Adaptation of the thylakoid membranes of pea chloroplasts to light intensities. I. Study on the distribution of chlorophyll-protein complexes

The effect of light intensity (16 h white light and 8 h dark) during growth of pea plants at 20°C on the chlorophyll composition and on the relative distribution of chlorophyll amongst the various chlorophyll-protein of pea thylakoids was studied. The chl a/chl b ratios increased from 2.1 to 3.2 as light intensity during growth varied from 10 to 840 Em^-2 s^-1. This function can be described by two straight lines intersecting at a transition point of approximately 200 Em^-2 s^-1. Similar discontinuities in the responses were observed in the changes in the relative distribution of chlorophyll amongst the various chlorophyll-protein complexes. This demonstrates that the chl a/chl b ratio of the various thylakoids is a good indicator of changes in the relative distribution of chlorophyll. As the chl a/chl b ratio decreased, the amount of chlorophyll associated with photosystem I complexes decreased, that with photosystem II core reaction centre complex was halved, and that with the main chl a/b-proteins of the light-harvesting complex was markedly increased.Abbreviations chl chlorophyll - PS photosystem - SDS sodium dodecyl sulphate - Tricine N-tris (hydroxymethyl) methylglycine     

The kinetics of photoinhibition and its recovery in the red alga Porphyridium cruentum

When Porphyridium cruentum cells were illuminated with high fluence rate between 1900 and 4800 mol photons m^-2s^-1, a decrease in the photosynthetic activity of the cells was observed. Within the time frame of 20 min, and under the fluence rates studied, the sum of photons to be absorbed by cells (mg of chlorophyll (Chl), sufficient to initiate photoinhibition was calculated to be 9235.8 mol. The minimal specific light absorption rate to initiate photoinhibition in P. cruentum ranges between 2.29 and 4.26 mol photons s^-1 mg^-1 chl.a. There was a linear relationship between the specific rate of photoinhibition and the specific light absorption rate. A photon number of 2.56×10⁴ mol mg^-1 chl.a photoinhibited photosynthesis instantaneously. At 15°C, no photoinhibitory effect was observed at 2300 mol photons m^-2 s^-1 even after 45 min of illumination. At the other extreme of 35°C, 84% inhibition of photosynthetic activity was observed within 10 min of exposure to 2300 mol photons m^-2 s^-1. Between 20 and 30°C, the photoinhibitory effect was comparable. Photoinhibited P. cruentum cells recovered readily when transferred to low light (90 mol photons m^-2 s^-1) and darkness, and the specific rate of recovery was independent of the light intensity to which the cells were exposed, during the photoinhibitory treatment.Abbreviations Chlorophyll QL, specific light absorption rate Publication No. 28 of the Microalgal Biotechnology Laboratory     

Transient state characteristics of adaptation to changes in light conditions for the cyanobacterium Oscillatoria agardhii

Transitions in growth irradiance level from 92 to 7 Em^-2 s^-1 and vice versa caused changes in the pigment contents and photosynthesis of Oscillatoria agardhii. The changes in chlorophyll a and C-phycocyanin contents during the transition from high to low irradiance (HL) were reflected in photosynthetic parameters. In the LH transition light utilization efficiencies of the cells changed faster than pigment contents. This appeared to be related to the lowering of light utilization efficiencies of photosynthesis. As a possible explanation it was hypothesized that excess photosynthate production led to feed back inhibition of photosynthesis. Time-scales of changes in the maximal rate of O₂ evolution were discussed as changes in the number of reaction centers of photosystem II in relation to photosynthetic electron transport. Parameters that were subject to change during irradiance transitions obeyed first order kinetics, but hysteresis occurred when comparing HL with LH transients. Interpretation of first order kinetic analysis was discussed in terms of adaptive response vs changes in growth rate.Non-standard abbreviations Chla chlorophyll a - CPC C-phycocyanin - PS II photosystem II - PS I photosystem I - RC II reaction center of photosystem II - P photosynthetic O₂-evolution - I irradiance, Em^-2 s^-1 - light utilization efficiency of cells, mmol O₂·mg dry wt^-1·h^-1/Em^-2 s^-1 - light utilization efficiency of photosynthetic apparatus, mol O₂·mol Chla ^-1·h^-1/Em^-2 s^-1 - P_max maximal rate of O₂ evolution by cells, mol O₂·mg dry wt^-1·h^-1 - P_max maximal rate of O₂ evolution by photosynthetic apparatus, mol O₂·mol·Chla ^-1·h^-1 - LL low light, E m^-2 s^-1 - HL high light, E m^-2 s^-1 - LH low to high light transition - HL high to low light transition - k specific rate of adaptation, h^-1 - specific growth rate, h^-1 - Q pool size of cell constituent, mol·mg dry wt^-1 - q net synthesis rate of cell constituent, mol·mg dry wt^-1·h^-1     

