How Tree Tobacco Overcomes the Extreme Bitterness of its Nectar

The nectar of tree tobacco (Nicotiana glauca) is composed of a particularly bitter combination of nicotine and anabasine – and yet sunbirds never stop visiting the plant. A new study conducted at the University of Haifa – Oranim Campus points to the reason why: the mixture of chemicals in the nectar affects the sunbird’s memory*

What does a plant whose nectar is bitter do to make pollinators return to visit its flowers and thereby ensure the plant’s continued propagation? Apparently it may cause its pollinators to forget the bitter taste of the nectar.

A new study by Dr. Shai Markman from the Department of Biology and the Environment at the University of Haifa – Oranim Campus reveals not only that the nicotine and anabasine found in the nectar of the tree tobacco’s (Nicotiana glauca) flowers are likely to give it a somewhat bitter taste, seemingly making it a problem for the plant, but they could also be the solution to this conundrum.

“Ostensibly the plant has conflicting interests – both materials give the nectar a bitter taste that is likely to repel rather than attract pollinators. But in practice the sunbird – the main pollinator of tree tobacco – never stops visiting the plant despite the fact that these materials interfere with the bird’s learning process, apparently through memory loss,” says Dr. Markman.

The survival of some plants depends on their ability to attract pollinators drawn to, among other things, the sweet nectar of its flowers. In addition, some plants also need protection from grazing animals which, for some, is achieved by the presence of bitter and toxic substances, like nicotine or caffeine, in their leaves. However, this mechanism is a double-edged sword, because the same substance that protects the plant’s leaves is also found in the nectar of its flowers, giving it a bitter taste – which may repel some of its pollinators.

It is exactly this problem that exists for the tree tobacco plant, as its leaves are protected not only by nicotine but by anabasine as well, a substance thought to be bitterer and more toxic than nicotine, and the two substances are also found in the nectar of its flowers. Surprisingly, the tree tobacco consistently attracts a pretty little pollinator – the sunbird. This strange behavior of the bird aroused curiosity in Dr. Markman’s laboratory.

In the present study, the researchers sought to examine whether substances found in nectar have any effect on the sunbird’s behavior. In the first part of the study, they prepared artificial yellow flowers (the color of the tree tobacco flowers), filling some of them with a solution of sugar water, while the rest of the flowers were filled with plain water. In order that the sunbirds would be able to identify the rewarding flowers from afar – those filled with the sugary solution – the researchers painted them a different hue of yellow than that of the non-rewarding flowers.

tobacco_tree_flowersxThe researchers waited for the sunbirds to learn which flowers were rewarding, and when the birds reached 80% accuracy, the researchers provided sugar water with a mixture of anabasine and nicotine in a feeder. After an hour they returned the sunbirds to the artificial flowers. At this point all the flowers contained only water, no sugar water. The assumption being that if there was no impairment in the birds’ learning process, they would sample more flowers with the rewarding color – at least until they learn that the situation has changed (namely all flowers are non-rewarding and are filled with water).

The results showed that sunbirds approached the flowers colored with the rewarding hue less frequently and the flowers colored with the non-rewarding hue more frequently – when compared to what had happened before they drank from the solution containing nicotine and anabasine. Or, in other words, the birds’ foraging performance had decreased, and their learning patterns seem to have been impaired. According to the researchers, despite the fact that consumption of low concentrations of nicotine for short periods of time is known to help focus attention and improve learning, the finding of the present study show that when this is combined with anabasine, learning ability decreases.

However, this mechanism of potential memory retention reduction is not the only way in which the plant causes the sunbirds to return. The researchers estimate that it takes about half an hour for the mixture of nicotine and anabasine to influence the sunbirds. Or, in other words, during the first half hour the birds can still learn that the nectar is bitter and avoid it, which means that they won’t consume enough of the mixture to impair their memory. The researchers offer a hypothesis explaining why this happens, linking it to the flower clusters of the plant and the curiosity of the sunbirds.

“The sunbird is curious by nature, and it samples a large number of flowers in its vicinity. The tree tobacco plant produces mixtures containing different proportions of anabasine and nicotine in each of the flowers in its clusters, so that in some of them the bitterness is much less pronounced, so much so that in some flowers the nectar may even be very sweet. In this way, the sunbird has an incentive to sample more and more flowers. It is also a mechanism that promotes pollination, not only increased consumption of the nectar mixture,” explains Dr. Markman.

“Perhaps the plant did not actually undergo a distinct process of evolutionary selection to produce bitter nectar, but because this ‘mistake’ may be advantageous to its reproduction – it results in the nectar remaining bitter, at least for now”, says Dr. Markman, and adds, “Apart from the limited analogy with the impact of smoking and nicotine consumption in humans, this research may also have implications for future studies of crops in which the nectar of the plants’ flowers contain secondary substances which may affect the performance of pollination by pollinators, and thereby affect crop yields.”

 

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