Constructive breathing and maintenance breathing

What makes up the costs of growth?

Creating new organic matter is not a simple gluing together of molecules. It is a cascade of complex reactions, each requiring energy. The main cost items on this "construction site" are as follows:

1. Creation of carbon skeletons: Various organic acids, amino acids, and other precursors are synthesized from simple sugars (hexoses). This process essentially "reformats" the carbon skeleton, requiring additional reactions and energy.

2. Reduction of mineral elements: Particularly costly is the reduction of nitrates ( \mathrm{NO3^-} ) to ammonium ( \mathrm{NH4^{+} ) and sulfates to sulfides. Nitrate is an oxidized form of nitrogen. To incorporate it into amino acids and proteins, it must be reduced. This process requires a huge amount of NADH (reduced nicotinamide adenine dinucleotide), which is mainly supplied by respiration (Lambers & Oliveira, 2019; Marschner, 2012). If the plant uses ammonium ( \mathrm{NH_4}+} ) as a nitrogen source, this cost item is significantly reduced.

3. Synthesis of macromolecules (polymerization): Assembling amino acids into proteins, sugars into polysaccharides (cellulose, starch), fatty acids into lipids—each step requires ATP expenditure. For example, adding each new amino acid to a growing protein chain requires about 4–5 ATP molecules (Schopfer & Brennicke, 2016; Taiz et al., 2023).

4. Maintaining "growth tools": Growth requires the constant synthesis of new enzymes and mRNA molecules. These are the so-called costs of "maintaining the tools," which are essentially part of growth, not the maintenance of already existing structures (Amthor, 1994; Penning de Vries et al., 1974).

Chemical composition of the yield — the main cost factor

The most important practical implication from all this is that growth respiration directly depends on what exactly we are building. The synthesis of different substances costs the plant different "prices" in terms of sugar expenditure.

The table below shows averaged data on the amount of glucose needed to synthesize 1 gram of various organic compounds (Penning de Vries et al., 1974; Lambers & Oliveira, 2019).

What does this mean for practice?

• Cereal crops (wheat, rice, corn): The main mass of the yield is starch (carbohydrates). Its synthesis is relatively "cheap." Therefore, such crops can have a high efficiency of photosynthate use and a high Harvest Index.
• Oilseed crops (sunflower, rapeseed, soybean): Their seeds are rich in oils (lipids). Lipid synthesis is a very "expensive" endeavor. Producing a gram of oil requires almost 2.5 times more sugar than producing a gram of starch.
• Legumes (peas, soybeans, alfalfa): These plants accumulate a lot of proteins. Moreover, they obtain nitrogen through symbiotic N₂ fixation, which itself is an extremely energy-consuming process. Protein synthesis, especially considering the cost of nitrogen fixation, also requires significantly more sugar than carbohydrate synthesis.

For example, according to calculations (Lambers & Oliveira, 2019; Sinclair & de Wit, 1975), the efficiency of glucose use for biomass synthesis drops sharply from rice to oilseed crops.

Conclusion: Growth respiration is an inevitable and vital cost for creating yield. However, its magnitude varies greatly. Fast and efficient growth requires fast growth respiration. This is why one cannot directly compare the yields of crops with different chemical compositions of the final product. A plant synthesizing oils and proteins simply must physiologically "spend" more photosynthetic products on the "construction" itself than a plant synthesizing starch. Consequently, a crop with higher photosynthesis but directing most assimilates towards lipid synthesis may produce a smaller mass but much more energy-dense product.

This is only one side of the coin. The second expenditure item—maintenance respiration—also strongly influences the final yield, but for entirely different reasons. We will cover it in the next chapter.

What makes up the costs of maintenance?

1. Protein Turnover (Renewal)

This is perhaps the most costly item of maintenance respiration, potentially consuming up to 50–70% of its energy (Penning de Vries, 1975; Lambers & Oliveira, 2019).

Proteins are the workhorses of the cell: enzymes, structural proteins, transporter proteins. But they are not eternal. During work, they become damaged (oxidized), lose activity, or simply become unnecessary when physiological tasks change. The cell must constantly break down old, used proteins (proteolysis) and synthesize new ones in their place.

