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The Nature of Nectar
Plants & Gardens News | Volume 20, Number 2 | Summer 2005
by Niall Dunne
It's widely believed, even among plant lovers, that nectar is a simple, sugary secretion whose sole purpose is to lure flower pollinators with a sweet-tooth reward. But saying nectar is just a high-carb aphrodisiac for bees and butterflies is the equivalent of saying that a pint of Guinness stout is just a beer (yes, I'm Irish).
A bumble bee forages in the flowers of a trumpetweed (Eupatorium fistulosum).
Nectar, you see, has a complex chemistry and ecology. It even has its own scientific discipline: "ambrosiology," named after ambrosia, the legendary food of the Greek gods. Two academic journals have recently devoted long, excruciatingly dry volumes to the biological investigation of nectar—so it must be important stuff.
Actually, researchers have been diving into open flower corollas and cavernous floral spurs (granted, with micropipettes) to plumb nectar's depths since the 1970s. They've found a lot more down there than just sugar and water, including significant amounts of proteins, lipids, antioxidants, minerals, vitamins—and even toxins.
Researchers have also been matching nectar characteristics such as sugar ratios and amino acid content with different types of pollinators. And there's been a lot of focus on nonfloral or "extrafloral" nectar, which can pop up almost anywhere on a plant and serve important defense functions.
In short, we're discovering that nectar inspires not only sweet symbiosis between plants and animals but also intriguing and even dangerous interactions: insects battling for the finest quaff, chemical warfare against microbial intruders, and more. Take a spyglass out into the garden and you'll see what I mean.
Nectar Biology
Nectar is produced by specialized glands called nectaries on or embedded in the surfaces of many plants. These glands can vary in structure and position from plant to plant. For instance, under an electron microscope, leaf nectaries of cotton look like sunken pits or doughnut holes, while the petiole nectaries of passionflowers (Passiflora species) resemble fluffy pancakes.
Though we generally associate nectar with flowers and pollinator attraction, many botanists believe that nectaries appeared well before the evolution of flowering plants. (The most ancient nectar-producing plant still living today is the bracken fern, Pteridium aquilinum, which has extrafloral nectaries at the base of its leaves.) Indeed, some botanists have speculated that nectaries originally evolved as a means of actually excreting excess sugar (nice trick, eh?) from mature foliage.
The surface of each nectary is connected to the plant's vascular system via a group of specialized cells, which are either chock-full of chloroplasts (for photosynthesizing sugar) or starch (stored sugar). Basically, these cells supercharge plant sap with supplemental sugars and other substances as it passes through on its way to the nectary surface and is secreted as nectar.
Extrafloral nectaries on the leaf petiole of a Ricinis (castor bean) species. (Photo by David Webb)
Secreted nectar may cling to the open surface of a nectary, or it may drain into a flower corolla or nectar spur—the elongated spurs of columbines comes to mind—where it's harder for undesirable pollinators to reach and less vulnerable to evaporation. Though floral nectaries can occur in almost any part of the flower, they are most often found at the base of petals and stamens.
Production of nectar has been found to vary with such factors as time of day, weather, flower age, plant size, and soil moisture. For many plants it's a major energy investment, costing them upwards of 37 percent of their daily photosynthetic effort. Not surprisingly, a lot of plants resorb and reuse their nectar.
Pollinator Preferences
Only three sugars occur in quantity in nectar—sucrose, fructose, and glucose—and their ratios vary widely depending on the plant species. Sucrose-rich nectars tend to be associated with concealed nectaries and long-tongued pollinators (bees, butterflies, moths, or birds), while fructose- and glucose-dominated nectars are linked to exposed nectaries and short-tongued bees and flies.
The sugar concentration of nectar is also important. Pollinating birds, bats, and butterflies prefer fairly dilute nectar with a 15 percent to 25 percent sugar concentration. Bees—big saps that they are—like it more syrupy, with concentrations at around 50 percent. Some plants, such as horse chestnut (Aesculus hippocastanum), secrete nectar that's up to 70 percent sugar; visiting bees need to dilute this nectar with their saliva before they can suck it up.
Sugar concentration is linked to the physical and behavioral traits of different pollinators. That's because as concentration goes up, so does the viscosity or gooeyness of the nectar. Animals with narrow feeding tubes (such as butterflies) or those that hover around a flower while eating (such as hummingbirds) are best served by a more liquid lunch. There's a general rule of thumb that the bigger a pollinator is, the more nectar it needs—and the more nectar its adapted plants have. Bird-pollinated species of Banksia in Australia, for example, produce so much nectar in their dense flower heads that Aborigines also collect it for food.
