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1.
To test the hypothesis that the contribution of phosphoribulokinase (PRK) to the control of photosynthesis changes depending
on the light environment of the plant, the response of transgenic tobacco (Nicotiana tabacum L.) transformed with antisense PRK constructs to irradiance was determined. In plants grown under low irradiance (330 μmol m−2 s−1) steady-state photosynthesis was limited in plants with decreased PRK activity upon exposure to higher irradiance, with a
control coefficient of PRK for CO2 assimilation of 0.25 at and above 800 μmol m−2 s−1. The flux control coefficient of PRK for steady-state CO2 assimilation was zero, however, at all irradiances in plant material grown at 800 μmol m−2 s−1 and in plants grown in a glasshouse during mid-summer (alternating shade and sun 300–1600 μmol m−2 s−1). To explain these differences between plants grown under low and high irradiances, Calvin cycle enzyme activities and metabolite
content were determined. Activities of PRK and other non-equilibrium Calvin cycle enzymes fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase
and ribulose-1,5-bisphosphate carboxylase-oxygenase were twofold higher in plants grown at 800 μmol m−2 s−1 or in the glasshouse than in plants grown at 330 μmol m−2 s−1. Activities of equilibrium enzymes transketolase, aldolase, ribulose-5-phosphate epimerase and isomerase were very similar
under all growth irradiances. The flux control coefficient of 0.25 in plants grown at 330 μmol m−2 s−1 can be explained because low ribulose-5-phosphate content in combination with low PRK activity limits the synthesis of ribulose-1,5-bisphosphate.
This limitation is overcome in high-light-grown plants because of the large relative increase in activities of sedoheptulose-1,7-bisphosphatase
and fructose-1,6-bisphosphatase under these conditions, which facilitates the synthesis of larger amounts of ribulose-5-phosphate.
This potential limitation will have maintained evolutionary selection pressure for high concentrations of PRK within the chloroplast.
Received: 15 November 1999 / Accepted: 27 January 2000 相似文献
2.
Inorganic carbon (Ci) uptake and efflux has been investigated in the marine microalga Nannochloropsis gaditana Lubian by monitoring CO2 fluxes in cell suspensions using mass spectrometry. Addition of H13CO3
− to cell suspensions in the dark caused a transient increase in the CO2 concentration in the medium far in excess of the equilibrium CO2 concentration. The magnitude of this release was dependent on the length of time the cells had been kept in the dark. Once
equilibrium between the Ci species had been achieved, a CO2 efflux was observed after saturating light intensity was applied to the cells. External carbonic anhydrase (CA) was not detected
nor does this species demonstrate a capacity to take up CO2 by active transport. Photosynthetic O2 evolution and the release CO2 in the dark depend on HCO3
− uptake since both were inhibited by the anion exchange inhibitor, 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS).
The bicarbonate uptake mechanism requires light but can also continue for short periods in the dark. Ethoxyzolamide, a CA
inhibitor, markedly inhibited CO2 efflux in the dark, indicating that CO2 efflux was dependent upon the intracellular dehydration of HCO3
−. These results indicate that Nannochloropsis possesses a bicarbonate uptake system which causes the accumulation of high intracellular Ci levels and an internal CA which
maintains the equilibrium between CO2 and HCO3
− and thus causes a subsequent release of CO2 to the external medium.
Received: 20 September 1999 / Accepted: 25 October 1999 相似文献
3.
