Why do plants produce fragrance? Floral scents are designed to attract pollinators, of course. Fruit scents help attract seed dispersers. However, plants emit a variety of scents even outside of these specific situations. Forest scents are an essential element of forest bathing, and their components have been demonstrated to have positive psychological and physiological effects on people. Trees in the forest emit these scents from their leaves and trunks, not to refresh people, but to bring some benefit to themselves. In ancient times, it was believed that such scents were merely wastes or surpluses of plant metabolites. It is true that some of the fragrance compounds emitted by plants are released without much meaning. For plants, which do not have excretory organs, volatilizing and discarding them in this way may have a positive meaning. However, the fact that plants have evolved the ability to produce thousands of volatile compounds must have a more positive meaning than simply disposing of them.
It has been estimated that plants release up to 36% of the carbon they have worked so hard to capture through photosynthesis as volatile compounds outside the plant body. Evolutionary selection pressures would not allow such waste for no reason. The volatile compounds produced and released by plants must be endowed with some function that provides some benefit to plants. The Blue Mountains of western Australia are so named because of the subtle blue color of their foothills. This blue color is said to be due to the absorption of part of the sunlight by isoprene emitted from the eucalyptus trees that grow wild here. Plants produce up to 300 million tons of isoprene per year, which is a nuisance and an air pollutant. Plants also produce and release volatile compounds when they are attacked by herbivores or infected by pathogens. Apparently, when plants are under some stress, they begin to produce volatile compounds and dissipate some of them. In this way, the production and dissipation of volatile compounds may be a way for plants to avoid stress. If these volatile compounds reach the surrounding plants and are accepted by them, and if the accepted plants understand the reason why the compounds are released, they should be able to take some measures to increase the probability of stress avoidance even before they are directly exposed to stress.
We have been studying how a group of volatile compounds called "Green Leaf Volatiles (GLVs)" are produced in plants and what their physiological significance is. It is not present in large amounts in intact plants, but it is rapidly biosynthesized and partly released into the atmosphere when plants are injured. These volatile compounds are the main cause of the green smell and weeds you get when you mow the grass in your garden. Previous studies have shown that the scent of greenery enhances the resistance of plants to pests and diseases. It has also become clear that plants communicate with each other using various odor compounds, including green fragrances. By studying the functions of these odor compounds in detail and clarifying their mechanisms, we may be able to develop a new agricultural system using odor compounds.
