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1.
The assembly, organization and function of the photosynthetic apparatus was investigated in the wild type and a chlorophyll
(Chl) b-less mutant of the unicellular green alga Chlamydomonas reinhardtii, generated via DNA insertional mutagenesis. Comparative analyses were undertaken with cells grown photoheterotrophically
(acetate), photomixotrophically (acetate and HCO −
3) or photoautotrophically (HCO −
3). It is shown that lack of Chl b diminished the photosystem-II (PSII) functional Chl antenna size from 320 Chl ( a and b) to about 95 Chl a molecules. However, the functional Chl antenna size of PSI remained fairly constant at about 290 Chl molecules, independent
of the presence of Chl b. Western blot and kinetic analyses suggested the presence of inner subunits of the Chl a-b light-harvesting complex of PSII (LHCII) and the entire complement of the Chl a-b light-harvesting complex of PSI (LHCI) in the mutant. It is concluded that Chl a can replace Chl b in the inner subunits of the LHCII and in the entire complement of the LHCI. Growth of cells on acetate as the sole carbon
source imposes limitations in the photon-use efficiency and capacity of photosynthesis. These are manifested as a lower quantum
yield and lower light-saturated rate of photosynthesis, and as lower variable to maximal (F v/F max) chlorophyll fluorescence yield ratios. This adverse effect probably originates because acetate shifts the oxidation-reduction
state of the plastoquinone pool, and also because it causes a decrease in the amount and/or activity of Rubisco in the chloroplast.
Such limitations are fully alleviated upon inclusion of an inorganic carbon source (e.g. bicarbonate) in the cell growth medium.
Further, the work provides evidence to show that transformation of green algae can be used as a tool by which to generate
mutants exhibiting a permanently truncated Chl antenna size and a higher (per Chl) photosynthetic productivity of the cells.
Received: 10 November 1999 / Accepted: 22 December 1999 相似文献
2.
Using iron stress to reduce the total amount of light-harvesting and electron transport components per unit leaf area, the influence of light-harvesting and electron transport capacity on photosynthesis in sugar beet ( Beta vulgaris L. cv F58-554H1) leaves was explored by monitoring net CO 2 exchange rate ( P) in relation to changes in the content of Chl. In most light/CO2 environments, and especially those with high light (≥1000 microeinsteins photosynthetically active radiation per square meter per second) and high CO2 (≥300 microliters CO2 per liter air), P per area was positively correlated with changes in Chl (a + b) content (used here as an index of the total amount of light-harvesting and electron transport components). This positive correlation of P per area with Chl per area was obtained not only with Fe-deficient plants, but also over the normal range of variation in Chl contents found in healthy, Fe-sufficient plants. For example, light-saturated P per area at an ambient CO2 concentration close to normal atmospheric levels (300 microliters CO2 per liter air) increased by 36% with increase in Chl over the normal range, i.e. from 40 to 65 micrograms Chl per square centimeter. Iron deficiency-mediated changes in Chl content did not affect dark respiration rate or the CO2 compensation point. The results suggest that P per area of sugar beet may be colimited by light-harvesting and electron transport capacity (per leaf area) even when CO2 is limiting photosynthesis as occurs under field conditions. 相似文献
3.