This "breakdown-synthesis" cycle requires energy:

• Energy for breakdown (proteolysis): Although a catabolic process, it also requires ATP to activate the ubiquitin-proteasome system.
• Energy for new protein synthesis: As we discussed regarding growth respiration, synthesizing protein from amino acids is a very expensive process, requiring about 4–5 ATP molecules per amino acid.

Why is this important for practice?

The rate of protein turnover, and hence the cost of maintenance, varies greatly and depends on several factors:

• Protein concentration in the tissue: Tissues rich in enzymatic proteins (e.g., young actively photosynthesizing leaves) have much higher turnover than storage tissues (seeds, tubers). The higher the nitrogen (N) content in the tissue, the higher the maintenance respiration rate. This is one of the main reasons why nitrogen fertilization, while stimulating growth, simultaneously increases maintenance respiration costs, especially under stress (Marschner, 2012). The plant is forced to pay a higher "rent" for its high protein status.
• Temperature: The rate of enzymatic reactions, including protein turnover, strongly depends on temperature. This factor is so important that we will dedicate a separate section to it.

2. Maintenance of Ion Gradients (Membrane Transport)

Inside the cell and its organelles, the concentration of various ions (potassium, calcium, protons, anions) is strictly regulated and differs significantly from the concentration in the external environment. For example, the concentration of potassium (K⁺) in the cytoplasm can be hundreds of times higher than in the soil solution. To hold these ions in the right places and create the electrochemical gradients necessary for life (which, in turn, are used for the active transport of sugars and amino acids), the cell must constantly work ion pumps (H⁺-ATPases).

These pumps actively transport ions against the concentration gradient, and this work requires a continuous expenditure of ATP. The greater the concentration difference and the larger the membrane surface area, the more energy is spent (Amthor, 1994; Penning de Vries, 1975). According to some estimates, this item can account for 20 to 30% of all maintenance costs (Bouma & De Visser, 1993; cited in Lambers & Oliveira, 2019).

3. Other "Maintenance" Processes

This category also includes:

• Renewal of mRNA and other nucleic acids.
• Maintenance of cell turgor and membrane fluidity. This is especially important during temperature changes.
• Repair of DNA and other macromolecules damaged by reactive oxygen species (ROS), which are inevitable byproducts of respiration and photosynthesis.

Maintenance Respiration as a Target for Breeding

Since maintenance respiration does not directly contribute to biomass growth, breeders and physiologists long considered it a "useless burden" and tried to find genotypes with minimal levels of this respiration.

A classic example is D. Wilson's work with ryegrass (Lolium perenne) (Wilson, 1982; Nelson, 1994). He managed to breed plant lines with different respiration rates in mature leaves. Lines with reduced respiration (and thus reduced maintenance costs) had significantly higher dry matter yields than lines with high respiration. This convincingly demonstrated that maintenance respiration is a real limiting factor for productivity and that reducing maintenance costs is a direct path to increasing yield.

However, there is another side to this coin. By lowering maintenance respiration, we risk weakening the plant's "immunity," its ability to respond quickly to stress (drought, salinity, pathogen attack), as synthesizing protective substances and repairing damage also requires energy. This is a complex balance that breeders face.

Key takeaway for practice:

Maintenance respiration is not a "waste" but a necessary cost for viability. It increases with rising temperatures and high protein (nitrogen) content in tissues. Consequently, variety selection, timing, and rates of nitrogen fertilization directly impact not only growth but also the magnitude of these inevitable "operating expenses," determining the final economic efficiency of photosynthesis.

In the next chapter, we will examine how temperature, especially nighttime temperature, becomes the main factor dictating the level of these expenses and explain why hot nights are so dangerous for crops.

Temperature Coefficient (Q₁₀): The Rule, Not the Exception

In plant physiology, the concept of the temperature coefficient Q₁₀ is used to describe the temperature dependence of respiration. Simply put, Q₁₀ indicates how many times the process rate will increase when the temperature rises by 10 °C.

For maintenance respiration, Q₁₀ in the physiological temperature range (e.g., from +15 °C to +35 °C) is approximately 2.0–2.6 (Lambers & Oliveira, 2019; Tretyakov, 2000).

What does this mean in practice?

It means that with an increase in nighttime temperature of just 5 °C, "maintenance" costs can grow by 2^{0.5} ≈ 1.4 times (by 40%). And with an increase of 10 °C, they almost double.

