Plant Respiration: Its Importance and Applications

Plant respiration is being studied in increasing depth and scope because of its importance for plants, the global atmosphere, and climate change. Despite intensive research, we still need to learn more about the factors that control the process, especially at larger scales. This ubiquitous process has essential applications in agriculture, climate action, and ecosystem management. So what do we know so far?

Respiration and Its Functions

Plant respiration occurs in all plants, and there are two kinds: aerobic and anaerobic.

Aerobic respiration occurs in all green plants and uses oxygen to burn photosynthates (glucose and starch). It occurs partly in the mitochondria of the cells and all parts of the plant, shoot, and roots. The results of this process are carbon dioxide (CO2) and chemical energy in the form of Adenosine triphosphate (ATP), see equation 1. The steps involved are 

• Glycolysis,
• The tricarboxylic acid (TCA) cycle or Citric Acid cycle,
• Mitochondrial electron transport chain
• Production of CO2 and Adenosine triphosphate (ATP).

Anaerobic respiration or fermentation occurs without oxygen and can also happen in higher-green plants. It also involves using glucose and starch in glycolysis but then follows another pathway to produce ethyl alcohol and lactic acid.

Besides producing energy, aerobic respiration has two other functions that must be considered.

Produce C precursors: Plants produce all the organic compounds they need by integrating inorganic compounds like nitrates or minerals. The basic compounds they start with are byproducts of the carbon metabolism in the glycolysis or Citric Acid Cycles. These byproducts act as building blocks and are called carbon skeletons or biosynthetic precursors.

Redox balancing: The mitochondrial electron chain, which produces electrons, is essential to balance oxidation-reduction or redox reactions in cellular processes, such as photosynthesis and neutralizing reactive oxygen species. 

The three functions could overlap, but they need different modes of the respiration pathway, and plants have to coordinate them. 

Scales of respiration

Figure 1. The innermost box represents cellular respiration, the middlebox gas exchange at the plant scale, and the outer box depicts respiration at ecosystem scales in terms of carbon loss, Leary et al ( 2018). (Image credits: https://doi.org/10.1111/nph.15576)

Respiration can also be defined based on the three scales on which it occurs in a plant.

• Cellular respiration occurs in the cells of plants and includes the biochemical processes driven by enzymes in the pathway: Glycolysis-TCA/Citric Acid cycle-Mitochondrial electron transport chain-Production of CO2 and Adenosine triphosphate (ATP).

Tissue-level respiration processes describe the gas exchange that occurs through stomatal conductance within the tissues. The intake of oxygen from the atmosphere and the fate of CO2 produced is important here. Although not all the CO2 a plant produces is released into the atmosphere, at least part of it is used for photosynthesis during the day. 

Plant-level respiration that considers the carbon budget of the plant. Here respiration is explained as carbon loss (see Equation 2) and includes shoot and root processes like photosynthesis, root exudation, biomass accumulation, etc. Here the results of respiration, like growth and maintenance, are essential.

Respiration = photosynthesis − plant biomass production Equation 2

This approach separates different aspects of respiration and is explained in Figure 1. 

Respiration is not uniform, and the rate will vary based on environmental conditions and intrinsic factors. Considering that different factors influence respiration rates, let’s explore the three scales. 

Factors Controlling Respiration Rate

Though the factors are listed separately below as external and internal, many interact to influence the respiration rate. 

Intrinsic Factors

The factors can be other processes or states of the plant.

Photosynthesis: Respiration is intricately connected to photosynthesis, as it provides the sugars and starches needed. So increased photosynthesis leads to higher respiration, but not linearly. The recent accumulation of photosynthates is more critical than the process of photosynthesis itself. Respiration will also use stored carbohydrates in roots, fruits, and seeds where there is no photosynthesis.

Water content in the plants: Drought and heat stress limit water content in plants. As a response, the leaves close their stomata, which reduces the intake of oxygen from the air and the rate of respiration. 

Age and type of tissue: Younger tissues have a higher respiration rate than older tissues. Thus, seedlings and young growing leaves respire more than older plants and leaves. The physiological activity in tissues will also influence respiration rate. For example, mature fruits or seeds respire at a lower rate.

Biological stress: Tissues damaged due to herbivory or diseases require more healing energy and therefore have a faster respiration rate. If the damage disrupts sap flow, respiration decreases in tissue deprived of photosynthates.

Genotype or species: The respiration rate and its changes will depend on the species and between individuals of a species.

Habitat: Each habitat is defined by various environmental factors like light, temperature, and water availability. Since these factors affect respiration, plants growing in each habitat will have varying respiration rates. 

External Factors

Oxygen: When oxygen levels decrease, so does respiration; as the concentration rises, so does respiration to a certain point. When there is no oxygen, anaerobic respiration takes over.

Temperature: Each species has an optimum range of temperature, between 18-40°C. When temperatures fall or rise beyond this range, respiration rates fall. However, increasing temperature will increase respiration rate until 50°C, after which damage to plant tissue stops respiration. Respiration changes are proportional for every 10°C change due to the Q10 effect, but the rate varies on species, drought, and growth stage. Increasing temperature enhances enzyme activity—temperatures below 0°C slow down respiration. The effect of temperature on plants will also depend on the plant part.

Carbon dioxide levels: Historically, it was thought that increasing CO2 in the air would dampen respiration levels due to changes induced by CO2 on mitochondrial enzymatic activities at cellular levels. However, this concept is now being challenged as the overall change in the rate of respiration at the plant level still needs to be clarified, though some indirect effects of high CO2 on other plant physiological processes could be slowing respiration. 

