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1.
Anatomy and some physiological characteristics of the leaves in Polygonum cuspidatum Sieb. et Zucc., a dioecious clonal herb, were compared between two populations, one from a lowland in Shizuoka City (10 m above sea level), and another from a highland on Mt. Fuji (2500 m above sea level). Leaf mass per area (LMA) of the highland plants was about twice that of the lowland plants. The greater leaf thickness, thicker mesophyll cell walls and higher mesophyll cell density in the highland leaves contributed to the larger LMA. Although mesophyll area exposed to intercellular airspaces was greater in the highland leaves than in the lowland leaves by 30%, the surface area of chloroplasts facing intercellular airspaces was similar between these leaves. CO 2 transfer conductance inside the leaf ( gi) of the highland leaves (0·75 μmol m ?2 s ?1 Pa ?1) is the lowest recorded for herbaceous plants and was only 40% of that in the lowland leaves. On the other hand, the difference in stomatal conductance was small. δ 13C values in the leaf dry matter were greater in the highland leaves by 4‰. These data and the estimation of CO 2 partial pressures in the intercellular air spaces and in the chloroplast suggested that the greater dry matter δ 13C in the highland leaves, indicative of lower long‐term ratio of the chloroplast stroma to the ambient CO 2 partial pressures, would be mainly attributed to their lower gi. 相似文献
2.
In C 3 leaves, the mesophyll conductance to CO 2 diffusion, gm, determines the drawdown in CO 2 concentration from intercellular airspace to the chloroplast stroma. Both gm and stomatal conductance limit photosynthetic rate and vary in response to the environment. We investigated the response of gm to changes in CO 2 in two Arabidopsis genotypes (including a mutant with open stomata, ost1), tobacco and wheat. We combined measurements of gas exchange with carbon isotope discrimination using tunable diode laser absorption spectroscopy with a CO 2 calibration system specially designed for a range of CO 2 and O 2 concentrations. CO 2 was initially increased from 200 to 1000 ppm and then decreased stepwise to 200 ppm and increased stepwise back to 1000 ppm, or the sequence was reversed. In 2% O 2 a step increase from 200 to 1000 ppm significantly decreased gm by 26–40% in all three species, whereas following a step decrease from 1000 to 200 ppm, the 26–38% increase in gm was not statistically significant. The response of gm to CO 2 was less in 21% O 2. Comparing wild type against the ost1 revealed that mesophyll and stomatal conductance varied independently in response to CO 2. We discuss the effects of isotope fractionation factors on estimating gm. 相似文献
3.
Abstract. It has been shown that atmospheric O 2 can either depress or stimulate the rate of apparent photosynthesis of white mustard depending on the environmental conditions: CO 2 concentration, light intensity and temperature. Stimulation by O 2 was observed only under high photon fluence rate and at high CO 2 concentrations. The critical CO 2 concentration below which O 2 was inhibiting and above which it was stimulating was dependent on the temperature of the assay: for plants grown at 12°C the critical CO 2 concentration was 13.35 mmol at 5° C and 21.92 mmol at 10° C. Stimulation by O 2 depended also on the growth temperature: for measurements at 26.31 mmol m ?3 CO 2, O 2 was stimulating at temperatures less than 12°C for plants grown at 12°C and less than 19°C for plants grown at 27°C. The efficiency of the O 2-dependent stimulation of net photosynthesis was maximum at 9.21 mol m ?3 O 2 at 26.31 mmol m ?3 CO 2. Oxygen-stimulation of net photosynthesis was detected in Nicotiana tabacum L. var Samsun, Lycopersicum esculentum L. and Chenopodium album L. At 5°C and under high photon fluence rate, O 2 increased the carboxylation capacity of the photosynthetic apparatus of mustard and decreased its affinity for CO 2. The O 2 inhibition of the net CO 2 uptake observed at low CO 2 concentrations was the result of a decrease in the affinity for carbon dioxide. The nature of the mechanism which causes the stimulation of photosynthesis is discussed. 相似文献
4.
