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1.
Abstract The acquisition of inorganic carbon for photosynthetic assimilation by leaf mesophyll cells and chloroplasts is discussed with particular reference to membrane permeation of CO 2 and HCO ?3. Experimental evidence indicates that at the apoplast pH normally experienced by leaf mesophyll cells (pH 6–7) CO 2 is the principal species of inorganic carbon taken up. Uptake of HCO ?3 may also occur under certain circumstances (i.e. pH 8.5), but its contribution to the net flux of inorganic carbon is small and HCO ?3 uptake does not function as a CO 2-concentrating mechanism. Similarly, CO 2 rather than HCO ?3 appears to be the species of inorganic carbon which permeates the chloroplast envelope. In contrast to many C 3 aquatic plants and C 4 plants, C 3 terrestrial plants lack specialized mechanisms for the acquisition and transport of inorganic carbon from the intercellular environment to the site of photosynthetic carboxylation, but rely upon the diffusive uptake of CO 2. 相似文献
2.
The net rate of CO 2 uptake for leaves of Gossypium hirsutum L. was reduced when the plants were grown at low concentrations of NO 3-, PO 42-, or K +. The water vapor conductance was relatively constant for all nutrient levels, indicating little effect on stomatal response. Although leaves under nutrient stress tended to be lower in chlorophyll and thinner, the ratio of mesophyll surface area to leaf area did not change appreciably. Thus, the reduction in CO 2 uptake rate at low nutrient levels was due to a decrease in the CO 2 conductance expressed per unit mesophyll cell wall area (g cellCO2). The use of g cellCO2 and nutrient levels expressed per unit of mesophyll cell wall provides a new means of assessing nutrient effects on CO 2 uptake of leaves. 相似文献
3.
Nocturnal CO 2 uptake by a Crassulacean acid metabolism succulent, Agave deserti Engelm. (Agavaceae), was measured so that the resistance properties of the mesophyll chlorenchyma cells and their CO 2 concentrations could be determined. Two equivalents of acidity were produced at night per mole of CO 2 taken up. The nocturnal CO 2 uptake became light-saturated at 3.5 mEinsteins cm −2 of photosynthetically active radiation (400-700 nm) incident during the preceding day; at least 46 Einsteins were required per mole of CO 2 fixed. Variations in the daytime leaf temperature between 20 and 37 C had little effect on nocturnal CO 2 uptake. After the first few hours in the dark, the leaf liquid phase CO 2 resistance (r liqCO2) and the CO 2 concentration in the chlorenchyma cells (c iCO2) both increased, the latter usually reaching the ambient external CO 2 level at the end of the dark period. Increasing the leaf surface temperature above 15 C at night markedly increased the stomatal resistance, r liqCO2, and c iCO2. The minimum rliqCO2 at night was about 1.6 seconds cm−1. Based on the ratio of chlorenchyma surface area to total leaf surface area of 82, this rliqCO2 corresponded to a minimum cellular resistance of approximately 130 seconds cm−1, comparable to values for mesophyll cells of C3 plants. The contribution of the carboxylation reaction and/or other biochemical steps to rliqCO2 may increase appreciably as the nighttime temperature shifts a few degrees from the optimum or after a few hours in the dark, both of which caused large increases in rliqCO2. This necessitates a large internal leaf area for CO2 diffusion into the chlorenchyma to support moderate nocturnal CO2 uptake rates by these succulent leaves. 相似文献
4.