Growth respiration also depends on temperature, but to a lesser extent. Moreover, on a hot night, when photosynthesis (the energy source) has ceased, the plant is in an extremely vulnerable position. It is forced to spend the very sugars that were meant for grain filling or fruit growth simply to "survive" the night. This is a direct robbery of the future harvest!

Day and Night: Two Different Worlds

To understand the tragedy of night heat, we must clearly separate the plant's daytime and nighttime physiology:

1. During the day: Photosynthesis is active. The plant actively produces sugars. Some are immediately used for respiration (both growth and maintenance), while the surplus is stored or transported to sink organs (grain, fruit, tubers). High daytime temperatures can accelerate photosynthesis (up to a point), and the plant "earns" more.

2. During the night: Photosynthesis stops. The influx of new assimilates is zero. But cells and tissues continue to live and require energy for maintenance. This "nightly tax" is paid exclusively from the carbohydrate reserves created during the day.

Key point: If nights are hot, the rate of consumption of these reserves (maintenance respiration) increases sharply. As a result, by morning, the plant has fewer resources for growth and grain filling (Taiz et al., 2023; Schopfer & Brennicke, 2016).

Why is Maintenance Respiration So Sensitive to Heat?

This is related to its biochemical nature:

1. Protein Turnover: Enzymes that break down and synthesize proteins, as well as the processes of denaturation and repair, have a high temperature dependence. The warmer it is, the faster proteins "wear out" and require replacement (Amthor, 1994; Lambers & Oliveira, 2019).

2. Membrane Leakage: As temperature rises, membrane fluidity increases, and they become more "leaky" to ions. To compensate for these leaks and maintain the necessary ion gradients, pumps (H⁺-ATPases) must work harder, consuming even more ATP (Nelson, 1994).

3. Enzymatic Kinetics: The rate of enzyme-catalyzed reactions follows the Van't Hoff rule. Since maintenance respiration is a cascade of enzymatic reactions, it inevitably accelerates with increasing temperature.

What Does This Mean for the Harvest?

A classic example is research on the effect of nighttime temperature on rice yields.

The Peng Effect and others: It was shown that an increase in nighttime temperature of only 1 °C (while maintaining daytime temperature) leads to a 10% decrease in rice yield! (Peng et al., 2004; cited in Lambers & Oliveira, 2019; Taiz et al., 2023). This effect is specifically attributed to the acceleration of nighttime maintenance respiration and the depletion of carbohydrate reserves needed for grain filling.

Similar data have been obtained for wheat: an increase in nighttime temperature during the grain-filling period significantly reduces the final 1000-grain weight (Foulkes et al., 2015; Sadras & Calderini, 2015).

A simple agronomic conclusion: In regions where nighttime temperatures during yield formation are high, plants are forced to spend much more assimilates on maintenance respiration. This reduces the Assimilate Use Efficiency (RUE) for forming the economically valuable part of the yield. Therefore, even with the same level of daytime photosynthesis, the yield in a hot climate will be significantly lower than in a cool one.

What About Global Warming?

This physiological property makes plants particularly vulnerable to global climate change. Since global average temperatures are rising and nights are warming faster than days (IPCC, 2013; cited in Asseng et al., 2015), we are facing a situation where losses from respiration will grow faster than gains from photosynthesis. This poses a serious threat to food security, especially for crops like wheat and rice, which are critically important for global agriculture.

In the next chapter, we will examine another tool that allows us to "look inside" the plant's respiration and understand what exactly it is "breathing"—the Respiratory Quotient.

What Do Different RQ Values Mean?

Theoretically, the RQ value depends on the degree of oxidation (reduction) of the respiratory substrate and can vary widely. Let's consider the main options.

1. RQ ≈ 1.0 : Substrate — Carbohydrates

This is the classic and most common case. If the plant is breathing "sweet"—sugars (glucose, sucrose, starch)—the oxidation process can be described by the overall equation:

In this equation, for every 6 molecules of O₂ absorbed, 6 molecules of CO₂ are released. The ratio 6 / 6 = 1.0.

Practical significance: This is the "standard" mode for most vegetating plants that are actively growing and using fresh photosynthetic products. For example, germinating cereal seeds rich in starch have an RQ close to one (Lambers & Oliveira, 2019; Tretyakov, 2000).

2. RQ < 1.0 : Substrate — Lipids or Proteins

If the plant is forced to breathe "fatty" or "protein" fuel, the RQ becomes less than one. Why?