Ultrastructure and freeze-fracture studies of the thylakoids ofMantoniella squamata (Prasinophyceae)

Summary The ultrastructure and the supramolecular organization of the thylakoids of the small green flagellate,Mantoniella squamata, were examined in thin sections and freeze-fracture preparations. The whole chloroplast is tightly packed with thylakoids, which show a pattern of meandering, branching and/or anastomosing membranes. In freeze-fracture preparations only two fracture-faces can be distinguished: the PF- and the EF-face. The PF-face has a much higher particle density than the EF-face (PF: 4086 particles/m²; EF: 865 particles/m²). The EF-face is not as uniform as the PF-face. The areas which are packed with particles probably correspond to closely appressed thylakoid regions or adhesive patches, noticed in thin sections in some areas. The mean particle size on both faces is also different (EF: 10.5 nm; PF: 8.6 nm), but no information about the classification of the particles to special protein complexes is available at this time.Abbreviations chl chlorophyll - EF exoplasmic fracture face - ER endoplasmic reticulum - LHC light-harvesting chlorophyll-protein complex - PF protoplasmic fracture face - PS I photosystem I - PS II photosystem II     

Inhibition of photosystem II activity by Cu++ ion. Choice of buffer and reagent is critical

Cupric ion (Cu⁺⁺) inhibits the rate of photosystem II electron transport and the intensity of the variable part of chl a fluorescence in isolated chloroplast thylakoids. The inhibition is markedly dependent on the nature of the buffer used in the assay medium. In MES and HEPES buffers, complete inhibition of photosystem II occurs at 30 M of Cu⁺⁺, while in Tricine no inhibition occurred even at 200 M Cu⁺⁺. In other buffers used (TES, Phosphate, Tris), the efficacy of Cu⁺⁺ inhibition is intermediate. The calculated binding constants are found to correspond to the observed I₅₀ values for the six buffers used. It is concluded that the previous reports on copper inhibition, where buffers have been used indiscriminately should be reconsidered. Certain reagents such as hydroxylamine, ascorbate and diphenyl carbazide, which react with Cu⁺⁺, should be avoided.Abbreviations Chl chlorophyll - DCIP 2,6-dichlorophenol indophenol - DCMU 3-(3,4 dichlorophenyl)-1,1-dimethyl urea - DAD diaminodurene - DPC diphenyl carbazide - Fv variable chl fluorescence - HEPES N-2-hydroxyethyl piperazine sulfonic acid - I ₃₀ inhibitor concentration causing 30% inhibition of Fv - MES 2-(N-morpholino) ethane sulfonic acid - MV Methyl viologen - PS II Photosystem II - PS I Photosystem I - TES N-tris(hydroxymethyl)-methyl-2-amino sulfonic acid - TMPD N,N,N,N-tetramethyl-p-phenylenediamine - Tricine N-tris(hydroxymethyl) ethylglycine - Tris N-tris(hydroxymethyl)amino ethane     

Effect of washing on the plasma membrane and on stress reactions of cultured rose cells

