So you thought plants are non-polluting? Three studies released recently by German scientists have turned this belief upside down. According to them, stressed plants release toluene, a smog-forming compound. This discovery could complicate pollution measurements. The results of the experiment show sunflowers and Scots pines emit toluene when stressed due to injury and nutrient deficiency. One of the scientists has been measuring plant emissions in the Amazonian forests. Suprisingly, he found toluene levels increasing in the rainforest canopy far from human sources. "However, at the moment, we cannot explain this by anthropogenic pollution,' he says.