Stomata are tiny structures found on the epidermis of a plant. The tiny pore called the stoma is surrounded by guard cells which are specialised cells. The main function of the stomata is to allow gases such as carbon dioxide, water vapour and oxygen to move rapidly into and out of the leaf. Since most of the water (90%) is lost through stomata, plants regulate the degree of stomatal opening to reduce the water loss. The guard cells have unevenly thickened walls. The cell wall around stoma is tough and flexible and the one away from stoma is thinner.
Stomata opening and closing:
most plants do not have the aforementioned facility and must therefore open and close their stomata during the daytime in response to changing conditions, such as light intensity, humidity, and carbon dioxide concentration. It is not entirely certain how these responses work. However, the basic mechanism involves regulation of osmotic pressure.
When conditions are conducive to stomatal opening (e.g., high light intensity and high humidity), a proton pump drives protons (H+) from the guard cells. This means that the cells' electrical potential becomes increasingly negative. The negative potential opens potassium voltage - gated channels and so an uptake of potassium ions (K+) occurs. To maintain this internal negative voltage so that entry of potassium ions does not stop, negative ions balance the influx of potassium. in some cases chloride ions enter, while in other plants the organic ion malate is produced in guard cells. This in turn increases the osmotic pressure inside the cell, drawing in water through osmosis. This increases the cell's volume and turgor pressure. Then, because of rings of cellulose microfibrils that prevent the width of the guard cells from swelling, and thus only allow the extra turgor pressure to elongate the guard cells, whose ends are held firmly in place by surrounding epidermal cells, the two guard cells lengthen by bowing apart from one another, creating an open pore through which gas can move.
When the roots begin to sense a water shortage in the soil, abscisic acid (ABA) is released. ABA binds to receptor proteins in the guard cells' plasma membrane and cytosol, which first raises the pH of the cytosol of the cells and cause the concentration of free Ca2+ to increase in the cytosol due to influx from outside the cell and release of Ca2+ from internal stores such as the endoplasmic reticulum and vacuoles. This causes the chloride (Cl-) and inorganic ions to exit the cells. Secondly, this stops the uptake of any further K+ into the cells and subsequentally the loss of K+. The loss of these solutes causes a reduction in osmotic pressure, thus making the cell flaccid and so closing the stomatal pores.
Interestingly, guard cells have more chloroplasts than the other epidermal cells from which guard cells are derived. Their function is controversial.
This diagram shows normal responses of stomata to light, CO2, pH, K+ ion and Water Deficiency
Kalanchoe daigremontiana and also sometimes called Mother of Thousands, is a plant from southwest Madagascar. The plant generally would reach up to 3 feet (1m) tall and consist of leaves that reach 6-8 inches long and about 1.25 inches wide. These are medium green above and blotched with purple underneath. The most interesting feature of this plant is the fact that it has spoon-shaped bulbiliferous spurs that bear young plants on its margins. This plant is distinguished by its ability to propagate via vegetative propagation. All parts of the plant are poisonous, which can even be fatal if ingested by infants or small pets.
Blooming: In the greenhouse, the plants bloom sporadically in late winter. The compound cymes have 1 inch (2.5 cm) long purplish flowers.
Culture: Kalanchoe daigremontiana needs full sun to partial shade with a well-drained soil mix. In the greenhouse, soil is mixed consisting of 1 part peat moss to 2 parts loam and sand. The plants are watered and allowed to dry slightly before watering again. They are generally fertilised only once during the season with a balanced fertiliser. During the winter months, water is somewhat restricted, but the plants are not allowed to dry out completely. The plants can become very weedy, so these plants should not be used around other plants. Plantlets are drought resistant, root readily, and if allowed to establish, can easily create a plant epidemic wherever the plantlets land (hence their common name).
Propagation: Kalanchoe daigremontiana is easily propagated from plantlets formed on the edges of leaves or from cuttings. Cuttings must be kept very dry to root.
- Eye piece graticule
- Distilled water
- Sugar solution
- Iodine (optional)
- Petri dish
- Clear nail vanish
1. Select the leaves which will be used
2. Put them in the different conditions over night
3. Next day, nail polish all the leaves with clear nail vanish
4. Allow to dry for about 2 hours
5. When completely dry, peal of the nail polish
6. Put it on the slide and observe with the microscope, also insert the eye piece graticule to measure the size of the stomata
7. (optional) if not very visible, apply iodine on the slide put the piece of nail polish on top and press down with tissue to clear of excess solution
8. Count the number of stomata visible and measure the size
Conclusion & Evaluation
From the results we can it is clear that the stomatal size is greatly dependent on the conditions in which it is located. For the geranium we can see that the stomata has the greatest size of 10mm in the condition with light, here it is possible to accept the hypothesis, the fact that it has a greater stomatal density in conditions with the most light. On the other hand the Mexican hat showed lower numbers such as .... This may be because of the origin of this sort of plant and the fact that they are generally found in hot countries where there is generally more light could have ment that these sort of plants undergo various adaptation and therefore have adapted to have smaller stomatal sizes even thought there is a lot of light present.
We also notice that the geranium has the most stomatal numbers (120 stomatal pores in a 1cm by 1cm sample of leaf) in the water condition, this shows that stomatal numbers increase in the presence of more water. Here also the Mexican hat leaf shows the opposite, here it has a very low stomatal number, this may be an anomaly or may be purely due to the fact that the plant is found in areas with water shortages and therefore here again it may have adapted to maintain low numbers of stomata pores in water
Another way to find out whether stomata are open or closed, or more accurately, how open they are, is by measuring leaf gas exchange. A leaf is enclosed in a sealed chamber and air is driven through the chamber. By measuring the concentrations of carbon dioxide and water vapor in the air before and after it flows through the chamber, one can calculate the rate of carbon gain (photosynthesis) and water loss (transpiration) by the leaf.
However, because water loss occurs by diffusion, the transpiration rate depends on two things: the gradient in humidity from the leaf's internal air spaces to the outside air, and the diffusion resistance provided by the stomatal pores. Stomatal resistance (or its inverse, stomatal conductance) can therefore be calculated from the transpiration rate and humidity gradient. (The humidity gradient is the humidity inside the leaf, determined from leaf temperature based on the assumption that the leaf's air spaces are saturated with vapor, minus the humidity of the ambient air, which is measured directly.) This allows scientists to learn how stomata respond to changes in environmental conditions, such as light intensity and concentrations of gases such as water vapor, carbon dioxide, and ozone.