The effects of plant growth regulators, light intensity, and end-of-day (EOD) light quality treatments on node and microtuber induction (% of cultures with microtubers) and development (fresh weight of microtubers) in yam (Dioscorea alata L. cv. Oriental) cultures were investigated. Nodal segments were excised from plantlets cultured on tuberization medium containing growth regulators and exposed to various light treatments. Absciscic acid (1 M) stimulated and cytokinins (2.5 M) inhibited microtuber development from yam nodal segments cultured on Mantell's and Hugo's full-strength tuberization medium under 8-h photoperiods. EOD far-red (FR) light inhibited microtuber induction and development and enhanced node formation. EOD FR light effects were nullified by immediately following the FR treatment with red light. This suggested the involvement of phytochrome in these processes. The lowest light intensity evaluated (12 mol m^–2 s^–1) inhibited microtuber, root and shoot production as compared to light intensities of 42, 72 and 102 mol m^–2 s^–1. Kinetin (2.5 m) in half-strength tuberization medium inhibited microtuber induction and development but did not affect node production in the light intensity evaluation.Abbreviations ABA abscisic acid - BA 6-benzylaminopurine - 2iP 6-(c,c-dimethylallylamino)-purine - NAA napthaleneacetic acid - R light red light - FR light far-red light - EOD light end-of-day light     

Chlorophyll fluorescence and photoinhibition in a tropical rainforest understory plant

The data presented here deal with the effects of high-light exposure on the 77 K fluorescence characteristics of Elatostema repens. It is shown that the decrease of the variable fluorescence during the treatment is biphasic. The reactions responsible for the first phase of fluorescence quenching are saturated under 700 mol photon m^-2 s^-1 and insensitive to streptomycin, whereas those responsible for the second phase are not yet saturated under 700 mol photon m^-2 s^-1 and sensitive to streptomycin. It is concluded that only the second phase of fluorescence quenching is associated with photoinhibitory processes. Rate and amplitude of recovery from photoinhibition are maximum under very low light (3.5 mol photon m^-2 s^-1), and very small at a moderate light (160 mol photon m^-2 s^-1) which does not cause photoinhibition. It is concluded that recovery processes are inhibited during photoinhibition. It is suggested that they could be associated with damage occuring on the oxidizing side of PSII.Abbreviations F_o, F_v, F_m initial, variable and maximum fluorescence, respectively - PFD photon flux density - PS II photosystem II     

Light-path length and population density in photoacclimation of Nannochloropsis sp. (Eustigmatophyceae)

Photoacclimation in the marine eustigmatophyte Nannochlropsis sp., used extensively as a food chaincomponent in aquaculture, was studied both in thelaboratory and outdoors. Cell-chlorophyll andcarotenoids were used as markers to assessphotoacclimation to strong light, as well as todecreasing growth irradiance due to cellproliferation. Focusing on practical aspects involvedin mass cultivation, three different approaches wereused as follows: (a) cultures initially exposed to lowlight (150 mol photon m^-2 s^-1) thentransferred to strong light (1000 to 3000 molphoton m^-2 s^-1); (b) initially low celldensity cultures grown in reactors of differentlight-paths, exposed to strong PFD, in the laboratoryand outdoors; (c) initially low or high cell densitycultures exposed to strong light. As has already beenestablished in many reports, cell-chlorophyllrepresented a sensitive parameter in assessing cellresponse to changes in the intensity of the lightsource as well as to modifications in the light regimeto which the cells were exposed. Cell-chlorophyllconcentration sharply decreased initially upontransferring the culture from low PFD cell^-1 tohigh PFD cell^-1 due to either culture dilution(i.e. decrease in cell density and mutual shading) orto an increase in PFD. After some 7 days ofphotoacclimating to 2000 and 3000 mol photonm^-2 s^-1, chlorophyll a content began to riseat a much faster rate than cell number, which alsoincreased in response to the higher irradiance.Cell-chlorophyll in the culture exposed to 2000mol photon m^-2 s^-1 increased afteracclimation earlier and at a faster rate than in theculture exposed to 3000 mol photon m^-2s^-1, indicating the later irradiance affected astronger stress. The length of the reactor's lightpath exerted a decisive effect on cell response tostrong light through its influence on the light regimein the culture. Upon a sharp increase in PFD,carotenoids in the 1-cm reactor increased in muchhigher rate than chlorophyll, compared with the 3-cmlight path reactors. This marked difference in cellresponse to a shift-up in light was attributed to thevast variations in the light regime associated withdifferences in the length of the light path and areal density. Growth oflow cell density cultures ceased temporarily upontransfer to strong light, in contrast with high celldensity cultures transferred to strong light, whichcontinued growth without a lag.     