The physiological properties of transgenic tobacco plants (Nicotiana tabacum L.) with decreased or increased transport capacities of the chloroplast triose phosphate/phosphate translocator (TPT) were
compared in order to investigate the extent to which the TPT controls metabolic fluxes in wild-type tobacco. For this purpose,
tobacco lines with an antisense repression of the endogenous TPT (αTPT) and tobacco lines overexpressing the TPT gene isolated
from the C4 plant Flaveria trinervia (FtTPT) were used. The F. trinervia TPT expressed in yeast cells exhibited transport characteristics identical to the TPT from C3 plants. Neither antisense TPT plants nor FtTPT overexpressors showed a phenotype when grown in a greenhouse in air. Contents
of starch and soluble sugars in upper source leaves were similar in TPT underexpressors and FtTPT overexpressors compared
to the wild type at the end of the photoperiod. The FtTPT overexpressors incorporated more 14CO2 in sucrose than the wild type, indicating that the TPT limits sucrose biosynthesis in the wild type. There were only small
effects on labelling of amino acids and organic acids. The mobilisation of starch was enhanced in αTPT lines but decreased
in FtTPT overexpressors compared to the wild type. Enzymes involved in starch mobilisation or utilisation, such as α-amylase
or hexokinase were increased in αTPT plants and, in the case of amylases, decreased in FtTPT overexpressors. Moreover, α-amylase
activity exhibited a pronounced diurnal variation in αTPT lines with a maximum activity after 8 h in the light. These changes
in starch hydrolytic activities were confirmed by activity staining of native gels. Activities of glucan phosphorylases were
unaffected by either a decrease or an increase in TPT activity. There were also effects of TPT activities on steady-state
levels of phosphorylated intermediates as well as total amino acids and malate. In air, there was no or little effect of altered
TPT transport activity on either rates of photosynthetic electron transport and/or CO2 assimilation. However, in elevated CO2 (1500 μl · l−1) and low O2 (2%) the rate of CO2 assimilation was decreased in the αTPT lines and was slightly higher in FtTPT lines. This shows that the TPT limits maximum
rates of photosynthesis in the wild type.
Received: 26 March 1999 / Accepted: 21 August 1999 相似文献
4.
Infiltrating detached maize (Zeamays L.) leaves with L-galactono-1,4-lactone (L-GAL) resulted in a 4-fold increase in the content of leaf ascorbate. Upon exposure to high irradiance (1000 μmol photons m−2 s−1) at 5 °C, L-GAL leaves de-epoxidized the xanthophyll-cycle pigments faster than the control leaves; the maximal ratio of de-epoxidized
xanthophyll-cycle pigments to the whole xanthophyll-cycle pool was the same in both leaf types. The elevated ascorbate content,
together with the faster violaxanthin de-epoxidation, did not affect the degree of photoinhibition and the kinetics of the
recovery from photoinhibition, assayed by monitoring the maximum quantum efficiency of photosystem II primary photochemistry
(Fv/Fm). Under the experimental conditions, the thermal energy dissipation seems to be zeaxanthin-independent since, in contrast
to the de-epoxidation, the decrease in the efficiency of excitation-energy capture by open photosystem II reaction centers (Fv′/Fm′) during the high-irradiance treatment at low temperature showed the same kinetic in both leaf types. This was also observed
for the recovery of the maximal fluorescence after stress. Furthermore, the elevated ascorbate content did not diminish the
degradation of pigments or α-tocopherol when leaves were exposed for up to 24 h to high irradiance at low temperature. Moreover,
a higher content of ascorbate appeared to increase the requirement for reduced glutathione.
Received: 20 May 1999 / Accepted: 29 October 1999 相似文献
5.
The green alga Chlamydomonas reinhardtii Dangeard CW-15 exhibited very low rates of plasma-membrane Fe(III) reductase activity when grown under Fe-sufficient conditions.
After switching the medium to an Fe-free formulation, both ferricyanide reductase and ferric chelate reductase activities
rapidly increased, reaching a maximum after 3 d under iron-free conditions. Both of the Fe(III) reductase activities increased
in parallel over time, they exhibited similar K
m values (approximately 10 μM) with respect to Fe(III), displayed the same pH profile of activity, and both exhibited the same
degree of light stimulation which could be inhibited by 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU). Furthermore, ferricyanide
competitively inhibited ferric chelate reduction by iron-limited cells. These results indicate that both Fe(III) reductase
activities were mediated by the same iron-limitation-induced plasma-membrane reductase. No evidence was found for the presence
of Fe(III)-reducing substances in the culture medium, or for the involvement of active oxygen species in the process of Fe(III)
reduction. Chlamydomonas reinhardtii appears to respond to iron limitation in a manner similar to Strategy I higher plants.
