Question

The portable instrument used to record photosynthesis in plants is

• A. Isotope-ratio mass spectrometer (IRMS)
• B. Inductively coupled plasma optical emission spectrometer (ICP-OES)
• C. Infrared Gas Analyzer (IRGA)
• D. Atomic absorption spectroscope (AAS)

Hint:

Remember that \( \text{CO}_2 \) absorbs infrared light at a specific wavelength (\( 4.26 \text{ }\mu\text{m} \)).
The Infrared Gas Analyzer (IRGA) exploits this physical property to calculate gas exchange rates directly in the field.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Measuring physiological processes in the field requires portable, non-destructive, and highly precise instruments.
The rate of photosynthesis can be quantified by monitoring the consumption of carbon dioxide (\( \text{CO}_2 \)) or the release of oxygen (\( \text{O}_2 \)) from a leaf enclosed within a specialized chamber.

Step 2: Detailed Explanation:
Let us evaluate the analytical functions of the listed instruments:
1. Infrared Gas Analyzer (IRGA) (C): IRGA is the heart of portable photosynthesis systems (such as the LI-6400 or LI-6800).
It works on the principle that heteronuclear gas molecules, like \( \text{CO}_2 \) and water vapor, absorb specific wavelengths of infrared (IR) radiation.
By measuring the difference in IR absorption between air entering the leaf chamber (reference) and air leaving the chamber (sample), the instrument calculates the rate of \( \text{CO}_2 \) uptake (photosynthesis) and water loss (transpiration) in real-time.
Its portability and accuracy make it the standard field instrument in plant physiology.
2. Isotope-ratio mass spectrometer (IRMS) (A): This is a large, laboratory-based instrument used to measure the abundance of stable isotopes (e.g., \( ^{13}\text{C}/^{12}\text{C} \) or \( ^{18}\text{O}/^{16}\text{O} \)) to study long-term water use efficiency, but it is not portable.
3. ICP-OES (B) and AAS (D): These are laboratory spectrometers used to determine the elemental composition (mineral nutrient analysis) of digested plant tissue samples.

Step 3: Final Answer:
The portable instrument used to measure real-time photosynthesis in the field is the Infrared Gas Analyzer (IRGA), corresponding to option (C).