Boxplot - how to rotate x-axis labels to 45°?

Hey all,

R-beginner question!

boxplot(disprt, main = "", xlab = "")

My x-axis labels are pretty long, so they or overlap or they are not shown competely in the plot (las=2).
I would like to rotate them to 35° or 45°, so they are not cut off anymore.
Adding par(srt=45)and theme(axis.text.x=element_text(angle=45)) was not successful and I run out of ideas now.

How is it possible to or rotate the labels or to tell R to show them completely?

Thank you!