Respiration is a cellular process by which plants, animals and other organisms produce energy. In the mitochondria of the cell, sugars are combined with oxygen in a series of chemical reactions. This results in the creation of carbon dioxide, water and energy in the form of an adenosine triphosphate (ATP) molecule, which is used to fuel other cellular processes.
In plants, respiration can be viewed as reversing the process of photosynthesis. Like photosynthesis, respiration requires an exchange of gases with the atmosphere. Unlike photosynthesis, however, respiration can take place during the day or night and occurs not just in the leaves, but in the stem and roots as well.
Respiration rates have a significant effect on plant health and growth. A major factor determining the respiration rate is plant physiology, with each species respiring according to its specific adaptations to the local environment. As well as the natural differences between plant types, there are many environmental factors affecting respiration rates in plants. This can have large implications for issues such as agriculture and horticulture, the ecology of natural areas and food preservation.
Tissue Age & Life Stage
Life stage is one of the most important factors affecting respiration in plants. When a seed has absorbed enough water, it produces a burst of respiration to germinate and power its emergence from the seed case. As leaves emerge, branches grow and root tips dig into the soil. These parts initially respire at a high rate as they use energy to grow. As the roots, branches and leaves mature, respiration rates decrease.
Respiration also increases when fruits and seed are developing, with energy use peaking when fully ripe. Respiration falls dramatically after this point, with many annual plants dying off completely after fruiting, while perennials may shed older and damaged leaves.
Sunlight or Artificial Light
Many plants have two forms of respiration. Plants, and all other living organisms, use dark respiration, which is not dependent on sunlight. However, plants can also use photorespiration, which is dependent on light to power the chemical reactions. Therefore, changes in light levels caused by clouds, shading or being covered by dust, paint or other materials can affect the rate of respiration.
Respiration Rates & Temperature
Respiration rate is directly linked to the ambient temperature, with respiration rate peaking at an optimal temperature. Changes in temperature from this optimum will decrease efficiency. Different plant species have varying optimal ranges that are generally between 18 degrees Celsius (65 degrees Fahrenheit) and 40 degrees Celsius (105 degrees Fahrenheit).
All plants will stop respiring when temperatures reach near freezing (zero degrees Celsius, 32 degrees Fahrenheit) or when temperatures of 50 degrees Celsius (122 degrees Fahrenheit) or higher cause cells and tissue to break down.
The respiration rates of fruits can be controlled by storage in cool, dry places. Lower storage temperatures are able to slow the respiration and ripening of fruit.
As respiration requires oxygen from the atmosphere, decreased available oxygen will reduce respiration rates. In plants, this normally occurs in the root zones in water-logged and poorly drained soils. Under these conditions, cells will use anaerobic respiration (fermentation), which does not require oxygen.
Anaerobic respiration is the same process used to produce breads, yogurts, wine and beer. However, it is a less efficient method of respiration, and plants cannot survive for long periods using this process alone. Some plants, such as mangrove trees, adapt to low-oxygen soils by growing aerial roots that emerge from the soil surface to increase oxygen availability.
The rate of respiration for most plants peaks around the normal oxygen level in the atmosphere.
Although drought has a much greater impact on the process of photosynthesis in plant cells, lack of available water also negatively affects respiration. Plants will attempt to limit their loss of water to the atmosphere, including reducing their exchange of gases with the atmosphere, which then reduces the amount of oxygen available to the respiration process.
Dry or desiccated tissue has a lower respiration rate than hydrated tissue, which is why many fruits can be preserved through drying.
Carbon Dioxide Levels
Carbon dioxide, one of the waste products of the respiration process, also affects respiration. The higher the concentration of carbon dioxide, the lower the rate of respiration. This can occur during times of reduced gas exchange in the atmosphere, such as when a plant is conserving water.
Carbon dioxide is also used to measure the rate of respiration, described as the quantity of carbon dioxide (in mg) produced by one kilogram of plant material in one hour.
Damage and Disease
When a plant is damaged or infected, it will begin a process of healing and repair. This can include the leaking of sap to fill a wound or the creation of a gall to trap a burrowing insect. This is an energy-intensive process, and respiration will increase in both the damaged tissue and the surrounding cells to supply the necessary fuel.
Additionally, some diseases can interfere with the transfer of sugars within a plant. In these cases, parts of the plant with restricted flows of sugar can experience reduced respiration.
Respiration requires sugars to convert into energy. All sugars a plant uses are produced from photosynthesis, which generally occurs in the green leaves and stems. An increased rate of photosynthesis often leads to an increased rate of respiration, with leaves in the upper canopy of trees and shrubs respiring faster than shaded material lower down. However, excess sugars can be stored in the roots and stem of the plant for use in times of reduced photosynthesis.
About the Author
Born and raised in Western Australia, I have been fascinated by the natural world for as long as I can remember. I studied Conservation Biology at Murdoch University before working as a government field researcher, project leader for community-led bushland regeneration programs, private botanical consultant and environmental educator. Although fascinated by all things ecological, a particular interest is regenerative agriculture and its potential to help transform our entire relationship with the natural world in which we depend.
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