Significance of Respiratory Quotient in Plants: A Guide to Respiration Types and Substrates

The respiratory quotient (RQ) of plants is the ratio of the volume of carbon dioxide released during respiration to the volume of oxygen absorbed. This indicator reflects which type of organic material is being oxidised in the cells and which pathway respiration takes. In the international literature it is denoted as the Respiratory Quotient (RQ), and it is calculated using the formula RQ = volume of CO₂ released / volume of O₂ absorbed.

What is the respiratory quotient (RQ) of plants?

The respiratory quotient of plants expresses the relationship between the gases a tissue exchanges with its environment per unit of time: how much carbon dioxide is released for every volume of oxygen absorbed. RQ is a dimensionless value, because both volumes are measured in the same units at standard temperature and pressure. From the RQ value, a physiologist can infer the nature of the respiratory substrate — carbohydrates, organic acids, fats or proteins — and whether respiration is proceeding fully aerobically or with the involvement of anaerobic processes.

Plant respiration is the oxidative breakdown of organic substances with the release of energy stored in the form of ATP. The carbon of the substrate is released as CO₂ (respiratory CO₂ efflux, RCO₂), while oxygen is absorbed for the final oxidation of hydrogen to water (respiratory O₂ uptake, RO₂). The respiratory quotient is the coupling coefficient of these two flows, RCO₂/RO₂, which is precisely why it carries information about the chemical nature of the oxidised material.

Formula and calculation of the respiratory quotient

The respiratory quotient is calculated as the ratio of the volume of carbon dioxide released to the volume of oxygen absorbed: RQ = V(CO₂) / V(O₂). To carry out the calculation, it is enough to know the stoichiometry of the oxidation reaction of a particular substrate — the coefficients of CO₂ and O₂ in the equation of complete oxidation directly give the value of the RQ.

To calculate the RQ for any substance, three steps are performed:

• write the equation of complete oxidation of the substrate to CO₂ and H₂O;
• find the number of molecules of CO₂ released and the number of molecules of O₂ absorbed;
• divide the first by the second — the resulting number is the respiratory quotient.

The value of the RQ depends on the ratio of carbon, hydrogen and oxygen in the substrate molecule. The more oxygen already present within the oxidised substance itself, the less external oxygen is required for its complete oxidation and the higher the RQ. Carbohydrates, rich in intramolecular oxygen, give an RQ of about unity; reduced fats, poor in oxygen, require a great deal of external O₂ and give a low RQ.

The significance of the respiratory quotient in plant physiology

The significance of the respiratory quotient in plant physiology lies in its role as an indirect but rapid indicator of the metabolic state of a tissue. From the RQ one can determine which storage material is being consumed at a given moment, whether the plant is switching to anaerobic respiration under oxygen deficiency, and how efficiently energy is being stored. This makes RQ a convenient tool for assessing the physiology of leaves, seeds, fruits and roots without destroying the tissue.

RQ is closely linked to leaf dark respiration, during which carbon is lost without photosynthetic compensation. The value of the leaf respiratory quotient (RQ) varies between species and over the course of the day: night-time and daytime values differ because the available substrates and the intensity of respiration change. Inter-species variability in leaf RQ is associated with nitrogen concentration and Rubisco content — the higher the respiratory activity, the more noticeable the deviations of the quotient.

Respiration and photosynthesis in plants are interconnected: photosynthesis supplies reduced carbon (sucrose, starch), which is then oxidised in respiration. This connection makes RQ an indicator of the leaf carbon balance. At the ecosystem scale, the same principles of gas exchange measurement underpin terrestrial biosphere models (TBMs), where assumptions about the value of the respiratory quotient influence estimates of respiratory CO₂ release.

The RQ value and the nature of the oxidised material

The value of the respiratory quotient indicates both the nature of the material being oxidised in respiration and the type of respiration; it can be equal to unity, greater than it, or less than it. The pattern is simple: the more oxidised the substrate, the higher the RQ, and the more reduced it is, the lower the RQ. This relationship is manifested only when there is enough oxygen in the environment and in the plant tissues for complete oxidation.

