Botany

Influences on Stomatal Openings in Plants



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Stomata are often found at higher frequencies on the lower surfaces of leaves, although they can usually be found to some degree on both sides. The higher frequencies of stomata on the lower leaf surfaces is an adaptive feature adopted by a wide range of plants as an attempt to reduce water loss, as shading from the sun leads to less evaporation. Adaptations to conserve water and Co2 are important because, water diffuses 1.6 times faster out of leaves, than Co2 uptake, this is partly due to low concentrations of Co2 in the atmosphere. Carbon dioxide is at around 375 ppm by volume in the Earth's atmosphere, this is a small concentration compared to di-oxygen (O2), which has a far greater proportion making up 20.95% of the atmosphere.

There are many physical stomatal adaptations to water loss, for example, certain species in the taxa Cactaceae have been found to have sunken or hidden stomata (Eggli, 1984). There are also adaptations to ensure Co2 uptake is not impaired by water films on leaves, such as the wet forest tree Drimys winteri which has stomatal plugs, which aid gas exchange by breaking up water films which sit on the leaf surface (Zwieniecki, et. al., 1998). In hot dry conditions some cacti such as Kalanchoe open their stomata at night and close them during the day to reduce water loss by transpiration. They fix Co2 into organic acids during the day and at night they open stomata (Neales, et. al., 1968).

Transpiration is the movement of water out of the stomata via evaporation, which crates a hydrostatic pressure which pulls water molecules and their dissolved minerals up the xylem. It is possible for plants to survive without a transpiration stream, but when it occurs the plant does seem to benefit from increased circulation of minerals and an optimal turgidity is maintained (Salisbury, and Ross 1992). When a stoma is open as well as Co2 being absorbed, a large amount of heat is removed from the leaves, this cools the leaves down, and is especially important in tropical climates. There are many resistances in the process of Co2 entry through the stoma, between the pore, cell walls, intracellular space, plasma membranes, cytoplasmic, chloroplast membrane and the biochemistry in the chloroplast, the Co2 encounters all these before it can processed for photosynthesis.

Low Co2 in most plants causes stomatal opening, and increased levels of Co2 causes the stomata to close slightly both in the light and the dark. Environmental influences on stomatal aperture are integrated, for example as light intensity increases stomatal conductance increases, and Co2 levels can also influence to what extent stomatal conductance increases. So at low levels of Co2 and high levels of light stomatal conductance is at its highest, however if Co2 levels were to increase, then conductance would be reduced, even at higher levels of light. Although there are some exceptions in some plant species, Wheeler (1999) found that very high levels of Co2 (i.e. 10000 mol mol-1) can actually increase stomatal conductance particularly in the dark.

Light influences on stomata

Increased intracellular Co2 increases as light intensity increases, the opening of stomata in light involves at least two reactions, the first is a response to the removal of Co2 by photosynthesis, and the second is a response to blue light not dependent on the removal of Co2 (Mansfield, and Meidner, 1966). Zeiger (1977) found that when blue light was shined on onion guard cell protoplasts, they swell, indicating the existence of blue photoreceptor's responding to the light. Blue light (430- 460nm) is ten times more effective as red light (630-680nm) in producing as stomatal opening (Nobel, 2005), stomatal opening is significantly greater in blue light than in red, even when blue light intensity is lower, and Co2 is higher than red light (Mansfield, and Meidner, 1966). There is only a slight response to green light, and red light is absorbed by chlorophyll, showing correlations with the chlorophyll involved in photosynthesis, the blue-light response is independent of photosynthesis. The guard cells contain only 3% of the average amount of chlorophyll of a mesophyll cell, showing that although products of photosynthesis can be found, they are too little to be significant (Salisbury, and Ross 1992) .

The effectiveness of alternate wavelengths of light on stomatal opening can be tested by shining alternate energy levels of light onto a leaf through using color filters, and measuring the average of stomatal apertures (mm) within a 1mm2. The results would then be used to create a graph, showing the width of stomatal apertures and their relations to the different colors of light. Another method would be to measure stomatal aperture over a period of time whilst red light is being shined onto the leaf, then at some point the red light is switched to blue light, the graph created from the results should show a dramatic increase in stomatal aperture at the point where blue light is introduced.

Arabidopsis mutants that have no blue-light responses have identified four blue light receptors (Kinoshita et. al., 2001). Blue light is sensed by two types of photoreceptors, the first are cryptochromes (cry1 and cry2), they contain a pterin chromophore which absorbs blue light, passing an electron to FAD as a signal. The second are phototropins, (phot1 and phot2) which contain Lov1 and Lov2 proteins which bind spontaneously in the dark. The importance of blue light in the process of opening stomata can be investigated experimentally through looking at phot1 and phot2 mutants, and cry1, and cry2 mutants which exhibit specific biological responses. Individual examples of phot1 and phot2 mutants have reduced stomatal pore apertures, and a double mutant of the two has a completely closed stomata, showing that these genes are important for opening the stomata, and that blue light is the trigger.

