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Concentration and isotopic composition (δC and δO) of ambient CO 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 δCO, δ COO and δ HO, 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. δC of organic matter input for δCO, δO of exchangeable water for δ COO and transpired water vapour for δ HO. [CO] and δCO showed strong coupling, where the released CO 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 CO, or temporal differences in ecosystem discrimination. δ COO was less well coupled to [CO], probably due to the heterogeneity and transient nature of water pools (soil, leaf and moss) within the forest. Similarly, δ HO was less coupled to [HO], again reflecting the transient nature of water transpired to the forest, seen as uncoupling during times of large changes in vapour pressure deficit. The δO 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 CO 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.

The regulation of Rubisco activity was investigated under high, constant photosynthetic photon flux density during the diurnal phases of Crassulacean acid metabolism in Kalanchoëdaigremontiana Hamet et Perr. During phase I, a significant period of nocturnal, C4-mediated CO2 fixation was observed, with the generated malic acid being decarboxylated the following day (phase III). Two periods of daytime atmospheric CO2 fixation occurred at the beginning (phase II, C4–C3 carboxylation) and end (phase IV, C3–C4 carboxylation) of the day. During the 1st h of the photoperiod, when phosphoenolpyruvate carboxylase was still active, the highest rates of atmospheric CO2 uptake were observed, coincident with the lowest rates of electron transport and minimal Rubisco activity. Over the next 1 to 2 h of phase II, carbamylation increased rapidly during an initial period of decarboxylation. Maximal carbamylation (70%–80%) was reached 2 h into phase III and was maintained under conditions of elevated CO2 resulting from malic acid decarboxylation. Initial and total Rubisco activity increased throughout phase III, with maximal activity achieved 9 h into the photoperiod at the beginning of phase IV, as atmospheric CO2 uptake recommenced. We suggest that the increased enzyme activity supports assimilation under CO2-limited conditions at the start of phase IV. The data indicate that Rubisco activity is modulated in-line with intracellular CO2 supply during the daytime phases of Crassulacean acid metabolism.

Numerous studies have found that proportional electoral rules significantly increase women’s representation in national parliaments relative to majoritarian and mixed rules. These studies, however, suffer from serious methodological problems including the endogeneity of electoral laws, poor measures of cultural variables, and neglect of time trends. This article attempts to produce more accurate estimates of the effect of electoral rules on women’s representation by using within-country comparisons of electoral rule changes and bicameral systems as well as matching methods. The main finding is that the effect of electoral laws is not as strong as in previous studies and varies across cases. The policy implication is that changes in electoral laws may not provide a quick and consistent fix to the problem of low women’s representation.

The magnitude and extent of Crassulacean acid metabolism (CAM) activity in two Clusia species was manipulated to investigate the regulation of the distinct CAM phases. First, in response to leaf-air vapor pressure deficit at night, changes in leaf conductance altered on-line carbon-isotope discrimination throughout the theoretical range for dark CO, uptake during CAM. These ranged from the limit set by phosphoenolpyruvate carboxylase (PEPc) (-6%0, 613C equivalent of -2%0) to that imposed by diffusion limitation (+4%0, 613C equivalent of -1 2%0), but the lowest carbon-isotope discrimination occurred when pJpa was only 0.7. Second, when the availability of external or internal sources of CO, was reduced for both field-and greenhouse-grown plants, CO, uptake by day via PEPc during phase I1 largely compensated. Third, by reducing the dark period, plants accumulated low levels of acidity, and CO, uptake occurred throughout the subsequent light period. Discrimination switched from being dominated by PEPc (phase II) to ribulose 1,s-bisphosphate carboxylase/oxygenase (phase III), with both enzymes active during phase IV. Under natural conditions, photochemical stability is maintained by extended PEPc activity in phase II, which enhances acid accumulation and delays decarboxylation until temperature and light stress are maximal at midday.