Welcome to Double Helix magazine’s Q&A section – Microscope. We take a close look at small questions full of big ideas.

Q: What makes some plants have purple leaves?
The Helix reader, Sudashree from South Australia

As Kermit the Frog once sang: “It’s not easy being green”. However, many plants are – although some leaves strike out with their own palette of yellows, reds, and even purples.

Plants sprout purple or green leaves for similar reasons – they need to absorb sunlight to grow. The difference in colour lies in the mix of pigments found inside leaves that allow them to do their job effectively.

The different colours that our eyes see are mostly caused by different wavelengths of light. The Sun emits a mix of wavelengths, which our brains interpret as white light. Some colours in sunlight are scattered in the atmosphere, making the Sun seem a little yellow and the sky look blue. However, the Sun emits more green light than any other colour.

When sunlight hits the leaves of most plants, a pigment called chlorophyll absorbs the red, purple and blue wavelengths of light. This causes chemical reactions that result in the production of glucose. But, green light bounces off the chlorophyll pigment. If you’re looking at green leaves, the green light will bounce right into your eye, which is why the plant looks green.

Why would plants reject green light, especially when there is so much of it in sunlight? The short answer is: nobody knows. Chlorophyll evolved to be rather good at its job, and simply happens to not use green light. If plants produced pigments that absorbed green, leaves would probably look black. However, it’s possible they would also overheat!

Purple leaves contain a lot of a pigment called anthocyanin. Many green plants have a small amount of anthocyanin inside their leaves. While the role of this pigment is still debated, it may be protective. Some plants live in shady areas and need to be sensitive to catch as much light as possible. Occasional bursts of light through the canopy might overwhelm their light-absorbing chemicals. These plants make so much anthocyanin their leaves look dark purple.

Purple plants might seem like the punks of the plant world, but it’s probably to help their survival while soaking up sunshine.

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