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  • 1
    Electronic Resource
    Electronic Resource
    Intraspecific variation in photosynthetic responses of trebouxioid lichens with reference to the activity of a carbon-concentrating mechanism (1998)
    Springer
    Oecologia 113 (1998), S. 360-369 
    Details
    ISSN: 1432-1939
    Keywords: Key words Lichen ; Phycobiont ; Photosynthesis ; Carbon-concentrating mechanism ; Carbon isotope discrimination
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The photosynthetic responses of a range of trebouxioid lichens were investigated to determine whether variations in net assimilation rates shown by populations of the same species collected from different habitats could be correlated with adjustments in carbon-concentrating mechanism (CCM) activity. The activity of a CCM was inferred from the high affinity for CO2 [i.e. low CO2 compensation point (Γ); low external CO2 concentration at which half-maximal assimilation rates are reached (K 0.5 CO2)], the release of a pool of accumulated dissolved inorganic carbon (Ci) during light/dark transient measurements of CO2 exchange and values for carbon isotope discrimination intermediate between those characteristic of C3 and C4 terrestrial plants. Higher net and gross assimilation rates were expressed by lichens collected from shaded woodland habitats. The higher rates were not accounted for by variations in chlorophyll content. Lichens with high assimilation rates also showed an increased affinity for CO2 as demonstrated by low CO2 compensation points and K 0.5 values and the magnitude of the Ci pool accumulated upon illumination and released after darkening of the thalli. However, there was no correlation between assimilation rates and organic matter or instantaneous carbon isotope discrimination measurements, with the latter remaining roughly consistent whatever the provenance or species of the lichen material. The data are discussed with reference to significant environmental factors which are likely to control photosynthesis in the habitats studied.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004420050387
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  • 2
    Electronic Resource
    Electronic Resource
    Determinants of isotopic coupling of CO2 and water vapour within a Quercus petraea forest canopy (1999)
    Springer
    Oecologia 119 (1999), S. 109-119 
    Details
    ISSN: 1432-1939
    Keywords: Key words Stable isotopes ; Carbon ; CO2 ; Water vapour ; Forest canopies
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Concentration and isotopic composition (δ13C and δ18O) of ambient CO2 and water vapour were determined within a Quercus petraea canopy, Northumberland, UK. From continuous measurements made across a 36-h period from three heights within the forest canopy, we generated mixing lines (Keeling plots) for δa 13CO2, δa C18O16O and δa H2 18O, to derive the isotopic composition of the signal being released from forest to atmosphere. These were compared directly with measurements of different respective pools within the forest system, i.e. δ13C of organic matter input for δa 13CO2, δ18O of exchangeable water for δa C18O16O and transpired water vapour for δa H2 18O. [CO2] and δa 13CO2 showed strong coupling, where the released CO2 was, on average, 4 per mil enriched compared to the organic matter of plant material in the system, suggesting either fractionation of organic material before eventual release as soil-respired CO2, or temporal differences in ecosystem discrimination. δa C18O16O was less well coupled to [CO2], probably due to the heterogeneity and transient nature of water pools (soil, leaf and moss) within the forest. Similarly, δa H2 18O was less coupled to [H2O], again reflecting the transient nature of water transpired to the forest, seen as uncoupling during times of large changes in vapour pressure deficit. The δ18O of transpired water vapour, inferred from both mixing lines at the canopy scale and direct measurement at the leaf level, approximated that of source water, confirming that an isotopic steady state held for the forest integrated over the daily cycle. This demonstrates that isotopic coupling of CO2 and water vapour within a forest canopy will depend on absolute differences in the isotopic composition of the respective pools involved in exchange and on the stability of each of these pools with time.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004420050766
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  • 3
    Electronic Resource
    Electronic Resource
    Short-term changes in carbon-isotope discrimination in the C3-CAM intermediate Clusia minor L. growing in Trinidad (1993)
    Springer
    Oecologia 95 (1993), S. 444-453 
    Details
    ISSN: 1432-1939
    Keywords: Clusia minor ; Carbon isotope discrimination ; Crassulacean acid metabolism ; Carboxylation pathway
