Felix Bast

Who doesn't like the smell of the first rain — such an enticing scent. This refreshing aroma has a formal name, "Petrichor", which in Greek translates to the “ether of rocks”. 

Science as early as the 1960s revealed that this scent is a combination of three distinct substances in the air that get released from the soil when the season's first raindrops hit it. First, a mixture of two volatile plant oils, stearic acid and palmitic acid, which the plants released during the dry season preceding the first rain. When the rain hits the dry earth, these oils transfer from the soil to the air. Second, a volatile organic molecule called geosmin is released by Actinomycete bacteria like Streptomyces and a few soil-inhabiting cyanobacteria. Finally, the third is the ozone molecule, formed in the lightning and brought down by rain droplets. The earthy, musty, and pleasant smell of petrichor is a combination of these three substances getting into our noses.

Just like petrichor, another unforgettable smell for many of us is the scent of cutting grasses. But, however pleasing it may seem to us, the smell is a distress signal for the grass. The scent is due to a group of compounds called Green Leaf Volatiles (GLVs), primarily a combination of aldehydes, alcohols, esters, and ketones. These molecules can trigger wound-healing responses in the cut parts of plants, which help the plants to heal faster. In addition, these compounds also have antibacterial and antifungal properties that contribute to minimizing the chances of infection in wounds.

A recent study found that GLVs can indirectly act as pesticides (Allmann and Baldwin 2010). This is because as GLVs get in the saliva of the caterpillar — the pest — the caterpillar becomes more attractive to its natural predators — Geocoris insects. The Geocoris eats the caterpillars, helping the plants to get rid of this pest. GLVs are also precursors to Ozone, which is a known pollutant in the lower atmosphere. GLVs can also cause urban smog during winter, significantly decreasing visibility (Kirstine, Galbally et al. 2002). This is one reason for the rampant smog episodes during winters across north India.

Research on petrichor and GLVs are examples of an enthralling field of science, phytosemiotics, the study of plant communication. The area emerged as a fringe idea in the 1980s when scientists conjectured trees communicate with each other via "W-waves" — the language of trees, a notion quickly discounted as pseudoscience by established scientists. Remember that Indian polymath Jagadish Chandra Bose did pioneering studies on the electrophysiology of plants since the 1900s and believed that…read more on NOPR