Light microscopic examination of leaf cross-sections showed that Flaveria brownii A. M. Powell exhibits Kranz anatomy, in which distinct, chloroplast-containing bundle sheath cells are surrounded by two types of mesophyll cells. Smaller mesophyll cells containing many chloroplasts are arranged around the bundle sheath cells. Larger, spongy mesophyll cells, having fewer chloroplasts, are located between the smaller mesophyll cells and the epidermis. F. brownii has very low CO 2 compensation points at different O 2 levels, which is typical of C 4 plants, yet it does show about 4% inhibition of net photosynthesis by 21% O 2 at 30°C. Protoplasts of the three photosynthetic leaf cell types were isolated according to relative differences in their buoyant densities. On a chlorophyll basis, the activities of phosphoenolpyruvate carboxylase and pyruvate, Pi dikinase (carboxylation phase of C 4 pathway) were highest in the larger mesophyll protoplasts, intermediate in the smaller mesophyll protoplasts, and lowest, but still present, in the bundle sheath protoplasts. In contrast, activities of ribulose 1,5-bisphosphate carboxylase, other C 3 cycle enzymes, and NADP-malic enzyme showed a reverse gradation, although there were significant activities of these enzymes in mesophyll cells. As indicated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the banding pattern of certain polypeptides of the total soluble proteins from the three cell types also supported the distribution pattern obtained by activity assays of these enzymes. Analysis of initial 14C products in whole leaves and extrapolation of pulse-labeling curves to zero time indicated that about 80% of the CO 2 is fixed into C 4 acids (malate and aspartate), whereas about 20% of the CO 2 directly enters the C 3 cycle. This is consistent with the high activity of enzymes for CO 2 fixation by the C 4 pathway and the substantial activity of enzymes of the C 3 cycle in the mesophyll cells. Therefore, F. brownii appears to have some capacity for C 3 photosynthesis in the mesophyll cells and should be considered a C 4-like species. 相似文献
5.
The appearance of transverse sections of maize leaves indicates the existence of two airspace systems serving the mesophyll, one connected to the stomata of the upper epidermis and the other to the stomata of the lower surface, with few or no connections between the two. This study tests the hypothesis that the air-space systems of the upper and lower mesophyll are separated by a defined barrier of measurable conductance. A mathematical procedure, based on this hypothesis, is developed for the quantitative separation of the contributions made by the upper and lower halves of the mesophyll to carbon assimilation using gasexchange data. Serial paradermal sections and three-dimensional scanning-electron-microscope images confirmed the hypothesis that there were few connections between the two air-systems. Simultaneous measurements of nitrous-oxide diffusion across the leaf and of transpiration from the two surfaces showed that the internal conductance was about 15% of the maximum observed stomatal conductance. This demonstrates that the poor air-space connections, indicated by microscopy, represent a substantial barrier to gas diffusion. By measuring the CO 2 and water-vapour fluxes from each surface independently, the intercellular CO 2 concentration ( c
i) of each internal air-space system was determined and the flux between them calculated. This allowed correction of the apparent CO 2 uptake at each surface to derive the true CO 2 uptake by the mesophyll cells of the upper and lower halves of the leaf. This approach was used to analyse the contribution of the upper and lower mesophyll to CO 2 uptake by the leaf as a whole in response to varying light levels incident on the upper leaf surface. This showed that the upper mesophyll was light-saturated by a photon flux of approx. 1000 mol·m -2·s -1 (i.e. about one-half of full sunlight). The lower mesophyll was not fully saturated by photon fluxes of nearly double full sunlight. At low photon fluxes the c
i of the upper mesophyll was significantly less than that of the lower mesophyll, generating a significant upward flux of CO 2. At light levels equivalent to full sunlight, and above, c
i did not differ significantly between the two air space systems. The physiological importance of the separation of the air-space systems of the upper and lower mesophyll to gas exchange is discussed.Abbreviations and symbols
A
net leaf CO 2 uptake rate
-
A
upper
app.
and A
lower
app.
net rates of CO 2 uptake across the upper and lower surfaces
-
A
upper and A
lower
derived net rates of CO 2 uptake by the upper and lower mesophyll
-
A
upward
net flux of CO 2 from the lower to upper mesophyll
-
c
a, c
a, upper and c
a, lower
the CO 2 concentrations in the air around the leaf above the upper surface and below the lower surface
-
c
N2O
the concentration of N 2O in the air around the leaf
-
c
i, c
i, upper and c
i, lower
the mesophyll intercellular CO 2 concentration of the whole leaf, the upper mesophyll and the lower mesophyll
-
g
i
leaf internal conductance to CO 2
-
g
s, g
s, lower and g
s, upper
the stomatal conductance of the whole leaf, the lower surface and the upper surface
-
g
the total conductance across the leaf
-
Q
the photosynthetically active photon flux density 相似文献
6.
