[R] bold face labelling/expression

[hagric]

Dear colleagues,

I have a great problem in using "expression" for axes labels. I want the 
labels in bold face (i.e.: par(font.lab=2)). When typing

boxplot(y ~ groups, names = "" , xlab = "", ylab = "", axes = F)
axis(side=1, at=c(1,2), xlab=c(expression(H[2]*O),"others")),

I do not get a bold face labelling. Does anyone know, how I could proceed?
Thank you very much.

Best regards
H.