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.

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.

Leaf structure and water vapour loss

Inside a leaf, the mesophyll cells have moist surfaces next to air spaces. These spaces connect with one another and expose a large internal surface area. Water can evaporate from these surfaces into the air spaces.

The water vapour then moves through the connected spaces towards the stomata: small pores in the leaf surface. When stomata are open, water vapour can diffuse out of the leaf into the surrounding air.

The number and size of open stomata affect the opportunity for water vapour to escape. When other conditions are similar, more open stomata, or larger open stomata, provide more space for diffusion and allow a greater rate of water vapour loss.

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.

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.

Root hair cells

Root hair cells take up water and mineral ions from the soil. Each cell has a long, thin extension that increases the surface area in contact with the soil. This larger surface area increases the rate at which water and mineral ions can enter the cell.

Water uptake by osmosis

Water enters a root hair cell through its partially permeable cell membrane by osmosis. The soil solution is more dilute than the cell contents, so there is a net movement of water from the soil into the cell.

Mineral ion uptake by active transport

Mineral ions can enter a root hair cell by active transport. This allows the cell to take up 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 many mitochondria, where respiration releases energy to support this uptake.

Phloem and translocation

Plants transport dissolved sugars from their leaves to other parts of the plant. There, the sugars can be used immediately or stored for later use. This movement of food molecules through the plant is called translocation.

Translocation takes place in phloem, which forms tubes running through the plant. Each tube consists of elongated living cells arranged end to end. Their end walls have pores, allowing cell sap containing dissolved sugars to pass from one cell to the next along the tube.

Stomata and guard cells

Stomata are small pores in the leaf epidermis, the layer covering the leaf. Each pore, or stoma, is surrounded by a pair of guard cells. Gases move between the air and the inside of the leaf through the pore: carbon dioxide can enter, while oxygen and water vapour can leave.

How guard cells change the pore size

Guard cells change shape as their water content changes:

• When guard cells take up water, they become firm, or turgid, and bend apart. This opens the stoma.
• When guard cells lose water, they become less turgid and the pore closes.

An open stoma allows gas exchange, but it also allows water vapour to escape. Closing the stoma reduces water loss while restricting gas exchange.

Constructing scientific graphs

A graph displays numerical data visually, making it easier to examine values and relationships. The type of graph and the way its axes are drawn should match the data being displayed.

Choosing a graph type

Use a line graph when the independent variable is continuous: it can take any value within a range, such as time or temperature. The plotted points show how the measured value changes across that range.

Use a bar chart when the independent variable consists of categories, such as different types of plant. Separate bars represent the categories, with gaps between them.

Axes and scales

Place the independent variable—the variable changed or the categories being compared—on the horizontal axis. Place the dependent variable—the quantity measured—on the vertical axis. Label each axis with the quantity and its unit, for example, Temperature (°C) or Growth (cm). A category axis needs category names rather than numerical units.

Choose a numerical scale that covers the full range of data and uses the available space effectively. Equal intervals on an axis must represent equal changes in value. For example, if each large square represents $$2\ \mathrm{cm}$$, it must represent that amount throughout the axis.

Plotting the data

For a line graph, plot each pair of values accurately: find the independent value on the horizontal axis and the corresponding dependent value on the vertical axis. Draw an appropriate line through the data, joining the points or drawing a line of best fit as appropriate.

For a bar chart, draw bars of equal width, with gaps between them. The height of each bar should reach the measured value for its category. Start the numerical axis at zero so that the bar lengths represent the values fairly.

Interpreting graphs, charts and tables

Graphs, charts and tables organise data so that individual values can be found and compared. A reliable interpretation describes what the displayed data show, using the quantities and units provided.

Reading a value

Before selecting a value, read the title, headings, labels and units to establish what has been measured. On a graph or chart, check the scale on each axis: identify the starting value and the amount represented by each division. The scale does not always start at zero, and each division may represent more than one unit. If there is a key, use it to identify the correct data series.

To read a graphical value, locate the relevant point or bar, then use its position against the appropriate scale. For a graph with two axes, use one axis to locate the requested position and read the corresponding value from the other axis. If the position falls between labelled values, use the scale divisions to determine or estimate the value.

In a table, follow the appropriate row and column to their intersection. Check the headings and units before reporting the value.

Comparing and interpreting data

Use the displayed values to describe a pattern, trend or difference. State what changes, the direction of change and, where useful, the size of the change. Include units rather than relying on vague descriptions such as “much higher”.

For example, if a graph shows a plant’s height increasing from 3 cm on day 0 to 8 cm on day 4, its height increased by 5 cm over those four days. The values support a description of the change, but they do not by themselves establish what caused it.

Keep interpretations within the evidence shown. Do not assume a value beyond the displayed data or claim a cause simply because two quantities change together.