Extrafloral nectaries (EFN) are secretory structures that can be found on most above-ground plant surfaces. The nectar is composed of sugars and amino acids that act as an attractant and nutrient reward for visiting defenders. This defense trait is distributed broadly across the angiosperm phylogeny, appearing to be an adaptation that has arisen multiple times. Passiflora is a species-rich genus in the family Passifloraceae that exhibits diverse variation in EFN morphology, nectar chemical composition, and associated insect defenders.
The effectiveness of EFNs as a defensive trait relies on the tri-trophic interactions (plant-herbivore-defender) of the species involved. These interactions will vary depending on herbivore pressure, plant life stage, and defender presence. Variation in an herbivore’s life history strategies, such as number of eggs laid and type of plant tissue where the eggs are laid, will affect herbivore load on an individual plant. Herbivore load should dictate EFN chemical composition and subsequently the type of defender attracted. A variety of defenders from small parasitoids to larger, aggressive ants and wasps have been associated with EFNs. Sugars found in nectar can be an important energy source and attractant, but extrafloral nectar amino acid composition having high ratios of essential amino acids would foster a strong relationship between the defending insect and associated plant.
Results/Conclusions
Passiflora is host to the specialist herbivore Heliconius butterfly, which has an array of egg laying strategies and can vary in herbivore pressure in relation to plant health. Interspecific and intraspecific variation in amino acid composition in the EFNs was explored to determine if the composition is maintained at an individual, population, and species level. EFN amino acid composition appears to be strongly maintained at a species level regardless of the type of nectary or associated Heliconius species life history. EFN composition across Passiflora species exhibit wide variation in total concentration and ratio of the amino acids present. Similarity in Heliconius-host plant relationships and not shared ancestry appear to be the main driver behind this EFN amino acid composition.