The effect of environmental factors on the post-illumination burst of CO 2 (PIB) and O 2 inhibition of apparent photosynthesis (APS) in wheat ( Triticum aestivum L.) was studied in an open gas exchange system utilizing the mathematics of non-steady-state systems. Two components of inhibition by O 2 are suggested: one is caused by photorespiration as measured from the maximum rate of the PIB, and the second is direct inhibition as taken as APS 2%O2— (APS x%O2+ PIB x%O2) where X is the oxygen concentration. A primary PIB which occurred from 16–28 s after the darkening of the foliage was attributed to photorespiration. No primary PIB was observed at 2% O 2. At a CO 2 concentration of 100 μ/1 in the atmosphere (about 2.5 μ M based on leaf intercellular concentration) and at 30°C and 145 nE/cm 2 nE/cm 2·s, APS decreased curve-linearly with increasing O 2 and reached an O 2 compensation point of 560 μ M (48% by volume), above which there was a net loss of CO 2 in the light. The PIB increased with increasing O 2 and became saturated at about 500 μ M O 2 but decreased above 900 μ M O 2. Direct inhibition of photosynthesis by O 2 increased with increasing O 2 concentration. Decreasing CO 2 concentration had an effect on the magnitude of the PIB similar to that of increasing O 2. At 30°C and 21% O 2, the PIB increased with decreasing CO 2 down to the CO 2 compensation point ( I) of 1.4 μ M (47 μ M/l). Below Γ, both PIB and CO 2 evolution into the air in the light (at 21% O 2) increased and then decreased at CO 2 below 0.8 μ M. The ratio of the PIB to APS 2% o O 2 increased linearly with increasing O 2/CO 2 ratio where O 2 was held constant at 21% and CO 2 was varied from 1.4 to 8.5 μ M, while direct inhibition of photosynthesis expressed as a proportion of APS 2%O2 remained constant over this range. At low CO 2 concentration photorespiration as estimated by the PIB is the major part of O 2 photosynthesis, while at atmospheric CO 2 levels, direct inhibition is the major component. The PIB and APS at 2% and 21% O 2 increased hyperbolically with increasing irradiance and all became light-saturated at about 65 nE/cm 2 s. The percentage total O 2 inhibition of photosynthesis remained constant with increasing irradiance as did the relative contribution of direct O 2 inhibition or photorespiration (PIB) to total O 2 inhibition. The PIB and APS at 21% O 2 had similar temperature optima of 30°C when experimental conditions were adjusted to provide a constant internal O 2/CO 2 solubility ratio at varying temperatures. However, with a constant external CO 2 concentration, the temperature optimum for the PIB shifted upward to 35°C while that for APS at 21% O 2 remained at 30°C, which may be due to an increased O 2/CO 2 concentration in the leaf with increasing temperature. 相似文献
5.
Abstract. The photosynthetic responses to temperature in C 3, C 3-C 4 intermediate, and C 4 species in the genus Flaveria were examined in an effort to identify whether the reduced photorespiration rates characteristic of C 3-C 4 intermediate photosynthesis result in adaptive advantages at warm leaf temperatures. Reduced photorespiration rates were reflected in lower CO 2 compensation points at all temperatures examined in the C 3-C 4 intermediate, Flaveria floridana, compared to the C 3 species, F. cronquistii. The C 3-C 4 intermediate, F. floridana, exhibited a C 3-like photosynthetic temperature dependence, except for relatively higher photosynthesis rates at warm leaf temperatures compared to the C 3 species, F. cronquistii. Using models of C 3 and C 3-C 4 intermediate photosynthesis, it was predicted that by recycling photorespired CO 2 in bundle-sheath