Theory suggests that the level of enrichment of 18 O above source water in plant organic material (⌬) may provide an integrative indicator of control of water loss. However, there are still gaps in our understanding of the processes affecting ⌬. One such gap is the observed discrepancy between modeled enrichment of water at the sites of evaporation within the leaf and measured enrichment of the leaf water as a whole (⌬ L ). Farquhar and Lloyd (1993) suggested that this may be caused by a Péclet effect. It is also unclear whether organic material formed in the leaf reflects enrichment of water at the sites of evaporation within the leaf or ⌬ L . To investigate this question castor bean (Ricinus communis L.) leaves, still attached to the plant, were sealed into a controlled-environment gas exchange chamber and subjected to a step change in leaf-to-air vapor pressure difference. Sucrose was collected from a cut on the petiole of the leaf in the chamber under equilibrium conditions and every hour for 6 h after the change in leaf-to-air vapor pressure difference. Oxygen isotope composition of sucrose in the phloem sap (⌬ suc ) reflected modeled ⌬ L . A model is presented describing ⌬ suc at isotopic steady state, and accounts for 96% of variation in measured ⌬ suc . The data strongly support the Péclet effect theory.

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Data from 57 permanent monitoring sites are used to document the growth in woody vegetation and estimate the carbon sink in 27 M ha of eucalypt woodlands (savannas), contained within c. 60 M ha of grazed woodlands in Queensland (northeast Australia). The study sites are shown to be representative of the environment and structure of the eucalypt woodlands in the defined study area. Mean basal area increment for all live woody plants in 30 long‐term sites, with an average initial basal area of 11.86 ± 1.38 (SE) m2 ha−1, was 1.06 m2 ha−1 over a mean 14 years timeframe. The majority of the measurement period, commencing between 1982 and 1988, was characterized by below‐average rainfall. The increase in live tree basal area was due primarily to growth of existing trees (3.12 m2 ha−1) rather than establishment of new plants (0.25 m2 ha−1) and was partly offset by death (2.31 m2 ha−1). A simple but robust relationship between stand basal area and stand biomass of all woody species was developed for the eucalypt dominant woodlands. Analysis of above‐ground carbon stocks in live and standing dead woody plants gave a mean net above‐ground annual carbon increment for all 57 sites of 0.53 t C ha−1 y−1, similar to values estimated elsewhere in world savannas. Published root : shoot ratios were used to infer C flux in woody root systems on these sites. This results in an estimated sink in above‐ and below‐ground biomass of 18 Mt C y−1 over the eucalypt woodlands studied, and potentially up to 35 Mt C y−1 if extended to all grazed woodlands in Queensland. It is suggested that introduction of livestock grazing and altered fire regimes have triggered the change in tree‐grass dominance in these woodlands. Thus, change in carbon stocks in the grazed woodlands of Queensland is identified as an important component of human‐induced greenhouse gas flux in Australia, equivalent in magnitude to c. 25% of the most recently published (1999) total estimated national net emissions. The latter inventory takes into account emissions from land clearing, but does not include the sink identified in the present study. This sequestration also represents a small but significant contribution to the global terrestrial carbon sink.

Abstract.Climate change presents a range of challenges for animal agriculture in Australia. Livestock production will be affected by changes in temperature and water availability through impacts on pasture and forage crop quantity and quality, feed-grain production and price, and disease and pest distributions. This paper provides an overview of these impacts and the broader effects on landscape functionality, with a focus on recent research on effects of increasing temperature, changing rainfall patterns, and increased climate variability on animal health, growth, and reproduction, including through heat stress, and potential adaptation strategies. The rate of adoption of adaptation strategies by livestock producers will depend on perceptions of the uncertainty in projected climate and regional-scale impacts and associated risk. However, management changes adopted by farmers in parts of Australia during recent extended drought and associated heatwaves, trends consistent with long-term predicted climate patterns, provide some insights into the capacity for practical adaptation strategies.Animal production systems will also be significantly affected by climate change policy and national targets to address greenhouse gas emissions, since livestock are estimated to contribute~10% of Australia's total emissions and 8-11% of global emissions, with additional farm emissions associated with activities such as feed production. More than two-thirds of emissions are attributed to ruminant animals. This paper discusses the challenges and opportunities facing livestock industries in Australia in adapting to and mitigating climate change. It examines the research needed to better define practical options to reduce the emissions intensity of livestock products, enhance adaptation opportunities, and support the continued contribution of animal agriculture to Australia's economy, environment, and regional communities.

Changes in the respiratory rate and the contribution of the cytochrome (Cyt) c oxidase and alternative oxidase (COX and AOX, respectively) were investigated in soybean (Glycine max L. cv Stevens) root seedlings using the 18 O-discrimination method. In 4-d-old roots respiration proceeded almost entirely via COX, but by d 17 more than 50% of the flux occurred via AOX. During this period the capacity of COX, the theoretical yield of ATP synthesis, and the root relative growth rate all decreased substantially. In extracts from whole roots of different ages, the ubiquinone pool was maintained at 50% to 60% reduction, whereas pyruvate content fluctuated without a consistent trend. In whole-root immunoblots, AOX protein was largely in the reduced, active form at 7 and 17 d but was partially oxidized at 4 d. In isolated mitochondria, Cyt pathway and succinate dehydrogenase capacities and COX I protein abundance decreased with root age, whereas both AOX capacity and protein abundance remained unchanged. The amount of mitochondrial protein on a dry-mass basis did not vary significantly with root age. It is concluded that decreases in whole-root respiration during growth of soybean seedlings can be largely explained by decreases in maximal rates of electron transport via COX. Flux via AOX is increased so that the ubiquinone pool is maintained in a moderately reduced state.The rate of plant respiration is linked to the rate of metabolism and growth due to requirements for ATP, reductant, and carbon skeletons during cell maintenance, division, and expansion (Hunt and Loomis, 1979; Lambers et al., 1983). For example, respiration rates are often lower in species with intrinsically slower growth rates (Poorter et al., 1991). Moreover, respiration is rapid in tissues with high energy demands, such as thermogenic floral spadices (Meeuse, 1975), and in rapidly growing tissues, such as the elongation zone of roots (Lambers et al., 1996). Plant respiration can also increase rapidly in response to both biotic and abiotic stress (for a recent review, see Lambers et al., 1996). Conversely, decreases in respiratory rate often occur as plant tissues age (Azcon-Bieto et al., 1983; McDonnell and Farrar, 1993; Atkin and Cummins, 1994;Winkler et al., 1994). Various factors may be responsible for these changes, including substrate availability, enzyme activation, specific protein degradation or de novo protein synthesis, and alterations in mitochondrial numbers.The extent to which such changes in respiration rate alter the rate of oxidative phosphorylation also depends on the partitioning of electron flux between the Cyt and the alternative pathways of electron transport. The Cyt pathway (terminating at COX) couples the reduction of O 2 to water with the translocation of protons across the inner mitochondrial membrane, thereby building a proton-motive force that drives ATP synthesis. The alternative pathway branches directly from Q and reduces O 2 to water without further proton translocation. This pathway appears to consist of a single-s...