I couldn’t locate a CSL file for Pakistan Journal of Botany on the journal’s website. If anyone has created or obtained one, I would greatly appreciate your assistance.
You could probably pick a style that is very similar and already in Mendeley, look at some of the articles in the journal and do the small edits yourself to get the proper citation style format through the CSL Editor. Otherwise you could probably contact the journal, since they must have the proper formatted style.