OCCURRENCE OF STOMATA:
- Scattered throughout the epidermis are pairs of bean-shaped cells, called guard cells. Guard cells contain chloroplasts, which are tiny granules filled with the green pigment chlorophyll. Chloroplasts enable leaves to carry on photosynthesis because they are able to absorb carbon dioxide and sunlight, which are required for the food-making process. In response to heat and light, each pair of guard cells pulls apart, and a tiny pore forms between them. The pores, called stomata, open to the outside atmosphere.
- Stomata occur mostly or exclusively on the lower leaf surface of most plants. Stomata occur on both surfaces of grasses, and in conifers they may be arranged in distinctive lines on either side of the mid vein of the needle (e.g. Abies spectabalis), or on one of more facets of a multi-sided pine needle. Hydrophytes (e.g. water lily) have stomata on the upper surface.
- There is a greater likelihood that the boundary layer will be thicker on the lower leaf surface because the layer is not so likely to be disrupted by convection. Concentrating stomata on the lower surface conserves water by taking advantage of the thicker boundary layer and thus lower transpiration.
FACTORS AFFECTING STOMATAL OPENING:
- When the stomata are open, carbon dioxide and oxygen pass either in or out—when carbon dioxide enters, it takes part in photosynthesis, the food-making process that releases oxygen as a waste product. This oxygen passes out of the leaf. At the same time, oxygen also enters the leaf, where it takes part in respiration, a process that forms carbon dioxide as a waste product. This carbon dioxide passes out through the stomata. Water also passes out of the open stomata in the form of a vapor. This process is called transpiration. Generally, there are more stomata on the under surface of a leaf than on the upper surface. This prevents water from evaporating too quickly or in excessive amounts from the leaf’s upper side, which is exposed to the sun. Stomata close at night, providing another level of water conservation.
- The main factors affecting Stomatal opening are:
Most stomata open in the light.
Most stomata open in CO2 free air, and close at high CO2 concentrations (e.g. >1,000 ppm). The mechanism of C02 action on stomata is not well understood.
Overrides all other factors. Water stress may be induced if transpiration exceeds water absorption because of high transpiration rates, or slow water absorption because of inadequate root growth or dry soil. Water stress often occurs at midday, and this may be accompanied by partial stomatal closure. If water stress is severe the leaf liberates abscissic acid (ABA) which will keep stomata closed for several days even after water stress has been relieved. The ABA response occurs in some crop plants with as little as Yleaf = -1.0 Mpa, thus it is prudent for the farmer with irrigation available to water crops well before wilting. Succulents are a special case because during water stress some keep their stomata closed during the daylight, but open them at night to absorb and store CO2 (as an organic acid) for photosynthesis conducted without gas exchange during the day.
In summary, darkness, high CO2, and most of all, water stress close the stomata of most plants, conserving water until a time more favorable for photosynthesis.
- Stomata are composed of two, opposing, guard cells which are surrounded by the accessory cells. All of these cells are a part of the leaf epidermis.
- Guard cells are unique among epidermal cells in that they contain chloroplasts – not as many as mesophyll cells, but enough to sense light and perform some photosynthesis (See Fig). They are also unique in having radially arranged cellulose microfibrils in their cell walls (radial micellation). The concentration of—radial microfibrils is much greater on the inner, concave cell walls (because of its decreased radius of curvature) than on the outer, convex cell wall. This causes the inner cell wall to be thicker.
- When turgor pressure increases in a guard cell the outer, thin wall expands, pushing into its accessory cell. The radially arranged microfibrils pull the thicker, inner wall toward the bulging outer wall. Since this happens to both of the paired guard cells, their touching walls pull apart and the pore opens. (Fig)
- In summary, when a pair of guard cells becomes turgid their outer walls expand and bulge into accessory cells. Radial microfibrils pull the inner wall toward the outer wall, opening the stomatal pore. Stomatal opening depends upon turgor changes in the guard cells.
- Water stress occurs in guard cells when transpiration is more rapid than the cells can absorb water. This may be caused by high soil moisture stress (i.e. dry soil), high resistance to the movement of water up the stem, or an excessively dry air in which the flow of water from vein to accessory cell to guard cell is slower than guard cell transpiration. This commonly occurs at midday, even in well-watered soils. Midday stress causes partial stomatal closure and reduced photosynthesis until the evapo-transpiration potential is lowered by decreased temperature in the late afternoon.
VARIATION IN STOMATAL SENSITIVITY:
- Stomata vary considerably in their sensitivity to water stress, and this variation reflects adaptive strategies to different environments.
- Some species adapted to arid conditions may keep stomata open at higher levels of soil moisture stress than species found in moist environments. Note that succulents, generally considered well adapted to xeric environments.
- Succulents are very sensitive to soil or atmospheric moisture stress, and quickly close their stomata at the slightest stress. They save water with their thick cuticles from one favorable time for photosynthesis to the next, but are not very productive because of the long intervals with closed stomata (and thus little photosynthesis). For this reason succulents are generally restricted to habitats too harsh for most plants, and thus compensate for low photosynthetic capacity by avoidance of competition.
- Species vary in conductance of water vapor. Conifers generally have low conductance regardless of the humidity of the air, thus conserving water. The other species close stomata with varying sensitivity as the relative humidity decreases, even though they are well-watered.
- Some desert species can maintain open stomata; and thus maintain photosynthesis. Other desert plants close stomata with slight water stress similar to succulents, conserving water at the expense of photosynthesis during drought. Many phreatophytes, e.g. some Populus species, maintain open stomata even during hot, dry days, but compensate by having root systems that tap the dependable soil water of the capillary fringe above ground water, and have low resistance conducting system in the stem.
- Stomata may also become less sensitive with age. Stomata in the older needles of many conifer species become so plugged with cutin that they no longer function.
- Stomata in aging grass leaves accumulate silicon, which reduces and eventually eliminates cell wall elasticity. Such stomata in mature grass leaves remain open, “curing” or drying the leaf as a part of senescence. This has the ecological advantage of also reducing the moisture content of the grain, helping to insure its long term viability.
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