Business Is Blooming
THEY’RE CERTAINLY BEAUTIFUL, but a flower’s purpose is more advertising than aesthetics. Blooming petals are billboards, grains of pollen are the goods exchanged, and nectar is the liquid currency beneath it all. This fascinating biological fluid drives a complex economy between the plant and animal kingdoms.
Nectar consists mainly of water and sugar, but, when examined closely, nectars display a remarkable chemical complexity that we are just now beginning to appreciate. Nectar-producing plants serve up unique cocktails to nurture mutually beneficial relationships with one or more animal visitors. In return, those who partake are expected to provide the accidental service of pollination, or sometimes even serve as bodyguards.
Many plants offer up edible rewards besides nectar, namely pollen, but this protein-rich powder is expensive to produce and risky to part with. Nectar, on the other hand, is cheap. A plant need only blend the product of photosynthesis (sugar) with water and offer it up.
Nectaries, the tissues that secrete nectar, are typically associated with flowers, but they can be found on essentially any part of a plant from stamen to stem. Flowers of the extensive mint family, or Lamiaceae (including Salvia spp. and Monarda spp.), conceal their nectaries at the base of each floral stalk, while milkweeds (Asclepias spp.) pool their nectar amidst a crown of petals. Each arrangement is presented to match a preferred pollinator’s anatomy.
Some plants, including various passionflowers (Passiflora spp.), offer nectar outside of flowers. These extrafloral nectaries attract ants, which in turn protect the plant from insect herbivores such as caterpillars. One legume, the introduced, non-native Senna occidentalis, has even adopted the dreaded red imported fire ant (Solenopsis invicta) as an extrafloral bodyguard.
Plants tweak nectar chemistry to suit their animal mutualists. Sucrose, or common table sugar, consists of two simpler sugar molecules linked together. Though not a hard and fast rule, sucrose tends to be the primary carbohydrate in nectars favored by hummingbirds, butterflies, moths and long-tongued bees (which include honeybees, bumblebees and carpenter bees). A majority of garden flowers favor this sugar chemistry, but columbines (Aquilegia spp.) and Indian paintbrushes (Castilleja spp.) are particularly sucrose rich. Nectars favored by flies and short-tongued bees (including sweat bees and many desert-dwelling bees) more often contain simple sugars like glucose and fructose. The nectars of yellow bells (Tecoma stans) and succulents in the genus Echeveria (both found in deserts) contain notably high levels of these.
Insects that feed solely on floral nectar as adults, such as butterflies, prefer their nectars laced with particular amino acids as a way to obtain essential protein building blocks. The flowers of Lantana spp. are particularly attractive to amino acid-seeking pollinators. Nectarivorous birds and bats, on the other hand, frequently supplement their diet with insects or fruits and show no preference for protein-laced nectars.
It does a flower no good to give away its nectar to a visitor unlikely to gather or deposit pollen. Jimsonweed (Datura wrightii) deters thieves by hiding its treasure at the bottom of a long flower tube where only the lengthy proboscises of certain hawkmoths (family Sphingidae) can reach it. Other flowers, like those of catalpa trees (Catalpa spp.), lace their nectar with toxins to which only their preferred pollinators are resistant.
Many flowers even spike their nectar with brain-altering chemicals. Nicotine in wild tobacco nectar (Nicotiana spp.) and the caffeine-laced nectar of non-native citrus flowers are thought to promote more frequent return visits. That might sound familiar to a few of us.
There are antimicrobial additives to keep nectar fresh, perfumes to lure pollinators close, even flavors to repel some and entice others. Whatever its recipe, nectar is clearly much more than sugar water. It holds nearly as much diversity as the plants and animals who exchange its sweet reward. Indeed, in every garden, nectar’s chemistry is what keeps business blooming.
Joe Hanson, Ph.D., is a science writer, biologist and YouTube educator. He is the creator and host of “It’s Okay To Be Smart,” an award-winning science education show from PBS Digital Studios that celebrates curiosity and the pleasure of finding things out.