The stomatal apparatuses, which often occupy 1% or more of the surface area of a leaf, regulate transpiration and gas exchange. Control of transpiration is, however, strongly influenced by the water-vapor concentration of the atmosphere. The guard cells bordering each stoma have relatively elastic walls with radially oriented microfibrils, making them analogous to pairs of sausage-shaped balloons joined at each end, each with a row of rubber bands around it. The part of the wall adjacent to the hole itself is considerably thicker than the remainder of the wall (Fig. 9.13). This thickness allows each stoma to be opened and closed by means of changes in the turgor of the guard cells. The stoma is closed when turgor pressure is low and open when turgor pressure is high. Changes in turgor pressures in the guard cells, which contain chloroplasts, take place when they are exposed to changes in light intensity, carbon dioxide concentration, or water concentration.
Changes in turgor pressure take place when osmosis and active transport between the guard cells and other epidermal cells bring about shifts in solute concentrations. While photosynthesis is occurring in the guard cells, they expend energy to acquire potassium ions from adjacent epidermal cells, leading to the opening of the stomata. When photosynthesis is not occurring in the guard cells, the potassium ions leave, and the stomata close. With an increase in potassium ions, the water potential in the guard cells is lowered, and the osmosis that takes place as a result brings in water that makes the cells turgid. The departure of potassium ions also results in water leaving, making the cells less turgid and causing the stomata to close (see Fig. 9.13).
Stomata will close passively whenever water stress occurs, but there is evidence that the hormone abscisic acid is produced in leaves subject to water stress and that this hormone causes membrane leakages, which induce a loss of potassium ions from the guard cells and cause them to deflate.
The stomata of most plants are open during the day and closed at night. However, the stomata of a number of desert plants are open only at night when there is less water stress on the plants. This conserves water but makes carbon dioxide needed for photosynthesis inaccessible during the day. Such plants convert the carbon dioxide available at night to organic acids, which are stored in cell vacuoles. The organic acids are then converted back to carbon dioxide during the day when photosynthesis occurs (Fig. 9.14). A specialized form of photosynthesis called CAM photosynthesis uses the carbon dioxide released from the organic acids. CAM photosynthesis is discussed in Chapter 10.
Other desert plants have their stomatal apparatuses recessed below the surface of the leaf or stem in small chambers. These chambers often are partially filled with epidermal hairs, which further reduce water loss. Similar recessed stomatal apparatuses are found in the leaves of pine trees, which have little water available to them in winter when the soil is frozen (see Fig. 7.12). A few tropical plants that occur in damp, humid areas (e.g., ruellias; see also Fig. 4.13B) have stomata that are raised above the surface of the leaf, while plants of wet habitats generally lack stomatal apparatuses on submerged surfaces.
Although light and carbon dioxide concentration affect transpiration rates, several other factors play at least an indirect role. For example, air currents speed up transpiration as they sweep away water molecules emerging from stomata.
Humidity plays an inverse but direct role in transpiration rates: high humidity reduces transpiration, and low humidity accelerates it. Temperature also plays a role in the movement of water molecules out of a leaf. The transpiration rate of a
subsidiary cells guard cells subsidiary cells guard cells open stoma closed stoma open stoma closed stoma
50 |im guard -cell stoma guard -cell
K+ enters guard cells and water follows
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.