There are correlations between the concentration of amino acids (the building blocks of proteins) in nectar and the type of floral visitor, indicating that nectar is a significant protein source for some animals (especially those that don't supplement their diet with pollen or other protein-rich material). For instance, butterflies need more protein in their nectar than birds, which can always eat the odd bug. I'm not sure what this says about the Atkins diet, but the most protein-rich nectars are found in nasty-smelling flowers that attract dung flies, like those of purple trillium (Trillium erectum).
Competition for Consumption
As an important energy and nutritional resource for many animals, nectar generates a good deal of competition. In many cases, animal hierarchies form around plants, with dominant species getting first dibs on the most desirable nectar. Such is the case, for instance, with weaver ants on extrafloral nectaries of barbwire vines (Smilax australis) in Queensland, Australia. The dominant ant species monopolizes the nectar with the highest amino acid and sugar concentrations and leaves the cruddy stuff for more subordinate species.
Researchers have found a similar hierarchy among four stingless bee species foraging on flowers of the tapi-tapi tree (Santiria laevigata) in the forests of Sarawak, Malaysia. In a slight variation of the "early bird catches the worm" adage, the most aggressive bee species feeds—and excludes other species from feeding—in the morning, when nectar flow from the flowers is at its peak.
Plant Defense
A Pseudomyrmex ant guard feeds on nectar from an Acacia extrafloral nectary. (Photo by the Max Plank Society; www.mpg.de)
Extrafloral nectar protects plants indirectly by attracting beneficial insects that subsequently defend foliage, flowers, and developing fruit against herbivores. The best-known example of this kind of arrangement is the one between African Pseudomyrmex ant guards and swollen-thorn Acacia plants. Nectar rewards keep the foliage of the Acacia plants swarming with the aggressive stinging ants all year round, and these ants can repel even the most leathery-lipped of hungry giraffes.
University of Arizona researchers looking at barrel cacti (Ferocactus wislizeni) in the Sonoran Desert have found that ants on plants with active extra-floral nectaries feed much more aggressively on foraging caterpillars compared with ants on plants with inactive or no floral nectaries. They concluded that when ants load up on carbohydrates from nectar, their craving for protein increases—as does their efficiency as bodyguards. (Call it the "meat-and-potato syndrome.")
But it's not just ants getting in on the action. For example, jumping spiders feed on the extrafloral nectar of the partridge pea (Chamaecrista nictitans) in old farm fields in New Jersey. In exchange for the sweet treat, the spiders defend the leaves against herbivorous insects—a behavior that's been shown by scientists at Rutgers to boost seed production in the plants.
When situated near flowers, extrafloral nectaries attract insect guards that defend the flowers against "nectar robbers"—organisms that feed on nectar without providing any pollination service. Nectar robbers generally drill or chew holes into flowers and drain off nectar without ever contacting the plant's reproductive organs. And this can create problems. For instance, bumblebee thieving of scarlet gilia (Ipomopsis aggregata) nectar causes a reduction in visits by the plant's bona fide hummingbird pollinators and lowers the average number of seeds in the plant's fruit. The gilia could learn a few things from the morning glory Ipomoea carnea, which uses extrafloral nectar to woo ants into playing defense against nectar-robbing carpenter bees.
Complex Chemistry
The nectar of some plants can take an active role in defense, too. For example, a protein (nectarin 1) produced by floral nectaries of ornamental tobacco plants (Nicotiana) generates high levels of hydrogen peroxide in the nectar, which protect flower parts from fungal infection. Toxic compounds in northern catalpa tree (Catalpa speciosa) nectar are known to repel nectar-thieving ants but not make the nectar unpalatable to legitimate bee pollinators.
Something even more complex takes place in almond tree nectar. The presence of amygdalin (a cyanide-containing chemical) increases during the season and makes the nectar less and less attractive to nectar robbers. But it also deters the plants' bee pollinators. What gives? Scientists at the University of Tel Aviv, in Israel, think the toxic buildup comes from amygdalin-laced pollen grains falling into the nectar due to bee activity—the more visited the flower, the more toxin in the nectar. The bees are forced to search for the least toxic nectar, and this may ultimately be a strategy by the plants to increase bee movement between individual trees and enhance cross-pollination.
Now, that's some food for thought while you're out admiring the unadulterated beauty of your flowers.
Niall Dunne is the editor of Plants & Gardens News