Ever since the evolution of the first flowers, nearly 140 million years ago during the Early Cretaceous, plants have been enticing insects with their nectar. Flowering plants in a very short duration rapidly diversified and began dominating most of Earth’s ecosystems. Charles Darwin himself couldn’t even wrap his head around the punctuated diversification of flowering plants. The other lifeforms he studied required far longer time scales to differentiate into a wide array of species. For years this perplexed Darwin. This bewilderment was simply known as his “Abominable mystery.” We now know that as plants and insects formed mutualisms, plants no longer had only herbivory and abiotic selection pressures working on their genome. Selection pressures from their insect counterpart allowed for the rapid selection of certain plant traits. This caused plants to rapidly diversify into the wide diversity we see today.
The Grandfather of modern day biology. Charles Darwin.
Some plants utilize their nectar for purposes other than fertilization. Two to four percent of the 300,000 species of flowering plants produce nectaries outside of their flowers. These extrafloral nectaries mainly attract eusocial insects likes ants. Colonies that locate plants with productive extrafloral nectaries defend the plant against other herbivores. Like all symbiotic relationships, it’s a win-win. The plants get eaten less, while the ant colony receives a steady supply of carbohydrate rich nectar. New research published a year ago wanted to look into this relationship a bit closer. Mariana Pereyra, Gabriel Grilli, and Leonardo Galetto wanted to view this interaction through a mycorrhizal lens.
These researchers from Argentina selected a native plant species (Croton lachnostachyus) that produces extrafloral nectaries. Not only does this plant entice eusocial insects that provide protection against herbivory, but it also engages in a mycorrhizal relationship with arbuscular mycorrhizal fungi (AMF). The two treatments used in this study included a fungicidal treatment that killed the symbiotic fungi living in the plants’ roots, and an herbivore treatment, whereby researchers simulated high and low levels of herbivory by removing leaf tissue with a hole puncher. With these two treatments these researchers analyzed the differences in pollen production, nectar volume and nectar sugar content.
Croton lachnostachyus. From INSTITUTO DE BOTÁNICA DARWINION.
Their study yielded some intriguing results taking place both aboveground and belowground. Croton lachnostachyus produced less pollen with the fungicidal treatment, highlighting the important phosphorus scavenging ability of AMF. With no AMF finding and sending phosphorus to the host plant, stigma produced significantly less pollen. With less pollen production in this particular species, there is a clear, direct fitness reduction seen in plants without their AMF symbiote. Additionally, in control plants with their AMF community intact, Croton lachnostachyus actually produced less nectar at its extrafloral nectaries. Even more interesting is that although it produced less extrafloral nectar compared to plants treated with fungicide, the nectar it did produce was richer in sugar content.
Plants in the high simulated herbivory treatment produced significantly less extrafloral nectar. You would think that in the face of voracious herbivory, plant individuals would increase their extrafloral nectary production. But for this species, as leaf photosynthetic output diminishes, less sugars become allocated to these non-reproductive nectaries. Ness 2003found the contrary to the results of this study, in which increased caterpillar herbivory enhanced the dissolved sugars in extrafloral nectar, enticing more protective ants to fend off unwanted herbivores.
An arbuscular mycorrhizal fungus.
This publication enriched my perspective of the forest floor. These interactions are extremely species specific, which makes conclusions to results challenging. Croton lachnostachyus is known to produce both physical and chemical defenses, so in the face of high herbivory, it most likely shifts its resource allocation to direct defenses instead of indirect ones like extrafloral nectaries. I now realize that the diversity of plant responses in nature are highly context specific, and infinitesimally numerous.