Transpiration

Transpiration is essential for transporting water and minerals up a plant and results in the loss of water vapour through tiny pores in leaves, called stomata. This process occurs exclusively in xylem vessels which have adapted their structure to facilitate effective water transport.

Transpiration in Plants

Transpiration is the evaporation of water from the spongy mesophyll layer in leaves and the loss of water vapour through the stomata. This occurs in xylem vessels, which make up half of the vascular bundle consisting of xylem and phloem. The xylem also carries ions dissolved in water, and this is crucial for plants as they need water for photosynthesis. Photosynthesis is the process by which plants absorb light energy and use it to form chemical energy. Below, you will find the word equation and the necessity of water in this process.

$Carbondioxide+Water\stackrel{Lightenergy}{\to }Glu\cos e+Oxygen$

As well as providing water for photosynthesis, transpiration also has other functions in the plant. For example, transpiration also helps keep the plant cool. As plants carry out exothermic metabolic reactions, the plant can heat up. Transpiration allows the plant to stay cool by moving water up the plant. As well as this, transpiration helps to keep cells turgid. This helps to maintain structure in the plant and prevent its collapse.

Fig. 1 - The directionality of xylem vessels

The Process of Transpiration

Transpiration refers to the evaporation and loss of water from the leaf's surface, but it also explains how water moves through the rest of the plant in the xylem. When water is lost from the leaves' surface, negative pressure forces water to move up the plant, often referred to as the transpiration pull. This allows water to be transported up the plant with no additional energy required. This means that water transport in the plant through the xylem is a passive process.

Fig. 2 - The process of transpiration

Factors Affecting Transpiration

Several factors affect the rate of transpiration. These include wind speed, humidity, temperature and light intensity. These factors all interact and work together to determine the rate of transpiration in a plant.

Table 1. The factors that affect the rate of transpiration.

Adaptations of the Xylem Vessel

There are many adaptations of the xylem vessel that allow them to efficiently transport water and ions up the plant.

Lignin

Lignin is a waterproof material found on the walls of xylem vessels and is found in different proportions depending on the age of the plant. Here is a summary of what we need to know about lignin;

• Lignin is waterproof
• Lignin provides rigidity
• There are gaps in the lignin to allow water to move between adjacent cells

Lignin is helpful in the process of transpiration too. The negative pressure caused by the loss of water from the leaf is significant enough to push the xylem vessel to collapse. However, the presence of lignin adds structural rigidity to the xylem vessel, preventing the vessel's collapse and allowing transpiration to continue.

Protaoxylem and Metaxylem

There are two different forms of xylem found at various stages of the plant's life cycle. In younger plants, we find protoxylem and in more mature plants, we find metaxylem. These different types of xylem have different compositions, allowing for different growth rates at different stages.

In younger plants, growth is crucial; protoxylem contains less lignin, enabling the plant to grow. This is because lignin is a very rigid structure; too much lignin restricts growth. However, it provides more stability for the plant. In older, more mature plants, we find metaxylem contain more lignin, providing them with a more rigid structure and preventing their collapse.

No Cell Contents in Xylem Cells

Xylem vessels are not living. The xylem vessel cells are not metabolically active, which allows them to have no cell contents. Having no cell contents allows for more room for water transport in the xylem vessel. This adaptation ensures that water and ions are transported as efficiently as possible.

Additionally, the xylem also has no end walls. This allows for the xylem cells to form one continuous vessel. Without cell walls, the xylem vessel can maintain a constant stream of water, also known as the transpiration stream.

Types of Transpiration

Water can be lost from the plant in more than one area. The stomata and cuticle are the two main areas of water loss in the plant, with water being lost from these two areas in slightly different ways.

Stomatal Transpiration

Around 85-95% of water loss happens through the stomata, known as stomatal transpiration. The stomata are small openings mostly found on the bottom surface of leaves. These stomata are closely bordered by guard cells. Guard cells control whether the stomata open or close by becoming turgid or plasmolysed. When the guard cells become turgid, they change shape allowing the stomata to open. When they become plasmolysed, they lose water and move closer together, causing the stomata to close.

Plasmolysed guard cells signify that the plant does not have enough water. So, the stomata close to prevent further water loss. Conversely, when the guard cells are turgid, this shows us that the plant has enough water. So, the plant can afford to lose water, and the stomata remain open to allow for transpiration.

Stomatal transpiration only occurs during the day because photosynthesis takes place; carbon dioxide needs to enter the plant via the stomata. At night, photosynthesis does not occur, and therefore, there is no need for carbon dioxide to be entering the plant. So, the plant closes the stomata to prevent water loss.

Cuticular Transpiration

Cuticular transpiration makes up for around 10% of transpiration in the plant. Cuticular transpiration is transpiration through the cuticles of a plant, which are layers at the top and bottom of the plant that serves a role in preventing water loss, highlighting why transpiration from the cuticle only accounts for around 10% of transpiration.

The extent to which transpiration happens through the cuticles depends on the thickness of the cuticle and whether the cuticle has a waxy layer or not. If a cuticle has a waxy layer, we describe it as a waxy cuticle. Waxy cuticles prevent transpiration from occurring and avoid water loss — the thicker the cuticle, the less transpiration can occur.

Differences Between Transpiration and Translocation

We must understand the differences and similarities between transpiration and translocation. It may be helpful to read our article on translocation to understand this section better. In short, translocation is the two-way active movement of sucrose and other solutes up and down the plant.

Solutes in Translocation and Transpiration

Translocation refers to the movement of organic molecules, such as sucrose and amino acids up and down the plant cell. In contrast, transpiration refers to the movement of water up the plant cell. The movement of water around the plant happens at a much slower speed than the movement of sucrose and other solutes around the plant cell.

Energy in Translocation and Transpiration

Translocation is an active process as it requires energy. The energy needed for this process is transferred by the companion cells accompanying each sieve tube element. These companion cells contain many mitochondria that help carry out the metabolic activity for each sieve tube element.

On the other hand, transpiration is a passive process as it does not require energy. This is because the transpiration pull is created by the negative pressure which follows water loss through the leaf.

Direction

The movement of water in the xylem is one way, meaning it is unidirectional. Water can only move up through the xylem to the leaf.

The movement of sucrose and other solutes in translocation is bidirectional. Due to this, it requires energy. Sucrose and other solutes can move both up and down the plant, aided by the companion cell of each sieve tube element. We can see that translocation is a two-way process by adding radioactive carbon to the plant. This carbon can be seen above and below the point where it was added to the plant.

Fig. 4 - The main differences between transpiration and translocation