cells, as occurs in many C 3-C 4 intermediates, photosynthesis rates at 35°C could be increased by 28%, compared to a C 3 plant. Without recycling photorespired CO 2, it was calculated that in order to improve photosynthesis rates at 35°C by this amount in C 3 plants, (1) intercellular CO 2 partial pressures would have to be increased from 25 to 31 Pa, resulting in a 57% decrease in water-use efficiency, or (2) the activity of RuBP carboxylase would have to be increased by 32%, resulting in a 22% decrease in nitrogen-use efficiency. In addition to the recycling of photorespired CO 2, leaves of F. floridana appear to effectively concentrate CO 2 at the active site of RuBP carboxylase, increasing the apparent carboxylation efficiency per unit of in vitro RuBP carboxylase activity. The CO 2-concentrating activity also appears to reduce the temperature sensitivity of the carboxylation efficiency in F. floridana compared to F. cronquistii. The carboxylation efficiency per unit of RuBP carboxylase activity decreased by only 38% in F. floridana, compared to 50% in F. cronquistii, as leaf temperature was raised from 25 to 35°C. The C 3-C 4 intermediate, F. ramosissima, exhibited a photosynthetic temperature temperature response curve that was more similar to the C 4 species, F. trinervia, than the C 3 species, F. cronquistii. The C 4-like pattern is probably related to the advanced nature of C 4-like biochemical traits in F. ramosissima The results demonstrate that reductions in photorespiration rates in C 3-C 4 intermediate plants create photosynthetic advantages at warm leaf temperatures that in C 3 plants could only be achieved through substantial costs to water-use efficiency and/or nitrogen-use efficiency. 相似文献
6.
We suggest a new technique for estimating the relative drawdown of CO2 concentration (c) in the intercellular air space (IAS) across hypostomatous leaves (expressed as the ratio cd/cb, where the indexes d and b denote the adaxial and abaxial edges, respectively, of IAS), based on the carbon isotope composition (δ13C) of leaf cuticular membranes (CMs), cuticular waxes (WXs) or epicuticular waxes (EWXs) isolated from opposite leaf sides. The relative drawdown in the intracellular liquid phase (i.e., the ratio cc/cbd, where cc and cbd stand for mean CO2 concentrations in chloroplasts and in the IAS), the fraction of intercellular resistance in the total mesophyll resistance (rIAS/rm), leaf thickness, and leaf mass per area (LMA) were also assessed. We show in a conceptual model that the upper (adaxial) side of a hypostomatous leaf should be enriched in 13C compared to the lower (abaxial) side. CM, WX, and/or EWX isolated from 40 hypostomatous C3 species were 13C depleted relative to bulk leaf tissue by 2.01–2.85‰. The difference in δ13C between the abaxial and adaxial leaf sides (δ13CAB ? 13CAD, Δb–d), ranged from ??2.22 to +?0.71‰ (??0.09 ± 0.54‰, mean ± SD) in CM and from ??7.95 to 0.89‰ (??1.17 ± 1.40‰) in WX. In contrast, two tested amphistomatous species showed no significant Δb–d difference in WX. Δb–d correlated negatively with LMA and leaf thickness of hypostomatous leaves, which indicates that the mesophyll air space imposes a non-negligible resistance to CO2 diffusion. δ13C of EWX and 30-C aldehyde in WX reveal a stronger CO2 drawdown than bulk WX or CM. Mean values of cd/cb and cc/cbd were 0.90 ± 0.12 and 0.66 ± 0.11, respectively, across 14 investigated species in which wax was isolated and analyzed. The diffusion resistance of IAS contributed 20 ± 14% to total mesophyll resistance and reflects species-specific and environmentally-induced differences in leaf functional anatomy. 相似文献
7.
