The effect of norflurazon on protein composition and chlorophyll organization in pigment–protein complex of Photosystem II

The pyridazinone-type herbicide norflurazon SAN 9789 inhibiting the biosynthesis of long-chain carotenoids results in significant decrease in PS II core complexes and content of light-harvesting complex (LHC) polypeptides in the 29.5–21 kDa region. The Chl a forms at 668, 676, and 690 nm that belong to LHC and antenna part of PS I disappear completely after treatment. The intensity of the Chl b form at 648 nm is sharply decreased in treated seedlings grown under 30 or 100 lx light intensity. The bands of carotenoid absorption at 421, 448 (Chl a), 452, 480, 492, 496 (β-carotene), and 508 nm also disappear. The band shift from 740 to 720 nm and decrease in its intensity relative to the 687 nm emission peak in the low-temperature fluorescence spectrum (77 K) suggests a disturbance of energy transfer from LHC to the Chla form at 710–712 nm.     

Calcium-induced formation of chlorophyll b and light-harvesting chlorophyll complex in cucumber cotyledons in the dark

Dark-grown cucumber seedlings were exposed to intermittent light (2 min light and 98 min dark) and then cotyledons were incubated with 50 mM CaCl₂ in the dark. Chlorophyll (Chl) a was selectively accumulated under intermittent light and Chl b was accumulated during the subsequent dark incubation with CaCl₂. The change in chlorophyll-protein complexes during Chl b accumulation induced by CaCl₂ in the dark was investigated by SDS-polyacrylamide gel electrophoresis. Chlorophyll-protein complex I and free chlorophyll were major chlorophyll-containing bands of the cotyledons intermittently illuminated 10 times. When these cotyledons were incubated with CaCl₂ in the dark, the light-harvesting Chl complex was formed. When the number of intermittent illumination periods was extended to 55, small amounts of Chl b and light-harvesting Chl complex were recognized at the end of intermittent light treatment, and these two pigments were further increased during the subsequent incubation of the cotyledons with CaCl₂ in the dark compared to water controls.     

Inhibition of carotenoid biosynthesis by the herbicide SAN 9789 and its consequences for the action of phytochrome on plastogenesis

Treatment of the mustard (Sinapis alba L.) seedling with the herbicide SAN 9789 inhibits synthesis of colored carotenoids and interferes with the formation of plastid membrane lipids without affecting growth and morphogenesis significantly. In farred light, which is hardly absorbed by chlorophyll, development of plastid ultrastructure, synthesis of ribulosebisphosphate carboxylase and synthesis of chlorophyll are not affected by SAN 9789. It is concluded that normal phytochrome actions on plastid structural development, protein and chlorophyll syntheses are not affected by the absence of carotenoids provided that there is no significant light absorption in chlorophyll. The findings show that the inhibition of synthesis of one set of plastid membrane components (the carotenoids) does not stop synthesis of other components such as chlorophyll and does not halt membrane assembly. Supplementary experiments with the closely related compound SAN 9785, which affects the amount and composition of plastid lipids but not carotenoid and chlorophyll syntheses, suggest that the effect of the herbicide SAN 9789 is due exclusively to its inhibition of synthesis of colored carotenoids. In the presence of SAN 9789 white or red light at high fluence rate causes photodestruction of chlorophyll and ribulosebisphosphate carboxylase and photodecomposition of thylakoids. These effects are interpreted as resulting exclusively from the self-photooxidation and photosensitizing action of chlorophyll once the protection by carotenoids of chlorophyll against self- and sensitized photooxidation is lost.Abbreviations Carboxylase ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) - Chl chlorophyll a plus chlorophyll b - PAL phenylalanine ammonia-lyase (EC 4.3.1.5) - SAN 9789 -chloro-5-(methylamino)-2-(, , -trifluoro-m-tolyl)-3 (2H) pyridazinone - SAN 9785 4-chloro-5-(dimethylamino)-2-phenyl-3(2H)-pyridazione. SAN 9789 is sold commercially under the trade name Norflurazon - fr far red - wl white light     

Structural and functional relationships in developing Pinus silvestris chloroplasts

