The loss of green colour in plants due to the presence of even a low concentration of `SO_2` is called .........

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the term used to describe the loss of green color in plants due to the presence of sulfur dioxide (SO₂) at low concentrations.

2. **Identifying the Cause**: The presence of SO₂ in the environment can lead to various physiological effects in plants. One of the significant effects is the reduction in the production of chlorophyll, which is the pigment responsible for the green color in plants.

3. **Defining Chlorophyll**: Chlorophyll is essential for photosynthesis, the process by which plants convert light energy into chemical energy. It gives leaves their green color.

...