Muhlenbergia sobolifera (Muhl.) Trin., a C 4 grass, occurs in understory habitats in the northeastern United States. Plants of M. sobolifera were grown at 23 and 30°C at 150 and 700 μmol photons m −2 s −1. The photosynthetic CO 2 compensation point, maximum CO 2 assimilation, dark respiration and the absorbed quantum use efficiency (QUE) were measured at 23 and 30°C at 2 and 20% O 2. Photosynthetic CO 2 compensation points ranged from 4 to 14mm 3 dm −3 CO 2 and showed limited O 2 sensitivity. The mean photosynthetic CO 2 compensation point of plants grown at 30°C (4·5 mm 3 dm −3) was 57% lower and 80% less inhibited by O 2 than that of plants grown at 23°C. Photosynthesis was similarly affected by growth temperature, with 70% more O 2 inhibition in plants grown at 23°C; suppression over all treatments ranging from 2 to 11%. Unlike typical C 4 species, plants of M. sobolifera from both temperature regimes exhibited higher CO 2 assimilation rates when grown at low light. Growth temperature and light also affected QUE; plants grown at low light and 23°C had the highest value (0·068 mol CO 2/mol quanta). Measurement temperature and growth light regime significantly affected dark respiration; however, O 2 did not affect QUE or dark respiration under any growth or measurement conditions. The results indicate that M. sobolifera is adapted to low PPFD, and that complete suppression of photorespiration is dependent upon high growth temperature. 相似文献
8.
The effect of leaf temperature on the post-illumination burst of CO 2 (PIB) in 15 day-old primary bean leaves ( Phaseolus vulgaris L.) was studied by means of infrared gas analysis in a closed gas exchange system. The amplitude and kinetic of PIB was different
with its own characteristic course for different temperature steps. The temperature optimum for the PIB at 21% O 2 near the carbon dioxide compensation concentration Λ (150 mg m -3) was 33 °C while that for net photosynthetic rate ( P
N) at 21% O 2 and 600 mg m -3 CO 2 was 24.5 °C. The PIB was observed to 12…33 s after the darkening of leaves. No PIB was observed at 2% O 2. The applicability of PIB as an estimate of photorespiration rate R
p has been proved by comparing it with extrapolation of the relationship between P
n and CO 2 concentration to zero. 相似文献
9.
Effects of growth light intensity on the temperature dependence of CO 2 assimilation rate were studied in tobacco ( Nicotiana tabacum) because growth light intensity alters nitrogen allocation between photosynthetic components. Leaf nitrogen, ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco) and cytochrome f (cyt f) contents increased with increasing growth light intensity, but the cyt f/Rubisco ratio was unaltered. Mesophyll conductance to CO 2 diffusion ( gm) measured with carbon isotope discrimination increased with growth light intensity but not with measuring light intensity. The responses of CO 2 assimilation rate to chloroplast CO 2 concentration ( Cc) at different light intensities and temperatures were used to estimate the maximum carboxylation rate of Rubisco ( Vcmax) and the chloroplast electron transport rate ( J). Maximum electron transport rates were linearly related to cyt f content at any given temperature (e.g. 115 and 179 µmol electrons mol ?1 cyt f s ?1 at 25 and 40 °C, respectively). The chloroplast CO 2 concentration ( Ctrans) at which the transition from RuBP carboxylation to RuBP regeneration limitation occurred increased with leaf temperature and was independent of growth light intensity, consistent with the constant ratio of cyt f/Rubisco. In tobacco, CO 2 assimilation rate at 380 µmol mol ?1 CO 2 concentration and high light was limited by RuBP carboxylation above 32 °C and by RuBP regeneration below 32 °C. 相似文献
10.
It has long been held that the low photosynthetic rates ( A) of coffee leaves are largely associated with diffusive constraints to photosynthesis. However, the relative limitations of the stomata and mesophyll to the overall diffusional constraints to photosynthesis, as well as the coordination of leaf hydraulics with photosynthetic limitations, remain to be fully elucidated in coffee. Whether the low actual A under ambient CO 2 concentrations is associated with the kinetic properties of Rubisco and high (photo)respiration rates also remains elusive. Here, we provide a holistic analysis to understand the causes associated with low A by measuring a variety of key anatomical/hydraulic and photosynthetic traits in sun- and shade-grown coffee plants. We demonstrate that leaf hydraulic architecture imposes a major constraint on the maximisation of the photosynthetic gas exchange of coffee leaves. Regardless of the light treatments, A was mainly limited by stomatal factors followed by similar limitations associated with the mesophyll and biochemical constraints. No evidence of an inefficient Rubisco was found; rather, we propose that coffee Rubisco is well tuned for operating at low chloroplastic CO 2 concentrations. Finally, we contend that large diffusive resistance should lead to large CO 2 drawdown from the intercellular airspaces to the sites of carboxylation, thus favouring the occurrence of relatively high photorespiration rates, which ultimately leads to further limitations to A. 相似文献
11.