Two types of PS II centres as manifested by light saturation of delayed fluorescence from DCMU-treated chloroplasts

Intensity of 2 s delayed fluorescence (DF) as a function of steady-state actinic light intensity was investigated in pea chloroplasts in the presence of 10 M DCMU. The light saturation curve of DF was approximated by a sum of two hyperbolic components which differ by an order of magnitude in the half-saturating incident light intensity. The relative contribution of the amplitudes of the components was practically independent of cation (Na⁺ and Mg²⁺) concentration and a short-term heating of the chloroplasts at 45°C. The component saturating at low incident light intensity was selectively suppressed by 100 M DCMU or by 1 mol g^-1 Chl oleic acid. DF intensity following excitation by a single saturating 15 s flash was equal to the intensity of the component saturating at a low incident light intensity. Upon flash excitation, the maximum steady-state DF level was found to be attained only after a series of saturating flashes. It is concluded that the two components of the DF light saturation curves are related to PS II centres heterogeneity in quantum yield of stabilization of the reduced primary quinone acceptor.Abbreviations DF Delayed fluorescence - L₁- and L₂-components DF components saturating at low and high incident light intensity, respectively - I incident light intensity - L DF intensity - P₆₈₀ reaction centre chlorophyll of PS II - Q_A and Q_B primary and secondary quinone acceptors of PS II, respectively     

Changes in the quantities of violaxanthin de-epoxidase,xanthophylls and ascorbate in spinach upon shift from low to high light

Zeaxanthin, a carotenoid in the xanthophyll cycle, has been suggested to play a role in the protection against photodestruction. We have studied the importance of the parameters involved in zeaxanthin formation by comparing spinach plants grown in low light (100 to 250 mol m^-2 s^-1) to plants transferred to high light (950 mol m^-2 s^-1). Different parameters were followed for a total of 11 days. Our experiments show that violaxanthin de-epoxidase decreased between 15 and 30%, the quantity of xanthophyll cycle pigments doubled to 100 mmol (mol Chl)^-1, corresponding to 27 mol m^-2, and the rate of violaxanthin to zeaxanthin conversion was doubled. Lutein and neoxanthin increased from 50 to 71 mol m^-2 and from 16 to 23 mol m^-2, respectively. On a leaf area basis, chlorophyll and -carotene levels first decreased and then after 4 days increased. The chlorophyll a/b ratio was unchanged. The quantity of ascorbate was doubled to 2 mmol m^-2, corresponding to an estimated increase in the chloroplasts from 25 to 50 mM. In view of our data, we propose that the increase in xanthophyll cycle pigments and ascorbate only partly explain the increased rate of conversion of violaxanthin to zeaxanthin, but the most probable explanation of the faster conversion is an increased accessibility of violaxanthin in the membrane.     

Decrease of polypeptides in the PS I antenna complex with increasing growth irradiance in the red alga Porphyridium cruentum

Thylakoids isolated from cells of the red alga Porphyridium cruentum exhibit an increased PS I activity on a chlorophyll basis with increasing growth irradiance, even though the stoichiometry of Photosystems I and II in such cells shows little change (Cunningham et al. (1989) Plant Physiol 91: 1179–1187). PS I activity was 26% greater in thylakoids of cells acclimated at 280 mol photons · m^–2 · s^–1 (VHL) than in cells acclimated at 10 mol photons · m^–2 · s^–1 (LL), indicating a change in the light absorbance capacity of PS I. Upon isolating PS I holocomplexes from VHL cells it was found that they contained 132±9 Chl/P700 while those obtained from LL cells had 165±4 Chl/P700. Examination of the polypeptide composition of PS I holocomplexes on SDS-PAGE showed a notable decrease of three polypeptides (19.5, 21.0 and 22 kDa) in VHL-complexes relative to LL-complexes. These polypeptides belong to a novel LHC I complex, recently discovered in red algae (Wolfe et al. (1994a) Nature 367: 566–568), that lacks Chl b and includes at least six different polypeptides. We suggest that the decrease in PS I Chl antenna size observed with increasing irradiance is attributable to changes occurring in the LHC I-antenna complex. Evidence for a Chl-binding antenna complex associated with PS II core complexes is lacking at this point. LHC II-type polypeptides were not observed in functionally active PS II preparations (Wolfe et al. (1994b) Biochimica Biophysica Acta 1188: 357–366), nor did we detect polypeptides that showed immunocross-reactivity with LHC II specific antisera (made to Chlamydomonas and Euglena LHC II).Abbreviations Bis-Tris bis(2-hydroxyethyl)imino-tris(hydroxymethyl)methane - DCPIP 2,6-dichlorophenol indophenol - -dm dodecyl--d-maltoside - HL high light of 150 mol photons · m^–2 · s^–1 - LGB lower green band - LHC I light-harvesting complex of PS I - LHC II light-harvesting complex of PS II - LL low light of 10 mol photons · m^–2 · s^–1 - ML medium light of 50 mol photons · m^–2 · s^–1 - MES 2-(N-morpholino) ethanesulfonic acid - P700 reaction center of PS I - PFD photon flux density - Trizma tris(hydroxymethyl)aminomethane - UGB upper green band - VHL very high light of 280 mol photons · m^–2 · s^–1     