Engineering C 4 photosynthesis into rice has been considered a promising strategy to increase photosynthesis and yield. A question that remains to be answered is whether expressing a C 4 metabolic cycle into a C 3 leaf structure and without removing the C 3 background metabolism improves photosynthetic efficiency. To explore this question, we developed a 3D reaction diffusion model of bundle‐sheath and connected mesophyll cells in a C 3 rice leaf. Our results show that integrating a C 4 metabolic pathway into rice leaves with a C 3 metabolism and mesophyll structure may lead to an improved photosynthesis under current ambient CO 2 concentration. We analysed a number of physiological factors that influence the CO 2 uptake rate, which include the chloroplast surface area exposed to intercellular air space, bundle‐sheath cell wall thickness, bundle‐sheath chloroplast envelope permeability, Rubisco concentration and the energy partitioning between C 3 and C 4 cycles. Among these, partitioning of energy between C 3 and C 4 photosynthesis and the partitioning of Rubisco between mesophyll and bundle‐sheath cells are decisive factors controlling photosynthetic efficiency in an engineered C 3–C 4 leaf. The implications of the results for the sequence of C 4 evolution are also discussed. 相似文献
7.
Haberlea rhodopensis is a homoiochlorophyllous resurrection plant that shows a low rate of leaf net CO 2 uptake (4–6 μmol m ?2 s ?1) under saturating photosynthetic photon flux densities in air (21% O 2 and about 390 ppm CO 2). However, leaf net CO 2 uptake reaches values of 17–18 μmol m ?2 s ?1 under saturating CO 2 and light. H. rhodopensis leaves have a very low mesophyll CO 2 conductance that can partly explain the low rate of leaf net CO 2 uptake in normal air. Experimental evidences suggest that mesophyll conductance is not sensitive to temperature in the 20–35 °C range. In addition, it is shown that the (1) transpiration rate of H. rhodopensis is nearly linearly related to the vapour pressure difference between the leaf and the ambient air within the interval from 0.5 kPa to 2.5 kPa at a leaf temperature of 25 °C and (2) leaf net CO 2 uptake in normal air under saturating light does not change much with leaf temperature (between 20 °C and 30 °C). At a leaf relative water content of between 90% and 30%, the decrease of leaf net CO 2 assimilation during drought can be explained by a decrease of leaf CO 2 diffusional conductance. Accordingly the non-photochemical chlorophyll fluorescence quenching decreases only at relative water contents lower than 20%, indicating that photosynthetic activity maintains a trans-thylakoidal proton gradient over a wide range of leaf water contents. Moreover, PSII photochemistry (as estimated by the Fv/Fm ratio and the thermoluminescence B band intensity) is only affected at leaf relative water contents lower than about 20%, thus confirming that primary photosynthetic reactions are resistant to drought. Interestingly, the effect of leaf desiccation on photosynthetic capacity, measured at very high ambient CO 2 molar ratios under saturating PPFD, is identical to that observed for three non-resurrection C 3 mesophytes. This demonstrates that the photosynthetic apparatus of H. rhodopensis is not more resistant to desiccation when compared to other C 3 plants. Since the leaf area decreases by more than 50% when the leaf relative water content is reduced to about 40% during drought it is supposed, following Farrant et al. [Farrant, J.M., Vander, W.C., Lofell, D.A., Bartsch, S., Whittaker, A., 2003. An investigation into the role of light during desiccation of three angiosperms resurrection plants. Plant Cell Environ. 26, 1275–1286], that H. rhodopensis leaf cells avoid mechanical stress. 相似文献
8.
Increasing salinity led to substantially higher ratios of mesophyll surface area to leaf area (A mes/A) for Phaseolus vulgaris and Gossypium hirsutum and a smaller increase for Atriplex patula, a salt-tolerant species. The increase in internal surface for CO 2 absorption did not lead to higher CO 2 uptake rates, since the CO 2 resistance expressed on the basis of mesophyll cell wall area (r cell) increased even more with salinity. The differences among species in the sensitivity of photosynthesis to salinity in part reflect the different A mes/A and r cell responses. 相似文献
9.