RQ during the oxidation of carbohydrates (RQ ≈ 1.0)

During the oxidation of carbohydrates, the volumes of the exchanged gases — carbon dioxide and oxygen — are equal, so the CO₂ : O₂ ratio equals unity. The oxygen consumed during respiration is used only to oxidise carbon to carbon dioxide, because the ratio of hydrogen to oxygen in the glucose molecule is such that there is enough oxygen within the sugar molecule itself to oxidise hydrogen to water.

The complete oxidation of glucose is described by the equation C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, and the RQ (6CO₂ : 6O₂) equals 1. Sucrose and starch — the main respiratory substrates of most actively growing tissues — give values that are likewise close to unity. Therefore an RQ of around 1.0 normally indicates carbohydrate respiration.

RQ during the oxidation of fats (RQ < 1.0)

When a plant respires using fats, whose molecules contain a great deal of hydrogen and carbon and little oxygen, the RQ is less than unity, because a large amount of oxygen must be absorbed from outside to oxidise all the carbon and hydrogen. During the oxidation of stearic acid the reaction proceeds as follows: C₁₈H₃₆O₂ + 26O₂ → 18CO₂ + 18H₂O, and the RQ (18CO₂ : 26O₂) is about 0.69.

Fats and lipids are the most reduced substrates, so their oxidation through the beta-oxidation of fatty acids gives the lowest RQ values, around 0.7. A low RQ is therefore typical, for example, of germinating oilseeds, in which storage fats are converted into sugars. Palmitic acid and stearic acid give similarly low values because both are poor in oxygen.

RQ during the oxidation of proteins

When proteins are oxidised, the respiratory quotient takes an intermediate value of approximately 0.8–0.9, because amino acids contain more hydrogen and carbon relative to oxygen than carbohydrates do, but they are less reduced than fats. The exact figure depends on the amino acid composition and on the fate of the nitrogen-containing groups, which are not fully oxidised within the respiratory pathway.

Protein-based respiration is less common in vigorously growing plant tissues, where carbohydrates dominate, but it becomes significant during senescence and under conditions where stored carbohydrates and lipids have been depleted. An RQ in the 0.8–0.9 range, accompanied by mobilisation of amino acids, signals that proteins are contributing to the respiratory substrate supply.

RQ during the oxidation of organic acids (RQ > 1.0)

During the oxidation of a number of organic acids the respiratory quotient is greater than unity, because these compounds are richer in oxygen than carbohydrates. Oxalic acid is a compound that is especially oxygen-rich: the oxygen present in the molecule is not only sufficient to oxidise hydrogen to water, but part of it also remains to oxidise carbon. Therefore one molecule of oxygen is enough for the complete oxidation of two molecules of oxalic acid: 2C₂H₂O₄ + O₂ → 4CO₂ + 2H₂O, and the RQ (4CO₂ : O₂) in this case equals 4.

A high RQ is characteristic of tissues that accumulate and consume organic acids, for example malic acid in succulents. In plants of the Crassulaceae family — such as Bryophyllum and Opuntia — the night-time accumulation and daytime oxidation of organic acids markedly shift the value of the respiratory quotient, which serves as a distinctive feature of this type of metabolism.

Typical ranges and a table of RQ values

Calculating the respiratory quotient from chemical equations clearly demonstrates how the stoichiometry of a substrate determines the value of the RQ. The table below lists typical substrates, their oxidation reactions and the corresponding values of the quotient.

To summarise the ranges: carbohydrates give an RQ equal to unity, organic acids give values greater than unity, and proteins and fats give values less than unity. This same approach — calculating the RQ through the equation of complete oxidation — applies equally to animal substrates, which makes the respiratory quotient a universal measure of the nature of the oxidised material.

Leaf dark respiration and the daily dynamics of RQ

Leaf respiratory quotient varies both between species and over the course of a day, reflecting changes in available substrates and in the intensity of respiration. Leaf dark respiration releases CO₂ and consumes O₂ without the compensating fixation of carbon by photosynthesis, so it represents a net carbon loss whose magnitude and substrate basis the RQ helps to reveal.

