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  • 1
    Electronic Resource
    Electronic Resource
    Photosynthetic oxygen exchange in C4 grasses: the role of oxygen as electron acceptor (2003)
    Oxford, UK : Blackwell Science Ltd
    Plant, cell & environment 26 (2003), S. 0 
    Details
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: C4 grasses of the NAD-ME type (Astrebla lappacea, Eleusine coracana, Eragrostis superba, Leptochloa dubia, Panicum coloratum, Panicum decompositum) and the NADP-ME type (Bothriochloa bladhii, Cenchrus ciliaris, Dichanthium sericeum, Panicum antidotale, Paspalum notatum, Pennisetum alopecuroides, Sorghum bicolor) were used to investigate the role of O2 as an electron acceptor during C4 photosynthesis. Mass spectrometric measurements of gross O2 evolution and uptake were made concurrently with measurements of net CO2 uptake and chlorophyll fluorescence at different irradiances and leaf temperatures of 30 and 40 °C. In all C4 grasses gross O2 uptake increased with increasing irradiance at very high CO2 partial pressures (pCO2) and was on average 18% of gross O2 evolution. Gross O2 uptake at high irradiance and high pCO2 was on average 3.8 times greater than gross O2 uptake in the dark. Furthermore, gross O2 uptake in the light increased with O2 concentration at both high CO2 and the compensation point, whereas gross O2 uptake in the dark was insensitive to O2 concentration. This suggests that a significant amount of O2 uptake may be associated with the Mehler reaction, and that the Mehler reaction varies with irradiance and O2 concentration. O2 exchange characteristics at high pCO2 were similar for NAD-ME and NADP-ME species. NAD-ME species had significantly greater O2 uptake and evolution at the compensation point particularly at low irradiance compared to NADP-ME species, which could be related to different rates of photorespiratory O2 uptake. There was a good correlation between electron transport rates estimated from chlorophyll fluorescence and gross O2 evolution at high light and high pCO2.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-3040.2003.01112.x
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  • 2
    Electronic Resource
    Electronic Resource
    The photosynthesis of young Panicum C4 leaves is not C3-like (1998)
    Oxford, UK : Blackwell Publishing Ltd
    Plant, cell & environment 21 (1998), S. 0 
    Details
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Evidence is presented contrary to the suggestion that C4 plants grow larger at elevated CO2 because the C4 pathway of young C4 leaves has C3-like characteristics, making their photosynthesis O2 sensitive and responsive to high CO2. We combined PAM fluorescence with gas exchange measurements to examine the O2 dependence of photosynthesis in young and mature leaves of Panicum antidotale (C4, NADP-ME) and P. coloratum (C4, NAD-ME), at an intercellular CO2 concentration of 5 Pa. P. laxum (C3) was used for comparison. The young C4 leaves had CO2 and light response curves typical of C4 photosynthesis. When the O2 concentration was gradually increased between 2 and 40%, CO2 assimilation rates (A) of both mature and young C4 leaves were little affected, while the ratio of the quantum yield of photosystem II to that of CO2 assimilation (ΦPSII/ΦCO2) increased more in young (up to 31%) than mature (up to 10%) C4 leaves. A of C3 leaves decreased by 1·3 and ΦPSII/ΦCO2 increased by 9-fold, over the same range of O2 concentrations. Larger increases in electron transport requirements in young, relative to mature, C4 leaves at low CO2 are indicative of greater O2 sensitivity of photorespiration. Photosynthesis modelling showed that young C4 leaves have lower bundle sheath CO2 concentration, brought about by higher bundle sheath conductance relative to the activity of the C4 and C3 cycles and/or lower ratio of activities of the C4 to C3 cycles.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-3040.1998.00348.x
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  • 3
    Electronic Resource
    Electronic Resource
    Spatial patterning of pigmentation in evergreen leaves in response to freezing stress (2003)
    Oxford, UK : Blackwell Science Ltd
    Plant, cell & environment 26 (2003), S. 0 
    Details
