Hidden poisons in rhododendron nectar

Chemical defence in Rhododendron

The toxins responsible for the poisonous effects of Rhododendron are grayanotoxins.

These are highly oxygentated diterpenoids that have been presumed to be produced elsewhere in the plant as a natural chemical defence against insects.

Recent research in the laboratories at Kew has now shown, for the first time, that grayantoxins do indeed act as a natural defence for the plant and protect Rhododendron plants from herbivore attack.

Thrips are tiny winged insects which include some species that are pests of agricultural and horticultural crops around the world, damaging plants by sucking the life out of them.

One particular species, the glasshouse thrip (Heliothrips haemorrhoidalis), frequently causes damage to plants in the glasshouse collections at Kew.

It has been recorded feeding on the leaves of Rhododendron simsii, a species native to China, Taiwan and Burma, growing in Kew’s Temperate House.

Curiously, the thrips rarely damage the terminal 5–10 leaves which seem always to be left healthy and which are renewed each year.

These are the youngest leaves and are produced in a whorl each spring prior to flowering. Arguably these are the most critical leaf tissues so more important to protect.

Chemical analysis of these terminal leaves at Kew revealed they contained very high concentrations of grayanotoxins – the same compounds that poisoned Pompey’s army.

Biological studies testing the toxin which we isolated from the plants against the thrips showed that the grayanotoxins were indeed toxic to them, protecting these precious terminal leaves.

It’s interesting that the presence of the compounds seems to be only expressed locally in the plant tissue – perhaps to conserve energy and save just the most important new leaves each year.

Kew's recent study provides the first scientific evidence that the grayanotoxins are a natural defence mechanism against herbivores of Rhododendron. 

What do nectar toxins in Rhododendron do to bees?

The buff-tailed bumblebee (Bombus terrestris) was unable to detect the toxin at natural levels so it was certainly not repellent. Even after 30 days feeding on a nectar containing the toxin at natural levels, bumblebees were fine.

However, when the nectar was fed to honeybees they died in a matter of hours.

This was surprising as we know that in Turkey the local sub-species of Apis mellifera visited Rhododendron and made honey from it.

Testing the compounds against a third species, the mining bee, Andrena scotica (previousy A. carantonica) showed it was harmful to this species too.

Although it didn’t increase mortality in the mining bee, it severely affected their normal behaviours important for foraging such as time spent flying and increased time spent grooming, as well as showing other symptomatic behaviours associated with toxicity.

The changing chemistry of invasive plants

But perhaps all is not lost.

We carried out a landscape scale chemical analysis of populations of Rhododendron ponticum in Ireland and compared this to the nectar toxin levels in its range in Spain and Portugal where the species is also native.

While the amounts of nectar toxins varied within the invasive and native ranges, expression was overall lower in the invasive range: in 1 out of 5 plants in the invasive range the toxins were absent from nectar altogether.

Other chemicals in the nectar which were not toxic to bees changed very little between the invasive and native ranges.

So in light of the specific change to the grayanotoxin, and its specific toxicity to some bee species, its expression may have been influenced during invasion by pollinator-mediated selection.

In other words, in the invasive range pollination was limited by the toxin so too few species were available for pollination. So the effect was for the plants to reduce or stop entirely the production of nectar toxins to ensure sufficient pollination success.

Synthesis

Rhododendrons produce toxins to protect against herbivores feeding on their leaves.

The toxins occur in nectar too where they ensure that only the best pollinators (that can tolerate the toxins) visit. 

The toxin levels in nectar of invasive plants are lower than that of native plants.

So what does this mean for the defence of the plant?

If toxin levels in nectar and leaves are correlated with one another, we can assume that lower overall toxin levels in invasive plants might make them more exposed to herbivores. 

Which function takes precedence for the plant: chemicals to optimise pollination or defence? Spatially explicit studies of nectar secondary compounds are needed to understand how chemical variation affects floral ecology.

Our understanding of plant invasion must consider plant defence not just in relation to protecting against herbivores but the consequences for beneficial interactions too.