Transpiration Occurs in Plants

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Transpiration

Transpiration-Transpiration, the loss of water vapor from leaves, is the force behind the movement of water in xylem. This evaporative water loss occurs mainly through the stomata (90%) and to a lesser extent through the cuticle (10%). When stomata are open, gas exchange occurs freely between the leaf and the atmosphere. Water vapor and oxygen (from photosynthesis) diffuse out of the leaf while carbon dioxide diffuses into the leaf. The amount of water vapor that is transpired is astounding, with estimates of 2 liters (0.5 gallons) of water per day for a single corn plant, 5 liters (1.3 gallons) for a sunflower, 200 liters (52 gallons) for a large maple tree, and 450 liters (117 gallons) for a date palm. Imagine the quantities of water lost each day from the acres of corn and wheat planted in the farm belt of the United States! Clearly, transpiration by plants is a major force in the global cycling of water.

Transpiration Occurs in Plants

Transpiration

The basis of transpiration is the diffusion of water molecules from an area of high concentration within the leaf to an area of lower concentration in the atmosphere. Unless the atmospheric relative humidity is 100%, the air is relatively dry compared with the interior of a leaf, where the intercellular spaces are saturated with water vapor. As long as stomata are open, a continuous stream of water vapor transpires from the leaf, creating a pull on the water column that extends from the leaf through the plant to the soil.
It is the action of the guard cells that regulates the rate of water lost through transpiration and, at the same time, regulates the rate of photosynthesis by controlling the CO 2 uptake. Each stoma is surrounded by a pair of guard cells, which have unevenly thickened walls. The walls of the guard cells that border the stoma are thicker than the outer walls. When guard cells become turgid they can only expand outward owing to the radial orientation of cellulose fibrils; this outward expansion of the guard cells opens the stomata. Stomata are generally open during daylight and closed at night. As long as the stomata are open, both transpiration and photosynthesis occur, but when water loss exceeds uptake, the guard cells lose turgor and close the stomata. On hot, dry, windy days the high rate of transpiration frequently causes the stomata to close early, resulting in a near shutdown of photosynthesis as well as transpiration. A fine balance must be struck in this photosynthesis-transpiration dilemma to allow enough CO 2 for photosynthesis while at the same time preventing excessive water loss. Some plants have evolved an alternate pathway for CO 2 uptake at night when rates of transpiration are lower (see CAM Pathway later in this chapter). Other plants have morphological or anatomical adaptations that reduce rates of transpiration while keeping the stomata open. These physiological and anatomical adaptations are most common in xerophytes, plants occurring in arid environments.

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