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract On-line instantaneous carbon isotope discrimination was measured in conjunction with net uptake of CO2 in leaves of exposed and shaded plants of the C3-CAM intermediate Clusia minor growing under natural conditions in Trinidad. At the end of the rainy season (late January-early February, 1992) C3 photosynthesis predominated although exposed leaves recaptured a small proportion of respiratory CO2 at night for the synthesis of malic acid. Citric acid was the major organic acid accumulated by exposed leaves at this time with a citric: malic acid ratio of 11:1. Values of instantaneous discrimination (Δ) in exposed leaves during the wet season rose from 17.1‰ shortly after dawn to 22.7‰ around mid-day just before stomata closed, suggesting that most CO2 was fixed by Rubisco at this time. During the late afternoon, instantaneous Δ declined from 22.2‰ to 17‰, probably reflecting the limited contribution from PEPc activity and an increase in diffusional resistance to CO2 in exposed leaves. Shaded leaves showed no CAM activity and CO2 uptake proceeded throughout the day in the wet season. The decrease in instantaneous Δ from 27‰ in the morning to 19.2‰ in the late afternoon was therefore entirely due to diffusional limitation. Leaves sampled in the dry season (mid-March, 1992) had by now induced full CAM activity with both malic and citric acids accumulated overnight and stomata closed for 4–5 h over the middle of the day. Values of instantaneous Δ measured over the first 3 h after dawn (6.4–9.1‰) indicated that C4 carboxylation dominated CO2 uptake for most of the morning when rates of photosynthesis were maximal, implying that under natural conditions, the down regulation of PEPc in phase II occurs much more slowly than laboratory-based studies have suggested. The contribution from C3 carboxylation to CO2 uptake during phase II was most marked in leaves which accumulated lower quantities of organic acids overnight. In exposed leaves, measurements of instantaneous Δ during the late afternoon illustrated the transition from C3 to C4 carboxylation with stomata remaining open during the transition from dusk into the dark period. Uptake of CO2 by shaded leaves during the late afternoon however appeared to be predominantly limited by decreased stomatal conductance. The short-term measurements of instantaneous Δ were subsequently integrated over 24 h in order to predict the leaf carbon isotope ratios (δp) and to compare this with the δp measured for leaf organic material. Whilst there was close agreement between predicted and measured δp for plants sampled in the wet season, during the dry season the predicted carbon isotope ratios were 5–9‰ higher than the measured isotope ratios. During the annual cycle of leaf growth most carbon was fixed via the C3 pathway although CAM clearly plays an important role in maintaining photochemical integrity in the dry season.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00321001
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  • 4
    Electronic Resource
    Electronic Resource
    Gas exchange and carbon isotope discrimination in lichens: Evidence for interactions between CO2-concentrating mechanisms and diffusion limitation (1995)
    Springer
    Planta 196 (1995), S. 95-102 
    Details
    ISSN: 1432-2048
    Keywords: Carbon isotope discrimination ; Carbon dioxide concentrating mechanism ; Lichen (photosynthesis) ; Photosynthesis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The characteristics of gas exchange and carbon isotope discrimination were determined for a number of lichen species, representing contrasting associations between fungal (mycobiont) and photosynthetic (photobiont) organism. These parameters were evaluated with regard to the occurrence of any CO2-concentrating mechanism (CCM) expressed specifically by the green algal (phycobiont) or cyanobacterial (cyanobiont) partner. Carbon isotope discrimination (Δ) fell into three categories. The highest Δ, found in lichens comprising a phycobiont plus cyanobacteria limited to pockets in the thallus (known as cephalodia), ranged from 24 to 28‰, equivalent to a carbon isotope ratio (δ13C) of around -32 to-36‰ vs. Pee Dee Belemnite (PDB) standard. Further evidence was consistent with CO2 supply to the carboxylating system entirely mediated by diffusion rather than a CCM, in that thallus CO2 compensation point and online instantaneous Δ were also high, in the range normally associated with C3 higher plants. For lichens consisting of phycobiont or cyanobiont alone, organic material Δ formed two distinct ranges around 15‰ (equivalent to a δ13C of -23%.). Thallus compensation point and instantaneous Δ were lower in the cyanobiont group, which also showed higher maximum rates of net photosynthesis, whether expressed on the basis of thallus dry weight, chlorophyll content or area. These data provide additional evidence for the activity of a CCM in cyanobiont lichens, which only show photosynthetic activity when reactivated with liquid water. Rates of net CO2 uptake were lower in both phycobiont associations, but were relatively constant across a wide working range of thallus water contents, usually in parallel with on-line Δ. The phycobiont response was consistent whether photosynthesis had been reactivated with liquid water or water vapour. The effect of diffusion limitation could generally be seen with a 3–4‰ decrease in instantaneous Δ at the highest water contents. The expression of a CCM in phycobiont algae, although reduced compared with that in cyanobacteria, has already been related to the occurrence of pyrenoids in chloroplasts. In view of the inherent requirement of cyanobacteria for some form of CCM, and the smaller pools of dissolved inorganic carbon (DIC = CO2 + HCO inf3 su− + CO inf3 su2− ) associated with phycobiont lichens, it appears that Δ characteristics provide a good measure of the magnitude of any CCM, albeit tempered by diffusion limitation at the highest thallus water contents.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00193222
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