The potential for glycolate and glycine metabolism and the mechanism of refixation of photorespiratory CO 2 in leaves of C 4 plants were studied by parallel inhibitor experiments with thin leaf slices, different leaf cell types and isolated mitochondria of C 3 and C 4
Panicum species. CO 2 evolution by leaf slices of P. bisulcatum, a C 3 species, fed glycolate or glycine was light-independent and O 2-sensitive. The C 4
P. maximum and P. miliaceum leaf slices fed glycolate or glycine evolved CO 2 in the dark but not in the light. In C 4 species, dark CO 2 evolution was abolished by the addition of phosphoenolpyruvate (PEP) 4. The addition of maleate, a PEP carboxylase inhibitor, resulted in photorespiratory CO 2 efflux by C 4 leaf slices in the light also. However, PEP and maleate had no effect on either glycolate-dependent O 2 uptake by the C 4 leaf slices or on glycolate and glycine metabolism in C 3 leaf slices. The rate of photorespiratory CO 2 evolution in the C 3
Panicum species was 3 times higher than that observed with the C 4 species. The ratio of glycolate-dependent CO 2 evolution to O 2 uptake in both groups was 1:2. Isolated C 4 mesophyll protoplasts or their mitochondria did not metabolize glycolate or glycine. However, both C 3 mesophyll protoplasts and C 4 bundle sheath strands readily metabolized glycolate and glycine in a light-independent, O 2-sensitive manner, and the addition of PEP or maleate had no effect. C 4 bundle sheath- and C 3-mitochondria were capable of oxidizing glycine. This oxidation was linked to the mitochondrial electron transport chain, was coupled to three phosphorylation sites and was sensitive to electron transport inhibitors. C 4 bundle sheath- and C 3-mitochondrial glycine decarboxylation was stimulated by oxaloacetate and NAD had no effect. In marked contrast, mitochondria isolated from C 4 mesophyll cells were incapable of oxidizing or decarboxylating added glycine. The results suggest that in leaves of C 4 plants bundle sheath cells are the primary site of O 2-sensitive photorespiratory CO 2 evolution and the PEP carboxylase present in the mesophyll cells has the Potential for efficiently refixing CO 2 before it escapes out of the leaf. The relative role of the PEP carboxylase mediated CO 2 pump and reassimilation of photorespiratory CO 2 are discussed in relation to the apparent lack of photorespiration in leaves of C 4 species.Abbreviations BSA
bovine serum albumin
- Chl
chlorophyll
- PEP
phosphoenolpyruvate
- Rbu- P
2
ribulose 1,5-bisphosphate
- Rib-5-P
ribose-5-phosphate
- Ru-5-P
ribuluse-5-phosphate
- FCCP
carbonyl cyanide p-trifluoromethoxyphenylhydrazone
Journal Series Paper, New Jersey Agricultural Experiment Station 相似文献
10.
Mesophyll cells and bundle sheath strands were isolated from Cyperus rotundus L. leaf sections infiltrated with a mixture of cellulase and pectinase followed by a gentle mortar and pestle grind. The leaf suspension was filtered through a filter assembly and mesophyll cells and bundle sheath strands were collected on 20-μm and 80-μm nylon nets, respectively. For the isolation of leaf epidermal strips longer leaf cross sections were incubated with the enzymes and gently ground as above. Loosely attached epidermal strips were peeled off with forceps. The upper epidermis, which lacks stomata, could be clearly distinguished from the lower epidermis which contains stomata. Microscopic evidence for identification and assessment of purity is provided for each isolated tissue.Enzymes related to the C 4-dicarboxylic acid cycle such as phosphoenolpyruvate carboxylase, malate dehydrogenase (NADP +), pyruvate, P i dikinase were found to be localized, ≥98%, in mesophyll cells. Enzymes related to operating the reductive pentose phosphate cycle such as RuDP carboxylase, phosphoribulose kinase, and malic enzyme are distributed, ≥99%, in bundle sheath strands. Other photosynthetic enzymes such as aspartate aminotransferase, pyrophosphatase, adenylate kinase, and glyceraldehyde 3- P dehydrogenase (NADP +) are quite active in both mesophyll and bundle sheath tissues.Enzymes involved in photorespiration such as RuDP oxygenase, catalase, glycolate oxidase, hydroxypyruvate reductase (NAD +), and phosphoglycolate phosphatase are preferentially localized, ≥84%, in bundle