New Research Challenges Long-Held Belief on Why Plants Are Green
A long-held belief about why plants appear green has been challenged by recent scientific research. Contrary to the common textbook explanation that chlorophyll reflects green light, a 2020 study indicates that chlorophyll primarily absorbs blue and red light.
Instead of reflection, green light is less efficiently absorbed by chlorophylls 'a' and 'b' than red or blue light. This increases its probability of being diffusely scattered from other plant structures, such as cell walls.
Molecular plant biologist Olli Virtanen and his team at the University of Turku in Finland clarified that chlorophylls do not reflect light.
The True Mechanism of Green Appearance
The traditional explanation, based on how uniform objects reflect light, does not fully apply to complex, heterogeneous structures like plant leaves. While chlorophyll's strong absorption of violet-blue and red light and weaker absorption of green light has been understood for decades, this weaker absorption does not equate to reflection.
Green Light's Vital Role in Photosynthesis
Green light is not entirely inactive in plant processes. Leaves absorb green wavelengths only 20 to 30 percent less efficiently than red or blue light. Its ability to penetrate deeper into leaves and plant canopies allows green light to contribute to photosynthesis in lower layers.
Experimental Evidence Unveils Scattering Structures
Experiments conducted by Virtanen's team involved comparing green, yellow, and white leaves, which have varying chlorophyll levels. The findings showed that yellow and white leaves, with less or no chlorophyll, reflected more green light than green leaves. Green leaves reflected less than 10 percent of incident green light, while yellow and white leaves reflected significantly more.
This suggests that structures other than chlorophyll, possibly cellulose in cell walls, are responsible for scattering green light.
The Human Perception of Vivid Green
The perception of green in plants is also influenced by human vision. While leaves without chlorophyll may reflect more green light, they also reflect light across the entire spectrum, leading to perceptions of white or yellow.
The human eye is most sensitive to green wavelengths, meaning green appears brighter even at lower intensities. Therefore, despite green leaves absorbing most green light, the small amount scattered is sufficient for humans to perceive a vivid green color.
These findings were published in the Journal of Biological Education, aiming to correct the common misconception regarding chlorophyll's role in plant coloration.