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Calorespirometry is a means for understanding how plants adapt and acclimate metabolically to their environment. Analysis of the energetics of respiration shows that measurements of metabolic heat and CO rates by calorespirometry, combined with estimates of substrate and biomass composition, are sufficient to calculate substrate carbon-conversion efficiencies, anabolic rates or rates of growth and development, and relative activities of metabolic paths. Calorespirometric measurements can thus be used to rapidly investigate the responses of plant growth and metabolism to varying conditions and to compare the responses of species and genotypes. Calorespirometric and calorimetric methods have been used to determine the temperature dependence of growth rate, temperature limits for growth, the kinetics of both chilling- and high-temperature responses, and the effects of toxins and nutrient deficiencies.

Calorespirometry is a means for understanding how plants adapt and acclimate metabolically to their environment. Analysis of the energetics of respiration shows that measurements of metabolic heat and CO rates by calorespirometry, combined with estimates of substrate and biomass composition, are sufficient to calculate substrate carbon-conversion efficiencies, anabolic rates or rates of growth and development, and relative activities of metabolic paths. Calorespirometric measurements can thus be used to rapidly investigate the responses of plant growth and metabolism to varying conditions and to compare the responses of species and genotypes. Calorespirometric and calorimetric methods have been used to determine the temperature dependence of growth rate, temperature limits for growth, the kinetics of both chilling- and high-temperature responses, and the effects of toxins and nutrient deficiencies.

The oxygen isotope technique is currently the only reliable method for studying relative electron partitioning between the cytochrome and alternative plant respiratory pathways. The theoretical background to this technique is described, as well as some of the difficulties that can complicate measurements. This chapter describes the development of systems over the last 15 years that currently allow measurement of respiration in both intact tissues and in the aqueous phase. Initially, the focus was on developing on-line systems for both gas and liquid phase measurements, but in recent years attention has shifted to the development of portable off-line systems which will allow measurements of respiratory electron partitioning in field studies. Measurements can now be made much more rapidly and accurately than a decade ago, however, the application of this technique is still limited by the availability of dedicated systems. Finally, a summary of data obtained with this technique is presented.

According to gas exchange measurements, mitochondrial oxygen consumption in the light is always fast, while respiratory CO evolution is markedly decreased (compared with rates in darkness). We analyze the metabolic events that lead to such contrasting responses. In the light, the generation of NADH in mitochondria, both in the glycine decarboxylase reaction and in the tricarboxylic acid cycle, leads to increased NAD(P)H levels, which may increase the activity of the rotenone-insensitive NAD(P)H dehydrogenases. The resulting increase of the reduction level of ubiquinone activates the alternative oxidase. Stabilization of (photo)respiratory flux during the transition from darkness to light takes place at higher NADH/NAD and ATP/ADP ratios. Maintenance of fast rates of mitochondrial electron transport in the light is facilitated by the import of oxaloacetate (OAA) from the cytosol to remove NADH, and by the export of citrate to the cytosol. This reduces the flow of metabolites in the tricarboxylic acid cycle, decreasing decarboxylation rates, while the rate of oxygen consumption reactions remain fast.

A positive correlation has been observed between dark respiration and carbohydrate status/light intensity during prior illumination in both leaves and roots of many species. This correlation is often ascribed to an indirect effect: changes in carbohydrate status/light intensity are thought to influence various ATP-consuming processes (growth, maintenance and ion uptake), and adenylate demands for these processes are thought to restrict respiration rates. However, some data clearly indicate that this correlation is partly caused by a direct effect of carbohydrate as substrates for respiration both in leaves and in roots. In leaves of some species, in vivo activity of the alternative oxidase (AOX) in mitochondria is high when carbohydrate status is high (e.g., leaves after illumination), and AOX would have an important role as an energy-overflow pathway, while this correlation between carbohydrate status and in vivo AOX activity does not exist in leaves of other species. These different responses to carbohydrate status among plant species may be related to their ecological traits. However, the significance and physiological mechanisms of these different responses are still unknown. Day respiration (non-photorespiratory mitochondrial CO production or O consumption in the light) also depends on light intensity, although measurements of day respiration are still hard to make. High-light intensity induces fast rates of O uptake in the light which would support fast rates of photosynthesis; rates of CO production in the light also depend on light intensities under low irradiances. Growth light intensity also has a direct influence on dark respiration, especially at photo-oxidative light intensities. If excess light intensity overwhelms avoiding and scavenging systems in leaves, photoinhibition in photosystems occurs in leaves. Under these conditions, non-phosphorylating pathways, such as AOX and uncoupling protein, would consume reducing equivalents efficiently, and prevent the over-reduction in the electron transfer of chloroplasts and mitochondria.

Plant growth can be limited by several factors, among which a lack of water is considered of major importance. Despite the vast knowledge of the effect of water stress on photosynthesis, there is much less known about its effect on respiration. Respiration, unlike photosynthesis, never halts, and it reflects the overall metabolism. However, the data available on the effect of water stress on respiration show large variation, from inhibition to stimulation under different water-stress conditions. This chapter combines a review of the latest studies of the effect of water stress on plant respiration with the compilation of data from different authors and recent results to develop a working hypothesis to explain how respiration is regulated under water stress. Leaf respiration shows a biphasic response to Relative Water Content (RWC), decreasing in the initial stages of water stress (RWC > 60%), and increasing as RWC decreases below 50%. Under this hypothesis, the initial decrease in respiration would be related to the immediate inhibition of leaf growth and, consequently, the growth respiration component. The increase of respiration at lower RWC would relate to an increasing metabolism as the plant triggers acclimation mechanisms to resist water stress. These mechanisms would increase the maintenance component of respiration, and, as such, the overall respiration rate. This hypothesis aims to give a metabolic explanation for the observed results, and to raise questions that can direct future plant respiration experiments.

