Chlorophyll, Plant Age and UV light

Viola spp illuminated with UV 365nm showing chlorophyll in non-scarred, younger leaves reflecting red wavelengths

Chlorophyll is the plant phytochemical primarily responsible for converting sunlight into energy.  Plants take this chlorophyll generated energy and make carbohydrates to be used for growth.  This entire process of converting water, CO2, nutrients and light into carbohydrates (like glucose) is commonly known as the photosynthesis process, or simply 'photosynthesis'.

Viola spp. under full day spectrum light (reflecting green wavelengths)

Under full spectrum light chlorophyll absorbs all colors except green.  Reds and blues are absorbed by chlorophyll and utilized during photosynthesis.  Because chlorophyll doesn't absorb green under full spectrum light, the green wavelengths are reflected back away from the plants.  This is why most plants appear green to us in the sunlight.

Interestingly however, when illuminated with ultraviolet light, chlorophyll reflects red wavelengths back away from the chloroplasts where it is stored.  When a plant is illuminated with light in the UVa and UVb range the chlorophyll will reflect red hues.

I find interesting the fact that younger, fresh leaves usually reflect back a brighter and fuller red color when illuminated under UVa and UVb than do older leaves.  

As is with human skin, the cuticle and upper epidermis layers of a leaf can become damaged and scarred, a result of the sun's harmful UV rays, as well as from abrasion, pest and wind damage too.  Once the cuticle and epidermis is damaged, light in any form is restricted from entering the leaf to be absorbed or reflected by the chloroplyll.  Where young, fresh leaves may reflect deep red colors when illuminated with UV light, older, damaged leaves may be so scarred that the UV light cannot reach the chlorophyll.

Recognition of these principles can provide clues to a leaf's age, state of health, presence of pests, and other facets of the overall plant's growth cycle.

I find it interesting to take a portable UV light, wearing yellow UV rated safety glasses, and walk across the yard at night.  Plants that have evolved elaborate leaf cuticle and upper epidermis protection systems usually glow red under UV light as the leaf surface is usually free from UV and scarring damage.

Finally, not only do typical plants contain UV red reflective chlorophyll, but as you will notice when out at night with a UV light, algae and some moss covered surfaces will reflect red also.  Algae are photosynthetic eukaryotes and contain chlorophyll too! Mosses also contain chlorophyll.  Flower pots, wrought iron and other objects covered with a thin layer of algae will 'glow red'  at night under UV light.

You may ask, what does the understanding of chlorophyll's reflective color really mean to me?  As one small but important part of nature's life processes, an understanding of chlorophyll and light can help further learn about and understand the deep complexity of Nature's Languages.