Changes in Photosystem II fluorescence in Chlamydomonas reinhardtii exposed to increasing levels of irradiance in relationship to the photosynthetic response to light

The effects of a 60 min exposure to photosynthetic photon flux densities ranging from 300 to 2200 mol m^–2s^–1 on the photosynthetic light response curve and on PS II heterogeneity as reflected in chlorophyll a fluorescence were investigated using the unicellular green alga Chlamydomonas reinhardtii. It was established that exposure to high light acts at three different regulatory or inhibitory levels; 1) regulation occurs from 300 to 780 mol m^–2s^–1 where total amount of PS II centers and the shape of the light response curve is not significantly changed, 2) a first photoinhibitory range above 780 up to 1600 mol m^–2s^–1 where a progressive inhibition of the quantum yield and the rate of bending (convexity) of the light response curve can be related to the loss of Q_B-reducing centers and 3) a second photoinhibitory range above 1600 mol m^–2s^–1 where the rate of light saturated photosynthesis also decreases and convexity reaches zero. This was related to a particularly large decrease in PS II centers and a large increase in spill-over in energy to PS I.Abbreviations Chl chlorophyll - DCMU 3,(3,4-dichlorophenyl)-1,1-dimethylurea - F_M maximal fluorescence yield - F_pl intermediate fluorescence yield plateau level - F₀ non-variable fluorescence yield - F_v total variable fluorescence yield (F_M-F₀) - initial slope to the light response curve, used as an estimate of initial quantum yield - convexity (rate of bending) of the light response curve of photosynthesis - LHC light-harvesting complex - P_max maximum rate of photosynthesis - PQ plastoquinone - Q photosynthetically active photon flux density (400–700 nm, mol m^–2s^–1) - PS photosystem - Q_A and Q_B primary and secondary quinone electron acceptor of PS II     

Xanthophyll cycle activity in detached barley leaves senescing under dark and light

Senescence-induced changes in the xanthophyll cycle activity and chlorophyll (Chl) fluorescence parameters were compared in detached barley (Hordeum vulgare L.) leaf segments kept for 6 d in darkness or under continuous white light (90 mol m^–2 s^–1). Before detachment of the leaf segments, the plants were grown at periodic regime [12 h light (90 mol m^–2 s^–1)/12 h dark]. The de-epoxidation state of the xanthophyll cycle pigments (DEPS) in the leaf samples was determined immediately (the actual DEPS), after 1 h of dark-adaptation (the residual DEPS), and during 14 min of a high-irradiance (HI) exposure (500 mol m^–2 s^–1) (HI-induced DEPS). In the light-senescing segments, senescence was delayed pronouncedly compared to dark-senescing ones as the Chl content, the photosystem 2 photochemistry, and electron transport processes were highly maintained. Further, the actual DEPS increased, probably due to the increased mean photon dose. The HI-induced increase in the DEPS was stimulated in the light-senescing segments, whereas it was slowed down in the dark-senescing ones. However, after the 14 min HI-exposure of the dark-senescing segments the HI-induced DEPS was not markedly lower than in the mature leaves, which indicated the maintenance of the xanthophyll cycle operation.     