Lipids (fats, oils): Fatty acid molecules are much more reduced (contain less oxygen) than carbohydrates. Their complete oxidation requires relatively more oxygen. For example, for stearic acid:

RQ = 18 / 26 ≈ 0.7.

Proteins: On average, the RQ for proteins is also less than one (about 0.8–0.9).

Practical significance: A low RQ is a sure sign that the plant has switched to using reserve substances. This occurs in several key situations:

1. Germination of oilseeds: Seeds of sunflower, rapeseed, castor bean are rich in oils. In the initial germination period, their RQ can drop to 0.4–0.7 while they "burn" their fat reserves for energy (Lambers & Oliveira, 2019). This is direct evidence that respiration is working on a substrate stored for future use.

2. Starvation or stress: Under prolonged shading, drought, or nutrient deficiency, when carbohydrate reserves are depleted, the plant may start using proteins and lipids to maintain life. This is an "SOS" signal—the plant is switching to internal reserves, often preceding tissue senescence and death (Fischer et al., 2015; cited in Lambers & Oliveira, 2019).

3. Senescing tissues: In senescing (aging) leaves, from which carbohydrates are actively exported and proteins are broken down for reuse, RQ also decreases.

3. RQ > 1.0 : Substrate — Organic Acids or Anaerobic Respiration

An increase in RQ above 1.0 signals two fundamentally different processes.

• Substrate — Organic acids (e.g., malic or oxaloacetic acid): These substances are more oxidized than carbohydrates. Their oxidation requires less oxygen. For example, for oxaloacetic acid, the RQ can reach 1.6 (Tretyakov, 2000). This often occurs in succulent fruits during ripening, when organic acids are actively metabolized. It is also characteristic of CAM plants (Crassulaceae) during the light phase when decarboxylation of the malate accumulated overnight occurs.
• Anaerobic respiration (fermentation): If the plant lacks oxygen (e.g., due to root flooding, in compacted soil), it switches to fermentation. In alcoholic fermentation:

In this process, carbon dioxide (CO₂) is released, but oxygen (O₂) is not absorbed because it is absent. As a result, the RQ becomes infinitely large (in practice, it increases sharply because very little O₂ is consumed, while much CO₂ is released).

Practical significance: A sharp increase in RQ is a clear diagnostic sign of oxygen starvation in the roots (Schopfer & Brennicke, 2016). This allows timely detection of soil waterlogging problems. A high RQ can also indicate a high rate of lipid synthesis from carbohydrates, as this process is accompanied by CO₂ release without oxygen consumption (Lambers & Oliveira, 2019).

The Respiratory Quotient as a Tool for Physiologists and Agronomists

Thus, by measuring the respiratory quotient, we can obtain crucial information:

1. Diagnosing the substrate type: Finding out what serves as "fuel" at the moment—fresh sugars (RQ ≈ 1) or stored fats/proteins (RQ < 1).

2. Assessing biosynthesis intensity: A high RQ > 1 may indicate active synthesis of fats or organic acids, which is relevant for assessing crop quality (e.g., seed oil content).

3. Identifying stress: A decrease in RQ may indicate depletion of carbohydrate reserves (starvation), while a sharp increase indicates oxygen deficiency in the root zone or disruption of oxidative phosphorylation, which could be caused by toxins or membrane damage (Tretyakov, 2000).

Thus, the respiratory quotient is not an abstract value from a textbook but an important physiological indicator that allows us to "look into" the plant's metabolism and understand how efficiently and on what "fuel" it is operating at any given moment.

In the next, concluding part of our lecture, we will bring all this knowledge together and answer the main question: Why is respiration the main limiter of yield?

1. Respiration Sets the "Efficiency" of Photosynthesis

We are used to evaluating photosynthesis by its intensity—the amount of CO₂ absorbed per hour. However, for the production process, the Net Photosynthetic Productivity (NPP) is much more important—this is the difference between true (gross) photosynthesis and respiration (Tretyakov, 2000).

The formula we mentioned in the first chapter: Yield = Photosynthesis – Respiration — works flawlessly. If two crops have the same gross photosynthesis, but one has respiratory losses of 30% of gross photosynthesis and the other has 50%, then the second crop, all other things being equal, will produce significantly less yield. It will simply "eat up" most of its income.

