Stomata

Definition of Stomata

The large number of tiny pores present in every leaves are called Stomata. These stomata are located on the surface of the epidermal layers of the leaves.

Opening and Closing of Stomata

Each stoma is composed of two special types of epidermal cells, called guard cells. The guard cells control the opening and closing of stomata, which in turn regulate the loss of water from the leaves to the atmosphere through transpiration. The size of typical stomatal pore ranges from 3 – 12 micrometer in width and from 10-14 micrometer in length. The number of such tiny pores.

Stomata varies from 1,000 to 60,000 per cm² of the leaf surface. Stomata are found on the lower (Abaxial) as well as (Adaxial) monocot leaves (grasses), the stomatal density and number is almost the same on both the surfaces. Leaves of dicot plants usually contain fewer stomata on the upper surface.

Opening and closing if stomata is controlled by accumulation of solutes in the guard cells. Solutes are taken in by the guard cells from the neighbouring epidermal cells and mesophyll cells. As a result both the osmotic potential and water potential of the guard cells are lowered. This would create a water potential gradient between the guard cells and the neighbouring cells, and make the water move into the guard cells. The guard cells become turgid and swell in size, resulting in the stomatal opening. With a decline in guard cell solutes, water moves out of the guard cells, making them Flaccid. As a result, the stomata close. Thus the stomatal opening and closing is controlled by osmotic movement of water in or out of the guard cells along the water potential gradient.
The major solute, which is taken in by the guard cells from the neighbouring cells, is potassium. The rise in potassium levels causes stomatal opening and its decrease causes stoamatal closing. The uptake in the potassium controls the gradient in water potential. This in turn, triggers osmotic flow of water into the guard cells raising the turgor pressure. Role of K in the stomatal opening is now universally accepted. The extent K accumulation in the guard cells determines the size of the stomatal opening.
The accumulation of large amounts of potassium in the guard cells is electrically balanced by the uptake of negatively charged ions- chloride and malate. The high amount o malate in the guard cells of open stomata accumulates by hydrolysis of starch.

Factors affecting Stomatal movement

Many environmental factors affect stomatal movement. Most important factor includes light, temperature, water availibilty to plants and CO₂ concentration. Some endogenous factors like K , Cl^- and H^- ions and organic acids, also influence stomatal movement.

• Light: Stomata open in the presence of light and close in darkness. Light intensity required to open the stomata is very low, as compared to the intensity required for photosynthesis. Even moonlight is sufficient to keep the stomata open in some plant species. In plants with Crassulacean Acid Metabilosm (CAM), stomata open during the dark and close during the day. This unique kind of behaviour of stomata is a kind of adaptation to conserve moisture in CAM plants, for example pineapple, agave etc.
• Temperature: Stomata tend to open more with temperature and close with a decrease in temperature. In some plant species, stomata remains closed even under Stomatal opening and closing continuous light at 0 degree Celsius. However if the temperature is increased, stomatal opening in these species increases. At temperatures higher than 30 degree Celsius, there is a decline in stomatal opening in some species.
• Water availability: If the water availability to plants is less and transpiration rate is high, plants undergo water stress. Water stress also called, water deficit or moisture deficit, includes stomatal closure. This happens to conserve moisture in plants by cutting down the transpirational loss of water.
• CO₂ concentration: With an increase in the carbon dioxide concentration inside the leaf, the stomata close. This happens even under the light. In some plant species stomata also close even if we merely breathe on their leaves. It is the internal leaf carbon dioxide concentration rather than the atmospheric carbon dioxide concentration that dictates the stomatal opening. If plants are transferred to carbon dioxide free environment, but kept in darkness, the stomata will still remain closed. This means that since the internal carbon dioxide is not utilized due to the absence of photosynthesis in the dark, it influences the stomata to remain closed. However if these plants are exposed to light, photosynthesis will utilize the carbon dioxide permitting the stomata to open.