Received: 24 June 1997 / Accepted: 2 August 1997 相似文献
6.
Lolium temulentum L. Ba 3081 was grown hydroponically in air (350 μmol mol−1 CO2) and elevated CO2 (700 μmol mol−1 CO2) at two irradiances (150 and 500 μmol m−2 s−1) for 35 days at which point the plants were harvested. Elevated CO2 did not modify relative growth rate or biomass at either irradiance. Foliar carbon-to-nitrogen ratios were decreased at elevated
CO2 and plants had a greater number of shorter tillers, particularly at the lower growth irradiance. Both light-limited and light-saturated
rates of photosynthesis were stimulated. The amount of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) protein was
increased at elevated CO2, but maximum extractable Rubisco activities were not significantly increased. A pronounced decrease in the Rubisco activation
state was found with CO2 enrichment, particularly at the higher growth irradiance. Elevated-CO2-induced changes in leaf carbohydrate composition were small in comparison to those caused by changes in irradiance. No CO2-dependent effects on fructan biosynthesis were observed. Leaf respiration rates were increased by 68% in plants grown with
CO2 enrichment and low light. We conclude that high CO2 will only result in increased biomass if total light input favourably increases the photosynthesis-to-respiration ratio.
At low irradiances, biomass is more limited by increased rates of respiration than by CO2-induced enhancement of photosynthesis.
Received: 23 February 1999 / Accepted: 15 June 1999 相似文献
7.
Interseasonal comparison of CO2 concentrations, isotopic composition, and carbon dynamics in an Amazonian rainforest (French Guiana) 总被引:2,自引:0,他引:2
Canopy CO2 concentrations in a tropical rainforest in French Guiana were measured continuously for 5 days during the 1994 dry season
and the 1995 wet season. Carbon dioxide concentrations ([CO2]) throughout the canopy (0.02–38 m) showed a distinct daily pattern, were well-stratified and decreased with increasing height
into the canopy. During both seasons, daytime [CO2] in the upper and middle canopy decreased on average 7–10 μmol mol−1 below tropospheric baseline values measured at Barbados. Within the main part of the canopy (≥ 0.7 m), [CO2] did not differ between the wet and dry seasons. In contrast, [CO2] below 0.7 m were generally higher during the dry season, resulting in larger [CO2] gradients. Supporting this observation, soil CO2 efflux was on average higher during the dry season than during the wet season, either due to diffusive limitations and/or
to oxygen deficiency of root and microbial respiration. Soil respiration rates decreased by 40% after strong rain events,
resulting in a rapid decrease in canopy [CO2] immediately above the forest floor of about 50␣μmol mol−1. Temporal and spatial variations in [CO2]canopy were reflected in changes of δ13Ccanopy and δ18Ocanopy values. Tight relationships were observed between δ13C and δ18O of canopy CO2 during both seasons (r
2 > 0.86). The most depleted δ13Ccanopy and δ18Ocanopy values were measured immediately above the forest floor (δ13C = −16.4‰; δ18O = 39.1‰ SMOW). Gradients in the isotope ratios of CO2 between the top of the canopy and the forest floor ranged between 2.0‰ and 6.3‰ for δ13C, and between 1.0‰ and 3.5‰ for δ18O. The δ13Cleaf and calculated c
i/c
a of foliage at three different positions were similar for the dry and wet seasons indicating that the canopy maintained a
constant ratio of photosynthesis to stomatal conductance. About 20% of the differences in δ13Cleaf within the canopy was accounted for by source air effects, the remaining 80% must be due to changes in c
i/c
a. Plotting 1/[CO2] vs. the corresponding δ13C ratios resulted in very tight, linear relationships (r
2 = 0.99), with no significant differences between the two seasons, suggesting negligible seasonal variability in turbulent
mixing relative to ecosystem gas exchange. The intercepts of these relationships that should be indicative of the δ13C of respired sources were close to the measured δ13C of soil respired CO2 and to the δ13C of litter and soil organic matter. Estimates of carbon isotope discrimination of the entire ecosystem, Δe, were calculated as 20.3‰ during the dry season and as 20.5‰ during the wet season.
