Leaf mouths trade carbon dioxide for precious water
A stoma is an epidermal pore edged by two guard cells that meter gas exchange in most land-plant sporophytes—liverworts excepted. Daylight carbon dioxide at about 425 ppm enters while water vapour escapes as transpiration, so plants cannot feed photosynthesis without risking drought.
Guard cells open and close the stomatal aperture; oxygen and carbon dioxide diffuse through, and water vapour leaves the saturated leaf interior. Dicots often pack more stomata on lower leaf faces, while monocots such as onion, oat, and maize may balance both sides. Number, size, and spread vary widely among vascular plants, and some mosses and hornworts lack the pores entirely.
RuBisCO's weak carbon dioxide grip and wasteful photorespiration push many species to keep stomata open in light despite water loss. Where water is scarce but light is strong, PEPcase pathways retrieve carbon at energy cost. CAM desert plants flip the schedule: night opening stores carbon in vacuoles, then daytime closure lets the Calvin cycle run behind sealed pores.
Ordinary plants instead tune apertures to light, humidity, and carbon dioxide, starting with proton pumps that shift guard-cell potential and draw ions and water to bow the pore open. That hydraulic choreography is how forests breathe without boiling dry—an invisible negotiation at every leaf surface.
Drought throws the switch the other way. When roots detect drying soil they release abscisic acid, which docks on guard-cell receptors, raises cytosolic pH and free calcium, and pushes chloride and organic ions out; potassium intake halts, water leaves with the solutes, and the pore sags shut. Leaf habit shapes the layout too: floating leaves may carry pores only on top, submerged leaves can have none, and most trees keep theirs on the underside. Scientists track the whole performance indirectly, calculating stomatal conductance from how fast a leaf transpires and how steep the humidity difference is between leaf and air.
Source: Stoma
