0610 · 8.3Transpiration

Water loss from leaves

0:00A plant has no heart pumping water upwards. Yet water travels from its roots to its leaves, sometimes against gravity over a considerable distance. The surprising part is that losing water from the leaves helps drive this movement. But if the plant loses water faster than it can replace it, it begins to wilt. By the end of this video, you will understand how water leaves a leaf, how that loss pulls more water upwards, and why temperature, wind speed and humidity change the rate.

0:26We will also investigate transpiration and explain why a thirsty plant droops. First, three useful reminders. Xylem carries water through the plant. Mesophyll cells are the cells inside a leaf. Stomata are tiny pores in the leaf surface, and a single pore is called a stoma. You also need diffusion. This is the net movement of particles from a region of higher concentration to a region of lower concentration. Keep that idea in mind, because it explains the final stage of water leaving a leaf.

0:57Transpiration is the loss of water vapour from leaves. To understand this definition properly, imagine following a small amount of water inside a leaf. The surfaces of the mesophyll cells are moist. Water evaporates from these surfaces into the air spaces between the cells. Evaporation means liquid water changes into water vapour. The water vapour then diffuses through the interconnected air spaces and out of the leaf through the stomata.

1:21Usually, the concentration of water vapour is higher inside the leaf than in the surrounding air, so there is a net movement outwards. Notice the two separate processes. Water evaporates from mesophyll cell surfaces. Water vapour then diffuses out through stomata. The stomata are exits, not the main surfaces where evaporation happens. And the water leaving through them is a gas, not a stream of liquid water. Why does the inside of a leaf allow so much evaporation?

1:48The interconnecting air spaces expose the surfaces of many mesophyll cells to air. Together, these surfaces provide a large internal surface area from which water can evaporate. Think of the difference between a single wet tile and many wet tiles spread out. More exposed wet surface allows more evaporation under the same conditions. Inside a leaf, it is the exposed moist cell surfaces that matter, not simply the volume of the air spaces.

2:13The number and size of stomata also affect water loss. With other conditions the same, more stomata provide more routes for water vapour to escape. Larger open pores provide a greater total opening for diffusion. Closing the pores reduces that opening and therefore reduces water loss.

Transpiration pull in xylem

2:29Now connect water loss at the leaf to upward movement in the stem. As water evaporates from the mesophyll surfaces, water is drawn from the leaf xylem to replace it. This creates a pulling force in the xylem called transpiration pull. The important detail is that water molecules attract one another. This attraction is called cohesion. It helps hold the water molecules together as a continuous column. So, when water is pulled towards the evaporating surfaces in the leaf, the pull is transmitted down the column of water in the xylem.

2:58The column is drawn upwards. Imagine a connected chain being pulled from its top. Pulling the top moves the links below because they remain connected. Water is not literally a metal chain, but the analogy helps explain why attraction between its molecules matters. Without those forces of attraction, pulling water at the top would not draw up a continuous column in the same way. The sequence is therefore evaporation from the mesophyll surfaces, replacement from the xylem, and transpiration pull drawing the water column upwards.

3:28This is a pull associated with water loss at the leaves, not a pump inside the stem.

Factors affecting transpiration

3:32We can now predict how the surroundings affect transpiration. Ask two questions. How quickly does water evaporate? And how easily can water vapour diffuse away? First, temperature. As temperature increases, water molecules gain kinetic energy. More molecules can escape from the moist mesophyll surfaces, so evaporation becomes faster. Water vapour also diffuses faster at higher temperatures. Therefore, increasing temperature usually increases transpiration rate, provided other conditions, including stomatal opening, remain comparable.

4:04The explanation is not simply that the plant feels hot or needs more water. It is that temperature affects the movement of water molecules and the rate of evaporation and diffusion. Second, wind speed. In still air, water vapour leaving a leaf can build up near its surface. This makes the air immediately outside the stomata more humid. The difference in water vapour concentration between the inside and outside becomes smaller, so diffusion out becomes slower.

4:30Wind removes this humid air and replaces it with less humid air. That maintains a steeper concentration gradient, so water vapour diffuses out faster. Increasing wind speed therefore usually increases transpiration rate. Third, humidity. Humidity describes how much water vapour is present in the air. In humid conditions, there is more water vapour outside the leaf. At the same temperature, increasing humidity makes the concentration gradient between the leaf air spaces and the surrounding air less steep.

