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

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.

Reference

http://env.licor.com/products/gas.htm