Stomata

A leaf must let in carbon dioxide (CO₂) to make sugar, but the cells that take it up are wet. A wet surface open to the air loses water. The leaf solves the problem with stomata (singular: stoma), tiny pores in the leaf surface, often mostly on the underside. A typical leaf has from dozens to several hundred of them per square millimetre.

Each stoma is bordered by two guard cells. When the guard cells take up ions and water by osmosis, they swell and bend apart and the pore opens. When they lose water, they go slack and the pore closes. Typical signals:

• Light (especially blue light) opens stomata, so that CO₂ is available while photosynthesis runs.
• Low CO₂ inside the leaf opens them.
• Water stress makes the roots and leaves release the hormone abscisic acid (ABA), which makes guard cells lose water and the pore close.

Gas moves through an open stoma by diffusion, from high concentration to low concentration (Fick’s law). Both gases use the same pore:

• CO₂ in. Outside air has only about 0.04% CO₂, so the gradient into the leaf is small.
• Water vapour out. The air inside a leaf is nearly saturated, while the outside air is usually drier, so the gradient out is much steeper.

Both rates are proportional to the opening of the pore, the stomatal conductance g_s. This is the trade-off: a more open stoma means more carbon gain and more water loss. For each CO₂ molecule fixed, a leaf loses hundreds of water molecules. In total, roughly 95% or more of the water a plant takes up is lost by transpiration, and only a small fraction ends up in the plant or in photosynthesis.

Big trees can transpire a lot. Depending on species, size, leaf area and weather, a large tree can lose from tens of litres to several hundred litres on a hot, dry day.

On a hot, dry afternoon, the air is very dry, so a given pore opening loses water faster. A tree with plenty of water in the soil can keep stomata open longer. A tree under drought closes them early. That protects the water columns from cavitation (lesson 4), but photosynthesis falls and the tree grows less that season. This is why a dry summer leaves a narrow tree ring (World 2).

Many plants therefore make a daily deal: open the stomata in the cool, humid morning, partly close them in the afternoon heat. Some desert plants (such as cacti) use a different trick: they open their stomata at night and store the CO₂ for the day.