New York, Apr 6 (IANS): A team of researchers has shown that ozone in the lower layers of the atmosphere decreases crop yields in maize and changes the types of chemicals that are found inside the leaves.

The researchers said that while stratospheric ozone protects us by filtering out the Sun's ultraviolet radiation, tropospheric ozone is a harmful pollutant.

"Ozone pollution is higher in the northern hemisphere and peaks in the warmer, summer months," said researcher Jessica Wedow from the University of Illinois Urbana-Champaign in the US.

"High concentrations of ozone pollution overlap temporally and spatially with crop growth, so it is important to study how the high ozone concentrations affect crop yields," Wedow added.

Ozone is formed when nitrous oxide, released from industries and tail pipes of cars, is broken down by sunlight and chemically reacts to form ozone.

For the study, published in the journal Plant Direct, the researchers looked at three types of maize -- two inbred lines B73 and Mo17 and the hybrid cross B73 × Mo17.

Surprisingly, they found that chronic ozone stress caused a 25 per cent decrease in yield in the hybrid crops while the inbred plants remained unaffected. The hybrid plants also aged faster than the inbred crops.

To understand why B73 × Mo17 was affected, the researchers measured the chemical composition of the leaves.

These results suggest that the since the hybrid maize is more sensitive to ozone exposure, they may be producing more chemicals that deal with the consequences of chronic ozone stress, the team said.