this reversibility is no longer evident. This is due to the fact that a further synthesis of anthocyanin occurs in unexposed seedlings and in FR and R/FR treated material in the period from 48 to 120 hr but does not occur in the R treatment after 48 hr. Reagents such as n-propanol which are believed to increase membrane permeability, greatly increase anthocyanin synthesis in dark grown material. n-PrOH also reverses the effect of 5 min FR irradiation but, by contrast with R light, does not promote PAL activity. It is concluded that the limitation to synthesis in material unexposed to light is substrate availability at the site of flavonoid biosynthesis, rather than the level of PAL activity. The evidence presented supports the hypothesis that R/FR reversible phytochrome action involves the control of the passage of substrate through a membrane to the site of anthocyanin biosynthesis.     

Accumulation of non-utilizable starch in laticifers of Euphorbia heterophylla and E. myrsinites

Starch biosynthesis and degradation was studied in seedlings and mature plants of Euphorbia heterophylla L. and E. myrsinites L. Mature embryos, which lack starch grains in the non-articulated laticifers, develop into seedlings that accumulate starch rapidly when grown either in the light or the dark. Starch accumulation in laticifers of dark-grown seedlings was ca. 47 and 43% of total starch in light-grown controls in E. heterophylla and E. myrsinites, respectively. In light-grown seedlings, starch was present in laticifers as well as parenchyma of stems and leaves, whereas in dark-grown seedlings starch synthesis was almost exclusively limited to laticifers. In 7-month-old plants placed into total darkness, the starch in chyma was depleted within 6 d, whereas starch in laticifers was not mobilized. The starch content of latex in plants during development of floral primordia, flowering, and subsequent fruit formation remained rather constant. The results indicate that laticifers in seedlings divert embryonal storage reserves to synthesize starch even under stress conditions (darkness) in contrast to other cells, and that starch accumulated in laticifers does not serve as a metabolic reserve. The laticifer in Euphorbia functions in the accumulation and storage of secondary metabolites yet retains the capacity to produce, but not utilize starch, a primary metabolite.     

A Close Relationship between Increases in Galactosyltransferase Activity and the Accumulation of Galactolipids during Plastid Development in Cucumber Seedlings

Changes in the activity of UDP-galactose:diacylglycerol galactosyltransferase(UDGT), a key enzyme in galactolipid biosynthesis, during germinationwere investigated in cucumber (Cucumis sativus L. cv. Aonagajibai)seedlings. After germination, UDGT activity increased duringgrowth in darkness for 4 days, reaching 10 times the activityin ungerminated seeds. Illumination of 4-day-old dark-grownseedlings strongly stimulated the activity. By contrast, inseedlings grown continuously in darkness, the increase in UDGTactivity ceased after 4 days and the activity remained constantthereafter. A similar increase in the specific activity of UDGTwas observed i n the envelope fraction from seedlings, indicatingthat the increase in the enzymatic activity preceded synthesisof other proteins in the envelope membrane. Coincident withthe change in the enzymatic activity, here was an increase inlevels of monogalactosyl diacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), two major constituents of chloroplastmembrane lipids, in the germinated seedlings. Cycloheximideinhibited the light-mediated increase in the enzymatic activityby illumination of 4-day-old dark-grown seedlings, and, as aconsequence, it inhibited the accumulation of MGDG and DGDG.It was clear, therefore, that protein synthesis was necessaryduring this activation. Addition of a cytokinin, benzyladenine(BA), stimulated the increase in the UDGT activity. The increasein the UDGT activity caused by BA was accompanied by the accumulationof galactolipids, as in the case of the activation by light.These results suggest that activation of the final reactionin the synthesis of MGDG, which is catalyzed by the galactosyl-transferase,contributes to the accumulation of galactolipids during thedevelopment of the chloroplast membrane. (Received December 3, 1994; Accepted July 3, 1995)     

Combined effect of elevated UVB,elevated temperature and fertilization on growth,needle structure and phytochemistry of young Norway spruce (Picea abies) seedlings

Simultaneously with warming climate, other climatic and environmental factors are also changing. Here, we investigated for the first time the effects of elevated temperature, increased ultraviolet‐B (UVB) radiation, fertilization and all combinations of these on the growth, secondary chemistry and needle structure of 1‐year‐old Norway spruce (Picea abies (L.) Karst.) seedlings in an outdoor experiment. After one growing season, elevated temperature increased root : shoot ratio and concentrations of needle piperidine alkaloids, while concentrations of needle catechins and acetophenones and bark flavonoids decreased compared with ambient temperature seedlings. UVB‐radiation increased concentrations of bark condensed tannins, while fertilization increased total biomass and concentrations of needle catechins. In addition to the main effects, concentrations of some individual phenolic compounds showed UV × temperature or UV × temperature × fertilization interactions, and fertilization modified temperature response on root : shoot ratio. All the treatments described here affected the defence chemistry profiles of the seedlings, which may imply some changes in plant‐herbivore interactions in connection with changing climate. The interactions between treatments indicate a need for further experiments involving several simultaneously affecting environmental changes.     