An experiment which changes the day length that a plant is subjected to can show the effects of night length on the rate of stomatal opening in light. Initial rapid closure of stomata in darkness seems to be independent of Co2 accumulation (similar to a blue light response), shown though artificial wind removing levels of Co2, this shows that stomatal closing can be both independent and dependent upon Co2 concentrations (Mansfield, and Meidner, 1966) .




Humidity and stomata

Guard cells are very sensitive to humidity, when water vapor of air and intracellular spaces is low, the guard cells close. This is usually due to water stresses, the effect is strong and overrides the opening effects of both Co2 and light. Large gradients can produce oscillations of the opening and closing of the guard cells (Salisbury, and Ross, 1992). At high temperatures the stomata close, although some species open their stomata in heat to cool their leaves, the wind across the stomata closes the stomata as higher Co2 levels move into the leaves.

Lange (1971) found that treatment of the outer side of the leaf with dry air led to a rapid closing of the stomata, and moist air caused opening. The guard cells act as humidity sensors, measuring the water potential both inside, and outside. Plants can reduce their transpiration through an increase in diffusion resistance of the stomata during decreasing humidity, controlling water loss in a localised manor.

Hormone effects

Hormones involved in stomatal opening are cytokinins which cause the opposite to ABA hormone, and open the guard cells. Stomatal opening is also induced by indoleacetic acid (IAA), and fusicoccin (FC) (Irving, et. al., 1992). Abscisic acid (ABA) induces closure of the stomata, it does this to reduce desiccation, and travels to the guard cells from the roots where water depletion is sensed, it then reaches the guard cells via the apoplast pathway from the mesophyll cells (Salisbury, and Ross, 1992). ABA is synthesised in the cytosol of cells, it accumulates in the chloroplasts, and is released into the cell walls outside the protoplasts. The degree of stomatal response to decreased levels of ABA depends on the concentrations of Co2 in the guard cells and the Co2 response depends on the levels of ABA. Ethylene is also a plant hormone which increases stomatal closure in tomato and carnation plants at concentrations of 60-70ppm (Madhavan, et. al., 1983).

Mechanism

There are two positive feed-back loops which contributed to the opening and closing of the guard cells, The first feedback involves light increasing photosynthesis, this decreases the Co2 in the leaf, which increases the K+ into the guard cells, which leads to H2O moving into the guard cells, which swells the cells and opens them. The second feed-back method closes the stomata through a loss of water through the pore, this increases the levels of ABA synthesised from the mesophyll cells, K+ moves out of the guard cells, and H2O also moves out of the guard cells by diffusion/osmosis.

Stomata cells open because the guard cells absorb water and swell. The guard cells are structured so that cellulose micro fibrils are arranged in circumference, as if radiating from the center of the stoma called radial micellation (Salisbury, and Ross, 1992) . So the guard cells can not expand much in diameter, but can increase their length , micro fibrils then pull at the wall (closest to the stoma pore) which opens the stoma wider, it also helps that the guard cells are thicker on this inner wall.

Guard cells are usually kidney shaped (40mm long, with a pore width of 5-15mm), and grasses are dumbbell shaped (Nobel, 2005). It is unusual for stomata to completely close. They usually contain some chloroplasts, although whether any significant levels of photosynthesis occurs in the guard cell is still a debatable subject. The waxy cuticle of the top of the leaves prevents evaporation of all gases, so all diffusion must take place through the stomata. As water evaporates Co2 follows. Accessory cells lie next to the guard cells. Most open at sunrise and close in the dark, opening require 21 hrs, closing is a gradual change over the afternoon, although they close faster if suddenly exposed to the dark (Salisbury, and Ross, 1992).

Proton efflux by the guard cells precedes stomatal opening (Edwards. et. al., 1988), and it is also accompanied by an increase in Ca2+ (Irving, et. al., 1992). Proton (H+) from the cells which is mediated through blue light phosphorylation of the plasma membrane H+-ATPase (Kinoshita et. al., 2001), induces an increase in potassium by 0.2 to 0.5 M inside the guard cell, K+ ions move into the cells through K+ channels driven by an inside-negative electrical potential (Kinoshita et. al., 2001).There is also an accumulation of potassium salts of organic acids, mainly Malate which accumulates in the guard cells upon illumination (Allaway, 1972), and H+ is produced from these organic acids stored in the guard cells. Starch is broken down into 3-carbon compound PEP (phosphoenol private), which is promote by blue light, PEP binds with Co2 to form four carbon acetic acid, which is converted to malaic acid, which provides the H+ ions needed to transport K+ into the guard cells.

The closure of stomata mainly involves K+ and other ions into the cell increases the internal the osmotic potential, which lowers the external which allows external water potential causing water to move into the guard cell. The increased water concentration inside the guard cell increases the hydrostatic pressure, and thus increases the turgidity of the guard cells. As the cells get more turgid they swell, and a kidney shape is made. In some species Cl- ions or other anions accompany K+ into and out of the guard cells. The process of stomatal opening is influenced by light, internal Co2 concentrations, plant water stress, and hormones.

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