ALBERT

Notes: The contribution which (photo)respiration makes to carbon isotope discrimination (Δ13C) was examined by conducting simultaneous gas exchange measurements and isotopic analysis of carbon dioxide passing over leaves of Triticum aestivum and Phaseolus vulgaris, via manipulations of the carbon isotope composition (δ13C) of source CO2 during growth and measurement. Dark respiration only altered net Δ13C (Δobs) at low CO2 assimilation, and was sensitive to source CO2δ13C during measurement. Photorespiration reduced Δobs relative to Δ13C predicted from pi/pa (Δi) over the full range of CO2 assimilation, to a greater degree under elevated oxygen partial pressure (pO2), indicating fractionation during photorespiration (f) in T. aestivum. For P. vulgaris, Δobs was insensitive to elevated pO2 at higher assimilation rates, suggesting that f was minimal. A model was developed to calculate gross discrimination (Δps), independent of (photo)respiration, from which estimates of f were obtained for T. aestivum (3.3‰) and P. vulgaris (0.5‰). Because photorespiratory fractionation varies interspecifically, and influences net Δ13C which is directly reflected in leaf δ13C, consideration of (photo)respiratory fractionation is necessary when interpreting δ13C of leaf material, especially under conditions where (photo)respiratory CO2 losses make a large relative contribution to total plant carbon budgets.

Notes: The interrelationships between the induction of CAM and the turnover of malate and citrate in the dicotyledenous tree Clusia minor were compared with seasonal changes in rainfall, leaf water status, PFD and photoinhibitory responses during the transition from wet to dry season in Trinidad. Over a period of 8 weeks, as rainfall declined from a maximum observed around week 3, leaf xylem tensions measured at dusk and dawn reflected the concurrent reduction in day-time carbon gain and an increase in the diel turnover of malate (exposed leaves) and citrate (shaded leaves). Clear seasonal trends were observed in the turnover of malate and citrate during the transition from wet to dry season. In contrast to the declining back-ground concentrations of citrate during the wet-dry season transition, malate accumulation was markedly enhanced and the ratio of malalc:citrate accumulated overnight increased as the dry season advanced. Photo-inhibitory responses, assessed by chlorophyll fluorescence, indicated that photochemistry was largely determined by the diurnal course of PFD incident on leaves, regardless of the magnitude of internal CO2 release from malate and citrate decarboxylation. In the long term, photochemical efficiency in both shaded and exposed leaves appeared to decline as the dry season progressed. Although there was a clear linear relationship between integrated PFD and overnight accumulation of malate, no such correlation was found for citrate. However, citrate breakdown during the day showed a much closer correlation with PFD as compared to malate, with levels of citrate measured at dusk declining in response to higher daily light intensities. Moreover, enhanced citrate decarboxylation during the day was strongly correlated with increased CAM and overnight accumulation of both malate and citrate.

Notes: Concurrent measurements of gas exchange, instantaneous isotope discrimination (Δ) against 13CO2 and C18O16O, and extent of 18O enrichment in H2O at the evaporative sites, were followed in a tropical forest pioneer, Piper aduncum, on two different days in Trinidad during February 1995. Δ13CO2 differed from that predicted from measurements of internal:external CO2 concentration (Ci/Ca) and showed a wide range of values which decreased throughout the course of the day. Derivation of Cc (the CO2 concentration at the carboxylation site) was not possible using carbon isotope discrimination under field conditions in situ and was derived assuming a constant value of internal transfer conductance (gw). Under low rates of assimilation the derived Cc/Ca, like Ci/Ca, remained relatively stable over the course of both days and ΔC18O16O followed evaporative demand. Lower values of ΔC18O16O on day 2 occurred in response to the indirect effect of increased leaf-to-air vapour pressure deficits (VPD) and reduced stomatal conductance. For the first time, direct determination of the δH218O of transpired water vapour (δt) allowed derivation of evaporative site enrichment without the prerequisite of isotopic steady state (ISS) defined in the Craig and Gordon model. Generally, δt was less enriched than the source water (δs) in the morning and more enriched in the afternoon, which would be predicted from an increase and decrease in ambient VPD, respectively. On both days, leaves of P. aduncum approached ISS (indicated where δt≈δs) between 1300 and 1500 h. Evaporative site enrichment was maintained into the late afternoon, despite a decrease in ambient VPD. The data presented provide a greater insight into the natural variation in isotopic discrimination under field conditions, which may help to refine models of terrestrial biome discrimination.

