Transpiration

Transpiration and the Mechanics of Stomatal Movement

Transpiration is the evaporative loss of water from a plant, occurring mainly through the stomata (singular stoma) of the leaves — the same pores through which oxygen and carbon dioxide are also exchanged with the atmosphere. Under normal conditions stomata are open by day and closed at night, and the immediate cause of this opening and closing is a change in the turgidity of the guard cells flanking each pore.

How a guard cell opens the pore: each guard cell's inner wall (facing the pore) is thicker and less elastic than its thin outer wall. When the guard cells become turgid, their thin outer walls bulge outward, and this bulging forces the thicker inner walls to curve into a crescent shape, opening the pore between the two cells. This is helped by the arrangement of cellulose microfibrils in the guard-cell wall — laid out radially, like spokes, rather than lengthwise, which makes bulging mechanically easier (Figure 11.8). When guard cells lose turgor (from water loss or water stress) their elastic inner walls spring back, the cells become flaccid, and the pore closes.

• Dorsiventral leaves (typical of dicots) carry more stomata on the lower surface than the upper.
• Isobilateral leaves (typical of monocots) have roughly equal numbers on both surfaces.

The rate of transpiration is shaped both by external environmental factors — temperature, light intensity, humidity, and wind speed — and by plant factors, including the number and distribution of stomata, what fraction are open at a given moment, the plant's overall water status, and its canopy structure.

Why the pull works so strongly: the rise of xylem sap depends on three physical properties of water acting together — cohesion (mutual attraction between water molecules), adhesion (attraction of water molecules to the polar walls of the tracheary elements), and surface tension (water molecules attracting each other more strongly than the surrounding air). Together these give water a high tensile strength (resistance to a pulling force without the column breaking) and high capillarity (ability to rise in narrow tubes) — further assisted, inside the plant, by the very small diameter of the tracheids and vessel elements that make up xylem. …