There is no change in the gas exchanged at tissue levels. Gas leaks during the measurement of gas exchange are also blamed for some of the reduction in respiration recorded. Also, a significant portion of respiration occurs in roots, increasing due to rising CO2 and enhanced new root growth. 

Light: Respiration does not need light. However, the stomata are open during daylight to allow photosynthesis, so as light intensity increases, respiration indirectly increases as oxygen intake increases. However, respiration will continue at night and is called Dark Respiration. 

Importance of Respiration

Respiration is essential for the plant and the atmosphere, where it, along with photosynthesis, plays a significant role in the global carbon and oxygen cycle. Some important aspects are discussed below.

Figure 2. “Respiration rates versus biomass for small and larger trees. (a) Aboveground respiration rates versus aboveground biomass, (b) total respiration rates versus total biomass at 248C. Open circles, US saplings; crosses, Japanese trees; filled circles, Chinese trees; dotted and dashed lines, regression for small trees; continuous lines, regression for larger trees,” Cheng et al. 2010. (Image credits: DOI:10.1098/rsbl.2010.0070) 

Biomass Accumulation

West, Brown, and Enquist proposed the WBE theory to find a relationship between respiration and biomass accumulation in plants. A modification to the WBE theory states that the whole plant respiration rate, R’s relation to plant mass M, changes with the growth stage/size. For seedlings, the regression of R scales close to 1, while in larger plants, it shifts to 0.82, see figure 2. 

Effect on Ecosystem-Level Carbon Budget 

About 40-60% of the carbon dioxide fixed by photosynthesis is released by respiration annually by terrestrial ecosystems. Hence respiration has an essential effect on the atmosphere and climate change. About 120 Gt of carbon moves through plants in the terrestrial system because of respiration and photosynthesis.

The bulk of research studying the effect of increased CO2 on respiration concluded that doubling present levels of CO2 in the atmosphere would reduce respiration by 15-20%. This would help vegetation sequester 3·4 Gt of carbon each year. 

The effects of higher CO2 on respiration at the ecosystem level still need to be well studied, and we also need to learn more about the processes controlling respiration. Hence, it is impossible to scale cellular, tissue, and plant-level respiration in ecosystems to predict the carbon budget. Nevertheless, given its importance for climate action, we can expect more studies on this topic. 

Effect of Global Warming on Respiration

Annual carbon release through terrestrial plant respiration is eight times more than anthropogenic fossil fuel combustion. Therefore, any slight change in plants’ respiration rates will significantly impact the atmosphere. In addition, increased respiration rates due to rising temperatures can further exacerbate climate change. Therefore, the combined effects of increasing temperature and CO2 from respiration need urgent attention.  

Root Respiration

Since a significant portion of respiration occurs through roots, this is important. It accounts for 50% of soil respiration and plays a vital role in Biogeochemical Cycles and soil carbon flux. 

In agriculture, waterlogging and soil oxygen availability, which affect root respiration, can be essential factors that affect plant growth. These are known to be an issue in greenhouses due to over-irrigation. The root respiration rates of plants are species-specific and need to be established for each plant.

Partitioning of respiration

Understanding the demands of respiration or energy use is an important area of research. The two component-model of growth and maintenance respiration is used to partition the respiration needs of the plant.

● Growth respiration covers the energy needs for growing new structures, nutrient uptake, and phloem loading. 

● Maintenance respiration refers to energy use needed for protein and membrane turnover, metabolic processes during acclimation to the environment, response to stress, and maintaining ion concentrations and gradients.

These types of respiration depend on plants’ age, size, chemical composition, and environment. 

Manipulating Respiration at the Cellular Level

Changing plant respiration through metabolic engineering by altering enzymes provides the chance to increase crop yields. Also, the increase in postharvest respiration due to temperature and moisture content, which can be deleterious for the quality of seeds and fresh produce, is manipulated by excellent storage or other means to improve the food supply. 

Similarly, reduced respiration in lower oxygen levels is used to preserve grains and fresh produce in controlled atmospheres during storage and in Modified Atmosphere Packaging (MAP). 

Respiration Helps in Stress Response

Plants experience many kinds of stress- low temperature, oxygen limitation, salinity, water, heavy metal exposure, UV radiation, etc. The byproducts formed in the respiratory pathways help in various ways. For example, TCA cycle intermediates like citrate and malate are released by roots into the soil to change pH or alleviate aluminum toxicity.

Respiration is flexible, and alternative pathways exist that use different enzymes in glycolysis, the TCA cycle, and the electron transport chain. The alternative enzymes use pyrophosphate instead of ATP. This has been proven to help plants adjust to stress and plays a vital role in adaptation and evolution. 

Both these aspects are helpful also to increase crop yield.

Applications of Respiration

Despite its importance, we still need to learn more about respiration and its effect on plant, crop, and ecosystem processes. Therefore we see many studies in the following areas:

Plant breeding for stress resistance

Increasing crop productivity 

Postharvest storage and packaging

Irrigation studies

Greenhouse management

Soil and root respiration

Soil carbon formation

Biomass production

Climate change studies

Carbon accounting

Measuring Respiration

Though the various cellular processes can be studied, respiration is usually measured as gas exchange using Infrared Gas Analyzers, like the CI-340 Handheld Photosynthesis System, manufactured by CID-Bio-science Inc. It is a small portable tool that can be used in fields to get results in a matter of seconds, with modules to control temperature, oxygen, and light. This makes the tool ideal for tackling some of the critical lacunae that currently exist in our understanding of the respiration control process.

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