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Guard Cell Function

According to Merriam Webster Dictionary, the “guard cell is one of the two crescent-shaped epidermal cells that border and open and close a plant stoma.” Stomata are minute openings in the epidermis of a plant organ (such as a leaf) through which gaseous exchange takes place. Carbon dioxide enters the plant body through stomata and oxygen produced as a resultant of photosynthesis diffuses out.

  1. Structure

    • In plants, the epidermis, a thin surface layer of tissue, is not continuous but interrupted by minute openings called stomata. Guard cells are bean-shaped cells covering the stomata opening. Guard cells are used to let gas exchange in a plant and help water move up the stem by means of cohesion and capillary action. In addition, guard cells have some influence on the capacity of the stomata to respond to changes in ambient humidity.

    Objective

    • During transpiration, watery vapor also escapes through stomata. However, excessive loss of water results in the death of a plant. To cope with this problem, a pair of guard cells has evolved on the either side of a stoma.

    Mechanisms

    • The guard cell controls the rate of transpiration by regulating the opening and closing of stomata. During stomatal opening, guard cell pressure rises, causing the cells in inflate. When stomata close, the potassium ions are pumped out of the guard cells, which changes vacuole volume. This also causes the chloride and inorganic ions to exit the cells.

    Use of Guard Cells

    • The walls surrounding the stoma are very flexible and thin; however, the opposite walls of stoma are rigid and thicker. Whenever the guard cells are inflated with internal water pressure, there is a difference in thickness in surrounding walls which results in the development of an opening. On a hot day, when the guard cells lose water, they deflate and start pushing toward each other, thus resulting in the closure of the stoma.

    Significance

    • Signal transduction mechanisms in guard cells coordinate sunlight, carbon dioxide, water status, hormone responses and other external environmental conditions (e.g., heat) in order to regulate the movement of stomata cells present in the leaves. In adverse environmental conditions, it enhances survival of plants and their growth. The guard cells surrounding the stomata are one of the well-developed model systems which help in understanding the various signaling components that interact within a network in a single plant cell.

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