Relationship Between Photosystem 2 Electron Transport and Photosynthetic CO2 Assimilation Responses to Irradiance in Young Apple Tree Leaves

The responses to irradiance of photosynthetic CO₂ assimilation and photosystem 2 (PS2) electron transport were simultaneously studied by gas exchange and chlorophyll (Chl) fluorescence measurement in two-year-old apple tree leaves (Malus pumila Mill. cv. Tengmu No.1/Malus hupehensis Rehd). Net photosynthetic rate (P _N) was saturated at photosynthetic photon flux density (PPFD) 600-1 100 (mol m^-2 s^-1, while the PS2 non-cyclic electron transport (P-rate) showed a maximum at PPFD 800 mol m^-2 s^-1. With PPFD increasing, either leaf potential photosynthetic CO₂ assimilation activity (F_d/F_s) and PS2 maximal photochemical activity (F_v/F_m) decreased or the ratio of the inactive PS2 reaction centres (RC) [(F_i – F_o)/(F_m – F_o)] and the slow relaxing non-photochemical Chl fluorescence quenching (q_s) increased from PPFD 1 200 mol m^-2 s^-1, but cyclic electron transport around photosystem 1 (RFp), irradiance induced PS2 RC closure [(F_s – F_o)/F_m – F_o)], and the fast and medium relaxing non-photochemical Chl fluorescence quenching (q_f and q_m) increased remarkably from PPFD 900 (mol m^-2 s^-1. Hence leaf photosynthesis of young apple leaves saturated at PPFD 800 mol m^-2 s^-1 and photoinhibition occurred above PPFD 900 mol m^-2 s^-1. During the photoinhibition at different irradiances, young apple tree leaves could dissipate excess photons mainly by energy quenching and state transition mechanisms at PPFD 900-1 100 mol m^-2 s^-1, but photosynthetic apparatus damage was unavoidable from PPFD 1 200 mol m^-2 s^-1. We propose that Chl fluorescence parameter P-rate is superior to the gas exchange parameter P _N and the Chl fluorescence parameter F_v/F_m as a definition of saturation irradiance and photoinhibition of plant leaves.     

Regulation of the distribution of excitation energy in Ochromonas danica,an organism containing a chlorophyll-A/C/carotenoid light harvesting antenna

A chlorophyll a, c-fucoxanthin pigment-protein complex8 functions as the major light harvesting antenna in the Chrysophyte Ochromonas danica. The regulated distribution of excitation energy between the two photosystems was investigated in these organisms and was shown to be strongly wavelength dependent. A light state transition was induced by pre-illumination of cells using light 2 (640 nm) and light 1 (700 nm) of equal absorbed intensity, and detected by reversible changes in the 77 K chlorophyll fluorescence emission spectra. Peaks at 690 nm and 720 nm in the low temperature spectra are most likely associated with PS2 and PS1 respectively. A room temperature fluorescence emission at 680 nm induced by modulated light 2 (500 nm) was strongly quenched in the presence of background light 1 (720 nm). Removal of light 1 led to an increase in fluorescence followed by a slow quenching. The room temperature fluorescence changes were directly correlated with changes in the 77 K emission spectra that indicated a change in the distribution of excitation energy between the two photosystems. It was established that DCMU (1 mol) prevented the state 2. The conversion to state 1 followed a simple photochemical dose dependence and had a half-time of 20 s-1.5 min at 6 W m^-2. In contrast, the conversion to state 2 was independent of light intensity. These data indicate that O. danica undergoes a light state transition in response to the preferential excitation of PS2 or PS1.Abbreviations PS2 photosystem 2 - PS1 photosystem 1 - LHC light harvesting chlorophyll a/b protein - fx fucoxanthin - PQ plastoquinone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea     

Nutrient content and photosynthesis of young yellowing Norway spruce trees (Picea abies L. Karst.) following calcium and magnesium fertilisation