sheath strands.Nitrate and nitrite reductase can be found only in mesophyll cells, while glutamate dehydrogenase is present, ≥96%, in bundle sheath strands.Starch- and sucrose-synthesizing enzymes are about equally distributed between the mesophyll and bundle sheath tissues, except that the less active phosphorylase was found mainly in bundle sheath strands. Fructose-1,6-diP aldolase, which is a key enzyme in photosynthesis and glycolysis leading to sucrose and starch synthesis, is localized, ≥90%, in bundle sheath strands. The glycolytic enzymes, phosphoglyceromutase and enolase, have the highest activity in mesophyll cells, while the mitochondrial enzyme, cytochrome c oxidase, is more active in bundle sheath strands.The distribution of total nutsedge leaf chlorophyll, protein, and PEP carboxylase activity, using the resolved leaf components, is presented. 14CO 2 Fixation experiments with the intact nutsedge leaves and isolated mesophyll and bundle sheath tissues show that complete C 4 photosynthesis is compartmentalized into mesophyll CO 2 fixation via PEP carboxylase and bundle sheath CO 2 fixation via RuDP carboxylase. These results were used to support the proposed pathway of carbon assimilation in C 4-dicarboxylic acid photosynthesis and to discuss the individual metabolic characteristics of intact mesophyll cells, bundle sheath cells, and epidermal tissues. 相似文献
11.
Leaves of twelve C 3 species and six C 4 species were examined to understand better the relationship between mesophyll cell properties and the generally high photosynthetic rates of these plants. The CO 2 diffusion conductance expressed per unit mesophyll cell surface area (g CO2cell) cell was determined using measurements of the net rate of CO 2 uptake, water vapor conductance, and the ratio of mesophyll cell surface area to leaf surface area (A mes/A). A mes/A averaged 31 for the C 3 species and 16 for the C 4 species. For the C 3 species g CO2cell ranged from 0.12 to 0.32 mm s -1, and for the C 4 species it ranged from 0.55 to 1.5 mm s -1, exceeding a previously predicted maximum of 0.5 mm s -1. Although the C 3 species Cammissonia claviformis did not have the highest g CO2cell, the combination of the highest A mes and highest stomatal conductance resulted in this species having the greatest maximum rate of CO 2 uptake in low oxygen, 93 μmol m -2 s -1 (147 mg dm -2 h -1). The high g CO2cell of the C 4 species Amaranthus retroflexus (1.5 mm s -1) was in part attributable to its thin cell wall (72 nm thick). 相似文献
12.
A ) depend not only on photosynthetic biochemistry but also on mesophyll structure. Because resistance to CO 2 diffusion from the substomatal cavity to the stroma is substantial, it is likely that mesophyll structure affects A through affecting diffusion of CO 2 in the leaf. To evaluate effects of various aspects of mesophyll structure on photosynthesis, we constructed a one-dimensional
model of CO 2 diffusion in the leaf. When mesophyll thickness of the leaf is changed with the Rubisco content per unit leaf area kept constant,
the maximum A occurs at an almost identical mesophyll thickness irrespective of the Rubisco contents per leaf area. On the other hand,
with an increase in Rubisco content per leaf area, the mesophyll thickness that realizes a given photosynthetic gain per mesophyll
thickness (or per leaf cost) increases. This probably explains the strong relationship between A and mesephyll thickness.
In these simulations, an increase in mesophyll thickness simultaneously means an increase in the diffusional resistance in
the intercellular spaces ( R
ias), an increase in the total surface area of chloroplasts facing the intercellular spaces per unit leaf area ( S
c
), and an increase in construction and maintenance cost of the leaf. Leaves can increase S
c
and decrease R
ias
also by decreasing cell size. Leaves with smaller cells are mechanically stronger. However, actual leaves do not have very
small cells. This could be because actual leaves exhibiting considerable rates of leaf area expansion, adequate heat capacitance,
high efficiency of N and/or P use, etc, are favoured. Relationships between leaf longevity and mesophyll structure are also
discussed.
Received 20 September 2000/ Accepted in revised form 4 January 2001 相似文献
13.
