The Stomatal Index In Tomato Plants

Published Date: 02 Nov 2017

In plants, the organs responsible for making food and one of the most important parts of the plant itself are the leaves. Leaves absorb light (usually from the sun) and carbon dioxide from their surroundings and convert them into food and energy in a process known as photosynthesis. Photosynthesis is the process where light energy is converted into chemical energy and is stored as a bond of sugar. The process of photosynthesis occurs mainly in plants and some algae. The plants require light energy, carbon dioxide and water in order to produce sugar. This process occurs in the chloroplasts of the plant cells, using the chlorophyll, i.e. the green pigment which is involved in photosynthesis.

The exchange of carbon dioxide and oxygen throughout the leaf, including the loss of water vapor during the process of transpiration, all occurs through the stomata, i.e. pores which are surrounded by pairs of cells called guard cells, which are responsible for controlling what enters and exits the cell. Gas exchange occurs when carbon dioxide molecules are taken into the leaves for photosynthesis, and then, after the process oxygen molecules are produced which are released out of the leaves. The pores/holes on the leaf allow for water to flow through the plant by sustaining a lower water pressure at the leaves than it is at the roots.

Figure 1: Labeled image of stomata. [1] 

The opening and closing of the stoma is adjusted by the swelling of the guard cells. The Stomata can open and close during the day to control water loss through evaporation on particularly hot days. There are more stomata on the bottom of the leaf because that lessens the amount of water loss in the process of transpiration.

Accordingly, plants are able to vary their stomata density by regulating the amount of water which is lost during the opening and closing process of the stomata.

The greater the stomata density, the more efficient the control of water loss will be. The density of the stomata of a leaf varies with many different factors such as the temperature, humidity, and light intensity in the surrounding environment of the plant. The stomatal index is defined as the number of stomata as a percentage when compared to the number of all of the epidermal cells, including the stomata, which are present within in a unit area of the leaf. 

Research question:

What is the effect of light intensity on the density of the stomata and the stomatal index in tomato leaves? The experiment will be separated into two parts, which will focus on recording the data of the amount of sunlight that reached the leaves. Tomato plants were divided into two groups – plants grown in a glasshouse under direct sunlight and plants grown under daylight conditions with no direct sunlight conditions. This investigation will be conducted in order to determine the number of epidermal stomata and epidermal cells present.

Aim:

The aim of this experiment will be to investigate and determine the effect of different light intensity on stomata density and the stomatal index in tomato leaves.

Hypothesis:

It is presumable that more stomata would be required in sun leaves per leaf area as they are denser with wider cell surroundings (palisade layer) than the shade leaves, so that the concentration of carbon dioxide would not limit the rate of photosynthesis. The light intensity for shade leaves may limit the rate of photosynthesis, and too much stoma may cause excessive water loss. As a result the leaves collected from tomato plants which will be exposed to direct sunlight (glasshouse) will have greater numbers of stomata compared to those collected from plants exposed to daylight (no glasshouse) conditions.

Variables:

Controlled variables:

Age of plants

Room temperature

Amount of water used for irrigation

The volume and quality of soil in which the plants were grown

Amount of leaves used in both glasshouse and daylight conditions

Dependent variable:

Number of epidermal cells

Stomata number

Independent variables:

The amount of sunlight that reached the leaves

Tomato plant groups –

- Plants grown in a glasshouse under direct sunlight &

- Plants grown under daylight, no direct sunlight.

Control of variables:

All plants which are to be used in the experiment will be planted and grown for a period of 12 months. They will be kept in similar conditions; only the glasshouse and daylight conditions will be altered and kept constant for each in order to test the experiment.

The temperature will be ensured by heating and keeping the environment of the experiment which is taking place in the glasshouse, and making it the same as the temperature which is present during the day, in which the other plant is exposed, therefore ensuring constant and equal temperatures throughout the whole experiment, for both plants.

25of water will be measured by measuring cylinder.

Plants will grow in natural fresh soil in both cases, and in the same volume of soil also (≈ 900).

There are 3 leaves of each, the 3 leaves from plants grown in a glasshouse under direct sunlight and the 3 leaves from plants grown under daylight conditions (no direct sunlight).

Materials:

3 leaves from plants grown in a glasshouse under direct sunlight

3 leaves from plants grown under daylight conditions

6 – 10 clear glass slides

Clear nail polish

Clear tape

Measuring cylinder

Microscope

Scissors

Soil

Tomato plant grown in a glasshouse (direct sunlight)

Tomato plant grown under daylight conditions (no direct sunlight)

Water for irrigation

Method:

Collect leaves from plants grown under direct sunlight in the glasshouse as well as those grown under daylight conditions.

Place clean nail enamel on the undersurface of the leaves.

After the nail polish dries up, remove the imprint by using clear tape and then place it on a slide.

Examined the leaf impressions under a light microscope at 400X magnification.

Count carefully the amount of the stomata present in one microscopic field and record the numbers.

Repeat counts for the two remaining microscopic fields.

Record all of the counts were recorded, hence determine an average number per microscopic field.

Calculate the Stomata and epidermal cells in a 1 mm2 unit area by multiplying the average number/400X microscopic field by 8.

Determine the stomatal index and record the data.

Repeated the procedure with the remaining five leaves from each of the two plant samples (plant grown in the glasshouse and under direct sunlight) and record data.