A potometer —sometimes known as a transpirometer— is a device used for measuring the rate of water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 uptake of a leaf

Leaf

A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....

y shoot

Shoot

Shoots are new plant growth, they can include stems, flowering stems with flower buds, and leaves. The new growth from seed germination that grows upward is a shoot where leaves will develop...

. The causes of water uptake are photosynthesis

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 and transpiration

Transpiration

Transpiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants , especially in leaves but also in stems, flowers and roots. Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants...

.

Potometers are often difficult to set up, as measurement-altering air bubbles in the xylem

Xylem

Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...

 of the plant or in the apparatus must be eliminated. Everything must be completely water tight so that no leakage of water occurs.

There are two main types of potometers used - the bubble potometer (as detailed below), and the mass potometer. The mass potometer consists of a plant with its root submerged in a beaker. This beaker is then placed on a digital balance; readings can be made to determine the amount of water lost by the plant. The mass potometer measures the water lost through transpiration of the plant and not the water taken up by the plant.

Design

Potometers come in a variety of designs, but all follow the same basic principle.

• A length of capillary tube A bubble is introduced to the capillary; as water is taken up by the plant, the bubble moves. By marking regular gradations on the tube, it is possible to measure water uptake.
• A reservoir. Typically a funnel
  Funnel
  A funnel is a pipe with a wide, often conical mouth and a narrow stem. It is used to channel liquid or fine-grained substances into containers with a small opening. Without a funnel, spillage would occur....
  
   with a tap; turning the tap on the reservoir resets the bubble. Some designs use a syringe
  Syringe
  A syringe is a simple pump consisting of a plunger that fits tightly in a tube. The plunger can be pulled and pushed along inside a cylindrical tube , allowing the syringe to take in and expel a liquid or gas through an orifice at the open end of the tube...
  
   instead.
• A tube for holding the shoot. The shoot must be held in contact with the water; additionally, the surface of the water should not be exposed to the air. Otherwise, evaporation will interfere with measurements. A rubber bung greased with petroleum jelly
  Petroleum jelly
  Petroleum jelly, petrolatum, white petrolatum or soft paraffin, CAS number 8009-03-8, is a semi-solid mixture of hydrocarbons , originally promoted as a topical ointment for its healing properties...
  
   suffices.

Preparation

1. Cut a leafy shoot from a plant and plunge its base into water. This prevents the xylem from taking up any air. Wetting the leaves themselves will alter the rate of transpiration.
2. Immerse the whole of the potometer into the sink. Move it about until all the air bubbles come out.
3. Recut the shoot's stem underwater. Put it into the bung; grease the bung with plenty of petroleum jelly (Vaseline) if it doesn't stay and then put the bung into the potometer.
4. Make sure the tap is closed, then lift the whole assembly out of the water.
5. Leave the end of the capillary tube out of the water until an air bubble forms then put the end into a beaker of water.

Use

1. Set up the conditions of the experiment. Alterations to lighting (placing the plant in bright light or shadow), wind (directing a fan at the plant), and humidity (placing the plant in a humid chamber) are typical.
2. Let the bubble reach a "zero" point in the tube.
3. Measure the movement of the bubble at regular intervals and record the results.

Precautions

1. When a twig is cut from a plant, it should be immediately put under water (only the cut portion). Then, a small part is cut under water. This prevents entry of air into the xylem vessels.
2. The conditions of the potometer, other than the alteration that is being tested, should not be changed during a test, as outside conditions (for example, temperature) determine water uptake.

Limitations

1. The Potometer does not measure the rate of transpiration accurately because not all of the water that is taken by the plant is used for transpiration ( water taken might be used for photosynthesis or by the cells to maintain turgidity). The potometer measures the rate of uptake of water. To measure transpiration rate directly, rather than the rate of water uptake, utilize a scientific instrument
   Photosynthesis system
   Photosynthesis systems are electronic scientific instruments designed for non-destructive measurement of photosynthetic rates in the field. Photosynthesis systems are commonly used in agronomic and environmental research, as well as studies of the global carbon cycle.- How photosynthesis systems...
   
   which quantifies water transfer at the leaves.