Certain plants change sex from year to year. The basis for this has eluded botanists, but it has now been revealed that, for a particularly intriguing group of plants, such a change can be triggered by the amount of light they receive.
Some species of flowering plants produce male and female individuals, like most animals. Others produce flowers with both male and female parts, which is handy for fertilization if there are no other members of the same species nearby, but risks inbreeding. Some species change sex from year to year, a phenomenon known as being sexually labile.
Gynodioecious plants complicate things further. Some members of a species are exclusively female, while others have both male and female sexual organs. Despite studies going back to Darwin, they are poorly understood. Particularly little research has been done on labile gynodioecious plants, those that are female one year, hermaphroditic another.
To address this, Dr Sandra Varga of the University of Lincoln experimented with 326 specimens of Geranium sylvaticum over a period of four years, moving them between locations to see what triggered sex changes. Like a lot of other gynodioecious species, G. sylvaticum, also known as woodland cranesbill, is a small plant known for its attractive flowers. They live in both well-lit meadows and under shaded forest canopies.
Varga's finding, published in the American Journal of Botany, is that the choice of sex depends at least in part on light availability. Movements between locations with very different amounts of light triggered shifts in flowering. Varga attributes this to the fact that seed production (female) requires more resources than pollen production (male).
Consequently, when plants are struggling for light, and therefore need to allocate resources with care, it is better to put some of what they have into producing pollen, rather than all of it into the more costly seed production.
The capacity to change sex “is one of the most important developments in the evolution of plant breeding systems,” Varga said in a statement. "Our research clearly showed that sex expression was changeable over the course of the study, and was directly related to light availability."
Varga noted that more species may be sexually labile, whether shifting between male and female or between single sexed and hermaphroditic, than we realize. Without tracking changes to individual plants brought on by substantial alterations in environmental conditions, we cannot know what capacities may exist.
Moreover, as gynodioecy is thought to be an evolutionary stepping stone between other methods of reproduction, an understanding of its causes can shed light on more common approaches to the eternal challenge of passing on genes.