Cuttings of Populus cathayana were exposed to three different alkaline regimes (0, 75, and 150 mM Na 2CO 3) in a semicontrolled environment. The net photosynthesis rate ( P N), mesophyll conductance ( g m), the relative limitations posed by stomatal conductance ( L s) and by mesophyll conductance ( L m), photosynthetic nitrogen-use efficiency (PNUE), carbon isotope composition (δ 13C), as well as specific leaf area (SLA) were measured. P N decreased due to alkaline stress by an average of 25% and g m decreased by an average of 57%. Alkaline stress caused an increase of L m but not L s, with average L s of 26%, and L m average of 38% under stress conditions. Our results suggested reduced assimilation rate under alkaline stress through decreased mesophyll conductance in P. cathayana. Moreover, alkaline stress increased significantly δ 13C and it drew down CO 2 concentration from the substomatal cavities to the sites of carboxylation ( C i- C c), but decreased PNUE. Furthermore, a relationship was found between PNUE and C i- C c. Meanwhile, no correlation was found between δ 13C and C i/ C a, but a strong correlation was proved between δ 13C and C c/ C a, indicating that mesophyll conductance was also influencing the 13C/ 12C ratio of leaf under alkaline stress. 相似文献
14.
Mature leaves of Cyperus rotundus, Cyperus polystachyos, Digitaria decumbens, and Digitaria sanguinalis were separated, using pectinase and cellulase, into pure preparations of mesophyll cells and bundle sheath strands. Assays on these distinct leaf cell types show a clear compartmentation of phosphoenolpyruvate carboxylase, >98%, into mesophyll cells and of ribulose-1, 5-diphosphate carboxylase and malic enzyme, >98%, into the bundle sheath strands. The results clearly establish that the major CO 2 uptake in mesophyll cells is via a β-carboxylation and that both a decarboxylation and a carboxylation reaction occurs in the bundle sheath strands of plants using C 4-dicarboxylic acid photosynthesis. 相似文献
15.
The use of mesophyll protoplast extracts from various C 4 species has provided an effective method for studying light-and substrate-dependent formation of oxaloacetate, malate, and asparate at rates equivalent to whole leaf C 4 photosynthesis. Conditions regulating the formation of the C 4 acids were studied with protoplast extracts from Digitaria sanguinalis, an NADP-malic enzyme C 4 species, Eleusineindica, an NAD-malic enzyme C 4 species, and Urochloa panicoides, a phosphoenolpyruvate (PEP) carboxykinase C 4 species. Light-dependent induction of CO 2 fixation by the mesophyll extracts of all three species was relatively low without addition of exogenous substrates. Pyruvate, alanine and α-ketoglutarate, or 3-phosphoglycerate induced high rates of CO 2 fixation in the mesophyll extracts with oxaloacetate, malate, and aspartate being the primary products. In all three species, it appears that pyruvate, alanine, or 3-phosphoglycerate may serve as effective precursors to the formation of PEP for carboxylation through PEP-carboxylase in C 4 mesophyll cells. Induction by pyruvate or alanine and α-ketoglutarate was light-dependent, whereas 3-phosphoglycerate-induced CO 2 fixation was not. 相似文献
16.
There are numerous studies describing how growth conditions influence the efficiency of C 4 photosynthesis. However, it remains unclear how changes in the biochemical capacity versus leaf anatomy drives this acclimation. Therefore, the aim of this study was to determine how growth light and nitrogen availability influence leaf anatomy, biochemistry and the efficiency of the CO 2 concentrating mechanism in Miscanthus × giganteus. There was an increase in the mesophyll cell wall surface area but not cell well thickness in the high-light (HL) compared to the low-light (LL) grown plants suggesting a higher mesophyll conductance in the HL plants, which also had greater photosynthetic capacity. Additionally, the HL plants had greater surface area and thickness of bundle-sheath cell walls compared to LL plants, suggesting limited differences in bundle-sheath CO 2 conductance because the increased area was offset by thicker cell walls. The gas exchange estimates of phospho enolpyruvate carboxylase (PEPc) activity were significantly less than the in vitro PEPc activity, suggesting limited substrate availability in the leaf due to low mesophyll CO 2 conductance. Finally, leakiness was similar across all growth conditions and generally did not change under the different measurement light conditions. However, differences in the stable isotope composition of leaf material did not correlate with leakiness indicating that dry matter isotope measurements are not a good proxy for leakiness. Taken together, these data suggest that the CO 2 concentrating mechanism in Miscanthus is robust under low-light and limited nitrogen growth conditions, and that the observed changes in leaf anatomy and biochemistry likely help to maintain this efficiency. 相似文献
17.