The photon use efficiencies and maximal rates of photosynthesis in Dunaliella salina (Chlorophyta) cultures acclimated to
different light intensities were investigated. Batch cultures were grown to the mid-exponential phase under continuous low-light
(LL: 100 μmol photon m -2 s -1) or high-light (HL: 2000 μmol photon m -2 s -1) conditions. Under LL, cells were normally pigmented (deep green) containing ∼500 chlorophyll (Chl) molecules per photosystem
II (PSII) unit and ∼250 Chl molecules per photosystem I (PSI). HL-grown cells were yellow-green, contained only 60 Chl per
PSII and 100 Chl per PSI and showed signs of chronic photoinhibition, i.e., accumulation of photodamaged PSII reaction centers
in the chloroplast thylakoids. In LL-grown cells, photosynthesis saturated at ∼200 μmol photon m -2 s -1 with a rate (P max) of ∼100 mmol O 2 (mol Chl) -1 s -1. In HL-grown cells, photosynthesis saturated at much higher light intensities, i.e. ∼2500 μmol photon m -2 s -1, and exhibited a three-fold higher P max (∼300 mmol O 2 (mol Chl) -1 s -1) than the normally pigmented LL-grown cells. Recovery of the HL-grown cells from photoinhibition, occurring prior to a light-harvesting
Chl antenna size increase, enhanced P max to ∼675 mmol O 2 (mol Chl) -1 s -1. Extrapolation of these results to outdoor mass culture conditions suggested that algal strains with small Chl antenna size
could exhibit 2–3 times higher productivities than currently achieved with normally pigmented cells.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
4.
Spectrophotometric and kinetic measurements were applied to yield photosystem (PS) stoichiometries and the functional antenna size of PSI, PSII α, and PSII β in Zea mays chloroplasts in situ. Concentrations of PSII and PSI reaction centers were determined from the amplitude of the light-induced absorbance change at 320 and 700 nm, which reflect the photoreduction of the primary electron acceptor Q of PSII and the photooxidation of the reaction center P700 of PSI, respectively. Determination of the functional chlorophyll antenna size ( N) for each photosystem was obtained from the measurement of the rate of light absorption by the respective reaction center. Under the experimental conditions employed, the rate of light absorption by each reaction center was directly proportional to the number of light-harvesting chlorophyll molecules associated with the respective photosystem. We determined NP700 = 195, Nα = 230, Nβ = 50 for the number of chlorophyll molecules in the light-harvesting antenna of PSI, PSII α, and PSII β, respectively. The above values were used to estimate the PSII/PSI electron-transport capacity ratio (C) in maize chloroplasts. In mesophyll chloroplasts C > 1.4, indicating that, under green actinic excitation when Chl a and Chl b molecules absorb nearly equal amounts of excitation, PSII has a capacity to turn over electrons faster than PSI. In bundle sheath chloroplasts C < 1, suggesting that such chloroplasts are not optimally poised for linear electron transport and reductant generation. 相似文献
5.
A computer model comprising light reactions, electron–proton transport, enzymatic reactions, and regulatory functions of C 3 photosynthesis has been developed as a system of differential budget equations for intermediate compounds. The emphasis is
on electron transport through PSII and PSI and on the modeling of Chl fluorescence and 810 nm absorptance signals. Non-photochemical
quenching of PSII excitation is controlled by lumenal pH. Alternative electron transport is modeled as the Mehler type O 2 reduction plus the malate-oxaloacetate shuttle based on the chloroplast malate dehydrogenase. Carbon reduction enzymes are
redox-controlled by the ferredoxin–thioredoxin system, sucrose synthesis is controlled by the fructose 2,6-bisphosphate inhibition
of cytosolic FBPase, and starch synthesis is controlled by ADP-glucose pyrophosphorylase. Photorespiratory glycolate pathway
is included in an integrated way, sufficient to reproduce steady-state rates of photorespiration. Rate-equations are designed
on principles of multisubstrate-multiproduct enzyme kinetics. The parameters of the model were adopted from literature or
were estimated from fitting the photosynthetic rate and pool sizes to experimental data. The model provided good simulations
for steady-state photosynthesis, Chl fluorescence, and 810 nm transmittance signals under varying light, CO 2 and O 2 concentrations, as well as for the transients of post-illumination CO 2 uptake, Chl fluorescence induction and the 810 nm signal. The modeling shows that the present understanding of photosynthesis
incorporated in the model is basically correct, but still insufficient to reproduce the dark-light induction of photosynthesis,
the time kinetics of non-photochemical quenching, ‘photosynthetic control’ of plastoquinone oxidation, cyclic electron flow
around PSI, oscillations in photosynthesis. The model may find application for predicting the results of gene transformations,
the analysis of kinetic experimental data, the training of students. 相似文献
6.
