MadSci Network: Botany

Re: How do you measure the rate of photosynthesis in a plant?

Date: Thu Nov 5 13:16:39 1998
Posted By: Karen Culver-Rymsza, Biological Oceanographer
Area of science: Botany
ID: 909594745.Bt

Dear photosynthesis explorer,

There are two phenomena that take place in the process of photosynthesis. One is the splitting of water (to obtain electrons and release oxygen) in the so-called 'light reactions' and the other is the fixation of carbon dioxide into an organic compound in 'dark reactions'. Both of these changes are used to measure photosynthetic rates. And both require some specialized equipment.

Measurement of oxygen evolution is probably the easiest method. It can be performed using an oxygen electrode or, in the case of aquatic algae, Winkler titration. Winkler titration relies on chemical changes to measure oxygen content in water after photosynthesis takes place upon exposure to light. Oxygen electrodes measure oxygen concentration in air or water and are quick, efficient and accurate. Since most researchers are interested in the amount of carbon fixed more so than oxygen released, the oxygen-based rate is converted to carbon fixed by the ratio known as the photosynthetic quotient. (This is often assumed to be 1:1,but actually varies a bit). Both techniques require a "dark" control sample to account for respiration.

The measurement of carbon fixation is a bit more difficult. It requires the use of radioactive carbon(C-14) dioxide. This assay measures the amount of radioactive carbon taken up from the medium (air or water) and incorporated into plant tissue. When the total amount of carbon dioxide is known, then the amount of radioactive carbon taken up is used to calculate the total carbon (both radioactive and non-radioactive) that was fixed. This technique also requires a dark control sample. It is a very nice and sensitive technique, but requires special equipment (scintillation counter), toxic chemicals, and experience with isotopes.

In addition to the incorporation of carbon dioxide into tissue, one could theoretically measure the removal of carbon dioxide from the air or water. This approach is not often used since it requires special equipment (like the other techniques), and in addition is much less sensitive and prone to contamination problems. Carbon dioxide is difficult to measure directly.

Both of these techniques measure NET photosynthetic rates. That means that respiration is already factored into the results. In other words, when one measures oxygen evolution on a whole plant or cell, respiration, which CONSUMES oxygen is also taking place and affects the measured oxygen evolution. Thus the need for a dark control sample. In the case of carbon fixation, it is generally agreed that this technique measures somewhere in between net and gross photosynthesis. This is because while respiration does release carbon dioxide, in this type of experiment it will release some non-radioactive carbon dioxide which cannot be easily accounted for.

In both cases a light dependent change is measured. This means that the results will vary due to differences in the quality and quantity of light. The quality of light affects photosynthesis by changing the wavelengths (or color) of light that reaches the photosynthetic pigments. These pigments have specific absorption characteristics that affect what colors of light are usable. The quantity of light simply increases or decreases the amount of energy based on brightness or total flux of light. Both quality and quantity of light must be controlled to obtain useful results.

There are some simple techniques for measuring productivity (rather than photosynthesis). In these cases the results of photosynthesis and growth are measured. Of course plants depend on photosynthesis for growth so there is a relationship. Some of these techniques are quite simple. They measure the mass of plant material. This can be done by weighing before and after treatment. Aquatic botanists also relate the length of the blade of seagrass or seaweed to growth as well. Both methods give reliable results BUT the changes in mass (or blade length) must be large enough to measure. This means short-term experiments are not appropriate.

On a personal note, I find photosynthesis to be one of the most fascinating processes of the living world. Without it there would be no oxygen atmosphere for life as we know it to develop and enough carbon dioxide in the atmosphere to really heat things up. Enjoy your exploration of this amazing process!

Admin note:
David Hershey adds the following:

Photosynthesis of intact leaves or potted plants is very often measured using an infra-red gas analyser which measures carbon dioxide without radioisotopes. It is a very accurate, easy, and nondestructive technique.


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