Transpiration in leaves

Transpiration is the loss of water vapour from leaves. It happens in two connected stages: water first evaporates inside the leaf, then water vapour diffuses out.

Water on the surfaces of mesophyll cells evaporates into the air spaces between the cells. During evaporation, liquid water changes into water vapour.

The water vapour then diffuses through the air spaces and leaves through small openings called stomata. Diffusion is the net movement of water vapour from a region of higher concentration to a region of lower concentration. Thus, evaporation supplies water vapour inside the leaf, and diffusion carries it out.

Pathway of water through a plant

Water enters the plant from the soil through root hair cells on the surface of the root. It then passes through the cells of the root cortex and enters the xylem.

The xylem carries water through the root and stem into the leaf. In the leaf, water leaves the xylem and reaches the mesophyll cells inside the leaf. Together, these structures provide a continuous route for water from the root to the leaf.

Root hair cell adaptations

Root hair cells absorb water and mineral ions from the soil. Each cell has a long, hair-like extension that gives it a large surface area for uptake. Its thin cell wall provides a short pathway for water and mineral ions to enter the cell.

Water uptake by root hair cells

Water enters a root hair cell by osmosis. Soil water has a higher water potential than the cell sap inside the root hair cell. Water therefore moves across the cell’s partially permeable membrane into the cell, towards the lower water potential.

Mineral ion uptake by root hair cells

Root hair cells can absorb mineral ions by active transport. This moves ions against their concentration gradient, from a lower concentration in the soil to a higher concentration inside the cell.

Active transport requires energy released by respiration. Root hair cells contain mitochondria, where respiration releases energy to support this uptake.

Xylem

Xylem is plant tissue that carries water and dissolved mineral ions from the roots, through the stem, to the leaves. Its vessels also help support the plant.

Xylem vessel structure

Xylem vessels are hollow tubes formed from dead cells. The end walls of the cells have broken down, leaving a continuous pathway through which water can move.

The vessel walls are strengthened with lignin. This helps prevent the vessels from collapsing as water is pulled through them and also provides support for the plant.

Water movement in xylem

Water travels upwards through the xylem in the transpiration stream. As water is lost from the leaves, it helps pull a continuous column of water up through the vessels from the roots. Dissolved mineral ions are carried with the water.

Phloem and translocation

Plants transport dissolved sugars and other food molecules through tissue called phloem. The movement of these substances in phloem is called translocation.

Sugars made in leaves are carried to other parts of the plant. There, they may be used immediately or stored for later use. Phloem can transport substances in different directions, depending on which parts are supplying sugars and which parts are receiving them.

Phloem structure

Phloem consists of tubes formed from elongated living cells joined end to end. The end walls of these cells have pores, allowing cell sap containing dissolved substances to pass from one cell to the next along the tube.

Stomata and guard cells

A stoma is a small pore in the leaf epidermis, the outer layer of the leaf. Each stoma is surrounded by a pair of guard cells, which control how wide the pore is. The plural of stoma is stomata.

Stomata allow gases to diffuse between the air and the leaf. Carbon dioxide can enter for photosynthesis, while oxygen and water vapour can leave.

How guard cells control a stoma

When guard cells take in water, they become firm, or turgid. Their shape causes them to curve apart, opening the pore. When they lose water, they become less turgid and the pore closes.

Opening a stoma allows gas exchange, but it also gives water vapour more opportunity to escape from the leaf. Closing the pore reduces this opportunity, while also restricting gas exchange.

Environmental factors affecting transpiration

Transpiration is the loss of water vapour from a plant, mainly through openings in its leaves called stomata. Water evaporates from moist surfaces inside the leaf and diffuses out through the stomata. Environmental conditions can change the rate by affecting evaporation, diffusion or stomatal opening.

Conditions that change transpiration rate

• Temperature: A higher temperature generally increases evaporation inside the leaf and diffusion of water vapour out of it, so transpiration increases.
• Humidity: Higher humidity means there is more water vapour in the surrounding air. This reduces the difference in water-vapour concentration between the inside of the leaf and the air, so diffusion out of the leaf is slower and transpiration decreases.
• Air movement: Greater air movement removes water vapour from around the leaf. This maintains a steeper concentration gradient between the leaf and the surrounding air, so water vapour diffuses out faster and transpiration increases.
• Light intensity: Higher light intensity generally causes stomata to open for gas exchange. More water vapour can then escape, so transpiration increases.

These comparisons describe the effect of changing one condition while other conditions remain comparable. When several conditions change together, their effects on transpiration may combine or oppose one another.

Calculating transpiration rate

A transpiration rate describes how much water a plant loses over a period of time. To calculate the average rate, divide the amount of water lost by the time taken.

The answer has a compound unit: a water-loss unit per unit of time. For example, a mass of water lost in grams over minutes gives a rate in $$\mathrm{g/min}$$. A volume lost in cubic centimetres over hours gives a rate in $$\mathrm{cm^3/h}$$.

Rate over a selected interval

For an interval that starts after the measurements began, use the change in water lost during that interval, not the total loss at the end. Subtract the starting value from the ending value, then divide by the duration of the interval.

For example, suppose cumulative water loss is $$2,\mathrm{g}$$ at $$5,\mathrm{min}$$ and $$6,\mathrm{g}$$ at $$10,\mathrm{min}$$. The average rate between these times is:

This is the average over the five-minute interval; the rate need not have been constant throughout it.

Transpiration rate from a graph

On a graph of cumulative water loss against time, the average rate over an interval is the gradient between the two points at its endpoints. Read the water-loss and time values at each point, then divide the change in water loss by the change in time. A steeper gradient means a greater average rate of water loss.