Severely yellowed ten-year-old spruce trees growing in the Vosges Mountains on an acidic soil were fertilised with Magnesium lime during the spring of 1990. The effects of this treatment were assessed 18 months later. A very significant improvement of the mineral status of the trees was detected, with increasing Mg contents in the needles, and as a consequence, reduced yellowing and improved chlorophyll content. Only slight differences with control trees were observed for height increase. Effects of this improved nutrition on photosynthesis were tested measuring net CO₂ assimilation rates and chlorophyll a fluorescence. Light-saturated net assimilation rates of current-year needles were high, reaching 5.3 mol m^–2 s^–1 on a total needle area basis. The improvement in chlorophyll and Mg content had no significant effect on net assimilation rates or on any parameter describing photochemical functions of both current-and previous-year needles. Despite the strong inter-individual variability in needle chlorophyll and Mg contents (ranging from 0.2 to 0.8 mg g^–1 fresh weight, and 0.05 to 0.5 mg g^-1 dry weight respectively), photochemical efficiency of PS II under limiting irradiance only decreased significantly on older needles displaying Mg contents below 0.1 mg g^–1. It is concluded from these results that spruce trees exhibit a high degree of plasticity with regard to Mg deficiency on acidic soils, and that improved Mg nutrition and increased chlorophyll content do not necessarily improve photosynthesis and height growth.Abbreviations A light-saturated net CO₂ assimilation rate (mol m^–2 s^–1) - g_w light-saturated needle conductance to water vapour (mmol m^–2 s^–1) - _wp and _wm pre-dawn and mid-day needle water potential (MPa) - osmotic potential of sap expressed from needles (MPa) - PFD photosynthetic photon flux density (mol m^–2 s^–1) - F_v/F_m photochemical efficiency of PS II after 20 min dark adaptation - F/F_m ^' photochemical efficiency of PS II reaction centres after 10 min at a PFD of 220 mol m^–2 s^–1     

Transient state characteristics of changes in light conditions for the cyanobacterium Oscillatoria agardhii

The cyanobacterium Oscillatoria agardhii was subjected to changes in irradiance and to changes in light period. During transient states parameters as growth rate, pigment contents, photosynthetic activities and pool sizes of carbohydrate and proteins were followed. The changes in pigments and photosynthesis were similar for irradiance transitions and transitions in light period length. Carbohydrates served for the supply of carbon and energy during adaptation to low light conditions until a basal level of 125 g · mg dry wt^-1 was reached. After transfer to high light conditions excess carbon fixation led to the storage of carbohydrate reserve polymers up to 600 g · mg dry wt^-1. During adaptation to longer light periods cells showed an overcapacity for carbohydrate accumulation even in the presence of a high carbohydrate content at the start of the light period. A model for the feed back repression of photosynthesis related to carbohydrate accumulation was presented. In all cases protein synthesis was directly maximized under the given conditions. Growth rate defined as specific rate of change in carbon showed the fastest response after a shift in light conditions. It was concluded that adaptation of O. agardhii to changes in light conditions was directed to the optimization of growth. The observation that carbohydrate is used to supply carbon and/or energy during adaptation leads to the conclusion that changes on survival in low light depend on carbohydrate level, the efficiency of its conversion in cell material and the maintenance requirements. Such a survival strategy enables cyanobacteria to cope succesfully with light limiting conditions.Abbreviations HL high irradiance Em^-2s^-1 - LL low irradiance Em^-2s^-1 - L/D light-dark cycle h - specific growth rate h^-1 - _e specific maintenance coefficient h^-1 - _max maximal specific growth rate h^-1 - _c specific rate of change of carbon h^-1 - _protein specific rate of change of protein h^-1 - Chl a chlorophyll a - CPC C-phycocyanin - dry wt dry weight mg - light utilization efficiency nmol O₂ · mg dry wt^-1 · min^-1/Em^-1 s^-1 - P _max photosynthetic capacity nmol O₂ · mg dry wt^-1 · min^-1 - PSI photosystem I - PS II photosystem II - RC II reaction center of PS II - PQ plastoquinone     