In this study, we have examined several physiological, biochemical and morphological features of Buddleja davidii plants growing at 1300 m above sea level (a.s.l.) and 3400 m a.s.l., respectively, to identify coordinated changes in leaf properties in response to reduced CO 2 partial pressure (P a). Our results confirmed previous findings that foliar δ 13C, photosynthetic capacity and foliar N concentration on a leaf area basis increased, whereas stomatal conductance (g s) decreased with elevation. The net CO 2 assimilation rate (A max), maximum rate of electron transport (J max) and respiration increased significantly with elevation, although no differences were found in carboxylation efficiency of Rubisco (V cmax). Consequently, also the J max to V cmax ratio was significantly increased by elevation, indicating that the functional balance between Ribulose‐1,5‐biphosphate (RuBP) consumption and RuBP regeneration changes as elevation increases. Our results also indicated a homeostatic response of CO 2 transfer conductance inside the leaf (mesophyll conductance, g m) to increasing elevation. In fact, with elevation, g m also increased compensating for the strong decrease in g s and, thus, in the P i (intercellular partial pressure of CO 2) to P a ratio, leading to similar chloroplast partial pressure of CO 2 (P c) to P a ratio at different elevations. Because there were no differences in V cmax, also A measured at similar PPFD and leaf temperature did not differ statistically with elevation. As a consequence, a clear relationship was found between A and g m, and between A and the sum of g s and g m. These data suggest that the higher dry mass δ 13C of leaves at the higher elevation, indicative of lower long‐term P c/P a ratio, cannot be attributed to changes either in diffusional resistances or in carboxylation efficiency. We speculate that because temperature significantly decreases as the elevation increases, it dramatically affects CO 2 diffusion and hence P c/P a and, consequently, is the primary factor influencing 13C discrimination at high elevation. 相似文献
12.
Short- and long-term effects of elevated CO 2 concentration and temperature on whole plant respiratory relationships are examined for wheat grown at four constant temperatures and at two CO 2 concentrations. Whole plant CO 2 exchange was measured on a 24 h basis and measurement conditions varied both to observe short-term effects and to determine the growth respiration coefficient (r g), dry weight maintenance coefficient (r m), basal (i.e. dark acclimated) respiration coefficient (r g), and 24 h respiration:photosynthesis ratio (R:P). There was no response of r g to short-term variation in CO2 concentration. For plants with adequate N supply, r g was unaffected by the growth-CO 2 despite a 10% reduction in the plant's N concentration (%N). However, r m was decreased 13%, and r b was decreased 20% by growth in elevated CO 2 concentration relative to ambient. Nevertheless, R:P was not affected by growth in elevated CO 2. Whole plant respiration responded to short-term variation of ± 5 °C around the growth temperature with low sensitivity (Q 10= 1.8 at 15 °C, 1.3 at 30 °C). The shape of the response of whole plant respiration to growth temperature was different from that of the short term response, being a slanted S-shape declining between 25 and 30 °C. While r m, increased, r g decreased when growth temperature increased between 15 and 20 °C. Above 20 °C r m became temperature insensitive while r g increased with growth temperature. Despite these complex component responses, R:P increased only from 0.40 to 0.43 between 15° and 30 °C growth temperatures. Giving the plants a step increase in temperature caused a transient increase in R:P which recovered to the pre-transient value in 3 days. It is concluded that use of a constant R:P with respect to average temperature and CO 2 concentration may be a more simple and accurate way to model the responses of wheat crop respiration to ‘climate change’ than the more complex and mechanistically dubious functional analysis into growth and maintenance components. 相似文献
13.