Diel (within-day) variation in leaf respiration rates means that night-time and daytime RQ values can differ, and temperature-dependent changes in leaf RQ are superimposed on inter-species variation. Because respiration is linked to leaf nitrogen concentration and Rubisco content, leaves with high respiratory activity tend to show the most pronounced departures of the quotient from unity. Precise measurements of leaf RQ require the separate determination of CO₂ efflux (RCO₂) and O₂ uptake (RO₂), and such work is published in journals including Physiologia Plantarum by researchers such as Andrew P Scafaro, Owen K Atkin, Kevin L Griffin, Dan Bruhn, Ian Max Møller, Daniel Cowan-Turner and Yuzhen Fan at institutions including the Australian National University, Aarhus University, Aalborg University and Columbia University.

Aerobic and anaerobic respiration in plants

Aerobic and anaerobic respiration differ in whether oxygen serves as the final electron acceptor, and they are therefore reflected differently in the value of the respiratory quotient. In aerobic respiration the substrate is fully oxidised to CO₂ and H₂O with the maximum yield of ATP; in anaerobic respiration oxygen is not used, oxidation is incomplete, and considerably less energy is released.

Mechanisms of aerobic respiration and its efficiency

Aerobic respiration in plant cells proceeds through glycolysis, the formation of acetyl-CoA, the Krebs cycle and the electron transport system (ETS), where oxygen acts as the final electron acceptor and energy is captured as ATP. The complete aerobic oxidation of glucose stores far more energy than anaerobic pathways because the entire reducing potential of the substrate passes through the ETS, and oxygen uptake at this stage defines the denominator of the respiratory quotient.

The efficiency of aerobic respiration depends on how many molecules of ATP are synthesised per atom of oxygen absorbed — the ATP : O ratio. Because every oxidisable atom of carbon and hydrogen is ultimately accounted for in CO₂ and water, complete carbohydrate respiration yields an RQ close to unity together with a high energy return, making aerobic respiration the dominant and most efficient mode of cellular respiration in well-aerated tissues.

Anaerobic respiration and fermentation

Fermentation and anaerobic respiration sharply raise the respiratory quotient because carbon dioxide continues to be released while oxygen is barely absorbed. During alcoholic fermentation, pyruvate formed in glycolysis is converted into ethanol and CO₂ without the participation of oxygen, so the formal RQ tends toward very high values or becomes indefinitely large because of the near-zero uptake of O₂.

A sharp rise in the RQ of fruits, seeds or flooded roots therefore serves as a sign that the tissue has switched to anaerobic metabolism. This is an important diagnostic signal during the storage of plant produce: an increase in the quotient points to oxygen starvation and the onset of fermentation, which leads to spoilage. Less energy is released as well, since anaerobic pathways extract only a fraction of the substrate's chemical energy.

How anaerobic processes change the RQ value

When the same respiratory material is oxidised but oxygen is in short supply in the environment and tissues, the value of the RQ changes. If oxygen is scarce, then during plant respiration oxidation does not go to completion, and besides carbon dioxide and water, organic acids — which are more oxidised than carbohydrates — are formed.

In this case the RQ will be less than unity, because part of the absorbed oxygen remains in the molecules of the organic acids that are formed, while less carbon dioxide is released. Such a picture is characteristic of waterlogging and soil saturation, when plant respiration and root respiration shift toward an oxygen deficit. Depending on whether the limited oxidation produces organic acids or whether glycolysis feeds fermentation, the same oxygen shortage can push the apparent RQ either below or far above unity.

RQ, ATP synthesis and the energy demands of respiration

The respiratory quotient is connected to ATP synthesis through the path that electrons and carbon follow in the cell. Oxidation of the substrate begins with glycolysis, continues in the Krebs cycle after the formation of acetyl-CoA, and concludes at the electron transport system (ETS), where oxygen acts as the final electron acceptor and energy is stored as ATP. The uptake of O₂ at this stage determines the denominator of the respiratory quotient.

