PLANT TRANSPIRATION: PROCESS, FACTORS AND IMPORTANCE - BIOLOGY - 2025

The transpiration of plants and remains of plant organisms is the process of loss of water in gaseous form that occurs through the stomata, which are specialized structures located in the leaf blades.

Perspiration is linked to various physiological processes in plants, which continuously absorb and lose water. Through this homeostatic mechanism, most of the evaporation of water occurs, as the atmospheric carbon dioxide necessary for photosynthetic processes is absorbed.

The stomata of Zebrina spp. (Source: AioftheStorm via Wikimedia Commons)

On average, a leaf can exchange up to 100% of its water content with the environment during a hot, dry and sunny day. Likewise, the calculations made by some authors allow estimating that, during the life of a plant, it can lose a mass equivalent to more than 100 times its fresh weight through the leaves due to perspiration.

Many plant physiologists and ecophysiologists are dedicated to "measuring" the transpiration rate of plants, as this can give them information about their physiological state and even some of the environmental conditions to which plants are continually subjected.

Where and why does perspiration occur?

Perspiration is defined as the loss of water in the form of vapor and is a process that occurs mainly through the leaves, although it can also occur, but to a much lesser extent, through small “openings” (lenticels) in the bark of the stems and branches.

It occurs thanks to the existence of a vapor pressure gradient between the leaf surface and the air, so it is deduced that it occurs due to an increase in the internal water vapor pressure in the leaves.

In this way it becomes greater than that of the vapor that surrounds the leaf blade, which can cause it to diffuse from the more concentrated zone to the less concentrated one.

Stomata

Stomata in lily epidermis. Viascos

This process is possible due to the existence of structures that "interrupt" the continuity of the leaf surface (epidermis) and are known as stomata.

The stomata allow the “controlled” release of water vapor from the leaves, avoiding evaporation by direct diffusion from the epidermal tissues, which occurs passively and without any type of control.

A stoma consists of two "guard" cells, which are shaped like a "sausage" or a "kidney", which form a pore-shaped structure, the closing or opening of which is controlled by different hormonal and environmental stimuli:

- It can be said that, in dark conditions, with internal water deficit and at extreme temperatures, the stomata remain closed, "trying" to avoid large losses of water through perspiration.

- The presence of sunlight, abundant availability of water (external and internal) and an "optimal" temperature promote stomatal opening and increased transpiratory rates.

When the guar cells fill with water, they become turgid, causing the stomatal pore to open; This is the opposite of what happens when there is not enough water, which is when the stomata remain closed.

Perspiration process

Scheme of the transpiration process in a plant (Source: Laurel Jules via Wikimedia Commons)

Having clarified the concept of stomata, the process of perspiration, then, occurs as follows:

1- The water transported in the xylem of vascular plants diffuses towards the foliar tissues, especially towards the mesophyll cells.

2- Said water can evaporate as a result of high temperatures and solar irradiation; The water vapor thus generated remains in characteristic air spaces found in the mesophyll (it is “concentrated”).

3- This water vapor moves by diffusion into the air when the stomata open, either in response to some phytohormone (a substance that regulates plant growth), an environmental condition, etc.

The opening of the stoma implies an exchange of water vapor from the plant to the atmosphere, but at the same time it allows the diffusion of carbon dioxide from the air to the foliar tissues, a process that occurs mainly due to a concentration gradient.

Factors that affect perspiration

There are multiple factors that affect transpiration, although their importance is relative to the type of plant considered.

Effect of wind speed on transpiration rate (Source: DGmann)

External factors

From an environmental point of view, perspiration depends considerably on solar radiation and temperature, as well as on the availability of water in the soil, the deficit in air vapor pressure, wind speed, etc.

Effect of wind speed on transpiration rate (Source: DGmann)

For some plants, the concentration of external carbon dioxide (CO2) is also a key element in regulating perspiration (stomatal opening). Some texts indicate that when the internal CO2 levels decrease considerably, the guard cells allow the opening of the stomatal pore to facilitate the entry of said gas.

Effect of temperature on the transpiratory rate (Source: DGmann)

Internal factors

In the anatomical context, transpiratory rates vary greatly depending on the external characteristics of the leaf surface (as well as the leaf surface area). In most vascular plants, the leaves are usually covered with "waxy layers" which are collectively known as the cuticle.

Effect of leaf area on transpiration rate (Source: DGmann via Wikimedia Commons)

The cuticle is a highly hydrophobic structure (which repels water), so it prevents perspiration by simple evaporation from the leaf parenchyma to the surface and thus prevents the total desiccation of the cells of the leaf tissue.

The presence or absence of an "efficient" cuticle in water vapor retention conditions the transpiratory rates of a vascular plant. Furthermore, the water absorption capacity of the roots can also be a conditioning factor for perspiration.

Abscisic acid (ABA) is a phytohormone related to perspiration: it promotes stomatal closure by inhibiting some of the enzymes necessary for water to enter the guard cells of the stomata, preventing their opening.

Usually it is a substance produced to "communicate" to the plant that there are water deficiencies from the root tissues.

Importance

Thermal homeostasis

Water is one of the most important natural resources for all living organisms, so plants are no exception. Therefore, all the processes that have to do with the water exchange between a plant and the environment that surrounds it are of utmost importance for its survival.

From the point of view of thermal homeostasis, perspiration is essential to dissipate the heat generated by solar radiation. This dissipation occurs thanks to the fact that the water molecules that escape into the atmosphere in the form of water vapor have a large amount of energy, which breaks the bonds that "retain" them in the liquid form.

The escape of the water molecules “leaves behind” a mass of molecules that has less energy than those that were dissipated, which encourages the cooling of the remaining “body” of water and, therefore, of the entire plant.

Water transport by negative hydrostatic pressure

When the transpiration rates in the leaves are very high, the water column in the xylem, which is part of the vascular system of many plants, rises rapidly from the roots, promoting the root absorption of water and other compounds and nutrients in the floor.

Thus, water moves from the ground to the atmosphere inside the plants thanks to the negative hydrostatic pressure exerted by the leaves during transpiration, which occurs thanks to the cohesive properties of water, which maintains high tensions throughout length of the water column in the xylem.

In other words, the evaporation of water and its release by perspiration provides most of the energy necessary for the upward movement of water, thanks to the existence of a water potential gradient between the leaf blades and the atmosphere.

Photosynthesis

Since perspiration is not only about the loss of water in the form of vapor, but also involves the entry of carbon dioxide into the foliar tissues, this process is also of utmost importance for photosynthesis, since CO2 is essential for the synthesis of food substances.

References

1. Azcón-Bieto, J., & Talón, M. (2000). Fundamentals of plant physiology (No. 581.1). McGraw-Hill Interamericana.
2. Encyclopaedia Britannica Inc. (2014). Encyclopaedia Britannica. Retrieved January 5, 2020, from www.britannica.com/science/transpiration
3. Taiz, L., & Zeiger, E. (2002). Plant physiology.
4. Taiz, L., Zeiger, E., Møller, IM, & Murphy, A. (2015). Plant physiology and development.
5. Turtenwald, K. (2018). Sciencing. Retrieved January 8, 2020, from www.sciencing.com