Received: 3 March 1996 / Accepted: 19 October 1996 相似文献
8.
The vertical profile of stable carbon isotope ratios (δ13C) of leaves was analyzed for 13 tree species in a cool-temperate deciduous forest in Japan. The vertical distribution of
long-term averaged δ13C in atmospheric CO2 (δa) was estimated from δ13C of dry matter from NADP-malic enzyme type C4 plant (Zea mays L. var. saccharata Sturt.) grown at a tower in the forest for 32␣days, assuming constant Δ value (3.3‰) in Z. mays against height. The δa value obtained from δ13C in Z.␣mays was lowest at the forest floor (−9.30 ± 0.03‰), increased with height, and was almost constant above 10␣m (−7.14 ± 0.14‰).
Then leaf Δ values for the tree species were calculated from tree leaf δ13 C andδa. Mean leaf Δ values for the three tall deciduous species (Fraxinus mandshurica, Ulmus davidiana, and Alnus hirsuta) were significantly different among three height levels in the forest: 23.1 ± 0.7‰ at the forest floor (understory), 21.4 ± 0.5‰
in lower canopy, and 20.5 ± 0.3‰ in upper canopy. The true difference in tree leaf Δ among the forest height levels might
be even greater, because Δ in Z. mays probably increased with shading by up to ∼‰. The difference in tree leaf Δ among the forest height levels would be mainly
due to decreasing intercellular CO2 (C
i) with the increase in irradiance. Potential assimilation rate for the three tree species probably increased with height,
since leaf nitrogen content on an area basis for these species also increased with height. However, the increase in stomatal
conductance for these tree species would fail to meet the increase in potential assimilation rate, which might lead to increasing
the degree of stomatal limitation in photosynthesis with height.
Received: 30 September 1995 / Accepted: 25 October 1996 相似文献
9.
Cell wall material (CWM) was prepared from sun-dried cocoa (Theobroma cacao L.) bean cotyledons before and after fermentation. The monosaccharide composition of the CWM was identical for unfermented
and fermented beans. Polysaccharides of the CWM were solubilised by sequential extraction with 0.05 M trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (CDTA), 0.05 M Na2CO3, and 1 M, 4 M and 8 M KOH. The non-cellulosic sugar composition for each fraction was similar for unfermented and fermented
samples, indicating that fermentation caused no significant modification of the structural features of individual cell wall
polysaccharides. Pectic polysaccharides accounted for 60% of the cell wall polysaccharides but only small amounts could be
solubilised in solutions of CDTA, Na2CO3, and 1 M and 4 M KOH. The bulk of the pectic polysaccharides were solubilised in 8 M KOH and were characterised by a rhamnogalacturonan
backbone heavily substituted with side-chains of 5-linked arabinose and 4-linked galactose. Linkage analysis indicated the
presence of additional acidic polysaccharides, including a xylogalacturonan and a glucuronoxylan. Cellulose, xyloglucan and
a galactoglucomannan accounted for 28%, 8% and 3% of the cell wall polysaccharides, respectively. It is concluded that the
types and structural features of cell wall polysaccharides in cocoa beans resemble those found in the parenchymatous tissue
of many fruits and vegetables rather than those reported for many seed storage polysaccharides.
Received: 29 May 1999 / Accepted: 19 October 1999 相似文献
10.
Geranylhydroquinone 3′′-hydroxylase, a cytochrome P-450 monooxygenase from Lithospermum erythrorhizon cell suspension cultures 总被引:2,自引:0,他引:2
Geranylhydroquinone 3′′-hydroxylase, which is likely to be involved in shikonin and dihydroechinofuran biosynthesis, was
identified in cell suspension cultures of Lithospermum erythrorhizon Sieb. et Zucc. (Boraginaceae). The enzyme hydroxylates the isoprenoid side chain of geranylhydroquinone (GHQ), a known precursor
of shikonin. Proton/proton correlation spectroscopic and proton/proton long-range correlation spectroscopic studies confirmed
that hydroxylation takes place specifically at position 3′′, i.e. at the methyl group involved in the cyclization reaction.
