A collection of papers on Extrafloral Nectaries has recently moved into Free Access at Annals of Botany. One of the papers raises the question, can a plant that never flowers have extrafloral nectaries?
Nectar secretion on fern fronds associated with lower levels of herbivore damage: field experiments with a widespread epiphyte of Mexican cloud forest remnants by Koptur et al. examines why ferns produce nectar. The paper starts with a brief review which includes a few facts that startled me. One is that extrafloral nectaries evolved before floral nectaries. This surprises me because I so deeply associate nectar with flowers. Another shock was that nectaries appear on ferns well before ants appear in the fossil record.
This shouldn’t be a surprise, but we’re so used to evolutionary stories being teleological, like plants evolved nectaries to reward insects, that it’s easy to forget that it’s a huge oversimplification that gets things very wrong. Nectaries didn’t evolve in order to do something with a purpose. Instead that plants with nectaries have a better chance of passing their traits to their offspring because they can reward insects. And what if there are no insects? Koptur et al. say that the early appearance of nectaries supports the ‘leaky phloem’ hypothesis, that sugars are forced out of the plant in weak developing tissues to ease hydrostatic pressure in the plant. This might explain how they formed, but once ants arrived did they help select ferns with better nectaries. Do the nectaries in ferns given them an evolutionary advantage?
The nectaries are on the leaves or fronds of the plant. Developing fronds are a prime target for herbivores, so if the ants were drawn into the leaves they could act as a defence. But do they. The experiment, like many of the best ones, sounds quite simple.
At its simplest, you find a plant with a suitable pair of young fronds. On one you paint over the nectaries with nail polish to prevent access to the nectar. You then see how the plants develop and compare the damage on the untreated leaf with the test leaf. Reality is messy, so they actually did a lot more than that to account for other factors — but the basic experiment was does access to the nectaries matter?
The results were clear. The fronds with blocked nectaries had four times the damage of the untreated fronds. The ferns benefited from hosting plants, and the ones that could attract them best got the best defence. The defence works best against invasive species that haven’t co-evolved with the fern and developed counter-defences against the ants.
It’s easy to see nectar as part of the plant’s reproductive strategy, or maybe as part of the reproductive system that’s been repurposed for something else. I think this paper neatly shows that there’s no need to assume any connection at all. There’s a lot more to nectar than bait for pollination.