UV Flowers
Honeybees and other insects with ultraviolet visual sensitivity are attracted to flowers containing nectar guides, which are patterns of pigments with ultraviolet absorption. Yellow pigments (and many white ones as well) look blue-violet or blue-green in ultraviolet light. What humans see as a plain white or yellow bloom is revealed to a bee as a complex diagram complete with lines and patches that serve as landing guides to efficiently move bees to nectar and pollen.
When plants began to colonize the land, plant pigments evolved to help screen plant tissues from the deadly power of ultraviolet radiation. Pigments suffuse leaves and stems and do for the plant what melanin does for us; absorb or reflect the ultraviolet wavelengths of light. (The ozone layer, which protects us from most UV radiation today, did not form until oxygen released through photosynthesis by early land plants was converted to ozone in the atmosphere.)
It was only later in evolutionary time that brightly colored flowers began to appear. As flowering plants enlisted insects and birds in their reproductive efforts, pigments that originally screened them from the sun began to serve a different purpose – alerting and guiding pollinators to receptive flowers.
The color of a flower is often a good clue as to what might pollinate it. Yellow and white are sort of default flower colors because they are highly reflective and easily seen by a number of potential pollinators. White, sweetly fragrant flowers that shine even in moonlight attract night-flying moths and bats.
Many yellow flowers contain nectar guides, which are patterns of pigments with ultraviolet absorption. The pigments belong to a group of organic compounds called flavanols. Honeybees and other insects with ultraviolet visual sensitivity are attracted to the center of these flowers. Interestingly, red flowers don’t contain these nectar guides as they tend to be pollinated by birds.
Yellow is certainly a common flower color in meadows and prairies in mid to late summer and fall, when bees, wasps, and flies are most numerous. However, the color they see is not the gold or chrome yellow that we perceive. Bees and many other insects can see ultraviolet light, an ability that likely evolved early on, when the only pigments available for floral advertisement where the ones that reflect ultraviolet light.
Yellow pigments (and many white ones as well) look blue-violet or blue-green in ultraviolet light. What we see as a plain white or yellow bloom is revealed to a bee as a complex diagram complete with lines and patches that serve as landing guides to efficiently move bees to nectar and pollen.
Some overly hybridized flowers bred for human purposes and not insects’ abilities can be very confusing to six-legged visitors. I have watched a honeybee stagger aimlessly on a florist’s lily. What to me was a lovely pattern of white, yellow and pink spotting on an ivory background was incomprehensible to the bee. It was as if someone had pulled out all the street signs and rearranged them randomly, and the bee could find no guidance to the nectar in the flower’s throat.
Red is probably the easiest color for us to see. A small red object stands out at a great distance, so it is no wonder that red and orange are the colors of warning and caution on our highways and byways. Birds see red very clearly as well, explaining why many bird dispersed fruits and hummingbird-pollinated flowers are crimson or scarlet.
Most red pigments are derived from a group of flavenoid pigments called anthocyanins (a few plant families, like the Cacti and Portulas, achieve brilliant reds and purples with another group of pigments called betalins). The word anthocyanin comes from the greek anthos, for flower, and kyanos, for blue. One of the fascinating things about this group of pigments is — through a bit of cellular wizardry — the color they express can be either brilliant red or sultry blue. Most of the pigment is contained in vacuoles (sacks) within the cell, and the pH of the vacuolar fluid affects the expression of color. Simply put, an acid environment will yield red and an alkaline blue or purple. Which brings us back (finally!) to the subject of this article: blue flowers.
Changes in cellular pH help explain why some flowers like those of Virginia Bluebells can change from fairly bee-invisible pink in bud to bee-attracting sky blue when the flowers are receptive. A quick change in acidity helps the bees to perform at their maximum efficiency – through a simple trick of chemistry trick that would make any office manager proud
All of this does not help to explain the rarity of delphinium blue flowers in nature. Perhaps it is because it is really the ultravioletly enhanced appearance that is most important for attracting insects, and blue violet or purple is more effective in this regard. Indeed, many true cobalt blue flowers (Salvia azurea, for example) have evolved for hummingbird, not insect pollination (like us, hummingbirds are blind to ultraviolet light)
It is, then, one of the beautiful accidents of nature that the color perception we humans developed evolved independently in birds and insects. If, like most mammals, they could only see shades of gray, thing of how drab our world would be. No kaleidoscopic butterfly wings, bird feathers, or beetle carapaces, and worst of all – no luminous, magnificent, brightly decorated flowers.
Many yellow flowers contain nectar guides, which are patterns of pigments with ultraviolet absorption. The pigments belong to a group of organic compounds called flavanols. Honeybees and other insects with ultraviolet visual sensitivity are attracted to the center of these flowers. Interestingly, red flowers don’t contain these nectar guides as they tend to be pollinated by birds.