4:58Water vapour diffuses out more slowly, so transpiration decreases. Dry air does the opposite. It produces a steeper concentration gradient and usually increases transpiration. These predictions assume the stomata remain similarly open. If water loss becomes excessive, stomata may close, limiting further loss.

Investigating with a potometer

5:17How could we investigate these effects? A potometer measures water uptake by a leafy shoot. As the shoot takes up water, an air bubble moves along a narrow capillary tube. We measure how far the bubble travels in a known time. This gives an estimate of transpiration rate. It does not measure water vapour loss directly, because some absorbed water is used by the plant rather than being transpired. Set up the apparatus filled with water.

5:42Cut the shoot under water to reduce the risk of air entering its xylem. Make sure all connections are watertight and airtight, so leaks do not move the bubble and produce misleading readings. To investigate wind speed, place a fan near the shoot and compare different measured wind speeds. Keep temperature, humidity, light intensity and the leafy shoot the same. Allow the shoot to adjust to each condition before measuring.

6:04Record bubble movement over the same time interval, repeat each condition, and calculate a mean. Reset the bubble between readings using the reservoir. To investigate temperature, use the same approach at different measured temperatures. Keep wind speed, humidity, light intensity and leaf area constant. Otherwise, a change in bubble movement might be caused by more than temperature alone. For example, a heater that also blows air would change both temperature and wind speed.

6:31That would not be a fair test of temperature unless airflow were controlled. Let us work through an illustrative calculation. These are practice values, not reported experimental results. Suppose the bubble moves twelve millimetres in four minutes in still air. To find its movement per minute, divide twelve by four. The rate is three millimetres per minute. With the fan running, suppose it moves twenty millimetres in four minutes. Dividing twenty by four gives five millimetres per minute.

6:58We divide by time because a rate tells us how much change occurs in each unit of time. Using the same capillary tube, faster bubble movement indicates faster water uptake. The higher rate with the fan is consistent with increased transpiration because moving air removes water vapour from around the leaf. These rates describe bubble distance per minute, not water volume per minute. Calculating a volume rate would also require the tube’s internal cross sectional area.

Wilting and transpiration recap

7:24Now return to the drooping plant. Wilting occurs when water loss exceeds water uptake for long enough that cells lose water and become less firm. Normally, water inside plant cells helps their contents press against the cell walls. This pressure is called turgor pressure. Cells with high turgor pressure are described as turgid. If cells lose water, their vacuoles become smaller and turgor pressure decreases. The cells become flaccid, meaning less firm.

7:50Leaves and soft stems depend partly on turgid cells for support. When that support decreases, they droop. That is wilting. A plant can therefore wilt even while its roots are still absorbing some water. The problem is the balance. Water is being lost faster than it is replaced. If sufficient water becomes available before serious damage occurs, cells can take up water by osmosis, regain turgor and restore support. Let us apply this to an exam-style question.

8:17Explain why a plant exposed to increased wind speed may wilt if its water uptake does not increase. Start with the effect of wind at the leaf surface. Then explain diffusion, the imbalance between loss and uptake, and finally the loss of cellular support. Increased wind speed removes humid air from around the leaves. This maintains a steeper water vapour concentration gradient between the leaf air spaces and the surrounding air.

8:41Water vapour diffuses out through the stomata faster, increasing transpiration. Water loss exceeds water uptake. The cells lose water and their turgor pressure decreases. The cells become flaccid, reducing support and causing the plant to wilt. Each sentence adds a distinct part of the explanation. Saying only that wind dries the plant misses the mechanism. Watch for three common mistakes. Transpiration is not liquid water flowing out through stomata. It is water vapour loss.

9:09Evaporation and diffusion are not interchangeable names. Evaporation changes liquid water into vapour, while diffusion moves that vapour down a concentration gradient. And high humidity does not normally increase transpiration. It reduces the concentration difference driving water vapour out. Finally, remember that a potometer measures water uptake, which estimates transpiration, and that wilting means reduced turgor, not necessarily immediate death. Here is the whole story. Water evaporates from moist mesophyll surfaces into interconnected air spaces, then diffuses out through stomata.

9:43A large internal surface area and greater total stomatal opening allow more water loss. That loss produces transpiration pull. Attraction between water molecules holds the xylem water column together as it is drawn upwards. Higher temperature and greater wind speed usually increase transpiration. Higher humidity usually decreases it. If water loss outpaces uptake, cells lose turgor and the plant wilts. Water movement, weather and plant support are all connected.