Effect of Al on root growth and callose formation in aspen. A Time series of the root growth rates of plants treated with no or increasing concentrations of Al for 2 d. Asterisks indicate significant differences between treated and control plants (repeated measures ANOVA; p < 0.05). B Callose content in root tips (RT) after 2 d of Al treatment. Callose content is expressed as a curdlan equivalent (CE). The means and standard error values from four independent roots are shown. Different letters indicate significant differences between treatments (ANOVA; p < 0.05).