The almond tree's secret weapon

The nectar of the almond tree produces an extraordinary and dangerous poison. This is the only known plant to have this poison in its flowers' nectar. A study carried out at the University of Haifa has revealed that bees are mysteriously drawn to the toxic substance.

Has the almond tree developed a unique way of drawing potential pollinators? A group of researchers at the Department of Environmental and Evolutionary Biology and the Department of Science Education at the University of Haifa-Oranim speculate that the toxin called amygdalin that is found in almond tree nectar is in fact an evolutionary development intended to give that tree an advantage over others in its surroundings.

Previous studies have already shown that amygdalin can be found in almond nectar at a concentration of 4-10 milligrams per liter. It also known that the almond tree is the only plant to have this toxin in its flowers' nectar; in fact, the tree's subgenus classification is Amygdalus, after the toxin it produces. For small mammals this is a deadly substance and as it is highly concentrated in the seeds of unripe wild bitter almonds, these almonds are also dangerous for human consumption.

A group of researchers, headed by Prof. Ido Izhaki along with Prof. Gidi Ne'eman, Prof. Moshe Inbar and Dr. Natarajan Singaravelan, investigated why it is that this plant produces such a potent toxin – a by-product of which is cyanide – in its nectar. They explain that the presence of amygdalin in the nectar is seemingly incompatible with the nectar's purpose of attracting insects to the flower to extract food and pollinate it and thereby contribute to the plant's reproduction.

The researchers exposed honey bees to plates of nectar that had varying concentrations of the toxin and a plate of nectar without the toxin. The team first monitored four different amygdalin concentrations, resembling the natural levels of the toxin in almond tree nectar: 2.5-10 milligrams per liter. A second experiment monitored levels much higher than those found in the natural form: 5-50 milligrams per liter. In both cases and for each of the compositions, the bees preferred nectar containing amygdalin over the amygdalin-free option.

"It is difficult – and sometimes impossible – to determine the workings of evolution, but it is likely that amygdalin is produced in the almond nectar so as to give the almond tree an advantage in reproduction. Based on our observations, we can make a guess at which mechanisms come into play for amygdalin to provide this advantage," Prof. Izhaki explains. For example, even though amygdalin is poisonous for mammals, it is not poisonous for insects, such as the honey bee, and it even produces a stimulant that attracts such insects. Therefore, it is possible that the plant produces it so as to attract potential pollinators. Another possibility is that the almond tree has developed this substance in its nectar as a form of filter: it repulses "non-expert" pollinators, but gives access to the "experts" that have built up resistance to the toxin while providing efficient pollination services for the plant.

The research team, in collaboration with Dr. Malka Halpern, Dr. Yoram Gerchman and research students Svetlana Friedman and Yana Gerstein, are presently examining the possibility of there being an additional mechanism in play: that the nectar toxin prevents inhabitation of bacteria that could spoil the nectar's quality and harm its appeal for potential pollinators, thereby impeding the tree's chances of pollination.

"Pollinating insects have always been lacking, so plants have had to develop ways to take the lead in attracting those that are available, in competition with other plants. Otherwise, they will not be able to reproduce. This is more than just a hypothesis: it is a very practical theory. For reasons that are not fully clear, there is a significant shortage of bees in the world. The worldwide scarcity of available pollinators severely harms agriculture and threatens supplies of produce for the human population. In California there are enormous almond groves that without bees will not produce fruit. Due to the scarcity of bees, the almond farmers in California are compelled to import – from as far away as Australia – truckloads of beehives during the almond's flowering season, so as to ensure pollination," Prof. Izhaki stated.

Source : University of Haifa