Basic Plant Physiology (Continued)

Transpiration in Plants

Transpiration is the loss of water from plants in the form of water vapor.  The water is lost through the stomata which are small openings in the epidermis of leaves and stems bordered by guard cells.  The stomata open primarily during the daylight hours to allow carbon dioxide to enter the plant and oxygen to escape during photosynthesis.  While the stomata are open for photosynthesis, water vapor is also allowed to escape from the plant.

As previously discussed, water is absorbed from the soil via the plant’s roots.  The absorbed water along with dissolved nutrients are transported as a liquid to all other parts of the plant via the xylem portion of the vascular system creating a column of water from roots to stems to leaves and flowers.  Water diffuses in plants or moves from areas of high concentration or pressure to areas of low concentration or pressure forming a concentration gradient.  

When water vapor escapes from the uppermost leaves, water moves from the stems into the leaves to replace the water lost and to establish an equilibrium in water concentration/pressure between the leaves and stems.  But this lowers the concentration of water in the stems, so water then moves from the roots to those stems causing the roots to absorb more water from the soil.  The cohesive properties of water (specifically, the way hydrogen bonds between adjacent water molecules) allow the column of water to be “pulled” up through the plant as transpiration occurs. 

Transpiration ceases when the stomata are closed which is typically at night for most plants.  How much water is lost via transpiration depends on light, temperature, humidity, wind, and soil water.