My last year of college, I landed a job at a local native plant nursery. I remember being so excited, but also having really bad imposter syndrome. On my first day, my boss handed me a manilla envelope and simply said, “Process this seed, then seed 300 D40’s,” he walked away and left me in the shade house to fend for myself. I had no clue what a D40 was, and absolutely no idea what this mysterious unlabeled seed was. Upon opening the envelope, I saw these rather large brown, semi-spiky, almost oblong seed pods. The seed pods had beautiful creamy white fluff inside, and I realized this was milkweed (Asclepias). I processed the seed (it’s not as easy to separate the fluff from the seed as you may think…), seeded what I assumed was 300 D40’s and began adding these new pots to the benches where the other milkweed was located.
As I approached the bench I noticed the green stems were yellow with aphids. I ran and got the Dr. Bronner’s and neem mixture I had noticed, and began spraying down the milkweed stems. While my boss had no desire to explain D40’s, or the best way to process milkweed seed, as soon as he saw me with that spray bottle he sure had some commentary… He threw out this term, extrafloral nectaries, and seemed disappointed that I didn’t have an understanding of the mutualistic relationship that was occurring between aphids, ants and the milkweed species. I went home that day and immediately began researching these mutualistic relationships. Google quickly sent me into a deep dive on these extrafloral nectaries and the many mysteries they bring to plant biology.
So what even are extrafloral nectaries…? Essentially extrafloral nectaries are plant parts, unrelated to pollination, that produce and excrete nectar. This nectar is rich in carbohydrates which is appealing to arthropod species (notoriously ants) leading to these really complex and not entirely understood mutualistic relationships. Mutualistic relationships are those that are mutually beneficial, where both organisms are advancing because of the relationship.
Another interesting aspect of these extrafloral nectaries is that these plant parts are considered a plant defense mechanism. There are physical, mechanical and chemical plant defense mechanisms. Plants are sessile, meaning rooted in place, so evolved these ways to combat herbivory. The extrafloral nectaries that plants may maintain are considered a mechanical plant defense mechanism.
In the left-most picture is a fungus gnat and on the right is an ant species. Both are feeding on extrafloral nectaries in Prunus species. Prunus species notoriously produce extrafloral nectaries where the leaf blade meets the petiole (Photo credit of left photo and right photo).
Let’s expand this working definition of extrafloral nectaries. Extrafloral nectaries are present in over one hundred plant families, including ferns. Despite this, almost half of the extrafloral nectary bearing species are from three families. The legume (Fabaceae) family comprises about 30% of the species richness. The remaining 20% is made up of the Passion-Flower (Passifloraceae) and Mallow (Malvaceae) families. The extrafloral nectar is essentially similar or the same among all plant taxa. Almost any above-ground portion of a plant can bear an extrafloral nectary, however, the extrafloral nectaries themselves are extremely diverse depending on the plant. The evolutionary origin, morphology, function, location, abundance, and more causes this variance. So you can understand how diverse these plant parts truly are, scientists have reported that extrafloral nectaries have independently evolved approximately 460 times!
The picture on the right shows a species of ant feeding on the legumes extrafloral nectary (Photo source). The flower on the right is PA native, partridge pea (Chamaecrista fasciculata). It is a member of the Fabaceae family, is a sensitive plant and maintains extrafloral nectaries (Photo source).
To add to the mystic of this plant part, extrafloral nectaries have been reported to actually shape plant community composition. The arthropods feeding on the nectar are effectively guaranteeing the success and reproduction of its host plant by protecting the emergence of new plant parts (leaves, buds, flowers, seeds and more). Essentially the invertebrate feeding on the extrafloral nectar can so relentlessly protect the plant from herbivory, that the herbivore will become conditioned to feed on other species of plants in the community. Adding to this conundrum, theories that contradict this notion are readily considered. Plants with recruited arthropods can influence the likelihood of a pollinator visiting a flower. Scientists noted that the density and aggressiveness of the invertebrate species plays into visitation. Ultimately, if a pollinator is reducing its visit rate to a species due to an aggressive protector, sexual reproduction and seed maturation is less likely to occur, further shaping community composition.
Theories surrounding the purpose of extrafloral nectaries are a lot like the chicken and the egg… Some scientists reflect the expectation that upon the occurrence of herbivory, extrafloral nectary production will increase and therefore arthropod recruitment will increase. Other scientists reported that the increased abundance of extrafloral nectaries will draw arthropods to nest close to the plant (sometimes even within the plant), which can decrease the likelihood of herbivory occurring. Can you see now why extrafloral nectaries are such puzzles to plant biologists?
A few Pennsylvania natives that feature extrafloral nectaries are: elderberry (Sambucus nigra), black cherry (Prunus serotina), American plum (Prunus americana), black locust (Robinia pseudoacacia), basswood (Tilia americana), Eastern redbud (Cercis canadensis), Hackberry (Celtis occidentalis) and many more!
So as it turns out my boss at the native plant nursery was wrong! The mutualistic relationship that was occurring between milkweed, aphids and ants (aphids produce “honeydew” from feeding off the milkweed plant that ants farm), has nothing to do with extrafloral nectaries. Mutualistic relationships in nature are highly complex and extrafloral nectaries are an excellent example of this. Regardless of this puzzle extrafloral nectaries bring to plant biology, next time you’re out exploring green spaces, try and locate these intricate plant parts!
