Midday stomatal closure

Why do plants shut the pores they feed through during the brightest hours of the day?

A leaf eats light and carbon dioxide. Noon delivers the most light of any hour, so noon ought to be the hour a leaf throws its pores wide and works hardest. Instead many plants do the opposite: measure gas exchange through a summer day and the curve rises through the morning, sags conspicuously around midday, and recovers in the late afternoon. Two peaks with a dip between them.

That looks like a machine switching itself off at the moment of greatest opportunity. Before calling it a design flaw, it is worth asking what else is coming through the same opening.

Start with the geometry of the pore. A stoma is a hole in a waterproof surface, and the leaf cannot make a hole that admits carbon dioxide without also letting water vapour out — the two gases share one door. So the real question is not "is the pore useful?" but "what is the exchange rate right now: how much water leaves per unit of carbon that enters?"

That rate is not fixed through the day, and this is the crux. What drives water out is the vapour pressure deficit: the gap between how much water vapour the air could hold and how much it does hold. Warm the air and its capacity to hold vapour climbs steeply — roughly seven per cent for every degree Celsius near ordinary temperatures — so by early afternoon, when air temperature peaks, the pull on the leaf is at its strongest of the day. Not double the morning's; often several times it.

Now what about the gain side? Ask whether a leaf at noon is actually short of light. In most species photosynthesis saturates well below full midday sun — past a certain irradiance, more photons buy no more carbon, because the limit has moved to the enzymes and to the supply of carbon dioxide itself. So between mid-morning and midday the numerator of the exchange has largely stopped rising while the denominator keeps climbing.

Do you see what that does to the accounting? Closing at noon forfeits a modest amount of carbon and avoids a large amount of water loss. A pore that stays wide during the driest, hottest hour is buying its worst-priced carbon of the day.

There is a harder constraint behind the economics. Water moves to the leaves in xylem conduits under tension — a pulled thread of liquid, not a pushed one. If transpiration outruns what the roots can supply, the tension rises, and past a threshold the thread breaks: air is drawn in and the conduit embolises, permanently in many species. So the leaf is not only economising, it is protecting its plumbing.

How is the closure actually triggered? Two routes, working together. When demand outruns supply, the leaf loses water faster than it gains it, guard cells lose turgor, and the pore narrows passively. Layered on that is a chemical route: the hormone abscisic acid accumulates in the dehydrating leaf and drives ions out of the guard cells, so they lose turgor actively and hold the pore closed. Which of these dominates — and how much of the abscisic acid arrives from the roots as a soil-dryness signal rather than being made in the leaf itself — is genuinely contested; the evidence has shifted substantially toward leaf-sourced acid in recent decades, but the weighting is still argued.

Three refinements worth holding onto.

First, closure is a dimmer, not a switch. Stomata narrow by degrees, and midday "closure" in most measurements is a partial reduction in conductance rather than a sealed leaf.

Second, it is conditional. A plant with wet soil in humid air often shows no midday dip at all; the dip appears when evaporative demand outruns supply, which is why it is most pronounced on hot, dry, windy days and in plants with limited rooting depth. Species also differ in policy: some hold leaf water potential nearly constant by closing early and readily, others let it fall and keep trading — the isohydric and anisohydric strategies — and the same day produces very different curves in each.

Third, and most often overlooked: the midday depression of photosynthesis is not entirely a stomatal story. Excess light can transiently damage or downregulate the photosynthetic apparatus, and biochemical capacity itself declines at high leaf temperature. Both would depress carbon uptake even with the pores held open, and separating the stomatal from the non-stomatal share of the dip requires careful work rather than assumption.

• How crassulacean acid metabolism inverts the schedule entirely, opening at night and fixing carbon in the dark.
• Why guard cells respond to blue light specifically, and what that buys over responding to total irradiance.

Every term the collection defines is gathered in the glossary.