2. Growth Respiration: An "Expensive" Yield Isn't Always Profitable

As we found out, growth respiration is the cost of building new biomass. Its magnitude depends on what exactly we are building.

• The synthesis of carbohydrates (starch, cellulose) is "cheap" for the plant. Therefore, cereal crops (wheat, rice, corn) have relatively low growth respiration costs and can use photosynthate efficiently for grain formation.
• The synthesis of proteins and lipids is a very "expensive" process. Oilseed and legume crops spend much more sugar to create their reserves. This is an objective physiological fact. Therefore, even with high photosynthesis, oilseed crops may produce a smaller mass but more energy-dense products. However, under conditions of assimilate deficiency (e.g., during stress), these high growth respiration costs can become an insurmountable obstacle to forming a full yield.

3. Maintenance Respiration: "Operating Expenses" Decide Everything

As we discussed in the third chapter, maintenance respiration is the cost of living. But this cost is not fixed. It can vary greatly.

• Stress (drought, salinity, overheating) sharply increases the need for repair and protection. The plant is forced to synthesize heat shock proteins, antioxidants, osmoprotectants. This requires additional energy, thus increasing maintenance respiration (Amthor, 1994; Lambers & Oliveira, 2019).
• Temperature, as we already discussed, is the main regulator. Hot nights force the plant to spend significantly more assimilates just to maintain cell viability.

Ultimately, maintenance respiration is the cost item that the agronomist and breeder can and should control. This is where the key lies to understanding why two crops with the same photosynthesis produce different yields. A variety with more economical "maintenance" under stress (drought-tolerant, with more stable membranes) will preserve more sugars for grain filling (Nelson, 1994; Foulkes et al., 2015).

4. Interconnections and Trade-offs

It is important to understand that growth respiration and maintenance respiration are not two independent "ledgers." They are closely linked.

1. Growth requires maintenance: By creating new tissues (e.g., a powerful photosynthetic apparatus), we simultaneously create new objects for maintenance. Plants with high leaf protein content (intensive varieties) photosynthesize better but are also forced to bear higher costs for renewing this protein.

2. Stress intensifies competition: Under stress (e.g., drought), growth slows down to save water. But maintenance respiration may not only fail to decrease but may even increase, as repair of damaged membranes and synthesis of protective substances are required (Marschner, 2012). As a result, sugar consumption for maintenance occurs at the expense of even the limited growth that is still possible. This is a classic example of carbon starvation.

5. Practical Conclusions: How Can Knowledge of Physiology Help Manage Yield?

So, knowing all this, how can we influence yield?

1. Breeding for low maintenance respiration: This is a direct path to creating varieties with more economical metabolism. Successful work with ryegrass shows this is possible. For field crops, the task is more complex, but it remains a key one (Nelson, 1994; Foulkes et al., 2015).

2. Managing the temperature regime: In regions with hot climates, especially during the grain-filling period, it is critically important to provide nighttime cooling. This can be done by choosing sowing dates (to avoid the hottest period) or through irrigation (evaporative cooling).

3. Optimizing nitrogen nutrition: Nitrogen is a powerful growth stimulant. But its excess, especially in later stages, leads to the accumulation of a lot of protein in the leaves, which needs to be "fed." This increases maintenance respiration and depletes the carbohydrate budget, reducing grain or fruit filling (Marschner, 2012). Nitrogen fertilization must be precisely balanced and timed to phases of active growth.

4. Stress management: Timely irrigation and protection against diseases and pests not only save the leaf apparatus but also prevent energy-consuming "emergency repairs" that stimulate additional respiration.

Final Conclusion

Respiration is not just a "loss" but a fundamental process determining the efficiency of using solar energy. A plant, like any organism, cannot spend 100% of its energy solely on growth. Some is inevitably spent on maintenance. However, the difference in the efficiency of this maintenance is the main reason for varying yields.

To increase the productivity of agricultural crops, we need not only to increase photosynthesis (improve the "income side") but also to learn how to manage respiratory losses (the "expenditure side"). Creating varieties with economical maintenance respiration, optimizing sowing dates and fertilization, and protecting against stress—all this allows the plant to channel as much carbon as possible into the final product. Therefore, the physiology of respiration is not an abstract science but a powerful practical tool in the hands of agronomists and breeders.