Photosynthetically active radiation. Photosynthetically active radiation, often abbreviated PAR, designates the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis

Photosynthetically active radiation (PAR) refers to the range of wavelengths of light that are essential for photosynthesis in plants. It is the range of light that is most effectively used by plants to drive the process of photosynthesis.

The correct answer is option 'A', which states that the range of PAR is 400-700 nm. Here's why:

Photosynthesis and Pigments:
- Photosynthesis is the process by which plants convert light energy into chemical energy, using pigments such as chlorophyll.
- Chlorophyll is the primary pigment responsible for capturing light energy during photosynthesis.
- Different pigments can absorb light at different wavelengths, but the most important pigments for photosynthesis are chlorophyll a and chlorophyll b.
- Chlorophyll a absorbs light most efficiently in the red (around 680 nm) and blue (around 430 nm) regions of the spectrum.

Range of PAR:
- The range of PAR is defined as the range of wavelengths of light that are absorbed by chlorophyll and other pigments to drive photosynthesis.
- The range of 400-700 nm includes the wavelengths of light that are most efficiently absorbed by chlorophyll a and b.
- This range covers the blue (around 430 nm), green (around 500 nm), yellow (around 570 nm), and red (around 680 nm) regions of the spectrum.
- Light outside this range, such as ultraviolet (UV) light below 400 nm or infrared (IR) light above 700 nm, is less effective for photosynthesis.

Importance of PAR:
- PAR is crucial for plant growth and development.
- It provides the energy required for photosynthesis, which is essential for the production of glucose and oxygen.
- The wavelengths within the PAR range stimulate various physiological processes in plants, including leaf expansion, flowering, and fruit ripening.
- PAR also plays a role in regulating plant responses, such as phototropism (growth towards light) and photoperiodism (timing of flowering based on day length).

Conclusion:
In conclusion, the range of wavelengths called Photosynthetically Active Radiation (PAR) is 400-700 nm. This range includes the wavelengths of light most effectively absorbed by chlorophyll a and b for photosynthesis. PAR is essential for plant growth, energy production, and various physiological processes.