Geum montanum L. is an alpine plant usually found at altitudes between 1700 and 2600 m. Its wintergreen leaves can be subjected to very
low temperatures and at the same time receive high photon flux densities at the beginning of the growth season when the snow
melts. We report results of a study, performed with classical methods of biophysics, showing that leaves of G. montanum were remarkably tolerant to sunlight even at low temperatures. This tolerance results from the interplay of photorespiration
and CO 2 photosassimilation. When temperatures approach 0°C, responses include stomatal opening and CO 2 uptake even under desiccation stress. This permits linear electron transport that is sufficient to avoid the excessive reduction
of the electron transport chain which is known to lead to photodamage. In addition, excitation energy was shifted from photosystem
(PS)II to PSI which is a very efficient energy quencher. Sensitivity of P700 in PSI to oxidation by far-red light was decreased
and rates of dark reduction of photooxidized P700 were increased by actinic illumination, suggesting activation of cyclic
electron transport. Consistent with this, far-red light was able to decrease the quantum yield of PSII (measured by the F
v/ F
m ratio of chlorophyll fluorescence). We suggest that cyclic electron transport decreases the lumenal pH under strong light.
In the presence of zeaxanthin, this increases energy dissipation at the PSII level. At low temperatures, P700 remained strongly
oxidized under high irradiation while the primary electron acceptor of PSII, Q A, was largely reduced. This shows efficient control of electron transport presumably at the level of the cytochrome b/f complex
and suggests formation of a protective transthylakoid proton gradient even when linear electron transport is much reduced
in the cold. Thus, several mechanisms cooperate to effectively protect the photosynthetic apparatus of G. montanum from photodamage. We see no indication of destructive “photostress” in this species during the growth season under alpine
low-temperature and drought conditions.
Received: 2 March 1998 / Accepted: 7 January 1999 相似文献
7.
Anacystis nidulans cells grown under high (3%) CO 2 partial pressure have greater phycocyanin to chlorophyll ratio (Phc/Chl) relative to cells grown under low (0.2%) CO 2 tension (Eley (1971) Plant Cell Physiol 12: 311-316). Absorbance difference spectrophotometry of A. nidulans thylakoid membranes in the ultraviolet (Δ A320) and red (Δ A700) regions of the spectrum reveal photosystem II/photosystem I (PSII/PSI) reaction center ratio (RCII/RCI) changes that parallel those of Phc/Chl. For cells growing under 3% CO 2, the Phc/Chl ratio was 0.48 and RCII/RCI = 0.40. At 0.2% CO 2, Phc/Chl = 0.38 and RCII/RCI = 0.24. Excitation of intact cells at 620 nm sensitized RCII at a rate approximately 20 times faster than that of RCI, suggesting that Phc excitation is delivered to RCII only. In the presence of DCMU, excitation at 620 nm induced single exponential RCII photoconversion kinetics, suggesting a one-to-one structural-functional correspondance between phycobilisome and PSII complex in the thylakoid membrane. Therefore, phycobilisomes may serve as microscopic markers for the presence of PSII in the photosynthetic membrane of A. nidulans. Neither the size of individual phycobilisomes nor the Chl light-harvesting antenna of PSI changed under the two different CO 2 tensions during cell growth. Our results are compatible with the hypothesis that, at low CO 2 concentrations, the greater relative amounts of PSI present may facilitate greater rates of ATP synthesis via cyclic electron flow. The additional ATP may be required for the active uptake of CO 2 under such conditions. 相似文献
8.
