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Notes: We investigated the extent to which leaf and root respiration (R) differ in their response to short- and long-term changes in temperature in several contrasting plant species (herbs, grasses, shrubs and trees) that differ in inherent relative growth rate (RGR, increase in mass per unit starting mass and time). Two experiments were conducted using hydroponically grown plants. In the long-term (LT) acclimation experiment, 16 species were grown at constant 18, 23 and 28 °C. In the short-term (ST) acclimation experiment, 9 of those species were grown at 25/20 °C (day/night) and then shifted to a 15/10 °C for 7 days. Short-term Q10 values (proportional change in R per 10 °C) and the degree of acclimation to longer-term changes in temperature were compared. The effect of growth temperature on root and leaf soluble sugar and nitrogen concentrations was examined. Light-saturated photosynthesis (Asat) was also measured in the LT acclimation experiment. Our results show that Q10 values and the degree of acclimation are highly variable amongst species and that roots exhibit lower Q10 values than leaves over the 15–25 °C measurement temperature range. Differences in RGR or concentrations of soluble sugars/nitrogen could not account for the inter-specific differences in the Q10 or degree of acclimation. There were no systematic differences in the ability of roots and leaves to acclimate when plants developed under contrasting temperatures (LT acclimation). However, acclimation was greater in both leaves and roots that developed at the growth temperature (LT acclimation) than in pre-existing leaves and roots shifted from one temperature to another (ST acclimation). The balance between leaf R and Asat was maintained in plants grown at different temperatures, regardless of their inherent relative growth rate. We conclude that there is tight coupling between the respiratory acclimation and the temperature under which leaves and roots developed and that acclimation plays an important role in determining the relationship between respiration and photosynthesis.

Notes: In this study we assessed the inherent relative growth rate (RGR) under controlled environment conditions of 10 contrasting Acacia species from semi-arid and mesic environments. For several of the species, compound pinnate leaves produced early in the seedling stage, were gradually replaced by phyllodes (expanded petioles that form simple lamina). Other species either did not form phyllodes, or only did so to a minor degree by the end of the study. Phyllode production was dominant in the four slow-growing Acacia species from semi-arid environments (A. aneura, A. colei, A. coriacea and A. tetragonophylla), with leaf production being exclusive or dominant in five (A. dealbata, A. implexa, A. mearnsii, A. melanoxylon and A. irrorata) of the six faster-growing species from mesic environments. The exception was A. saligna which was fast growing but did produce phyllodes. From a carbon economy perspective, slow growth in the semi-arid species was not associated with lower net assimilation rates or less plant mass allocated to foliage. Rather, the primary factor associated with their slow growth was a smaller foliage area per unit foliage mass. This was true for comparisons based on the mean over all harvests or at set plant masses. The production of phyllodes by the semi-arid species substantially reduced foliage area per unit foliage mass, as this was lower for phyllodes than leaves in all species. To assess the impact that phyllode production had on ontogenetic changes in RGR, we modelled the situation where only leaves were formed. This analysis showed that changing from leaves to phyllodes substantially reduced the RGR. There was little difference in plant nitrogen concentration or the ratio of foliage nitrogen to plant nitrogen between the species. This resulted in foliage nitrogen productivity (dry mass gain per unit foliage nitrogen and time) being directly proportional to foliage area per unit foliage mass between species. We concluded that a smaller foliage area per unit foliage mass and phyllode production are the primary factors associated with lower RGR in contrasting Acacia species.

