Stomata are minute aperture structures on plants found typically on the outer leaf skin layer, also known as the epidermis. They consist of two specialized cells, called guard cells that surround a tiny pore called a stoma. Their main function is to allow gases such as carbon dioxide, water vapor and oxygen to move rapidly into and out of the leaf. The size and shape of stomata also vary with different plant species and environmental conditions. After the diffusion of water through sub-stomatal cavity and stomatal pores to the air outside the leaf, the water potential gradient develops in the sub-stomatal cavity, stomatal pore, boundary layer and the atmosphere.
During transpiration the sub-stomatal cavity has relatively much higher water potential as compared to the atmosphere forces the water vapors to move out. Because of this, the water potential of the sub-stomata cavity get lowers and the water is evaporated through the cell walls-of cells surrounding the sub-stomatal cavity. The decrease in the water potential of the sub-stomatal cavity and the surrounding cells acts as a ‘pull’ on the water column for maintaining continuity through the vascular bundles of the leaf. The intercellular spaces also help in making the continuity with the sub-stomatal cavity and cause a gradient quickly.
The resistance experiences by leaf during the evaporation of water can be categorized as internal (leaf resistance) and external resistance (air boundary resistance). The components of leaf resistance are cuticle, mesophyll cells, intercellular spaces of the leaf and stomata. The cuticle forms outermost surface of the leaf and offers resistance to the evaporation of water vapor and entry of carbon dioxide necessary for photosynthesis. This is called cuticular resistance and it is maximum. The main function of stomata includes allowing CO2 to enter for photosynthesis, to control water loss through transpiration and to facilitate transpiration at higher temperature (above 35°C).
Stomatal resistance is most important because gas exchange between leaves and external atmosphere takes place entirely through stomatal pores. Stomatal resistance depends mainly on the size and shape of the stomatal cavity and size of stomatal aperture. Water loss from the leaf also depends on the number of stomata per leaf. The agricultural scientists are trying to find a way for minimizing the water loss from the plants. Stomatal frequency (i.e. number of stomata per unit area) and Stomatal index (i.e. ratio of number of stomata to the total number of cells per unit area) are two parameters for expressing the distribution of stomata.
MisKen and Co workers in i 972 found that stomatal resistance and transpiration rates differed significantly between barley lines but photosynthetic rates were the same. The two lines were high and low in stomatal frequency. Stomata comprise a pair of highly specialized guard cells that are encompassed by a pair of larger and thinner subsidiary cells. Kidney shaped guard cells are found in dicotyledons whereas dumb-bell-shaped guard cells are found in monocots. In nearly all vascular plants, guard cells differ significantly from other epidermal cells in having chloroplasts. These also contain mitochondria and can synthesize starch.