Our work has shown for the first time that rather than causing the pores, or stomates, on a plant's leaves to close, as was generally assumed, ozone actually inhibits stomatal opening by directly affecting the 'guard cells' that control the opening. Torsethaugen adds that knowing ozone's specific cellular targets may make it possible in the future to breed or to genetically engineer new plant varieties to improve productivity in geographic regions, such as California, with significant ozone exposure. Plants take in the carbon dioxide they need for photosynthesis through their stomates, Torsethaugen explains. They release oxygen made in photosynthesis through the same pores.
Ozone can also enter the plant through the stomates and can affect photosynthesis via that route.
Torsethaugen's experiments point to direct action on the guard cells as an additional path that ozone takes to decrease carbon dioxide assimilation and reduce plant productivity. Torsethaugen conducted the experiments with fava bean plants, an important world food source and a species scientists favor for guard cell studies. Using various techniques, she examined the pores on the leaves of whole plants and portions of leaf surfaces and then studied the isolated guard cells. In whole plants and the leaf surfaces, she found that ozone directly affects the stomatal opening. Using isolated guard cells, she monitored the flow of potassium, in a positively charged or ion form, into and out of the cells.