Temperature, activating metal ions, and amino-acid substitutions are known to influence the CO 2/O 2 specificity of the chloroplast enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase. However, an understanding of the physical basis for enzyme specificity has been elusive. We have shown that the temperature dependence of CO 2/O 2 specificity can be attributed to a difference between the free energies of activation for the carboxylation and oxygenation partial reactions. The reaction between the 2,3-enediolate of ribulose 1,5-bisphosphate and O 2 has a higher free energy of activation than the corresponding reaction of this substrate with CO 2. Thus, oxygenation is more responsive to temperature than carboxylation. We have proposed possible transition-state structures for the carboxylation and oxygenation partial reactions based upon the chemical natures of these two reactions within the active site. Electrostatic forces that stabilize the transition state of the carboxylation reaction will also inevitably stabilize the transition state of the oxygenation reaction, indicating that oxygenase activity may be unavoidable. Furthermore, the reduction in CO 2/O 2 specificity that is observed when activator Mg 2+ is replaced by Mn 2+ may be due to Mg 2+ being more effective in neutralizing the negative charge of the carboxylation transition state, whereas Mn 2+ is a transition-metal ion that can overcome the triplet character of O 2 to promote the oxygenation reaction.Abbreviations CABP
2-carboxyarabinitol 1,5-bisphosphate
- enol-RuBP
2,3-enediolate of ribulose 1,5-bisphosphate
- K c
K mfor CO 2
- K o
K mfor O 2
- Rubisco
ribulose-1,5-bisphosphate carboxylase/oxygenase
- RuBP
ribulose 1,5-bisphosphate
- V c
V
max for carboxylation
- V o
V
max for oxygenation 相似文献
14.
The source of glycolate in photorespiration and its control, a particularly active and controversial research topic in the
1970s, was resolved in large part by several discoveries and observations described here. George Bowes discovered that the
key carboxylation enzyme Rubisco (ribulosebisphosphate carboxylase/oxygenase) is competitively inhibited by O 2 and that O 2 substitutes for CO 2 in the initial `dark' reaction of photosynthesis to yield glycolate-P, the substrate for photorespiration. William Laing
derived an equation from basic enzyme kinetics that describes the CO 2, O 2, and temperature dependence of photosynthesis, photorespiration, and the CO 2 compensation point in C 3 plants. Jerome Servaites established that photosynthesis cannot be increased by inhibiting the photorespiratory pathway prior
to the release of photorespiratory CO 2, and Douglas Jordan discovered substantial natural variation in the Rubisco oxygenase/carboxylase ratio. A mutant Arabidopsis plant with defective glycolate-P phosphatase, isolated by Chris Somerville, definitively established the role of O 2 and Rubisco in providing photorespiratory glycolate. Selection techniques to isolate photorespiration-deficient plants were
devised by Jack Widholm and by Somerville, but no plants with reduced photorespiration were found. Somerville's approach,
directed mutagenesis of Arabidopsis plants, was subsequently successful in the isolation of numerous other classes of mutants and revolutionized the science
of plant biology.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
15.
Seedlings (70-d-old) of two tall fescue ( Festuca arundinacea Schreb.) genotypes, heat-tolerant Jaguar 3 and heat-sensitive TF 66, were exposed to a high temperature stress of 35/30 °C
(day/night) for 20 d and both light-saturated and CO 2-saturated leaf stomatal conductance decreased, especially in TF 66. Higher reductions of quantum efficiency, carboxylation
efficiency and maximum photochemical efficiency of photosystem 2 in dark adapted leaves (measured as F v/F m) occurred in TF 66 than in Jaguar 3. High temperature stress increased photorespiration in the two plants, but more in TF
66. Moreover, high temperature stress also reduced the growth, chlorophyll content and caused cell membrane injuries in the
two cultivars, the changes were again more pronounced in TF 66 than in Jaguar 3. 相似文献
16.