Notes: The Arctic is often assumed to be an NH4+-dominated ecosystem. This review assesses the validity of this assumption. It also addresses the question of whether Arctic plant growth is limited by the ability to use the forms of nitrogen that are available. The review demonstrates that several sources of soil nitrogen are available to Arctic plants, including soluble organic nitrogen (e.g. glycine, aspartic acid and glutamic acid), NH4+ and NO−3. In mesic Arctic soils, soluble organic nitrogen is potentially more important than either NH+4 or NO−3. Many Arctic species are capable of taking up soluble organic nitrogen (either directly and/or in association with ectomycorrhizae), with the greatest potential for soluble organic nitrogen uptake being exhibited by deciduous species. The ability to take up soluble organic nitrogen may enable some Arctic plants to avoid nitrogen limitations imposed by the slow rate of organic matter decomposition. NO−3 is also present in many Arctic soils, especially calcareous soils and soils near flowing water, animal burrows and bird cliffs. Arctic species characteristic of mesic and xeric habitats are capable of taking up and assimilating NO−3. Even when present in lower concentrations in soils than NH+4, NO−3 is still an important source of nitrogen for some Arctic plants. Arctic-plants therefore have a variety of nitrogen sources available to them, and are capable of using those nitrogen sources. Taken together, these findings demonstrate that the Arctic is not an NH+4dominated ecosystem. Symbiotic fixation of atmospheric N2 does not appear to be an important source of nitrogen for Arctic plants. The reliance of Arctic plants on internal recycling of nitrogen substantially reduces their dependence on soil nitrogen uptake (this is particularly the case for slow-growing evergreens). Despite the high level of internal nitrogen recycling, Arctic plant growth remains limited by the low levels of available soil nitrogen. However, Arctic plant growth is not limited by an inability to utilize any of the available forms of nitrogen. The potential effects of climatic warming on nitrogen availability and use are discussed. The question of whether the Arctic ecosystem is uniquely different from temperate nitrogen-deficient ecosystems is also assessed.

Notes: We investigated the relationship between daily and seasonal temperature variation and dark respiratory CO2 release by leaves of snow gum (Eucalyptus pauciflora Sieb. ex Spreng) that were grown in their natural habitat or under controlled-environment conditions. The open grassland field site in SE Australia was characterized by large seasonal and diurnal changes in air temperature. On each measurement day, leaf respiration rates in darkness were measured in situ at 2–3 h intervals over a 24 h period, with measurements being conducted at the ambient leaf temperature. The rate of respiration at a set measuring temperature (i.e. apparent ‘respiratory capacity’) was greater in seedlings grown under low average daily temperatures (i.e. acclimation occurred), both in the field and under controlled-environment conditions. The sensitivity of leaf respiration to diurnal changes in temperature (i.e. the Q10 of leaf respiration) exhibited little seasonal variation over much of the year. However, Q10 values were significantly greater on cold winter days (i.e. when daily average and minimum air temperatures were below 6° and –1 °C, respectively). These differences in Q10 values were not due to bias arizing from the contrasting daily temperature amplitudes in winter and summer, as the Q10 of leaf respiration was constant over a wide temperature range in short-term experiments. Due to the higher Q10 values in winter, there was less difference between winter and summer leaf respiration rates measured at 5 °C than at 25 °C. The net result of these changes was that there was relatively little difference in total daily leaf respiratory CO2 release per unit leaf dry mass in winter and summer. Under controlled-environment conditions, acclimation of respiration to growth temperature occurred in as little as 1–3 d. Acclimation was associated with a change in the concentration of soluble sugars under controlled conditions, but not in the field. Our data suggest that acclimation in the field may be associated with the onset of cold-induced photo-inhibition. We conclude that cold-acclimation of dark respiration in snow gum leaves is characterized by changes in both the temperature sensitivity and apparent ‘capacity’ of the respiratory apparatus, and that such changes will have an important impact on the carbon economy of snow gum plants.

Notes: This study investigates the nitrogen economy of six altitudinally contrasting Poa species which differ in their relative growth rate (R). Two alpine (Poa fawcettiae and P. costiniana), one sub-alpine (P. alpina)and three temperate lowland species (P. pratensis, P. campressa and P. trivialis) were grown hydroponically under identical conditions in a growth room. The low R exhibited by the alpine species was associated with lower plant organic nitrogen concentration (np) and lower nitrogen productivity (Πp, amount of biomass accumulation per mol organic nitrogen and time). The differences in Πp between the alpine and lowland species did not appear to be due to differences in the carbon concentration or the proportion of total plant organic nitrogen allocated to the leaves, stems or roots. Variations in ΠP were also not due to variations in photosynthetic nitrogen use efficiency (ΨN, the rate of photosynthesis per unit organic leaf nitrogen) or shoot or root respiration rates per unit organic nitrogen (ΛSH and ΛR, respectively) per se. Rather, the lower Λp in the alpine species was probably due to a combination of small variations in several of the parameters (e.g. slightly lower ΨN, slightly higher ΛSH and ΛR, and slightly higher proportions of total plant organic nitrogen allocated to the roots). The alpine species exhibited lower organic acid and mineral concentrations. However, no differences in whole-plant construction costs (grams of glucose needed to synthesize one gram of biomass) were observed between She alpine and lowland Poa species. The lack of sub-stantial differences in ΨN between the alpine and lowland species contrasts with the large differences in ΨN between slow- and fast-growing lowland species that have been reported in the literature. The reasons for the unusually high ΨN values exhibited by the alpine Poa species are discussed.

