Question

Write a note on photosynthetically active radiation.

Hint:

PAR is the visible-light slice of the full solar spectrum, not all of sunlight.

Answer 1

Step 1: Understanding the Concept:
Sunlight reaching the earth contains a broad range of wavelengths, but plants can only use a specific part of that range for photosynthesis.

Step 2: Define the term.
Photosynthetically Active Radiation (PAR) is the portion of solar radiation, roughly in the wavelength range of 400 to 700 nanometres (the visible light range), that green plants and other photosynthetic organisms can actually absorb and use to drive photosynthesis.

Step 3: State how much of it is actually used.
Even though PAR itself is a big share of sunlight, plants only manage to capture about 2 to 10 percent of it, because of factors like leaf reflection, transmission of light through leaves, and losses during the chemical reactions of photosynthesis.

Final Answer:
PAR is the 400-700 nm band of sunlight usable for photosynthesis, of which plants convert only a small fraction, about 2-10 percent, into chemical energy.