The light dependent chloroplast development of dark grown seedlings of Pinus silvestris L. was followed by analyses of chlorophyll content, chlorophyll a/b ratios, chlorophyll/P700 ratios, chlorophyll-protein complexes and structural changes. Low-temperature fluorescence emission spectra of isolated chloroplasts and separation of sodium dodecyl sulphate solubilized chlorophyll-protein complexes by gel electrophoresis showed that the chlorophyll-protein complexes of photosystem 1 (P700-CPa), photosystem II (PS II-CPa) and the light-harvesting complex LH–CPa/b were present in dark grown seedlings. The low-temperature fuoorescence emission maxima of isolated P700–CPa and PS II–CPa shifted towards longer wavelengths during greening in light, indicating a light induced change of the chlorophyll organisation in the two photosystems. Illumination caused LH–CPa/b to increase relative to P700–CPa, whereas the ratio between LH–CPa/b and PS II–CPa remained essentially constant. Analyses of low-temperature fluorescence spectra with or without 0.01 M Mg²⁺ showed that the Mg²⁺ controlled distribution of excitation energy into PS I was activated upon illumination of the seedlings. The photosynthetic unit size, as defined by the chlorophyll/P700 ratio, did not change over a 96 h illumination period, although the chlorophyll content increased about 6–fold during that time. This result and the constant electron transport rate per unit chlorophyll and time during chlorophyll accumulation provided evidence for a sequential development of the photosynthetic units when illuminating dark grown pine cotyledons. Electron micrographs showed that exposure of dark grown seedlings to light for 2 h caused the prolamellar body to disappear and grana to form. These changes occurred prior to substantial accumulation of chlorophyll or change in the ratio between LH–CPa/b and P700–CPa. However, both the water-splitting system of photosystem II and the Mg²⁺ controlled redistribution of excitation energy was activated during this period.     

The Pool of Chlorophyllous Pigments in Barley Seedlings of Different Ages under Heat Shock and Water Deficit

The effects of a high temperature (3 h, 40°C) and water deficit (45 h on 3% PEG 6000) on the pool of chlorophyllous pigments in the leaves of 4-, 7-, and 11-day-old barley (Hordeum vulgare L.) seedlings were studied. Heating resulted in a decrease in the total content of chlorophylls (Chl) (a + b) in 4-day-old plants but not in the older leaves. Water deficit induced an increase in the pigment content in young seedlings but reduced it in the leaves of 11-day-old plants. In young seedlings, hyperthermia and dehydration affected similarly Chl (a + b) degradation, leading to a marked inhibition of the chlorophyllase (Chlase) activity hydrolyzing Chl to chlorophyllides and phytol. In old leaves, an activation of this enzyme was observed. The stress factors under study affected different stages of pigment biosynthesis. High temperature inhibited the activity of dark and light stages of Chl(a + b) biosynthesis. Dehydration did not change markedly the resynthesis of protochlorophyllide, while the enzymes of the light stage of Chl biosynthesis were activated in young but inhibited in old barley leaves. The results thus obtained allowed us to conclude that heat treatment and dehydration specifically affected the Chl biosynthesis. At the same time, the Chlase response was nonspecific.     

Interconversions of Chlorophylls a and b Synthesized from Exogenous Chlorophyllides a and b in Etiolated and Post-Etiolated Rye Seedlings

A comparative study of reciprocal conversions of chlorophylls a and b (Chl aand Chl b) in etiolated and post-etiolated rye seedlings (Secale cereale L.) was performed. The production of these pigments was initiated by infiltration of exogenous chlorophyllides a and b (Chlide a and b). It was shown that Chlide b, when infiltrated into etiolated rye seedlings, was esterified, producing Chl b. A major portion of Chl b (more than 80%) was transformed into Chl aduring long-term seedling dark exposure. The high rate of Chl b conversion into Chl a in the pool of pigments of exogenous origin was also observed during the lag-phase when there was no chlorophyll formation from endogenous precursors. The infiltration of Chlide a resulted in Chl a formation. The efficiency of its conversion into Chl b was low (about 1%) in the etiolated seedlings but increased during their greening. In the post-etiolated seedlings infiltrated with Chlide b, which were preliminary illuminated for 6–12 h, the Chl /Chl a ratio was almost similar in the pools of pigments synthesized from both exogenous and endogenous precursors. The rates of direct and reverse reactions responsible for the interconversion of Chl aand Chl b depended on the stage of the formation of the photosynthetic apparatus during greening of etiolated seedlings, when the particular structural components are formed in a definite sequence.     

