What are stomata?
Figure 1: Stomata of Tall fescue grasses.
Stomata are tiny plant structures found on the outer skin layer, also known as the epidermis, of plants (Figure 1). They consist of two specialized cells, called guard cells that surround a tiny pore called a stoma. The word stomata means “mouth” in Greek because they allow communication between the internal and external environments of the plant. Their main function is to allow gases such as carbon dioxide, water vapor and oxygen to move rapidly into and out of the leaf. Stomata are found on all above-ground parts of plants including the petals of flowers, petioles, soft herbaceous stems and leaves. They are formed during the initial stages of the development of these various plant organs and therefore reflect the environmental conditions under which they grew.
Stomatal density, size and shape
Stomatal density refers to the number of stomata per squared millimeter. Typical densities can vary from 100 to 1000 depending on the plant species and the environmental conditions during development. More stomata are made on plant surfaces under higher light, lower atmospheric carbon dioxide concentrations and moist environments. Grasses typically have lower stomatal densities than deciduous trees. The size and shape of stomata also vary with different plant species and environmental conditions. For example, grasses have guard cells that resemble slender dumbbells whereas trees and shrubs have guard cells that resemble kidney beans.
Physiological function of stomata
Leaves are the main “food manufacturing” organs of plants. They make food from carbon dioxide and water in the presence of light during a process called photosynthesis. As stomata open in the presence of light, carbon dioxide will diffuse into the leaf as it is converted to sugars through photosynthesis inside the leaf. At the same time, water vapor will exit the leaf along a diffusive gradient through the stomata to the surrounding atmosphere through the process of transpiration. Consequently, plants face the dilemma of taking up carbon dioxide while losing water vapor through their stomata. If this water loss remains unchecked, they can deplete their water reserve. This depletion can become catastrophic to the physiological functioning of the plant given that is the most essential solvent in which biochemical and growth processes occur. Based on Darwinian principles, it is presumed that selective adaptation has driven plants to acquire characteristics which enable them to grow more quickly without diminishing the probability of survival. If plants have not acquired the characteristics to withstand changes in water availability in their growth environment, plants may exacerbate their water shortage by not regulating the size of their stomatal apertures in an optimal manner and may fail to survive when water availability declines.
Optimal size of stomatal apertures
The Optimisation Theory, first proposed by Ian Cowan and Graham Farquhar (1977) suggests that the gas exchange of a plant is optimal if the plant is maximizing photosynthesis at a given average rate of transpiration. This ratio of photosynthesis to transpiration defines the instantaneous water-use efficiency (WUE) of the plant. The WUE of the leaf, when compared to economic principles, can be considered to be analogous to the interest rate on an invested resource. The invested resource in this case is water transpired, while the interest is the carbon gained through photosynthesis. The optimal stomatal aperture size is one in which the interest rate, WUE, is maximized as the environmental conditions change.
Stomatal apertures will typically vary in response to changes in light intensity, saturation deficit of ambient water vapor and soil moisture availability. As stomatal aperture size changes, rates of photosynthesis and transpiration will vary because the pore size will provide a corresponding resistance to the diffusion of CO2 into and H2O out of the leaf. The inverse of this resistance can be calculated as the conductance to these two gases across a leaf surface.
Plant biologists typically measure stomatal conductance using a specialized instrument called an IRGA (Infra Red Gas Analyzer).
Figure2: Leaf chamber attached to Infra red gas analyzer connected to computer console
This instrument allows one to clamp a leaf into a chamber and the relative mole fractions of CO2 and water vapor entering and leaving the chamber are monitored over time. The leaf touches a temperature thermocouple inside the chamber so that leaf temparature can be monitored in conjunction with air temperature that exits the chamber. The leaf temperature is used to determine the saturated molar concentration of water vapor inside the leaf intercellular air spaces. The resistance to the diffusion of water vapor from inside the leaf to the air stream passing over the leaf is calculated from the difference between the molar concentrations of saturated water vapor inside the leaf air spaces in the air stream passing over the leaf. The resistance will increase as the stomatal aperture size decreases. The stomatal conductance to water vapor decreases as the resistance increases. Because H2O has a lighter molecular mass than CO2, water typically diffuses 1.6 times faster than CO2. The conductance to CO2 into a leaf is going to be 0.625 times the conductance to H2O out of the leaf.
As stomatal conductance declines, WUE will increase if the reduction in photosynthesis is lower than the reduction in transpiration. Plant species typically show this physiological response under mild drought stress. Under severe drought stress, the photosynthetic biochemical machinery can become damaged. As a result, WUE will decrease as stomatal conductance declines if the reduction in photosynthesis becomes larger than the reduction in transpiration.