The energy demands of a tissue — growth, the transport of substances, the synthesis of proteins — set the intensity of respiration, while the nature of the consumed substrate is reflected in the value of the RQ. Fats, the Krebs cycle and the glyoxylate cycle are linked by amphibolic pathways: the same intermediate products serve both for the production of energy and for biosynthesis. The glyoxylate cycle is especially important in germinating oilseeds, where fatty acids are converted into sugars, which explains the characteristic shifts in the respiratory quotient during this period.

Engagement of the alternative oxidase and its influence on RQ

The alternative oxidase is an enzyme of the plant respiratory chain that reduces oxygen to water while bypassing some of the energy-conserving coupling sites of ATP synthesis. When it is engaged, electrons skip the sites where energy is normally stored, so oxygen uptake continues but the ATP : O ratio falls and part of the energy is dissipated as heat.

The operation of the alternative oxidase complicates the interpretation of the respiratory quotient because it changes the relationship between absorbed oxygen and synthesised ATP without directly affecting CO₂ release. This pathway is activated under stress, with an excess of reductants, and when heat production is required, and its contribution must be taken into account in the precise modelling of respiration and the calculation of energy yield.

The respiratory quotient in ecosystems and carbon-exchange models

Biological modelling of plant respiration describes CO₂ efflux (RCO₂) and O₂ uptake at different scales — from a single leaf to an entire ecosystem. Mechanistic models link the intensity of respiration to nitrogen concentration, Rubisco content, temperature and available substrate, while an assumption about the value of the respiratory quotient converts the measured flux of one gas into the flux of the other.

Terrestrial biosphere models (TBMs) use the value of the RQ as a key assumption when calculating carbon exchange. If the real respiratory quotient deviates from the value adopted in the model, estimates of respiratory CO₂ release at the ecosystem level are distorted — which is why the inter-species and temperature variability of leaf RQ remains an important subject of research and a limitation of such models.

The apparent respiratory quotient (ARQ) in aquatic systems

The apparent respiratory quotient (ARQ) describes the covariance of oxygen and carbon dioxide in water and is the aquatic counterpart of the terrestrial RQ. In lakes and oceans, where dissolved gases are influenced by physical mixing as well as biology, the ARQ helps to interpret the coupling of respiratory CO₂ production and O₂ consumption.

Bacterioplankton respiration is a major driver of these gas exchanges and of ecosystem functioning in aquatic systems, so the ARQ provides insight into the substrates that microbial communities are oxidising. As with leaf RQ, values that deviate from unity reveal departures from simple carbohydrate respiration, including the contribution of more oxidised or more reduced substrates and of anaerobic pathways.

Practical application of RQ in agriculture

The agricultural application of the respiratory quotient is based on the fact that RQ reflects the type of substrate being consumed and the availability of oxygen, and therefore the condition of stored produce and seeds. By measuring RQ, agronomists assess the intensity of respiration, the risk of a switch to fermentation, and the loss of dry matter during the storage of fruits, vegetables and grain.

RQ in seed storage and the control of germination

The respiratory quotient in seed storage helps to monitor seed viability and the state of dormancy. In maturing oilseeds, such as groundnut, carbohydrates are converted into fats and the RQ falls below unity; during germination, by contrast, stored fats are converted into sugars through the glyoxylate cycle, and the character of gas exchange changes.

The principles of cold storage and modified-atmosphere techniques rely directly on the respiratory quotient:

• a lower temperature slows respiration (the Q10 effect) and reduces carbon loss;
• controlling the concentrations of O₂ and CO₂ in a modified atmosphere prevents oxygen starvation and fermentation;
• a rise in RQ signals the onset of anaerobic metabolism and spoilage, serving as a criterion for adjusting storage conditions.

RQ of fruits, roots and the assessment of tissue condition

The respiratory quotient of fruits and roots indicates which substrates these organs are using and whether they are experiencing oxygen stress. Factors that affect the rate of respiration — and hence the measured RQ — include temperature, oxygen availability, CO₂ concentration, moisture and wounding, all of which alter both the intensity and the type of metabolism in harvested tissue.