Assembly of Pigment–Protein Complexes in Carotenoid-Deficient Membranes of Plastids from Wheat Seedlings Treated with Norflurazon

The pyridazinone-type herbicide norflurazon SAN 9789 inhibiting the biosynthesis of long-chain carotenoids results in significant decrease in PS II core complexes and content of light-harvesting complex (LHC) polypeptides. At the same time, early light-induced proteins (ELIP) with molecular masses of 20.5-16.5 and 13.5 kD disappear in norflurazon-treated seedlings grown under intermittent (pulsed) light, confirming the hypothesis that they are carotenoid-binding proteins. Full disappearance of Chl a forms at 668, 676, and 690 nm and a sharp decrease in Chl b form at 648 nm in treated seedlings grown under 30 or 100 lx light intensity shows close contact of these forms with carotenoids in the thylakoid membrane. The band shift from 740 to 720 nm in the low-temperature fluorescence spectrum (77 K) suggests a disturbance of energy transfer from LHC to the Chl a form at 710-712 nm.     

Photosynthetic Characteristics of Chloroplasts of Primary Wheat Leaves Grown under Different Irradiance

The rate of accumulation of total chlorophyll (Chl) and carotenoids (Car) of leaves grown under high irradiance, HI (30 and 45 W m^–2) was faster than at moderate irradiance, MI (15 W m^–2). However, the senescence phase started earlier in the samples and proceeded at a faster rate. Chl a/b and Chl (a+b)/Car values showed faster loss of Chl a (compared to Chl b) and Chl (a+b) (compared to Car) in HI leaves. Protein accumulation and loss were also similar to that of Chl (a+b) content. Increase in Chl fluorescence during the development phase may suggest a gradual change in thylakoid organisation, however, the temporal kinetics were different in HI and MI samples. Increase in fluorescence polarisation during senescence of HI leaves compared to the control (MI) suggests conversion of thylakoid membranes to gel phase. Chloroplasts prepared from HI seedlings showed higher rate of photochemical activities, however, the activity declined earlier and at faster rate compared to the control.     

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.     

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.     

Carotenoid composition in Zea mays developed at sub-optimal temperature and different light intensities

The content and composition of pigments were examined in the third leaf of Zea mays L. plants grown under controlled environment at near-optimal temperature (24°C) or sub-optimal temperature (14°C) at a light intensity of either 200 or 600 μmol m^?2 s^?1. Compared to leaves grown at 24°C, leaves grown at 14°C showed a large reduction in the chlorophyll (Chl) content, a marked decrease in the Chl a/b ratio, and a large increase in the ratio of total carotenoids/Chl a+b. Leaves grown at 14°C showed a much lower content of β-carotene than leaves grown at 24°C, while the content of the carotenoids of the xanthophyll cycle (violaxanthin [V] + antheraxanthin [A] + zeaxanthin [Z]) was markedly higher in the former leaves as compared to the latter leaves; neoxanthin and lutein were affected by the growth temperature to a much lesser extent. The xanthophylls/β-carotene ratio was about three times higher in leaves grown at 14°C as compared to leaves grown at 24°C. On a chlorophyll basis, the two types of leaves hardly differed in their level of β-carotene, while the levels of the xanthophylls (including lutein and neoxanthin) were higher in 14°C-grown leaves as compared to 24°C-grown leaves. In leaves grown at 14°C, 40 and 56% of the V+A+Z pool was in the form of zeaxanthin at low light intensity and high light intensity, respectively. Only trace amounts of zeaxanthin, if any, were present in leaves grown at 24°C. The changes in the pigment composition induced by growth at sub-optimal temperature were more pronounced at a light intensity of 600 as compared to 200 μmol m^?2 s^?1. In the given range, the light intensity slightly affected the composition of pigments in leaves grown at 24°C. The physiological significance of the modifications to the pigment composition induced by growth at sub-optimal temperature is discussed.     

Use of a SPAD meter to estimate chlorophyll content in Eugenia uniflora L. leaves as affected by contrasting light environments and soil flooding

In three separate experiments, the effectiveness of a SPAD-502 portable chlorophyll (Chl) meter was evaluated for estimating Chl content in leaves of Eugenia uniflora seedlings in different light environments and subjected to soil flooding. In the first experiment, plants were grown in partial or full sunlight. In the second experiment plants were grown in full sunlight for six months and then transferred to partial sunlight or kept in full sunlight. In the third experiment plants were grown in a shade house (40% of full sunlight) for six months and then transferred to partial shade (25–30% of full sunlight) or full sunlight. In each experiment, plants in each light environment were either flooded or not flooded. Non-linear regression models were used to relate SPAD values to leaf Chl content using a combination of the data obtained from all three experiments. There were no significant effects of flooding treatments or interactions between light and flooding treatments on any variable analyzed. Light environment significantly affected SPAD values, chlorophyll a (Chl a), chlorophyll b (Chl b), and total chlorophyll [Chl (a+b)] contents in Experiment I (p≤0.01) and Experiment III (p≤0.05). The relationships between SPAD values and Chl contents were very similar among the three experiments and did not appear to be influenced by light or flooding treatments. There were high positive exponential relationships between SPAD values and Chl (a+b), Chl a, and Chl b contents.     