Notes: The effect of light on the development of the capacity for alternative pathway respiration was investigated in leaf slices of Belgium endive (Cichorum intybus L. cv. deliva). Dark-grown plants possessed little capacity for the cyanide-insensitive alternative pathway. In contrast, plants grown in continuous light had significant alternative pathway capacity. Light-grown plants also had substantially higher concentrations of ethanol-soluble carbohydrates in their leaves than plants grown in complete darkness. Despite these differences in leaf carbohydrate status and alternative pathway capacity of light- and dark-grown leaf tissue, no differences were found in the activity of the alternative pathway, which was negligible in both treatments. Dark-grown plants were adenylate restricted, as indicated by the increase in cytochrome pathway activity following uncoupling. Adenylates did not limit respiration in light-grown leaf tissue. Plants that had been grown for 8d in complete darkness were also transferred to continuous light. Respiration of dark controls steadily declined over 11d following the transfer of plants to the light, due primarily to a decrease in cytochrome pathway activity. No such decline was observed in the plants transferred to continuous light. Transfer to continuous light led to significant increases in alternative pathway capacity relative to the dark controls. Alternative pathway activity remained negligible in both the dark controls and in plants transferred to continuous light. The results of this study suggest then that light per se may be responsible for the induction of alternative pathway capacity in Belgium endive.

Notes: In the present study the effect of elevated CO2 on growth and nitrogen fixation of seven Australian Acacia species was investigated. Two species from semi-arid environments in central Australia (Acacia aneura and A. tetragonophylla) and five species from temperate south-eastern Australia (Acacia irrorata, A. mearnsii, A. dealbata, A. implexa and A. melanoxylon) were grown for up to 148 d in controlled greenhouse conditions at either ambient (350 µmol mol−1) or elevated (700 µmol mol−1) CO2 concentrations. After establishment of nodules, the plants were completely dependent on symbiotic nitrogen fixation. Six out of seven species had greater relative growth rates and lower whole plant nitrogen concentrations under elevated versus normal CO2. Enhanced growth resulted in an increase in the amount of nitrogen fixed symbiotically for five of the species. In general, this was the consequence of lower whole-plant nitrogen concentrations, which equate to a larger plant and greater nodule mass for a given amount of nitrogen. Since the average amount of nitrogen fixed per unit nodule mass was unaltered by atmospheric CO2, more nitrogen could be fixed for a given amount of plant nitrogen. For three of the species, elevated CO2 increased the rate of nitrogen fixation per unit nodule mass and time, but this was completely offset by a reduction in nodule mass per unit plant mass.

Notes: This study investigates the effect of short- and long-term changesin temperature on the regulation of root respiratory O2 uptakeby substrate supply, adenylate restriction and/or the capacityof the respiratory system. The species investigated were the lowland Plantagolanceolata L. and alpine Plantago euryphylla Briggs, Carolin& Pulley, which are inherently fast- and slow-growing, respectively. Theplants were grown hydroponically in a controlled environment (constant23 °C). The effect of long-term exposure to lowtemperature on regulation of respiration was also assessed in P.lanceolata using plants transferred to 15/10 °C(day/night) for 7 d. Exogenous glucose and uncoupler (CCCP)were used to assess the extent to which respiration rates were limitedby substrate supply and adenylates. The results suggest that adenylatesand/or substrate supply exert the greatest control overrespiration at moderate temperatures (e.g. 15–30 °C)in both species. At low temperatures (5–15 °C),CCCP and glucose had little effect on respiration, suggesting thatrespiration was limited by enzyme capacity alone. The Q10 (proportionalincrease of respiration per 10 °C) of respirationwas increased following the addition of CCCP and/or exogenousglucose. The degree of stimulation by CCCP was considerably lowerin P. euryphylla than P. lanceolata. This suggeststhat respiration rates operate much closer to the maximum capacity in P.euryphylla than P. lanceolata. When P. lanceolata wastransferred to 15 °C for 7 d, respirationacclimated to the lower growth temperature (as demonstrated by an increasein respiration rates measured at 25 °C). In addition,the Q10 was higher, and the stimulatory effectof exogenous glucose and CCCP lower, in the cold-acclimated rootsin comparison with their warm-grown counterparts. Acclimation of P.lanceolata to different day/night-time temperatureregimes was also investigated. The low night-time temperature wasfound to be the most important factor influencing acclimation. The Q10 valueswere also higher in plants exposed to the lowest night-time temperature.The results demonstrate that short- and long-term changes in temperaturealter the importance of substrate supply, adenylates and capacityof respiratory enzymes in regulating respiratory flux.

