Radioisotopes can be used to help understand chemical and biological processes in plants. This is true for two reasons: 1)radioisotopes are chemically identical with other isotopes of the same element and will be substituted in chemical reactions and 2)radioactive forms of the element can be easily detected with a Geiger counter or other such device.

Example:
A solution of phosphate, containing radioactive phosphorus-32, is injected into the root system of a plant. Since phosphorus-32 behaves indentically to that of phosphorus-31, the more common and non-radioactive form of the element, it is used by the plant in the same way. A Geiger counter is then used to detect the movement of the radioactive phosphorus-32 throughout the plant. This information helps scientists understand the detailed mechanism of how plants utilized phosphorus to grow and reproduce.

How would a biochemist use radioactive carbon to trace the path of carbon atoms in the photosyntheses process of green plants?

Original source of page:  http://www.chem.duke.edu/~jds/cruise_chem/nuclear/agriculture.html