Kinetics of Chlorophyll Accumulation and Formation of Chlorophyll-Protein Complexes during Greening of Chlamydomonas reinhardtii y-1 at 38 degrees C

The initial kinetics of accumulation of chlorophylls (Chl) were analyzed during optimal greening of Chlamydomonas reinhardtii y-1 at 38°C. Acetate was required for maximal synthesis of Chl, which occurred at a linear rate when degreened cells were exposed to light. During the first hour Chl a and b accumulated predominantly as geranylgeraniol esters, with lesser amounts of the species with more reduced alcohol side chains. When Chl synthesis was blocked either by treatment with gabaculine or by transfer to the dark, the distribution shifted to the more reduced forms. Similar kinetic patterns indicated that a common pool of chlorophyllides a and b provided substrate for the enzymatic system that performs esterification and reduction of the sldechain for each group of Chl. Chl b was essentially quantitatively integrated into light-harvesting complexes as indicated by energy transfer to Chl a. In the presence of cycloheximide, an inhibitor of cytoplasmic protein synthesis, Chl b did not accumulate and Chl a production was reduced about one-half. The results demonstrate that Chl a/b-protein complexes assemble rapidly during greening and that reduction of the alcohol side chain of the Chl is not required for assembly of these complexes.     

A comparative study of the responsivity of Sinapis alba L. seedlings to pulsed and continuous irradiation

Anthocyanin formation in 36h dark grown Sinapis alba L. seedlings and inhibition of hypocotyl elongation in 36h and 54h dark grown and 54h and 7 day light grown seedlings in response to continuous red light could be substituted for by hourly 5 min light pulses where the total fluence over the irradiation period is the same. These pulses are partially (36h) or almost totally (54h and 7 day) reversible by subsequent far-red (RG 9) light pulses. In contrast to 654 nm light, hourly light pulses with 552 nm, 449 nm and 715 nm can at best only partially substitute for continuous irradiation. These data are discussed with respect to the commonly used models for the phytochrome high irradiance response.Abbreviations P_tr tar-red absorbing form of phytochrome - SAN 9789 4-chloro-5-(methyl-amino)-2-(,,-trifluoro-m-tolyl)-3(2H)-pyridazinone=Norflurazon - HIR High irradiance response     

Contrasting patterns of sun-red and shade-green leaves of <Emphasis Type="Italic">Buxus microphylla</Emphasis> in response to gradients of excess light during winter acclimation

Leaf reddening in overwintering evergreens largely restricts their application in landscapes and is generally triggered in response to excess light. To explore how leaves respond to excess light and examine the potential relevance of leaf reddening in this process, a comparative field study was conducted on the sun leaves (SUL), shade leaves (SHL) and three levels of artificially shaded sun leaves (SSUL) of Buxus microphylla ‘Wintergreen’. The seasonal changes in leaf colorations, chlorophyll (Chl) and carotenoid contents, leaf absorbance and chlorophyll fluorescence characteristics were investigated. The results showed that SUL upregulated Chl a/b with increased reductions in Chl b compared with Chl a, accumulated red pigments in the upper palisade mesophyll with reduced absorption in blue and red light but increased absorption in green light, and additionally, significantly downregulated photochemical activities through the sustained enhancement of energy dissipation in PSII antenna (ΦD) from fall to midwinter. In the SSUL, as the light intensity decreased, all of the above processes were mitigated except that the SSUL maintained constant absorptions in blue light region and whose levels were similar to those of the SUL and SHL. In contrast, the SHL maintained relatively high levels of Chl a and Chl b, remained completely green and showed regulated ΦD and ΦE (energy dissipation in PSII reaction centers) to maintain relatively high photochemical activity in the winter. We conclude that the sun leaves downregulate Chl contents to reduce the light absorption and simultaneously enhance sustained ΦD to dissipate most of the light energy, whereas shade leaves maintain relatively high Chl contents and demonstrate regulated proportions of ΦD and ΦE to match the extent to which the absorbed light can be utilized through photochemical reactions. The accumulated red pigments in sun phenotypes may provide a shading effect on Chls by directing energy to non-photosynthetic reaction centers in the blue light region where the absorption is offset by the reduced Chls.     

