What is a Ganong’s Potometer?
A potometer is an apparatus which is used to measure the amount of moisture absorption by the leafy shoot during transpiration. It is also referred to as a transpirometer. The reasons for moisture uptake are transpiration and photosynthesis. The Ganong’s potometer finds its purpose in the measurement of the transpiration rate in a cut twig in lab settings.
Ganong's potometer is a glass tube that is bent twice and consists of a broad-mouthed glass cylinder, a horizontal glass rod with a capillary tube inserted within and a reservoir connected with the horizontal rod.
The broad-mouthed glass cylinder is present towards the front end of the entire setup. The opening of the cylinder is attached with a single-holed cork (rubber). The newly cut twig for the experiment is placed via this hole in the rubber cork.
The horizontal rod of the setup is a long glass tube within which a capillary tube is placed and has graduated readings marked on it. This horizontal rod has a bent end containing a nozzle mouth. This bent side is placed in a beaker that consists of colored liquid (water).
As we move towards the front end of the horizontal rod, a vertically placed reservoir is present that is attached with the horizontal rod in which water can be accumulated. A pinch cock or stop cock is placed at the bottom of the reservoir to enable or halt the flow of water from reservoir to the horizontal rod. The entire apparatus is mounted on a flat surface like the table with the help of a stand.
Length of Capillary Tube - A bubble is forced into the capillary. As the water is absorbed by the plants, the bubble starts moving. By spotting regular markings on the tube, it is possible to calculate the amount of water absorbed.
Reservoir – It is generally a funnel with a tap attached to it. Rotating the tap on the reservoir adjusts the bubble. Some other designs make use of a syringe instead.
Tube for Handling the Leafy Shoot - The leafy shoot should be kept in contact with the water. In addition, the surface of the water is not supposed to be exposed to the air. Or else, evaporation will interfere with the calculations. A rubber bung lubricated with petroleum jelly is sufficient for the setup.
Transpiration is the mechanism where there is loss of moisture or water in the form of water vapor from the aerial organs of the plants, particularly via the leaves. In every green plant, the process of transpiration takes place via the stomata of the leaves, in most cases. The rate of transpiration relies on different climatic factors such as temperature, intensity of sunlight, humidity, velocity of wind, availability of water to the plant, atmospheric pressure, and many more.
There are various types of transpiration which are:
Water loss mechanism
The rate of transpiration can be calculated in the following ways:
Directly – By calculating the weight of the Ganong’s potometer after a definite duration of time. In this case, it is assumed that any loss that takes place is because of transpiration.
Indirectly – By calculating the path the water level drops in the graduated tube after a measured period of time. In this case, it is assumed that this phenomenon takes place because of the taking in water that, in turn, is important to substitute an equal amount of moisture or water lost due to transpiration.
The entire setup or apparatus is full of colored water. The colored water is made by pouring a couple of drops of Eosin oil to the water. A newly cut twig is placed with the help of the single-holed rubber cork to the opening of the glass cylinder. The entire setup or apparatus is supposed to be mounted on a flat surface like a table under bright sunlight.
Prior to the beginning of the experiment, a lone air bubble is supposed to be kept at zero reading in the capillary tube of the horizontal glass rod. To achieve this, the bent side of the horizontal rod is supposed to be shifted from the container or the beaker. Doing this will enable a couple of air bubbles to get held in the capillary tube of the horizontal glass rod. However, we need to observe that there must be only a single air bubble to get held at the zero reading.
To do this modification, we need to take off the stop cock of the reservoir to enable water from the reservoir flow through the horizontal rod. The water now compels the extra air bubbles to move out from the nozzle. We need to do numerous trails until a single air bubble get modified at the zero reading.
After a certain time duration post beginning of the experiment, we observe that the air bubble from the zero reading begins to move. As the moisture or water is transpired from the aerial regions of the leafy shoot, a transpirational pull is generated by the leafy shoot to compensate for the loss of moisture. Because of this force, the air bubble begins to move from the zero reading of the horizontal rod towards the newly cut twig.
The rate of water absorption can be calculated by measuring the distance travelled by the air bubble in a certain period of time and numerous measurements are recorded by adjusting the air bubble to the zero reading again. The average or mean of these measurements will provide us with the rate of transpiration. To adjust the air bubble to the zero reading and to perform the experiment again, the water stored in the reservoir is to be utilized.
The following precautions must be taken before using Ganong’s potometer:
When a twig is freshly cut from a plant, make sure that it is immediately placed under water (only the cut region). After that, a tiny portion is cut while it is still under water. Doing this avoids the entrance of the air into the vessels of the xylem.
The state of the potometer, except the modification that is being measured, must not be manipulated during a test, as external factors such as temperature, and pressure can affect the water absorption.
Each and everything should be absolutely water-tight in order to avoid any leakage.
The Ganong’s potometer comes with the following limitations:
The newly cut twig positioned on the glass cylinder may not stay alive for a significant period of time.
The forcing of an air bubble into the horizontal rod is not an easy task and takes a lot of time.
The Ganong’s potometer does not calculate the rate of transpiration with the required accuracy as not the entire water absorbed by the plants is utilized for transpiration. The device generally estimates the rate of moisture uptake.
A little alteration in the atmospheric temperature can have some impact on the position of the air bubble.