Transgenic tobacco ( Nicotiana tabacum L. cv. W38) with an antisense gene directed against the mRNA of the ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit was used to determine the kinetic properties of Rubisco in vivo. The leaves of these plants contained only 34% as much Rubisco as those of the wild type, but other photosynthetic components were not significantly affected. Consequently, the rate of CO 2 assimilation by the antisense plants was limited by Rubisco activity over a wide range of CO 2 partial pressures. Unlike in the wild-type leaves, where the rate of regeneration of ribulose bisphosphate limited CO 2 assimilation at intercellular partial pressures above 400 ubar, photosynthesis in the leaves of the antisense plants responded hyperbolically to CO 2, allowing the kinetic parameters of Rubisco in vivo to be inferred. We calculated a maximal catalytic turnover rate, k cat, of 3.5+0.2 mol CO 2·(mol sites) –1·s –1 at 25° C in vivo. By comparison, we measured a value of 2.9 mol CO 2·(mol sites) –1· –1 in vitro with leaf extracts. To estimate the Michaelis-Menten constants for CO 2 and O 2, the rate of CO 2 assimilation was measured at 25° C at different intercellular partial pressures of CO 2 and O 2. These measurements were combined with carbon-isotope analysis ( 13C/ 12C) of CO 2 in the air passing over the leaf to estimate the conductance for transfer of CO 2 from the substomatal cavities to the sites of carboxylation (0.3 mol·m –2·s –1·bar –1) and thus the partial pressure of CO 2 at the sites of carboxylation. The calculated Michaelis-Menten constants for CO 2 and O 2 were 259 ±57 bar (8.6±1.9M) and 179 mbar (226 M), respectively, and the effective Michaelis-Menten constant for CO 2 in 200 mbar O 2 was 549 bar (18.3 M). From measurements of the photocompensation point ( * = 38.6 ubar) we estimated Rubisco's relative specificity for CO 2, as opposed to O 2 to be 97.5 in vivo. These values were dependent on the size of the estimated CO 2-transfer conductance.Abbreviations and Symbols A
CO 2-assimilation rate
- g w
conductance for CO 2 transfer from the substomatal cavities to the sites of carboxylation
- K c, K o
Michaelis-Menten constants for carboxylation, oxygenation of Rubisco
- k cat
V cmax/[active site]
- O
partial pressure of O 2 at the site of carboxylation
- p c
partial pressure of CO 2 at the site of carboxylation
- p i
intercellular CO 2 partial pressure
- R d
day respiration (non-photorespiratory CO 2 evolution)
- Rubisco
ribulose 1,5-bisphosphate carboxylase/oxygenase
- RuBP
ribulose-1,5-bisphosphate
- S c/o
relative specificity factor for Rubisco
- SSu
small subunit of Rubisco
- V cmax, V omax
maximum rates of Rubisco carboxylation, oxygenation
- *
partial pressure of CO 2 in the chloroplast at which photorespiratory CO 2 evolution equals the rate of carboxylation 相似文献
17.
In leaves of C 3 plants, the rate of nonphotorespiratory respiration appears to be higher in darkness than in the light. This change from a high to a low rate of carbon loss with increasing photon flux density leads to an increase in the apparent quantum yield of photosynthetic CO 2 assimilation at low photon flux densities (Kok effect). The mechanism of this suppression of nonphotorespiratory respiration is not understood, but biochemical evidence and the observation that a Kok effect is often not observed under low O 2, has led to the suggestion that photorespiration might be involved in some way. This hypothesis was tested with snowgum ( Eucalyptus pauciflora Sieb. ex Spreng.) using gas exchange methods. The test was based on the assumption that if photorespiration were involved, then it would be expected that the intercellular partial pressure of CO 2 would also have an influence on the Kok effect. Under normal atmospheric levels of CO 2 and O 2, a Kok effect was found. Changing the intercellular partial pressure of CO 2, however, did not affect the estimate of nonphotorespiratory respiraton, and it was concluded that its decrease with increasing photon flux density did not involve photorespiration. Concurrent measurements showed that the quantum yield of net assimilation of CO 2 increased with increasing intercellular partial pressure of CO 2, and this increase agreed closely with predictions based on recent models of photosynthesis. 相似文献
18.
