PLANT BIOPHYSICS - Y. I. Posudin - 2004

II. TRANSPORT PROCESSES IN THE SOIL-PLANT-ATMOSPHERE SYSTEM

7. GAS TRANSPORT

7.4. MEASUREMENT OF OXYGEN CONTENT CHANGES

Photosynthesis is accompanied by the photolysis of Water, which results in the release of molecular oxygen. To measure oxygen during photosynthesis, the polarographic method is used [Walker, 1987], which relies on an oxygen electrode (Clark electrode) (Fig. 7.17). A plant sample (an 8 mm diameter leaf disc) is placed on a perforated plate inside the chamber. Directly beneath the sample is the Clark electrode itself, consisting of a platinum cathode and a silver ring-shaped anode immersed in an electrolyte (a saturated KCl^- solution). Both electrodes are separated from the reaction mixture by an oxygen-permeable membrane. Typical membrane Materials include polyethylene or Teflon with a thickness of 12.5-25 µm. The reaction mixture is continuously stirred with a magnetic stirrer. Under an applied voltage, the following electrochemical reactions take place:

a) at the cathode:

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b) at the anode:

Fig. 7.17. Clark electrode for measuring changes in oxygen content during photosynthesis.

The KCl^- electrolyte forms a closed electrical circuit (Fig. 7.18) through which an electric current flows when voltage is applied to the electrodes. At a voltage of 0.6-0.7 V, the electric current is linearly dependent on the partial pressure of oxygen in the solution. This electric current is measured when investigating The Effect of illumination or chemical agents on the photosynthetic activity of plants.

Fig. 7.18. Closed electrical circuit formed by the oxygen electrode: 1 - oxygen supply system, 2 - membrane, 3 - cathode, 4 - power source, 5 - KCl bridge, 6 - KCl solution, 7 — anode.

The oxygen electrode is characterized by high sensitivity; it is capable of detecting oxygen at concentrations as low as 10^-8 mol·cm^-3. An advantage of this device is The ability to perform rapid and continuous measurements of oxygen content changes within the system. The instrument is compact, portable, and serves as a cost-effective alternative to IRGA. Furthermore, unlike IRGA, it can measure oxygen even under conditions of CO₂ saturation.

Review Questions

1. What is Transpiration, and what processes does it comprise?

2. How does stomatal transpiration differ from cuticular transpiration?

3. Define the processes of vaporization and Condensation.

4. What is referred to as saturated vapor?

5. State Raoult's law.

6. What factors determine the resistance to diffusive flux?

7. Characterize the main Methods for measuring transpiration.

8. What is the operating principle of a gas analyzer?

9. Name the main methods for measuring gas exchange.