The effects of short- and long-term changes in temperature on plant respiration () are reviewed. We discuss the methods available for quantifying the short- and long-term temperature-dependence of . The extent to which the Q (the proportional change in with a 10 °C increase in temperature) and the degree of thermal acclimation (change in the temperature-response curve of following a long-term change in growth temperature) vary within and amongst plant species are assessed. We show that Q values are highly variable (e.g., being affected by measuring and growth temperature, irradiance and drought), but most plant species exhibit similar Q values (in darkness) when grown and measured under identical conditions (i.e. little evidence of inherent differences in the Q of plant ). The possible mechanisms responsible for variability in the Q are discussed; high Q values occur in tissues where respiratory flux is substrate saturated (i.e. capacity limited). This is illustrated using plots of reduced ubiquinone versus O uptake in isolated mitochondria. The degree of acclimation is also highly variable amongst plant species. This variability is due, in part, to some studies exposing pre-existing roots/leaves to a new growth temperature, whereas others compare roots/leaves that develop at different temperature. In most cases, maximal acclimation requires that new leaves and/or roots be developed following a change in growth temperature. In addition to its link with development, acclimation is also often associated with changes in the Q, particularly in pre-existing leaves/roots transferred from one environment to another. The importance of acclimation in determining annual rates of as a component of net primary productivity and net ecosystem CO exchange is discussed. The importance of acclimation for future atmospheric CO concentrations is highlighted, including a positive feedback effect of climate warming on the carbon cycle. This review shows that the assumptions of coupled global circulation models (that Q values are constant and that does not acclimate to long-term changes in temperature) are incorrect, and this may lead to overestimation of the effects of climate warming on respiratory CO flux.

Soil flooding is a severe abiotic stress for many plant species (e.g., most crops), whereas well adapted species (e.g., rice () and other wetland species) usually thrive. Flooded soils are usually anaerobic, so that internal O transport from shoot to roots is crucial for sustaining respiration in submerged organs. Formation of aerenchyma, together with a barrier impermeable to radial O loss in basal zones of roots, act synergistically to enhance O diffusion to the apex of the main axis of roots of many wetland species. In some situations the soil is not anoxic, but provides a restricted O supply to the roots. For roots with aerenchyma, O is usually much more readily available from the intercellular gas-filled pathway, than exogenously from the soil. In both types of situations some cells/tissues may become anoxic, especially the apical regions and the stele since these are at the ends of the longitudinal (gas phase) and radial (predominately liquid phase) diffusion paths, respectively. Anoxia in root tissues becomes even more likely when shoots of plants are submerged by floodwaters. The inhibition of oxidative phosphorylation due to anoxia causes a severe energy crisis. Tolerance of anoxia requires a carbohydrate supply to fuel anaerobic catabolism, and apportionment of the scarce available energy to processes essential to survival, whereas several energy-consuming processes typical for aerobic cells may be reduced. In addition to O-deficiency, roots in waterlogged soils must also tolerate high CO partial pressures (e.g., up to 43 kPa); however, information on this topic is scant.

Interactions among external (soil) pH, cellular pH, and their effects on respiratory metabolism are complex. While the effects of changes in the apoplastic pH on the cytosolic pH are not clearly understood, pH directly affects enzymatic reactions in the cell, and pH-regulated ion uptake has profound indirect effects on cellular respiratory metabolism. A major consequence of soil acidification is the release of aluminum in solubilized forms from its insoluble forms, which, in turn, adversely affects the uptake of cations, causes organic acid secretion, and inhibits cell division and growth in the roots. Consequently, the respiratory metabolism is redirected to meet the needs of organic acid efflux from the roots. The effects of changes in external pH on cellular pH and consequent effects of this change on respiratory metabolism, particularly through effects on soil aluminum are summarized.

A revolution in plant modeling in the 1970s highlighted the need for better respiration algorithms. Subsequent research enhanced conceptual insights into processes underlying growth and maintenance respiration. This chapter offers an overview of the most important basic concepts used to partition respiration into energy-utilizing components for growth, maintenance and ion uptake, both with respect to modeling and for experimental measurements.

Conceptual models can offer a simplified representation of the mechanisms of respiratory energy partitioning in plant. Comparing different conceptual models demonstrates that plant growth can be simulated with different equations, each without containing detailed information on the underlying respiratory processes. For that reason, process-based models are more useful to quantify the relative importance of energy-consuming processes and identifying quantitatively important gaps in our knowledge. Allometric modeling is a rapidly developing research area, and seems to offer promising perspectives for the future.

Experimental methods to relate respiration to underlying energy-utilizing components for growth, maintenance and ion uptake may be divided in three distinct approaches: i) correlative or regression approaches, ii) black-box approaches, and iii) process-based approaches. The first method especially has enhanced our insight into the differences between fast- and slow-growing species. Although useful in the past, the second approach is outdated, and should no longer be used. More studies should use process-based approaches to further progress our understanding. Especially combining experimental and theoretical process-based approaches seems to offer interesting perspectives. Our understanding of the relation between the energy-utilizing components with environmental conditions is still limited, but gradually increasing by including environmental conditions in process-based research. In addition to down-scaling by studying energy-utilizing processes in depth, up-scaling as done in allometric modeling may offer valuable insights into our understanding of the use of respiratory energy.