Increasing photosynthetic photon flux density (PPFD) received during development from 5.5 to 31.2 mol m -2 d -1 resulted in greater leaf and mesophyll cell surface areas in cotton ( Gossypium hirsutum L.). The relationships between the amounts of these surface areas and potential CO 2 assimilation by these leaves were evaluated. Leaf area (epidermal surface area of one side of a leaf), mesophyll cell surface area, and net rate of CO 2 uptake (P n) were measured from the time leaves first unfolded until P., was substantially reduced. At the higher PPFD, leaf and mesophyll surface areas increased more rapidly during expansion, and P n per unit leaf area was greater than at the lower PPFD. Although leaves at the higher PPFD reached the maximum P., per unit mesophyll cell surface area 4 to 5 days earlier than leaves at the lower PPFD, the maxima for these P., were similar. Leaves grown at the higher PPFD had the potential to assimilate 2.2, 3.5, or 5.8 times the amount of CO 2 as leaves from the lower PPFD when P., was expressed per unit mesophyll surface, per unit leaf surface, or per whole leaf, respectively. Greater and earlier development of both P., and mesophyll cell surface area at higher PPFD apparently had a compounding effect on the potential for carbon assimilation by a leaf. 相似文献
18.
The resistance to diffusion of CO 2 from the intercellular airspaces within the leaf through the mesophyll to the sites of carboxylation during photosynthesis was measured using three different techniques. The three techniques include a method based on discrimination against the heavy stable isotope of carbon, 13C, and two modeling methods. The methods rely upon different assumptions, but the estimates of mesophyll conductance were similar with all three methods. The mesophyll conductance of leaves from a number of species was about 1.4 times the stomatal conductance for CO 2 diffusion determined in unstressed plants at high light. The relatively low CO 2 partial pressure inside chloroplasts of plants with a low mesophyll conductance did not lead to enhanced O 2 sensitivity of photosynthesis because the low conductance caused a significant drop in the chloroplast CO 2 partial pressure upon switching to low O 2. We found no correlation between mesophyll conductance and the ratio of internal leaf area to leaf surface area and only a weak correlation between mesophyll conductance and the proportion of leaf volume occupied by air. Mesophyll conductance was independent of CO 2 and O 2 partial pressure during the measurement, indicating that a true physical parameter, independent of biochemical effects, was being measured. No evidence for CO 2-accumulating mechanisms was found. Some plants, notably Citrus aurantium and Simmondsia chinensis, had very low conductances that limit the rate of photosynthesis these plants can attain at atmospheric CO 2 level. 相似文献
19.
A study was conducted on a C 4 ( Panicum maximum) and a C 3 ( Panicum bisulcatum) species to determine the nature of the dark release of 14CO 2 with respect to its responses to changes in temperature and O 2 tension during light CO 2 uptake of 14CO 2. 相似文献
20.
We studied plants of five species with hypostomatous leaves, and six with amphistomatous leaves, to determine the extent to which gaseous diffusion of CO 2 among the mesophyll cells limits photosynthetic carbon assimilation. In helox (air with nitrogen replaced by helium), the diffusivities of CO 2 and water vapor are 2.3 times higher than in air. For fixed estimated CO 2 pressure at the evaporating surfaces of the leaf ( pi), assimilation rates in helox ranged up to 27% higher than in air for the hypostomatous leaves, and up to 7% higher in the amphistomatous ones. Thus, intercellular diffusion must be considered as one of the processes limiting photosynthesis, especially for hypostomatous leaves. A corollary is that CO 2 pressure should not be treated as uniform through the mesophyll in many leaves. To analyze our helox data, we had to reformulate the usual gas-exchange equation used to estimate CO 2 pressure at the evaporating surfaces of the leaf; the new equation is applicable to any gas mixture for which the diffusivities of CO 2 and H 2O are known. Finally, we describe a diffusion-biochemistry model for CO 2 assimilation that demonstrates the plausibility of our experimental results. 相似文献
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