Mesophyll conductance ( gm) generally correlates with photosynthetic capacity, although the causal relationship between the two is unclear. The response of gm to various CO 2 regimes was measured to determine its relationship to environmental changes that affect photosynthesis. The overall effect of CO 2 growth environment on gm was species and experiment dependent. The data did not statistically differ from the previously shown A– gm relationship and was unaffected by CO 2 treatment. The consequences of the CO 2 effect on gm for interpreting photosynthesis in individual cases were investigated. Substantial effects of assumed versus calculated g m on leaf properties estimated from gas‐exchange measurements were found. This differential error resulted in an underestimation in ratio of maximum carboxylation to electron transport, especially in plants with high photosynthetic capacity. Including gm in the calculations also improved the agreement between maximum carboxylation rates and in vitro Rubisco measurements. It is concluded that gm is finite and varies with photosynthetic capacity. Including gm when calculating photosynthesis parameters from gas‐exchange data will avoid systematic errors. 相似文献
19.
Abstract Some characteristics of photosynthetic inorganic carbon uptake by Palmaria palmata, a marine red macroalga, have been measured under physiological conditions in artificial seawater. The apparent affinity of thallus for CO 2 [K 1/2(CO 2)] at pH 8.0 and 15°C was 21.4±3.0mmol m ?3 CO 2 under air, and 25.7±70mmol m ?3 CO 2 under N 2. The corresponding values of Vmax were 2.98 ± 0.42 and 3.65±0.87 mmol O 2 evolved g Chr ?1 s ?l. The apparent K m(CO 2) of isolated ribulose bisphosphate carboxylase was determined at pH 8.0 and 30 °C to be 30.2 mmol m ?3 CO 2, and the corresponding value of V max was 19.67 μniol CO 2 g protein ?1 s ?1. The CO 2 compensation points of the thallus were measured in artificial seawater at pH 8.0 under air and N 2, using a gas-chromatographic method. The values were relatively low, rising from 10 cm 3 m ?3 at 15°C, to 35 cm 3 m ?3 at 25°C, but were not affected by the O 2 concentration. The lack of an effect of O 2 on photosynthesis and on compensation point indicates that there is little photorespiratory CO 2 loss in this macroalga. The high affinity of the thallus for CO 2, and the low CO 2 compensation concentrations, are consistent with the occurrence of bicarbonate uptake in this alga. 相似文献
20.
Because photosynthetic rates in C 4 plants are the same at normal levels of O 2 ( c, 20 kPa) and at c, 2 kPa O 2 (a conventional test for evaluating photorespiration in C 3 plants) it has been thought that C 4 photosynthesis is O 2 insensitive. However, we have found a dual effect of O 2 on the net rate of CO 2 assimilation among species representing all three C 4 subtypes from both monocots and dicots. The optimum O 2 partial pressure for C 4 photosynthesis at 30 °C, atmospheric CO 2 level, and half full sunlight (1000 μmol quanta m ?2 s ?1) was about 5–10 kPa. Photosynthesis was inhibited by O 2 below or above the optimum partial pressure. Decreasing CO 2 levels from ambient levels (32.6 Pa) to 9.3 Pa caused a substantial increase in the degree of inhibition of photosynthesis by supra-optimum levels of O 2 and a large decrease in the ratio of quantum yield of CO 2 fixation/quantum yield of photosystem II (PSII) measured by chlorophyll a fluorescence. Photosystem II activity, measured from chlorophyll a fluorescence analysis, was not inhibited at levels of O 2 that were above the optimum for CO 2 assimilation, which is consistent with a compensating, alternative electron How as net CO 2 assimilation is inhibited. At suboptimum levels of O 2, however, the inhibition of photosynthesis was paralleled by an inhibition of PSII quantum yield, increased state of reduction of quinone A, and decreased efficiency of open PSII centres. These results with different C 4 types suggest that inhibition of net CO 2 assimilation with increasing O 2 partial pressure above the optimum is associated with photorespiration, and that inhibition below the optimum O 2 may be caused by a reduced supply of ATP to the C 4 cycle as a result of inhibition of its production photochemically. 相似文献
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