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Evergreen leaves of temperate climate plants are often subject to frosts. Changes in carbon gain patterns arise from freezing-related tissue damage, and from interactions between light and temperature stress. We examined relationships between spatial patterns in freezing and concentrations of chlorophyll. Spatial patterns in pigmentation in leaves that had or had not been exposed to naturally occurring frosts were determined by conventional extraction techniques combined with high-resolution hyperspectral imaging of reflectance from intact leaves. Predictive indices were developed to relate reflectance to chlorophyll content and chlorophyll a/b ratios within intact leaves. Leaves exposed to frosts had lower chlorophyll contents and more variable a/b ratios than protected leaves. In frost-affected leaves, chlorophyll content was highest near leaf centres and decreased toward leaf tips and margins. Decline in chlorophyll content was associated with shifts in chlorophyll a/b ratios and increases in red pigmentation due to anthocyanin, with effects being greater on leaf sides exposed directly to the sun. These altered pigmentation patterns were consistent with patterns in freezing. The present results illustrate the fine scale of spatial variation in leaf response to freezing, and raise important questions about impacts of freezing on photosynthetic function in over-wintering evergreens.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-3040.2003.01106.x
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  • 4
    Electronic Resource
    Electronic Resource
    Control of photosynthesis in leaves as revealed by rapid gas exchange and measurements of the assimilatory force FA (1990)
    Springer
    Planta 182 (1990), S. 513-522 
    Details
    ISSN: 1432-2048
    Keywords: Assimilatory force ; Enzyme regulation ; Gas exchange ; Helianthus (photosynthesis) ; Photosynthesis ; Proton gradient ; Stroma alkalization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The rapid transients of CO2 gas exchange have been measured in leaves ofHelianthus annuus L. In parallel experiments the assimilatory force FA, which is the product of the phosphorylation potential and the redox ratio NADPH/NADP, has been calculated from measured ratios of dihydroxyacetone phosphate to phosphoglycerate in the chloroplast stroma and in leaves. The following results were obtained: (i) When the light-dependent stroma alkalization was measured under steady-state conditions for photosynthesis in air containing 2000 μl · l-1 CO2, alkalization increased with photosynthesis as the quantum flux density (irradiance) was increased. This contrasts to the light-dependent stroma alkalisation measured in dark-adapted leaves during the dark-light transient (Laisk et al. 1989, Planta177, 350–358) which reached a maximum at a quantum flux density far below that necessary to saturate photosynthesis. This maximum was about three times higher than the maximum stroma alkalization at light- and CO2-saturated photosynthesis. (ii) Accurate calculations of the assimilatory force FA require a consideration of the stromal pH. However, under many conditions, changes in the stromal pH resulting from changes in photosynthetic flux can be neglected because they are small. (iii) Stromal ratios of dihydroxyacetone phosphate to phosphoglycerate are generally lower than ratios measured in leaf extracts. The value of FA calculated from stromal metabolites was about 30% lower than FA calculated from cellular metabolites. Still, it appears sufficient for many purposes to calculate FA from metabolite measurements in leaf extracts. (iv) In the light, the catalytic capacity of the photosynthetic apparatus is adjusted to the level of irradiance. The response of carbon assimilation to large increases in irradiance is slow because it requires enzyme activation. Deactivation of the Calvin cycle induced by decreases in irradiance is slower than activation. (v) Changes in catalytic capacity and in the availability or level of substrates such as CO2 alter the flux resistance of the Calvin cycle. A decrease in flux resistance explains why FA often does not increase by much and may actually decrease when carbon flux is increased. Adjustments of flux resistances in the Calvin cycle and of photosystem-II activity in the electron-transport chain permit varying rates of photosynthesis at low levels of ATP and NADPH. As NADP remains available, the danger of over-reduction which leads to photoinactivation of electron transport is minimized.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02341026
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