What changes the rate of transpiration, and measuring it

A pot of basil, its soil sealed in a plastic bag, weighs less at teatime than it did at breakfast. Where did the mass go? Nothing was cut, nothing was eaten, and nothing fell off the plant. Move the pot next to an open window, in a breeze, and it loses even more. The missing mass and the breeze both trace back to the leaves.

Start with where the mass went. Three explanations tend to get offered for it. Which explains the lost mass: bits of leaf blown off, energy used up, or water vapour? Water vapour. It escapes from the leaves through the stomata, and that loss is transpiration. Nothing fell off the basil, so no leaf was lost. Using energy or pausing photosynthesis would hardly change its weight in a day. In investigations like this, the mass that disappears is water, leaving as vapour through the stomata. One examiner's report on a Foundation paper, about a leaf in front of a fan, records "many other incorrect descriptions of how parts of the leaf could be lost or how the pressure of the wind would affect the leaf." It adds: "Others wrote about the leaf using energy as it was blown around by the fan." The way to avoid it is a rule written before any numbers: the mass that goes is water vapour, out through the stomata. Now, why is the soil sealed in a bag? What would the balance measure without it? Water from the soil as well as from the leaves. The bag stops the soil drying out, so the balance measures the leaves alone. How fast that water goes is the rate of transpiration. A rate is an amount divided by a time. So the rate of water loss is the mass lost divided by the time taken. With a tube of water instead, it is the volume the shoot takes up, divided by the time. No exam hands you this, so learn it in words.

One way to measure the rate is a potometer: a cut leafy shoot sealed into a tube full of water. As the shoot takes up water, an air bubble moves along a scale. How far it moves in a measured time gives the rate. For a fair reading, the shoot is cut and fitted under water so no air gets in, the joints are sealed, and the shoot is left to settle before timing starts. A potometer measures water taken up, not water lost. Why might the two differ slightly? The plant uses a little of the water it takes up, for example in photosynthesis. So uptake is close to the water lost, but not exactly the same. Now some readings. A lilac shoot sits in a potometer whose scale reads in cubic centimetres, and at the start it reads nought point one two. Fifteen minutes later it reads nought point five seven. The rate is wanted in cubic centimetres per hour. First, the volume taken up. Nought point five seven take away nought point one two is nought point four five cubic centimetres. Then the time goes into hours, on its own line. Fifteen minutes is fifteen divided by sixty, which is nought point two five hours. Now divide. What is nought point four five divided by nought point two five? One point eight. That gives a rate of water uptake of one point eight cubic centimetres per hour. Always name what it is a rate of: water uptake, which is close to, but not the same as, the rate of transpiration.

A balance can measure it too. Two leafy shoots of the same size, cut from one plant, hang side by side, one in still air and one in front of a fan. Each is weighed every thirty minutes for two hours. Both start at twenty-five grams. After a hundred and twenty minutes, the still-air shoot is at twenty-three point eight grams, and the fan shoot at twenty-two grams. Take each end point away from twenty-five grams. How much did each shoot lose? One point two grams lost without the fan, and three grams with it. Both losses are water, gone as vapour through the stomata. The rate is still the mass lost divided by the time. The time goes on its own line first: a hundred and twenty minutes is two hours. What is each shoot's rate of mass loss, in grams per hour? Without the fan, one point two divided by two is nought point six grams per hour. With it, three divided by two is one point five grams per hour. One more number for the fan shoot: its percentage loss of mass. Which mass goes on the bottom of the fraction: twenty-five grams or twenty-two grams? Twenty-five grams, the starting mass. You always compare with what you started with. That makes it three divided by twenty-five, which is nought point one two. Times a hundred, that is twelve per cent. Why did the shoot by the fan lose water faster? The moving air carries water vapour away from the leaf surface. The air next to the leaf stays drier, the concentration gradient stays steep, and water vapour diffuses out faster. That's the basil by the open window. The breeze keeps sweeping the vapour away, so it loses water faster than it would on a still windowsill. One examiner's report on a Higher paper, on a fan and water uptake, notes that "Most candidates gained the mark for recognising that the water uptake had increased." It adds: "Some went on to provide some linkage as to why this occurred." That why is the steep gradient.