Notes: Dimers of plasmid ColE1 are converted to monomers by site-specific recombination, a process that requires 240 bp of DNA (cer ) and four host-encoded proteins (XerC, XerD, ArgR and PepA). Here, we propose structures for nucleoprotein complexes involved in cer–Xer recombination based upon existing knowledge of the structures of component proteins and computational analyses of protein structure and DNA curvature. We propose that, in the nucleoprotein complex at a single cer site, a PepA hexamer acts as an adaptor, connecting the heterodimeric recombinase (XerCD) to an ArgR hexamer. This provides a protein core around which the cer site wraps, its exact path being defined by strong sequence-specific interactions with ArgR and XerCD, weak interactions with PepA and sequence-dependent flexibility of cer. The initial association of single-site complexes (pairing) is proposed to occur via an ArgR–PepA interaction. Pairing between sites in a plasmid dimer is stabilized by DNA supercoiling and is followed by a structural isomerization to form a recombination-proficient synaptic complex. We propose that paired structures formed between sites in trans are too short-lived to permit synaptic complex formation. There is thus an energetic barrier to inappropriate recombination reactions. Our proposals are consistent with a wide range of experimental observations.

Notes: Abstract Above-ground biomass production in seedling and semi-mature individual plants of Sesbania sesban varieties were compared to non-destructive measurements of stem allometrics. The results indicate that measurements of stem characteristics (diameter and total height) are suitable for estimating biomass of S. sesban varieties and therefore useful in the selection of particular varieties for inclusion in short-rotation agroforestry systems in arid- and semi-arid regions. Less labour-intensive methods for accurately assessing the productivity of agroforestry species, such as the use of allometric regressions, could significantly increase the number of individual plants that could be screened in any particular trial. A multiple polynomial regression accurately estimated above-ground biomass in all plants ranging in age from six weeks to eighteen months (r 2=0.99;p〈0.001).

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.

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.

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.

Notes: Abstract The organic-matter carbon isotope discrimination (Δ) of lichens with a wide range of photobiont and/or cyanobiont associations was used to determine the presence or absence of a carbon-concentrating mechanism (CCM). Two groups were identified within the lichens with green algal photobionts. One group was characterised by low, more C4-like Δ values (Δ 〈 15‰), the other by higher, more C3-like Δ values (Δ 〉 18‰). Tri-partite lichens (lichens with a green alga as the primary photobiont and cyanobacteria within internal or external cephalodia) occurred in both groups. All lichens with cyanobacterial photobionts had low Δ values (Δ 〈 15‰). The activity of the CCM, organic-matter Δ values, on-line Δ values and gas-exchange characteristics correlated with the presence of a pyrenoid in the algal chloroplast. Consistent with previous findings, lichens with Trebouxia as the primary photobiont possessed an active CCM while those containing Coccomyxa did not. Organic Δ values for lichens with Stichococcus as the photobiont varied between 11 and 28‰. The lichen genera Endocarpon and Dermatocarpon (Stichococcus + pyrenoid) had C4-like organic Δ values (Δ = 11 to 16.5‰) whereas the genus Chaenotheca (Stichococcus — pyrenoid) was characterised by high C3-like Δ values (Δ = 22 to 28‰), unless it associated with Trebouxia (Δ = 16‰). Gas-exchange measurements demonstrated that Dermatocarpon had an affinity for CO2 comparable to those species which possessed the CCM, with K0.5 = 200–215 μ1 · 1−1, compensation point (Γ) = 45–48 μl · l−1, compared with K0.5 = 195 μ1 · 1−1, Γ = 44μ1 · 1−1 for Trebouxioid lichens. Furthermore, lichens with Stichococcus as their photobiont released a small pool (24.2 ± 1.9 to 34.2 ± 2.5 nmol · mg−1 Chl) of inorganic carbon similar to that released by Trebouxioid lichens [CCM present, dissolved inorganic carbon (DIC) pool size = 51.0 ± 2.8 nmol · mg−1 Chl]. Lichens with Trentepohlia as photobiont did not possess an active CCM, with high C3-like organic Δ values (Δ = 18‰ to 23‰). In particular, Roccella phycopsis had very high on-line Δ values (Δ = 30 to 33‰), a low affinity for CO2 (K0.5 = 400 μ1 · 1−1,Γ = 120 μ1 · −1) and a negligible DIC pool. These responses were comparable to those from lichens with Coccomyxa as the primary photobiont with Nostoc in cephalodia (organic Δ = 17 to 25‰, on-line Δ = 16 to 21‰, k0.5 = 388 μ1 · 1−1, Γ = 85 μ1 · 1−1, DIC pool size = 8.5 ± 2.4 nmol · mg−1 Chl). The relative importance of refixation of respiratory CO2 and variations in source isotope signature were considered to account for any variation between on-line and organic Δ. Organic Δ was also measured for species of Anthocerotae and Hepaticae which contain pyrenoids and/or Nostoc enclosed within the thallus. The results of this screening showed that the pyrenoid is correlated with low, more C4-like organic Δ values (Δ = 7 to 12‰ for members of the Anthocerotae with a pyrenoid compared with Δ = 17 to 28‰ for the Hepaticae with and without Nostoc in vesicles) and confirms that the pyrenoid plays a fundamental role in the functioning of the CCM in microalgal photobionts and some bryophytes.