Plant eaters are called herbivores and they pose a threat for plants. Understandably, nobody would like to get eaten. In order to prevent tissue loss and damage from herbivores plants have evolved defensive adaptations such as hard to digest tissues and poisonous chemicals. Some plants however, have evolved a different solution.
Plants can use nectar as drivers of beneficial behaviors such as pollination and protection from herbivores. Nectar is an attractive fluid for many animals. It is a rich calorie source and contains nutrients. Many adult insects can only feed on liquids. While animals feed on nectar secreted from flowers, they provide pollination service. Here you can see a carpenter bee covered in pollen grains of the passion flower from head to toe.
Nectar can be secreted from any part of the plant except the roots. It has a complex chemistry containing water, sugars, amino acids, secondary compounds, antimicrobial proteins and even enzymes. Chemical composition of nectar and it’s location of secretion can adjust the interactions of plants with animals.
Nectar production and secretion in nectaries (video has no sound):
Nectar secreted from flowers of jewelweed (Impatiens capensis) can attract hummingbirds for pollination. Hummingbirds, butterflies, moths and long-tongued bees usually prefer sucrose-rich nectar secreted from flowers. Nectar can also attract another group of insects: Those are Ants.
Ants make up only 3 percent of animal diversity yet they may constitute up to half of the total animal biomass on land. Ants are great resource utilizers. They eat anything they can harvest, carry and digest. In nature, if you look around carefully you can observe ants tending aphids. Aphids provide ants with honeydew. In return ants protect aphids from predators such as lady beetles and wasps. Aphids must be changing the chemistry of plants sap in ways that make them attractive to ants. Increased ratios of digestible sugars in honeydew might be one of the changes. Some nectar eating animals and many species of ants lack the enzyme called invertase. Invertase is needed for digestion of sucrose. An Acacia tree from Mexico is known to attract ants by secreting nectar containing invertase.
From the plant point of view, provision of nectar is quite an effective way of getting rid of herbivores such as aphids and recruit ants as guards. Many plants have evolved specialized secretion organs called nectaries. If ants are not bribed properly they can even behave like herbivores. Leafcutter ants (Acromyrmex lundii) can defoliate plants and feed those leaves to a special mushroom inside their nest. Ants are not able to eat the leaves directly but they feed on the fruiting bodies of the mushroom who digests those leaves.
Nectar can turn enemies into allies. The scene where you see a green ground beetle (Calleida punctata) coming up to drink nectar from a passionflower is quite telling. The flower is not yet open but the nectaries at it’s base are active secreting nectar. Ants defend the nectaries and squirt formic acid to repel the beetle. Once ants discover a resource they will defend it aggressively. Notice the surge in ant numbers after the green ground beetle appears. The following video is a longer version of ants defending nectaries:
Nectaries can be found in many different plant parts such as the leaves. Biologists generally distinguish two types of nectaries. Those that are found inside the flowers are called FLORAL NECTARIES. All others are grouped as EXTRAFLORAL NECTARIES. This documentary focuses on the extrafloral nectaries (EFNs) and gives examples from three plant species belonging to three different families.
An evolutionary tree, taken from a 2013 study shows ancestry of plant families and gives us an overview of distribution of EFNs in the plant world. In this tree, names are only given for those families containing EFNs. Branches leading directly to families with EFNs are coloured in red. Branches that split into families with and without EFNs are indicated in black. As we can see EFNs are widely scattered across vascular plant lineages.
However they are entirely absent in gymnosperms, early splitting angiosperms and early ferns. Out of more than 55.000 species of vascular plants living today, less than 4% have evolved EFNs. It may seem like a trivial number but when we think at family level 21% of vascular plant families have at one point evolved these secretory structures. 99% of species containing EFNs are found in flowering plants. Close to 60% of all species with EFNs are found in one major clade of rosids. Rosids form a quarter of all flowering plants.
The family having the highest number of species with EFNs is the legume family with more than 1000 species. Next comes the Mallows and then the Passion flower family. There are three plant families in which all species contain EFNs. The first two are carnivorous plant families called the pitcher plant family (Cephalotaceae) and the sundew family (Drosophyllaceae). The third is a strictly African family endemic to Central and West Africa called Thomandersiaceae. Evolution of nectaries has a dynamic nature with multiple gains and losses across the history of many plant families.
Upland cotton (Gossypium hirsitum)
Extrafloral nectaries are found in many crop plants and upland cotton is one of them. The upland cotton is native to central America and constitutes around 95% of all cotton production in the United States. All species of cotton except one endemic to Hawaii have extrafloral nectaries. Note that Hawaii never had ants until recently introduced by humans.
On the first day of flowering nectar production at the base of the upland cotton flowers shows a distinct peak. This initial high volume of nectar may be necessary for ants to mark the location as high value resource. Nectar production at these nectaries exceeds those found on leaves by up to one hundred times. Nectaries on leaves only become active after herbivores attack.
Garden vetch (Vicia sativa)
Garden vetch is a legume especially attractive to herbivores. This is because legumes can fix nitrogen from the air with the help of bacteria in their roots. Nitrogen is essential for building proteins. EFNs provide an effective way of protecting nitrogen rich tissues using ants. Experiments have shown that Vetches produce additional EFNs when their leaves are damaged by herbivores. However unlike cotton, the volume of nectar secretion remains the same. Vetch nectaries have a distinct purple coloration which may be more visible to ants. Color of the nectaries can turn black if sooty mold starts growing and could be a sign of reduced ant activity.
Passion flower (Passiflora incarnata)
Passion flowers carry the defensive repertoire of extrafloral nectaries to a different level. Passion flower leaves are defended chemically but Heliconius butterflies have evolved tolerance against the toxins. Caterpillars of the Heliconius can be cannibalistic. For this reason, female butterflies must be very careful to avoid laying eggs on plants that have already been visited by other females. Extrafloral nectaries have evolved to mimic the butterfly eggs. This visual deception can behaviorally deter butterflies from laying eggs. Avoiding predators using egg decoys is known as Gilbertian mimicry.
In this documentary, we tried to look at the evolutionary role of extrafloral nectaries. Witnessing the on going coevolution between insects and plants is fascinating and helps us to understand ecological interactions in the natural world.
Many tropical species such as Macaranga (Euphorbiaceae) of the Old World tropical forests or the Cecropia (Urticaceae) of the Neotropical forests are myrmecophytic enable ants to live inside their woody tissues. Macaranga also possess extrafloral nectaries. Researchers hypothesize that extrafloral nectaries may have evolved from plants that fail to close their wounds inflicted by herbivores. Plants tend to close wounds rapidly to prevent infections. Greenhouse experiments on the bittersweet nightshade Solanum dulcamara have shown that wounds caused by herbivores do not heal and continue to “bleed” sugary wound secretions which form droplets just like in extrafloral nectaries. Foraging ants successfully locate these secretions and reduce herbivory on S. dulcamara from slugs and flea beetle larvae.
Music used in the background is from a social experiment called Darwin Tunes which explores how music can evolve without composers using mechanisms of evolution.