Sweet nectar gives ferns a bitter taste

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?

An unwanted caterpillar

Photo: Koptur et al.

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.

Alun Salt. ORCID 0000-0002-1261-4283

When he's not the web developer for AoB Blog, Alun Salt researches something that could be mistaken for the archaeology of science. His current research is about whether there's such a thing as scientific heritage and if there is how would you recognise it?

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