The photosynthetic linear electron transport rate in excess of that used for CO 2 reduction was evaluated in Sorghum bicolor Moench. [NADP-malic enzyme (ME)-type C 4 plant], Amaranthus cruentus L. (NAD-ME-type C 4 plant) and Helianthus annuus L. (C 3 plant) leaves at different CO 2 and O 2 concentrations. The electron transport rate ( J
F) was calculated from fluorescence using the light partitioning factor (relative PSII cross-section) determined under conditions
where excess electron transport was assumed to be negligible: low light intensities, 500 μmol CO 2 · mol −1 and 2% O 2. Under high light intensities there was a large excess of J
F/4 at 10–100% O 2 in the C 3 plant due to photorespiration, but very little in sorghum and somewhat more in amaranth, showing that photorespiration is
suppressed, more in the NADP-ME- and less in the NAD-ME-type species. It is concluded that when C 4 photosynthesis is limited by supply of atmospheric CO 2 to the C 4 cycle, the C 3 cycle becomes limited by regeneration of ribulose 1,5-bisphosphate (RuBP) which in turn limits RuBP oxygenase activity and
photorespiration. The rate of excess electron transport over that consumed for CO 2 fixation in C 4 plants was very sensitive to the presence of O 2 in the gas phase, rapidly increasing between 0.01 and 0.1% O 2, and at 2% O 2 it was about two-thirds of that at 21% O 2. This shows the importance of the Mehler O 2 reduction as an electron sink, compared with photorespiration in C 4 plants. However, the rate of the Mehler reaction is still too low to fully account for the extra ATP which is needed in C 4 photosynthesis.
Received: 8 November 1997 / Accepted: 26 December 1997 相似文献
9.
The effect of temperature on the rate of electron transfer through photosystems I and II (PSI and PSII) was investigated
in leaves of barley ( Hordeum vulgare L.). Measurements of PSI and PSII photochemistry were made in 21% O 2 and in 2% O 2, to limit electron transport to O 2 in the Mehler reaction. Measurements were made in the presence of saturating CO 2 concentrations to suppress photorespiration. It was observed that the O 2 dependency of PSII electron transport is highly temperature dependent. At 10 °C, the quantum yield of PSII (ΦPSII) was insensitive
to O 2 concentration, indicating that there was no Mehler reaction operating. At high temperatures (>25 °C) a substantial reduction
in ΦPSII was observed when the O 2 concentration was reduced. However, under the same conditions, there was no effect of O 2 concentration on the ΔpH-dependent process of non-photochemical quenching. The rate of electron transport through PSI was
also found to be independent of O 2 concentration across the temperature range. We conclude that the Mehler reaction is not important in maintaining a thylakoid
proton gradient that is capable of controlling PSII activity, and present evidence that cyclic electron transport around PSI
acts to maintain membrane energisation at low temperature.
Received: 6 July 2000 / Accepted: 3 August 2000 相似文献
10.
The contribution of changes in stomatal conductance and metabolism in determining heterogeneous photosynthesis inhibition
during dehydration and abscisic acid (ABA) feeding was investigated using detached leaves of Rosa rubiginosa L. The steady-state and maximal rates of electron transport under a transient high CO 2 concentration were monitored using chlorophyll fluorescence imaging. The decrease in electron transport rate induced by dehydration
and ABA treatment almost reverted to the control rate under transient high CO 2 availability. Therefore, inhibition of photosynthesis was mainly mediated through stomatal closure. However, since reversion
was not complete, a metabolic inhibition was also identified as a decrease in the maximal electron transport rate driven by
carboxylation. Under dehydration or ABA feeding, as under low ambient CO 2 treatment, in 21% or 0.4% O 2, the lower the steady-state electron transport was, the lower was the maximal electron transport rate during transient high
CO 2 availability. We conclude that low CO 2 availability reduced the capacity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) to drive electron transport.
The potential contribution of Rubisco deactivation mediated by stomatal closure is discussed.
Received: 1 February 1999 / Accepted: 15 June 1999 相似文献
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