The enzyme is membrane-bound and was found in the microsomal fraction. It requires NADPH and molecular oxygen as cofactors,
and is inhibited by cytochrome P-450 inhibitors such as cytochrome c and CO. The inhibitory effect of CO is reversed by illumination.
These data suggest that the enzyme is a cytochrome P-450-dependent monooxygenase. The optimum pH of GHQ 3′′-hydroxylase is
7.4, and the apparent K
m value for GHQ is 1.5 μM. The reaction velocity obtained with 3-geranyl-4-hydroxybenzoic acid was more than 100 times lower
than that obtained with geranylhydroquinone.
Received: 20 March 1999 / Accepted: 20 July 1999 相似文献
11.
Q. Chen L. F. Wang N. Su H. D. Qin H. B. Niu J. L. Wang H. Q. Zhai J. M. Wan 《Photosynthetica》2008,46(1):35-39
A yellow leaf colouration mutant (named ycm) generated from rice T-DNA insertion lines was identified with less grana lamellae and low thylakoid membrane protein contents.
At weak irradiance [50 μmol(photon) m−2 s−1], chlorophyll (Chl) contents of ycm were ≈20 % of those of WT and Chl a/b ratios were 3-fold that of wild type (WT). The leaf of ycm showed lower values in the actual photosystem 2 (PS2) efficiency (ΦPS2), photochemical quenching (qP), and the efficiency of excitation capture by open PS2 centres 1 (Fv′/Fm′) than those of WT, except no difference in the maximal efficiency of PS2 photochemistry (Fv/Fm). With progress in irradiance [100 and 200 μmol(photon) m−2 s−1], there was a change in the photosynthetic pigment stoichiometry. In ycm, the increase of total Chl contents and the decrease in Chl a/b ratio were observed. ΦPS2, qP, and Fv′/Fm′ of ycm increased gradually along with the increase of irradiance but still much less than in WT. The increase of xanthophyll ratio
[(Z+A)/(V+A+Z)] associated with non-photochemical quenching (qN) was found in ycm which suggested that ycm dissipated excess energy through the turnover of xanthophylls. No significant differences in pigment composition were observed
in WT under various irradiances, except Chl a/b ratio that gradually decreased. Hence the ycm mutant developed much more tardily than WT, which was caused by low photon energy utilization independent of irradiance. 相似文献
12.
Regulation by irradiance level of the mechanism for dissolved inorganic carbon (DIC) acquisition was examined in the red
macroalga Gracilaria tenuistipitata Zhang et Xia. For this purpose, affinity for external DIC, carbonic anhydrase (CA; EC 4.2.1.1) activity and content of ribulose-1,5-bisphosphate
carboxylase/oxygenase (Rubisco; EC 4.1.1.39) were determined in thalli grown at 45 and 500 μmol photons m−2 s−1. Oxygen evolution rates declined by 50% when the medium pH was changed from 8.1 to 8.7, and the pH compensation point attained
was ca. 9.2. These characteristics were unaffected by the light treatments. In contrast, photosynthetic conductance for DIC
at pH 8.7 was doubled in thalli grown at high irradiance compared with those grown at low irradiance (to 0.74 × 10−6 from 0.33 × 10−6 m s−1). Photosynthetic rates at saturating DIC concentration were also higher by 60% in thalli grown at high irradiance. These
differences could not be attributed to changes in the use of external DIC, since external CA activity did not vary. Although
the irradiance level did not modify the pool size of Rubisco, Rubisco content expressed on a chlorophyll a basis was almost doubled at high irradiance. These results likely indicate that the internal transport of DIC towards the
active-site of Rubisco, rather than the external use of DIC, is enhanced in the thalli grown at high irradiance.
Received: 7 June 1999 / Accepted: 16 October 1999 相似文献
13.
