In Lomatia, the pollen is naturally poisoned

When you think of floral chemistry, you probably think of perfume, but there should be another side to it. Edita Ritmejerytė and colleagues point out that plant defence strategies suggest that it’s the most accessible and the most important parts of a plant that should have the strongest defences. Given the importance of flowers to reproduction, that means that they should be strongly defended. The botanists decided to look for a hidden defence, cyanogenic glycosides (CNglycs). These chemicals break down on contact with water into hydrogen cyanide. This is what happens when a hungry herbivore starts chewing on a plant.

Ritmejerytė and colleagues used florets from the genus Lomatia, everygreen shrubs from the southern hemisphere, to see how the plants used CNglycs in the flowers. They searched for evidence that the plants used the CNglycs strategically. Seeing exactly where these these chemicals were used would give clues to how the plant fought off attackers.

The team found high concentrations of CNglycs in the flowers, which fits well with optimal defence theory. However, the authors say that the chemical defences se not evenly distributed in the flowers. “There was interspecific variation in floral CNglyc concentrations among eight congeneric Lomatia species, however, all had high concentrations in the pollen presenter, intermediate concentrations in ovaries, and based on more detailed analysis of three species, extremely high concentrations of CNglycs were restricted to pollen/anthers and especially the spiral-walled cells on the pollen presenter.”

“Other prominent and persistent pistillate tissues such as the style, however, contained relatively low CNglyc concentrations, whereas loose, more transient (i.e. less apparent) specialised cells on the pollen presenter, the fitness value of which is unclear, had the highest CNglyc concentrations. It does appear, from this and previous studies, that fine scale optimal allocation of floral defences is species-specific.”

The authors conclude that the tissue-specific distribution of defences show that strong selection is happening. However, the concentration of these defences in transient cells remains a puzzle.