Effects of Short-term Red Radiation and Choline Compounds on Cytokinin Content, Chlorophyll Accumulation and Photomorphogenesis in Wheat Seedlings

Effects of choline compounds (2-chloroethyltrimethylammonium chloride and 2-ethyltrimethylammonium chloride) as well as red radiation (R) pulse on the dynamics of cytokinin changes, growth and chlorophyll (a + b) accumulation were studied during the growth and greening of etiolated wheat seedlings (Triticum aestivum L., var. Mironovskaya-808). The seedlings were grown for 120 h in the dark and then exposed for 72 h to white light. Pre-treatment of caryopses with cholines and pre-irradiation of etiolated seedlings with R inhibited elongation of both coleoptile and first leaf; but the same factors accelerated these growth responses when seedlings were exposed to white light. Chlorophyll (Chl) accumulation and the first leaf appearance from coleoptile were accelerated by the pre-treatments as well. Far-red radiation (FR) reversed all effects of R but choline pre-treatment eliminated partly R/FR photoreversibility. Two compounds with high cytokinin activity (tested on a fresh weight basis by the bioassay with Amaranthus caudatus L.) were found in shoots and first leaves. One of them had R_f, UV absorbance spectrum and the biological activity similar to N⁶-(Δ²-isopentenyl)adenosine. Another cytokinin-like substance was not identified with the used standards. Stimulation of greening by R pulse and cholines was accompanied with accelerated accumulation of both cytokinin-like substances. We conclude that the influence of R and cholines on the concentration of substances with cytokinin activities detected in the leaves might be involved in the stimulation of Chl accumulation.     

Contrasting effect of dark-chilling on chloroplast structure and arrangement of chlorophyll-protein complexes in pea and tomato: plants with a different susceptibility to non-freezing temperature

The effect of dark-chilling and subsequent photoactivation on chloroplast structure and arrangements of chlorophyll–protein complexes in thylakoid membranes was studied in chilling-tolerant (CT) pea and in chilling-sensitive (CS) tomato. Dark-chilling did not influence chlorophyll content and Chl a/b ratio in thylakoids of both species. A decline of Chl a fluorescence intensity and an increase of the ratio of fluorescence intensities of PSI and PSII at 120 K was observed after dark-chilling in thylakoids isolated from tomato, but not from pea leaves. Chilling of pea leaves induced an increase of the relative contribution of LHCII and PSII fluorescence. A substantial decrease of the LHCII/PSII fluorescence accompanied by an increase of that from LHCI/PSI was observed in thylakoids from chilled tomato leaves; both were attenuated by photoactivation. Chlorophyll fluorescence of bright grana discs in chloroplasts from dark-chilled leaves, detected by confocal laser scanning microscopy, was more condensed in pea but significantly dispersed in tomato, compared with control samples. The chloroplast images from transmission-electron microscopy revealed that dark-chilling induced an increase of the degree of grana stacking only in pea chloroplasts. Analyses of O-J-D-I-P fluorescence induction curves in leaves of CS tomato before and after recovery from chilling indicate changes in electron transport rates at acceptor- and donor side of PS II and an increase in antenna size. In CT pea leaves these effects were absent, except for a small but irreversible effect on PSII activity and antenna size. Thus, the differences in chloroplast structure between CS and CT plants, induced by dark-chilling are a consequence of different thylakoid supercomplexes rearrangements. Dedicated to Prof. Zbigniew Kaniuga on the 25th anniversary of his initiation of studies on chilling-induced stress in plants.     

The Spectral Response of Light Dependent Chlorophyll b Formation

Dark grown seedlings of barley will obtain a high ratio chlorophyll a/chlorophyll b when exposed to intermittent light (1 min of incandescent light or one electronic flash every hour). Such material has been exposed to monochromatic light of different wavelengths for 1 h. The ratio a/b gets a minimum in light of 670 nm, indicating the highest rate of chlorophyll b formation at this wavelength. The possibility is discussed that the light absorber (and also precursor) could be a short-lived chlorophyll a form, existing prior to the forms in the Shibata-shift. Chlorophyll b formation in darkness is discussed from the findings that the rate of formation of chlorophyll b is higher in intermittent light than it should be, calculated from the rate in continuous light, where the saturation intensity is rather low.