The role of cyclic electron transport has been re-examined in leaves of C3 plants because the bioenergetics of chloroplasts (H+/e = 3 in the presence of a Q-cycle; H+/ATP = 4 of ATP synthesis) had suggested that cyclic electron flow has no function in C3 photosynthesis. After light activation of pea leaves, the dark reduction of P700 (the donor pigment of PSI) following far-red
oxidation was much accelerated. This corresponded to loss of sensitivity of P700 to oxidation by far-red light and a large
increase in the number of electrons available to reduce P700+ in the dark. At low CO2 and O2 molar ratios, far-red light was capable of decreasing the activity of photosystem II (measured as the ratio of variable to
maximal chlorophyll fluorescence, Fv/Fm) and of increasing light scattering at 535 nm and zeaxanthin synthesis, indicating formation of a transthylakoid pH gradient.
Both the light-induced increase in the number of electrons capable of reducing far-red-oxidised P700 and the decline in Fv/Fm brought about by far-red in leaves were prevented by methyl viologen. Antimycin A inhibited CO2-dependent O2 evolution of pea leaves at saturating but not under limiting light; in its presence, far-red light failed to decrease Fv/Fm. The results indicate that cyclic electron flow regulates the quantum yield of photosystem II by decreasing the intrathylakoid
pH when there is a reduction in the availability of electron acceptors at the PSI level (e.g. during drought or cold stresses).
It also provides ATP for the carbon-reduction cycle under high light. Under these conditions, the Q-cycle is not able to maintain
a H+/e ratio of 3 for ATP synthesis: we suggest that the ratio is flexible, not obligatory.
Received: 23 February 1999 / Accepted: 19 August 1999 相似文献
14.
The effect of extractants on degradation of L-glutamate and L-arginine in the course of shaking and filtration at low temperature 总被引:1,自引:0,他引:1
Summary. The effects of demineralized water (DEMI H2O) and 0.5 M ammonium acetate (0.5 M AAc) on losses of L-glutamic acid and L-arginine in the course of shaking and filtration
at low temperature (6 °C) were tested. The concentration of L-glutamic acid decreased by 6.3% in DEMI H2O and by 4.9% in 0.5 M AAc, whereas the L-arginine concentration decreased by 6.0% (DEMI H2O) and 10.7% (0.5 M AAc). We found a significantly (P < 0.05) higher degradation of L-arginine in 0.5 M AAc compared with that of DEMI H2O. 相似文献
15.
The aim of this work was to examine the role of fructose 2,6-bisphosphate (Fru-2,6-P2) in photosynthetic carbon partitioning. The amount of Fru-2,6-P2 in leaves of tobacco (Nicotiana tabacum L. cv. Samsun) was reduced by introduction of a modified mammalian gene encoding a functional fructose-2,6-bisphosphatase
(EC 3.1.3.46). Expression of this gene in transgenic plants reduced the Fru-2,6-P2 content of darkened leaves to between 54% and 80% of that in untransformed plants. During the first 30 min of photosynthesis
sucrose accumulated more rapidly in the transgenic lines than in the untransformed plants, whereas starch production was slower
in the transgenic plants. On illumination, the proportion of 14CO2 converted to sucrose was greater in leaf disks of transgenic lines possessing reduced amounts of Fru-2,6-P2 than in those of the control plants, and there was a corresponding decrease in the proportion of carbon assimilated to starch
in the transgenic lines. Furthermore, plants with smaller amounts of Fru-2,6-P2 had lower rates of net CO2 assimilation. In illuminated leaves, decreasing the amount of Fru-2,6-P2 resulted in greater amounts of hexose phosphates, but smaller amounts of 3-phosphoglycerate and dihydroxyacetone phosphate.
These differences are interpreted in terms of decreased inhibition of cytosolic fructose-1,6-bisphosphatase resulting from
the lowered Fru-2,6-P2 content. The data provide direct evidence for the importance of Fru-2,6-P2 in co-ordinating chloroplastic and cytosolic carbohydrate metabolism in leaves in the light.
Received: 8 February 2000 / Accepted: 25 April 2000 相似文献
16.
Photoinhibition has been often evaluated with leaf discs floated on water or placed on wet papers to prevent desiccation.