Effect of growth irradiance on plastocyanin levels in barley

Plastocyanin levels in barley (Hordeum vulgare cv Boone) were found to be dependent on growth irradiance. An immunochemical assay was developed and used to measure the plastocyanin content of isolated thylakoid membranes. Barley grown under 600 mole photons m^–2s^–1 contained two- to four-fold greater quantities of plastocyanin per unit chlorophyll compared with plants grown under 60 mole photons m^–2s^–1. The plastocyanin/Photosystem I ratio was found to be 2 to 3 under high irradiance compared with 0.5 to 1.5 under low irradiance. The reduced plastocyanin pool size in low light plants contributed to a two-fold reduction in photosynthetic electron transport activity. Plastocyanin levels increased upon transfer of low light plants to high irradiance conditions. In contrast, plastocyanin levels were not affected in plants transferred from high to low irradiance, suggesting that plastocyanin is not involved in the acclimation of photosynthesis to shade.Abbreviations: BSA bovine serum albumin - chl chlorophyll - cyt cytochrome - DCIP 2,6-dichlorophenolindophenol - PS I Photosystem I - PS II Photosystem II - P700 reaction center of Photosystem I - TBS 20 mM Tris-HCl pH 7.5, 500 mM NaCl - TTBS 20 mM Tris-HCl pH 7.5, 500 mM NaCl, 0.5% (w/v) polyoxyethylenesorbitan monolaurate (Tween-20)     

Acclimation of barley to changes in light intensity: photosynthetic electron transport activity and components

Barley seedlings (Hordeum vulgare L. Boone) were grown at 20°C with 16 h/8 h light/dark cycle of either high (H) intensity (500 mole m^-2 s^-1) or low (L) intensity (55 mole m^-2 s^-1) white light. Plants were transferred from high to low (H L) and low to high (L H) light intensity at various times from 4 to 8 d after leaf emergence from the soil. Primary leaves were harvested at the beginning of the photoperiod. Thylakoid membranes were isolated from 3 cm apical segments and assayed for photosynthetic electron transport, Photosystem II (PS II) atrazine-binding sites (Q_B), cytochrome f(Cytf) and the P-700 reaction center of Photosystem I (PS I). Whole chain, PS I and PS II electron transport activities were higher in H than in L controls. Q_B and Cytf were elevated in H plants compared with L plants. The acclimation of H L plants to low light occurred slowly over a period of 7 days and resulted in decreased whole chain and PS II electron transport with variable effects on PS I activity. The decrease in electron transport of H L plants was associated with a decrease in both Q_B and Cytf. In L H plants, acclimation to high light occurred slowly over a period of 7 days with increased whole chain, PS I and PS II activities. The increase in L H electron transport was associated with increased levels of Q_B and Cytf. In contrast to the light intensity effects on Q_B levels, the P-700 content was similar in both control and transferred plants. Therefore, PS II/PS I ratios were dependent on light environment.Abbreviations Asc ascorbate - BQ 2,5-dimethyl-p-benzoquinone - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCIP 2,6-dichlorophenolindophenol - H control plants grown under high light intensity - H L plants transferred from high to low light intensity - L low control plants grown under low light intensity - L H plants transferred from low to high light intensity - MV methyl viologen - P-700 photoreaction center of Photosystem I - Q_B atrazine binding site - TMPD N,N,N,N-tetramethyl-p-phenylenediamine Cooperative investigations of the United States Department of Agriculture, Agricultural Research Service, and the North Carolina Agricultural Research Service, Raleigh, NC. Paper No. 11990 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7643, USA.     

Recovery from photoinhibition: effect of light and inhibition of protein synthesis of 32-kD chloroplast protein

Recovery from photoinhibition of photosynthesis in intact Lemna gibba was studied in presence of the protein synthesis inhibitors chloramphenicol and cycloheximide. Exposure to an irradiance of 1000 mol m^-2s^-1 in N₂ for 90 min induced 80% photoinhibition. The plants recovered photosynthesis when transfered to normal irradiances (210 mol m^-2s^-1) and air. Chloramphenicol added to the medium was taken up by the plant and reduced photosynthesis slightly. Recovery from photoinhibition was more inhibited than photosynthesis. Cycloheximide was also taken up by the plants and reduced synthesis of light harvesting chlorophyll protein: however, neither photosynthesis nor recovery were much affected. Synthesis of 32-kD chloroplast protein during recovery was inhibited by chloramphenicol, but not by cycloheximide. Synthesis of 32-kD protein was enhanced by 20–210 mol m^-2s^-1 light. The results support the hypothesis that synthesis of 32-kD protein is important for recovery of photosynthesis after photoinhibition.