Notes: The contribution of individual plant mitochondrial respiratory pathways to total respiration is commonly assessed by titration with specific inhibitors of different components in the branched electron transport chain. A pathway's contribution is equal to the activity when the other branch is blocked by an inhibitor multiplied by the degree (0-1.0) to which this activity is engaged when both pathways are operating. According to Bahr and Bonner (1973. J. Biol. Chem. 218: 3441–3445) the plot of the activities of identical titrations, one performed in the absence and the other in the presence of a specific inhibitor of the other branch of the respiratory chain, yields a straight line whose slope indicates the engagement of the titrated pathway during uninhibited respiration. An initial slope of zero may occur if electron flux is diverted between pathways during titrations. However, beyond the breakpoint (representing the point of pathway saturation), a straight line is obtained with a slope representing engagement. This technique assumes that the kinetics of inhibiting a specific component of the respiratory chain are independent of the absolute rate of electron flux through the total pathway. To test this assumption, the activity of respiratory pathways in isolated soybean (Glycine max [L]. Merr. cv. Stevens) mitochondria was titrated with specific inhibitors of the cytochrome and alternative oxidases. Under these conditions, the electron flux through a given pathway was manipulated by poising the rate of succinate oxidation with the succinate dehydrogenase inhibitor malonate. Construction of activity plots in the presence versus absence of malonate failed to result in straight lines for either KCN (when titrating the cytochrome pathway) or salicylhydroxamic acid (when titrating the alternative pathway). Rather, the resultant plots were always curvilinear whenever the activity in the presence of malonate divided by the activity in the absence of malonate was less than 1.0. In no case could the real engagement of the pathway be precisely estimated from the titration data. Titrations of cytochrome pathway activity in isolated potato tuber (Solanum tuberosum L. cv. Sabago and Canabex) mitochondria (which lack the alternative oxidase) showed that as the inhibitor concentration was increased, so did the reduction status of the ubiquinone pool, to a new steady state. The dependence of inhibition kinetics on the rate of flux through the pathway, and the increase in ubiquinone pool reduction upon KCN addition, are explained in terms of the elasticity of component enzymes as outlined in the theory of metabolic control analysis. The implications of this finding for the use of titrations to estimate engagement of plant respiratory pathways are discussed.

Notes: We examined the effect of growth temperature on the underlying components of growth in a range of inherently fast- and slow-growing plant species. Plants were grown hydroponically at constant 18, 23 and 28 °C. Growth analysis was conducted on 16 contrasting plant species, with whole plant gas exchange being performed on six of the 16 species. Inter-specific variations in specific leaf area (SLA) were important in determining variations in relative growth rate (RGR) amongst the species at 23 and 28 °C but were not related to variations in RGR at 18 °C. When grown at 18 °C, net assimilation rate (NAR) became more important than SLA for explaining variations in RGR. Variations in whole shoot photosynthesis and carbon concentration could not explain the importance of NAR in determining RGR at the lower temperatures. Rather, variations in the degree to which whole plant respiration per unit leaf area acclimated to the different growth temperatures were responsible. Plants grown at 28 °C used a greater proportion of their daily fixed carbon in respiration than did the 18 and 23 °C-grown plants. It is concluded that the relative importance of the underlying components of growth are influenced by growth temperature, and the degree of acclimation of respiration is of central importance to the greater role played by NAR in determining variations in RGR at declining growth temperatures.