We are all aware of the process of transpiration that happens in leaves via the stomata. Many factors impact transpiration rate, which can be classed as external or internal. Plants are assumed to need transpiration because it helps them to collect water and nutrients from the soil; yet, excessive transpiration may be exceedingly damaging to the plant, even leading to death due to dehydration. Transpiration is a normal bodily function. Let us now look at the numerous environmental and internal elements that impact transpiration in this article.

What is transpiration?

Transpiration is the mechanism through which water vapour is lost via the stomata of plants. When the temperature is particularly hot, the plant’s loss of water vapour cools it down, and water from the stem and roots flows upwards or is ‘drawn’ into the leaves. When there is less water available for the plants, dehydrated mesophyll cells produce the plant hormone abscisic acid, which forces the stomatal pores to shut and reduces water loss during oxygen release and carbon dioxide absorption.

Factor Affecting Transpiration Rate

The rate of transpiration is affected by two variables, which are classified as follows:

1. Internal 
2. External

• Factors that are internal, structural, or plant-related

Internal variables include plant structural elements that influence transpiration rate, which are as follows:

• Surface Area of the Leaf

If the surface area of the leaf is larger, the rate of transpiration is quicker. However, because smaller leaves have a greater number of stomata than bigger leaves, the rate of transpiration per unit surface area is greater in smaller leaves. However, the rate of transpiration per unit leaf surface area falls in a canopy due to foliage density, shade, and decreased air movement inside the canopy.

• Stomata 

Stomata are leaf pores that promote gas exchange by allowing water vapour to leave the plant and carbon dioxide to enter. Guard cells are special cells that govern the opening and closure of each pore. When stomata open, transpiration rates rise; when they close, transpiration rates fall.

• Boundary layer 

The boundary layer is a tiny layer of calm air that hugs the surface of the leaf. This layer of air is stationary. To transpiration, water vapour must flow through this stationary layer from the stomata to the atmosphere, where it will be eliminated by moving air. The slower the rate of transpiration, the bigger the boundary layer.

• Ratio of Root-Shoot

Water intake from the soil is more efficient with a large root system. The rate of transpiration rises as water supply increases. Thus, a low root-shoot ratio reduces transpiration, whereas a high root-shoot ratio promotes transpiration. As a result, the rate of transpiration is related to the root-shoot ratio.

• Leaf Orientation

When leaves are positioned transversely on the shoot, they lose more water when they are exposed to direct sunlight, but the rate of transpiration reduces when the leaves are oriented perpendicularly on the stem.

External Factors

External factors are environmental factors that influence the rate of transpiration, and they are as follows:

1. Relative Humidity – Relative humidity (RH) is the quantity of water vapour in the air in comparison to the amount of water vapour the air could contain at a given temperature. Any decrease in atmospheric water produces a gradient for water to migrate from the leaf to the atmosphere. As lower the relative humidity will be, the less wet the environment and, as a result, the stronger the pushing force for transpiration. When the relative humidity (RH) is high, the environment contains more moisture, which reduces the driving force for transpiration.
2. Temperature – Rather than having a direct effect on stomata, temperature has a large influence on the amount of the driving force for water transport out of a plant. As the temperature rises, so does the air’s capacity to hold water. Only the capacity of the air to hold water changes, not the volume of water. Because warmer air can store more water, its relative humidity is lower than that of the same air sample at a lower temperature, indicating that it is ‘drier air.’ Because colder air carries less water, its relative humidity rises, or it is referred to as moist air.’ As a result, warmer air increases the driving force for transpiration, whereas colder air decreases the driving force for transpiration.
3. Soil water – The soil is the source of water for the plant’s transpiration. Plants with enough soil moisture will often transpire at a high pace since the earth provides the water for the plant to pass through. Plants cannot continue to transpire without wilt if the soil is too dry, since the water in the xylem that passes out via the leaves is not replenished by soil water. The leaf loses turgor or rigidity as a result of this situation, and the stomata close. If this turgor loss persists throughout the plant, it will wilt.
4. Light — In the presence of light, stomata open, allowing carbon dioxide to enter the light-dependent process of photosynthesis. In the dark, most plants’ stomata close. Very low amounts of light at dawn can trigger stomata to open, allowing carbon dioxide to enter the leaves for photosynthesis as soon as the sun touches them. Stomata are particularly sensitive to blue light, which is most prevalent at sunrise.
5. Wind – Wind can change transpiration rates by removing the boundary layer, which is a motionless layer of water vapour hugging the surface of the leaves. When the boundary layer is reduced, the channel for water to reach the atmosphere is shortened, which promotes the migration of water from the leaf surface.
6. Soil Water Content – The rate of transpiration is largely influenced by the availability of water in the soil. If there isn’t enough water available to the plants, the leaves get dehydrated, producing stomatal closure and a drop in transpiration rate. This situation frequently develops during a drought or when the soil becomes frozen or so cold that the roots are unable to absorb water.
7. Pressure in the Atmosphere – The rate of transpiration is inversely related to the pressure in the atmosphere. The rate of transpiration rises when atmospheric pressure decreases because air can readily flow out of the plant owing to the formation of pressure deficit.

Conclusion 

The rate of transpiration is affected by two variables first is internal such as, Surface Area of the Leaf, Stomata and Boundary Layer and the second one is external factor i.e. Humidity, temperature etc. Transpiration is the mechanism through which water vapour is lost via the stomata of plants. When the temperature is particularly hot, the plant’s loss of water vapour cools it down, and water from the stem and roots flows upwards or is wouldrawn’ into the leaves. If the surface area of the leaf is larger, the rate of transpiration is quicker. The boundary layer is a tiny layer of calm air that hugs the surface of the leaf. The slower the rate of transpiration, the bigger the boundary layer. Relative humidity is the quantity of water vapour in the air in comparison to the amount of water vapour the air could contain at a given temperature.