Under these conditions, there is a possibility that CO2 diffusion is blocked by water, which may lead to reduction in photosynthetic CO2 assimilation. Using Chenopodium album L. grown at two irradiances, photosynthesis, quantum yield of Photosystem II (ΔF/F
m′), non-photochemical quenching (qN), and photoinhibition were compared between detached leaves and leaf discs. In low-light-grown plants, photoinhibition was
greater in leaf discs than in detached leaves, while in high-light-grown plants, there was little difference. Leaf discs showed
lower rates of photosynthesis and ΔF/F
m′, and higher qN. The ΔF/F
m′ in leaf discs increased when leaf discs were exposed to high concentration of CO2, suggesting that CO2 diffusion to chloroplasts was limited in leaf discs floated on water.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
17.
Regulation of the photosynthetic electron transport chain 总被引:20,自引:1,他引:19
The regulation of electron transport between photosystems II and I was investigated in the plant Silene dioica L. by means of measurement of the kinetics of reduction of P700 following a light-to-dark transition. It was found that, in this species, the rate constant for P700 reduction is sensitive to light intensity and to the availability of CO2. The results indicated that at 25 °C the rate of electron transport is down-regulated by approximately 40–50% relative to
the maximum rate achievable in saturating CO2 and that this down-regulation can be explained by regulation of the electron transport chain itself. Measurements of the
temperature sensitivity of this rate constant indicated that there is a switch in the rate-limiting step that controls electron
transport at around 20 °C: at higher temperatures, CO2 availability is limiting; at lower temperatures some other process regulates electron transport, possibly a diffusion step
within the electron transport chain itself. Regulation of electron transport also occurred in response to drought stress and
sucrose feeding. Measurements of non-photochemical quenching of chlorophyll fluorescence did not support the idea that electron
transport is regulated by the pH gradient across the thylakoid membrane, and the possibility is discussed that the redox potential
of a stromal component may regulate electron transport.
Received: 4 March 1999 / Accepted: 25 May 1999 相似文献
18.
Atomic force microscopy (AFM) enables the topographical structure of cells and biological materials to be resolved under
natural (physiological) conditions, without fixation and dehydration artefacts associated with imaging methods in vacuo. It
also provides a means of measuring interaction forces and the mechanical properties of biomaterials. In the present study,
AFM has been applied for the first time to the study of the mechanical properties of a natural adhesive produced by a green
plant cell. Swimming spores of the green alga Enteromorpha linza (L.) J. Ag. (7–10 μm) secrete an adhesive glycoprotein which provides firm anchorage to the substratum. Imaging of the adhesive in its
hydrated state revealed a swollen gel-like pad, approximately 1 μm thick, surrounding the spore body. Force measurements revealed
that freshly released adhesive has an adhesion strength of 173 ± 1.7 mN m−1 (mean ± SE; n=90) with a maximum value for a single adhesion force curve of 458 mN m−1. The adhesive had a compressibility (equivalent to Young's modulus) of 0.54 × 106 ± 0.05 × 106 N m−2 (mean ± SE; n=30). Within minutes of release the adhesive underwent a progressive `curing' process with a 65% reduction in mean adhesive
strength within an hour of settlement, which was also reflected in a reduction in the average length of the adhesive polymer
strands (polymer extension) and a 10-fold increase in Young's modulus. Measurements on the spore surface itself revealed considerably
lower adhesion-strength values but higher polymer-extension values than the adhesive pad, which may reflect the deposition
of different polymers on this surface as a new cell wall is formed. The study demonstrates the value of AFM to the imaging
of plant cells in the absence of fixation and dehydration artefacts and to the characterisation of the mechanical properties
of plant glycoproteins that have potential utility as adhesives.
Received: 22 February 2000 / Accepted: 20 April 2000 相似文献
19.
Maize (Zea mays L.) cell cultures incorporated radioactivity from [14C]cinnamate into hydroxycinnamoyl-CoA derivatives and then into polysaccharide-bound feruloyl residues. Within 5–20 min, the
CoA pool had lost its 14C by turnover and little or no further incorporation into polysaccharides then occurred. The system was thus effectively a
pulse–chase experiment. Kinetics of radiolabelling of diferulates (also known as dehydrodiferulates) varied with culture age.