Dim-Red-Light-Induced Increase in Polar Auxin Transport in Cucumber Seedlings : I. Development of Altered Capacity, Velocity, and Response to Inhibitors

We have developed and characterized a system to analyze light effects on auxin transport independent of photosynthetic effects. Polar transport of [³H]indole-3-acetic acid through hypocotyl segments from etiolated cucumber (Cucumis sativus L.) seedlings was increased in seedlings grown in dim-red light (DRL) (0.5 μmol m⁻² s⁻¹) relative to seedlings grown in darkness. Both transport velocity and transport intensity (export rate) were increased by at least a factor of 2. Tissue formed in DRL completely acquired the higher transport capacity within 50 h, but tissue already differentiated in darkness acquired only a partial increase in transport capacity within 50 h of DRL, indicating a developmental window for light induction of commitment to changes in auxin transport. This light-induced change probably manifests itself by alteration of function of the auxin efflux carrier, as revealed using specific transport inhibitors. Relative to dark controls, DRL-grown seedlings were differentially less sensitive to two inhibitors of polar auxin transport, N-(naphth-1-yl) phthalamic acid and 2,3,5-triiodobenzoic acid. On the basis of these data, we propose that the auxin efflux carrier is a key target of light regulation during photomorphogenesis.     

Chlorophyll accumulation and breakdown in light-grown barley transferred to darkness: effect of seedling age

Diurnally grown barley (Hordeum vulgare L. cv. Clipper) seedlings of various ages (3–4, 5–6 and 10–11-days-old) were transferred to darkness for 17 h and changes in leaf fresh weight, chlorophyll a, chlorophyll b and protochlorophyllide measured. The results were consistent with previous evidence of a light-independent chlorophyll biosynthetic pathway in light-grown barley. There was a net gain in chlorophyll (μg leaf^-1) in 5–6- and 10–11-day-old plants after 17 h dark treatment. The amounts of chlorophyll that accumulated were similar (5.9 and 4.3 μg Chl leaf^-1), despite a twofold difference in leaf size at T₀. The rate of leaf expansion in 5–6-day-old plants greatly exceeded the rate of chlorophyll accumulation and leaves were noticeably paler after dark treatment i.e. there was a reduction in chlorophyll concentration (μg g fresh weight^-1) in spite of an increase in chlorophyll content (μg leaf^-1). The ability of light-grown barley to accumulate chlorophyll in darkness was a function of seedling age. Very young seedlings (3–4-day-old) generally lost chlorophyll in darkness. The decrease in chlorophyll per leaf resulted mainly from loss of chlorophyll b. Preferential loss of chlorophyll b resulted in dramatic increases in the chlorophyll a:b ratio. Since 3–4-day-old seedlings (1) accumulated 5-aminolevulinic acid in the presence of levulinic acid at a rate comparable to older seedlings, and (2) converted exogenous 5-aminolevulinic acid to chlorophyll in the absence of light, it is unlikely that failure of the youngest plants to accumulate chlorophyll in darkness was due to blocks at these steps in the pathway. Net loss of chlorophyll (μg leaf^-1) in 3–4-day-old seedlings in darkness was eliminated by the addition of chloramphenicol, which occasionally produced a small, but significant, gain in total chlorophyll. These results imply that chlorophyll degradation in young barley leaves is strongly influenced by the chloroplast genome, and is a major factor influencing changes in chlorophyll levels in darkness. The present findings are consistent with the suggestion that the failure of 3–4-day-old barley seedlings to accumulate chlorophyll in darkness may be due to chlorophyll turnover in which the rate of degradation exceeds the rate of synthesis.     

Osmotic relations during elongation growth in hypocotyls of Helianthus annum L.

The relationship between growth, change in cell osmotic pressure and accumulation of osmotic solutes was investigated in hypocotyls of sunflower (Helianthus annum L.) seedlings. During growth in darkness the osmotic pressure decreased by 50% between days 2 and 6 after sowing. After irradiation of dark-grown seedlings with continuous white light (WL) an inhibition of hypocotyl growth was measured, but the osmotic pressure of the growing cells was not lower than in the dark-grown control. Growth in darkness and after WL irradiation was accompanied by an increase in the amount of osmotic substances (soluble sugars) which was proportional to the increase in length of the organ. During growth in continuous WL the cell osmotic pressure decreased by 45 % between days 2 and 6 after sowing. The transfer of WL-grown seedlings to darkness (“re-etiolation”) resulted in a rapid acceleration of hypocotyl growth, but the cell osmotic pressure was the same as that of the WL grown control. Growth in continuous WL was accompanied by a corresponding accumulation of osmotic substances (soluble sugars). The transition from WL to darkness resulted in an enhanced accumulation of osmotica and an increase in cell-wall extensibility. The results indicate that the relative maintenance of cell osmotic pressure during rapid hypocotyl growth in darkness is caused by an enhanced accumulation of soluble sugars into the growing cells of the organ.     

Assembly of the D1 precursor in monomeric photosystem II reaction center precomplexes precedes chlorophyll a-triggered accumulation of reaction center II in barley etioplasts

Assembly of plastid-encoded chlorophyll binding proteins of photosystem II (PSII) was studied in etiolated barley seedlings and isolated etioplasts and either the absence or presence of de novo chlorophyll synthesis. De novo assembly of reaction center complexes in etioplasts was characterized by immunological analysis of protein complexes solubilized from inner etioplast membranes and separated in sucrose density gradients. Previously characterized membrane protein complexes from chloroplasts were utilized as molecular mass standards for sucrose density gradient separation analysis. In etiolated seedlings, induction of chlorophyll a synthesis resulted in the accumulation of D1 in a dimeric PSII reaction center (RCII) complex. In isolated etioplasts, de novo chlorophyll a synthesis directed accumulation of D1 precursor in a monomeric RCII precomplex that also included D2 and cytochrome b(559). Chlorophyll a synthesis that was chemically prolonged in darkness neither increased the yield of RCII monomers nor directed assembly of RCII dimers in etioplasts. We therefore conclude that in etioplasts, assembly of the D1 precursor in monomeric RCII precomplexes precedes chlorophyll a-triggered accumulation of reaction center monomers.