The percentage of photosynthetically active radiation (PAR) in the incident solar radiation is less than 50%. PAR refers to the portion of sunlight that is within the wavelength range of 400 to 700 nanometers, which is the range most effectively used by plants for photosynthesis.

Here is a detailed explanation:

1. Definition of PAR:
- PAR is the range of light wavelengths that are absorbed by chlorophyll and other pigments in plants, enabling them to carry out photosynthesis.
- PAR is typically measured in micromoles per square meter per second (µmol/m²/s).

2. Components of solar radiation:
- Solar radiation is composed of various wavelengths, including ultraviolet (UV), visible, and infrared (IR) light.
- Only a small portion of solar radiation falls within the PAR range.

3. Wavelengths of PAR:
- The PAR range is defined as 400 to 700 nanometers (nm), which corresponds to the visible light spectrum.
- Within this range, different wavelengths of light have varying effects on plant growth and development.

4. Importance of PAR for photosynthesis:
- Photosynthesis is the process by which plants convert light energy into chemical energy, using water and carbon dioxide to produce glucose and oxygen.
- Chlorophyll, the primary pigment in plants, absorbs light most efficiently in the blue (400-500 nm) and red (600-700 nm) regions of the spectrum.
- Therefore, light within the PAR range is crucial for driving photosynthesis.

5. Percentage of PAR in incident solar radiation:
- While the exact percentage of PAR in incident solar radiation varies depending on factors such as atmospheric conditions and time of day, it is generally accepted to be less than 50%.
- This means that more than half of the solar radiation that reaches the Earth's surface falls outside the PAR range.

6. Utilization of non-PAR wavelengths:
- Plants can also utilize certain wavelengths of light outside the PAR range, such as UV and IR light, for various physiological processes.
- UV light, for example, can stimulate the production of protective compounds in plants, while IR light can affect plant growth and development.

In conclusion, the percentage of photosynthetically active radiation (PAR) in the incident solar radiation is less than 50%. This indicates that a significant portion of solar radiation falls outside the PAR range, highlighting the importance of considering the entire spectrum of light when studying plant responses to sunlight.