Moving air is one of four factors that change the rate, and each one is always said with its direction, up or down. Humidity is a measure of how damp the air is. If the air outside the leaf gets more humid, does transpiration speed up or slow down? It slows down, the mirror image of air movement. With more vapour already outside, the gradient from inside to outside is shallower, so water vapour diffuses out more slowly. Humidity is on some courses' lists and not others, but the reasoning is the same as for air movement, so learn it either way. Now turn the temperature up. What happens to the rate, and why? The rate speeds up. Water evaporates faster inside the leaf, and the vapour particles move faster, so they diffuse out faster. One examiner's report on a Foundation paper records that "Only forty-five per cent of students were able to identify that increased temperature would increase the rate of transpiration. The most common incorrect answer was increased humidity, a concept that many students continue to find difficult." Instead of guessing, picture the air just outside the leaf. Fill it with vapour, and the gradient flattens, so the rate falls. Light intensity is the last factor, and it works on the stomata themselves. In brighter light, what do the stomata do, and what does that do to the rate? The stomata open wider. More water vapour can diffuse out, so the rate goes up. In an experiment you change light intensity by moving a lamp. A nearer lamp gives brighter light. Light has its own reason, separate from temperature. Brighter light opens the stomata wider; it is not about warming the leaf. If the rate stops rising as the light keeps getting brighter, the stomata are already fully open. Change one factor at a time, and keep the other three the same, with the same shoot and the same timing. A warmer room or a draught would change the rate on its own, and you could not tell which factor did it. Now the factors meet one plant. A pepper plant, its pot sealed in a bag, needs to lose water faster. The choices are: leave it on the balance twice as long, move it to a more humid room, raise the temperature from fifteen to twenty-five degrees, or take the bag off. Which one change would make it lose water faster, and why not the others? Raising the temperature. Water evaporates faster inside the leaf, and the vapour diffuses out faster. Leaving it longer changes how much water is lost, not how fast, so the rate stays the same. A more humid room makes it slower, because the gradient gets shallower. And taking the bag off lets the soil lose water too, so the balance stops measuring the leaves alone. The bag is there on purpose. One examiner's report on a Foundation paper, on a change to increase the rate of water loss, notes that "A factor that would affect the rate did not gain credit unless it was clear whether the factor should be increased or decreased." It adds: "Quite a lot of students said remove the bag. They did not realise that transpiration is from the leaves and the bag was there to prevent loss of water from the soil." The habit is three checks: every factor gets its direction, a rate is never a total, and the bag stays on.

Before the end, see whether the numbers and the reasons are still with you. A bubble moves forty-five millimetres in a quarter of an hour. What's that per minute? Three millimetres per minute. A quarter of an hour is fifteen minutes, and forty-five divided by fifteen is three. Now a plant moves from dry air into damp air, same temperature. Faster or slower, and why? Slower. The damp air outside flattens the gradient, so water vapour diffuses out of the leaf more slowly. One more: why does brighter light speed up transpiration? It opens the stomata wider, so more water vapour can diffuse out. And the basil by the open window lost its mass as water vapour, swept off by the breeze faster than on a still windowsill.

Every video up to this one is listed. If you've got this down, leave a like on it, and it's one fewer video to come back to. If the rate working still wobbles, save the playlist, and write that working out once by hand; that's what makes it stick. For combined science, and for anyone whose list stops here, this is the final video of the chapter. Your next stop is the chapter on photosynthesis and respiration.

Next in the chapter: plants adapted to extreme environments. Hopefully that's next in your feed, but if not, search for ScholaFly B I oh five, oh five.

Every other video is on scholafly.com too, easy to search by video code or by keyword. Whatever you're revising, go there and have a look, or, watch the next video.