Stomata Density

Stomata are tiny pores found on the epidermis of the leaf, surrounded by guard cells. [1] Their main function is gas exchange [1] for photosynthesis and respiration. The development of stomata on the leaves of a plant is determined by interaction between different genes and environmental factors. A few studies have been conducted in order to establish a relationship between stomatal densities and given environmental factors. Research has shown that stomatal densities are controlled by environmental conditions during leaf development, but are fixed after the leaf matures. [2]

The article “The influence of light on stomatal density of a tomato” by A. P. Gay and R. G. Hurd describes their findings that plants grown under high light intensity have more stomata per 1 mm 2 than plants grown under low light intensity. [3] The purpose of my investigation is to determine whether there is a correlation between the light intensity and the stomatal density on lavender leaves and whether the initial height of the plants influences the stomatal densities. The hypothesis is that an increase in the light intensity will lead to an increase in the stomatal density of the lavender leaf.

The first aim of this investigation was to find whether there is a significant correlation between the stomatal density of lavender plants and the light intensity under which they are grown. The second aim of the investigation was to find out whether the initial height of the plant influences its stomatal density. Cuttings were taken from lavender plants to ensure that all the plants were genetically identical and that the only changes occurring in the stomatal density would be due to environmental conditions. Four cuttings were short (3 cm initial height) and four were tall (6 cm initial height).

The cuttings were put under compact fluorescent light bulbs with four different power ratings (8, 11, 14, and 20 W). One short and one tall cutting were put under each of the four light bulbs for 28 days in order to grow them. Both the short and the tall plants showed a positive correlation between their stomatal densities and the light intensity. The correlation was statistically significant at a 0. 025 significance level according to the Pearson product-moment correlation test.

The short and the tall plants grown under the same light intensity did not show any statistically significant difference between their stomatal densities. The first aim of this investigation was to find whether there is a significant correlation between the stomatal density of lavender plants and the light intensity under which they are grown. The second aim of the investigation was to find out whether the initial height of the plant influences its stomatal density. Cuttings were taken from lavender plants to ensure that all the plants were genetically identical and that the only changes occurring in the stomatal density would be due to environmental conditions.

Four cuttings were short (3 cm initial height) and four were tall (6 cm initial height). The cuttings were put under compact fluorescent light bulbs with four different power ratings (8, 11, 14, and 20 W). One short and one tall cutting were put under each of the four light bulbs for 28 days in order to grow them. Both the short and the tall plants showed a positive correlation between their stomatal densities and the light intensity. The correlation was statistically significant at a 0. 025 significance level according to the Pearson product-moment correlation test.

The short and the tall plants grown under the same light intensity did not show any statistically significant difference between their stomatal densities. When trying to explain the correlation, it is important to consider what stomata are in the first place and what their most important functions are. Stomata are tiny pores [1] found on the epidermis of the plants and their main role is gas exchange between the leaf and the environment. Although stomatal development is essentially controlled by different genes, the environment also has a significant effect on stomatal development.

Using plants that are clones in the investigation means that they all have the same genetic material and any changes in stomatal density on their leaves should be due to environmental factors. [9] Both light intensity and carbon dioxide concentration have been shown to influence the frequency at which stomata develop on the leaves of plants. [8] Plants can respond to changes in environmental conditions by changing their stomatal frequency. Recent research has shown that signals from older leaves can influence the development of stomata on the younger leaves.

In that way, if the environmental conditions to which the older leaves are exposed change, then the younger leaves can increase or decrease their stomatal density; this physiological adaptation can help the plant cope with the changing environment. Why is the increased light intensity leading to increased stomatal density? Photosynthesis is the process by which plants synthesize glucose from carbon dioxide and water. The energy of the reaction is supplied by the sunlight. However, there are two main stages in photosynthesis – light-dependent and light-independent stages.

The light-dependent stage depends on the light because the energy from the light is used to split water in the process of photolysis and excite electrons in the chlorophyll. [11] The products from the light-dependent stage are ATP and the electron acceptor – reduced NADP. [11] The products from the light-dependent stage are fed into the light-independent stage of photosynthesis, the Calvin cycle. [11] Carbon dioxide is fixed in the light-independent stage and converted to glucose; in the Calvin cycle, the products of the light-dependent stage are needed. So, more ATP and reduced NADP will result in an increased rate of carbon fixation.

If the rate of carbon fixation increases, the rate at which carbon dioxide diffuses in and out of the leaf will increase. The light intensity is simply the energy per second per unit area carried by the incident light and it is proportional to the number of photons per second carried by the incident light. [12] Higher light intensity means more photons per second resulting in more electrons per second that would be excited during the light-dependent stage of photosynthesis, and more ATP and reduced NADP are produced. Therefore, increasing the light intensity will increase the overall rate of photosynthesis.

The rate of gas exchange will increase as a result. Coming back to the main function of the stomata, increasing the rate of gas exchange may lead to increased stomatal density on the epidermis of the leaf. The adaptation leads to higher carbon dioxide assimilation as the results of recent studies have shown. [2] However, the energy of the incident light arriving per second is also proportional to the wavelength of the light. Therefore, the light intensity depends on the light wavelength. Plants have combinations of chlorophyll pigments [11] that absorb sunlight from the visible spectrum.

The light of wavelengths 400-500 nm and 650-700 nm [11] is absorbed the most. These are blue and red light, respectively. Lavender grows well under compact fluorescent light bulbs. [13] By placing colored filters in front of the light bulbs, it can be established which color of light is most suitable for growing lavender and whether the color of light affects the stomatal density. To determine whether there is a statistically significant difference between the stomatal densities on the tall and short plants grown under the same light intensity, the Mann-Whitney U test is used.

The null hypothesis is that there is no statistically significant difference between the stomatal densities of the tall and the short plants grown under the same light intensity. The null hypothesis may be rejected if the calculated value of U is equal to or smaller than the critical value. The critical value for U for 10 sets of data is 16. [7] Looking back at the results section, all the calculated values of U are bigger than the critical value, so the null hypothesis is accepted. The initial height did not seem to influence the stomatal development in my investigation.