Practical observations on living tissues confirm that a respiratory quotient drifting away from the value expected for carbohydrate respiration is an early warning sign in post-harvest management. A falling RQ may indicate a shift toward lipid use, while a sharply rising RQ points to incipient anaerobiosis caused by inadequate ventilation or excessive moisture during storage.

Bioengineering approaches to enhance crop yield through respiration

Bioengineering approaches that aim to increase yield by managing respiration focus on reducing unproductive losses of carbon in leaf respiration while preserving ATP synthesis. Because leaf dark respiration returns to the atmosphere part of the carbon assimilated in photosynthesis, reducing these losses or improving the ATP : O efficiency can raise the net carbon balance of the plant.

The link between respiration and the nitrogen and Rubisco content of the leaf makes the respiratory quotient and its associated indicators a target for breeding and engineering: by regulating the contribution of the alternative oxidase and the efficiency of the respiratory chain, it is possible to influence the balance between the cost of respiration and the accumulation of biomass.

Methods of measuring gas exchange and RQ

The respiratory quotient of plants is measured by determining the volume of oxygen absorbed and the volume of carbon dioxide released per unit of time at standard temperature and pressure. The classic laboratory instrument for this is the respirometer; in the educational practice described in the NCERT curriculum, a simple Ganong's Respirometer is used to demonstrate gas exchange, and the topic features in CBSE, ICSE and ISC syllabi and examination papers.

Accurate measurement of the leaf respiratory quotient requires the separate determination of CO₂ release (RCO₂) and O₂ uptake (RO₂), because it is precisely their ratio that constitutes the RQ. Modern protocols measure leaf dark respiration and take diurnal dynamics into account in order to separate genuine substrate differences from temperature effects. The Q10 coefficient describes how the activity of respiratory enzymes responds to temperature — roughly a doubling of rate for every 10 °C rise — and changes in temperature can shift not only the rate of respiration but sometimes the preferred substrate, and therefore the RQ.

Comparison of RQ in plants, animals and microorganisms

The respiratory quotient is a universal measure applicable to plants, animals and microorganisms, although it is called by different names and measured by different methods. In animals and humans the measured ratio of gases at the level of the whole organism is called the Respiratory Exchange Ratio (RER), and at rest it coincides with the tissue-level RQ; during intense exercise, however, RER exceeds the true RQ because of the additional CO₂ released by the bicarbonate buffer system.

In human medicine and physiology the respiratory quotient is applied widely:

• in indirect calorimetry, to assess resting energy expenditure and basal metabolic rate (BMR);
• in VO₂ max tests and exercise physiology, to evaluate the ratio of carbohydrate to fat oxidation;
• in the nutritional support of critically ill and morbidly obese patients, to detect overfeeding and underfeeding;
• in the management of patients with chronic obstructive pulmonary disease (COPD), sepsis and polytrauma;
• in the assessment of liver function in cirrhosis and non-alcoholic fatty liver disease, and to predict weight gain in non-insulin-dependent (type 2) diabetic patients.

The ratio of macronutrients also affects metabolism: a carbohydrate-rich diet raises RER closer to 1, whereas a fat-rich diet lowers it toward 0.7, and insulin and insulin sensitivity modulate the choice of substrate. Reference material on these clinical uses is published by sources such as StatPearls Publishing, the National Library of Medicine and the National Institutes of Health, with contributors including Abhishek Bhardwaj, Hiran Patel and Vanshika Anand affiliated with institutions such as the American University of Antigua and the University of California, Riverside. In microorganisms and aquatic ecosystems, related concepts apply: bacterioplankton respiration and the apparent respiratory quotient (ARQ) describe the covariance of oxygen and carbon dioxide in water, making the respiratory quotient a common language for describing metabolism across the most diverse organisms.

For more on related topics in physiology and agriculture, read the section on Agronomy, while further material on biology and science is collected in the Medicine section.