A warming climate could lead to a more fragrant world, but it might disturb an intricate communication system used by plants, according to a review published recently in Trends in Plant Science.
When Jarmo Holopainen grew white cabbages in a greenhouse in Finland, he found that over many years of sunlight and elevated levels of carbon dioxide, the plants' communication with the world was altered.
Cabbages and most vegetation emit chemicals called biogenic volatile organic compounds, or BVOCs, that are mostly undetectable by humans. But they notify other organisms of danger and opportunity, and also function as methods of plant-plant communication. When we can smell them, they manifest as fragrances.
Over the past 30 years, higher temperatures have increased emissions of the compounds by 10 percent, according to the review. And an increase in temperature of 2 to 3 degrees Celsius could lead to a further 30 to 45 percent increase.
Studies with a heath plant native to the sub-Arctic showed that a 3- to 4-degree temperature rise would increase emissions between 56 and 83 percent, according to the review.
"Plants emit more of the message, but also change the wording of the message," said Josep Penuelas, lead author of the report and director at the Global Ecology Unit at Universitat Autonoma de Barcelona in Spain. "Pollinators and predators could be confused."
BVOCs are extremely volatile, and at higher temperatures, the plants not only synthesize more of them, but the chemicals also vaporize easily. There are many different types of volatile compounds with exotic names such as isoprenes, monoterpenes, green leaf volatiles, herbivore-induced volatiles, and others. They have different origins and functions, and serve essential roles in plant growth, defense and reproduction.
Self-protection and reproduction through scents
Some plant species, for instance, protect themselves from predators by emitting volatile compounds when their leaves get damaged. These scents attract the predators of herbivores to the scene.
"It is a form of intelligent self-protection," said Penuelas.
Other scents can attract pollinators such as bees to help with reproduction.
But the effects of altered communication signals are not straightforward, and more research is necessary, said Penuelas. It could also affect the timing of lifecycle events, or phenology, he said.
"We know that biological spring is advancing now," he said. "If the odors arise earlier and insects are not there, there could be a mismatch in biological processes."
Holopainen found that with his cabbages, higher levels of carbon dioxide could weaken the signals given out by the plants to specific predators when an insect herbivore fed on its leaves. Other predators were unaffected. The work was published in the journal Plant Physiology.
This could be because an excess of emissions creates enough background noise to mask the signal for certain predators, said Holopainen.
BVOCs have also been shown to have a cooling effect on plants. They get degraded to aerosols that can cool plants down, serving a function similar to respiration.