How External Factors Affect the Respiration Process in Plants
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External factors influence plant respiration mainly through temperature, water content, light, aeration, and a handful of additional conditions such as toxins or wounding. Each of these factors changes how fast a plant takes in oxygen and releases carbon dioxide, and together they determine whether respiration proceeds at a steady, healthy rate or slows to the point that the plant is endangered.
How does temperature affect plant respiration?
Temperature has a powerful effect on plant respiration, just as it does on other life processes. As a general rule, raising the temperature by 10° roughly doubles the rate of the chemical reactions involved in respiration.
This acceleration of reaction rate is called the temperature coefficient and is written as Q10. The lower temperature limit of respiration lies well below 0°. According to N. A. Maximov, the buds of deciduous trees and the needles of conifers can still respire at 20–25° of frost.
Respiration intensity rises quickly as temperature climbs toward 40°, then declines with further heating; above 50° respiration falls sharply and the plant dies. The increase in respiration at high temperatures is unstable, because such heat lowers the viability of the tissues.
A change in temperature also affects the chemistry of respiration, since one set of oxidative ферментов may be replaced by another that has a different temperature minimum. For example, at the onset of ripening in яблок, respiration proceeds with the participation of cytochrome oxidase, and later of polyphenol oxidase.
The optimal temperature for most plants is 30–40°, the range in which respiration stays at a single, fairly high and stable level. This is why moderate warmth supports vigorous metabolic activity without pushing tissues toward heat damage.
How does water content influence respiration?
Water content in plant cells strongly affects respiration intensity, and noticeable respiration occurs only in sufficiently hydrated cells. Dry tissues respire so weakly that their gas exchange is barely measurable.
There is no straightforward, linear relationship between respiration intensity and the amount of water, however. A slight drop in leaf moisture actually intensifies respiration, while greater dehydration weakens it; seeds in an air-dry state respire negligibly, yet respiration increases as seed moisture rises.
During seed ripening, respiration intensity falls as moisture content declines. When grain reaches the late waxy stage of ripeness and its water content drops to about 16%, respiration intensity decreases sharply, because the water remaining in the grain is held firmly by colloids and is no longer freely available for metabolic reactions.
Does light affect plant respiration directly?
Light affects respiration indirectly rather than directly. Its most immediate influence comes from the rise in temperature that occurs when plants are warmed by the sun's rays, and this warming, not the light itself, speeds up respiration.
Light also shapes respiration through the accumulation of assimilates that serve as the respiratory substrate. By driving photosynthesis, illumination builds up the sugars that respiration later breaks down, so the effect of light reaches respiration through two separate, indirect pathways.
How does aeration and oxygen supply affect respiration?
The oxygen content of the atmosphere surrounding a plant is essential for respiration. The normal oxygen level of air, up to about 21%, is enough for healthy plant respiration. Respiration begins to be suppressed when oxygen falls below 5%, and at that point анаэробное дыхание may set in.
Roots and rhizomes obtain oxygen from the air held in the soil, where its content ranges from 7 to 12% in soil with a good crumb structure. In poorly worked, structureless, or heavily waterlogged soils the oxygen content drops sharply, which can leave the roots short of the oxygen they need to respire.
When soil oxygen runs low, anaerobic respiration begins, and once the plant's reserve substances are exhausted the roots die off and the plant perishes. This is why drainage and soil structure are so closely tied to root health.
What other factors influence plant respiration?
Beyond the four major environmental factors, toxins and physical injury also alter respiration. When a plant is exposed to poisonous substances in large doses, respiration rapidly declines and the plant dies; in small doses, by contrast, the same poisons stimulate respiration.
Respiration intensity rises sharply when a plant is wounded, which is probably a protective reaction of the organism. The division of cells on the surface of the wound likely plays a part here, as it does, for example, when trees are grafted.
- Temperature — roughly doubles respiration per 10° rise up to about 40°, then becomes damaging above 50°.
- Water content — respiration needs hydrated cells, but the relationship is non-linear.
- Light — acts indirectly through warming and through building respiratory substrate.
- Aeration — below 5% oxygen, respiration is suppressed and may turn anaerobic.
- Toxins and wounding — small toxin doses stimulate, large doses kill; injury intensifies respiration.