In young (1–3 d) cultures, polysaccharide-bound [14C]feruloyl- and [14C]diferuloyl residues were both detectable within 1 min of [14C]cinnamate feeding. Thus, feruloyl residues were dimerised <1 min after their attachment to polysaccharides. For at least
the first 2.3 h after [14C]cinnamate feeding, polysaccharide-bound [14C]diferuloyl residues remained almost constant at ≈7% of the total polysaccharide-bound [14C]ferulate derivatives. Since feruloyl residues are attached to polysaccharides <1 min after the biosynthesis of the latter,
and >10 min before secretion, the data show that extensive feruloyl coupling occurred intra-protoplasmically. Exogenous H2O2 (1 mM) caused little additional feruloyl coupling; therefore, wall-localised coupling may have been peroxidase-limited. In
older (e.g. 4 d) cultures, less intraprotoplasmic coupling occurred: during the first 2.5 h, polysaccharide-bound [14C]diferuloyl residues were a steady 1.4% of the total polysaccharide-bound [14C]ferulate derivatives. In contrast to the situation in younger cultures, exogenous H2O2 induced a rapid 4- to 6-fold increase in all coupling products, indicating that coupling in the walls was H2O2-limited. In both 2- and 4-d-old cultures, polysaccharide-bound 14C-trimers and larger coupling products exceeded [14C]diferulates 3- to 4-fold, but followed similar kinetics. Thus, although all known dimers of ferulate can now be individually
quantified, it appears to be trimers and larger products that make the major contribution to cross-linking of wall polysaccharides
in cultured maize cells. We argue that feruloyl arabinoxylans that are cross-linked before and after secretion are likely
to loosen and tighten the cell wall, respectively. The consequences for the control of cell expansion and for the response
of cell walls to an oxidative burst are discussed.
Received: 19 January 2000 / Accepted: 13 April 2000 相似文献
20.
M. E. Kubiske Kurt S. Pregitzer Carl J. Mikan Donald R. Zak Jennifer L. Maziasz A. Teeri 《Oecologia》1997,110(3):328-336
We tested the hypothesis that elevated CO2 would stimulate proportionally higher photosynthesis in the lower crown of Populus trees due to less N retranslocation, compared to tree crowns in ambient CO2. Such a response could increase belowground C allocation, particularly in trees with an indeterminate growth pattern such
as Populus tremuloides. Rooted cuttings of P. tremuloides were grown in ambient and twice ambient (elevated) CO2 and in low and high soil N availability (89 ± 7 and 333 ± 16 ng N g−1 day−1 net mineralization, respectively) for 95 days using open-top chambers and open-bottom root boxes. Elevated CO2 resulted in significantly higher maximum leaf photosynthesis (A
max) at both soil N levels. A
max was higher at high N than at low N soil in elevated, but not ambient CO2. Photosynthetic N use efficiency was higher at elevated than ambient CO2 in both soil types. Elevated CO2 resulted in proportionally higher whole leaf A in the lower three-quarters to one-half of the crown for both soil types. At elevated CO2 and high N availability, lower crown leaves had significantly lower ratios of carboxylation capacity to electron transport
capacity (V
cmax/J
max) than at ambient CO2 and/or low N availability. From the top to the bottom of the tree crowns, V
cmax/J
max increased in ambient CO2, but it decreased in elevated CO2 indicating a greater relative investment of N into light harvesting for the lower crown. Only the mid-crown leaves at both
N levels exhibited photosynthetic down regulation to elevated CO2. Stem biomass segments (consisting of three nodes and internodes) were compared to the total A
leaf for each segment. This analysis indicated that increased A
leaf at elevated CO2 did not result in a proportional increase in local stem segment mass, suggesting that C allocation to sinks other than the
local stem segment increased disproportionally. Since C allocated to roots in young Populus trees is primarily assimilated by leaves in the lower crown, the results of this study suggest a mechanism by which C allocation
to roots in young trees may increase in elevated CO2.
Received: 12 August 1996 / Accepted: 12 November 1996 相似文献