Stomatal characteristics and infection biology of Pyrenopeziza betulicola in Betula pendula trees grown under elevated CO2 and O3

Two silver birch clones were exposed to ambient and elevated concentrations of CO2 and O3, and their combination for 3 years, using open-top chambers. We evaluated the effects of elevated CO2 and O3 on stomatal conductance (gs), density (SD) and index (SI), length of the guard cells, and epidermal cell size and number, with respect to crown position and leaf type. The relationship between the infection biology of the fungus (Pyrenopeziza betulicola) causing leaf spot disease and stomatal characteristics was also studied. Leaf type was an important determinant of O3 response in silver birch, while crown position and clone played only a minor role. Elevated CO2 reduced the gs, but had otherwise no significant effect on the parameters studied. No significant interactions between elevated CO2 and O3 were found. The infection biology of P. betulicola was not correlated with SD or gs, but it did occasionally correlate positively with the length of the guard cells.